Intervalley collisions, which scatter electrons from one valley or band to another, can be detrimental to thermoelectric performance in materials with multiple valleys/bands. In this study, density functional theory is used to investigate the electron-phonon scattering characteristics of three lead chalcogenides (PbS, PbSe, PbTe) and three half-Heuslers (ScNiBi, ScPdSb, ZrNiSn), which all possess multiple equivalent conduction valleys, in order to characterize and analyze their intravalley/intervalley components. To elucidate what controls the degree of intravalley and intervalley transitions, the scattering rates are decomposed into the product of the phase space (a measure of how much scattering is possible) and the average electron-phonon coupling. To help guide the search for improved thermoelectric and high-conductivity materials, simple and approximate approaches are demonstrated that can be adopted to identify materials with reduced intervalley scattering, which circumvent the need for computationally demanding electron-phonon scattering calculations. In addition, the benefits of selecting materials with large-energy zone-edge phonons are explored in the limit $\hslash \omega$ $\gg$ $k_{B}T$, and found to potentially suppress intervalley processes by up to an order of magnitude, leading to a 70% and 100% increase in conductivity and power factor, respectively.

Carbon biphenylene has stimulated substantial research because of extraordinary properties introduced by the metallic character, e.g., the ultrahigh electron thermal transport. Here, inspired by the synthesis of carbon biphenylene [Fan et al., Science 372, 852 (2021)], we identify the stability of ${\mathrm{C}}_x{\left(\mathrm{BN}\right)}_{1\text{−}\mathrm{x}}$ biphenylene as $\text{CBN}$ and ${\mathrm{C}}_4\mathrm{BN}$ semiconductors with four-, six-, and eight-membered periodic rings of irregularly $s{p}^2$-hybridized atoms via structural searches. Unexpectedly, we confirm that $\text{CBN}$ biphenylene exhibits a peculiar funnel-shaped band structure, which is a direct consequence of the delocalization/localization of $\pi$ bonds formed by B-, N-, or C-$p_z$ electrons. The band structure greatly improves the thermoelectric performance by enhancing the power factor, although the lattice thermal conductivity is relatively large after including four-phonon scattering resistance because of low atomic masses. The similar behaviors are absent in ${\mathrm{C}}_4\mathrm{BN}$ biphenylene because of the localization of $\pi$ bonds formed by C-$p_z$ electrons, although with a stronger anharmonicity and thus a lower lattice thermal conductivity. The anomalous power factor can be explained by the constant $\tau$ approximation: the $p$-type doping controls the carrier group velocities and thus realizes the tunability of tensor ratio $K_1/K_0$. Our analysis suggests that the funnel-shaped electronic structure could be reproduced in two-dimensional semiconductor systems with the small electronegativity difference and the comparable stoichiometry. Our work realizes the thermoelectric improvement through controlling the shape of band structure, which provides insights for designing promising two-dimensional thermoelectric materials.

Ferrovalley materials with spontaneous valley polarization are crucial to valleytronic application. Based on first-principles calculations, we demonstrate that two-dimensional (2D) $\mathrm{Y}{X}_2(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$ in a 2H structure constitutes a series of promising ferrovalley semiconductors with large spontaneous valley polarization and high magnetic transition temperature. Our calculations reveal that $\mathrm{Y}{X}_2$ are dynamically, energetically, thermally and mechanically stable 2D ferromagnetic semiconductors with a magnetic transition temperature about 200 K. Due to the natural noncentrosymmetric structure, intrinsic ferromagnetic order and strong spin orbital coupling, the large spontaneous valley polarizations of 108.98, 57.70, and 22.35 meV are also predicted in single-layer $\mathrm{Y}{X}_2(X=\mathrm{I}, \mathrm{Br}, \text{and} \mathrm{Cl})$, respectively. The anomalous valley Hall effect is also proposed based on the valley contrasting Berry curvature. Moreover, the ferromagnetism and valley polarization are found to be effectively tuning by applying a biaxial strain. Interestingly, the suppressed valley physics of ${\mathrm{YBr}}_2$ and ${\mathrm{YCl}}_2$ can be switched on via applying a moderate compression strain. The present findings promise $\mathrm{Y}{X}_2$ as competitive candidates for the further experimental studies and practical applications in valleytronics.

