Table of contents

The neutron-induced positron source NEPOMUC at the FRM II provides a mono-energetic positron beam of high intensity of the order of 10⁹ moderated positrons per second. The new layout of NEPOMUC upgrade is presented and the constraints for operating an in-pile positron source at a research reactor are discussed. Inside the tip of the new beam tube, 80% ¹¹³Cd-enriched Cd is used as a neutron-γ-converter that has a projected lifetime of 25 years of reactor operation and thus ensures positron beam experiments in the long term. The source consists of Pt foils that both generate positrons, by pair production, and moderate them. The layout of these foils, the electric lenses and the magnetic fields for positron extraction and beam formation have been improved. In addition to a higher beam intensity, it is expected that the beam brightness will improve by at least one order of magnitude. The present and planned experiments range from fundamental studies in nuclear, atomic and plasma physics to high-sensitivity and element-selective investigations in surface and solid state physics to applications in materials science. The upgrade of several positron spectrometers as well as new positron beam experiments are presented. In addition, a new switching and remoderation unit will allow us to toggle from the high-intensity primary beam to a brightness enhanced remoderated positron beam.

We demonstrate the feasibility to completely characterize entanglement by negativities of quasiprobabilities. This requires the complete solution of a sophisticated mathematical problem, the so-called separability eigenvalue problem. Its solution is obtained for a non-Gaussian continuous variable quantum state, a two-mode squeezed state undergoing dephasing. This is a standard scenario for experiments with quantum-correlated radiation fields.

In this paper, we explore how potential biomechanical influences on cell cycle entrance and cell migration affect the growth dynamics of cell populations. We consider cell populations growing in free, granular and tissue-like environments using a mathematical single-cell-based model. In a free environment we study the effect of pushing movements triggered by proliferation versus active pulling movements of cells stretching cell–cell contacts on the multi-cellular kinetics and the cell population morphotype. By growing cell clones embedded in agarose gel or cells of another type, one can mimic aspects of embedding tissues. We perform simulation studies of cell clones expanding in an environment of granular objects and of chemically inert cells. In certain parameter ranges, we find the formation of invasive fingers reminiscent of viscous fingering. Since the simulation studies are highly computation-time consuming, we mainly study one-cell-thick monolayers and show that for selected parameter settings the results also hold for multi-cellular spheroids. Finally, we compare our model to the experimentally observed growth dynamics of multi-cellular spheroids in agarose gel.

Verifying correlations within a quantum state is an important task in quantum state engineering, while realistic detectors with imperfect efficiency usually introduce errors into the detection results. With these detection errors, genuine multi-partite entanglement can be invisible. In this paper, we propose a simple method for correcting arbitrary uncorrelated detection errors. The method is based on the processing of data from repetitive experiments and can correct detection errors of any magnitude as long as the error magnitude is calibrated. The method is illustrated with its application in the detection of multi-partite entanglement from quantum state engineering.

It is demonstrated that the bootstrap kernel [1] for finite values of q crucially depends upon the matrix character of the kernel and gives results of the same good quality as in the q → 0 limit. The bootstrap kernel is further used to study the electron loss as well as the dielectric tensor for Si, LiF and Ar for various values of q. The results show that the excitonic effects in LiF and Ar are enhanced for values of q away from the Γ-point. The reason for this enhancement is the interaction between the exciton and high-energy inter-band electron–hole transitions. This fact is validated by calculating the absorption spectra under the influence of an external electric field. The electron energy loss spectra are shown to change dramatically as a function of q.

Recently, there have been significant new insights concerning the conditions under which closed systems equilibrate locally. The question of whether subsystems thermalize—if the equilibrium state is independent of the initial state—is, however, much harder to answer in general. Here, we consider a setting in which thermalization can be addressed: a quantum quench under a Hamiltonian whose spectrum is fixed and whose basis is drawn from the Haar measure. If the Fourier transform of the spectral density is small, almost all bases lead to local equilibration to the thermal state with infinite temperature. This allows us to show that, under almost all Hamiltonians that are unitarily equivalent to a local Hamiltonian, it takes an algebraically small time for subsystems to thermalize.

We examine the optical helicity, the optical spin and the ij-infra-zilches in electromagnetic theory and show that these conserved quantities can be combined to form a new description of the angular momentum associated with optical polarization: one that is analogous to the familiar description of optical energy and linear momentum. The symmetries of Maxwell's equations that underlie the conservation of our quantities are presented and discussed. We explain that a similar but distinct set of quantities, Lipkin's zilches, describe the 'angular momentum' of the curl of the electromagnetic field, rather than the angular momentum of the electromagnetic field itself.

We describe a versatile toolbox for the quantum simulation of many-body lattice models, capable of exploring the combined effects of background Abelian and non-Abelian gauge fields, bond and site disorder and strong on-site interactions. We show how to control the quantum dynamics of particles trapped in lattice potentials by the photon-assisted tunneling induced by periodic drivings. This scheme is general enough to be applied to either bosons or fermions with the additional advantage of being non-perturbative. It finds an ideal application in microfabricated ion trap arrays, where the quantized vibrational modes of the ions can be described by a quantum lattice model. We present a detailed theoretical proposal for a quantum simulator in that experimental setup, and show that it is possible to explore phases of matter that range from the fractional quantum Hall effect, to exotic strongly correlated glasses or flux-lattice models decorated with arbitrary patterns of localized defects.

We report evidence that the body-centered cubic (bcc)–face-centered cubic (fcc) transition that occurs during Ni film growth on a Fe(001) substrate is preceded by a pre-martensitic phase, as demonstrated by low-energy electron diffraction. The corresponding film superstructure is characterized by a displacement of Ni atoms along the main 〈100〉 crystallographic axes of iron, without any rotation of the unit cell with respect to the (001) plane, in contrast with the martensitic transition that shows four fcc Ni domains with the Ni〈211〉 crystallographic directions aligned with the Fe〈110〉 axes. In addition, the martensitic transition is detected not at 6 ML, as previously believed, but above 20 ML if the Ni sample is rigorously kept at room temperature. The surface morphology of the bcc–fcc transition is characterized by the development of Ni mounds oriented along the 〈110〉 directions, as shown by scanning tunneling microscopy.

We report on excitonic spectra of armchair graphene nanoribbons (AGNRs) obtained from a full many-body exact diagonalization of the Hubbard model within low and intermediate correlation regimes and with a complete characterization of the spin multiplicity of the calculated eigenstates. Our results allow us to group these systems into three different families according to the sequence of the one- and two-photon allowed states and the magnitude of the respective optical oscillator strengths within the investigated correlation regime. The oscillator strengths for the one-photon allowed transitions are found to be lower than those obtained previously for zigzag semiconducting single-walled carbon nanotubes, pointing out a qualitatively different photophysical behaviour of AGNRs.

The energy and time correlation, i.e. the chirp, of imaging electron pulses in dispersive propagation is measured by time-slicing (temporal hole burning) using photon-induced near-field electron microscopy. The chirp coefficient and the degree of correlation are obtained in addition to the duration of the electron pulse and its energy spread. Improving temporal and energy resolutions by time-slicing and energy-selection is discussed here and we explore their utility in imaging with time and energy resolutions below those of the generated ultrashort electron pulse. Potential applications for these imaging capabilities are discussed.

