Two-dimensional electron gas (2DEG) bilayers provide suitable platforms for electronic phases and transitions that are fundamental to both theoretical physics and practical applications in device technology. In bilayer systems, the additional pseudospin, representing the layer degree of freedom, enables the emergence of interlayer coherence, which is a direct consequence of the interlayer Coulomb interaction. This study presents a comprehensive Hartree-Fock (HF) mean-field investigation of the interlayer coherence in 2D bilayers, uncovering ground-state behaviors and temperature-dependent phase transitions that are distinct from single-layer 2DEG. This interlayer coherence signals a spontaneous breaking of the U(1) symmetry in layer pseudospin. We explore the zero-temperature phase diagrams as a function of the electron density and interlayer separation within the HF formalism. We also calculate the critical temperature ($T_c$) of the interlayer coherence onset by self-consistently solving the HF gaplike equation. We contrast this interlayer coherent phase in electron-electron (e-e) bilayers with the closely related excitonic superfluid phase in electron-hole (e-h) bilayers. Although both e-e and e-h bilayers spontaneously break the pseudospin U(1) symmetry, e-h bilayers produce Bardeen-Cooper-Schrieffer–Bose-Einstein condensates crossover intrinsic to the excitons acting as effective bosons or Cooper pairs, whereas the symmetry-broken phase in e-e bilayers is akin to the XY or easy-plane pseudospin ferromagnetism. Using the same system parameters and a similar theoretical framework, we find that $T_c$ of the interlayer coherent phase in e-e bilayers is about one-third of that in exciton condensates, suggesting a weaker interlayer coherence in e-e bilayers. In addition, we examine the effect of a weak interlayer tunneling on the interlayer coherence order parameter, drawing parallels with the influence of an effective in-plane magnetic field on the XY pseudospin ferromagnetism. Our findings provide a comparative theoretical framework that bridges the gap between the interlayer coherence physics in e-e and e-h bilayers, contributing to a unified understanding of phase transitions in low-dimensional electron-hole systems and establishing in particular the same universality class for interlayer phase coherence in both e-e and e-h bilayers.

Recent experiments performed nonreciprocal magnetotransport studies in ${\mathrm{ZrTe}}_5$ and obtained a giant magnetochiral anisotropy (MCA) coefficient ${\gamma }^{\prime }$. The existing theoretical analysis was based on the semiclassical Boltzmann equation. In this paper, we develop a full quantum theory to calculate ${\gamma }^{\prime }$ and further explore the underlying physics. We reveal that the $xz$-mirror symmetry-breaking term also breaks the parity symmetry of the system and leads to mixed selection rules and nonvanishing second-order conductivity ${\sigma }_{xxx}$. The calculations show that ${\gamma }^{\prime }$ decreases with the magnetic field, survives only to the weak impurity scatterings, and exhibits a nonmonotonous dependence on the strength of the $xz$-mirror symmetry breaking. In this paper, we provide deeper insights into the intrinsic nonreciprocal magnetotransport phenomena in the topological semimetal material.

Transition metal dichalcogenides (TMDCs) combine both strong light-matter interaction and strong Coulomb interaction for the formation of optically excitable excitons. Through radiative feedback control, a mechanism to control the linewidth can be applied, which modifies optical transition spectra. Here, we extend these investigations to the absorption spectra of TMDCs in a variety of geometries with respect to non-Markovian exciton-phonon-scattering contributions. Our approach is based on the self-consistent solution of the microscopic Bloch equations and the macroscopic solution of the wave equation. We discuss the formation of a phonon sideband for ${\mathrm{MoSe}}_2$ embedded in ${\mathrm{SiO}}_2$, and two setups for enhancing or suppressing the phonon sideband in the spectrum.

