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We have shown experimentally, in a real-world setting, that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion.

We propose and experimentally demonstrate a zero-sum game that is in a fair Nash equilibrium for classical players, but has the property that a quantum player can always win using an appropriate strategy. The gain of the quantum player is measured experimentally for different quantum strategies and input states. It is found that the quantum gain is maximized by a maximally entangled state, but does not decrease to zero when entanglement disappears. Instead, it links with another kind of quantum correlation described by discord for the qubit case and the connection is demonstrated both theoretically and experimentally.

Silicene is a monolayer of silicon atoms forming a two-dimensional (2D) honeycomb lattice and shares almost all the remarkable properties of graphene. The low-energy structure of silicene is described by Dirac electrons with relatively large spin–orbit interactions owing to its buckled structure. A key observation is that the band structure can be controlled by applying an electric field to a silicene sheet. In particular, the gap closes at a certain critical electric field. Examining the band structure of a silicene nanoribbon, we show that a topological phase transition occurs from a topological insulator to a band insulator with an increase of electric field. We also show that it is possible to generate helical zero modes anywhere in a silicene sheet by adjusting the electric field locally to this critical value. The region may act as a quantum wire or a quantum dot surrounded by topological and/or band insulators. We explicitly construct the wave functions for some simple geometries based on the low-energy effective Dirac theory. These results are also applicable to germanene, which is a 2D honeycomb structure of germanium.

We study the inhomogeneity of the pairing gap near the surface of Bi₂Sr₂CaCu₂O_8+δ due to interstitial oxygen dopants. Within the slave boson mean-field theory of a disordered t–t'–J model, we identify three aspects of the oxygen dopants related to inhomogeneity: (i) the superexchange interaction is locally enhanced in the vicinity of oxygen dopants, which enhances the local pairing gap and reduces the coherence peak; (ii) oxygen dopants donate holes into the CuO₂ plane, which reduces the spinon density of states and hence the average gap at large doping; and (iii) holes are locally attracted to the vicinity of oxygen dopants, which causes an impurity bound state and further reduces the coherence peak. The interplay of these mechanisms simultaneously explains the locally enhanced pairing gap around oxygen dopants and the reduction of average gap with increasing oxygen concentration, as observed by scanning tunneling microscopy.

To understand the abnormal behavior of the superconducting transition temperature (T_c) because of the presence of a non-magnetic Zn impurity in the (F, Zn)-codoped LaFeAsO system (Li et al 2010 New J. Phys.12 083008), we investigated its unique electronic and local structures via x-ray absorption spectroscopy and first-principles calculations. The data obtained showed that the presence of a Zn impurity induces an electron transfer from As to Fe atoms in both the F-underdoped and -overdoped regions. Moreover, due to the lattice mismatch, the local lattice structure is finely modulated by both F and Zn impurities. Actually, in the F-underdoped region doping by Zn is associated with regular FeAs₄ tetrahedra, while distorted FeAs₄ tetrahedra occur in the F-overdoped region where superconductivity is significantly suppressed.

Crystalline MgO films with a defined level of Cr dopants (MgO_Cr) are prepared by either subsequent or simultaneous deposition of Cr and Mg atoms in an oxygen ambience onto a Mo(001) support. The structural and morphological parameters of the doped films are investigated using low-energy electron diffraction and scanning tunnelling microscopy (STM). Whereas at low Cr concentration the doped oxide has similar properties to pristine MgO(001), a new Cr/Mg mixed oxide develops at higher Cr load. The nature of the Cr impurities in the MgO matrix is deduced from cathodoluminescence spectroscopy performed by electron injection from the STM tip into the oxide film. In agreement with earlier studies on MgO_Cr bulk crystals, the majority of Cr adopts a 3 +  charge state and occupies Mg substitutional sites. The dopants give rise to sharp emission lines at about 700 nm, arising from radiative electron transitions between the Cr 3d levels split in the MgO crystal field. From the spectral evolution upon annealing the MgO_Cr films, we deduce a strong tendency of the dopants to accumulate in a near-surface region. Our experiments demonstrate that high-quality MgO_Cr films with similar optical properties as bulk samples can be prepared on conductive supports, being a first step to make doped oxide materials accessible to surface science studies.

We investigate the potential of transformation optics for the design of novel electromagnetic cavities. First, we determine the dispersion relation of bound modes in a device performing an arbitrary radial coordinate transformation and we discuss a number of such cavity structures. Subsequently, we generalize our study to media that implement azimuthal transformations, and show that such transformations can manipulate the azimuthal mode number. Finally, we discuss how the combination of radial and azimuthal coordinate transformations allows for perfect confinement of subwavelength modes inside a cavity consisting of right-handed materials only.

Engineering and controlling well-defined states of light for quantum information applications is of increasing importance as the complexity of quantum systems grows. For example, in quantum networks, high multi-photon interference visibility requires properly devised pure photon sources. In this paper, we present a theoretical model for a spontaneous parametric down conversion source based on an integrated cavity-waveguide, where single narrow-band, possibly distinct, resonant modes for the idler and the signal fields can be generated. This mode selection takes advantage of the clustering effect, due to the intrinsic dispersion of the nonlinear material. We show that, by engineering the clustering effect in an integrated cavity-waveguide and by using a standard detector, one can efficiently generate heralded pure single photons even with a continuous-wave pumping mode. The photon source proposed in this paper is extremely flexible and could easily be adapted to a wide variety of wavelengths and applications, such as long-distance quantum communication.

The nonadiabatic saddle-point method is used to investigate the process of high-order harmonic generation in a gas, driven by few-optical-cycle pulses with above-saturation intensity and controlled electric field. The peculiar effects produced on the generation process by temporal reshaping of the driving field, induced by propagation in a highly ionized gas cell, can be used to control the electron quantum paths, which contribute to the harmonic generation process. It is shown that complete spectral tunability of the harmonic peak position over the entire extreme-ultraviolet spectrum, obtained by changing the carrier-envelope phase of the driving pulses, can be understood by considering the effects of driving pulse distortions on the phase of the relevant electron quantum paths.

The dynamics of metastable magnetic domain walls in straight ferromagnetic nanowires under spin waves, external magnetic fields and current-induced spin-transfer torque are studied by means of micromagnetic simulations. It is found that in contrast to a stable wall, it is possible to displace a metastable domain wall in the absence of external excitation. In addition, independent of the domain wall excitation method, the velocity of a metastable wall is much smaller than that of a stable wall and its displacement direction could be different from that of the stable wall depending on the structure of metastable walls. Under current-induced spin-transfer torque excitation, the direction of domain wall displacement is directly related to the intensity of non-adiabatic spin-transfer torque. In a rough nanowire, it is found that the displacement of a metastable wall could happen much below the critical excitation of a stable wall. Furthermore, we show that it is possible to have either a forward or a backward displacement of a metastable domain wall by changing the pulse width of the excitation.

