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(NH₄)₂[FeCl₅(H₂O)], a member of the family of antiferromagnetic A₂[FeX₅(H₂O)] compounds (X = halide ion and A = alkali metal or ammonium ion) is classified as a new multiferroic material. We report the onset of ferroelectricity below ≃6.9 K within an antiferromagnetically ordered state (T_N ≃ 7.25 K). The corresponding electric polarization can drastically be influenced by applying magnetic fields. Based on measurements of pyroelectric currents, dielectric constants and magnetization we characterize the magnetoelectric, dielectric and magnetic properties of (NH₄)₂[FeCl₅(H₂O)]. By combining these data with measurements of thermal expansion, magnetostriction and specific heat, we derive detailed magnetic field versus temperature phase diagrams. Depending on the direction of the magnetic field up to three different multiferroic phases are identified, which are separated by a magnetically ordered, but non-ferroelectric phase from the paramagnetic phase. Besides these low-temperature transitions, we observe an additional phase transition at ≃79 K, which we suspect to be of structural origin.

Self-interference correlation imaging is a recently discovered method that takes advantage of holographic reconstruction when using a spatially incoherent light. Although the temporal coherence of light significantly influences the resolution of the method, it has not been studied either theoretically or experimentally. We present the first systematic study of the resolution in a broadband correlation imaging based on the concept of coherence-induced diffraction. We show that the physical limits of the resolution are reached in a non-dispersive experiment and their examination can be performed by the coherence aperture whose width depends on the coherence length of light and the optical path difference of interfering waves. As the main result, the optimal configuration of the non-dispersive experimental system is found in which the sub-diffraction image resolution previously demonstrated for monochromatic light can be retained even when the white light is used. Dispersion effects that prevent reaching the physical resolution limits are discussed and the dispersion sensitivity of the currently available experiments examined. The proposed concept of the coherence aperture is verified experimentally and its generalization to the concept of the dispersion-induced aperture suggested. As a challenge for future research, possible methods of dispersion elimination are outlined that allow the design of advanced optical systems enabling implementation of the high-resolution white light correlation imaging.

The quantum capacity of bosonic Gaussian quantum channels can be non-additive in a particularly striking way: a pair of such optical fiber type channels can individually have zero quantum capacity but super-activate each other such that the combined channel has strictly positive capacity. This has been shown in Smith et al (2011 Nature Photon.5 624) where it was conjectured that squeezing is a necessary resource for this phenomenon. We provide a proof of this conjecture by showing that for gauge covariant channels a Choi matrix with positive partial transpose implies that the channel is entanglement-breaking. In addition, we construct an example which shows that this implication fails to hold for Gaussian channels which arise from passive interactions with a squeezed environment.

We study numerically time evolution in classical lattices with weak or moderate nonlinearity which leads to interactions between linear modes. Our results show that in a certain strength range a moderate nonlinearity generates a dynamical thermalization process which drives the system to the quantum Gibbs distribution of probabilities, or average oscillation amplitudes. The effective dynamical temperature of the lattice varies from large positive to large negative values depending on the energy of the initially excited modes. This quantum Gibbs distribution is drastically different from the usually expected energy equipartition over linear modes corresponding to a regime of classical thermalization. Possible experimental observations of this dynamical thermalization are discussed for cold atoms in optical lattices, nonlinear photonic lattices and optical fiber arrays.

We study the effective dipole–dipole interactions in ultracold quantum gases on optical lattices as a function of asymmetry in confinement along the principal axes of the lattice. In particular, we study the matrix elements of the dipole–dipole interaction in the basis of lowest band Wannier functions which serve as a set of low-energy states for many-body physics on the lattice. We demonstrate that, for shallow lattices in quasi-reduced dimensional scenarios, the effective interaction between dipoles in an optical lattice is non-algebraic in the inter-particle separation at short to medium distance on the lattice scale and has a long-range power-law tail, in contrast to the pure power-law behavior of the dipole–dipole interaction in free space. The modifications to the free-space interaction can be sizable; we identify differences of up to 36% from the free-space interaction at the nearest-neighbor distance in quasi-one-dimensional arrangements. The interaction difference depends essentially on asymmetry in confinement, due to the d-wave anisotropy of the dipole–dipole interaction. Our results do not depend on statistics, applying to both dipolar Bose–Einstein condensates and degenerate Fermi gases. Using matrix product state simulations, we demonstrate that use of the correct lattice dipolar interaction leads to significant deviations from many-body predictions using the free-space interaction. Our results are relevant to up and coming experiments with ultracold heteronuclear molecules, Rydberg atoms and strongly magnetic atoms in optical lattices.

We developed a classical potential to model phase-change materials based on the binary chalcogenide alloy of GeTe that are currently exploited for memory applications. Our potential is based on the recently proposed extension of the Tersoff potential plus additional terms to better reproduce the structure of the amorphous and the crystalline phases of GeTe. The parameters defining the potential reported in this work were fitted to reproduce the energies and forces of a database of reference structures obtained via density-functional theory molecular-dynamics simulations. This paper reports on the method used to construct the potential and on its validation against first-principles calculations either available in literature or part of this work. We found that the structural properties of amorphous GeTe were well reproduced. The advantage of the current implementation toward more flexible neural network-based methods is that most of the parameters can be reconnected to physical properties. Moreover, the relatively small number of parameters results in a simple implementation and facilitates the introductions of further interactions among additional species.

We have developed a method of coherent x-ray diffractive imaging to surmount its inability to image the structure of strongly strained crystals. We used calculated models from finite–element analysis to guide an iterative algorithm to fit experimental data from a series of increasingly bent wires cut into silicon-on-insulator films. Just before mechanical fracture, the wires were found to contain new phase structures, which are identified as dislocations associated with crossing the elastic limit.

The interaction of a cavity with an external field is symmetric under time reversal. Thus, coupling to a resonator is most efficient when the incident light is the time reversed version of a free cavity decay, i.e. when it has a rising exponential shape matching the cavity lifetime. For light entering the cavity from only one side, the maximally achievable coupling efficiency is limited by the choice of the cavity mirrors' reflectivities. Such an empty-cavity experiment serves also as a model system for single-photon single-atom absorption dynamics. We present experiments coupling exponentially rising pulses to a cavity system which allows for high coupling efficiencies. The influence of the time constant of the rising exponential is investigated as well as the effect of a finite pulse duration. We demonstrate coupling 94% of the incident TEM₀₀ mode into the resonator.

We propose a theory of optical absorption in monolayer graphene–hexagonal boron nitride (hBN) heterostructures. In highly oriented heterostructures, the hBN underlay produces a long-range moiré superlattice potential for the graphene electrons which modifies the selection rules for absorption of incoming photons in the infrared to visible frequency range. The details of the absorption spectrum modification depend on the relative strength of the various symmetry-allowed couplings between the graphene electrons and the hBN, and the resulting nature of the reconstructed band structure.

We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference ΔT = (20 ± 10) mK across the device and find a maximum thermovoltage signal of several μV in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott's relation and can be well understood within a model of sequential transport.