This article aims to study the linear and nonlinear optical response of inversion symmetric graphene quantum dots (GQDs) in the presence of on-site disorder or vacancies. The presence of disorder or vacancy breaks the special inversion symmetry, leading to the emergence of intense Hall-type anomalous harmonics. This phenomenon is attributed to the intrinsic time-reversal symmetry breaking in quantum dots with a pseudo-relativistic Hamiltonian, even in the absence of an external magnetic field. We demonstrate that the effects induced by disorder or vacancy have a distinct impact on the optical response of GQDs. In the linear response we observe significant Hall conductivity. In the presence of an intense laser field, we observe the radiation of strong anomalous odd- and even-order harmonics already for relatively small levels of disorder or monovacancy. Both the disorder and vacancy lift the degeneracy of states, thereby creating new channels for interband transitions and enhancing the emission of near-cutoff high-harmonic signals.

Directed transport is a key concept for many ongoing applications including nanoscale heat management, current rectification, source protection, and energy harvesting. Within the context of quantum transport, we here explore the use of nonlinear effects introduced by current-induced forces (CIFs) as a practical way to effectively break charge and heat transport reciprocities. In particular, we consider a simple model consisting of a mobile quantum dot (QD) coupled to two leads, where the charge (or heat) current develops an asymmetric behavior under inversion of voltage (or temperature) bias, thereby turning the system into a quantum diode (or quantum thermal diode). Furthermore, we find multiple stable positions for the QD and we show how the extraction of useful work is possible by modulating the nonequilibrium sources along well-established hysteresis loops. Finally, we explore a particular case where the nonlinearity of the CIFs can be exploited to pump heat or charge, even for systems that preserve inversion symmetry. This counterintuitive result is attributed to a spontaneous breaking of the inversion symmetry due to the intrinsic system's dynamics.

It is known that intrinsic currents in magnetic metals often appear in the direction perpendicular to the external field for linear and nonlinear responses. Distinct from three kinds of known nonlinear currents, namely, the Drude contribution, the Berry curvature dipole induced current and the Berry connection polarization induced current, here we report a intrinsic nonlinear current with breaking time-reversal symmetry. This new kind of intrinsic nonlinear current from the nontrivial Berry connection polarizability may emerge in the longitudinal or transverse direction, and both are dissipative Ohmic currents. We unveil 66 magnetic point group symmetries that can accommodate such nonlinear current, and possible candidate materials are proposed. This theory is also applied to observe the nonlinear current we proposed in one- and two-dimensional Dirac systems as examples.

In recent decade, structural superlubricity has been established as one of the most effective methods to achieve extremely low friction when two crystalline surfaces slide over each other in dry incommensurate contact, which however may be blocked to commensurate configurations during the sliding and thus lead the failure of superlubricity. Here, our first-principles calculations predict negative differential static friction coefficient in the commensurate contact of bilayer two-dimensional (2D) electride (such as ${\mathrm{Ca}}_2\mathrm{N}$, ${\mathrm{Sr}}_2\mathrm{N}$, and ${\mathrm{Y}}_2\mathrm{C}$), which was essentially sustained by the concept of electronic lubricity, where the lubricity was dominated by the electronic structures, rather than the structural effect. Specifically, it is demonstrated that, in the range of 0–10 GPa, the pressure-enhanced charge transfer from the vicinity of surface Ca and interfacial Ca atoms to the uniformly distributed interstitial anionic electron regime collectively screens the corrugation of the sliding potential energy surface (PES) and thus leads to negative differential friction coefficient μ. However, beyond 10 GPa, the accumulated interstitial anionic electrons become significantly localized when sliding to the saddle point of the PES, thus leading to much enhanced electronic kinetic energy, leading to positive μ. The present findings on electronic lubricity are expected to play an instrumental role in developing high-performance solid lubricants.

Clocks are inherently out-of-equilibrium because, due to friction, they constantly consume free energy to keep track of time. The thermodynamic uncertainty relation (TUR) quantifies the trade-off between the precision of any time-antisymmetric observable and entropy production. In the context of clocks, the TUR implies that a minimum entropy production is needed in order to achieve a certain level of precision in timekeeping. But the TUR has only been proven for overdamped systems. Recently, a toy model of a classical underdamped pendulum clock was proposed that violated this relation [Phys. Rev. Lett. 128, 130606 (2022)], thus demonstrating that the TUR does not hold for underdamped dynamics. We propose an electronic implementation of such a clock, using a resistor-inductor-capacitor circuit and a biased CMOS inverter (not gate), which can work at different scales. We find that in the nanoscopic single-electron regime of the circuit, we essentially recover the toy model violating the TUR bound. However, in different macroscopic regimes of the circuit, we show that the TUR bound is restored and analyze the thermodynamic efficiency of timekeeping.

