The Seebeck coefficient, $S=L_{12}/\left(T{L}_{11}\right)$, which is proportional to a ratio of the thermoelectric conductivity $L_{12}$ to the electric conductivity $L_{11}$, with $T$ being temperature, is examined for two-dimensional Dirac electrons in the three-quarter-filled organic conductor $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ [BETS = BEDT-TSeF = bis(ethylenedithio)tetraselenafulvalene] at ambient pressure. Using a tight-binding model obtained with the first-principles relativistic density functional theory method [Tsumuraya and Suzumura, Eur. Phys. J. B 94, 17 (2021)], we calculate $S$ in the presence of the impurity and electron-phonon scatterings. We show that $S_x<0$ and $S_y>0$ at high temperatures, where $S_x$ ($S_y$) denotes $S$ perpendicular (parallel) to the molecular stacking axis. There is a sign change of $S_y$ with increasing $T$. We find that at low temperatures the absolute value of $S$ is enhanced by the spin-orbit coupling. The Seebeck coefficient is examined by dividing it into components of the conduction and valence bands; we find that the electron and hole contributions compete with each other. Such $T$ dependence of $S$ is clarified using the spectral conductivity, which determines $L_{12}$ and $L_{11}$.

We present a theoretical framework to investigate quantum magnetotransport in monolayer jacutingaite, focusing on its response to external electric fields and off-resonant circularly polarized laser irradiation. Our analysis reveals a sequence of topological phase transitions triggered by tuning these external parameters. Applying a perpendicular magnetic field, we study Landau level (LL) formation, spin- and valley-polarized splitting, and magneto-optical response in distinct topological phases. We find that the zeroth LL exhibits spin- and valley-polarized splitting, leading to four distinct peaks in the DOSs for the $K$ and $K^{\prime }$ valleys. We demonstrate that reversing the electric field or flipping the light helicity changes the Dirac mass sign in specific spin-valley sectors, which in turn reverses both the Berry curvature and the magnetic moment. Our results reveal that external electric, magnetic, and off-resonant optical fields can control these conductivities. These findings highlight monolayer jacutingaite as a highly tunable platform with strong potential for future applications in photonics, optoelectronics, and topological quantum devices.

It has been realized over the past two decades that topological nontriviality can be present not only in insulators but also in gapless semimetals, the most prominent example being Weyl semimetals in three dimensions. Key to the topological classification schemes are the three “internal” symmetries, time reversal $\mathcal{T}$, charge conjugation $\mathcal{C}$, and their product, called chiral symmetry $\mathcal{S}=\mathcal{T}\mathcal{C}$. In this work, through explicit calculations, we show that robust topological semimetal phases occur in $d=3$ without invoking crystalline symmetries other than translations. These semimetals appear as not only intermediate gapless phases between the topological and the trivial gapped phases in symmetry classes that have nontrivial topology, but also in symmetry classes that have only trivial gapped phases in $d=3$. We show that all classes that lack chiral symmetry always have topologically stable and robust phases with Weyl nodes, classified by their Chern number on an enclosing surface. Furthermore, all classes that possess chiral symmetry always have topologically protected and robust nodal-line semimetal phases, classified by the winding number of a linking loop. Just as point nodes imply Fermi arcs on the surface Brillouin zone (BZ), a nonzero winding number on a nodal loop implies robust gapless drumhead states on the surface BZ.

Higher-order phonon scatterings beyond fourth order remain largely unexplored despite their potential importance in strongly anharmonic materials at elevated temperatures. We develop a theoretical formalism for first-principles calculation of five- and six-phonon scatterings using Green's function techniques based on a diagrammatic formalism and systematically investigate multiphonon interactions in Si, MgO, and BaO from room temperature to near melting points. Our calculations reveal dramatically different material-dependent behaviors: while five- and six-phonon processes remain negligible in Si even at high temperatures, they become increasingly important in MgO near its melting point (3100 K) and in BaO at intermediate temperatures (1200 K). Most remarkably, five- and six-phonon scatterings surpass three- and four-phonon scattering intensity in BaO near its melting point (2100 K), reducing lattice thermal conductivity by over 50%. We demonstrate that the strength of higher-order interactions is primarily governed by interatomic force constants, with BaO exhibiting five- and six-phonon scattering rates over one order of magnitude stronger than MgO despite identical crystal structures, due to large scattering phase space arising from softened harmonic interactions. Our work provides theoretical insights into the lattice dynamics and thermal transport in strongly anharmonic materials and at elevated temperatures.