Electrical resonators are widely used in quantum information processing, by engineering an electromagnetic interaction with qubits based on real or virtual exchange of microwave photons. This interaction relies on strong coupling between the qubits' transition dipole moments and the vacuum fluctuations of the resonator in the same manner as cavity quantum electrodynamics (QED), and has consequently come to be called 'circuit QED' (cQED). Great strides in the control of quantum information have already been made experimentally using this idea. However, the central role played by photon exchange induced by quantum fluctuations in cQED does result in some characteristic limitations. In this paper, we discuss an alternative method for coupling qubits electromagnetically via a resonator, in which no photons are exchanged, and where the resonator need not have strong quantum fluctuations. Instead, the interaction can be viewed in terms of classical, effective 'forces' exerted by the qubits on the resonator, and the resulting resonator dynamics used to produce qubit entanglement are purely classical in nature. We show how this type of interaction is similar to that encountered in the manipulation of atomic ion qubits, and we exploit this analogy to construct two-qubit entangling operations that are largely insensitive to thermal or other noise in the resonator, and to its quality factor. These operations are also extensible to larger numbers of qubits, allowing interactions to be selectively generated among any desired subset of those coupled to a single resonator. Our proposal is potentially applicable to a variety of physical qubit modalities, including superconducting and semiconducting solid-state qubits, trapped molecular ions, and possibly even electron spins in solids.

Quantum computers hold the promise to solve certain problems exponentially faster than their classical counterparts. Trapped atomic ions are among the physical systems in which building such a computing device seems viable. In this work we present a small-scale quantum information processor based on a string of ⁴⁰Ca⁺ ions confined in a macroscopic linear Paul trap. We review our set of operations which includes non-coherent operations allowing us to realize arbitrary Markovian processes. In order to build a larger quantum information processor it is mandatory to reduce the error rate of the available operations which is only possible if the physics of the noise processes is well understood. We identify the dominant noise sources in our system and discuss their effects on different algorithms. Finally we demonstrate how our entire set of operations can be used to facilitate the implementation of algorithms by examples of the quantum Fourier transform and the quantum order finding algorithm.

We present a theoretical analysis of the standing wave patterns in scanning tunneling microscope (STM) images, which occur around surface point defects. We consider arbitrary dispersion relations for the surface states and calculate the conductance for a system containing a small-size tunnel contact and a surface impurity. We find rigorous theoretical relations between the interference patterns in the real-space STM images, their Fourier transforms and the Fermi contours of two-dimensional electrons. We propose a new method for reconstructing Fermi contours of surface electron states, directly from the real-space STM images around isolated surface defects.

A schematic model for baryon excitations is presented in terms of a symmetric Dirac gyroscope, a relativistic model solvable in closed form, that reduces to a rotor in the non-relativistic limit. The model is then mapped on a nearest neighbour tight binding model. In its simplest one-dimensional form this model yields a finite equidistant spectrum. This is experimentally implemented as a chain of dielectric resonators under conditions where their coupling is evanescent and a good agreement with the prediction is achieved.

The influence has been studied of the ionization laser polarization on the effective temperature of an ultracold electron source, which is based on near-threshold photoionization. This source is capable of producing both high-intensity and high-coherence electron pulses, with applications in, for example, electron diffraction experiments. For both nanosecond and femtosecond photoionization, a sinusoidal dependence of the temperature on the polarization angle has been found. For most experimental conditions, the temperature is minimal when the polarization coincides with the direction of acceleration. However, surprisingly, for nanosecond ionization, a regime exists when the temperature is minimal when the polarization is perpendicular to the acceleration direction. This shows that in order to create electron bunches with the highest transverse coherence length, it is important to control the polarization of the ionization laser. The general trends and magnitudes of the temperature measurements are described by a model, based on the analysis of classical electron trajectories; this model further deepens our understanding of the internal mechanisms during the photoionization process. Furthermore, for nanosecond ionization, charge oscillations as a function of laser polarization have been observed; for most situations, the oscillation amplitude is small.

Studying the phonon dispersion of the ferromagnetic shape memory alloy system Ni–Mn–Ga gives insight into the mechanism of the martensite transition and the forces driving the transition. Transformation of austenite single crystals under uniaxial stress results in the coexistence of two martensitic variants with perpendicular modulation vector. Here we report on inelastic neutron scattering studies of martensite crystals with off-stoichiometric compositions, varying from non-modulated (NM) to five- (5M) and seven- (7M) layer modulated martensite phases. Both the 5M and 7M crystals show fully commensurate satellite peaks along [$\xi \bar {\xi }$0], corresponding to the five- and seven-layer modulation. These superstructure peaks become Γ-points of the modulated structure. Due to the coexistence of two variants within the (001) plane, both new acoustic phonons reflecting the modulation vector [$\xi \bar {\xi }$0] and acoustic TA₂[ξξ0] phonons corresponding to the non-modulated direction are observed. The latter display a pronounced softening around ξ = 0.2–0.4 when approaching the martensite–austenite transition from above and below, i.e. this soft mode has lowest frequency at the transition temperature. Overall the phonon dispersion of the austenite and martensite phase resemble each other very much. The coexistence of two martensitic variants after uniaxial transformation explains the particular behaviour of the low-energy excitations, in contrast to previous interpretations involving charge-density waves and associated phason modes.

A method to generate an isolated single-cycle attosecond pulse from the interaction of a high-power femtosecond laser pulse with a nano-tube array is demonstrated using a two-dimensional relativistic particle-in-cell simulation. The radiation mechanism is relativistic nonlinear Thomson scattering from the electrons in a target material. Coherent radiation is emitted in the direction of specular reflection for the incident laser pulse while the electrons make a bunch size smaller than a wavelength of the laser pulse. Maintaining the coherence of the radiation from the electrons is essential to get an intense attosecond duration, which is achieved by using a nano-tube array target and a sharply increasing laser pulse. Optimal conditions for attosecond pulse generation are investigated by parameter scanning over plasma density, target thickness and laser pulse duration.

Using a first-principles density functional theory method, we have investigated the chemical and the hydrostatic pressure effects on the charge density wave (CDW) properties of the quasi-one-dimensional (1D) compound SmNiC₂. With increasing pressure, the relative 1D anisotropy of the electronic structure along a direction is enhanced because of its Ni chain structures. From the analysis of the Fermi surface and the generalized susceptibility, we also find that the Fermi surface nesting is enhanced along the modulation vector q₁ = (0.5, 0.52, 0) but is suppressed along q_R = (0.5, 0.5, 0.5) under pressure. The enhancement of 1D anisotropy of SmNiC₂ under pressure is responsible for increasing CDW strength along q₁. We suggest that this quantitative analysis could be used for analysis of the pressure effect on CDW materials.

Weyl semimetals (WS) are a new class of Dirac-type materials exhibiting a phase with bulk energy nodes and an associated vanishing density of states (DOS). We investigate the stability of this nodal DOS suppression in the presence of local impurities and consider whether or not such a suppression can be lifted by impurity-induced resonances. We find that while a scalar (chemical potential type) impurity can always induce a resonance at arbitrary energy and hence lift the DOS suppression at Dirac/Weyl nodes, for many other impurity types (e.g. magnetic or orbital mixing), resonances are forbidden in a wide range of energy. We investigate a four-band tight-binding model of WS adapted from a physical heterostructure construction due to Burkov et al (2011 Phys. Rev. B 84 235126), and represent a local impurity potential by a strength g as well as a matrix structure Λ. A general framework is developed to analyze this resonance dichotomy and make connection with the phase shift picture in scattering theory, as well as to determine the relation between resonance energy and impurity strength g. A complete classification of impurities based on Λ, based on their effect on nodal DOS suppression, is tabulated. We also discuss the differences between continuum and lattice approaches.

