We calculate the electronic structure of the narrow gap semiconductors PbTe, SnTe, and GeTe in the cubic phase using density functional theory (DFT) and the $G_0{W}_0$ method. Within DFT, we show that the band ordering obtained with a conventional semilocal exchange-correlation approximation is correct for SnTe and GeTe but wrong for PbTe. The correct band ordering at the high-symmetry point $L$ is recovered adding $G_0{W}_0$ quasiparticle corrections. However, one-shot $G_0{W}_0$ produces artifacts in the band structure due to the wrong orbital character of the DFT single-particle states at the band edges close to $L$. We show that in order to correct these artifacts it is enough to consider the off-diagonal elements of the $G_0{W}_0$ self-energy corresponding to these states. We also investigate the pressure dependence of the band gap for these materials and the possibility of a transition from a trivial to a nontrivial topology of the band structure. For PbTe, we predict the band crossover and topological transition to occur at around 4.8 GPa. For GeTe, we estimate the topological transition to occur at 1.9 GPa in the constrained cubic phase, a pressure lower than that of the structural phase transition from rombohedral to cubic. SnTe is a crystalline topological insulator at ambient pressure, and the transition into a trivial topology would take place under a volume expansion of approximately $10\%$.

We study the effect of the boundary on a system of weakly interacting bosons in one dimension. It strongly influences the boson density, which is completely suppressed at the boundary position. Away from it, the density is depleted over the distances on the order of the healing length at the mean-field level. Quantum fluctuations modify the density profile considerably. The local density approaches the average one as an inverse square of the distance from the boundary. We calculate an analytic expression for the density profile at arbitrary separations from the boundary. We then consider the problem of localization of a foreign quantum particle (impurity) in the potential created by the inhomogeneous boson density. At the mean-field level, we find exact results for the energy spectrum of the bound states, the corresponding wave functions, and the condition for interaction-induced localization. The quantum contribution to the boson density gives rise to small corrections of the bound-state energy levels. However, it is fundamentally important for the existence of a long-range Casimir-like interaction between the impurity and the boundary.

In the quest for realizations of quantum spin liquids, the exploration of Kitaev materials (spin-orbit-entangled Mott insulators with strong bond-directional exchanges) has taken center stage. However, in these materials the local spin-orbital $j=1/2$ moments typically show long-range magnetic order at low temperature, thus defying the formation of a spin-liquid ground state. It is an intriguing question how far the properties of the Kitaev spin liquid survive in these materials despite the presence of long-range magnetic order. Using resonant inelastic x-ray scattering (RIXS), we here report on a proximate spin-liquid regime with distinctive fingerprints of Kitaev physics in the magnetic excitations of the honeycomb iridates $\alpha \text{−}{\mathrm{Li}}_2\mathrm{Ir}{\mathrm{O}}_3$ and ${\mathrm{Na}}_2\mathrm{Ir}{\mathrm{O}}_3$. We observe a broad continuum of magnetic excitations and prove its magnetic character via the resonance behavior. The continuum is insensitive to temperature up to about 90 K and persists up to at least 300 K, more than an order of magnitude larger than the magnetic ordering temperatures. This striking temperature dependence of the RIXS intensity agrees with the behavior of the nearest-neighbor spin-spin correlations of the Kitaev model. This comparison is motivated by momentum-dependent RIXS measurements of the dynamical structure factor for energies within the continuum which show that dynamical spin-spin correlations are restricted to nearest neighbors, which is a key property of the Kitaev model. Notably, these spectroscopic observations are also present in the magnetically ordered state for excitation energies above the conventional magnon excitations. Phenomenologically, our data agree with inelastic neutron scattering results on the related honeycomb compound $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$, establishing a common ground for a proximate Kitaev spin-liquid regime in these materials.