We analyze the “ferron” excitations in order-disorder ferroelectrics by a microscopic pseudospin model. We demonstrate that analogous to magnons, the quanta of spin waves in magnetic materials, ferrons carry both static and oscillating electric dipole moments, exhibit a Stark effect, and may be parametrically excited by THz radiation. The anticrossing gap of the ferron-photon hybrid depends strongly on propagation direction and an applied static electric field. We predict ferron diffusion lengths that can reach centimeters, which implies efficient transport of electric polarization by temperature gradients. These properties suggest that ferroelectric materials may be useful for information technology beyond data storage applications.

We probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin-$1/2$ antiferromagnetic (AFM) chain compound ${\mathrm{NaVOPO}}_4$ using thermodynamic as well as local $\mu \mathrm{SR}$ and ${}^{31}\mathrm{P}$ NMR probes down to mK temperatures in magnetic fields up to 14 T. The $\mu \mathrm{SR}$ and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behavior, enabling us to directly determine the interaction parameter $K$ as a function of the magnetic field. These estimates are crosschecked using magnetization and specific heat data. The field-dependent $K$ lies in the range of $0.4<K<1$ and indicates the repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders ${\mathrm{NaVOPO}}_4$ the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied $S=1/2$ bond-alternating AFM-AFM chain compounds.

Understanding the nematic phase observed in the iron-chalcogenide materials is crucial for describing their superconducting pairing. Experiments on ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ showed that one of the slow Shubnikov-de Haas quantum oscillation frequencies disappears when tuning the material out of the nematic phase via chemical substitution or pressure, which has been interpreted as a Lifshitz transition [Coldea et al., npj Quantum Mater. 4, 2 (2019); Reiss et al., Nat. Phys. 16, 89 (2020)]. Here, we present a generic, alternative scenario for a nematicity-induced sharp quantum oscillation frequency, which disappears in the tetragonal phase and is not connected to an underlying Fermi surface pocket. We show that different microscopic interband scattering mechanisms—for example, orbital-selective scattering—in conjunction with nematic order can give rise to this quantum oscillation frequency beyond the standard Onsager relation. We discuss implications for iron-chalcogenides and the interpretation of quantum oscillations in other correlated materials.

A two-dimensional system with extra degrees of freedom, such as spin and valley, is of great interest in the study of quantum phase transitions. The critical condition when a transition between different multicomponent fractional quantum Hall states appears is one of the very few junctions for many-body problems between theoretical calculations and experiments. In this work, we present that uniaxial strain induces pseudospin transitions of composite fermions in a two-dimensional hole gas. Determined from transport behavior, strain along ⟨111⟩ effectively changes pseudospin energy levels. We deduce that diagonal strain dominates these variations. Our experiment provides a wedge for manipulating two-dimensional interacting systems mechanically.

We identify an exotic quasiuniversal behavior near the all-in–all-out Weyl quantum critical point in three-dimensional Luttinger semimetals, such as the pyrochlore iridates $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, with $R$ a rare-earth element. The quasiuniversal behavior is characterized by power laws with exponents that vary slowly over several orders of magnitude in energy or length. However, in contrast to the quasiuniversality discussed in the context of deconfined criticality, the present case is characterized by a genuinely universal ultra-low-temperature behavior. In this limit, the pertinent critical exponents can be computed exactly within a renormalization group analysis. Experimental implications for the pyrochlore iridates are outlined.

