Exploring hidden effects that have been overlooked given the nominal global crystal symmetry but are indeed visible in solid-state materials has been a fascinating subject of research recently. Here, we introduce a hidden Zeeman-type spin polarization (HZSP) in nonmagnetic bulk crystals with sublattice structures. In the momentum space of these crystals, the doubly degenerate bands formed in a certain plane can exhibit a uniform spin configuration with opposite spin orientations perpendicular to this plane, whereas such degenerate states are spatially separated in a pair of real-space sectors. Interestingly, we find that HZSP can manifest itself in both centrosymmetric and noncentrosymmetric materials. We further demonstrate the important role of nonsymmorphic twofold screw-rotational symmetry played in the formation of HZSP. Moreover, two representative material examples, i.e., centrosymmetric $\mathrm{W}{\mathrm{Se}}_2$ and noncentrosymmetric $\mathrm{Ba}{\mathrm{Bi}}_4{\mathrm{O}}_7$, are identified to show HZSP via first-principles calculations. Our finding thus not only opens different perspectives for hidden spin polarization research but also significantly broadens the range of materials towards spintronics applications.

It has been well established that (i) the thermopower of semiconductors can be enhanced through a phenomenon known as the drag effect, and (ii) the drag enhancement involves only low-frequency acoustic phonons and benefits from low electron densities and low temperatures. Using first-principles calculations we show that large drag enhancements to the thermopower are possible at high carrier density even at room temperature and arise from high-frequency acoustic phonons. A fascinating example is cubic boron arsenide (BAs) for which the calculated room temperature drag enhancement of the thermopower exceeds an order of magnitude at a high hole density of ${10}^{21}\mathrm{c}{\mathrm{m}}^{-3}$. This remarkable behavior stems from the simultaneously weak phonon-phonon and phonon-hole scattering of the high-frequency phonons in BAs that become drag active at high carrier densities through electron-phonon interactions. This work advances our understanding of coupled electron-phonon nanoscale transport and introduces an unexpected paradigm for achieving large thermopowers.

Amorphous silicon (a-Si) is an important thermal-management material and also serves as an ideal playground for studying heat transport in strongly disordered materials. Theoretical prediction of the thermal conductivity of a-Si in a wide range of temperatures and sample sizes is still a challenge. Herein we present a systematic investigation of the thermal transport properties of a-Si by employing large-scale molecular dynamics (MD) simulations with an accurate and efficient machine learned neuroevolution potential (NEP) trained against abundant reference data calculated at the quantum-mechanical density-functional-theory level. The high efficiency of NEP allows us to study the effects of finite size and quenching rate in the formation of a-Si in great detail. We find that a simulation cell up to $64000$ atoms (a cubic cell with a linear size of 11 nm) and a quenching rate down to ${10}^{11}$ K ${\mathrm{s}}^{-1}$ are required for almost convergent thermal conductivity. Structural properties, including short- and medium-range order as characterized by the pair-correlation function, angular-distribution function, coordination number, ring statistics, and structure factor are studied to demonstrate the accuracy of NEP and to further evaluate the role of quenching rate. Using both the heterogeneous and homogeneous nonequilibrium MD methods and the related spectral decomposition techniques, we calculate the temperature- and thickness-dependent thermal conductivity values of a-Si and show that they agree well with available experimental results from 10 K to room temperature. Our results also highlight the importance of quantum effects in the calculated thermal conductivity and support the quantum-correction method based on the spectral thermal conductivity.

We study the phase diagram of the one-dimensional Bose-Fermi-Hubbard model at unit filling for the scalar bosons and half filling for the $S=1/2$ fermions using quantum Monte Carlo simulations. The bare interaction between the fermions is set to zero. A central question of our study is what type of interactions can be induced between the fermions by the bosons, for both weak and strong interspecies coupling. We find that the induced interactions can lead to competing instabilities favoring phase separation, superconducting phases, and density wave structures, in many cases at work on length scales of more than 100 sites. Marginal bosonic superfluids with a density matrix decaying faster than what is allowed for pure bosonic systems with on-site interactions, are also found.

In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using synchrotron x-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen content in $\mathrm{Tc}{\mathrm{H}}_x$ phases increases when the pressure rises, and at 27 GPa we found a hexagonal (hcp) nonstoichiometric hydride $\mathrm{Tc}{\mathrm{H}}_{1.3}$. The formation of technetium hydrides is also confirmed by the emergence of a reflective band at 450–600 nm in the reflectance spectra of $\mathrm{Tc}{\mathrm{H}}_{1+x}$ samples synthesized at 45 GPa. On the basis of theoretical analysis, we propose crystal structures for the phases $\mathrm{Tc}{\mathrm{H}}_{0.45\pm 0.05} \left({\mathrm{Tc}}_{16}{\mathrm{H}}_7\right)$ and $\mathrm{Tc}{\mathrm{H}}_{0.75\pm 0.05} \left({\mathrm{Tc}}_4{\mathrm{H}}_3\right)$ previously obtained at 1–2 GPa. Calculations of the electron-phonon interaction show that technetium hydrides $\mathrm{Tc}{\mathrm{H}}_{1+x} \left(x=0–0.3\right)$ do not possess superconducting properties due to low electron-phonon interaction parameter $\left(\lambda \sim 0.23\right)$.