MXenes and MBenes, which are two-dimensional (2D) transition metal carbides/nitrides and borides, have been extensively studied for their impressive properties. Recently, we reported a family of transition metal sulfides MSene ($M_2\mathrm{S}$) with rich properties, it is worth studying whether selenides with similar structure also have rich properties. In this work, through high-throughput screening, we present a novel family of 2D transition metal selenides, $M_2\mathrm{Se}$. In this family, there are 58 candidate materials, of which 10 are stable and metallic. Notably, eight exhibit superconductivity, among which four are superconducting topological metals. Besides, 8 show charge density wave (CDW) behavior, among which 5 also exhibit antiferromagnetism. It is revealed that CDW originates from electron-phonon coupling rather than Fermi surface nesting. Moreover, strain can be applied to regulate the competition between the CDW and superconductivity. Our findings reveal the rich properties of superconductivity, band topology, CDWs, and magnetism in $M_2\mathrm{Se}$, providing a new platform for the controllable integration of multifunctional quantum states.

Spin relaxation via electron-phonon interaction is an important decoherence mechanism for spin qubits. In this work, we study spin relaxation in hexagonal (2H) germanium, a direct-gap semiconductor showing great potential to combine highly coherent spin qubits with optical functionality. Focusing on electrostatically defined quantum dots in hexagonal germanium nanowires, we (i) identify geometries where spin qubit experiments are feasible, (ii) compute the nanowire phonon modes, and (iii) describe spin relaxation of hole spin qubits due to phonon-induced lattice reorientation, a direct spin-phonon coupling mechanism that is absent in cubic semiconductors typically used for spin qubits (GaAs, cubic Si, and cubic Ge). We obtain the spin relaxation time as a function of nanowire cross-section, quantum dot confinement length, and magnetic field. For realistic parameters, we find relaxation times above 10 ms, and reveal that the magnetic field direction maximizing the relaxation time depends on the qubit Larmor frequency. Our results facilitate the design of nanowire quantum dot experiments with long qubit relaxation times.

We investigate the attosecond transient absorption spectroscopy (ATAS) in gapped graphene by numerically solving the four-band density matrix equations. Our results reveal that, in contrast to pristine graphene, whose fishbone-shaped spectra oscillate at twice the pump laser frequency, the ATAS of gapped graphene exhibits an additional component oscillating at the pump laser frequency, induced by the Berry connection. To gain insight into these interesting results, we employ a simplified model to derive an analytical expression for the spectral component stemming from the Berry connection. Our analytical results qualitatively reproduce the key features observed in the numerical simulations, revealing that the intensity of the fundamental-frequency spectral component depends not only on the Berry connection but also on the energy shifts associated with the effective mass of electrons at the van Hove singularities. These results shed light on the complex generation mechanism of the ATAS in inversion-symmetry-broken materials.

We introduce a model of an active quantum particle and discuss its properties. The particle has a set of internal states that mediate exchanges of heat with external reservoirs. Heat is then converted into motion by means of a spin-orbit term that couples internal and translational degrees of freedom. The quantum features of the active particle manifest both in the motion and in the heat-to-motion conversion. Furthermore, the stochastic nature of heat exchanges impacts the motion of the active particle and fluctuations can be orders of magnitude larger than the average values. The combination of spin-orbit interaction under nonequilibrium driving may bring active matter into the realm of cold atomic gases where our proposal can be implemented.

High-resolution neutron spectroscopy on ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ reveals a correlated state characterized by distinct dipolar scattering signals—quasielastic and inelastic contributions consistent with ‘photon’ and ‘spinon’ excitations in quantum spin ice. These signals coexist with weak octupolar scattering. Fits of thermodynamic data using numerical methods indicate a dominant octupolar exchange, $J_x$ or $J_y$, with substantial dipolar $J_z$ and minute dipole-octupole $J_{xz}$ couplings. The $J_{xz}$ value is corroborated by an independent fit of the neutron scattering amplitude balance between dipolar and octupolar ‘photon’ contributions, highlighting its importance to understand neutron scattering results in this family. ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ enriches the landscape of dipole-octupole pyrochlore physics, and reveals a ‘quantum multipolar liquid’ where hybrid correlations involve multiple terms in the moment series expansion, opening questions regarding their intertwining and hierarchy in quantum phases.