In this paper, we report a systematic investigation of the crystal structure and superconducting properties of iron-based superconductors Eu_1−xNa_xFe₂As₂ (x = 0–0.5). X-ray diffraction patterns indicate that the compounds form the ThCr₂Si₂-type structure with space group I4/mmm. The systematic evolution of the lattice constant demonstrates that the Eu ions are successfully replaced by Na. The use of alkali metal substitution into the Eu site allows us to suppress the magnetic/structural phase transition in the parent compounds and superconductivity reaches as high as 35 K with a doping level of x = 0.5. In addition, single crystals of Eu_1−xNa_xFe₂As₂ (x = 0, 0.5) have been successfully synthesized using the self-flux method. The upper critical fields have been determined with the magnetic field along the ab-plane and the c-axis, yielding an anisotropy of 1.7. The high upper critical fields and the low superconducting anisotropy of the Na-doped EuFe₂As₂ compounds indicate a potential for applications such as the generation of high magnetic fields.

We report the first evidence for double-slit interferences in a polyatomic molecule, which we have observed in the experimental carbon 1s photoelectron spectra of acetylene (or ethyne). The spectra have been measured over the photon energy range of 310–930 eV and show prominent oscillations in the intensity ratios σ_g(υ)/σ_u(υ) for the vibrational quantum numbers υ = 0,1 and for the ratios σ_s(υ = 1)/σ_s(υ = 0) for the symmetry s = g,u. The experimental findings are in very good agreement with ab initio density functional theory (DFT) calculations and are compatible with the Cohen–Fano mechanism of coherent emission from two equivalent atomic centers. This interpretation is supported by the qualitative predictions of a simple model in which the effect of nuclear recoil is taken into account to the lowest order. Our results confirm the delocalized character of the core hole created in the primary photoionization event and demonstrate that intramolecular core-hole coherence can survive the decoherent influence associated with the asymmetric nuclear degrees of freedom which are characteristic of polyatomic molecules.

We propose that the Hawking radiation energy and entropy flow rates from a black hole can be viewed as a one-dimensional (1D), non-equilibrium Landauer transport process. Support for this viewpoint comes from previous calculations invoking conformal symmetry in the near-horizon region, which give radiation rates that are identical to those of a single 1D quantum channel connected to a thermal reservoir at the Hawking temperature. The Landauer approach shows in a direct way the particle statistics independence of the energy and entropy fluxes of a black hole radiating into vacuum, as well as one near thermal equilibrium with its environment. As an application of the Landauer approach, we show that Hawking radiation gives a net entropy production that is 50% larger than that obtained assuming standard 3D emission into vacuum.

A multiple scattering analysis in a non-viscous fluid is developed in order to predict the effective constitutive parameters of certain suspensions of disordered particles or bubbles. The analysis is based on an effective field approach, and uses suitable pair-correlation functions to account for the essential features of densely distributed particles. The effective medium that is equivalent to the original suspension of particles is a medium with space and time dispersion, and hence, its parameters are functions of the frequency of the incident acoustic wave. Under the quasi-crystalline approximation, novel expressions are presented for effective constitutive parameters, which are valid at any frequency and wavelength. The emerging possibility of designing fluid–particle mixtures to form acoustic metamaterials is discussed. Our theory provides a convenient tool for testing ideas in silico in the search for new metamaterials with specific desired properties. An important conclusion of the proposed approach is that negative constitutive parameters can also be achieved by using suspensions of particles with random microstructures with properties similar to those shown in periodic arrays of microstructures.

To understand the main spin relaxation mechanism in graphene, we investigate the spin relaxation with random Rashba field (RRF) induced by both adatoms and a substrate using the kinetic spin Bloch equation approach. The charged adatoms, on the one hand, enhance the Rashba spin–orbit coupling locally and, on the other hand, serve as Coulomb potential scatterers. Both effects contribute to spin relaxation limited by the D'yakonov–Perel' (DP) mechanism. In addition, the RRF also causes spin relaxation by spin-flip scattering, manifesting itself as an Elliott–Yafet (EY)-like mechanism. Both mechanisms are sensitive to the correlation length of the RRF, which may be affected by environmental parameters such as electron density and temperature. Fitting and comparing the experiments of the Groningen group (Józsa et al 2009 Phys. Rev. B 80 241403(R)) and the Riverside group (Pi et al 2010 Phys. Rev. Lett.104 187201; Han and Kawakami 2011 Phys. Rev. Lett.107 047207), which show either DP (with the spin relaxation rate being inversely proportional to the momentum scattering rate) or EY-like (with the spin relaxation rate being proportional to the momentum scattering rate) properties, we suggest that the DP mechanism dominates the spin relaxation in graphene. The latest experimental finding of a nonmonotonic dependence of spin relaxation time on diffusion coefficient by Jo et al 2011 (Phys. Rev. B 84 075453) is also well reproduced by our model.

We experimentally demonstrate a single-qubit decohering quantum channel using linear optics. We implement the channel, whose special cases include the familiar amplitude-damping channel and the bit-flip channel, using a single, static optical setup. Following a recent theoretical result (Piani et al 2011 Phys. Rev. A 84 032304), we realize the channel in an optimal way, maximizing the probability of success, i.e. the probability for the photonic qubit to remain in its encoding. Using a two-photon entangled resource, we characterize the channel using ancilla-assisted process tomography and find average process fidelities of 0.98 ± 0.01 and 0.976 ± 0.009 for amplitude-damping and the bit-flip case, respectively.

The OPERA neutrino experiment in the underground Gran Sasso Laboratory (LNGS) was designed to perform the first detection of neutrino oscillations in direct appearance mode in the ν_μ → ν_τ channel, the ν_τ signature being the identification of the τ-lepton created in its charged current interaction. The hybrid apparatus consists of a large mass emulsion film/lead target complemented by electronic detectors. Placed in the LNGS, it is exposed to the high-energy long-baseline CERN Neutrino beam to Gran Sasso (CNGS) 730 km away from the neutrino source. The observation of a first ν_τ candidate event was reported in 2010. In this paper, we discuss the result of the analysis of the data taken during the first two years of operation (2008–2009) underlining the major improvements brought to the analysis chain and to the Monte Carlo simulations. The statistical significance of the one event observed so far is then evaluated to 95%.