The structure of mineral glasses at high pressure is often used as a model to understand corresponding melts. In this regard, silica, a prototypical oxide glass, has been studied extensively by theoretical simulations and experiments. A recent study of the oxygen $K$-edge x-ray Raman scattering (XRS) spectra of ${\mathrm{SiO}}_2$ glass suggested a relationship between energies at the centers of gravity of the XRS spectra with the bulk density and electronic density of states at Mbar pressure. It was further proposed that the oxygen atoms are 4-coordinated to silicon, so-called quadclusters, at pressures above 1 Mbar. Here, we investigate the theoretical basis of this empirical observation from the simulation of the XRS spectra and analysis of the structure and bonding of a silica glass up to 2 Mbar. The theoretical results reproduce the observed x-ray absorption spectra, and the relationship between the center of gravity of the O $K$-edge absorption with pressure. However, we show that the trend is not due to the proposed occurrence of oxygen quadclusters. It is simply a compaction effect and there are no characteristic spectral features that can be attributed to the additional oxygen-silicon coordination.

Hydrogen-rich ternary compounds are promising candidates for realizing room-temperature superconductivity due to the synergistic effects of crystal structure and electronic properties under high-pressure conditions. Here, the high-pressure structures, electronic properties, and superconductivity of the ternary ${\mathrm{YScH}}_{2n}$ ($n=3$–6) system are investigated by using the prediction method of particle swarm optimization structure combined with first-principles calculations. We find four stable structures, each with different hydrogen sublattices: $\mathit{Pm}\overline{3}\text{−}{\mathrm{YScH}}_6, P4$/mmm-${\mathrm{YScH}}_8$, Cmmm-${\mathrm{YScH}}_{10}$, and $\mathit{Pm}\overline{3}m\text{−}{\mathrm{YScH}}_{12}$. All these ${\mathrm{YScH}}_{2n}$ structures are predicted to be high-temperature superconductors. The electron local function (ELF) results indicate a lack of interaction between hydrogen atoms in ${\mathrm{YScH}}_6$, while the weak H-H covalent interactions are observed in the other stoichiometric ratios. Strikingly, ${\mathrm{YScH}}_6$ maintains dynamic stability down to ambient pressure and keeps a high superconducting critical temperature ($T_c$) of 66 K. At 140 GPa, the pressure-stabilized ${\mathrm{YScH}}_8$ and ${\mathrm{YScH}}_{10}$ structures exhibit high $T_c$ of 110 and 116 K, respectively. Upon further increasing the content of hydrogen, the lowest dynamically stable pressure of ${\mathrm{YScH}}_{12}$ is increased to 200 GPa, and the calculated $T_c$ is up to 179 K. In all ${\mathrm{YScH}}_{2n}$ structures, $\mathit{Pm}\overline{3}\text{−}{\mathrm{YScH}}_6$ (stabled from 1 atm to 47 GPa), $P4$/mmm-${\mathrm{YScH}}_8$ and Cmmm-${\mathrm{YScH}}_{10}$ (stabled from 140 to 250 GPa), $\mathit{Pm}\overline{3}m\text{−}{\mathrm{YScH}}_{12}$ (stabled from 200 to 286 GPa), strong electron-phonon coupling (EPC) and large electronic density of states of hydrogen at the Fermi level play important roles in their high-temperature superconductivity. It is discussed that phonon softening in the midfrequency region induced mainly by Fermi surface nesting effectively enhances the EPC. In this paper, we potentially discover high-temperature superconducting hydrides that can be stable at atmospheric pressure, taking an important step toward understanding the superconductivity and structural stability of ternary hydrides.

Quasi-one-dimensional materials are a promising system for future applications in electronics and optoelectronics. The refinement of the strong interaction mechanism in quasi-one-dimensional crystals under extreme conditions is still challenging. Here we report a thorough study of superconductivity and phase transition in pressurized ${\mathrm{Ta}}_2{\mathrm{PdS}}_6$ by electrical transport, in situ x-ray diffraction (XRD), Raman spectroscopy, and density functional calculation. ${\mathrm{Ta}}_2{\mathrm{PdS}}_6$ experiences valence bands crossing the Fermi level to form a set of small hole pockets at $P\sim 3.7$ GPa, which drives the system from a semiconductor to a metal. The pressure-induced superconductivity occurs with decreasing temperature, accompanied by a monoclinic distortion at the critical pressure $P_c\sim 30.8$ GPa. Both XRD measurements and theoretical calculations provide evidence that this structural transition is determined to be isosymmetric, mainly resulting from rearrangement of the S atoms along the $a$ and $c$ axes, indicative of the importance of electron-lattice coupling. When the pressure increases to 82 GPa, the high pressure phase has a very high density of electron states at the Fermi level with $T_{\text{c}}^{\text{onset}}\sim$ 5.2 K, likely mediated by strongly electron-coupled phonons. The present study proposes an alternative approach to investigate the effects of pressure on crystal distortion and superconductivity.