Symmetry fractionalization is a ubiquitous feature of topologically ordered states that can be used to classify different symmetry-enriched topological phases and reveal some of their unique experimental signatures. Despite its vast popularity, there is currently no available framework to study symmetry fractionalization of quantum spin ice (QSI)—a $\text{U}\left(1\right)$ quantum spin liquid (QSL) on the pyrochlore lattice supporting emergent photons—within the most widely used theoretical framework to describe it, gauge mean-field theory (GMFT). In this work, we provide an extension of GMFT that allows for the classification of space-time symmetry fractionalization. The construction classifies all GMFT Ansätze that yield physical wave functions invariant under given symmetries and a specific low-energy gauge structure. As an application of the framework, we first show that the only two Ansätze with emergent U(1) gauge fields that respect all space group symmetries are the well-known 0- and $\pi$-flux states. We then showcase how the framework may describe QSLs beyond the currently known ones by classifying chiral U(1) QSI. We find a new chiral QSL described by $\pi /2$ fluxes of the emergent gauge field threading the hexagonal plaquettes of the pyrochlore lattice. We finally discuss how the different ways translation symmetries fractionalize for all these states lead to unique experimentally relevant signatures and compute their respective inelastic neutron scattering cross-section to illustrate the argument.

We study the phase diagram of two different mixed-dimensional $t\text{−}{J}_z\text{−}{J}_{\perp }$ models on the square lattice, in which the hopping amplitude $t$ is only nonzero along the $x$ direction. In the first model, which is bosonic, the spin-exchange amplitude $J_{\perp }$ is negative and isotropic along the $x$ and $y$ directions of the lattice, and $J_z$ is isotropic and positive. The low-energy physics is characterized by spin-charge separation: the holes hop as free fermions in an easy-plane ferromagnetic background. In the second model, $J_{\perp }$ is restricted to the $x$ axis while $J_z$ remains isotropic and positive. The model is agnostic to particle statistics, and shows stripe patterns with antiferromagnetic Néel order at low temperature and high hole densities, in resemblance of the mixed-dimensional $t\text{−}{J}_z$ and $t\text{−}J$ models. At lower hole density, a very strong first-order transition and hysteresis loop is seen extending to a remarkably high 14(1)% hole doping.

We study the real-time dynamics of the Kondo effect after a quantum quench in which a magnetic impurity is coupled to two metallic Hubbard chains. Using an effective field theory approach, we find that for noninteracting electrons the charge current across the impurity is given by a scaling function that involves the Kondo time. In the interacting case, we show that the Kondo time decreases with the strength of the repulsive interaction and the time dependence of the current reveals signatures of the Kondo effect in a Luttinger liquid. In addition, we verify that the relaxation of the impurity magnetization does not exhibit universal scaling behavior in the perturbative regime below the Kondo time. Our results highlight the role of nonequilibrium dynamics as a valuable tool in the study of quantum impurities in interacting systems.

Motivated by recent experiments on Mott insulators, in both iridates and ultracold atoms, we theoretically study the effects of magnetic order on the Mott-Hubbard excitons. In particular, we focus on spin-mediated doublon-holon pairing in Hubbard materials. We use several complementary theoretical techniques: Mean-field theory to describe the spin degrees of freedom, the self-consistent Born approximation to characterize individual charge excitations across the Hubbard gap, and the Bethe-Salpeter equation to identify bound states of doublons and holons. The binding energy of the Mott exciton is found to increase with increasing the Néel order parameter, whereas the exciton mass decreases. We observe that these trends rely significantly on the retardation of the effective interaction, and require consideration of multiple effects from changing the magnetic order. Our results are consistent with the key qualitative trends observed in recent experiments on iridates. Moreover, the findings could have direct implications on ultracold atom Mott insulators where the Hubbard model is the exact description of the system and the microscopic degrees of freedom can be directly accessed.