Simulations performed with the particle-in-cell code Calder Circ show the feasibility of injection and acceleration of electrons in the laser wakefield created by few-femtosecond laser pulses with moderate energy at the few mJ level. A detailed study of the effect of the carrier-envelope phase of the pulse on the injection is presented. It is shown that using ionization injection with nitrogen as the target gas, the control of the optical phase allows production of high-quality and shot-to-shot stable electron beams. The electron bunches obtained have a relative energy spread of a few per cent, a bunch duration in the sub-fs domain, a divergence close to 10 mrad and a peak energy in the 10 MeV range, and could be produced in the near future at kHz repetition rates.

We report ⁵⁷Fe Mössbauer spectral results in pure and doped Ba(Fe_1−xNi_x)₂As₂ with x = 0.01 and 0.03. We show that all these materials present a first-order magnetic transition towards a magnetically ordered state. In the doped compounds, a broad distribution of Fe hyperfine fields is present in the magnetic phase. We successfully fit the Mössbauer data in Ba(Fe_1−xNi_x)₂As₂ in the framework of two different models: (i) an incommensurate spin density wave (IC-SDW); (ii) a dopant-induced perturbation of the Fe polarization, recently proposed to interpret ⁷⁵As NMR data in Ba(Fe_1−xNi_x)₂As₂, which is valid only in the very dilute limit x = 0.01. Moreover, we show here that these NMR data can also be successfully analysed in terms of the 'incommensurate model' for all doping contents by using the parameters obtained from the Mössbauer spectral analysis. Therefore it is not possible to rule out the presence of an IC-SDW on the basis of the ⁷⁵As NMR data.

Very recently (Cai et al 2010 Phys. Rev. E 82 021921), a simple mechanism was presented by which a molecule subjected to forced oscillations, out of thermal equilibrium, can maintain quantum entanglement between two of its quantum degrees of freedom. Crucially, entanglement can be maintained even in the presence of very intense noise, so intense that no entanglement is possible when the forced oscillations cease. This mechanism may allow for the presence of nontrivial quantum entanglement in biological systems. Here we significantly enlarge the study of this model. In particular, we show that the persistent generation of dynamic entanglement is not restricted to the bosonic heat bath model, but can also be observed in other decoherence models, e.g. the spin gas model, and in non-Markovian scenarios. We also show how conformational changes can be used by an elementary machine to generate entanglement even in unfavorable conditions. In biological systems, similar mechanisms could be exploited by more complex molecular machines or motors.

We investigate the application of metamaterials made of dielectrics with embedded metal nanoparticles for transformation-optics and gradient-index devices. We demonstrate that the permittivity and permeability distribution required for cylindrical cloaking at optical frequencies can be obtained by a composite material made of silver spheres embedded in a PMMA host. We show that this approach can be applied also to devices with an isotropic permittivity distribution and illustrate this for the optical black hole. The key ingredient of our system is the tunability of the plasmonic interactions between the metallic nanoparticles, depending on distance and size. In addition, we discuss possible realizations of the proposed structures.

Noise-assisted transport in quantum systems occurs when quantum time evolution and decoherence conspire to produce a transport efficiency that is higher than what would be seen in either the purely quantum or purely classical cases. In disordered systems, it has been understood as the suppression of coherent quantum localization through noise, which brings detuned quantum levels into resonance and thus facilitates transport. We report several new mechanisms of environment-assisted transport in ordered systems, in which there is no localization to overcome and where one would naively expect that coherent transport is the fastest possible. Although we are particularly motivated by the need to understand excitonic energy transfer in photosynthetic light-harvesting complexes, our model is general—transport in a tight-binding system with dephasing, a source and a trap—and can be expected to have wider application.

We present our recent experimental studies on anomalous luminescence and its connection to Bose–Einstein condensation (BEC) transition of dark excitons in a bulk semiconductor. Our sensitive and quantitative detection of this nonluminous quasi-particle using hydrogen-like internal transitions allows obtaining continuous spectra of dark excitons using a quantum cascade laser. According to quantitative measurements on the two-body inelastic collision cross section of excitons, the system needs to be cooled to sub-Kelvin temperatures. We discuss in detail our recent observation of an explosive phenomenon when the BEC criterion is satisfied (Yoshioka et al 2011 Nature Commun. 2 328) for trapped excitons using a helium-3 refrigerator, and outline a plausible scenario when the BEC transition occurs in an inelastic environment. We also discuss how to increase the condensate fraction in order to study the unique ground state of many-body electric excitations in solids.

We report on electrically switchable polarization and ferroelectric domain scaling over a thickness range of 5–100 nm in BiFeO₃ films deposited on [110] vicinal substrates. The BiFeO₃ films of variable thickness were deposited with SrRuO₃ bottom layer using the pulsed laser deposition technique. The domains are engineered into preferentially oriented patterns due to substrate vicinality along the [110] direction. The domain width scales closely with the square root of film thickness, in agreement with the Landau–Lifschitz–Kittel (LLK) law. Switching spectroscopy piezo-response force microscopy provides clear evidence for the ferroelectric switching behavior in all the films.

We investigate the collective dynamics of electrons in ultracold neutral plasmas driven by an oscillating radio-frequency field. We point out the importance of a sharp density drop at the plasma boundary that arises due to unavoidable charge imbalances, and show that this plasma edge provides the major mechanism for energy absorption from the external field. Using a cold fluid theory, we derive the corresponding absorption frequency and validate our findings by microscopic molecular dynamics simulations. The proposed edge-mode is shown to provide a consistent explanation for the observed absorption spectra measured in different experiments. Understanding the response of the electronic plasma component to weak external driving is essential since it grants experimental access to the time-evolving density and temperature of ultracold plasmas.

Recently, our group demonstrated an ultrafast, low-loss, fiber-loop switch based on a nonlinear Sagnac-interferometer design, using which entangled photons were shown to be routed without any measurable degradation in their entanglement fidelity (Hall et al 2011 Phys. Rev. Lett.106 053901). Such a device represents an enabling technology for a rich variety of networked quantum applications. In this paper, we develop a comprehensive quantum theory for such switches in general, i.e. those based on nonlinear Sagnac interferometers, where the in-coupling of quantum noise is carefully modeled. When applied to the fiber-loop switch, the theory shows good agreement with the experimental results without using any fitting parameter. This theory can serve as an important guiding tool for configuring switches of this kind for future quantum networking applications.

We calculate the renormalized effective two-, three- and four-body interactions for N neutral ultracold bosons in the ground state of an isotropic harmonic trap, assuming two-body interactions modeled with the combination of a zero-range and energy-dependent pseudopotential. We work to third-order in the scattering length a_t(0) defined at zero collision energy, which is necessary to obtain both the leading-order effective four-body interaction and consistently include corrections for realistic two-body interactions. The leading-order, effective three- and four-body interaction energies are $U_{3}\left ( \omega \right ) =-(0.85576\ldots )[a_{\mathrm {t}}(0)/\sigma (\omega )]^{2}+2.7921(1)[a_{\mathrm {t} }(0)/\sigma (\omega )]^{3}+\mathcal {O}(a_{\mathrm {t}}^{4})$ and $U_{4} (\omega )=+(2.43317\ldots )[a_{\mathrm {t}}(0)/\sigma (\omega )]^{3}+\mathcal {O} (a_{\mathrm {t}}^{4})$, where ω and σ(ω) are the harmonic oscillator frequency and length, respectively, and energies are in units of ℏω. The one-standard deviation error ±0.0001 for the third-order coefficient in U₃(ω) is due to numerical uncertainty in estimating a slowly converging sum; the other two coefficients are either analytically or numerically exact. The effective three- and four-body interactions can play an important role in the dynamics of tightly confined and strongly correlated systems. We also performed numerical simulations for a finite-range (FR) boson–boson potential, and it was comparison to the zero-range predictions which revealed that finite-range effects must be taken into account for a realistic third-order treatment. In particular, we show that the energy-dependent pseudopotential accurately captures, through third order, the finite-range physics, and in combination with the multi-body effective interactions gives excellent agreement with the numerical simulations, validating our theoretical analysis and predictions.