In this study we have investigated the quantum tunneling of two repulsive bosons in a triple-well potential, subject to a high-frequency driving field. By means of the multiple-time-scale asymptotic analysis, we evidence a far-off-resonant strongly interacting regime in which the dominant tunneling of doublons (paired states) is a second-order process and the selected coherent destruction of tunneling occurs either between doublons or between unpaired states. Two Floquet quasienergy bands of both kinds of states are given analytically, where a fine structure up to the second-order corrections is displayed. The analytical results are confirmed numerically, based on the exact model, and may be particularly relevant to controlling correlated tunneling in experiments.

We analyze the description of quantum many-body mixed states using matrix product states and operators. We consider two such descriptions: (i) as a matrix product density operator of bond dimension D; and (ii) as a purification that is written as a matrix product state of bond dimension D'. We show that these descriptions are inequivalent in the sense that D' cannot be upper bounded by D only. Then we provide two constructive methods to obtain (ii) out of (i). The sum of squares (sos) polynomial method scales exponentially in the number of different eigenvalues, and its approximate version is formulated as a semidefinite program, which gives efficient approximate purifications whose D' only depends on D. The eigenbasis method scales quadratically in the number of eigenvalues, and its approximate version is very efficient for rapidly decaying distributions of eigenvalues. Our results imply that a description of mixed states which is both efficient and locally positive semidefinite does not exist, but that good approximations do.

Optomechanical systems couple light to the motion of nanomechanical objects. Intriguing new effects are observed in recent experiments that involve the dynamics of more than one optical mode. There, mechanical motion can stimulate strongly driven multi-mode photon dynamics that acts back on the mechanics via radiation forces. We show that even for two optical modes Landau–Zener–Stueckelberg oscillations of the light field drastically change the nonlinear attractor diagram of the resulting phonon lasing oscillations. Our findings illustrate the generic effects of Landau–Zener physics on back-action induced self-oscillations.

Multi-dimensional spectroscopies with vacuum ultraviolet (VUV)/x-ray free-electron laser (FEL) sources would open up unique capabilities for dynamic studies of matter at the femtosecond–nanometer time–length scales. Using sequences of ultrafast VUV/x-ray pulses tuned to electron transitions enables element-specific studies of charge and energy flow between constituent atoms, which embody the very essence of chemistry and condensed matter physics. A remarkable step forward towards this goal would be achieved by extending the four wave mixing (FWM) approach at VUV/soft x-ray wavelengths, thanks to the use of fully coherent sources, such as seeded FELs. Here, we demonstrate the feasibility of VUV/soft x-ray FWM at Fermi@Elettra and we discuss its applicability to probe ultrafast intramolecular dynamics, charge injection processes involving metal oxides and electron correlation and magnetism in solid materials. The main advantage in using VUV/soft x-ray wavelengths is in adding element-sensitivity to FWM methods by exploiting the core resonances of selected atoms in the sample.

We examine the feasibility of creating and measuring large relative number squeezing in multicomponent trapped Bose–Einstein condensates. In the absence of multimode effects, this squeezing can be arbitrarily large for arbitrarily large condensates, but a range of processes limit the measurable squeezing in realistic trap configurations. We examine these processes, and suggest methods to mitigate them. We conclude that high levels of squeezing with large numbers of atoms is feasible, but can realistically only be achieved in particular trap geometries. We also introduce a method of maximizing the measurable squeezing by using a π-pulse during the process to improve spatial mode-matching.

A striking multiple-step behavior has been observed in magnetoresistance measurements during magnetization reversal in anti-ferromagnetically coupled GaMnAs/GaAs:Be multilayers. This behavior arises from the splitting of the energy degeneracy of spin configurations established by nearest-neighbor interlayer exchange coupling (NN IEC) with contribution from the next-nearest-neighbor (NNN) IEC. This observation reveals that NNN IEC plays a crucial role in the magnetic behavior of these multilayer structures.

An estimator is a state that represents one's best guess of the actual state of the quantum system for the given data. Such estimators are points in the state space. To be statistically meaningful, they have to be endowed with error regions, the generalization of error bars beyond one dimension. As opposed to standard ad hoc constructions of error regions, we introduce the maximum-likelihood region—the region of largest likelihood among all regions of the same size—as the natural counterpart of the popular maximum-likelihood estimator. Here, the size of a region is its prior probability. A related concept is the smallest credible region—the smallest region with pre-chosen posterior probability. In both cases, the optimal error region has constant likelihood on its boundary. This surprisingly simple characterization permits concise reporting of the error regions, even in high-dimensional problems. For illustration, we identify optimal error regions for single-qubit and two-qubit states from computer-generated data that simulate incomplete tomography with few measured copies.

The epsilon-near-zero (ENZ) condition in natural and artificial plasmas is considered for spatially dispersive materials. In the presence of spatial dispersion the ENZ condition must be judged by vanishing of the electric displacement field in real-space. Unlike the simple case of local materials where ENZ occurs at the plasma frequency, in spatially dispersive materials the matter is more complicated. To consider the spatially dispersive case, we obtain the momentum-dependent permittivity in real-space, and define a characteristic length parameter, in addition to the Debye length, which governs polarization screening. Using this formulation, conditions are investigated under which the electric displacement field (equivalently, the real-space permittivity) can vanish or be strongly diminished, even in the presence of spatial dispersion, implementing an ENZ material.

We investigate the molecular acceptors 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 4,5,9,10-pyrenetetraone (PYTON) on Ag(111) using density-functional theory (DFT). For two groups of the Heyd–Scuseria–Ernzerhof (HSE(α, ω)) family of exchange-correlation functionals (ω = 0 and 0.2 Å) we study the isolated components as well as the combined systems as a function of the amount of exact-exchange (α). We find that hybrid functionals favour electron transfer to the adsorbate. Comparing with experimental work function data, for α ≈ 0.25 we report a notable but small improvement over (semi) local functionals for the interface dipole. Although Kohn–Sham eigenvalues are only approximate representations of ionization energies, incidentally, at this value also the density of states agrees well with the photoelectron spectra. However, increasing α to values for which the energy of the lowest unoccupied molecular orbital matches the experimental electron affinity in the gas phase worsens both the interface dipole and the density of states. Our results imply that semi-local DFT calculations may often be adequate for conjugated organic molecules on metal surfaces and that the much more computationally demanding hybrid functionals yield only small improvements.

We propose and analyze a graphene-based cloaking metasurface aimed at achieving widely tunable scattering cancelation in the terahertz (THz) spectrum. This 'one-atom-thick' mantle cloak is realized by means of a patterned metasurface comprised of a periodic array of graphene patches, whose surface impedance can be modeled with a simple yet accurate analytical expression. By adjusting the geometry and Fermi energy of graphene nanopatches, the metasurface reactance may be tuned from inductive to capacitive, as a function of the relative kinetic inductance and the geometric patch capacitance, enabling the possibility of effectively cloaking both dielectric and conducting objects at THz frequencies with the same metasurface. We envision applications for low-observable nanostructures and efficient THz sensing, routing and detection.