We explore theoretically how the coupling to cavity vacuum fields affects the electron transport in quantum conductors due to the counter-rotating-wave terms of light-matter interaction. We determine the quantum conductance in terms of the transmission coefficients predicted by an effective electron Hamiltonian. The coupling between bare electronic states is mediated by virtual processes involving intermediate states with one electron (or one hole) on top of the Fermi sea and one virtual cavity photon. We study the behavior of the quantum conductance in the presence of artificial or disordered single-particle potentials, as well as a spatially varying cavity mode. As illustrative examples, we apply our theory to one-dimensional conductors and to disordered two-dimensional quantum Hall systems. We show how the cavity vacuum fields can lead to both large enhancement or suppression of electron conductance in the ballistic regime, as well as modification of the conductance quantization and fluctuations.

Bernal bilayer graphene exhibits a band gap that is tunable through the infrared with an electric field. We show that sublattice odd commensurate twisted bilayer graphene (C-TBG) exhibits a band gap that is tunable through the terahertz with an electric field. We show that from the perspective of terahertz optics the sublattice odd and even forms of C-TBG are “inflated” versions of Bernal and AA-stacked bilayer graphene, respectively, with energy scales reduced by a factor of 110 for the $21.{79}^{\circ }$ commensurate unit cell. This lower energy scale is accompanied by a correspondingly smaller gate voltage, which means that the strong-field regime is more easily accessible than in the Bernal case. Finally, we show that the interlayer coherence energy is a directly accessible experimental quantity through the position of a power-law divergence in the optical conductivity.

We present pressure $\left(P\right)$ and temperature $\left(T\right)$ variations of elastic and anisotropic properties, solid-solid (rocksalt, B1 to cesium chloride, B2) and solid-liquid structural phase transitions of CaO. We employed first-principles density functional theory supplemented with an anharmonic contribution to phonon dynamics. Good agreement is obtained for all properties up to the pressure and temperature range relevant to the Earth's mantle and outer core. Born elastic criteria are generalized for arbitrary stress to elaborate the structural phase diagram from a thermoelastic viewpoint. We propose a self-consistent computational scheme to incorporate the effect of thermal hysteresis into Lindemann's melting law. This improvised melting law exhibits quantitative agreement with reported findings for the high-$P$ melting curve. We find the triple point (i.e., the coexistence of B1, B2, and liquid phases) at 23 GPa and 4600 K. By examining the pressure-term included elastic properties, we can show that the solid-solid transition is mechanical in a pressure range of 0–200 GPa and temperature up to 3000 K. The softening of shear elastic constant $C_{44}$ drives the B1-B2 phase transition. This assertion is corroborated by examining the phonon dispersion curve and mode Grüneisen parameter at different pressures for both B1 and B2 phases. The solid-liquid phase boundary can be treated accurately through the temperature-dependent thermodynamic Grüneisen parameter. Furthermore, in this paper, we predict a negative melting slope $>140\mathrm{GPa}$ with a peak temperature of 7800 K, suggesting a smaller molar volume on the liquid side than that of the solid phase. This finding is supported by electronic band structure calculation. It is proposed that the hybridization of the empty $3d$ band of the cation with $s$ orbitals lowers the conduction band to cross the Fermi energy at the Brillouin zone center and leads the insulator-to-metal/semimetal phase transition $\sim 200\mathrm{GPa}$. Different electronic states on solid and liquid sides make the liquid phase more compressible than the solid phase, eventually reducing the melting temperature with pressure.

The process by which open quantum systems thermalize with an environment is both of fundamental interest and relevant to noisy quantum devices. As a minimal model of this process, we consider a qudit chain evolving under local random unitaries and local depolarization channels. After mapping to a statistical mechanics model, the depolarization (noise) acts like a symmetry-breaking field, and we argue that it causes the system to thermalize within a timescale independent of system size. We show that various bipartite entanglement measures—mutual information, operator entanglement, and entanglement negativity—grow at a speed proportional to the size of the bipartition boundary. As a result, these entanglement measures obey an area law: Their maximal value during the dynamics is bounded by the boundary instead of the volume. In contrast, if the depolarization only acts at the system boundary, then the maximum value of the entanglement measures obeys a volume law. We complement our analysis with scalable simulations involving Clifford gates, for both one- and two-dimensional systems.