The recent discovery of intrinsic di-interstitial stability against the isolated self-interstitial point defects in GaAs has evidenced the importance of such complexes in, for instance, irradiated GaAs. In this paper, we illustrate and discuss diffusion of such complexes in comparison with isolated self-interstitials. In particular, the diffusion barriers of neutral di-interstitials have been calculated in the framework of density functional theory, showing that, in addition to their being stable, di-interstitials can also diffuse rapidly through the lattice, similarly to isolated self-interstitials.

We identify the optimal operating conditions of an entangling two-qubit gate realized by capacitive coupling of two superconducting charge qubits in a transmission line resonator (the so-called 'transmons'). We demonstrate that the sensitivity of the optimized gate to 1/f flux and critical current noise is suppressed to leading order. The procedure only requires a preliminary estimate of the 1/f noise amplitudes. No additional control or bias line beyond those used for the manipulation of individual qubits is needed. The proposed optimization is effective also in the presence of relaxation processes and of spontaneous emission through the resonator (Purcell effect).

Recently, Chen et al (2011 New J. Phys.13 083018) presented experimental results, accompanied by quantum-mechanical analysis, showing that the quantum interference behavior of Bell states could be simulated in a modified Mach–Zehnder interferometer whose inputs are pseudothermal light beams obtained by passing laser light through a rotating ground-glass diffuser. Their experiments and their theory presumed low-flux operation in which the simulated quantum interference is observed via photon-coincidence counting. This work is a comment on the paper by Chen et al (2011). We first show that the Chen et al photon-coincidence counting experiments can be fully explained with semiclassical photodetection theory, in which light is taken to be a classical electromagnetic wave, and the discreteness of the electron charge leads to shot noise as the fundamental photodetection noise. We then use semiclassical photodetection theory to show that the same simulated quantum interference pattern can be observed in high-flux operation when photocurrent cross-correlation is used instead of photon-coincidence counting.

We report on specific-heat and resistivity experiments performed in parallel in a Bridgman-type pressure cell in order to investigate the nature of pressure-induced superconductivity in the iron pnictide compound CaFe₂As₂. The presence of a pronounced specific-heat anomaly at T_c reveals the bulk nature of the superconducting state. The thermodynamic transition temperature differs dramatically from the onset of the resistive transition. Our data indicate that superconductivity occurs in the vicinity of a crystallographic phase transition. We discuss the discrepancy between the two methods as caused by strain-induced superconducting precursors formed above the bulk thermodynamic transition due to the vicinity of the structural instability.

Hydrocarbon decomposition on transition metals provides a practical way of producing graphene. Here, ethylene deposition on Rh (111) is taken as an example. In-situ scanning tunneling microscopy measurements, under various growth conditions and at temperatures up to 1100 K, were carried out, revealing the processes of graphene formation at the atomic level. The initial nucleation stage nearly completely determines the phase in which further C is deposited, graphene or rhodium carbide, and the orientation of the growing graphene patches. We demonstrate that by separating the stages of nucleation and further growth and controlling other growth parameters, we obtain graphene of higher quality, while avoiding carbide formation and controlling the dissolved C to form graphene. Based on these observations, a universal physical picture emerges for graphene formation on metal surfaces.

Entanglement occupies a peculiar position in quantum mechanics (QM). It occurs in quantum systems that consist of space-like separated parts or—more generally—in systems whose observables belong to disjoint Hilbert spaces. The latter is the case with single-neutron systems. Here, we report on a neutron polarimetric experiment, where a triply entangled Greenberger–Horne–Zeilinger state is exploited. The entanglement of spin, momentum and total energy degree of freedom is generated utilizing a suitable combination of radio-frequency and static magnetic fields. An average deviation of expectation values from theory—ideal circumstances—of 0.016(1) confirms the predictions of QM with high accuracy, demonstrating the high-efficiency manipulation of the entangled single-neutron system.

We study the dynamics of a multispecies mixture of laser-illuminated polarizable particles moving inside an optical resonator. Above a certain pump threshold the collective enhanced scattering of laser light into the cavity induces a phase transition from a homogeneous spatial distribution to a common crystalline order. We analytically show that adding particles of any mass and temperature always strictly lowers the minimum pump power required for self-ordering and trapping. This allows to capture and trap new species of atoms, molecules or even polarizable nanoparticles in combination with proven examples, for which a high phase-space density is readily available. Cooperative light scattering mediates effective energy exchange and thus sympathetic cooling between different species without the need for direct collisional interaction. The predicted ordering thresholds and cooling timescales are in the range of current technology for particles with a wide range of mass, polarizability and initial temperature.

Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there remains the crucial challenge of simultaneously closing all major loopholes—the locality, freedom-of-choice and detection loopholes—in a single experiment. An important sub-class of local realistic theories can be tested with the concept of 'steering'. The term 'steering' was introduced by Schrödinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein–Podolsky–Rosen (EPR) argument. Einstein called this 'spooky action at a distance'. EPR-steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization-entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude—for the first time loophole-free—an important class of local realistic theories considered by EPR. Besides its foundational importance, loophole-free steering also allows the distribution of quantum entanglement secure event in the presence of an untrusted party.

Very recently Farhat et al (2011, Phys. Rev. B 84 235105) suggested that arrays of invisibility cloaks may find important applications in low-interference communication, noninvasive probing, sensing and communication networks and so on. We report on the first experimental realization of such an array of broadband invisibility cloaks that operates in the visible frequency range. The wavelength and angular dependences of the cloak array performance have been studied.

The radiative cooling timescales at the centers of hot atmospheres surrounding elliptical galaxies, groups and clusters are much shorter than their ages. Therefore, hot atmospheres are expected to cool and to form stars. Cold gas and star formation are observed in central cluster galaxies but at levels below those expected from an unimpeded cooling flow. X-ray observations have shown that wholesale cooling is being offset by mechanical heating from radio active galactic nuclei. Feedback is widely considered to be an important and perhaps unavoidable consequence of the evolution of galaxies and supermassive black holes. We show that cooling x-ray atmospheres and the ensuing star formation and nuclear activity are probably coupled to a self-regulated feedback loop. While the energetics are now reasonably well understood, other aspects of feedback are not. We highlight the problems of atmospheric heating and transport processes, accretion, and nuclear activity, and we discuss the potential role of black hole spin. We discuss x-ray imagery showing that the chemical elements produced by central galaxies are being dispersed on large scales by outflows launched from the vicinity of supermassive black holes. Finally, we comment on the growing evidence for mechanical heating of distant cluster atmospheres by radio jets and its potential consequences for the excess entropy in hot halos and a possible decline in the number of distant cooling flows.

A visualization scheme for quantum many-body wavefunctions is described that we have termed qubism. Its main property is its recursivity: increasing the number of qubits results in an increase in the image resolution. Thus, the plots are typically fractal. As examples, we provide images for the ground states of commonly used Hamiltonians in condensed matter and cold atom physics, such as Heisenberg or ITF. Many features of the wavefunction, such as magnetization, correlations and criticality, can be visualized as properties of the images. In particular, factorizability can be easily spotted, and a way to estimate the entanglement entropy from the image is provided.

