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Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered1,2. In GeTe and related IV–VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications1. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations3–5, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the <100>cdirection. We show that this anisotropy naturally emerges from a Ginzburg–Landau model that couples polarization fluctuations through long-range elastic interactions6. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes1,7–9and show that spontaneous anisotropy is ubiquitous in cubic IV–VI materials.

Out of equilibrium, a lack of reciprocity is the rule rather than the exception. Non-reciprocity occurs, for instance, in active matter1–6, non-equilibrium systems7–9, networks of neurons10,11, social groups with conformist and contrarian members12, directional interface growth phenomena13–15and metamaterials16–20. Although wave propagation in non-reciprocal media has recently been closely studied1,16–20, less is known about the consequences of non-reciprocity on the collective behaviour of many-body systems. Here we show that non-reciprocity leads to time-dependent phases in which spontaneously broken continuous symmetries are dynamically restored. We illustrate this mechanism with simple robotic demonstrations. The resulting phase transitions are controlled by spectral singularities called exceptional points21. We describe the emergence of these phases using insights from bifurcation theory22,23and non-Hermitian quantum mechanics24,25. Our approach captures non-reciprocal generalizations of three archetypal classes of self-organization out of equilibrium: synchronization, flocking and pattern formation. Collective phenomena in these systems range from active time-(quasi)crystals to exceptional-point-enforced pattern formation and hysteresis. Our work lays the foundation for a general theory of critical phenomena in systems whose dynamics is not governed by an optimization principle.

Abstract

Metal-to-insulator transitions1driven by strong electronic correlations occur frequently in condensed matter systems, and are associated with remarkable collective phenomena in solids, including superconductivity and magnetism. Tuning and control of the transition holds the promise of low-power, ultrafast electronics2, but the relative roles of doping, chemistry, elastic strain and other applied fields have made systematic understanding of such transitions difficult. Here we show that existing data3–5on the tuning of metal-to-insulator transitions in perovskite transition-metal oxides through ionic size effects provides evidence of large systematic effects on the phase transition owing to dynamical fluctuations of the elastic strain, which have usually been neglected6. We illustrate this using a simple yet quantitative statistical mechanical calculation in a model that incorporates cooperative lattice distortions coupled to the electronic degrees of freedom. We reproduce the observed dependence of the transition temperature on the cation radius in the well studied manganite7and nickelate8materials. Because elastic couplings are generally strong, we anticipate that these conclusions will generalize to all metal-to-insulator transitions that couple to a change in lattice symmetry.

We present a theoretical study of a new type of two-dimensional material exhibiting a pentagonal arrangement of C and Si atoms. Pentagonal SiC2is investigated with density functional theory-based calculations to show that the buckled nanostructure is dynamically stable, and exhibits an indirect energy band gap and an enhanced electronic dispersion with respect to the all-carbon counterpart. Computed Born effective charges exhibit a significant anisotropy for C and Si atoms that deviates substantially from their static effective charges. We establish an accurate tunability of the vertical location of the p-p-σ and p-p-π bands and show that under compressive biaxial strain the density of states decreases, and conversely for tensile biaxial strain. This coupling between the tunability of strain-mediated density of states and semiconducting properties in a monolayered structure may allow for the development of applications in semiconducting stretchable electronics.

We present the theory of the dynamics of a two-dimensional Wigner crystal in a high magnetic field, assuming charge-density-wave correlations extend over a distance large compared to the interelectronic spacing. We consider the effects of both weak disorder and screening by uncondensed carriers. In an electric field large compared to the threshold field for sliding, a perturbative theory is used to calculate the nonlinear conductivity and correlation functions of the elastic strain. We compare our results to the behavior of conventional charge-density-wave systems, and stress the role of the large magnetic field, soft shear modulus, and screening by the background linear conductivity.

Local electrochemical phenomena on the surfaces of the LaAlO3–SrTiO3heterostructure are explored using unipolar and bipolar dynamic electrochemical strain microscopy (D-ESM). The D-ESM suggests the presence of at least two distinct electrochemical processes, including fast reversible low-voltage process and slow high-voltage process. The latter process is associated with static surface deformations in the sub-nanometer regime. These behaviors are compared with Kelvin probe force microscopy hysteresis data. The possible origins of observed phenomena are discussed, and these studies suggest that charge-writing behavior in LAO–STO includes a strong surface/bulk electrochemical component and is more complicated than simple screening by surface adsorbates.

We present transmission electron microscopy observations of domain wall motion in thin freestanding KNbO 3 crystals under applied electric fields. Since there is no substrate, there is no elastic clamping of 90 domains. We observe that curved and tilted 90 domain walls are the most mobile, whereas untilted 90 domain walls are resistant to field-induced motion. We explain this result in terms of two factors. First, the switching pressure on a domain wall ( P 2 m P 1 ) E is determined by the relative electrostatic energies of the neighboring polarizations P 1 and P 2 . Consequently, some 90 domain walls are immobile under certain field directions, leading to domain interlocking. Second, domain walls experiencing a high switching pressure move by a ripple mechanism, and do not move as rigid sheets. The tilted wall region in such a ripple has a polarization charge, and an associated depolarization field, which reduces the local switching barrier. An accumulation of polarization charge can result in a tilted or curved wall, as occurs at the mobile tips of 90 domain needles. Any increase in density of immobile wall configurations with cycle time represents an inherent contribution to fatigue. Uniaxial ferroelectrics, with polarizations parallel to the field, should not experience such domain interlocking.

We calculate the optical conductivity in an s-wave superconductor interacting with a low density of magnetic impurities. Interactions are treated beyond the Born approximation so that an impurity band develops inside the superconducting gap. We find that the contribution of these states to the low frequency conductivity is large, and that they have a clear spectroscopic signature which would would make them observable in conventional superconductors.