We develop an analytical framework to study the influence of a weakly intercoupled in-plane spherical metal nanoparticle (MNP) assembly on a coherently illuminated quantum emitter (QE). We reduce the analytical expressions derived for the aforementioned generic planar setup into simple and concise expressions representing a QE mediated by a symmetric MNP constellation, by exploiting the symmetry. We use the recently introduced generalized nonlocal optical response (GNOR) theory that has successfully explained plasmonic experiments to model the MNPs in our system. Due to the use of GNOR theory, and our analytical approach, the procedure we suggest is extremely computationally efficient. Using the derived model, we analyze the absorption rate, resultant Rabi frequency, effective excitonic energy shift, and dephasing rate shift spectra of an exciton bearing QE at the center of a symmetric MNP setup. We observe that the QE experiences plasmon-induced absorption rate spectral linewidth variations that increase in magnitude with decreasing MNP-QE center separation and increasing number of MNPs. Our results also suggest that parameter regions where the QE exhibits trends of decreasing linewidth against decreasing MNP-QE center separation are likely to be associated with plasmon-induced excitonic energy redshifts. Similarly, regions where the QE absorption rate linewidth tends to increase against decreasing MNP-QE center separation are likely to be accompanied by plasmon-induced excitonic energy blueshifts. In both these cases, the magnitude of the observed redshift and blueshift was seen to increase with the number of MNPs in the constellation, due to enhancement of the plasmonic influence. We show that even when the magnitude of the QE absorption rate spectrum is much smaller compared to the isolated (collective) MNP spectra, it is sufficient to dramatically modify the spectrum of the MNP-QE nanohybrid, causing sharp Fano-type interference patterns.

Recent interest in topological nature in condensed matter physics has revealed the essential role of Berry curvature in the anomalous Hall effect (AHE). However, since a large Hall response originating from Berry curvature has been reported in quite limited materials, the detailed mechanism remains unclear at present. Here, we report the discovery of a large AHE triggered by a pressure-induced magnetic phase transition in elemental $\alpha$-Mn. The AHE is absent in the noncollinear antiferromagnetic phase at ambient pressure, whereas a large AHE is observed in the weak ferromagnetic phase under high pressure despite the small magnetization of $\approx 0.02{\mu }_B$/Mn. Our results indicate that the emergence of the AHE in $\alpha$-Mn is governed by the symmetry of the underlying magnetic structure, providing a direct evidence of a switch between a zero and nonzero contribution of the Berry curvature across the phase boundary. $\alpha$-Mn can be an elemental and tunable platform to reveal the role of Berry curvature in AHE.

A global symmetry (0-symmetry) in an $n$-dimensional space acts on the whole space. A higher symmetry acts on closed submanifolds (i.e., loops and membranes, etc.), and those transformations form a higher group. In this paper, we introduce the notion of algebraic higher symmetry, which generalizes higher symmetry and is beyond higher group. We show that an algebraic higher symmetry in a bosonic system in $n$-dimensional space is characterized and classified by a local fusion $n$-category. We find that, when restricted to its symmetric sub-Hilbert space, an algebraic higher symmetry can be fully characterized by a noninvertible gravitational anomaly (i.e., a topological order in one higher dimension). Thus, we also refer to a noninvertible gravitational anomaly as categorical symmetry to stress its connection to symmetry. This provides a holographic and entanglement view of symmetries. For a system with a categorical symmetry, its gapped state must spontaneously break part (not all) of the symmetry, and the state with the full symmetry must be gapless. Using such a holographic point of view, we obtain (1) the gauging of the algebraic higher symmetry; (2) the classification of anomalies for an algebraic higher symmetry; (3) the equivalence between classes of systems, with different (potentially anomalous) algebraic higher symmetries or different sets of low-energy excitations, as long as they have the same categorical symmetry; and (4) the classification of gapped liquid phases for bosonic and/or fermionic systems with a categorical symmetry, as gapped boundaries of a topological order in one higher dimension (that corresponds to the categorical symmetry). This classification includes symmetry protected trivial (SPT) orders and symmetry enriched topological (SET) orders with an algebraic higher symmetry.