We give a pedagogical introduction to the basic ideas and concepts of the Asymptotic Safety program in quantum Einstein gravity. Using the continuum approach based upon the effective average action, we summarize the state of the art of the field with a focus on the evidence supporting the existence of the non-trivial renormalization group fixed point at the heart of the construction. As an application, the multifractal structure of the emerging space-times is discussed in detail. In particular, we compare the continuum prediction for their spectral dimension with Monte Carlo data from the causal dynamical triangulation approach.

This focus issue on membrane biophysics presents a collection of papers illustrating new developments in modern biophysical research on cell membranes. The work described here addresses questions from a broad range of areas, including cell adhesion, membrane trafficking and activation of cells of the immune system. It also presents recent views on membrane mechanics, the effect of electric fields, as well as on the interplay of mechanics and chemistry and organization at many different scales.

We calculate the dynamic structure factor (DSF) in warm dense beryllium by means of ab initio molecular dynamics simulations. The dynamic conductivity is derived from the Kubo–Greenwood formula, and a Drude-like behaviour is observed. The corresponding dielectric function is used to determine the DSF. Since the ab initio approach is so far only applicable for wavenumbers k = 0, the k-dependence of the dielectric function is modelled via the Mermin ansatz. We present the results for the dielectric function and DSF of warm dense beryllium and compare these with perturbative treatments such as the Born–Mermin approximation. We found considerable differences between the results of these approaches; this underlines the need for a first-principles determination of the DSF of warm dense matter.

Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schrödinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for near-perpendicular emission the qualitative appearance of the spectrum is unaffected.

Electronic nematicity, proposed to exist in a number of transition metal materials, can have different microscopic origins. In particular, the anisotropic resistivity and meta-magnetic jumps observed in Sr₃Ru₂O₇ are consistent with an earlier proposal that the isotropic–nematic transition is generically first order and accompanied by meta-magnetism when tuned by a magnetic field. However, additional striking experimental features such as a non-Fermi liquid resistivity and critical thermodynamic behaviour imply the presence of an unidentified quantum critical point (QCP). Here we show that orbital degrees of freedom play an essential role in revealing a nematic QCP, even though it is overshadowed by a nearby meta-nematic transition at low temperature. We further present a finite temperature phase diagram including the entropy landscape and discuss our findings in light of the phenomena observed in Sr₃Ru₂O₇.

It is theoretically shown that the coherent hopping dynamics of a non-relativistic particle in engineered tight-binding lattices subjected to combined ac and dc driving forces can mimic in Fock space relativistic quantum field theories (QFTs) of strongly interacting fields, enabling access to extreme dynamical regimes beyond weak coupling and perturbative predictions. In particular, the simulation of a QFT model describing a Dirac field strongly coupled to a scalar bosonic field via a Yukawa coupling is proposed, suggesting the possibility of implementing this model using engineered lattices of evanescently coupled optical waveguides with a bent optical axis.

It was predicted a few years ago that a medium with negative index of refraction would allow for perfect imaging. Although no material has been found so far that behaves as a perfect lens, some experiments confirmed the theoretical predictions in the near-field, or quasi-static, regime where the behaviour of a negative index medium can be mimicked by a thin layer of noble metal, such as silver. These results are normally attributed to the excitation of surface plasmons in the metal, which only leads to the restoration of p-polarized evanescent waves. In this work, we show that the restoration of s-polarized evanescent waves and, correspondingly, sub-wavelength imaging by a single dielectric slab are possible. Specifically, we show that at λ = 632 nm a thin layer of GaAs behaves as a superlens for s-polarized waves. Replacing the single-metal slab by a dielectric is not only convenient from a technical point of view, it being much easier to deposit and control the thickness and flatness of dielectric films than metal ones, but also invites us to re-think the connection between surface plasmon excitation and the theory of negative refraction.

A numerical approach is employed to explain transport characteristics in realistic, quantum Hall-based Aharonov–Bohm (AB) interferometers. Firstly, the spatial distribution of incompressible strips, and thus the current channels, are obtained by applying a self-consistent Thomas–Fermi method to a realistic heterostructure under quantized Hall conditions. Secondly, the time-dependent Schrödinger equation is solved for electrons injected in the current channels. Distinctive AB oscillations are found as a function of the magnetic flux. The oscillation amplitude strongly depends on the mutual distance between the transport channels and on their width. At an optimal distance the amplitude and thus the interchannel transport is maximized, which determines the maximum visibility condition. On the other hand, the transport is fully suppressed at magnetic fields corresponding to half-integer flux quanta. The results confirm the applicability of realistic AB interferometers as controllable current switches.

The method of optical conformal mapping is used to design two isotropic devices through which light travels in a unidirectional manner. The first device is a directional emitter. By setting a line current source in a properly tuned refractive index profile, fields can be radiated in only one direction or its opposite without using any reflector or metallic structure. The second proposal is a dual-functional device. It works not only as a directional emitter for an embedded source but also as a quasi-diode for beams, thus having potential on-chip applications. Functionalities of the two designs are verified by finite-element-based simulations. We further investigate the spatial dependence of the refractive index near singularities, and corresponding optimization is proposed in the interests of experimental consideration. Numerical results show that the one-way property is well preserved after the parameter reduction.

We propose various schemes for the dissipative preparation of a maximally entangled steady state of two atoms in an optical cavity. Harnessing the natural decay processes of cavity photon loss and spontaneous emission, we use an effective operator formalism to identify and engineer effective decay processes, which reach an entangled steady state of two atoms as the unique fixed point of the dissipative time evolution. We investigate various aspects that are crucial for the experimental implementation of our schemes in present-day and future cavity quantum electrodynamics systems and analytically derive the optimal parameters, the error scaling and the speed of convergence of our protocols. Our study shows promising performance of our schemes for existing cavity experiments and favorable scaling of fidelity and speed with respect to the cavity parameters.

We present a comparison between weak-lensing and x-ray mass estimates of a sample of numerically simulated clusters. The sample consists of the 20 most massive objects at redshift z = 0.25 and M_vir > 5 × 10¹⁴M_⊙ h⁻¹. They were found in a cosmological simulation of volume 1 h⁻³ Gpc³, evolved in the framework of a WMAP-7 normalized cosmology. Each cluster has been resimulated at higher resolution and with more complex gas physics. We processed it through Skylens and X-MAS to generate optical and x-ray mock observations along three orthogonal projections. The final sample consists of 60 cluster realizations. The optical simulations include lensing effects on background sources. Standard observational tools and methods of analysis are used to recover the mass profiles of each cluster projection from the mock catalogue. The resulting mass profiles from lensing and x-ray are individually compared to the input mass distributions. Given the size of our sample, we could also investigate the dependence of the results on cluster morphology, environment, temperature inhomogeneity and mass. We confirm previous results showing that lensing masses obtained from the fit of the cluster tangential shear profiles with Navarro–Frenk–White functionals are biased low by ∼5–10% with a large scatter (∼10–25%). We show that scatter could be reduced by optimally selecting clusters either having regular morphology or living in substructure-poor environment. The x-ray masses are biased low by a large amount (∼25–35%), evidencing the presence of both non-thermal sources of pressure in the intra-cluster medium (ICM) and temperature inhomogeneity, but they show a significantly lower scatter than weak-lensing-derived masses. The x-ray mass bias grows from the inner to the outer regions of the clusters. We find that both biases are weakly correlated with the third-order power ratio, while a stronger correlation exists with the centroid shift. Finally, the x-ray bias is strongly connected with temperature inhomogeneities. Comparison with a previous analysis of simulations leads to the conclusion that the values of x-ray mass bias from simulations are still uncertain, showing dependences on the ICM physical treatment and, possibly, on the hydrodynamical scheme adopted.