We investigate topological spin-up triplet excitonic condensation and its competition with other stabilities in a two-dimensional interacting electron-hole system taking into account Rashba spin-orbit coupling and external magnetic fields. Using an unrestricted Hartree-Fock approach, we self-consistently evaluate spin-selective excitonic condensate order parameters and the Chern number. The ground state phase diagram in the dependence on magnetic field and Coulomb interaction shows a spin-up triplet excitonic condensate (EC) with a nonzero Chern number, emerging uniquely away from the topologically trivial singlet and spin-down triplet EC regions. Strong spin-polarized triplet excitonic fluctuations preceding the condensation are further revealed through the signatures of the dynamical excitonic susceptibility spectra. Our results establish a class of topological quantum phases driven by excitonic coherence and suggest a realistic pathway to its realization in a distorted Janus monolayer of transition metal dichalcogenides or some twisted van der Waals heterostructures.

At low temperatures, additional phase transitions have been detected in the antiferroelectric crystals ${\mathrm{PbZrO}}_3$ and ${\mathrm{PbHfO}}_3$ near 270 K. In this paper we investigate these transitions theoretically and experimentally and arrive at the conclusion that it is of first order, using the free energy expanded to a sixth order in the order parameter. In addition to recently published pyroelectric and dielectric properties, the presence of this transition has been proven by measuring the temperature dependence of the specific heat and natural absorbance, and compared with the literature data of the structural investigations by neutron scattering. Opposite to the previous articles, the origin of this transition is not connected with the Pb ion displacements, but it is solely caused by the p-d interaction in the transition metal-oxides ${\mathrm{ZrO}}_6$ and ${\mathrm{HfO}}_6$ substructures, typical for a broad number of ferroelectric oxide perovskites.

Chiral magnons in antiferromagnets offer a unique bosonic platform to explore quantum tunneling phenomena, including Klein tunneling, in experimentally accessible regimes. Here, we investigate the tunneling dynamics of magnon wave packets in ${\mathrm{MnF}}_2$ nanowires exhibiting anti-parity-time $\left(\text{anti}\text{−}\mathcal{PT}\right)$ symmetry under magnetic-field-controlled potential steps. By varying the wave vector $k$ and external magnetic fields, we design distinct channels for chiral and Klein tunneling. Our theoretical and numerical analysis shows that helicity and chirality are preserved during tunneling, and the magnonic particle–antiparticle pair creation occurs—a definitive signature of Klein tunneling. We explore how key parameters, such as magnetic field strength, wave vector, and wave packet width, influence tunneling dynamics and identify resonant transmission conditions across barriers and wells. Notably, in the Klein tunneling regime, the combined transmission and reflection exceed unity, as barriers effectively act as potential wells for antiparticles. These findings pave the way for approaches in the manipulation and amplification of chiral magnons, as well as in the engineering of bosonic tunneling effects in magnonic systems.

Quantum geometry plays a crucial role in the nonlinear transport of quantum materials. Here, we use the Boltzmann transport formalism to study the magnetic control of nonlinear transport induced by the quantum metric in two-dimensional systems with different types of spin-orbit coupling (SOC). It is shown that the nonlinear conductivity is strongly dependent on the direction of a field and reveals significant spatial anisotropy. Moreover, the field-direction-dependent relations are distinct for different SOCs. In addition, it is demonstrated that the contributions from the quantum metric and the Drude mechanism are distinguishable due to their opposite signs or distinct anisotropy relations. We further derive the analytical formulas for the anisotropic nonlinear conductivity, in exact agreement with numerical results. Finally, we exemplify those phenomena in the ${\mathrm{Ag}}_2\mathrm{Te}$ monolayer in the presence of an external magnetic field based on density functional theory calculations. Our work shines more light on the interplay between nonlinear transport and quantum geometry.