An ultracold gas of interacting fermionic atoms in a three-dimensional optical lattice is considered, where the lattice potential strength is periodically modulated. This non-equilibrium system is non-perturbatively described by means of a Keldysh–Floquet–Green's function approach for Mott–Hubbard systems employing a generalized dynamical mean field theory (DMFT). Strong repulsive interactions between different atoms lead to a Mott insulator state for the equilibrium system, but the additional external driving at zero temperature yields a non-equilibrium quantum critical behavior, where an infinite number of Floquet states arise and a transition to the liquid and conducting phase is given.

We report the discovery of superconductivity with T_c = 5.5 K in Nb₂Pd_xSe₅, in which one-dimensional (1D) Nb–Se chains existing along the b-direction hybridize each other to form the conducting b–c* plane. The magnetic susceptibility and specific heat data in both single crystal and polycrystal show evidence of bulk superconductivity. The resistivity, Hall coefficient and magneto-resistance data all indicate the presence of an energy scale T* =  ∼ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to the formation of a density wave or a stabilization of low-dimensional electronic structure. The zero-temperature upper critical field, H_c2(0), of the single crystal is found to be 10.5, 35 and 22 T in the a', b and c* directions, respectively. While the linearly increasing H_c2(T) for H//c* indicates the multi-band effect, H_c2(0) for H//b and c* are found to be much bigger than the Bardeen–Cooper–Schrieffer (BCS) Pauli limiting field, 1.84 T_c ∼ 9 T. The suppressed Pauli paramagnetic effect points to the possibility of enhanced spin–orbit scattering related to the low-dimensional electronic structure or the presence of heavy elements such as Pd.

Quantum phase transitions (QPTs) describe when a many-body quantum system displays non-analytic behavior associated with a discontinuous change in a property of the ground state as a parameter is varied. The QPT in prototypical Dicke model is difficult to reach experimentally as the spin–field coupling strength must be quite large. In this work we describe a new model—the off-resonant Tavis–Cummings model where we drive the common mode, and discover a new type of QPT at quite low coupling strengths which are comparable with the geometric mean of the atomic and field detunings $\lambda \sim \lambda _{\mathrm { c}}\equiv \sqrt {\Delta _{\mathrm { a}}\Delta _{\mathrm { c}}}$. Through analytic methods we demonstrate this QPT for both finite and infinite numbers of spins and show that |〈J_x(J_z)〉|/(N/2) ∼ |λ/λ_c − 1|^γ_x(γ_z) and 〈a^†a〉/N ∼ |λ/λ_c − 1|^γ_a for λ ⩾ λ_c, with critical exponents γ_x ≈ 1/2, γ_z ≈ 1 and γ_a ≈ 1. We show that this QPT can be immediately observed by laboratory cavity-QED setups such as Bose–Einstein condensate in optical cavity and superconducting circuit-QED as well as a line of trapped ultracold ions.

We present a series of numerically tabulated atom-centered orbital (NAO) basis sets with valence-correlation consistency (VCC), termed NAO-VCC-nZ. Here the index 'nZ' refers to the number of basis functions used for the valence shell with n = 2, 3, 4, 5. These basis sets are constructed analogous to Dunning's cc-pVnZ, but utilize the more flexible shape of NAOs. Moreover, an additional group of (sp) basis functions, called enhanced minimal basis, is established in NAO-VCC-nZ, increasing the contribution of the s and p functions to achieve the valence-correlation consistency. NAO-VCC-nZ basis sets are generated by minimizing frozen-core random-phase approximation (RPA) total energies of individual atoms from H to Ar. We demonstrate that NAO-VCC-nZ basis sets are suitable for converging electronic total-energy calculations based on valence-only (frozen-core) correlation methods which contain explicit sums over unoccupied states (e.g. the RPA or second-order Møller–Plesset perturbation theory). The basis set incompleteness error, including the basis set superposition error, can be gradually reduced with the increase of the index 'n', and can be removed using two-point extrapolation schemes.

Growth of nanocrystalline graphene films on (6√3 × 6√3)R30°-reconstructed SiC surfaces was achieved by molecular beam epitaxy, enabling the investigation of quasi-homoepitaxial growth. The structural quality of the graphene films, which is investigated by Raman spectroscopy, increases with growth time. X-ray photoelectron spectroscopy proves that the SiC surface reconstruction persists throughout the growth process and that the synthesized films consist of sp²-bonded carbon. Interestingly, grazing incidence x-ray diffraction measurements show that the graphene domains possess one single in-plane orientation, are aligned to the substrate, and offer a noticeably contracted lattice parameter of 2.450 Å. We correlate this contraction with theoretically calculated reference values (all-electron density functional calculations based on the van der Waals corrected Perdew–Burke–Ernzerhof functional) for the lattice parameter contraction induced in ideal, free-standing graphene sheets by: substrate-induced buckling, the edges of limited-size flakes and typical point defects (monovacancies, divacancies, Stone–Wales defects).

Ion acceleration from relativistic laser solid interactions has been of particular interest over the last decade. While beam profiles have been studied for target normal sheath acceleration (TNSA), such profiles have yet to be described for other mechanisms. Here, experimental data is presented, investigating ion beam profiles from acceleration governed by relativistic transparent laser plasma interaction. The beam shape of carbon C⁶⁺ ions and protons has been measured simultaneously with a wide angle spectrometer. It was found that ion beams deviate from the typical Gaussian-like shape found with TNSA and that the profile is governed by electron dynamics in the volumetric laser–plasma interaction with a relativistically transparent plasma; due to the ponderomotive force electrons are depleted from the center of the laser axis and form lobes affecting the ion beam structure. The results are in good agreement with high resolution three-dimensional-VPIC simulations and can be used as a new tool to experimentally distinguish between different acceleration mechanisms.

We calculate the correction to the electronic density of states in a disordered ferromagnetic metal induced by spin-wave mediated interaction between the electrons. Our calculation is valid for the case that the exchange splitting Δ in the ferromagnet is much smaller than the Fermi energy, but we make no assumption on the relative magnitude of Δ and the elastic electronic scattering time τ_el. In the 'clean limit' Δτ_el/ℏ ≫ 1 we find a correction with a T^d/2 temperature dependence, where d is the effective dimensionality of the ferromagnet. In the 'dirty limit' Δτ_el/ℏ ≪ 1, the density-of-states correction is a non-monotonous function of energy and temperature.

We calculate optical forces and torques exerted on a chiral dipole by chiral light fields and reveal genuine chiral forces in combining the chiral contents of both light field and dipolar matter. Here, the optical chirality is characterized in a general way through the definition of optical chirality density and chirality flow. We show, in particular, that both terms have mechanical effects associated, respectively, with reactive and dissipative components of the chiral forces. Remarkably, these chiral force components are directly related to standard observables: optical rotation for the reactive component and circular dichroism for the dissipative one. As a consequence, the resulting forces and torques are dependent on the enantiomeric form of the chiral dipole. This suggests promising strategies for using chiral light forces to mechanically separate chiral objects according to their enantiomeric form.