The mode inside a laser cavity may be understood as the interference of two counter-propagating waves, referred to as the forward and backward waves, respectively. We outline a simple experimental procedure, which does not require any additional components, to study the forward and backward propagating waves everywhere inside a laser cavity. We verify the previous theoretical-only prediction that the two fields may differ substantially in their amplitude profile, even for stable resonator systems, a result that has implications for how laser resonators are conceptualized and how the disparate traveling waves interact with nonlinear intra-cavity elements, for example, passive Q-switches and gain media.

The efficient implementation of electronic structure methods is essential for first principles modeling of molecules and solids. We present here a particularly efficient common framework for methods beyond semilocal density-functional theory (DFT), including Hartree–Fock (HF), hybrid density functionals, random-phase approximation (RPA), second-order Møller–Plesset perturbation theory (MP2) and the GW method. This computational framework allows us to use compact and accurate numeric atom-centered orbitals (NAOs), popular in many implementations of semilocal DFT, as basis functions. The essence of our framework is to employ the 'resolution of identity (RI)' technique to facilitate the treatment of both the two-electron Coulomb repulsion integrals (required in all these approaches) and the linear density-response function (required for RPA and GW). This is possible because these quantities can be expressed in terms of the products of single-particle basis functions, which can in turn be expanded in a set of auxiliary basis functions (ABFs). The construction of ABFs lies at the heart of the RI technique, and we propose here a simple prescription for constructing ABFs which can be applied regardless of whether the underlying radial functions have a specific analytical shape (e.g. Gaussian) or are numerically tabulated. We demonstrate the accuracy of our RI implementation for Gaussian and NAO basis functions, as well as the convergence behavior of our NAO basis sets for the above-mentioned methods. Benchmark results are presented for the ionization energies of 50 selected atoms and molecules from the G2 ion test set obtained with the GW and MP2 self-energy methods, and the G2-I atomization energies as well as the S22 molecular interaction energies obtained with the RPA method.

Porous media are ubiquitous in our environment and their application is extremely broad. The common connection between these diverse materials is the importance of the microstructure in determining the physical, chemical and biological functions and properties. Magnetic resonance and its imaging modality have been essential for noninvasive characterization of these materials in the development of catalysts, understanding cement hydration, fluid transport in rocks and soil, geological prospecting and characterization of tissue properties for medical diagnosis. This focus issue highlights recent NMR/MRI technical development, the underlying physics associated with the new technology, as well as novel applications of the technologies.

Scattering experiments on xenon nanoclusters with high-intensity soft x-ray laser pulses from the Free-Electron LASer in Hamburg (FLASH) are performed to investigate different cluster morphologies in the gas phase. Three different types of scattering patterns can be identified. The most frequent pattern of concentric rings reflects the event of a single spherical cluster in focus. Fine interference rings similar to Newton rings appear when two clusters are illuminated at μm distance, revealing three-dimensional information about the location of the clusters. Between 10 and 30% of all hits show a previously unknown twin cluster configuration with two clusters in direct contact. Simulations of scattering patterns for twin clusters with different sizes of the two particles, degree of fusion and orientation in space allow us to explain all the observed patterns.

Size-selected metal-cluster dianions of the elements gold, silver and copper have been photoexcited by nanosecond-pulse and continuous laser irradiation, which leads to electron emission and monomer evaporation. In addition to the observation of these competing decay pathways, there is a reduction of the total cluster-ion intensity, which indicates the neutralization of dianions, i.e. the loss of both surplus electrons. In contrast, the activation of singly charged anionic clusters of the same type results primarily in dissociation by monomer evaporation and not by electron emission. These decay processes as observed for doubly and singly charged cluster anions suggest that the dianions emit two electrons simultaneously, i.e. in a correlated fashion. A classical conducting-sphere approximation confirms that the Coulomb barrier for symmetric two-electron emission is lower than the Coulomb barrier for the emission of a single electron.

It is now well established that, under intense laser illumination, clusters undergo enhanced ionization compared to their isolated atomic and molecular counterparts being subjected to the same pulses. This leads to extremely high charge states and concomitant Coulomb explosion. Until now, the cluster size necessary for ionization enhancement has not been quantified. Here, we demonstrate that through the comparison of ion signal from small covalently bound silicon clusters exposed to low intensity laser pulses with semi-classical theory, their ionization potentials (IPs) can be determined. At moderate laser intensities the clusters are not only atomized, but all valence electrons are removed from the cluster, thereby producing up to Si⁴⁺. The effective IPs for the production of the high charge states are shown to be ∼40% lower than the expected values for atomic silicon. Finally, the minimum cluster size responsible for the onset of the enhanced ionization is determined utilizing the magnitude of the kinetic energy released from the Coulomb explosion.

The quantum Wigner function of an electron scattered by an ion in a strong laser field is considered in the framework of a one-dimensional scattering model with a soft-core Coulomb potential. The Wigner function contains much more information on the scattering process than the projected probability distributions in position and momentum space considered previously. The formation of the above-threshold ionization (ATI) energy spectrum, including ATI peaks, modulations and transients, can be easily explained by using the interference of phase-space trajectories.

The screening of a 2p core-hole in Na clusters is investigated using density functional theory (DFT) applied to an extended jellium model with an all-electron atom in the center. The study is related to recent experiments at the free-electron laser at DESY in which photoelectron spectra from mass-selected, core-shell-ionized metal clusters have been recorded. Relaxed and unrelaxed binding energies as well as Kohn–Sham (KS) orbital energies are calculated in Perdew–Zunger self-interaction-corrected exchange-only local spin-density approximation for valence and 2p core electrons in Na clusters up to 58 atoms. The relaxed binding energies follow approximately the metal-sphere behavior. The same behavior is seen in the experiment for sufficiently big clusters, indicating perfect screening and that the relaxation energy due to screening goes to the photoelectron. Instead, calculating the kinetic energy of the photoelectrons using unrelaxed binding energies or KS orbital energies yields the wrong results for core-shell electrons. The screening dynamics are investigated using time-dependent DFT. It is shown that screening occurs on two time scales, a core-shell-dependent inner-atomic and an inter-atomic valence electron time scale. In the case of Na 2p ionization the remaining electrons in the 2p shell screen within tens of attoseconds, while the screening due to cluster valence electrons occurs within several hundreds of attoseconds. The screening time scales may be compared with the photon energy and cluster size-dependent escape times of the photoelectron in order to estimate whether the photoelectron is capable of picking up the relaxation energy or whether the residual system is left in an excited state.

Transient nanoplasmas in laser-excited metal clusters open the route to probing ultrafast collisional and collective laser–plasma processes in a wide and well-tunable range of densities and temperatures. The transition from a fully degenerate to a nearly classical plasma can occur within a few femtoseconds, accompanied by fundamental changes in the relaxation processes driven by electron–electron collisions (EECs). To investigate the resulting implications for laser–metal–cluster interactions, we developed an extended semiclassical Vlasov–Uehling–Uhlenbeck approach where the collision term resolves time-dependent Pauli blocking and local screening effects for arbitrary levels of degeneracy. Our simulation results for resonant dual-pulse excitations of Na₅₅ reveal an unexpected synergy effect of EECs and collective laser–plasma processes, i.e. a strongly enhanced electron acceleration via plasmon-assisted rescattering in the presence of EECs.