The nonequilibrium carrier and phonon dynamics in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ was studied by a combination of Raman scattering and ultrafast optical pump-probe spectroscopy at high pressures up to 31 GPa. Detailed analysis reveals that the pressure dependence of the relaxation time and the damping rate of the $A_{1g}^1$ phonon mode exhibit an anomaly around 3 GPa supporting the occurrence of an electronic topological transition. At higher pressure, the transient reflectivity spectra manifest discontinuous changes around 11.5, 18.9, and 28 GPa, which can be assigned to the structure transitions as evidenced by the Raman spectra. Especially, low-frequency phonon modes, corresponding to the $A_g$ phonon modes in the monoclinic $C2/m$ phase, were identified in both Raman and transient reflectivity spectra in the pressure range of 11.5 to 18.9 GPa. These results not only provide crucial insights into carrier-phonon interaction of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ under high pressure, but also pave the way for investigating the pressure-induced phase transitions from transient reflection spectroscopy.

The generation of wave packets with well-defined momentum and energy distribution constitutes an experimental challenge in diverse areas of physics. The excitation of nontrivial states, on different lattice configurations, becomes mandatory nowadays to unveil new and exotic properties. In this work, we propose and demonstrate experimentally a straightforward technique to engineer on-chip wave packets by exploiting the radiation of an atomlike waveguide into a one-dimensional lattice. We show that the radiation modes generated after exciting this pseudoatom can be tuned at desired band energies by simply controlling the on-site properties, such that wave packets with precise quasimomentum can be generated. As a benchmark, we use the wave-packet generator for a precise excitation of the well-known topological edge states of a Su-Schrieffer-Heeger photonic lattice.

${\mathrm{MgCr}}_2{\mathrm{O}}_4$ is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below $T_{\mathrm{N}}\approx 13$ K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermomagnetic measurements of ${\mathrm{MgCr}}_2{\mathrm{O}}_4$ samples under applied hydrostatic pressure up to $P=1.7$ GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the $\mathit{k}=(0,0,1)$ magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with $\mathit{k}=(0,0,1)$, previously not observed in magnetometry measurements.

By solving the renormalization of the $s\text{−}d$ interaction from magnetic impurities embedded in conventional superconductors at low concentration, we derive the macroscopic superconducting phase fluctuation and electromagnetic linear-response properties within the path-integral approach. It is found that there exist two superconducting phase modes, both exhibiting similar behaviors of the Nambu-Goldstone mode. The existence of two phase modes suggest that in addition to the conventional free Cooper pairs as in the BCS case, there emerges a small part of the localized Cooper pairs around magnetic impurities due to the quantum correlation by the $s\text{−}d$ interaction, acting as Josephson islands. The emerging impurity Shiba bands inside the superconducting gap then correspond to the excitation of the ground state of the localized Cooper pairs, associated with the breaking of these Cooper pairs. In the diamagnetic response, the state of the free Cooper pairs gives rise to the conventional real contribution in the generated supercurrent, whereas the one of the localized Cooper pairs results in an imaginary contribution, leading to the superconducting Friedel oscillation, i.e., oscillation in the decay of the vector potential in the Meissner effect. As for the optical absorption of a conventional superconductor lying in the anomalous-skin-effect region, it is found that besides the conventional interband transition of Bogoliubov quasiparticles as revealed by Mattis-Bardeen theory, there also exist the interband transition between the impurity Shiba bands as well as all interband transitions between Bogoliubov quasiparticle and impurity Shiba bands. These transitions exhibit clear and separate resonance characters, providing a feasible scheme for the experimental detection.