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We investigate the influence of boundaries and spatial nonreciprocity on nonequilibrium driven-dissipative phase transitions. We focus on a one-dimensional lattice of nonlinear bosons described by a Lindblad master equation, where the interplay between coherent and incoherent dynamics generates nonreciprocal interactions between sites. Using a mean-field approach, we analyze the phase diagram under both periodic and open boundary conditions. For periodic boundaries, the system always forms a condensate at nonzero momentum and frequency, resulting in a time-dependent traveling wave pattern. In contrast, open boundaries reveal a far richer phase diagram, featuring multiple static and dynamical phases, as well as exotic phase transitions, including the spontaneous breaking of particle-hole symmetry associated with a critical exceptional point and phases with distinct bulk and edge behavior. Our model avoids post-selection or unphysical non-Hermitian Hamiltonians and is experimentally realizable in platforms such as superconducting circuits., Comment: 5+7 pages, 4+5 figures

Non-equilibrium dynamics are present in many aspects of our lives, ranging from microscopic physical systems to the functioning of the brain. What characterizes stochastic models of non-equilibrium processes is the breaking of the fluctuation-dissipation relations as well as the existence of non-static stable states, or phases. A prototypical example is a dynamical phase characterized by a limit cycle - the order parameter of finite magnitude rotating or oscillating at a fixed frequency. Consequently, birhythmicity, where two stable limit cycles coexist, is a natural extension of the simpler single limit cycle phase. Both the abundance of real systems exhibiting such states as well as their relevance for building our understanding of non-equilibrium phases and phase transitions are strong motivations to build and study models of such behavior. Field theoretic tools can be used to provide insights into either phase and the transition between them. In this work we explore a simple linear model of the single limit cycle phase with phase-amplitude coupling. We demonstrate how such non-equilibrium coupling affects the fluctuation spectrum of the theory. We then extend this model to include a continuous transition to a two-cycle phase. We give various results, such as an appearance of a critical exceptional point, the destruction of the transition, enhancement of noise for the phase and the presence of KPZ dynamics. Finally, we qualitatively demonstrate these results with numerics and discuss future directions., Comment: 11 pages, 7 figures

A magnetoresistance (MR) anomaly at low temperatures has been observed in a variety of systems, ranging from low-dimensional chalcogenides to spin and charge density wave (SDW/CDW) metals and, most recently, topological semimetals. In some systems parabolic magnetoresistance can rise to hundreds of thousands of times its low-temperature, zero-field value. While the origin of such a dramatic effect remains unresolved, these systems are often low-carrier-density compensated metals, and the physics is expected to be quasi-classical. Here we demonstrate that this MR anomaly in temperature also exists in high conductivity good metals with large Fermi surfaces, namely Cr, Mo, and W, for both linear and quadratic field-dependent regimes with their non-saturation attributed to open orbit and electron-hole compensation, respectively. We provide evidence that quantum transport across sharp Fermi surface arcs, but not necessarily the full cyclotron orbit, governs this low-temperature MR anomaly. In Cr, extremely sharp curvatures are induced by superposed lattice and SDW band structures. One observes an overlay of the temperature dependence of three phenomena: namely, MR at a constant high field, linear MR in the low-field limit, and Shubnikov-de Haas (SdH) oscillations of the lightest orbit. In Mo, the temperature dependence of low-T MR anomaly extends beyond those of its SdH oscillations but disappears at temperatures where Kohler's scaling reemerges. In the low-temperature and high-field limit, large magnetoresistance from carriers circling quantum orbits is the three-dimensional analogy to the zero-conductance state of carrier localization in the integer quantum Hall effect, especially with regard to the adverse effect of disorder.

Non-reciprocal systems can be thought of as disobeying Newtons third law - an action does not cause an equal and opposite reaction. In recent years there has been a dramatic rise in interest towards such systems. On a fundamental level, they can be a basis of describing non-equilibrium and active states of matter, with applications ranging from physics to social sciences. However, often the first step to understanding complex nonlinear models is to linearize about the steady states. It is thus useful to develop a careful understanding of linear non-reciprocal systems, similar to our understanding of Gaussian systems in equilibrium statistical mechanics. In this work we explore simplest linear non-reciprocal models with noise and spatial extent. We describe their regions of stability and show how non-reciprocity can enhance the stability of a system. We demonstrate the appearance of exceptional and critical exceptional points with the respective enhancement of fluctuations for the latter. We show how strong non-reciprocity can lead to a finite-momentum instability. Finally, we comment how non-reciprocity can be a source of colored, $1/f$ type noise., Comment: 11 pages, 6 figures