In density functional–theoretic studies of photoionized water-based systems, the role of charge localization in proton-transfer dynamics is not well understood. This is due to the inherent complexity in extracting the contributions of coupled electron-nuclear nonadiabatic dynamics in the presence of exchange and correlation functional errors. In this work we address this problem by simulating a model system of ionized linear H-bonded water clusters using real-time time-dependent density functional theory–based Ehrenfest dynamics. Our aim is to understand how self-interaction error in semilocal exchange and correlation functionals affects the probability of proton transfer. In particular, we show that the proton-transfer probability is largely underestimated for short H-bonded chains but becomes comparable to that predicted by hybrid functionals for ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n^{+}$ chains with $n>3$. This is because the formation of hemibonded-type geometries is largely suppressed in extended H-bonded structures. We also show how the degree of localization of the initial photo-hole is connected to the probability of a proton-transfer reaction, as well as to the hole–proton separation. These results are compared to those obtained with adiabatic dynamics where the initial wave function is allowed to relax to the ground state of the ion cluster, explaining why different functionals and dynamical approaches lead to quantitatively different results.

Violation of parity symmetry gives rise to various physical phenomena such as nonlinear transport and cross-correlated responses. In particular, the nonlinear conductivity has been attracting a lot of attentions in spin-orbit coupled semiconductors, superconductors, topological materials, and so on. In this paper we present a theoretical study of the nonlinear conductivity in odd-parity magnetic multipole ordered systems whose $\mathcal{PT}$ symmetry is essentially distinct from the previously studied acentric systems. Combining microscopic formulation and symmetry analysis, we classify the nonlinear responses in the $\mathcal{PT}$-symmetric systems as well as $\mathcal{T}$-symmetric (nonmagnetic) systems, and uncover nonlinear conductivity unique to the odd-parity magnetic multipole systems. A giant nonlinear Hall effect, nematicity-assisted dichroism, and magnetically induced Berry curvature dipole effect are proposed and demonstrated in a model for Mn-based magnets.

Ultrafast laser irradiation of silicon can significantly modify charge densities and optical indices, impacting the formation and the development of nanoscale-arranged periodic structures. Photoexcitation degree as well as thermodynamic, hydrodynamic, and structural aspects are reported for crossed orientation of laser-induced periodic surface structures generated on single-crystal silicon after multiple-pulse femtosecond laser irradiation. The periodic topography and microstructure generated by light scattering on surface nanoroughness were characterized to gain insights into the regime of photoexcitation, subsequent thermodynamic conditions, and inhomogeneous energy deposition related to periodic nanostructure formation and growth. A generated free-carrier density around $(3\pm 2)\times {10}^{21}{\mathrm{cm}}^{-3}$ is estimated from time-resolved ellipsometry and supported by time-dependent density-functional theory calculations. A finite-difference time-domain solution of the far-field interference of the surface scattered light and the refracted laser wave confirms the periodically crossed energy deposition for this excitation degree. The interference process does not necessarily involve surface plasmon polaritons, and quasicylindrical evanescent waves are identified as plausible scattered waves requiring less restrictive conditions of photoexcitation. Ab initio calculations are also performed to evaluate the transient state of silicon under strong electron-phonon nonequilibrium upon fs laser irradiation. For the reached excitation degree, an electron temperature up to 8000 K is deduced, supporting local amorphization processes observed as a result of high mechanical stress and quenching rates. Ab initio combined with electromagnetic calculations agree well with the results of topography and structural characterization.

We present a theory of a hybrid quantum liquid state, the quantum spin-dipole liquid (QSDL), in a hydrogen-bonded electron system, by combining a quantum proton ice and Anderson's resonating valence bond spin liquid theory, motivated by the recent experimental discovery of a quantum spin liquid with proton fluctuations in $\kappa \text{−}{\mathrm{H}}_3{\left(\mathrm{Cat}\text{−}\mathrm{E}\mathrm{D}\mathrm{T}\text{−}\mathrm{T}\mathrm{T}\mathrm{F}\right)}_2$ (a.k.a. H-Cat). In our theory, an electron spin liquid and a proton dipole liquid are realized simultaneously in the ground state called quantum valence bond ice. In this state, neither of them can be established independently of the other. Analytical and numerical calculations reveal that this state has a large entanglement entropy between spins and dipoles, which is far beyond the (crude) Born-Oppenheimer approximation. We also examine the stability of QSDL with respect to perturbations and discuss implications for experiments in H-Cat and its deuterated analog D-Cat.