We present a new concept for the generation of optical lattice waves. For all four families of nondiffracting beams, we are able to realize corresponding nondiffracting intensity patterns in a single setup. The potential of our approach is shown by demonstrating the optical induction of complex photonic discrete, Bessel, Mathieu and Weber lattices in a nonlinear photorefractive medium. However, our technique itself is very general and can be transferred to optical lattices in other fields such as atom optics or cold gases in order to add such complex optical potentials as a new concept to these areas as well.

We propose an approach for quantum simulation of electron–phonon interactions using Rydberg states of cold atoms and ions. We show how systems of cold atoms and ions can be mapped onto electron–phonon systems of the Su–Schrieffer–Heeger type. We discuss how properties of the simulated Hamiltonian can be tuned and how to read physically relevant properties from the simulator. In particular, use of painted spot potentials offers a high level of tunability, enabling all physically relevant regimes of the electron–phonon Hamiltonian to be accessed.

We report size-dependent trapping and delivery of polystyrene submicro-spheres using a 600 nm diameter fibre. Theoretical results show that both gradient and scattering forces exerted on polystyrene submicro-spheres by the evanescent wave field around the submicrofibre increase with an increase in the sphere diameter, and the delivery velocity of the bigger spheres is also higher than that of smaller spheres. To support the theoretical predictions, experiments were performed using polystyrene spheres with diameters of 230, 400, 530 and 700 nm by injecting a 532 nm green laser into the fibre. The results demonstrate that spheres with larger diameter can be more easily trapped to the surface of the fibre and delivered in the propagation direction of the laser at a low input laser power.

We show that multi-orbital and density-induced tunneling have a significant impact on the phase diagram of bosonic atoms in optical lattices. Off-site interactions lead to density-induced hopping, the so-called bond-charge interactions, which can be identified with an effective tunneling potential and can reach the same order of magnitude as conventional tunneling. In addition, interaction-induced higher-band processes also give rise to strongly modified tunneling, on-site and bond-charge interactions. We derive an extended occupation-dependent Hubbard model with multi-orbitally renormalized processes and compute the corresponding phase diagram. It substantially deviates from the single-band Bose–Hubbard model and predicts strong changes of the superfluid-to-Mott-insulator transition. In general, the presented beyond-Hubbard physics plays an essential role in bosonic lattice systems and has an observable influence on experiments with tunable interactions.

We present experimental observations of interference between an atomic spin coherence and an optical field in a Λ-type gradient echo memory. The interference is mediated by a strong classical field that couples a weak probe field to the atomic coherence through a resonant Raman transition. Interference can be observed between a prepared spin coherence and another propagating optical field, or between multiple Λ transitions driving a single-spin coherence. Our scheme can behave as a controllable time-delayed beamsplitter with dynamically tuneable splitting ratio and allows, in principle, for unity interference visibility.

We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al–Si electrode. Pure cobalt electrodes were compared with a Co–Fe–B alloy and the Heusler compound Co₂FeAl. The polarization of the tunneling electrons was determined using the Maki–Fulde model and is discussed along with the spin–orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co–Fe–B/MgO/Co–Fe–B junctions.

A novel reverse design schematic for designing a metamaterial magnifier with graded negative refractive index for both the two-dimensional and three-dimensional cases has been proposed. Photorealistic rendering is integrated with trace ray trajectories in example designs to visualize the scattering magnification as well as imaging of the proposed graded-index magnifier with negative-index metamaterials. The material of the magnifying shell can be uniquely and independently determined without knowing beforehand the corresponding domain deformation. This reverse recipe and photorealistic rendering directly tackles the significance of all possible parametric profiles and demonstrates the performance of the device in a realistic scene, which provides a scheme to design, select and evaluate a metamaterial magnifier.

Recent experiments with 100 terawatt-class, sub-50 femtosecond laser pulses show that electrons self-injected into a laser-driven electron density bubble can be accelerated above 0.5 gigaelectronvolt energy in a sub-centimetre-length rarefied plasma. To reach this energy range, electrons must ultimately outrun the bubble and exit the accelerating phase; this, however, does not ensure high beam quality. Wake excitation increases the laser pulse bandwidth by red-shifting its head, keeping the tail unshifted. Anomalous group velocity dispersion of radiation in plasma slows down the red-shifted head, compressing the pulse into a few-cycle-long piston of relativistic intensity. Pulse transformation into a piston causes continuous expansion of the bubble, trapping copious numbers of unwanted electrons (dark current) and producing a poorly collimated, polychromatic energy tail, completely dominating the electron spectrum at the dephasing limit. The process of piston formation can be mitigated by using a broad-bandwidth (corresponding to a few-cycle transform-limited duration), negatively chirped pulse. Initial blue-shift of the pulse leading edge compensates for the nonlinear frequency red-shift and delays the piston formation, thus significantly suppressing the dark current, making the leading quasi-monoenergetic bunch the dominant feature of the electron spectrum near dephasing. This method of dark current control may be feasible for future experiments with ultrahigh-bandwidth, multi-joule laser pulses.

We report a comprehensive study of the electrical and magneto-transport properties of nanocrystals of La_0.67Ca_0.33MnO₃ (LCMO) (with size down to 15 nm) and La_0.5Sr_0.5CoO₃ (LSCO) (with size down to 35 nm) in the temperature range 0.3–5 K and magnetic fields up to 14 T. The transport, magneto-transport and nonlinear conduction (I–V curves) were analysed using the concept of spin-polarized tunnelling in the presence of Coulomb blockade. The activation energy of transport, Δ, was used to estimate the tunnelling distances and the inverse decay length of the tunnelling wave function (χ) and the height of the tunnelling barrier (Φ_B). The magneto-transport data were used to find the magnetic field dependences of these tunnelling parameters. The data taken over a large magnetic field range allowed us to separate out the magneto-resistance (MR) contributions at low temperatures arising from tunnelling into two distinct contributions. In LCMO, at low magnetic field, the transport and MR are dominated by the spin polarization, while at higher magnetic field the MR arises from the lowering of the tunnel barrier by the magnetic field, leading to an MR that does not saturate even at 14 T. In contrast, in LSCO, which does not have substantial spin polarization, the first contribution at low field is absent, while the second contribution related to barrier height persists. The idea of inter-grain tunnelling has been validated by direct measurements of the nonlinear I–V data in this temperature range, and the I–V data were found to be strongly dependent on magnetic field. We made the important observation that a gap-like feature (with magnitude ∼E_C, the Coulomb charging energy) shows up in the conductance g(V ) at low bias for the systems with the smallest nanocrystal size at the lowest temperatures (T ⩽ 0.7 K). The gap closes when the magnetic field and temperature are increased.