The scaling of the intensity, angular and material dependence of bremsstrahlung radiation from an intense (I > 10¹⁸ W cm⁻²) laser–solid interaction has been characterized at energies between 100 keV and 1 MeV. These are the first high resolution (E/ΔE > 200) measurements of bremsstrahlung photons from a relativistic laser–plasma interaction. The measurement was performed using a high purity germanium detector at the high-repetition rate (500 Hz) λ³ laser facility. The bremsstrahlung spectra were observed to have a two effective temperature energy distribution which ranged between 80 (± 10) and 550 (± 60) keV depending on laser intensity and observation angle. The two temperatures were determined to result from separate populations of accelerated electrons. One population was isotropic and produced the lower effective bremsstrahlung temperature. The higher bremsstrahlung temperature was produced by an energetic electron beam directed out of the front of the target in the direction of the specular laser reflection, which was also the direction the bremsstrahlung effective temperature peaked. Both effective bremsstrahlung temperatures scaled consistently with a previously measured experimental electron temperature scaling on λ³. The electron populations and bremsstrahlung temperatures were modeled in the particle-in-cell code OSIRIS and the Monte Carlo code MCNPX and were in good agreement with the experimental results. The observed directionality and intensity scaling suggest a significant difference between picosecond and femtosecond duration pulse interactions.

We analyze the noise properties of both electric charge and heat currents as well as their correlations in a quantum-dot based thermoelectric engine. The engine is a three-terminal conductor with crossed heat and charge flows where heat fluctuations can be monitored by a charge detector. We investigate the mutual influence of charge and heat dynamics and how it is manifested in the current and noise properties. In the presence of energy-dependent tunneling, operating conditions are discussed where a charge current can be generated by heat conversion. In addition, heat can be pumped into the hot source by driving a charge current in the coupled conductor. An optimal configuration is found for structures in which the energy dependence of tunneling maximizes asymmetric transmission with maximal charge–heat cross-correlations. Remarkably, at a voltage that stalls the heat engine we find that in the optimal case the non-equilibrium state is maintained by fluctuations in the heat and charge currents only.

We discuss how continuous probing of a quantum system allows estimation of unknown classical parameters embodied in the Hamiltonian of the system. We generalize the stochastic master equation associated with continuous observation processes to a Bayesian filter equation for the probability distribution of the desired parameters, and we illustrate its application by estimating the direction of a magnetic field. In our example, the field causes a ground state spin precession in a two-level atom which is detected by the polarization rotation of off-resonant optical probes, interacting with the atomic spin components.

We develop a theory of spin-dependent phenomena in the streaming regime characterized by ballistic acceleration of electrons in the moderate electric field until they achieve the optical phonon energy and abruptly emit phonons. It is shown that the Dyakonov–Perel spin relaxation is drastically modified in this regime, the current-induced spin orientation remarkably increases, reaches a high value ≈2% in the electric field ∼1 kV cm⁻¹ and falls with further increase in the field. The spin polarization enhancement is caused by squeezing of the electron momentum distribution in the direction of drift. We also predict field-induced oscillatory dynamics of spin polarization of the photocarriers excited into the conduction band by a short circularly polarized optical pulse.

The principle of maximum likelihood reconstruction has proven to yield satisfactory results in the context of quantum state tomography for many-body systems of moderate system sizes. Until recently, however, quantum state tomography has been considered to be infeasible for systems consisting of a large number of subsystems due to the exponential growth of the Hilbert space dimension with the number of constituents. Several reconstruction schemes have been proposed since then to overcome the two main obstacles in quantum many-body tomography: experiment time and post-processing resources. Here we discuss one strategy to address these limitations for the maximum likelihood principle by adopting a particular state representation to merge a well established reconstruction algorithm maximizing the likelihood with techniques known from quantum many-body theory.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are gaining increasing attention as an alternative to graphene for their very high potential in optoelectronics applications. Here, we consider two prototypical metallic 2D TMDs, NbSe₂ and TaS₂. Using a first-principles approach, we investigate the properties of the localized intraband d plasmon that cannot be modeled on the basis of the homogeneous electron gas. Finally, we discuss the effects of the reduced dimensionality on the plasmon dispersion through the interplay between the interband transitions and the local-field effects. This result can be exploited to tune the plasmonic properties of these novel 2D materials.

Nitride light-emitting diodes are a promising solution for efficient solid-state lighting, but their performance at high power is affected by the efficiency-droop problem. Previous experimental and theoretical work has identified Auger recombination, a three-particle nonradiative carrier recombination mechanism, as the likely cause of the droop. In this work, we use first-principles calculations to elucidate the dependence of the radiative and Auger recombination rates on temperature, carrier density and quantum-well confinement. Our calculated data for the temperature dependence of the recombination coefficients are in good agreement with experiment and provide further validation on the role of Auger recombination in the efficiency reduction. Polarization fields and phase-space filling negatively impact device efficiency because they increase the operating carrier density at a given current density and increase the fraction of carriers lost to Auger recombination.

We report on the spectrum of a superconducting transmon device coupled to a planar superconducting resonator in the strong dispersive limit where discrete peaks, each corresponding to a different number of photons, are resolved. A thermal population of 5.474 GHz photons at an effective resonator temperature of T = 120 mK results in a weak n = 1 photon peak along with the n = 0 photon peak in the qubit spectrum in the absence of a coherent drive on the resonator. Two-tone spectroscopy using independent coupler and probe tones reveals an Autler–Townes splitting in the thermal n = 1 photon peak. The observed effect is explained accurately using the four lowest levels of the dispersively dressed qubit–resonator system and compared to results from numerical simulations of the steady-state master equation for the coupled system.

Preliminary results of the self-emission of charged particles from magnetically driven plasma jets has been investigated. The jets were launched and driven by a toroidal magnetic field generated by introducing a  ∼ 1.4 MA, 250 ns electrical current pulse from the MAGPIE generator into a radial wire array. This configuration has shown to reproduce some aspects of the astrophysical magnetic-tower jet launching model, in which a jet is collimated by a toroidal magnetic field inside a magnetic cavity. The emission of ions and protons from the plasma was recorded onto Columbia Resin 39 plates using time-integrated pinhole cameras. In addition a fly-eye camera, an array of 25–496 cylindrical apertures allowed estimating the location of the ion emitting source. The results show the ion emission comes from both the jet and its surrounding magnetic cavity, with the emission extending to a height of at least  ∼ 9 cm from the initial position of the wires. The emission of ions is consistent with the dynamics of the jet obtained from time-resolved imaging diagnostics, i.e. optical laser probing and self-emission of the plasma in the extreme ultra-violet. These preliminary results suggest the ions are trapped inside the cavity due to the strong toroidal magnetic field which drives the jet. In addition these studies provide first estimates of the energy and fluence of protons for future laser-driven proton probing diagnostics aimed at measuring the magnetic field in these experiments.

We develop a multiscale hybrid scheme for simulations of soft condensed matter systems, which allows one to treat the system at the particle level in selected regions of space, and at the continuum level elsewhere. It is derived systematically from an underlying particle-based model by field theoretic methods. Particles in different representation regions can switch representations on the fly, controlled by a spatially varying tuning function. As a test case, the hybrid scheme is applied to simulate colloid–polymer composites with high resolution regions close to the colloids. The hybrid simulations are significantly faster than reference simulations of a pure particle-based model, and the results are in good agreement.