The resilience of quantum entanglement to a classicality-inducing environment is tied to fundamental aspects of quantum many-body systems. The dynamics of entanglement has recently been studied in the context of measurement-induced entanglement transitions, where the steady-state entanglement collapses from a volume law to an area law at a critical measurement probability $p_c$. Interestingly, there is a distinction in the value of $p_c$ depending on how well the underlying unitary dynamics scramble quantum information. For strongly chaotic systems, $p_c>0$, whereas for weakly chaotic systems, such as integrable models, $p_c=0$. In this work, we investigate these measurement-induced entanglement transitions in a system where the underlying unitary dynamics are many-body localized (MBL). We demonstrate that the emergent integrability in an MBL system implies a qualitative difference in the nature of the measurement-induced transition depending on the measurement basis, with $p_c>0$ when the measurement basis is scrambled and $p_c=0$ when it is not. This feature is not found in Haar-random circuit models, where all local operators are scrambled in time. When the transition occurs at $p_c>0$, we use finite-size scaling to obtain the critical exponent $\nu =1.3\left(2\right)$, close to the value for (2+0)-dimensional percolation. We also find a dynamical critical exponent of $z=0.98\left(4\right)$ and logarithmic scaling of the Rényi entropies at criticality, suggesting an underlying conformal symmetry at the critical point. This work further demonstrates how the nature of the measurement-induced entanglement transition depends on the scrambling nature of the underlying unitary dynamics. This leads to further questions on the control and simulation of entangled quantum states by measurements in open quantum systems.

We investigate spinon band topology and engineering from the interplay between long-ranged magnetic order and fractionalized spinons, as well as Zeeman coupling under external magnetic fields, in honeycomb lattice magnets. The synergism of Néel order and magnetic fields could reconstruct the spinon bands and drive a topological phase transition from the coexisting phase of long-ranged order and chiral spin liquid with semion topological order to the conventional magnetic order. Our prediction can be immediately tested through thermal Hall transport measurements among the honeycomb lattice magnets that are tuned to be proximate to the quantum critical point. Our theory should also shed light on the critical behavior of honeycomb Kitaev materials with emergent Majorana fermion bands. We suggest a possible relevance to the spin-1/2 honeycomb spin liquid candidate material ${\mathrm{In}}_3{\mathrm{Cu}}_2{\mathrm{VO}}_9$.

A class of photocurrents is predicted to occur in both type-I and type-II Weyl semimetals. Unlike the previously studied photocurrents in chiral materials, the proposed current requires neither circularly polarized light nor an absence of symmetry with respect to a plane of reflection. We show that if a Weyl semimetal has a broken inversion symmetry then linearly polarized light can induce a photocurrent transverse to the direction of an applied magnetic field, in spite of the symmetry with respect to a reflection plane and the time reversal symmetry. The class of materials in which we expect this to occur is sufficiently broad and includes the transition metal monopnictides such as TaAs. The effect stems from the dynamics of Weyl chiral quasiparticles in a magnetic field, restricted by the symmetries described above; because the resulting current is transverse to the direction of magnetic field, we call it the transverse chiral magnetic photocurrent. The magnitude of the resulting photocurrent is predicted to be significant in the THz frequency range, about $0.75\mu \mathrm{A}$ for type-I and $2.5\mu \mathrm{A}$ for type-II Weyl semimetals. This opens the possibility to utilize the predicted transverse chiral magnetic photocurrent for sensing unpolarized THz radiation.