Charge transport in solids at low temperature reveals a material's mesoscopic properties and structure. Under a magnetic field, Shubnikov-de Haas (SdH) oscillations inform complex quantum transport phenomena that are not limited by the ground state characteristics. Here, in elemental metal Cr with two incommensurately superposed lattices of ions and a spin-density-wave ground state, we reveal that the phases of several low-frequency SdH oscillations in sigma_xx (rho_xx) and sigma_yy (rho_yy) are opposite, contrast with oscillations from normal cyclotron orbits that maintain identical phases. We trace the origin of the low frequency SdH oscillations to quantum interference effects arising from the incommensurate orbits of Cr's superposed reciprocal lattices, and explain the observed pi-phase shift by the reconnection of anisotropic joint open and closed orbits.

Complex dynamical systems may exhibit multiple steady states, including time-periodic limit cycles, where the final trajectory depends on initial conditions. With tuning of parameters, limit cycles can proliferate or merge at an exceptional point. Here we ask how dynamics in the vicinity of such a bifurcation are influenced by noise. A pitchfork bifurcation can be used to induce bifurcation behavior. We model a limit cycle with the normal form of the Hopf oscillator, couple it to the pitchfork, and investigate the resulting dynamical system in the presence of noise. We show that the generating functional for the averages of the dynamical variables factorizes between the pitchfork and the oscillator. The statistical properties of the pitchfork in the presence of noise in its various regimes are investigated and a scaling theory is developed for the correlation and response functions. The analysis is done by perturbative calculations as well as numerical means. Finally, observables illustrating the coupling of a system with a limit cycle to a pitchfork are discussed and the phase-phase correlations are shown to exhibit non-diffusive behavior with universal scaling., Comment: 10 pages, 8 figures

Previously, we introduced a method for systematically correcting a quasiparticle green's function via a power series expansion. Here we present an ODE based formalisms of power series correction that goes beyond the cumulant approximation and implement it to 1D Holstein chain for a wide range of coupling strengths in a scalable and inexpensive fashion at both zero and finite temperature. We show that this first differential formalism of the power series is both qualitatively and quantitatively in excellent agreement with exact diagonalization results on 1D Holstein chain with dispersive bosons for a large range of electron-boson coupling strength. We investigate carrier mass growth rate and carrier energy displacement across a wide range of coupling strength. Finally, we present a heuristic argument which predicts most of the rich satellite structure without explicit calculation.

We study a class of bifurcations generically occurring in dynamical systems with non-mutual couplings ranging from models of coupled neurons to predator-prey systems and non-linear oscillators. In these bifurcations, extended attractors such as limit cycles, limit tori, and strange attractors merge and split in a similar way as fixed points in a pitchfork bifurcation. We show that this merging and splitting coincides with the coalescence of covariant Lyapunov vectors with vanishing Lyapunov exponents, generalizing the notion of exceptional points to non-linear dynamical systems. We distinguish two classes of bifurcations, corresponding respectively to continuous and discontinuous behaviors of the covariant Lyapunov vectors at the transition. We outline some physical consequences of generalized exceptional points on the dynamics of the system, including non-reciprocal responses, the destruction of isochrons, and enhanced sensitivity to noise. We illustrate our results with concrete examples from neuroscience, ecology, and physics. When applied to interpret existing experimental observations, our analysis suggests a simple explanation for the non-trivial phase delays observed in the population dynamics of plankton communities and the recently measured statistics of rotation reversals for a solid body immersed in a Rayleigh-B\'enard convection cell., Comment: 32 pages, 16 figures

We consider a standard Ginzburg-Landau model of a ferroelectric whose electrical polarization is coupled to gradients of elastic strain. At the harmonic level, such flexoelectric interaction is known to hybridize acoustic and optic phonon modes and lead to phases with modulated lattice structures that precede the symmetry broken state for sufficiently large couplings. Here, we use the we use the self-consistent phonon approximation to calculate the effects of thermal and quantum polarization fluctuations on the bare modes to show that such long-range modulated order is unstable at all temperatures. We discuss the implications for the nearly ferroelectric SrTiO$_3$ and KTaO$_3$, and propose that these systems are melted versions of an underlying modulated state which is dominated by non-zero momentum thermal fluctuations except at the very lowest temperatures., Comment: 17 pages, 3 figures, 1 table, supplementary information included