Recently, perovskite hydrides have received great attention due to their excellent superconductivity at moderate pressures. Based on perovskite structure $Pm\overline{3}m\text{−}{\mathrm{KInH}}_3$, a series of quaternary double perovskite hydrides $A_2{\mathrm{In}Z\mathrm{H}}_6$ $(A=\mathrm{Li}, \mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs}, Z=\mathrm{Cu}, \mathrm{Ag} \mathrm{Au}, \mathrm{Zn}, \mathrm{Cd}, \mathrm{Hg}, \mathrm{Ga}, \mathrm{Tl}, \mathrm{Ge}, \mathrm{Sn}, \mathrm{Pb}, \mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$ were designed. High-throughput screening show that 27 compounds are dynamically stable below 50 GPa. Double perovskite hydrides $A_2{\mathrm{In}Z\mathrm{H}}_6$ which are dynamically stable at moderate pressures are all superconductors. Four hydrides exhibit high-temperature superconductivity, including ${\mathrm{K}}_2{\mathrm{InCuH}}_6$ (92 K at 0 GPa), ${\mathrm{K}}_2{\mathrm{InAuH}}_6$ (108 K at 6 GPa), ${\mathrm{Cs}}_2{\mathrm{InGeH}}_6$ (82 K at 20 GPa), and ${\mathrm{Cs}}_2{\mathrm{InAuH}}_6$ (104 K at 20 GPa). Additionally, ${\mathrm{K}}_2{\mathrm{InAgH}}_6$ has a superconducting $T_c$ of 75 K, which is approaching liquid nitrogen temperature. The flat band of ${\mathrm{K}}_2{\mathrm{InCuH}}_6$ at Γ point is governed by $\mathrm{H}\text{−}s$ and $\mathrm{Cu}\text{−}{e}_{1g}$ orbitals hybridization. Further analysis indicates that there is strong coupling between electrons and hydrogen-dominated optical phonons. This strong electron-phonon coupling (EPC) plays a critical role in high-temperature superconductivity for ${\mathrm{K}}_2{\mathrm{InCuH}}_6$. Our results enhance confidence in the exploration of high-temperature superconducting perovskite hydrides at moderate pressure, and stimulates further exploration of quaternary superconducting hydrides.

Motivated by recent experiments on superfluid ${}^3\mathrm{He}$ in nanoscale-confined geometries, we theoretically investigate the associated phase diagram in a slab geometry and perpendicular magnetic field as the size of confinement is varied. Our analysis is based on minimizing the Ginzburg-Landau free energy for the $3\times 3$ matrix superfluid order parameter for three different boundary conditions. We observe a smooth crossover from the phase diagram of the three-dimensional system to the quasi-two-dimensional limit for slab heights of several hundred nanometers and magnetic fields of several kilogauss. We illuminate that, despite the apparent complexity of the underlying equations, many precise numerical and even analytical statements can be made about the phase structure for general values of the coefficients of the free-energy functional, which can in turn be used to constrain or measure these parameters. To guide future experimental studies, we compute the phase diagram in dependence of pressure, temperature, slab height, and magnetic field.

Chromium trihalides ($\mathrm{Cr}{X}_3$, with $X=\mathrm{I},\mathrm{Br},\mathrm{Cl}$) are layered ferromagnetic materials with rich physics and possible applications. Their structure consists of magnetic Cr atoms sandwiched between two layers of halide atoms. Different halide atoms result in distinct magnetic properties, but their effect on spin-wave (magnon) excitations is not fully understood. Here we show first-principles calculations of magnon dispersions and wave functions in monolayer Cr trihalides using the finite-momentum Bethe-Salpeter equation (BSE) to describe collective spin-flip excitations. We study the dependence of magnon dispersions on the halide species and resolve the small topological gap at the Dirac point in the magnon spectrum by including spin-orbit coupling. Analysis of magnon wave functions reveals that magnons are made up of electronic transitions with a wider energy range than excitons in $\mathrm{Cr}{X}_3$ monolayers, providing insight into magnon states in real and reciprocal space. We analyze Heisenberg exchange parameters extracted from the BSE and discuss the convergence of BSE magnon calculations. Our work advances the quantitative modeling of magnons, providing the starting point for studying magnon interactions in a first-principles BSE framework.

Exploring the controllable aspects of local atomic structure and chemical ordering and their correlations with functional properties is crucial for harnessing the potential of complex oxides in the development of advanced materials. In this work, we have investigated the sensitivity of the magnetic properties in a nanostructured metastable spinel compositionally complex oxide (CCO) composition, $\left({\mathrm{Mg}}_{0.2}{\mathrm{Mn}}_{0.2}{\mathrm{Fe}}_{0.2}{\mathrm{Cu}}_{0.2}{\mathrm{Zn}}_{0.2}\right){\mathrm{Co}}_2{\mathrm{O}}_4$, to local chemical segregation and phase evolution introduced through variation in post-processing heat treatment temperature. A combination of x-ray diffraction, scanning transmission electron microscopy with energy dispersive x-ray spectroscopy, first-order reversal curve (FORC) magnetometry, and neutron diffraction and total scattering analyses was employed to understand both average and local structure-property evolution. Structure analysis shows that the postannealing process triggers local and long-range cation diffusion, resulting in changes in the distribution of atoms residing on the tetrahedral and octahedral sites of the spinel structure as well as nanoscale chemical heterogeneity. FORC analysis shows that redistribution of magnetic cations induces subtle magnetic phase separation and soft to hard magnetic phase transformations, and demonstrates incipient demixing of the as-synthesized material well before detection by neutron total scattering. This work additionally highlights the necessity of a combination of advanced characterization techniques for understanding the broader crystal-chemical class of compositionally complex oxides.