In recent theoretical and experimental investigations, researchers have linked the low-energy field theory of a Weyl semimetal gapped with a charge-density wave (CDW) to high-energy theories with axion electrodynamics. However, it remains an open question whether a lattice regularization of the dynamical Weyl-CDW is in fact a single-particle axion insulator (AXI). In this Rapid Communication, we use analytic and numerical methods to study both lattice-commensurate and incommensurate minimal (magnetic) Weyl-CDW phases in the mean-field state. We observe that, as previously predicted from field theory, the two inversion ($\mathcal{I}$)-symmetric Weyl-CDWs with $\varphi =0,\pi$ differ by a topological axion angle $\delta {\theta }_{\varphi }=\pi$. However, we crucially discover that neither of the minimal Weyl-CDW phases at $\varphi =0,\pi$ is individually an AXI; they are instead quantum anomalous Hall (QAH) and “obstructed” QAH insulators that differ by a fractional translation in the modulated cell, analogous to the two phases of the Su-Schrieffer-Heeger model of polyacetylene. Using symmetry indicators of band topology and non-Abelian Berry phase, we demonstrate that our results generalize to multiband systems with only two Weyl fermions, establishing that minimal Weyl-CDWs unavoidably carry nontrivial Chern numbers that prevent the observation of a static magnetoelectric response. We discuss the experimental implications of our findings and provide models and analysis generalizing our results to nonmagnetic Weyl- and Dirac-CDWs.

In solid-state physics, the Grüneisen parameter (GP) was first introduced to study the effect of volume change of a crystal lattice on its vibrational frequencies and has since been widely used to investigate the characteristic energy scales of systems associated with the changes of external potentials. However, the GP is less investigated in gas systems and especially strongly interacting quantum gases. Here we report on some general results on the origin of the GP, an identity, and caloric effects in ultracold quantum gases. We prove that there exists a simple identity among three different types of GPs, quantifying the caloric effect induced by variations of volume, magnetic field, and interaction, respectively. Using exact Bethe ansatz solutions, we present a rigorous study of these different GPs and the quantum refrigeration in one-dimensional Bose and Fermi gases. Based on the exact equations of states of these systems, we further obtain analytic results for singular behavior of the GPs and the caloric effects at quantum criticality. We also predict the existence of the lowest temperature for cooling near a quantum phase transition. It turns out that the interaction ramp up and down in quantum gases provide a promising protocol of quantum refrigeration in addition to the usual adiabatic demagnetization cooling in solid-state materials.

The honeycomb compound $\alpha \text{−}{\mathrm{RuCl}}_3$ is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled $j$ = 1/2 moments arising for a $t_{2g}^5$ electron configuration with strong spin-orbit coupling $\lambda$ and a large cubic crystal field. The actual low-energy electronic structure of $\alpha \text{−}{\mathrm{RuCl}}_3$, however, is still puzzling. In particular, infrared absorption features at 0.30, 0.53, and 0.75 eV seem to be at odds with a $j$ = 1/2 scenario. Also the energy of the spin-orbit exciton, the excitation from $j$ = 1/2 to 3/2, and thus the value of $\lambda$, are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find $\lambda$ = 0.16 eV and $\mathrm{\Delta }$ = 42(4) meV for the observed noncubic crystal-field splitting, supporting the validity of the $j$ = $1/2$ picture for $\alpha \text{−}{\mathrm{RuCl}}_3$. The unusual strength of the double excitation is related to the underlying hopping interactions, which form the basis for dominant Kitaev exchange.

We propose emulation of Hawking radiation (HR) by means of acoustic excitations propagating on top of persistent current in an atomic Bose-Einstein condensate (BEC) loaded in an annular confining potential. The setting is initially created as a spatially uniform one, and then switches into a nonuniform configuration, while maintaining uniform BEC density. The eventual setting admits the realization of sonic black and white event horizons with different slopes of the local sound speed. A smooth slope near the white-hole horizon suppresses instabilities in the supersonic region. It is found that tongue-shaped patterns of the density-density correlation function, which represent the acoustic analog of HR, are strongly affected by the radius of the ring-shaped configuration and number of discrete acoustic modes admitted by it. There is a minimum radius that enables the emulation of HR. We also briefly discuss a possible similarity of properties of the matter-wave sonic black holes to the known puzzle of the stability of Planck-scale primordial black holes in quantum gravity.