Inspired by the recent experimental observation of strongly coupled polaritons in a Moir\'e heterobilayer, we study a model of dipole-interacting excitons localized on sites of a lattice and coupled to planar cavity photons. We calculate the phase diagram of this system by computing fluctuations around the mean field and determining the stability of the excitation spectrum. We find that the transition from the normal state to a polariton condensate is intermediated by a series of ordered states at partial fillings of the exciton lattice, stabilized by the exciton interactions. In particular we predict a supersolid phase in which a polariton condensate coexists with spatial order., Comment: 5 pages, 4 figures

Insulating-metallic transition mediated by substitutional atoms is predicted in a series of two-dimensional carbon-based structures. Introducing Si atoms in selected sites of tetrahexcarbon [Carbon 137 (2018) 266] according to rational chemical rules, metallicity by trivial band inversion without band gap opening is induced. Additional substitution of remaining C atoms by BN dimers introduces no changes in the metallic properties. A series of isomorphous two-dimensional materials with isoelectronic structures derived by exchanging group IV elements exhibiting various band gaps is obtained. Dynamical stability is verified with phonon analysis and beyond the harmonic approximation with molecular dynamics up to room temperature. The semi-metallic compounds have well-nested pockets of carriers and are good candidates for the formation of an excitonic insulator., Comment: 7 pages, 5 figures

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. 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 interpretations, 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 $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials., Comment: Published version, substantial changes, many references removed due to journal limits :-( Please see the supporting information online. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format (see conditions)

In the context of a single electron two orbital Holstein system coupled to dispersionless bosons, we develop a general method to correct single particle Green's function using a power series correction(PSC) scheme. We then outline the derivations of various flavors of cumulant approximation through the PSC scheme and explain the assumptions and approximations behind them. Finally, we compute and compare PSC spectral function with cumulant and exact diagonalized spectral functions and elucidate three regimes of this problem - two that cumulant explains and one where cumulant fails. We find that the exact and the PSC spectral functions match within spectral broadening across all three regimes., Comment: 13 pages, 7 figures

Following a resurgence of interest in dilute superconductivity in polar semiconductors, we perform a variational calculation to probe the existence of Fr{\"o}hlich bipolarons in these materials. Our solution is capable of interpolating between the weak- and strong-coupling limits of the electron-phonon interaction. We predict bipolaron formation in solely the strong-coupling regime, and we are not aware of any existing materials at these parameter values. However, imposing an extrinsic electron size constraint to mimic confinement on the sub-micron scale produces binding in new parts of the phase diagram, including at weak coupling, within reach of the near-ferroelectric perovskites., Comment: 12 pages, 9 figures

We analyze a minimal model of SrTiO3, such as has been historically used to analyze the superconducting state. Treating the electron-phonon and electron-electron interactions on an equal footing, we calculate the normal state properties, focusing on the spectral function. We find a density-driven crossover caused mostly by the adiabaticity of the bosonic modes, but the spectral features of our theory are dominated by coupled plasmon-phonon oscillations rather than the bare phonon features seen in experiment., Comment: 5 pages, 4 figures

A detailed exploration of the $f$-atomic orbital occupancy space for UO$_2$ is performed using a first principles approach based on density functional theory (DFT), employing a full hybrid functional within a systematic basis set. Specifically, the PBE0 functional is combined with an occupancy biasing scheme implemented in a wavelet-based algorithm which is adapted to large supercells. The results are compared with previous DFT+U calculations reported in the literature, while dynamical mean field theory (DMFT) is also performed to provide a further base for comparison. This work shows that the computational complexity of the energy landscape of a correlated $f$-electron oxide is much richer than has previously been demonstrated. The resulting calculations provide evidence of the existence of multiple previously unexplored metastable electronic states of UO$_2$, including those with energies which are lower than previously reported ground states.