A detailed theoretical model for thermoelectric transport perpendicular to the multilayers of a Si–Ge heterostructure is presented. The electronic structure of a three-dimensional superlattice, consisting of a regular array of Ge quantum dots in each layer, capped by Si layers, is calculated using an atomistic tight-binding approach. The Seebeck coefficient, the electric conductivity and the contribution of the electrons to the thermal conductivity for n-doped samples are worked out within Boltzmann transport theory. Using experimental literature data for the lattice thermal conductivity, we determine the temperature dependence of the figure of merit ZT. A nonlinear increase of ZT with temperature is found, with ZT > 2 at T = 1000 K in highly doped samples. Moreover, we find an enhanced thermoelectric power factor already at room temperature and below, which is due to highly mobile electrons in strain-induced conductive channels.

We investigate the nonlinear wave-mixing in gases between intense, short optical pulses and long-wavelength fields (mid infrared and terahertz). We show numerically that the beating between the sum- and difference-frequency generation components can be isolated in the spectrogram of the interaction, and can be used to sample the electric field oscillations of the long-wavelength pulses. This, in turn, could be employed as a possible characterization method that provides information on the real electric field amplitude. Our numerical model is supported by spectrally resolved measurements of the four-wave mixing signals obtained from the interaction of intense, single-cycle terahertz fields (λ > 15 μm) and optical pulses (λ ≃ 800 nm, 50 fs duration) in air.

We relate the information exchange between two stochastic systems to the nonequilibrium entropy production in the whole system. By deriving a general formula that decomposes the total entropy production into the thermodynamic and informational parts, we obtain nonequilibrium equalities such as the fluctuation theorem in the presence of information processing. Our results apply not only to situations under measurement and feedback control but also to those under multiple information exchanges between two systems, giving the fundamental energy cost for information processing and elucidating the thermodynamic and informational roles of a memory in information processing. We point out a dual relationship between measurement and feedback.

Josephson parametric amplifiers (JPA) are promising devices for applications in circuit quantum electrodynamics and for studies on propagating quantum microwaves because of their good noise performance. In this work, we present a systematic characterization of a flux-driven JPA at millikelvin temperatures. In particular, we study in detail its squeezing properties by two different detection techniques. With the homodyne setup, we observe the squeezing of vacuum fluctuations by superposing signal and idler bands. For a quantitative analysis, we apply dual-path cross-correlation techniques to reconstruct the Wigner functions of various squeezed vacuum and thermal states. At 10 dB signal gain, we find 4.9 ± 0.2 dB squeezing below the vacuum. In addition, we discuss the physics behind squeezed coherent microwave fields. Finally, we analyze the JPA noise temperature in the degenerate mode and find a value smaller than the standard quantum limit for phase-insensitive amplifiers.

The spin structure of quantum well states (QWSs) in three-monolayer-thick gold overlayers on W(110) and Mo(110) is studied experimentally by spin- and angle-resolved photoelectron spectroscopy and theoretically by ab initio calculations. The spin–orbit coupling effects manifest themselves differently in the spin structure of atomic-like d and delocalized sp QWSs depending on how strongly the states are influenced by the substrate. The QWSs of a d character show a strong spin polarization with an almost identical structure for W(110) and Mo(110), suggesting a weak interaction with the substrate, whereas for sp QWSs, the interaction is much stronger, which to a large extent determines their splitting and spin polarization. The theoretical model yields a qualitative agreement with the experiment and explains the observed behavior.

We obtain Markovian equations of motion for a many body system of interacting coarse-grained (CG) variables and additional fluxes. The investigated CG variables belong to the special family of linear combinations of atomistic degrees of freedom. The system of Markovian equations of motion approximates Mori's exact non-Markovian generalized Langevin equation and is easy to solve by computer simulation. All parameters of the equations can be obtained from equilibrium molecular dynamics simulations of the investigated microscopic system. These parameters are either equal to the famous static covariances from Mori's continued fraction or they represent generalized constant friction matrices. We propose two different ways to compute these friction matrices based on Mori's continued fraction. Finally, some of the parameters are computed numerically for the special case of centre of mass variables in the graphene lattice and it is found that the CG variables interact with their additional fluxes in a spatially very local way.

Sessile bacteria adhere to engineered surfaces and host tissues and pose a substantial clinical and economical risk when growing into biofilms. Most engineered and biological interfaces are of chemically heterogeneous nature and provide adhesive islands for bacterial attachment and growth. To mimic either defects in a surface coating of biomedical implants or heterogeneities within mucosal layers (Peyer's patches), we embedded micrometre-sized adhesive islands in a poly(ethylene glycol) biopassive background. We show experimentally and computationally that filamentation of Escherichia coli can significantly accelerate the bacterial surface colonization under physiological flow conditions. Filamentation can thus provide an advantage to a bacterial population to bridge non-adhesive distances exceeding 5 μm. Bacterial filamentation, caused by blocking of bacterial division, is common among bacterial species and can be triggered by environmental conditions or antibiotic treatment. While great awareness exists that the build-up of antibiotic resistance serves as intrinsic survival strategy, we show here that antibiotic treatment can actually promote surface colonization by triggering filamentation, which in turn prevents daughter cells from being washed away. Our combined microfabrication and computational approaches provide quantitative insights into mechanisms that enable biofouling of biopassive surfaces with embedded adhesive spots, even for spot distances that are multiples of the bacterial length.

We study the spin-dependent transport of spin-1/2 electrons through an interferometer made of two elongated quantum dots or quantum nanowires, which are subject to both an Aharonov–Bohm flux and (Rashba and Dresselhaus) spin–orbit interactions. Similar to the diamond interferometer proposed in our previous papers (Aharony et al 2011 Phys. Rev. B 84 035323; Matityahu et al 2013 Phys. Rev. B 87 205438), we show that the double-dot interferometer can serve as a perfect spin filter due to a spin interference effect. By appropriately tuning the external electric and magnetic fields which determine the Aharonov–Casher and Aharonov–Bohm phases, and with some relations between the various hopping amplitudes and site energies, the interferometer blocks electrons with a specific spin polarization, independent of their energy. The blocked polarization and the polarization of the outgoing electrons is controlled solely by the external electric and magnetic fields and do not depend on the energy of the electrons. Furthermore, the spin filtering conditions become simpler in the linear-response regime, in which the electrons have a fixed energy. Unlike the diamond interferometer, spin filtering in the double-dot interferometer does not require high symmetry between the hopping amplitudes and site energies of the two branches of the interferometer and thus may be more appealing from an experimental point of view.

We use density functional theory (DFT) calculations to determine the interaction between phenyl-modified silicene nanosheets. The adhesion energy curves between the nanosheets are compared for different van der Waals density functional (vdW-DF) functionals and the DFT-D2 Grimme method. Our results show that there is a weak attraction between the sheets at close separations, that is stronger when vdWs forces are included. Without including vdWs forces the interaction is negligible and occurs at a much larger separation, highlighting the need to include such forces when modelling these nanosheets. Of the vdWs methods, the optB88 functional gives the strongest interaction energy while the Grimme gives the weakest, with the separation at which the nanosheets adhere more strongly varying between 10.04 and 11.24 Å, as measured by the distance between the silicene layers. As the modified nanosheets are brought closer together at separations as close at  ∼ 8 Å, the phenyl groups on the bottom of one nanosheet can fit in between the phenyl groups on the top of the adjacent nanosheet allowing some π − π interaction between the phenyl groups. We showed that the band gap can be modified by compressing the nanosheets together while retaining a small attraction between them. There is also a change from a direct to an indirect band gap. Such a property may be exploited for the application of these nanomaterials in optoelectronic devices.