Current breakthrough research on cold atmospheric plasma (CAP) demonstrates that CAP has great potential in various areas, including medicine and biology, thus providing a new tool for living tissue treatment. In this paper, we explore potential mechanisms by which CAP alters cell migration and influences cell adhesion. We focus on the study of CAP interaction with fibroblasts and corneal epithelial cells. The data show that fibroblasts and corneal epithelial cells have different thresholds (treatment times) required to achieve maximum inhibition of cell migration. Both cell types reduced their migration rates by ∼30–40% after CAP compared to control cells. Also, the impact of CAP treatment on cell migration and persistence of fibroblasts after integrin activation by MnCl₂, serum starvation or replating cells onto surfaces coated with integrin ligands is assessed; the results show that activation by MnCl₂ or starvation attenuates cells' responses to plasma. Studies carried out to assess the impact of CAP treatment on the activation state of β1 integrin and focal adhesion size by using immunofluorescence show that fibroblasts have more active β1 integrin on their surface and large focal adhesions after CAP treatment. Based on these data, a thermodynamic model is presented to explain how CAP leads to integrin activation and focal adhesion assembly.

Carrier scattering is known to crucially affect the dynamics of quantum dot (QD) laser devices. We show that the dynamic properties of a QD laser under optical injection are also affected by Coulomb scattering processes and can be optimized by band structure engineering. The nonlinear dynamics of optically injected QD lasers is numerically analyzed as a function of microscopically calculated scattering lifetimes. These lifetimes alter the turn-on damping of the solitary QD laser as well as the complex bifurcation scenarios of the laser under optical injection. Furthermore, we find a pump current sensitivity of the frequency-locking range, which is directly related to the nonlinearity of the carrier lifetimes.

Direct visualization of electronic-structure symmetry within each crystalline unit cell is a new technique for complex electronic matter research (Lawler et al 2010 Nature466 347–51, Schmidt et al 2011 New J. Phys.13 065014, Fujita K et al 2012 J. Phys. Soc. Japan81 011005). By studying the Bragg peaks in Fourier transforms of electronic structure images and particularly by resolving both the real and imaginary components of the Bragg amplitudes, distinct types of intra-unit-cell symmetry breaking can be studied. However, establishing the precise symmetry point of each unit cell in real space is crucial in defining the phase for such a Bragg-peak Fourier analysis. Exemplary of this challenge is the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+δ for which the surface Bi atom locations are observable, while it is the invisible Cu atoms that define the relevant CuO₂ unit-cell symmetry point. Here we demonstrate, by imaging with picometer precision the electronic impurity states at individual Zn atoms substituted at Cu sites, that the phase established using the Bi lattice produces a ∼2%(2π) error relative to the actual Cu lattice. Such a phase assignment error would not diminish reliability in the determination of intra-unit-cell rotational symmetry breaking at the CuO₂ plane (Lawler et al 2010 Nature466 347–51, Schmidt et al 2011 New J. Phys.13 065014, Fujita K et al 2012 J. Phys. Soc. Japan81 011005). Moreover, this type of impurity atom substitution at the relevant symmetry site can be of general utility in phase determination for the Bragg-peak Fourier analysis of intra-unit-cell symmetry.

The theoretical description of complex (dusty) plasmas requires multiscale concepts that adequately incorporate the correlated interplay of streaming electrons and ions, neutrals and dust grains. Knowing the effective dust–dust interaction, the multiscale problem can be effectively reduced to a one-component plasma model of the dust subsystem. The goal of this paper is a systematic evaluation of the electrostatic potential distribution around a dust grain in the presence of a streaming plasma environment by means of two complementary approaches: (i) a high-precision computation of the dynamically screened Coulomb potential from the dynamic dielectric function and (ii) full 3D particle-in-cell simulations, which self-consistently include dynamical grain charging and nonlinear effects. The range of applicability of these two approaches is addressed.

We have studied the resistance of a large number of highly oriented graphite samples with areas ranging from several mm² to a few μm² and thickness from ∼ 10 nm to several tens of micrometers. The measured resistance can be explained by the parallel contribution of semiconducting graphene layers with low carrier density <10⁹ cm⁻² and the one from metallic-like internal interfaces. The results indicate that ideal graphite with Bernal stacking structure is a semiconductor with a narrow band gap E_g ∼ 40 meV.

This paper describes gravity experiments, where the outcome depends upon both the gravitational acceleration g and the Planck constant ℏ. We focus on the work performed with an elementary particle, the neutron.

Ground-state fidelity (GSF) and quantum renormalization group (QRG) theory have proven to be useful tools in the study of quantum critical systems. Here we lay out a general, unified formalism of GSF and QRG; specifically, we propose a method for calculating GSF through QRG, obviating the need for calculating or approximating ground states. This method thus enhances the characterization of quantum criticality as well as scaling analysis of relevant properties with system size. We illustrate the formalism in the one-dimensional Ising model in a transverse field (ITF) and the anisotropic spin-1/2 Heisenberg (XXZ) model. Explicitly, we find the scaling behavior of the GSF for the ITF model in both small- and large-size limits, the corresponding critical exponents, the exact value of the GSF in the thermodynamic limit and a closed form for the GSF for arbitrary size and system parameters. In the case of the XXZ model, we also present an analytic expression for the GSF, which captures well the criticality of the model, hence excluding doubts that GSF might be an insufficient tool for signaling criticality in this model.

We have proposed an experiment for testing the violation of the Leggett–Garg (LG) inequality using an auxiliary probe [1]. In our scheme, a single microscopic qubit (ancillary) is correlated to a quantum ensemble. The operation of the method was demonstrated using liquid-state nuclear magnetic resonance (NMR) [1]. Recently, a new experiment was performed using a very similar approach [2]. A special case in [2], the three-correlation case, is identical to our proposal [1].

A recent paper by Souza, Oliveira and Sarthour (SOS) reports the experimental violation of a Leggett–Garg (LG) inequality (sometimes referred to as a temporal Bell inequality). The inequality tests for quantum mechanical superposition: if the inequality is violated, the dynamics cannot be explained by a large class of classical theories under the heading of macrorealism. Experimental tests of the LG inequality are beset by the difficulty of carrying out the necessary so-called 'non-invasive' measurements (which for the macrorealist will extract information from a system of interest without disturbing it). SOS argue that they nevertheless achieve this difficult goal by putting the system in a maximally mixed state. The system then allegedly undergoes no perturbation during their experiment. Unfortunately, the method is ultimately unconvincing to a skeptical macrorealist and so the conclusions drawn by SOS are unjustified.

Laser-controlled charge exchange is a promising method for producing cold antihydrogen. Caesium atoms in Rydberg states collide with positrons and create positronium. These positronium atoms then interact with antiprotons, forming antihydrogen. Laser excitation of the caesium atoms is essential to increase the cross section of the charge-exchange collisions. This method was demonstrated in 2004 by the ATRAP collaboration by using an available copper vapour laser. For a second generation of charge-exchange experiments we have designed a new semiconductor laser system that features several improvements compared to the copper vapour laser. We describe this new laser system and show the results from the excitation of caesium atoms to Rydberg states within the strong magnetic fields in the ATRAP apparatus.