We develop a comprehensive quantitative theory for magnetoelectricity in magnetically ordered quasi-two-dimensional (quasi-2D) systems whereby in thermal equilibrium an electric field can induce a magnetization and a magnetic field can induce an electric polarization. This effect requires that both space-inversion and time-reversal symmetry are broken. Antiferromagnetic order plays a central role in this theory. We define a Néel operator $\mathit{\tau }$ such that a nonzero expectation value $\langle \mathit{\tau }\rangle$ signals collinear antiferromagnetic order in the same way a magnetization signals ferromagnetic order. While a magnetization is even under space inversion and odd under time reversal, the operator $\mathit{\tau }$ describes a toroidal moment that is odd both under space inversion and under time reversal. Thus the magnetization and the toroidal moment $\langle \mathit{\tau }\rangle$ quantify complementary aspects of collinear magnetic order in solids. Focusing on quasi-2D systems, itinerant-electron ferromagnetic order can be attributed to dipolar equilibrium currents that give rise to a magnetization. In the same way, antiferromagnetic order arises from quadrupolar equilibrium currents that generate the toroidal moment $\langle \mathit{\tau }\rangle$. In the magnetoelectric effect, the electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar equilibrium currents. We develop a $\mathrm{k}·\mathrm{p}$ envelope-function theory for the antiferromagnetic diamond structure that allows us to derive explicit expressions for the Néel operator $\mathit{\tau }$. Considering ferromagnetic zincblende structures and antiferromagnetic diamond structures, we derive quantitative expressions for the magnetoelectric responses due to electric and magnetic fields that reveal explicitly the inherent duality of these responses required by thermodynamics. Magnetoelectricity is found to be small in realistic calculations for quasi-2D electron systems. The magnetoelectric response of quasi-2D hole systems turns out to be sizable, however, with moderate electric fields being able to induce a magnetic moment of one Bohr magneton per charge carrier. Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.

In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at $T^{*}$, which is tens of degrees Kelvin higher than the structural transition temperature $T_S$. In this “hidden” nematic state at $T_S<T<T^{*}$, the orthorhombicity is tiny $[\varphi =(a-b)/(a+b)\ll 0.1\%]$, but clear evidence of a bulk phase transition has been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the AF wave vector $\mathit{q}=(0,\pi )$ at $T=T^{*}$, by which the characteristic phenomena below $T^{*}$ are satisfactorily explained. This AFB order originates from the interorbital nesting between the $d_{xy}$-orbital hole pocket and the electron pocket, and this interorbital bond order naturally explains the pseudogap, band folding, and tiny nematicity that is linear in $T^{*}-T$. The hidden AFB order explains key experiments in both ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and NaFeAs, but it is not expected to occur in FeSe because of the absence of the $d_{xy}$-orbital hole pocket.

Van der Waals semiconductor heterostructures could be a platform to harness hot photoexcited carriers in the next generation of optoelectronic and photovoltaic devices. The internal quantum efficiency of hot-carrier devices is determined by the relation between photocarrier extraction and thermalization rates. Using ab initio methods we show that the photocarrier thermalization time in single-layer transition metal dichalcogenides strongly depends on the peculiarities of the phonon spectrum and the electronic spin-orbit coupling. In detail, the lifted spin degeneracy in the valence band suppresses the hole scattering on acoustic phonons, slowing down the thermalization of holes by one order of magnitude as compared with electrons. Moreover, the hole thermalization time behaves differently in $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{Se}}_2$ because spin-orbit interactions differ in these seemingly similar materials. We predict that the internal quantum efficiency of a tunneling van der Waals semiconductor heterostructure depends qualitatively on whether $\mathrm{Mo}{\mathrm{S}}_2$ or $\mathrm{W}{\mathrm{Se}}_2$ is used.