Bacterial swarming resulting in collective navigation over surfaces provides a valuable example of cooperative colonization of new territories. The social bacterium Paenibacillus vortex exhibits successful and diverse swarming strategies. When grown on hard agar surfaces with peptone, P. vortex develops complex colonies of vortices (rotating bacterial aggregates). In contrast, during growth on Mueller–Hinton broth gelled into a soft agar surface, a new strategy of multi-level organization is revealed: the colonies are organized into a special network of swarms (or 'snakes' of a fraction of millimeter in width) with intricate internal traffic. More specifically, cell movement is organized in two or three lanes of bacteria traveling between the back and the front of the swarm. This special form of cellular logistics suggests new methods in which bacteria can share resources and risk while searching for food or migrating into new territories. While the vortices-based organization on hard agar surfaces has been modeled before, here, we introduce a new multi-agent bacterial swarming model devised to capture the swarms-based organization on soft surfaces. We test two putative generic mechanisms that may underlie the observed swarming logistics: (i) chemo-activated taxis in response to chemical cues and (ii) special align-and-push interactions between the bacteria and the boundary of the layer of lubricant collectively generated by the swarming bacteria. Using realistic parameters, the model captures the observed phenomena with semi-quantitative agreement in terms of the velocity as well as the dynamics of the swarm and its envelope. This agreement implies that the bacteria interactions with the swarm boundary play a crucial role in mediating the interplay between the collective movement of the swarm and the internal traffic dynamics.

Quantum tomography has come a long way from early reconstructions of Wigner functions from projections along quadratures to the full characterization of multipartite systems. Now, it is routinely carried out in a wide variety of systems. And yet, many fundamental questions remain unanswered. In recent years, a spate of radical new experimental, theoretical and mathematical developments have occurred. The appeal of the subject lies largely in the breadth of techniques that must be brought together in order to fully understand the problem. This 'focus on' collection provides a platform for facilitating the exchange of ideas between the different communities involved in this process.

This paper demonstrates a method for screening transition metal and metal alloy catalysts based on their predicted rates and stabilities for a given catalytic reaction. This method involves combining reaction and activation energies (available to the public via a web-based application 'CatApp') with a microkinetic modeling technique to predict the rates and selectivities of a prospective material. This paper illustrates this screening technique using the steam reforming of methane to carbon monoxide and hydrogen as a test reaction. While catalysts are already commercially available for this process, the method demonstrated in this paper is very general and could be applied to a wide range of catalytic reactions. Following the steps outlined herein, such an analysis could potentially enable researchers to understand reaction mechanisms on a fundamental level and, on this basis, develop leads for new metal alloy catalysts.

Emperor penguins breed during the Antarctic winter and have to endure temperatures as low as −50 °C and wind speeds of up to 200 km h⁻¹. To conserve energy, they form densely packed huddles with a triangular lattice structure. Video recordings from previous studies revealed coordinated movements in regular wave-like patterns within these huddles. It is thought that these waves are triggered by individual penguins that locally disturb the huddle structure, and that the traveling wave serves to remove the lattice defects and restore order. The mechanisms that govern wave propagation are currently unknown, however. Moreover, it is unknown if the waves are always triggered by the same penguin in a huddle. Here, we present a model in which the observed wave patterns emerge from simple rules involving only the interactions between directly neighboring individuals, similar to the interaction rules found in other jammed systems, e.g. between cars in a traffic jam. Our model predicts that a traveling wave can be triggered by a forward step of any individual penguin located within a densely packed huddle. This prediction is confirmed by optical flow velocimetry of the video recordings of emperor penguins in their natural habitat.

In this paper a computational high-throughput screening (HTS) approach to the search for alternative permanent magnetic materials is presented. Systems considered for a start are binary intermetallic compounds composed of rare-earth (RE) and transition metal (TM) elements. With the tight-binding-linear muffin-tin-orbital-atomic-sphere-approximation (TB-LMTO-ASA) method of density functional theory (DFT) a variety of RE–TM intermetallic phases is investigated and their magnetic properties are obtained at rather low computational costs. Next, interstitial elements such as boron, carbon and nitrogen in these phases are considered. For promising candidate phases with high and stable spontaneous ferromagnetic polarization, the calculated local magnetic moments and exchange coupling parameters, as obtained from TB-LMTO-ASA calculations, are then used for Monte Carlo simulations to identify candidates with sufficiently high Curie temperatures (T_c). Finally, magnetocrystalline anisotropy constants (K₁) of the most promising candidate phases are calculated with accurate, potential-shape-unrestricted DFT calculations using the Vienna ab initio simulation package. The computational HTS procedure is illustrated by results for a selection of hard-magnetic RE–TM phases like RETM₅, RE₂TM₁₇ and RE₂TM₁₄B.

The nature of the lowest-energy electronic excitations in prototypical molecular solids is studied here in detail by combining electron energy loss spectroscopy (EELS) experiments and state-of-the-art many-body calculations based on the Bethe–Salpeter equation. From a detailed comparison of the spectra in picene, coronene and tetracene we generally find a good agreement between theory and experiment, with an upshift of the main features of the calculated spectrum of 0.1–0.2 eV, which can be considered the error bar of the calculation. We focus on the anisotropy of the spectra, which illustrates the complexity of this class of materials, showing a high sensitivity with respect to the three-dimensional packing of the molecular units in the crystal. The differences between the measured and the calculated spectra are explained in terms of the small differences between the crystal structures of the measured samples and the structural model used in the calculations. Finally, we discuss the role played by the different electron–hole interactions in the spectra. We thus demonstrate that the combination of highly accurate experimental EELS and theoretical analysis is a powerful tool to elucidate and understand the electronic properties of molecular solids.

The insufficient parameterization of low clouds which are caused by shallow convection remains one of the biggest sources of uncertainty in large-scale models of global atmospheric motion. One way to overcome this lack of understanding is to develop Boussinesq models of moist convection with simplified thermodynamics which allow for systematic studies of the cloud formation in different dynamical regimes and depend on a small set of system parameters only. This route makes the problem accessible to direct numerical simulations of turbulence without subgrid-scale modeling and provides an ideal testing bed for systematic and stepwise reductions of degrees of freedom. Such systematic reductions are studied here for a recently developed moist Rayleigh–Bénard convection model in the conditionally unstable regime. Our analysis is based on the proper orthogonal decomposition (POD) and determines the corresponding modes by a direct solution of the eigenvalue problem in form of an integral equation. The resulting reduced-order dynamical systems are obtained by a projection of the original equations of motion onto a finite set of POD modes. These modes are selected with respect to their energy as well as their ability to transport energy from large to small scales and to dissipate the energy at smaller scales efficiently such that an additional modal viscosity can be omitted for most cases. The reduced models reproduce important statistical quantities such as cloud cover, liquid water flux and global buoyancy transport to a very good degree. Furthermore we investigate different pathways to reduce the number of degrees of freedom in the low-dimensional models. The number of degrees of freedom can be compressed by more than two orders of magnitude until the models break down and cause significant deviations of essential mean transport quantities from the original fully resolved simulation data.