Out of equilibrium, the lack of reciprocity is the rule rather than the exception. Non-reciprocal interactions occur, for instance, in networks of neurons, directional growth of interfaces, and synthetic active materials. While wave propagation in non-reciprocal media has recently been under intense study, less is known about the consequences of non-reciprocity on the collective behavior of many-body systems. Here, we show that non-reciprocity leads to time-dependent phases where spontaneously broken symmetries are dynamically restored. The resulting phase transitions are controlled by spectral singularities called exceptional points. We describe the emergence of these phases using insights from bifurcation theory and non-Hermitian quantum mechanics. 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 non-reciprocal systems range from active time-(quasi)crystals to exceptional-point enforced pattern-formation and hysteresis. Our work paves the way towards a general theory of critical phenomena in non-reciprocal matter., Comment: Supplementary movies at https://home.uchicago.edu/~vitelli/videos.html

The Higgs amplitude mode is a collective excitation studied and observed in a broad class of matter, including superconductors, charge density waves, antiferromagnets, 3He p-wave superfluid, and ultracold atomic condensates. In all the observations reported thus far, the amplitude mode was excited by perturbing the condensate out of equilibrium. Studying an exciton-polariton condensate, here we report the first observation of this mode purely driven by intrinsic quantum fluctuations without such perturbations. By using an ultrahigh quality microcavity and a Raman spectrometer to maximally reject photoluminescence from the condensate, we observe weak but distinct photoluminescence at energies below the condensate emission. We identify this as the so-called ghost branches of the amplitude mode arising from quantum depletion of the condensate into this mode. These energies, as well as the overall structure of the photoluminescence spectra, are in good agreement with our theoretical analysis.

Critical phenomena arise ubiquitously in various context of physics, from condensed matter, high energy physics, cosmology, to biological systems, and consist of slow and long-distance fluctuations near a phase transition or critical point. Usually, these phenomena are associated with the softening of a massive mode. Here we show that a novel, non-Hermitian-induced mechanism of critical phenomena that do not fall into this class can arise in the steady state of generic driven-dissipative many-body systems with coupled binary order parameters such as exciton-polariton condensates and driven-dissipative Bose-Einstein condensates in a double-well potential. The criticality of this ``critical exceptional point'' is attributed to the coalescence of the collective eigenmodes that convert all the thermal-and-dissipative-noise activated fluctuations to the Goldstone mode, leading to anomalously giant phase fluctuations that diverge at spatial dimensions $d\le 4$. Our dynamic renormalization group analysis shows that this gives rise to a strong-coupling fixed point at dimensions as high as $d<8$ associated with a new universality class beyond the classification by Hohenberg and Halperin, indicating how anomalously strong the many-body corrections are at this point. We find that this anomalous enhancement of many-body correlation is due to the appearance of a sound mode at the critical exceptional point despite the system's dissipative character., Comment: 16 pages, 3 figures including appendices

We report on superconductivity in single crystals of SrTiO$_{3-\delta}$ with carrier densities $\textit{n} < 1.4 \times10^{18}cm^{-3}$, where only a single band is occupied. For all samples in this regime, the resistive transition occurs at $T_{c} \approx 65 \pm 25 \ mK$. We observe a zero resistance state for $\textit{n}$ as low as $1.03 \times10^{17}cm^{-3}$, and a partial resistive transition for $\textit{n} = 3.85 \times10^{16}cm^{-3}$. We observe low critical current densities, relatively high and isotropic upper critical fields, and an absence of diamagnetic screening in these samples. Our findings suggest an inhomogeneous superconducting state, embedded within a homogeneous high-mobility 3-dimensional electron gas. $T_{c}$ does not vary appreciably when $\textit{n}$ changes by more than an order of magnitude, inconsistent with conventional superconductivity., Comment: 5 pages, 4 figures

The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects [1], unusual topological band structures [2], and inhomogeneities that lead to wandering current paths [3, 4] can induce a crossover from quadratic to linear magnetoresistance with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear magnetoresistance. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semi-classical narrative for a broad category of materials with partially-gapped Fermi surfaces due to density waves.