We compare Fermi-liquid and marginal-Fermi-liquid prescriptions for the spin susceptibility of an itinerant model of quasiparticles relevant to the CuO2 superconductors. Features in the band stucture lead to peaks in the magnetic structure factor at incommensurate wavevectors. The frequency and temperature dependence of these peaks is controlled by quasiparticle lifetimes. We discuss the distinction between these models and a model of magnetic fluctuations with a long correlation length.

A computer program for processing gas-chromatographic data is described in which tailed peaks consisting of the convolution of an exponential decay and a Gaussian peak are fitted by iterative non-linear regression analysis to chromatograms of peaks which may overlap considerably. It is considered that in ordinary analytical practice many peaks approximate closely to the convoluted shape. In experimental evaluation it is shown that peaks from packed columns can be fitted very closely, e.g., to 1 part in 1000, by the convoluted shape and that all peaks have at least a small exponential decay component. The program enables area analyses of sets of overlapping peaks to be made more accurately than when pure Gaussian peaks are fitted, and it can give accurate analyses even when the time constant of the exponential component greatly exceeds the standard deviation of the Gaussian component, so that peaks are greatly tailed. The convoluted shape fits chromatograms from capillary columns better than a Gaussian shape, but the fit is not so precise as with peaks from packed columns.

Gas from the column joins a secondary stream of gas carrying electrolytically generated hydrogen. The combined stream passes through a reactor of hydrogenation catalyst to an electrochemical cell which is sensitive to hydrogen but not to products of hydrogenation. The output of the cell is applied to an amplifier which controls the supply of electricity to the hydrogen generator so as to form part of a feedback loop. In the steady state in the absence of vapor, the amplifier is adjusted so that there is a steady small generation of hydrogen, thus giving at the cell a concentration of hydrogen held steady by the servomechanism. When a vapor appears, there is a momentary fall in the concentration of hydrogen at the cell due to reaction in the reactor, and this is compensated by the servomechanism, which increases the hydrogen generation. As the peak is eluted, the servomechanism causes extra hydrogen to be generated which is exactly equivalent to the vapor according to the hydrogenation reaction. Thus, the peak in hydrogen generation gives an absolute measure of the quantity of the vapor. The use of the apparatus for measurement of oxygen and alkenes is described. A modification is its use for the physico-chemical study of dehydrogenation reactions and catalysts.

Excellent photovoltaic performance is predicted in a pentagonal covalent network of Si in a hollow structure exhibiting both thermal and dynamical stability. Consisting of a combination of sp2and sp3hybridized Si atomic orbitals, the GW0 computed band structure shows an indirect band gap near the zone edge and also a manifold of directly absorbing transitions at frequencies in the window of visible light, in distinction with conventional Si. Hydrogenation of a single sp2site is predicted to lead to a robust local magnetic moment. We find a low formation energy at low pressure that is compatible with other experimentally known phases, suggesting that a stable phase might be obtained.

Ferroelectrics are now being used as non-volatile memories, infrared detectors, phased array radar and optical switches. Although the development of commercial ferroelectric devices has advanced in recent years, we still do not have a clear understanding of the basic physics underpinning the behavior of actual ferroelectric devices. Trapped charge and oxygen vacancies are believed to strongly influence domain motion. In order to study these issues, we have designed an in situTEM holder that can subject ferroelectric crystals to voltage, heat and UV irradiation. Preliminary results on BaTiO3have been presented in previous reports.In this study, bulk KNbO3crystals were mechanically polished and thinned in hot orthophosphoric acid. The resulting thin flakes were attached to copper rings with conductive carbon paint, and electrical wires were glued to the copper rings which act as electrodes. The sample was placed in the cradle of the in situTEM holder and examined in a Hitachi H9000NAR TEM.

IN a recent communication, Ray1has given the equations: and which he then combines and discusses. In these equations, the left-hand side of (1) is proportional to the sensitivity of a catharometer ; λ1and λ2are the thermal conductivities of carrier gas and vapour respectively; λ12is the thermal conductivity of the mixture of the carrier gas with a fraction (assumed here to be molar) x2of vapour. Authors discussing this communication2all appear to assume that equation 2 is sufficiently accurate to enable them to use it in discussion of experimental results. This assumption is not correct.

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Charge density waves pinned by disorder have many possible metastable configurations. The existence of metastable states give rise to generic glasslike behavior including hysteresis and power law time decays of the polarization. The characteristic behavior is controlled by high-energy metastable states which are marginally stable to perturbations. One consequence is a dynamic memory effect, whereby the system can be trained to remember the length of a voltage pulse.

Two stationary liquids, one a pure substance and one simulated by making the equivalent mixture with the same type and proportion of polar groups by appropriate mixing of a pair of liquids, give the same specific retention volumes for solutes independently of solute type.

An informal symposium of the Gas Chromatography Discussion Group of the Institute of Petroleums Hydrocarbon Research Group was held at the Chelsea College of Science and Technology, London, England, on Friday, April 5th, 1963.

Against expectations, robust switchable ferroelectricity has been recently observed in ultrathin (1 nm) ferroelectric films exposed to air [V. Garcia et al., Nature (London) 460, 81 (2009)]. Based on first-principles calculations, we show that the system does not polarize unless charged defects or adsorbates form at the surface. We propose electrochemical processes as the most likely origin of this charge. The ferroelectric polarization of the film adapts to the external ionic charge generated on its surface by redox processes when poling the film. This, in turn, alters the band alignment at the bottom electrode interface, explaining the observed tunneling electroresistance. Our conclusions are supported by energetics calculated for varied electrochemical scenarios. [ABSTRACT FROM AUTHOR]