Nodes in a complex networked system often engage in more than one type of interactions among them; they form a multiplex network with multiple types of links. In real-world complex systems, a node's degree for one type of links and that for the other are not randomly distributed but correlated, which we term correlated multiplexity. In this paper, we study a simple model of multiplex random networks and demonstrate that the correlated multiplexity can drastically affect the properties of a giant component in the network. Specifically, when the degrees of a node for different interactions in a duplex Erdős–Rényi network are maximally correlated, the network contains the giant component for any nonzero link density. In contrast, when the degrees of a node are maximally anti-correlated, the emergence of the giant component is significantly delayed, yet the entire network becomes connected into a single component at a finite link density. We also discuss the mixing patterns and the cases with imperfect correlated multiplexity.

We propose a scheme for simulating the dynamics of neutrino oscillations using trapped ions. For neutrinos in 1 + 1 dimensions, our scheme is experimentally implementable with existing trapped-ion technology. We show that the three-generation neutrino oscillations can be realized with three ions for 1 + 3 and 1 + 1 dimensions where the latter case only requires experimentally proven two-ion interactions. For this case, we discuss two setups utilizing different types of spin–spin interactions. Our method can be readily applied to two-generation neutrino oscillations requiring fewer ions and lasers. We give a brief outline of a possible experimental scenario.

Located beyond the resolution limit of nanoindentation, contact resonance atomic force microscopy (CR-AFM) is employed for nano-mechanical surface characterization of single crystalline 14M modulated martensitic Ni–Mn–Ga (NMG) thin films grown by magnetron sputter deposition on (001) MgO substrates. Comparing experimental indentation moduli—obtained with CR-AFM—with theoretical predictions based on density functional theory (DFT) indicates the central role of pseudo plasticity and inter-martensitic phase transitions. Spatially highly resolved mechanical imaging enables the visualization of twin boundaries and allows for the assessment of their impact on mechanical behavior at the nanoscale. The CR-AFM technique is also briefly reviewed. Its advantages and drawbacks are carefully addressed.

Optimal open-loop control, i.e. the application of an analytically derived control rule, is demonstrated for nanooptical excitations using polarization-shaped laser pulses. Optimal spatial near-field localization in gold nanoprisms and excitation switching is realized by applying a π shift to the relative phase of the two polarization components. The achieved near-field switching confirms theoretical predictions, proves the applicability of predefined control rules in nanooptical light–matter interaction and reveals local mode interference to be an important control mechanism.

The coupling effects of subwavelength high-permittivity (ε_r > 100) arrayed ceramics which exhibit magnetic and electric Mie resonances are investigated by electromagnetic full-wave analysis. Special attention was paid to the symmetry properties of both magnetic- and electric-induced dipoles by varying independently the array periodicity. In agreement with the interactions between electric and magnetic dipoles, it is shown that resonance frequency shifts toward lower (higher) frequencies can be obtained, which depends on the longitudinal (transverse) dipole coupling strengths. Moreover, the emergence of quasi-bound states between tightly coupled basic cells is pointed out for the electric Mie resonances, which shows an unexpected frequency shift with a reverse variation.

To what extent do the characteristic features of a chemical reaction network reflect its purpose and function? In general, one argues that correlations between specific features and specific functions are key to understanding a complex structure. However, specific features may sometimes be neutral and uncorrelated with any system-specific purpose, function or causal chain. Such neutral features are caused by chance and randomness. Here we compare two classes of chemical networks: one that has been subjected to biological evolution (the chemical reaction network of metabolism in living cells) and one that has not (the atmospheric planetary chemical reaction networks). Their degree distributions are shown to share the very same neutral system-independent features. The shape of the broad distributions is to a large extent controlled by a single parameter, the network size. From this perspective, there is little difference between atmospheric and metabolic networks; they are just different sizes of the same random assembling network. In other words, the shape of the degree distribution is a neutral characteristic feature and has no functional or evolutionary implications in itself; it is not a matter of life and death.

The number of citations is a widely used metric for evaluating the scientific credit of papers, scientists and journals. However, it so happens that papers with fewer citations from prestigious scientists have a higher influence than papers with more citations. In this paper, we argue that by whom the paper is being cited is of greater significance than merely the number of citations. Accordingly, we propose an interactive model of author–paper bipartite networks as well as an iterative algorithm to obtain better rankings for scientists and their publications. The main advantage of this method is twofold: (i) it is a parameter-free algorithm; (ii) it considers the relationship between the prestige of scientists and the quality of their publications. We conducted real experiments on publications in econophysics, and used this method to evaluate the influence of related scientific journals. The comparison between the rankings by our method and simple citation counts suggests that our method is effective in distinguishing prestige from popularity.

We analyze multipartite entanglement between atomic ensembles within quantum matter–light interfaces. In our proposal, a polarized light beam crosses sequentially several polarized atomic ensembles impinging on each of them at a given angle α_i. These angles are crucial parameters for shaping the entanglement since they are directly connected to the appropriate combinations of the collective atomic spins that are squeezed. We exploit such a scheme to go beyond the pure state paradigm proposing realistic experimental settings to address multipartite mixed state entanglement in continuous variables.

We consider the synchronization of coupled dynamical systems when different types of interactions are simultaneously present. We assume that a set of dynamical systems is coupled through the connections of two or more distinct networks (each of which corresponds to a distinct type of interaction), and we refer to such a system as a dynamical hypernetwork. Applications include neural networks made up of both electrical gap junctions and chemical synapses, the coordinated motion of shoals of fish communicating through both vision and flow sensing, and hypernetworks of coupled chaotic oscillators. We first analyze the case of a hypernetwork made up of m = 2 networks. We look for the necessary and sufficient conditions for synchronization. We attempt to reduce the linear stability problem to a master stability function (MSF) form, i.e. decoupling the effects of the coupling functions from the structure of the networks. Unfortunately, we are unable to obtain a reduction in an MSF form for the general case. However, we show that such a reduction is possible in three cases of interest: (i) the Laplacian matrices associated with the two networks commute; (ii) one of the two networks is unweighted and fully connected; and (iii) one of the two networks is such that the coupling strength from node i to node j is a function of j but not of i. Furthermore, we define a class of networks such that if either one of the two coupling networks belongs to this class, the reduction can be obtained independently of the other network. As an example of interest, we study synchronization of a neural hypernetwork for which the connections can be either chemical synapses or electrical gap junctions. We propose a generalization of our stability results to the case of hypernetworks formed of m ⩾ 2 networks.