The metal-semiconductor contact is a major factor limiting the shrinking of transistor dimension to further increase device performance. In-plane edge contacts have the potential to achieve lower contact resistance due to stronger orbital hybridization compared to conventional top contacts. However, a quantitative understanding of the electron transport properties in the edge contact is still lacking. In this work, we present full-band atomistic quantum transport simulations of the graphene/MoS$_2$ edge contact. By using a Wannier function basis to accurately describe the electronic bands, together with a full self-consistent solution of the electrostatics, we are able to efficiently model device structures on a micron scale, but with atomic level accuracy. We find that the potential barrier created by trapped charges decays fast with distance away from the interface, and is thus thin enough to enable efficient injection of electrons. This results in Ohmic behavior in its I-V characteristics, which agrees with experiments. Our results demonstrate the role played by trapped charges in the formation of a Schottky barrier, and how one can reduce the Schottky barrier height (SBH) by adjusting the relevant parameters of the edge contact system. Our framework can be extended conveniently to incorporate more general nanostructure geometries. For example, a full 3D solution of the electrostatics will also lead to better modeling of the electrical potential. Furthermore, better ab-initio calculations can be conveniently added to our methods to further improve their accuracy.

We propose a novel mechanism for a nonequilibrium phase transition in a $U(1)$-broken phase of an electron-hole-photon system, from a Bose-Einstein condensate of polaritons to a photon laser, induced by the non-Hermitian nature of the condensate. We show that a (uniform) steady state of the condensate can always be classified into two types, namely, arising either from lower or upper-branch polaritons. We prove (for a general model) and demonstrate (for a particular model of polaritons) that an exceptional point where the two types coalesce marks the endpoint of a first-order-like phase boundary between the two types, similar to a critical point in a liquid-gas phase transition. Since the phase transition found in this paper is not in general triggered by population inversion, our result implies that the second threshold observed in experiments is not necessarily a strong-to-weak-coupling transition, contrary to the widely-believed understanding. Although our calculation mainly aims to clarify polariton physics, our discussion is applicable to general driven-dissipative condensates composed of two complex fields., Comment: Revised version. 6 pages and 4 figures + 13 pages and 6 figures of Supplemental Material

We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present., Comment: 25 pages, 21 figures

Perovskite structured materials contain myriad tunable ordered phases of electronic and magnetic origin with proven technological importance and strong promise for a variety of energy solutions. An always-contributing influence beneath these cooperative and competing interactions is the lattice, whose physics may be obscured in complex perovskites by the many coupled degrees of freedom which makes these systems interesting. Here we report signatures of an approach to a quantum phase transition very near the ground state of the nonmagnetic, ionic insulating, simple cubic perovskite material ScF3 and show that its physical properties are strongly effected as much as 100 K above the putative transition. Spatial and temporal correlations in the high-symmetry cubic phase determined using energy- and momentum-resolved inelastic X-ray scattering as well as X-ray diffraction reveal that soft mode, central peak and thermal expansion phenomena are all strongly influenced by the transition., Comment: 6 pages, 4 figures

The structural phase transitions of MF$_3$ (M=Al, Cr, V, Fe, Ti, Sc) metal trifluorides are studied within a simple Landau theory consisting of tilts of rigid MF$_6$ octahedra associated with soft antiferrodistoritive optic modes that are coupled to long-wavelength strain generating acoustic phonons. We calculate the temperature and pressure dependence of several quantities such as the spontaneous distortions, volume expansion and shear strains as well as $T-P$ phase diagrams. By contrasting our model to experiments we quantify the deviations from mean-field behavior and found that the tilt fluctuations of the MF$_6$ octahedra increase with metal cation size. We apply our model to predict giant barocaloric effects in Sc substituted TiF$_3$ of up to about $15\,$JK$^{-1}$kg$^{-1}$ for modest hydrostatic compressions of $0.2\,$GPa. The effect extends over a wide temperature range of over $140\,$K (including room temperature) due to a large predicted rate $dT_c/dP = 723\,$K GPa$^{-1}$, which exceeds those of typical barocaloric materials. Our results suggest that open lattice frameworks such as the trifluorides are an attractive platform to search for giant barocaloric effects., Comment: 11 pages, 8 figures

Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of LaNiO$_3$ and LaMnO$_3$. For comparison we also study a rocksalt ((111) double perovskite) structure and bulk LaNiO$_3$ and LaMnO$_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition metal d-level energy, which has a contribution from the change in d-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in the superlattice, indicating metallic behavior, in contrast to the insulating behavior found in recent experiments, while the rocksalt structure is found to be insulating with a large Mn-Ni charge transfer. We suggest a high density of cation antisite defects may account for the insulating behavior experimentally observed in short-period superlattices., Comment: 9 pages, 7 figures