Bacteria living on surfaces form heterogeneous three-dimensional consortia known as biofilms, where they exhibit many specific properties one of which is an increased tolerance to antibiotics. Biofilms are maintained by a polymeric network and display physical properties similar to that of complex fluids. In this work, we address the question of the impact of antibiotic treatment on the physical properties of biofilms based on recently developed tools enabling the in situ mapping of biofilm local mechanical properties at the micron scale. This approach takes into account the material heterogeneity and reveals the spatial distribution of all the small changes that may occur in the structure. With an Escherichia coli biofilm, we demonstrate using in situ fluorescent labeling that the two antibiotics ofloxacin and ticarcillin—targeting DNA replication and membrane assembly, respectively—induced no detectable alteration of the biofilm mechanical properties while they killed the vast majority of the cells. In parallel, we show that a proteolytic enzyme that cleaves extracellular proteins into short peptides, but does not alter bacterial viability in the biofilm, clearly affects the mechanical properties of the biofilm structure, inducing a significant increase of the material compliance. We conclude that conventional biofilm control strategy relying on the use of biocides targeting cells is missing a key target since biofilm structural integrity is preserved. This is expected to efficiently promote biofilm resilience, especially in the presence of persister cells. In contrast, the targeting of polymer network cross-links—among which extracellular proteins emerge as major players—offers a promising route for the development of rational multi-target strategies to fight against biofilms.

The integration of superconducting qubits and resonators in one circuit offers a promising solution for quantum information processing (QIP), which also realizes the on-chip analogue of cavity quantum electrodynamics (QED), known as circuit QED. In most prototype circuit designs, qubits are active processing elements and resonators are peripherals. As resonators typically have better coherence performance and more accessible energy levels, it is proposed that the entangled qubit–resonator hybrid can be used as a processing element. To achieve such a goal, an accurate measurement of the hybrid is first necessary. Here we demonstrate a joint quantum state tomography (QST) technique to fully characterize an entangled qubit–resonator hybrid. We benchmarked our QST technique by generating and accurately characterizing multiple states, e.g. |gN〉 + |e(N − 1)〉 where (|g〉 and |e〉) are the ground and excited states of the qubit and (|0〉,...,|N〉) are Fock states of the resonator. We further provided a numerical method to improve the QST efficiency and measured the decoherence dynamics of the bipartite hybrid, witnessing dissipation coming from both the qubit and the N-photon Fock state. As such, the joint QST presents an important step toward actively using the qubit–resonator element for QIP in hybrid quantum devices and for studying circuit QED.

Laser cooling of molecules employing broadband optical pumping involves a timescale separation between laser excitation and spontaneous emission. Here, we optimize the optical pumping step using shaped laser pulses. We derive two optimization functionals to drive population into those excited state levels that have the largest spontaneous emission rates to the target state. We show that, when using optimal control, laser cooling of molecules works even if the Franck–Condon map governing the transitions is preferential to heating rather than cooling. Our optimization functional is also applicable to the laser cooling of other degrees of freedom provided the cooling cycle consists of coherent excitation and dissipative de-excitation steps whose timescales are separated.

A system can be driven to nonequilibrium behavior in the presence of thermodynamic imbalances in its environment, such as temperature or chemical potential differences. In derivations of far-from-equilibrium fluctuation theorems, such imbalances are typically modeled as fixed, externally imposed thermodynamic forces. Here, we argue that fluctuation theorems can instead be understood in terms of the equilibrium dynamics of a larger supersystem, containing both the system of interest and its thermal surroundings. To this end, we consider rare fluctuations that spontaneously produce imbalances in the surroundings. In the aftermath of such fluctuations, the system of interest transiently behaves as though it were in the presence of an externally applied thermodynamic force. By applying the principle of detailed balance to these rare events, we recover the fluctuation theorem in both its transient and steady-state formulations.

The existence of an optimal thermal bath to facilitate robust energy transfer between the spectrally separated B800 and B850 rings in light-harvesting complex 2 (LH2) of purple bacteria is investigated via the multichromophoric Förster theory. Due to the inherent energy bias between the two rings, the energy transfer rate from B800 to B850 is maximized as a function of the bath coupling strength, establishing an optimization criterion. Critically, upon inclusion of energetic disorder, this maximum is averaged out. However, noting the distribution of transfer rates, we find that the bath coupling strength can yield a minimal dispersion for the rate distribution, i.e. a maximum ratio of mean to standard deviation, thus achieving maximum energy transfer robust to the effects of static disorder.

Using spin and angle-resolved photoemission spectroscopy we investigate a momentum region in Pb quantum well states on Si(111) where hybridization between Rashba-split bands alters the band structure significantly. Starting from the Rashba regime where the dispersion of the quasi-free two-dimensional electron gas is well described by two spin-polarized parabolas, we find a breakdown of the Rashba behavior which manifests itself (i) in a spin splitting that is no longer proportional to the in-plane momentum and (ii) in a reversal of the sign of the momentum splitting. Our experimental findings are well explained by including interband spin–orbit coupling that mixes Rashba-split states with anti-parallel rather than parallel spins. Similar results for Pb/Cu(111) reveal that the proposed hybridization scenario is independent on the supporting substrate.

While adiabatic quantum computing (AQC) has some robustness to noise and decoherence, it is widely believed that encoding, error suppression and error correction will be required to scale AQC to large problem sizes. Previous works have established at least two different techniques for error suppression in AQC. In this paper we derive a model for describing the dynamics of encoded AQC and show that previous constructions for error suppression can be unified with this dynamical model. In addition, the model clarifies the mechanisms of error suppression and allows the identification of its weaknesses. In the second half of the paper, we utilize our description of non-equilibrium dynamics in encoded AQC to construct methods for error correction in AQC by cooling local degrees of freedom (qubits). While this is shown to be possible in principle, we also identify the key challenge to this approach: the requirement of high-weight Hamiltonians. Finally, we use our dynamical model to perform a simplified thermal stability analysis of concatenated-stabilizer-code encoded many-body systems for AQC or quantum memories. This work is a companion paper to 'Error suppression and error correction in adiabatic quantum computation: techniques and challenges (2013 Phys. Rev. X 3 041013)', which provides a quantum information perspective on the techniques and limitations of error suppression and correction in AQC. In this paper we couch the same results within a dynamical framework, which allows for a detailed analysis of the non-equilibrium dynamics of error suppression and correction in encoded AQC.

We exploit dipolar coupling to control the magnetic states in assemblies of single-domain magnetic nanoislands, arranged in one, two and three adjacent hexagonal rings. On tailoring the shape anisotropy of specific islands, and thus their switching fields, we achieve particular target states with near perfect reliability, and are able to control the chirality of the vortex target states. The magnetic states are observed during magnetization reversal with x-ray photoemission electron microscopy and our results are generally in excellent agreement with a numerical model based on point dipoles and realistic values of disorder. We conclude with a quantitative discussion of how our results depend on disorder and the chosen bias in shape anisotropy.