A system with unequal populations of up and down fermions may exhibit a Larkin–Ovchinnikov (LO) phase characterized by periodic domain walls across which the order parameter changes sign and the excess polarization is localized. Despite fifty years of theoretical and experimental work, there has so far been no unambiguous observation of an LO phase. Here we propose an experiment in which two fermion clouds, prepared with unequal population imbalances, are allowed to expand and interfere. We show that a pattern of staggered fringes in the interference is unequivocal evidence of LO physics.

Actomyosin contractility is essential for biological force generation, and is well understood in highly organized structures such as striated muscle. Additionally, actomyosin bundles devoid of this organization are known to contract both in vivo and in vitro, which cannot be described by standard muscle models. To narrow down the search for possible contraction mechanisms in these systems, we investigate their microscopic symmetries. We show that contractile behavior requires non-identical motors that generate large-enough forces to probe the nonlinear elastic behavior of F-actin. This suggests a role for filament buckling in the contraction of these bundles, consistent with recent experimental results on reconstituted actomyosin bundles.

We develop a numerical technique for simulating metamaterial electromagnetic response based on an adaptation of the discrete dipole approximation (DDA). Our approach reduces each constituent metamaterial element within the composite to a point dipole with electric and magnetic polarizabilities, rather than assuming a homogenized effective material. We first validate the approach by computing the scattering cross-section for a collection of densely spaced isotropic dipole moments arranged within a cylindrical area, and compare with the known result from Mie theory. The discrete dipole approach has considerable advantages for the design of gradient and transformation optical media based on metamaterials, since the absence of local periodicity in other common design approaches leaves them with questionable validity. Several variants of iconic cloaking structures are investigated to illustrate the method, in which we study the impact that different configurations of dipolar elements can have on cloak performance. The modeling of a complex medium as polarizable dipoles provides a much closer connection to actual metamaterial implementations, and can address key nonlocal phenomena, such as magnetoelectric coupling, not accessible to most current numerical metamaterial approaches.

We derive rigorous conditions for the synchronization of all-optically coupled lasers. In particular, we elucidate the role of the optical coupling phases for synchronizability by systematically discussing all possible network motifs containing two lasers with delayed coupling and feedback. By these means we explain previous experimental findings. Further, we study larger networks and elaborate optimal conditions for chaos synchronization. We show that the relative phases between lasers can be used to optimize the effective coupling matrix.

A rigorous lower bound is obtained for the average resolution of any estimate of a shift parameter, such as an optical phase shift or a spatial translation. The bound has the asymptotic form k_I/〈2|G|〉 where G is the generator of the shift (with an arbitrary discrete or continuous spectrum), and hence establishes a universally applicable bound of the same form as the usual Heisenberg limit. The scaling constant k_I depends on prior information about the shift parameter. For example, in phase sensing regimes, where the phase shift is confined to some small interval of length L, the relative resolution $\delta \hat {\Phi }/L$ has the strict lower bound (2πe³)^−1/2/〈2m|G₁| + 1〉, where m is the number of probes, each with generator G₁, and entangling joint measurements are permitted. Generalizations using other resource measures and including noise are briefly discussed. The results rely on the derivation of general entropic uncertainty relations for continuous observables, which are of interest in their own right.

Quantum dots are an important model system for thermoelectric phenomena, and may be used to enhance the thermal-to-electric energy conversion efficiency in functional materials, by tuning the Fermi energy relative to the dots' transmission resonances. It is therefore important to obtain a detailed understanding of a quantum dot's thermopower as a function of the Fermi energy. However, so far it has proven difficult to take the effects of interactions into account in the interpretation of experimental data. In this paper, we present detailed measurements of the thermopower of quantum dots defined in heterostructure nanowires. We show that the thermopower lineshape is described well by a Landauer-type transport model that uses as its input experimental values of the dot conductance, which contains information about interaction effects.

Wave propagation and vibration transmission in metamaterial-based elastic rods containing periodically attached multi-degree-of-freedom spring–mass resonators are investigated. A methodology based on a combination of the spectral element (SE) method and the Bloch theorem is developed, yielding an explicit formulation for the complex band structure calculation. The effects of resonator parameters on the band gap behavior are investigated by employing the attenuation constant surface plots, which display information on the location, the width and the attenuation performance of all band gaps. It is found that Bragg-type and resonance-type gaps co-exist in these systems. In some special situations, exact coupling between Bragg and resonance gaps occurs, giving rise to super-wide coupled gaps. The advantage of multi-degree-of-freedom resonators in achieving multiband and/or broadband gaps in metamaterial-based rods is demonstrated. Band gap formation mechanisms are further examined by analytical and physical models, providing explicit formulae to locate the band edge frequencies of all the band gaps.

We report a new method for the design of kagome lattices using zigzag-edged triangular graphene nanoflakes (TGFs) linked with B, C, N or O atoms. Using spin-polarized density functional theory we show that the electronic and magnetic properties of the designed kagome lattices can be modulated by changing their size and the linking atoms. The antiferromagnetic coupling between the two directly linked TGFs becomes ferromagnetic coupling when B, C or N is used as the linking atoms, but not for O atom linking. All the designed structures are semiconductors which can be synthesized from graphene atomic sheets by using electron etching and block copolymer lithography techniques. This study is a good example of how mathematical models can be used to construct magnetic nanostructures involving only s, p elements.

We have studied the dynamics of a generalized toric code based on qudits at finite temperature by finding the master equation coupling the code's degrees of freedom to a thermal bath. We find that in the case of qutrits, new types of anyons and thermal processes appear that are forbidden for qubits. These include creation, annihilation and diffusion throughout the system code. It is possible to solve the master equation in a short-time regime and find expressions for the decay rates as a function of the dimension d of the qudits. While we provide an explicit proof that the system relaxes to the Gibbs state for arbitrary qudits, we also prove that above a certain crossover temperature the qutrits' initial decay rate is smaller than the original case for qubits. Surprisingly, this behavior only happens for qutrits and not for other qudits with d > 3.