Obtaining strong magnetoelectric couplings in bulk materials and heterostructures is an ongoing challenge. We demonstrate that manganite heterostructures of the form ${\rm (Insulator)/(LaMnO_3)_n/(CaMnO_3)_n/(Insulator)}$ show strong multiferroicity in magnetic manganites where ferroelectric polarization is realized by charges leaking from ${\rm LaMnO_3}$ to ${\rm CaMnO_3}$ due to repulsion. Here, an effective nearest-neighbor electron-electron (electron-hole) repulsion (attraction) is generated by cooperative electron-phonon interaction. Double exchange, when a particle virtually hops to its unoccupied neighboring site and back, produces magnetic polarons that polarize antiferromagnetic regions. Thus a striking giant magnetoelectric effect ensues when an external electrical field enhances the electron leakage across the interface., Comment: This version supersedes arXiv:1203.3283

Metal to insulator transitions (MITs) driven by strong electronic correlations are common in condensed matter systems, and are associated with some of the most remarkable collective phenomena in solids, including superconductivity and magnetism. Tuning and control of the transition holds the promise of novel, low power, ultrafast electronics, but the relative roles of doping, chemistry, elastic strain and other applied fields has made systematic understanding difficult to obtain. Here we point out that existing data on tuning of the MIT in perovskite transition metal oxides through ionic size effects provides evidence of systematic and large effects on the phase transition due to dynamical fluctuations of the elastic strain, which have been usually neglected. This is illustrated by 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 cation radius in the well-studied manganite and nickelate materials. Since the elastic couplings are generically quite strong, these conclusions will broadly generalize to all MITs that couple to a change in lattice symmetry., Comment: 14 pages, 4 figures, supplementary information included

We present a stability analysis on a driven-dissipative electron-hole condensate in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein-condensation)-crossover region. Extending the combined BCS-Leggett theory with the generalized random phase approximation (GRPA) to the non-equilibrium case by employing the Keldysh formalism, we show that the pumping-and-decay of carriers causes a depairing effect on excitons. This phenomenon gives rise to an attractive interaction between excitons in the BEC regime, as well as a supercurrent that anomalously flows anti-parallel to $\nabla \theta({\bf r})$ (where $\theta({\bf r})$ is the phase of the condensate) in the BCS regime, both leading to dynamical instabilities of an exciton-BEC. Our result suggests that substantial region of the exciton-BEC phase in the phase diagram (in terms of the interaction strength and the decay rate) is unstable., Comment: 20 pages, 14 figures

First-principles calculations on monolayer 8-{\it Pmmn} borophene are reported to reveal unprecedented electronic properties in a two-dimensional material. Based on a Born effective charge analysis, 8-{\it Pmmn} borophene is the first single-element based monolayered material exhibiting two sublattices with substantial ionic features. The observed Dirac cones are actually formed by the p$_z$ orbitals of one of the inequivalent sublattices composed of uniquely four atoms, yielding an underlying hexagonal network topologically equivalent to distorted graphene. A significant physical outcome of this effect includes the possibility of converting metallic 8-{\it Pmmn} borophene into an indirect band gap semiconductor by means of external shear stress. The stability of the strained structures are supported by a phonon frequency analysis. The Dirac cones are sensitive to the formation of vacancies only in the inequivalent sublattice electronically active at the Fermi level., Comment: 5 pages, 5 figures

We theoretically investigate a Bose-condensed exciton gas out of equilibrium. Within the framework of the combined BCS-Leggett strong-coupling theory with the non-equilibrium Keldysh formalism, we show how the Bose-Einstein condensation (BEC) of excitons is suppressed to eventually disappear, when the system is in the non-equilibrium steady state. The supply of electrons and holes from the bath is shown to induce quasi-particle excitations, leading to the partial occupation of the upper branch of Bogoliubov single-particle excitation spectrum. We also discuss how this quasi-particle induction is related to the suppression of exciton BEC, as well as the stability of the steady state., Comment: 7 pages, 2 figures, Proceedings of QFS-2015

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here we study under what circumstances similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO$_3$/SrTiO$_3$ heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first principles simulations of free standing slabs of PbTiO$_3$. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a non-volatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces., Comment: 19 pages, 10 figures