In most instances, tumors have to push their surroundings in order to grow. Thus, during their development, tumors must be able to both exert and sustain mechanical stresses. Using a novel experimental procedure, we study quantitatively the effect of an applied mechanical stress on the long-term growth of a spherical cell aggregate. Our results indicate the possibility to modulate tumor growth depending on the applied pressure. Moreover, we demonstrate quantitatively that the cells located in the core of the spheroid display a different response to stress than those in the periphery. We compare the results to a simple numerical model developed for describing the role of mechanics in cancer progression.

We investigate an experimental method for imprinting Skyrmion spin textures in a spinor Bose–Einstein condensate by rapidly moving the zero-field center of a three-dimensional (3D) quadrupole magnetic field through the condensate. Various excitations such as 2D Skyrmions and coreless vortices were created in spin-1 sodium condensates, initially prepared in a uniform polar or ferromagnetic phase. The spin textures were characterized with the spatial distribution of the spin tilt angle, which is found to be in good quantitative agreement with the local description of single spins under the field rotation. We demonstrate the creation of a highly charged Skyrmion in a trapped condensate by applying the imprinting process multiple times.

We establish an effective Markov theory for the rotational Brownian motion of hot nanobeads and nanorods. Compact analytical expressions for the effective temperature and friction are derived from the fluctuating hydrodynamic equations of motion. They are verified by comparison with recent measurements and with parallel molecular dynamics simulations over a wide temperature range. This provides unique insights into the physics of hot Brownian motion and an excellent starting point for further experimental tests and applications involving laser-heated nanobeads, nanorods and Janus particles.

We report on the systematic characterization of conductive micro-channels fabricated in single-crystal diamond with direct ion microbeam writing. Focused high-energy (∼MeV) helium ions are employed to selectively convert diamond with micrometric spatial accuracy to a stable graphitic phase upon thermal annealing, due to the induced structural damage occurring at the end-of-range. A variable-thickness mask allows the accurate modulation of the depth at which the microchannels are formed, from several μm deep up to the very surface of the sample. By means of cross-sectional transmission electron microscopy (TEM), we demonstrate that the technique allows the direct writing of amorphous (and graphitic, upon suitable thermal annealing) microstructures extending within the insulating diamond matrix in the three spatial directions, and in particular, that buried channels embedded in a highly insulating matrix emerge and electrically connect to the sample surface at specific locations. Moreover, by means of electrical characterization at both room temperature and variable temperature, we investigate the conductivity and the charge-transport mechanisms of microchannels obtained by implantation at different ion fluences and after subsequent thermal processes, demonstrating that upon high-temperature annealing, the channels implanted above a critical damage density convert into a stable graphitic phase. These structures have significant impact for different applications, such as compact ionizing radiation detectors, dosimeters, bio-sensors and more generally diamond-based devices with buried three-dimensional all-carbon electrodes.

In this paper, we present experimental results on the interplay between two different symmetry breaking mechanisms in a pattern forming system, namely inclined layer convection (ILC) with a spatially modulated heated plate. By varying the relative strength and relative orientation, we explored in detail the interplay of these symmetry breaking mechanisms. We found a stabilization of spatio-temporal chaos and resonant interactions that led to superlattice patterns. The fundamental mechanisms observed should be equally applicable to other pattern forming systems.

Due to the increasing popularity of collaborative tagging systems, the research on tagged networks, hypergraphs, ontologies, folksonomies and other related concepts is becoming an important interdisciplinary area with great potential and relevance for practical applications. In most collaborative tagging systems the tagging by the users is completely 'flat', while in some cases they are allowed to define a shallow hierarchy for their own tags. However, usually no overall hierarchical organization of the tags is given, and one of the interesting challenges of this area is to provide an algorithm generating the ontology of the tags from the available data. In contrast, there are also other types of tagged networks available for research, where the tags are already organized into a directed acyclic graph (DAG), encapsulating the 'is a sub-category of' type of hierarchy between each other. In this paper, we study how this DAG affects the statistical distribution of tags on the nodes marked by the tags in various real networks. The motivation for this research was the fact that understanding the tagging based on a known hierarchy can help in revealing the hidden hierarchy of tags in collaborative tagging systems. We analyse the relation between the tag-frequency and the position of the tag in the DAG in two large sub-networks of the English Wikipedia and a protein–protein interaction network. We also study the tag co-occurrence statistics by introducing a two-dimensional (2D) tag-distance distribution preserving both the difference in the levels and the absolute distance in the DAG for the co-occurring pairs of tags. Our most interesting finding is that the local relevance of tags in the DAG (i.e. their rank or significance as characterized by, e.g., the length of the branches starting from them) is much more important than their global distance from the root. Furthermore, we also introduce a simple tagging model based on random walks on the DAG, capable of reproducing the main statistical features of tag co-occurrence. This model has high potential for further practical applications, e.g., it can provide the starting point for a benchmark system in ontology retrieval or it may help pinpoint unusual correlations in the co-occurrence of tags.

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] in-plane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, non-lattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films.

We have investigated systematically the influence on electronic properties of anatase-TiO₂ of codoping by N and Si at different concentrations using Heyd–Scuseria–Ernzerhof (HSE06) hybrid density functional theory calculations. The optimized total energy shows that TiO₂ codoping by N and Si favours a configuration of two substitutional N atoms located at two adjacent O sites, with one substitutional Si atom at their neighbouring Ti site. We show that N–Si codoping can harvest longer-wavelength visible-light than either those of N and Si monodoping, owing to the contribution from N 2p in the 'forbidden gap' and Si 3s–3p at the tail of the conduction band. Increasing the N doping concentration leads to a larger extent of gap narrowing, which is directly related to coupling between N atoms. Our results suggest that double-hole coupling plays a key role in similar systems to obtain high visible-light photoactivity in TiO₂-based photocatalysts.

We consider spin transport in a two-component atomic Bose gas in three dimensions, at temperatures just above the critical temperature for Bose–Einstein condensation. In these systems, the spin conductivity is determined by spin drag, i.e. frictional drag between the two spin components due to interactions. We find that in the critical region, the temperature dependence of the spin conductivity deviates qualitatively from the Boltzmann result and is fully determined by the critical exponents of the phase transition. We discuss the size of the critical region where these results may be observed experimentally.

In superconducting films under an applied dc current, we analyze experimentally and theoretically the influence of engineered pinning on the vortex velocity at which the flux-flow dissipation undergoes an abrupt transition from low to high resistance. We argue, based on a nonuniform distribution of vortex velocity in the sample, that in strongly disordered systems the mean critical vortex velocity for flux-flow instability (i) has a nonmonotonic dependence on magnetic field and (ii) decreases as the pinning strength is increased. These findings challenge the generally accepted microscopic model of Larkin and Ovchinnikov (1979 J. Low. Temp. Phys.34 409) and all subsequent refinements of this model which ignore the presence of pinning centers.

Numerical simulations of the kinematic induction equation are performed on a model configuration of the Cadarache von-Kármán-sodium dynamo experiment. The effect of a localized axisymmetric distribution of relative permeability μ_r that represents soft iron material within the conducting fluid flow is investigated. The critical magnetic Reynolds number Rm^c for dynamo action of the first non-axisymmetric mode roughly scales like Rm^c_μr − Rm^c_∞∝μ^−1/2_r, i.e. the threshold decreases as μ_r increases. This scaling law suggests a skin effect mechanism in the soft iron discs. More important with regard to the Cadarache dynamo experiment, we observe a purely toroidal axisymmetric mode localized in the high-permeability discs which becomes dominant for large μ_r. In this limit, the toroidal mode is close to the onset of dynamo action with a (negative) growth rate that is rather independent of the magnetic Reynolds number. We qualitatively explain this effect by paramagnetic pumping at the fluid/disc interface and propose a simplified model that quantitatively reproduces numerical results. The crucial role of the high-permeability discs in the mode selection in the Cadarache dynamo experiment cannot be inferred from computations using idealized pseudo-vacuum boundary conditions (H × n = 0).