We analyze the thermoelectric behavior, using first principles and Boltzmann transport calculations, of very heavily electron-doped CrSi₂ and find that at temperatures of 900–1250 K and electron dopings of 1–4 × 10²¹ cm⁻³, thermopowers as large in magnitude as 200 μV K⁻¹ may be found. Such high thermopowers at such high carrier concentrations are extremely rare, and suggest that excellent thermoelectric performance may be found in these ranges of temperature and doping.

The spontaneous creation of electron–positron pairs out of the vacuum due to a strong electric field is a spectacular manifestation of the relativistic energy–momentum relation for the Dirac fermions. This fundamental prediction of quantum electrodynamics has not yet been confirmed experimentally, as the generation of a sufficiently strong electric field extending over a large enough space–time volume still presents a challenge. Surprisingly, distant areas of physics may help us to circumvent this difficulty. In condensed matter and solid state physics (areas commonly considered as low-energy physics), one usually deals with quasi-particles instead of real electrons and positrons. Since their mass gap can often be freely tuned, it is much easier to create these light quasi-particles by an analogue of the Sauter–Schwinger effect. This motivates our proposal for a quantum simulator in which excitations of ultra-cold atoms moving in a bichromatic optical lattice represent particles and antiparticles (holes) satisfying a discretized version of the Dirac equation together with fermionic anti-commutation relations. Using the language of second quantization, we are able to construct an analogue of the spontaneous pair creation which can be realized in an (almost) table-top experiment.

We present the convergent close-coupling formulation for positron scattering from noble gases (Ne, Ar, Kr and Xe) within the single-center approximation. Target functions are described in a model of six p-electrons above an inert Hartree–Fock core with only one-electron excitations from the outer p⁶ shell allowed. Target states have been obtained using a Sturmian (Laguerre) basis in order to model coupling to ionization and positronium (Ps) formation channels. Such an approach is unable to yield explicit Ps-formation cross sections, but is valid below this threshold and above the ionization threshold. The present calculations are found to show good agreement with recent measurements.

Transport properties of positron swarms in water vapour under the influence of electric and magnetic fields are investigated using a Monte Carlo simulation technique and a multi-term theory for solving the Boltzmann equation. Special attention is paid to the correct treatment of the non-conservative nature of positronium (Ps) formation and its explicit and implicit influences on various positron transport properties. Many interesting and atypical phenomena induced by these influences are identified and discussed. Calculated transport properties for positrons are compared with those for electrons, and the most important differences are highlighted. The significant impact of a magnetic field on non-conservative positron transport in a crossed field configuration is also investigated. In general, the mean energy and diffusion coefficients are lowered, while for the measurable drift velocity an unexpected phenomenon arises: for certain values of the reduced electric field, the magnetic field enhances the drift. The variation of transport coefficients with the reduced electric and magnetic fields is addressed using physical arguments with the goal of understanding the synergistic effects of Ps formation and magnetic field on the drift and diffusion of positrons in neutral gases.

Characteristic features of the positron-binding structure of some carbonyl and aldehyde species such as formaldehyde, acetaldehyde, acetone and propionaldehyde are discussed with the configuration interaction scheme of multi-component molecular orbital (MC_MO) calculations. This method can take the electron–positron correlation contribution into account through single electronic–single positronic excitation configurations. Our vertical positron affinity (PA) values of acetaldehyde and acetone with electronic 6-31 + +G(2df,2pd) and positronic [15s15p3d2f1g] basis sets are as 52 and 92 meV, which can be compared to the recent experimental values of 90 and 173 meV by Danielson et al (2010 Phys. Rev. Lett.104 233201). For formaldehyde we have also found that the PA values are enhanced by including the local ${\rm C}= \!\! ={\rm O}$ vibrational contribution from the vertical PA value of 15 meV to 17, 21 and 25 meV after averaging over the zeroth, first and second vibrational states, respectively, due to the anharmonicity of the potential.

This work reports on precise measurements of two-quantum positron annihilation-in-flight using a digital coincidence Doppler broadening spectrometer. Annihilation-in-flight was measured for positrons emitted by ⁶⁸Ge/⁶⁸Ga and ²²Na radioisotopes and for various targets. Experimental data were compared with theoretical prediction by quantum electrodynamics. It was found that two-quantum positron annihilation-in-flight can be clearly recognized in two-dimensional coincidence Doppler broadening spectra as a hyperbolic curve with shape described well by the relativistic theory. The contribution of annihilation-in-flight is determined predominantly by the energy of incident positrons and is only weakly dependent on the target material. The profile of the positron annihilation-in-flight contribution for positron kinetic energies above 100 keV is well described by theory.

The Large Hadron Collider (LHC) at CERN outside Geneva, Switzerland provides Pb + Pb beams at a nucleon–nucleon center-of-mass energy of 2.76 TeV, which is nearly 14 times higher than the energy available at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven (200 GeV). The first LHC heavy ion run ended in December 2010, and already several results are available which give some indications of future directions in heavy ion physics. Results have been released both for bulk observables, the charged particle multiplicity and elliptic flow, as well as for 'hard probes' such as jets and J/ψ. The charged particle multiplicity near mid-rapidity shows no anomalous rise relative to lower energies, although an extrapolation to full phase space using extended longitudinal scaling may be revealing the first known violation of the Landau–Fermi multiplicity formula. Elliptic flow is found to agree surprisingly well with lower-energy data when measured as a function of transverse momentum, agreeing with viscous hydrodynamic calculations that treat the matter at the LHC similarly to that found at RHIC. Despite the similar features found at lower energies, the higher center-of-mass energies provide much higher rates of high-p_T 'hard' probes, to study the medium microscopically. It makes it possible for the first time to study ultrahigh-energy jets, which are found to show dramatic event-by-event asymmetries in their energies, possibly reflecting strong energy loss in the hot, dense medium. Measurements of the spectra of charged tracks within jets are reported, to indicate a softening of fragmentation of the lower-energy jet. J/ψ rates have also been measured, and were found to be suppressed at a similar level as lower-energy results. In total, the medium formed at the LHC appears to be qualitatively similar to that measured at lower energies. However, future measurements will certainly take even more advantage of the particular strengths of the LHC program, in particular the higher multiplicities and higher rates of large-momentum processes, so it is too early to draw strong conclusions.

It is theoretically known that a pair of phase-conjugating surfaces can function as a perfect lens, focusing propagating waves and enhancing evanescent waves. However, the known experimental approaches based on thin sheets of nonlinear materials cannot fully realize the required phase conjugation boundary condition. In this paper, we show that the ideal phase-conjugating surface is, in principle, physically realizable and investigate the necessary properties of nonlinear and nonreciprocal particles which can be used to build a perfect lens system. The physical principle of the lens operation is discussed in detail and directions of possible experimental realizations are outlined.