Correlated band theory is employed to investigate the magnetic and electronic properties of different arrangements of oxygen di- and tri-vacancy clusters in SrTiO$_3$. Hole and electron doping of oxygen deficient SrTiO$_3$ yields various degrees of magnetization as a result of the interaction between localized magnetic moments at the defected sites. Different kinds of Ti atomic orbital hybridization are described as a function of the doping level and defect geometry. We find that magnetism in SrTiO$_{3-\delta}$ is sensitive to the arrangement of neighbouring vacancy sites, charge carrier density, and vacancy-vacancy interaction. Permanent magnetic moments in the absence of vacancy doping electrons are observed. Our description of the charged clusters of oxygen vacancies widens the previous descriptions of mono and multi-vacancies and points out the importance of the controled formation at the atomic level of defects for the realization of transition metal oxide based devices with a desirable magnetic performance., Comment: 6 pages, 5 figures, continuation of arXiv:1408.3103

Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit which provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipole forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization., Comment: 6 pages, 5 figures, supplemental material included

First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient SrTiO$_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices., Comment: 5 pages, 3 figures, Figure 2 amended

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated an enormous activity due to the variety of interesting effects it gives rise to. A case in point is the discovery of the electron gas at the interface between LaAlO3 and SrTiO3, which has since been shown to be quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite these findings, the origin of the two-dimensional electron gas is highly debated and several possible mechanisms remain. Here we review the main proposed mechanisms and attempt to model expected effects in a quantitative way with the ambition of better constraining what effects can/cannot explain the observed phenomenology. We do it in the framework of a phenomenological model for understanding electronic and/or redox screening of the chemical charge in oxide heterostructures. We also discuss the effect of intermixing, both conserving and non-conserving the total stoichiometry., Comment: 34 pages, 9 figures

We study the free energy landscape of a minimal model for relaxor ferroelectrics. Using a variational method which includes leading correlations beyond the mean-field approximation as well as disorder averaging at the level of a simple replica theory, we find metastable paraelectric states with a stability region that extends to zero temperature. The free energy of such states exhibits an essential singularity for weak compositional disorder pointing to their necessary occurrence. Ferroelectric states appear as local minima in the free energy at high temperatures and become stable below a coexistence temperature $T_c$. We calculate the phase diagram in the electric field-temperature plane and find a coexistence line of the polar and non-polar phases which ends at a critical point. First-order phase transitions are induced for fields sufficiently large to cross the region of stability of the metastable paraelectric phase. These polar and non-polar states have distinct structure factors from those of conventional ferroelectrics. We use this theoretical framework to compare and to gain physical understanding of various experimental results in typical relaxors., Comment: 10 pages, 7 figures

Using a Ginzburg--Landau--Devonshire model that includes the coupling of polarization to strain, we calculate the fluctuation spectra of ferroelectric domain walls. The influence of the strain coupling differs between 180 degree and 90 degree walls due to the different strain profiles of the two configurations. The finite speed of acoustic phonons, $v_s$, retards the response of the strain to polarization fluctuations, and the results depend on $v_s$. For $v_s \to \infty$, the strain mediates an instantaneous electrostrictive interaction, which is long-range in the 90 degree wall case. For finite $v_s$, acoustic phonons damp the wall excitations, producing a continuum in the spectral function. As $v_s\ to 0$, a gapped mode emerges, which corresponds to the polarization oscillating in a fixed strain potential.

Polaritons, mixed light-matter quasiparticles, undergo a transition to a condensed, macroscopically coherent state at low temperatures or high densities. Recent experiments show that coupling light to organic molecules inside a microcavity allows condensation at room temperature. The molecules act as saturable absorbers with transitions dressed by molecular vibrational modes. Motivated by this we calculate the phase diagram and spectrum of a modified Tavis-Cummings model, describing vibrationally dressed two-level systems, coupled to a cavity mode. Coupling to vibrational modes can induce re-entrance, i.e. a normal-condensed-normal sequence with decreasing temperature and can drive the transition first order., Comment: 6 pages, 3 figures

In this paper we study an effective model for the normal state of iron-based superconductors. It has separate, but interacting itinerant and localized degrees of freedom, originating from the d_xz and d_yz, and from d_xy iron orbitals respectively. At low temperatures, below a mean-field phase transition, these different states condense together in an excitonic order parameter. We show that at even lower temperature, after another phase transition, this ordered state can spontaneously break the C_4 lattice symmetry and become nematic. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds., Comment: 5 pages, 3 figures, new references added, replaced Fig. 1, fixed several typos