Through detailed analysis of neutron powder diffraction data, we confirm that the double perovskite ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ adopts a monoclinic $P{2}_1/n$ structure with nearly complete $B$-site ordering of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ cations. Below $T_1=198\mathrm{K}$, magnetic susceptibility measurements and further analysis of neutron diffraction data reveal that the ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ sublattices undergo ferromagnetic ordering, as expected of strong $3d\text{−}3d$ exchange interactions. Upon cooling through $T_2 =22\mathrm{K}$, a secondary magnetic transition is observed, below which we have discovered an additional noncollinear symmetry-breaking order of ${\mathrm{Nd}}^{3+}$ moments. We argue that the canting of the rare-earth moments naturally arises through the competition of $f\text{−}d$ and $f\text{−}f$ Heisenberg exchange interactions, which may be finely balanced in the double perovskite framework. Furthermore, the symmetry of the ground state magnetic structure implies significant ${\mathrm{Nd}}^{3+}$ easy plane anisotropy, and an effective decoupling of the antiferromagnetic spin canting from the transition metal sublattice. Between $T_1$ and $T_2, {\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ shows anomalous behavior in the frequency-dependent ac magnetic susceptibility that is characteristic of reentrant spin-glass-like properties attributed to antisite disorder and competing interactions. Finally, analysis of isothermal magnetization yields magnetic entropy changes that suggest ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ and related compounds have potential for magnetic refrigeration, showing a peak in the magnetic entropy change of $2.25\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ at $T_1$ under a 7 T field. The scaling behavior of the magnetic entropy, paired with analysis of other critical exponents, shows that the ferromagnetic transition at $T_1$ is consistent with a mean-field second-order phase transition. Taken together, our results provide crucial details on the magnetic properties of ${\mathrm{Nd}}_2{\mathrm{NiMnO}}_6$ and further consolidate the expectation that double perovskites may serve as model systems for investigating competing magnetic interactions, magnetocaloric effects, and reentrant spin-glass behavior.

The supersolid phase of soft-core bosons in two dimensions is investigated using the self-consistent Hartree-Fock and quantum Monte Carlo methods. An approximate phase diagram at finite temperatures is initially constructed using the mean-field approach, which is subsequently validated through precise path-integral simulations, enabling a microscopic characterization of the various phases. Superfluid and melting/freezing transitions are analyzed through the superfluid density and the long-range behavior of correlation functions associated with positional and orientational order, in accordance with the general picture of Berezinskii-Kosterlitz-Thouless transitions. A broad region at low temperatures is identified where the supersolid phase exists, separating the uniform superfluid phase from the normal quasicrystal phase. Additionally, a potential intermediate hexatic phase with quasi-long-range orientational order is identified in a narrow region between the normal solid and fluid phases. These findings establish self-consistent Hartree-Fock theory beyond the local density approximation as an effective tool, complementary to computationally intensive quantum Monte Carlo simulations, for investigating the melting of the supersolid phase and the possible emergence of the hexatic superfluid phase in bosonic systems with various interaction potentials.

We demonstrate the emergence of an intrinsic space-time crystalline order in a long ferromagnetic ${\phi }_0$ Josephson junction on a topological insulator without any external periodic modulation. The presence of the exchange and Dzyaloshinskii-Moriya interactions in a ferromagnetic layer with broken inversion symmetry internally modulates the critical current due to the coupling between the magnetic moment and the Josephson phase. This breaks the time translation symmetry, leading to the appearance of the space-time crystalline pattern in the spatiotemporal dependence of the in-plane current, which oscillates with almost twice the ferromagnetic resonance frequency. In the limit where the critical current is not modulated internally, the space-time crystalline order does not occur. In this case, only when an external parametric modulation is applied, the system exhibits a typical classical discrete space-time crystalline order that oscillates at half of the modulation frequency. Considering the still-pending problem of experimental detection, we demonstrate that a recently developed magnetometry device, which visualizes the supercurrent flow at the nanoscale, can be used to detect space-time crystalline patterns in hybrid Josephson junctions experimentally.