The time-dependent optical emission of GaAs quantum dots (QDs) is studied using single-dot photoluminescence (PL) spectroscopy with quasi-resonant excitation into the QD d-shell. The QDs are fabricated with a very recently developed method, i.e. by local droplet etching of self-assembled nanoholes in an epitaxial AlAs/AlGaAs heterostructure surface and subsequent filling with GaAs. The PL data are interpreted in terms of a three-level rate model, which yields lifetimes of 390 and 426 ps for the excitons and biexcitons, respectively. The strong dependences of both the PL peak intensities and decay times on the excitation power are quantitatively reproduced by the model. The comparison with various other types of self-assembled QDs shows the trend of a decreasing exciton lifetime with increasing emission energy. This behavior and the short lifetime of the GaAs QDs are discussed on the basis of common models of the exciton radiative lifetime.

Hawking's 1974 prediction that black holes radiate and evaporate has been hinting at a hidden connection between general relativity, quantum mechanics and thermodynamics. Recently, laboratory analogues of the event horizon have reached the level where tests of Hawking's idea are possible. In this paper we show how to go beyond Hawking's theory in such laboratory analogues in a way that is experimentally testable.

We study the dynamics of dark-bright (DB) solitons in binary mixtures of Bose gases at finite temperature using a system of two coupled dissipative Gross–Pitaevskii equations. We develop a perturbation theory for the two-component system to derive an equation of motion for the soliton centers and identify different temperature-dependent damping regimes. We show that the effect of the bright ('filling') soliton component is to partially stabilize 'bare' dark solitons against temperature-induced dissipation, thus providing longer lifetimes. We also study analytically thermal effects on DB soliton 'molecules' (i.e. two in-phase and out-of-phase DB solitons), showing that they undergo expanding oscillations while interacting. Our analytical findings are in good agreement with results obtained via a Bogoliubov–de Gennes analysis and direct numerical simulations.

We study photon condensation phenomena in a driven and dissipative array of superconducting microwave resonators. Specifically, we show that by using an appropriately designed coupling of microwave photons to superconducting qubits, an effective dissipative mechanism can be engineered, which scatters photons towards low-momentum states while conserving their number. This mimics a tunable coupling of bosons to a low-temperature bath, and leads to the formation of a stationary photon condensate in the presence of losses and under continuous-driving conditions. In this paper, we propose a realistic experimental setup to observe this effect in two or multiple coupled cavities, and study the characteristics of such an out-of-equilibrium condensate, which arise from the competition between pumping and dissipation processes.

We discuss a method for constructing generalized Wannier functions that are maximally localized at the minima of a one-dimensional periodic potential with a double well per unit cell. By following the approach of Marzari and Vanderbilt (1997 Phys. Rev. B 56 12847), we consider a set of band-mixing Wannier functions with minimal spread and design a specific two-step gauge transformation of the Bloch functions for a composite two-band system. This method is suited for efficiently computing the tight-binding coefficients needed for mapping the continuous system to a discrete lattice model. The behaviour of the tight-binding coefficients is analyzed here as a function of the symmetry properties of the double well (including the possibility of parity-breaking), in a range of feasible experimental parameters.

We use the exact diagonalization method to analyze the possibility of generating strongly correlated states in two-dimensional clouds of ultracold bosonic atoms that are subjected to a geometric gauge field that was created by coupling two internal atomic states to a laser beam. On tuning the gauge field strength, the system undergoes stepwise transitions between different ground states (GSs), which we describe by using analytical trial wave functions, including the Pfaffian (Pf), the Laughlin and a Laughlin quasiparticle many-body state. Whereas for an infinitely strong laser field, the internal degree of freedom of the atoms can adiabatically follow their center-of-mass movement, a finite laser intensity gives rise to non-adiabatic transitions between the internal states, which are shown to break the cylindrical symmetry of the Hamiltonian. We study the influence of the asymmetry on the GS properties of the system. The main effect is to reduce the overlap of the numerical solutions with the analytical trial expressions by occupying states with higher angular momentum. Thus, we propose generalized wave functions arising from the Laughlin and Pf wave functions by including components where extra Jastrow factors appear while preserving important features of these states. We analyze quasihole excitations over the Laughlin and generalized Laughlin states and show that they possess effective fractional charge and obey anyonic statistics. Finally, we discuss the observability of the Laughlin state for increasing numbers of particles.

We discuss the preparation of many-body states of cold fermionic atoms in an optical lattice via controlled dissipative processes induced by coupling the system to a reservoir. Based on a mechanism combining Pauli blocking and phase locking between adjacent sites, we construct complete sets of jump operators describing coupling to a reservoir that leads to dissipative preparation of pairing states for fermions with various symmetries in the absence of direct inter-particle interactions. We discuss the uniqueness of these states, and demonstrate it with small-scale numerical simulations. In the late-time dissipative dynamics, we identify a 'dissipative gap' that persists in the thermodynamic limit. This gap implies exponential convergence of all many-body observables to their steady-state values. We then investigate how these pairing states can be used as a starting point for the preparation of the ground state of the Fermi–Hubbard Hamiltonian via an adiabatic state preparation process also involving the parent Hamiltonian of the pairing state. We also provide a proof-of-principle example for implementing these dissipative processes and the parent Hamiltonians of the pairing states, based on ¹⁷¹Yb atoms in optical lattice potentials.

Absorption imaging with quasi-resonant laser light is a commonly used technique for probing ultra-cold atomic gases in various geometries. In this paper, we investigate some non-trivial aspects of this method when applying the method to in situ diagnosis of a quasi-two-dimensional (2D) gas. Using Monte Carlo simulations we study the modification of the absorption cross-section of a photon when it undergoes multiple scattering in the gas. We determine the variations of the optical density with various parameters, such as the detuning of the light from the atomic resonance and the thickness of the gas. We compare our results to the known 3D result (the Beer–Lambert law) and outline the specific features of the 2D case.

Cyclotron resonance has been measured in far-infrared transmission of GaAs/Al_xGa_1−xAs heterostructures with an etched trigonal lateral superlattice intended to mimic graphene with lattice constant of the order of 100 nm (about 1000 times larger than that of natural graphene). Nonlinear dependence of the resonance position on magnetic field was observed, as well as its splitting into several modes. Our explanation, based on a perturbative calculation, describes the observed phenomena as a weak effect of the lateral potential on the two-dimensional electron gas. Using this approach, we found a correlation between parameters of the lateral patterning and the created effective potential and obtained thus insights into how the electronic miniband structure has been tuned. The miniband dispersion was calculated using a simplified model and allowed us to formulate four basic criteria that have to be satisfied to reach graphene-like physics in such systems.

High transmission efficiency at terahertz (THz) frequency is reported for a single aperture with sub-wavelength dimensions having a Siemens-star shape, microfabricated in the metal film and surrounded by periodic surface corrugations. Compared to theoretical predictions for a simple circular hole of equivalent area, up to ∼10⁶ transmission enhancements were observed experimentally. Such a pointed-shape aperture was also used to obtain the detailed profile of the electric field distribution in the focal plane of a linearly polarized focused THz beam. Applications could be extended to other regions of the electromagnetic spectrum by appropriate scaling of aperture microstructure.