We study zigzag interfaces between insulating compounds that are isostructural to graphene, specifically II-VI, III-V and IV-IV two-dimensional (2D) honeycomb insulators. We show that these one-dimensional interfaces are polar, with a net density of excess charge that can be simply determined by using the ideal (integer) formal valence charges, regardless of the predominant covalent character of the bonding in these materials. We justify this finding on fundamental physical grounds, by analyzing the topology of the formal polarization lattice in the parent bulk materials. First principles calculations elucidate an electronic compensation mechanism not dissimilar to oxide interfaces, which is triggered by a Zener-like charge transfer between interfaces of opposite polarity. In particular, we predict the emergence of one dimensional electron and hole gases (1DEG), which in some cases are ferromagnetic half-metallic., Comment: 5 pages, 3 figures

The elastic response of suspended NbSe3 nanowires is studied across the charge density wave phase transition. The nanoscale dimensions of the resonator lead to a large resonant frequency (10-100 MHz), bringing the excited phonon frequency in close proximity of the plasmon mode of the electronic condensate - a parameter window not accessible in bulk systems. The interaction between the phonon and plasmon modes strongly modifies the elastic properties at high frequencies. This is manifested in the nanomechanics of the system as a sharp peak in the temperature dependence of the elastic modulus (relative change of 12.8%) in the charge density wave phase., Comment: 6 pages, 3 figures

We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation between NdMnO3 and LaMnO3, and obtain the relations between distortions. We find good agreement between theoretical results and experimental data., Comment: 6 figures

We present a new methodology to solve the Anderson impurity model, in the context of dynamical mean-field theory, based on the exact diagonalization method. We propose a strategy to effectively refine the exact diagonalization solver by combining a finite-temperature Lanczos algorithm with an adapted version of the cluster perturbation theory. We show that the augmented diagonalization yields an improved accuracy in the description of the spectral function of the single-band Hubbard model and is a reliable approach for a full d-orbital manifold calculation.

Solid state quantum condensates can differ from other condensates, such as Helium, ultracold atomic gases, and superconductors, in that the condensing quasiparticles have relatively short lifetimes, and so, as for lasers, external pumping is required to maintain a steady state. In this chapter we present a non-equilibrium path integral approach to condensation in a dissipative environment and apply it to microcavity polaritons, driven out of equilibrium by coupling to multiple baths, describing pumping and decay. Using this, we discuss the relation between non-equilibrium polariton condensation, lasing, and equilibrium condensation., Comment: 14 pages, Unedited version of chapter to appear in Quantum Gases: Finite Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms Series). N.P. Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds. Imperial College Press, London (in press)

We propose a mechanism for binding of diatomic ligands to heme based on a dynamical orbital selection process. This scenario may be described as bonding determined by local valence fluctuations. We support this model using linear-scaling first-principles calculations, in combination with dynamical mean-field theory, applied to heme, the kernel of the hemoglobin metalloprotein central to human respiration. We find that variations in Hund's exchange coupling induce a reduction of the iron 3d density, with a concomitant increase of valence fluctuations. We discuss the comparison between our computed optical absorption spectra and experimental data, our picture accounting for the observation of optical transitions in the infrared regime, and how the Hund's coupling reduces, by a factor of five, the strong imbalance in the binding energies of heme with CO and O_2 ligands., Comment: 5 pages, 4 figures. Supplementary material 12 pages, 5 figures. This version (v2) matches that accepted for Physical Review Letters on 31 January 2013

Atoms, molecules or excitonic quasiparticles, for which excitations are induced by external radiation fields and energy is dissipated through radiative decay, are examples of driven open quantum systems. We explain the use of commutator-free exponential time-propagators for the numerical solution of the associated Schr\"odinger or master equations with a time-dependent Hamilton operator. These time-propagators are based on the Magnus series but avoid the computation of commutators, which makes them suitable for the efficient propagation of systems with a large number of degrees of freedom. We present an optimized fourth order propagator and demonstrate its efficiency in comparison to the direct Runge-Kutta computation. As an illustrative example we consider the parametrically driven dissipative Dicke model, for which we calculate the periodic steady state and the optical emission spectrum., Comment: 23 pages, 11 figures