We undertake first steps in making a class of discrete models of quantum gravity, spin foams, accessible to a large-scale analysis by numerical and computational methods. In particular, we apply the Migdal–Kadanoff and tensor network renormalization (TNR) schemes to spin net and spin foam models based on finite Abelian groups and introduce 'cutoff models' to probe the fate of gauge symmetries under various such approximated renormalization group flows. For the TNR analysis, a new Gauß constraint preserving algorithm is introduced to improve numerical stability and aid physical interpretation. We also describe the fixed point structure and establish the equivalence of certain models.

Artificial spin ice arrays of micromagnetic islands are a means of engineering additional energy scales and frustration into magnetic materials. Here we demonstrate a magnetic phase transition in an artificial square spin ice and use the symmetry of the lattice to verify the presence of excitations far below the ordering temperature. We do this by measuring the temperature-dependent magnetization in different principal directions and comparing it with simulations of idealized statistical mechanical models. Our results confirm a dynamical pre-melting of the artificial spin ice structure at a temperature well below the intrinsic ordering temperature of the island material. We thus create a spin ice array that has the real thermal dynamics of artificial spins over an extended temperature range.

Electron–positron plasmas are unique in their behavior due to the mass symmetry. Strongly magnetized electron–positron, or pair, plasmas are present in a number of astrophysical settings, such as astrophysical jets, but they have not yet been created in the laboratory. Plans for the creation and diagnosis of pair plasmas in a stellarator are presented, based on extrapolation of the results from the Columbia Non-neutral Torus stellarator, as well as recent developments in positron sources. The particular challenges of positronium injection and pair plasma diagnostics are addressed.

We numerically study the focusing and bending effects of light and sound waves through heterogeneous isotropic cylindrical and spherical devices. We first point out that transformation optics and acoustics show that the control of light requires spatially varying anisotropic permittivity and permeability, while the control of sound is achieved via spatially anisotropic density and isotropic compressibility. Moreover, homogenization theory applied to electromagnetic and acoustic periodic structures leads to such artificial (although not spatially varying) anisotropic permittivity, permeability and density. We stress that homogenization is thus a natural mathematical tool for the design of structured metamaterials. To illustrate the two-step geometric transform-homogenization approach, we consider the design of cylindrical and spherical electromagnetic and acoustic lenses displaying some artificial anisotropy along their optical axis (direction of periodicity of the structural elements). Applications are sought in the design of Eaton and Luneburg lenses bending light at angles ranging from 90° to 360°, or mimicking a Schwartzchild metric, i.e. a black hole. All of these spherical metamaterials are characterized by a refractive index varying inversely with the radius which is approximated by concentric layers of homogeneous material. We finally propose some structured cylindrical metamaterials consisting of infinitely conducting or rigid toroidal channels in a homogeneous bulk material focusing light or sound waves. The functionality of these metamaterials is demonstrated via full-wave three-dimensional computations using nodal elements in the context of acoustics, and finite edge-elements in electromagnetics.

We examine the prospects of discrete quantum walks (QWs) with trapped ions. In particular, we analyze in detail the limitations of the protocol of Travaglione and Milburn (2002 Phys. Rev. A 65 032310) that has been implemented by several experimental groups in recent years. Based on the first realization in our group (Schmitz et al 2009 Phys. Rev. Lett.103 090504), we investigate the consequences of leaving the scope of the approximations originally made, such as the Lamb–Dicke approximation. We explain the consequential deviations from the idealized QW for different experimental realizations and an increasing number of steps by taking into account higher-order terms of the quantum evolution. It turns out that these already become significant after a few steps, which is confirmed by experimental results and is currently limiting the scalability of this approach. Finally, we propose a new scheme using short laser pulses, derived from a protocol from the field of quantum computation. We show that this scheme is not subject to the above-mentioned restrictions and analytically and numerically evaluate its limitations, based on a realistic implementation with our specific setup. Implementing the protocol with state-of-the-art techniques should allow for substantially increasing the number of steps to 100 and beyond and should be extendable to higher-dimensional QWs.

We calculate cross sections for helium–antihydrogen scattering for energies up to 0.01 atomic unit. Our calculation includes elastic scattering, direct antiproton–alpha particle annihilation and rearrangement into ${\rm He}^+\bar {\rm p}$ and ground-state positronium. Elastic scattering is calculated within the Born–Oppenheimer approximation using the potential calculated by Strasburger et al (2005 J. Phys. B: At. Mol. Opt. Phys.38 3091). Matrix elements for rearrangement are calculated using the T-matrix in the distorted wave approximation, with the initial state represented by Hylleraas-type functions. The strong force, leading to direct annihilation, was included as a short-range boundary condition in terms of the strong-force scattering length.

The thermally driven formation and evolution of vertex domains is studied for square artificial spin ice. A self-consistent mean-field theory is used to show how domains of ground state ordering form spontaneously, and how these evolve in the presence of disorder. The role of fluctuations is studied using Monte Carlo simulations and analytical modelling. Domain wall dynamics are shown to be driven by a biasing of random fluctuations towards processes that shrink closed domains, and fluctuations within domains are shown to generate isolated small excitations, which may stabilize as the effective temperature is lowered. Domain dynamics and fluctuations are determined by interaction strengths, which are controlled by inter-element spacing. The role of interaction strength is studied via experiments and Monte Carlo simulations. Our mean-field model is applicable to ferroelectric 'spin' ice, and we show that features similar to those of magnetic spin ice can be expected, but with different characteristic temperatures and rates.

Dipolar spin ice has attracted much attention because of its intriguing ground state ordering and elementary excitation properties. We present experimental realizations of magnetic dipolar spin ice on periodic lattices with honeycomb symmetry. We have analyzed in particular the evolution and distribution of excitations with magnetic charges ±3 per vertex as a function of magnetic field and the distance b between the dipoles ranging from b = 0.4 to 1.7 μm. In all the dipole patterns investigated, we observe a surprisingly high abundance of ±3 magnetic charges at coercivity in the descending and ascending branches of the magnetic hysteresis. At the same time, these ±3 vertices form a charge ordered state with large domains, resembling an ionic crystal. Monte Carlo simulations of the magnetization reversal confirm in the framework of a macrospin model an enhanced abundance of ±3 magnetic charges at the coercive field. But much better agreement is achieved by taking into account the micromagnetic reversal mechanism, which proceeds via nucleation and domain wall propagation for dipoles aligned with the field and via coherent rotation for all others.