We study a ($1+1$)-dimensional quantum circuit consisting of Haar-random unitary gates and projective measurements, both of which conserve a total $U\left(1\right)$ charge and thus have $U\left(1\right)$ symmetry. In addition to a measurement-induced entanglement transition between a volume-law and an area-law entangled phase, we find a phase transition between two phases characterized by bipartite charge fluctuation growing with the subsystem size or staying constant. At this charge-fluctuation transition, steady-state quantities obtained by evolving an initial state with a definitive total charge exhibit critical scaling behaviors akin to Tomonaga-Luttinger-liquid theory for equilibrium critical quantum systems with $U\left(1\right)$ symmetry, such as logarithmic scaling of bipartite charge fluctuation, power-law decay of charge correlation functions, and logarithmic scaling of charge-resolved entanglement whose coefficient becomes a universal quadratic function in a flux parameter. These critical features, however, do not persist below the transition, in contrast to a recent prediction based on replica field theory and mapping to a classical statistical mechanical model.

Topological spin textures, such as a skyrmion crystal, are a source of unusual physical phenomena owing to the interplay between magnetism and topology. Since physical phenomena depend on the topological property and the symmetry of underlying spin structures, the search for new topological spin textures and emergent phenomena is one of the challenges in condensed matter physics. In this study, we theoretically explore topological spin textures arising from the synergy between spin, charge, and sublattice degrees of freedom in an itinerant magnet. By performing simulated annealing for an effective spin model of the honeycomb Kondo lattice model, we find a plethora of skyrmion crystal instabilities at low temperatures, whose topological spin textures are classified into three types: ferrochiral, antiferrochiral, and ferrichiral skyrmion crystals. We show that the obtained skyrmion crystals are the consequence of the spin-orbit-coupling-free honeycomb structure. Our results reveal the potential for itinerant honeycomb magnets to host a wide variety of skyrmion crystal and emergent phenomena.

In a recent paper [Phys. Rev. Lett. 129, 120601 (2022)], we have shown that the dynamics of interfaces, in the symmetry-broken phase of the two-dimensional ferromagnetic quantum Ising model, displays a robust form of ergodicity breaking. In this paper, we elaborate more on the issue. First, we discuss two classes of initial states on the square lattice, the dynamics of which is driven by complementary terms in the effective Hamiltonian and may be solved exactly: (a) Strips of consecutive neighboring spins aligned in the opposite direction of the surrounding spins and (b) a large class of initial states, characterized by the presence of a well-defined “smooth” interface separating two infinitely extended regions with oppositely aligned spins. The evolution of the latter states can be mapped onto that of an effective one-dimensional fermionic chain, which is integrable in the infinite-coupling limit. In this case, deep connections with noteworthy results in mathematics emerge, as well as with similar problems in classical statistical physics. We present a detailed analysis of the evolution of these interfaces both on the lattice and in a suitable continuum limit, including the interface fluctuations and the dynamics of entanglement entropy. Second, we provide analytical and numerical evidence supporting the conclusion that the observed nonergodicity—arising from Stark localization of the effective fermionic excitations—persists away from the infinite-Ising-coupling limit, and we highlight the presence of a timescale $T\sim e^{cLlnL}$ for the decay of a region of large linear size $L$. The implications of our work for the classic problem of the decay of a false vacuum are also discussed.

Kuzkin's angular momentum balance method is implemented in the LAMMPS SPIN package for atomistic spin-lattice dynamics, along with shifted-force exchange and Néel Hamiltonians parameterized to minimize energy drifts in the simulations. Angular momentum contributions arising from two mechanisms are quantified using this method: particle transport across the boundaries of a periodic simulation domain and external torques applied to the domain by periodic image atoms. When these mechanisms are accounted for, lattice angular momentum is exactly conserved in lattice systems and in spin-lattice systems with isotropic exchange interactions. The calculations show that spin-lattice angular momentum exchange only occurs when the Néel anisotropy energy is added to the exchange energy, and that with this addition, total angular momentum is approximately conserved in the magnetization direction but not in other directions. Inclusion of the Néel anisotropy increases the energy drifts observed in simulations of iron nanoparticles. These drifts are linearly proportional to the magnitude of the anisotropy energy and the simulation time step.

We present the results of high magnetic field experiments in pure ${}^3\mathrm{He}$ (in the absence of ${}^4\mathrm{He}$ coverage) in nematic aerogel. In this case the aerogel strands are covered with few atomic layers of solid paramagnetic ${}^3\mathrm{He}$, which enables the spin-exchange mechanism for ${}^3\mathrm{He}$ quasiparticles scattering. Our earlier NMR experiments showed that in low fields, instead of the polar phase, the $A$ phase is expected to emerge in nematic aerogel. We use a vibrating wire resonator with the sample of aerogel attached to it and measure temperature dependencies of resonance properties of the resonator at different magnetic fields. A superfluid transition temperature of ${}^3\mathrm{He}$ in aerogel, obtained from the experiments, increases nonlinearly in applied magnetic field. And this increase is suppressed compared with that for the bulk $A_1$ phase, which we attribute to an influence of the magnetic scattering channel, previously considered theoretically for the case of ${}^3\mathrm{He}$ confined in isotropic silica aerogel. However, we observe the essential quantitative mismatch with theoretical expectations.

Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors (SCs) and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome this difficulty, here, we introduce the form factor that generalizes the nonlocal order parameter into the Luttinger-Ward (LW) Fermi liquid theory. We then construct a rigorous formalism of the density-wave equation that gives the thermodynamically stable form factor, like the SC gap equation. In addition, a rigorous expression of the Ginzburg-Landau free energy for the unconventional order is presented to calculate various thermodynamic properties. In the next stage, we apply the derived formalism to a typical Fe-based SC FeSe, by using the one-loop LW function that represents the free-energy gain due to the interference among paramagnons. The following key experiments are naturally explained: (i) Lifshitz transition (=disappearance of an electron pocket) due to the bond $+$ orbital order below $T_{\mathrm{c}}$; (ii) Curie-Weiss (CW) behavior of the nematic susceptibility at higher $T$, and the deviation from the CW behavior at lower $T$ near the nematic quantum critical point; and (iii) scaling relation of the specific heat jump at $T_{\mathrm{c}}, \mathrm{\Delta }C/T_{\mathrm{c}}\propto T_{\mathrm{c}}^b$ with $b\sim 3$. (Note that $b=0$ in the Bardeen-Cooper-Schrieffer theory.) These results lead to a conclusion that the nematicity in FeSe is the bond $+$ orbital order due to the paramagnon interference mechanism. The present theory paves the way for solving various unconventional phase transition systems.

In addition to the ferromagnetic materials with inversion symmetry breaking, the symmetric antiferromagnetic materials also exhibit intrinsic valley splitting due to the spin-valley coupling. Using first-principles calculations, we investigated the manipulation of valley splitting of antiferromagnetic monolayer ${\mathrm{MnPTe}}_3$ via biaxial strain. It is shown that two ${\mathrm{MnPTe}}_3$ monolayers with different structures are both stable antiferromagnetic semiconductors, and exhibit valley splitting between $K$ and $K^{\prime }$. The spin-valley coupling strength can be greatly tuned by in-plane strain, which is due to the changes of orbital composition of electronic state and the different contribution of two sublattice atoms. The proportions of $d$ orbitals of Mn atoms determine the orbital angular momentum of the electronic state, and the different contribution of different sublattices results in the change of Berry curvature at $K$ and $K^{\prime }$ points. The combination of two factors leads to the same changes of valley splitting and the proportion of the $d_{xz}$, $d_{yz}$ orbitals. These results of are some significance for the design of materials with greater valley splitting and the understanding of its mechanism.

Topological heavy-fermion systems in three dimensions are usually classified as topological insulators or semimetals. Here, we theoretically predict a different type of heavy-fermion system (dubbed exceptional heavy-fermion semimetal) by studying a three-dimensional periodic Anderson model consisting of strongly correlated localized $f$ electrons and itinerant conduction $c$ electrons in a zinc-blende lattice. Due to the breaking of inversion symmetry, the quasiparticle lifetimes at different sublattices are distinct, leading to the emergence of Weyl exceptional rings in the complex pole of the Green's function at finite temperatures; such rings lead to the appearance of bounded Fermi surfaces (bulk Fermi disks). As temperatures rise, two pairs of Weyl exceptional rings merge into two exceptional rings with one bounded bulk Fermi surface (bulk Fermi tube), which are experimentally measurable by angle-resolved photoemission spectroscopy. Finally, we use the dynamical mean-field theory to calculate the spectral functions which illustrate the emergence of bulk Fermi tubes. Our work thus opens the door for studying exceptional heavy-fermion semimetal phases in three dimensions.

Two-dimensional (2D) materials demonstrate fascinating thickness-dependent optical properties due to their van der Waals interaction and quantum confinement in the linear optical regime. However, the thickness-dependent nonlinear optical response is more complicated as virtual carriers (dipoles) and real carriers can be generated by below- and above-band-gap excitation, which calls for a more comprehensive understanding of these carriers in 2D materials. Herein, a direct band gap ${\mathrm{Bi}}_2{\mathrm{S}}_3$ is utilized to investigate the thickness-dependent nonlinear optical response by a terahertz (THz) emission spectroscopy. The results suggest that the THz emission intensity decreases with the increase of band gap energy under an 800-nm femtosecond laser excitation, which can be described by an empirical exponential equation. Under below-band-gap excitation, the optical rectification effect induced by instantaneous polarization (virtual carriers) dominates the THz emission mechanism in thin ${\mathrm{Bi}}_2{\mathrm{S}}_3$ films. In contrast, under above-band-gap excitation, real carriers are controlled by both resonant optical rectification and surface depletion electric field effects, resulting in shift and drift currents in thick ${\mathrm{Bi}}_2{\mathrm{S}}_3$ films. The contribution ratio of the shift and drift currents is $\sim 1:1$ at ${45}^{\circ }$ oblique incidence. This competition between shift and drift currents results in the elliptically polarized THz wave generation, and the major axis orientation and ellipticity of the ellipse could be manipulated by changing the polarization of the pump light. In this paper, we imply the potential for designing tunable on-chip THz sources and nonlinear optoelectronic devices based on thickness-dependent 2D materials.

By solving the Bogoliubov–de Gennes Hamiltonian, the electron-hole coherence within a partially proximitized $n$-doped semiconductor shell of a core-shell nanowire heterostructure is investigated numerically and compared with the Andreev reflection interpretation of proximity induced superconductivity. Partial proximitization is considered to quantify the effects of a reduced coherence length. Three cases of partial proximitization of the shell are explored: radial, angular, and longitudinal. For the radial case, it is found that the boundary conditions impose localization probability maxima in the center of the shell in spite of off-center radial proximitization. The induced superconductivity gap is calculated as a function of the ratio between the proximitized shell thickness and the total shell thickness. In the angular case, the lowest-energy state of a hexagonal wire with a single proximitized side is found to display the essence of Andreev reflection, only by lengthwise summation of the localization probability. In the longitudinal case, a clear correspondence with Andreev reflection is seen in the localization probability as a function of length along a half-proximitized wire. The effect of an external magnetic field oriented along the wire is explored.

In this work we investigate non-Hermitian topological phase transitions using real-space edge states as a paradigmatic tool. We focus on the simplest non-Hermitian variant of the Su-Schrieffer-Heeger model, including a parameter that denotes the degree of non-Hermiticity of the system. We study the behavior of the zero-energy edge states in the nontrivial topological phases with integer and semi-integer topological winding numbers, according to the distance to the critical point. We find that, depending on the parameters of the model, the edge states may penetrate into the bulk, as expected in Hermitian topological phase transitions. We also show that, using the topological characterization of the exceptional points, we can describe the intricate chiral behavior of the edge states across the whole phase diagram. Moreover, we characterize the criticality of the model by determining the correlation length critical exponent directly from numerical calculations of the penetration length of the zero-mode edge states.

To coherently control a few-level quantum emitter, typically pulses with an energy resonant to the transition energy are applied, making use of the Rabi mechanism, while a single off-resonant pulse does not result in a population inversion. Surprisingly, a two-color excitation with a combination of two off-resonant pulses is able to invert the system. In this paper, we provide an in-depth analysis of two-color excitation schemes within the dressed-state picture. We show that such schemes can be understood as a driving of the transition between the dressed states. In the two-level system this allows us to derive analytic expressions for the pulse parameters yielding a population inversion. We relate our findings to the Swing-UP of quantum EmitteR population (SUPER) mechanism, where the two pulses are red detuned. Our interpretation also holds in the case where one pulse is red detuned while the other pulse is blue detuned, as in dichromatic excitation schemes. We extend our considerations of the SUPER mechanism to the three-level system, where a strong mixing between the bare states takes place. The insights gained from these results help in finding accessible regimes for further experimental realizations of two-color excitation schemes.

We present a theoretical study of the quantum states of two repelling spinless particles in a one-dimensional tight-binding model with a simple periodic lattice and open boundary conditions. We demonstrate that, when the particles are not identical, their interaction drives nontrivial correlated two-particle states, such as bound states and edge states, and induces interaction-induced flat bands. We show that the localization of the center of mass of the two particles enforces the localization of their relative motion, which means formation of the bound states. While the considered system is Hermitian, an insight into the bound states is provided by an approximate effective non-Hermitian model for the relative motion that features the non-Hermitian skin effect.