The effects of low molecular weight diluents (namely water and glycerol) on the nanostructure and thermodynamic state of low water content gelatin matrices are explored systematically by combining positron annihilation lifetime spectroscopy (PALS) with calorimetric measurements. Bovine gelatin matrices with a variation in the glycerol content (0–10 wt.%) are equilibrated in a range of water activities (a_w = 0.11–0.68, T = 298 K). Both water and glycerol reduce the glass transition temperature, T_g, and the temperature of dissociation of the ordered triple helical segments, T_m, while having no significant effect on the level of re-naturation of the gelatin matrices. Our PALS measurements show that over the concentration range studied, glycerol acts as a packing enhancer and in the glassy state it causes a nonlinear decrease in the average hole size, v_h, of the gelatin matrices. Finally, we report complex changes in v_h for the gelatin matrices as a function of the increasing level of hydration. At low water contents (Q_w ∼ 0.01–0.10), water acts as a plasticizer, causing a systematic increase in v_h. Conversely, for water contents higher than Q_w ∼ 0.10, v_h is found to decrease, as small clusters of water begin to form between the polypeptide chains.

We propose a model to predict and control the statistical ensemble of magnetic degrees of freedom in artificial spin ice (ASI) during thermalized adiabatic growth (Wang et al 2006 Nature439 303; Morgan et alNature Phys.7 75). We predict that as-grown arrays are controlled by the temperature at fabrication and by their lattice constant, and that they can be described by an effective temperature. If the geometry is conducive to a phase transition, then the lowest-temperature phase is accessed in arrays of lattice constant smaller than a critical value, which depends on the temperature at deposition. Alternatively, for arrays of equal lattice constant, there is a temperature threshold at deposition and the lowest-temperature phase is accessed for fabrication temperatures larger rather than smaller than this temperature threshold. Finally, we show how to define and control the effective temperature of the as-grown array and how to measure critical exponents directly. We discuss the role of kinetics at the critical point, and applications to experiments, in particular to as-grown thermalized square ASI and to magnetic monopole crystallization in as-grown honeycomb ASI.

We have calculated the rates for the processes that result from collisions of positrons with Rydberg atoms in strong magnetic fields. This is the first step in a two-stage charge transfer, which is being explored as a mechanism for the formation of cold anti-hydrogen. Unlike previous theoretical explorations we have also investigated the cases when the energy of the positrons can be comparable to or larger than the binding energy of the atom. We have also examined how big an electric field is needed to destroy the resulting positronium. We find that the charge transfer does not scale as $\sqrt {T}$ for n = 40 states for positron temperatures above T ∼ 25 K. Also, we find that the positronium atoms are more easily destroyed by electric fields if they emerge from the charge transfer along the magnetic field lines, contrary to what might be expected for the $\vec {v}\times \vec {B}$ effective electric field. Both results have implications for the formation of anti-hydrogen atoms.

We show how to exchange (braid) Majorana fermions in a network of superconducting nanowires by control over Coulomb interactions rather than tunneling. Even though Majorana fermions are charge-neutral quasiparticles (equal to their own antiparticle), they have an effective long-range interaction through the even–odd electron number dependence of the superconducting ground state. The flux through a split Josephson junction controls this interaction via the ratio of Josephson and charging energies, with exponential sensitivity. By switching the interaction on and off in neighboring segments of a Josephson junction array, the non-Abelian braiding statistics can be realized without the need to control tunnel couplings by gate electrodes.

The Fermi surface of the ferromagnetic shape-memory alloy Ni₂MnGa has been determined experimentally with two-dimensional angular correlation of electron–positron annihilation radiation. Our results are supported by first principles electronic structure calculations. The measured electron occupancy within the Brillouin zone is consistent with the existence of two nesting features present in the Fermi surfaces calculated in previous studies. The nesting vectors of the calculated Fermi surface match the modulation of the pre-martensitic intermediate structure and that of the martensitic structure.

Calculations of γ-spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation γ-spectra characterise the momentum distribution of the electron–positron pair at the instant of annihilation. The contribution to the γ-spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation γ-spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective 'narrowing' of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron–positron and positron–molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives γ-spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation γ-spectra, can be approximated by a relatively simple scaling factor.

We model the dynamics of magnetization in an artificial analogue of spin ice specializing to the case of a honeycomb network of connected magnetic nanowires. The inherently dissipative dynamics is mediated by the emission and absorption of domain walls in the sites of the lattice, and their propagation in its links. These domain walls carry two natural units of magnetic charge, whereas sites of the lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between these charges play a major role in the physics of the system, as does quenched disorder caused by imperfections of the lattice. We identify and describe different regimes of magnetization reversal in an applied magnetic field determined by the orientation of the applied field with respect to the initial magnetization. One of the regimes is characterized by magnetic avalanches with a 1/n distribution of lengths.

In this paper, we report positron lifetime results obtained under high-power steady-state and transient optical excitation. We present a model for analysing the results. The method has been applied to vacancy clusters in natural diamond, for which we self-consistently analyse optoelectronic constants such as optical absorption cross-section and hole recombination cross-section. The temperature dependences of transient and steady-state measurements are studied, suggesting the possibility of analysing the positron trapping to extended defects and vacancy clusters in semiconductors.

We study topological phase transitions in discrete gauge theories in two spatial dimensions induced by the formation of a Bose condensate. We analyse a general class of euclidean lattice actions for these theories which contain one coupling constant for each conjugacy class of the gauge group. To probe the phase structure we use a complete set of open and closed anyonic string operators. The open strings allow one to determine the particle content of the condensate, whereas the closed strings enable us to determine the matrix elements of the modular S-matrix, in both the unbroken and broken phases. From the measured broken S-matrix we may read off the sectors that split or get identified in the broken phase, as well as the sectors that are confined. In this sense the modular S-matrix can be employed as a matrix valued nonlocal order parameter from which the low-energy effective theories that occur in different regions of parameter space can be fully determined. To verify our predictions, we studied a non-abelian anyon model based on the quaternion group $H=\skew3\bar {D_2}$ of the order of eight by Monte Carlo simulation. We probe part of the phase diagram for the pure gauge theory and find a variety of phases with magnetic condensates leading to various forms of (partial) confinement in complete agreement with the algebraic breaking analysis. Also the order of various transitions is established.