We consider compact $U^{\kappa }\left(1\right)$ gauge theory in $3+1$ dimensions with a general $2\pi$-quantized topological term ${\sum }_{I,J=1}^{\kappa }\frac{K_{IJ}}{4\pi }{\int }_{M^4}{F}^I\wedge F^J$, where $K$ is an integer symmetric matrix with even diagonal elements and $F^I=d{A}^I$. At energies below the gauge charges' gaps but above the monopoles' gaps, this field theory has an emergent ${\mathbb{Z}}_{k_1}^{\left(1\right)}\times {\mathbb{Z}}_{k_2}^{\left(1\right)}\times \cdots$ 1-symmetry, where $k_i$ are the diagonal elements of the Smith normal form of $K$ and ${\mathbb{Z}}_0^{\left(1\right)}$ is regarded as a $U\left(1\right)$ 1-symmetry. In the $U^{\kappa }\left(1\right)$ confined phase, the boundary can have a phase whose infrared (IR) properties are described by Chern-Simons field theory. Such a phase has a ${\mathbb{Z}}_{k_1}^{\left(1\right)}\times {\mathbb{Z}}_{k_2}^{\left(1\right)}\times \cdots$ 1-symmetry that can be anomalous. To show these results, we develop a bosonic lattice model whose IR properties are described by this continuum field theory, thus acting as its ultraviolet completion. The lattice model in the aforementioned limit has an exact ${\mathbb{Z}}_{k_1}^{\left(1\right)}\times {\mathbb{Z}}_{k_2}^{\left(1\right)}\times \cdots$ 1-symmetry. We find that the short-range entangled gapped phase of the lattice model, corresponding to the confined phase of the $U^{\kappa }\left(1\right)$ gauge theory, is a symmetry protected topological (SPT) phase for the ${\mathbb{Z}}_{k_1}^{\left(1\right)}\times {\mathbb{Z}}_{k_2}^{\left(1\right)}\times \cdots$ 1-symmetry, whose SPT invariant is $e^{i\pi {\sum }_{I,J}{K}_{IJ}{\int }_{{\mathcal{M}}^4}{B}_I⌣B_J+B_I\underset{1}{⌣}d{B}_J}{e}^{i\pi {\sum }_{I<J}{K}_{IJ}{\int }_{{\mathcal{M}}^4}d{B}_I\underset{2}{⌣}d{B}_J}$. Here, the background $\mathbb{R}/\mathbb{Z}$-valued 2-cochains $B_I$ satisfy $d{B}_I={\sum }_I{B}_I{K}_{IJ}=0mod1$ and describe the symmetry twist of the ${\mathbb{Z}}_{k_1}^{\left(1\right)}\times {\mathbb{Z}}_{k_2}^{\left(1\right)}\times \cdots$ 1-symmetry. We apply this general result to a few examples with simple $K$ matrices. We find the nontrivial SPT order in the confined phases of these models and discuss its classifications using the fourth cohomology group of the corresponding 2-group.

According to the three-step model for solid high-order harmonic generation, there is a one-to-one correspondence between the emitted photon energy and the band gap where the electron-hole pair is annihilated. In the tunneling excitation regime, as the electron-hole pair is mostly created in the vicinity of the minimum band gap, the conversion efficiency of the high-energy photon should be approximately proportional to the square of the transition dipole moment at the $\mathbf{k}$ point where the high-harmonic photon is emitted. Based on this picture we propose that a high-order harmonic spectrum could be a strong tool to reconstruct the shape of $\mathbf{k}$-dependent transition dipole moments with the band dispersion and the laser field known. Two real systems, e.g., MgO and ZnO, are taken as samples to verify our idea. The reconstructed shape of the transition dipole moments shows small variation as the laser parameters, such as intensity, wavelength, and pulse duration, are tuned in wide ranges, which proves this scheme is robust.

Composite quantum materials form a unique class of materials which exhibit more than one apparently distinct quantum phenomenon entangled by common requirement of different symmetries in a single material. In search of composite quantum phases in two dimensions, we investigate the electronic properties of computationally designed transition-metal carbide Janus compounds, with strong spin-orbit coupling and inversion symmetry broken by asymmetric surface passivation. The ab initio calculated electronic structure of these Janus structures, show giant Rashba effect with Rashba coefficients as large as $\sim 4\mathrm{eV}\AA$. Interestingly, the same Janus structures are shown to be potential candidates to exhibit nonlinear anomalous Hall conductivity owing to the nonzero Berry curvature on the nonequilibrium Fermi surface. Our computational study predicts that such Janus structures can be promising platform for exploring intertwined quantum phenomena in a single two-dimensional system. The two-dimensionality of the proposed systems make them ideally suited for device fabrication with technological implication of composite quantum materials.

The formation of the composite photonic-excitonic particle, known as a polariton, is a phenomenon emerging in materials possessing strong coupling to light. The organic-based materials besides the strong light-matter coupling also demonstrate strong interaction of electronic and vibrational degrees of freedom. We study the vibration-assisted polariton wave-function evolution treating both types of interactions as equally strong. Using the multiconfiguration Hartree approach we derive the equations of motion for the polariton wave function, where the vibration degrees of freedom interact with the polariton quantum field through the mean-field Hartree term. For the conventional quadratic polariton Hamiltonian and the Holstein-type vibration Hamiltonian (Tavis-Cummings-Holstein model), the obtained equations are in one-to-one correspondence with the original Schrödinger equation. In the second part of the paper, we show that our theory reproduces the physical properties of the polariton light emission spectrum. In particular, the theory explains experimental observations of the molecular Stokes shift in the polariton fluorescence spectra in the systems with strong light-matter coupling. We also investigate the behavior of the polariton wave function in the vicinity of the anticrossing point and demonstrate that the Hartree term can produce an infinite potential barrier of a dynamical origin, which is responsible for the formation of the mixed upper-lower polariton states. The nonlinear nature of the polariton theory reflects their collective behavior. We expect that the multiconfiguration Hartree approach being applied to polaritons and similar systems will result in a manifestation of new physical phenomena.

We present a systematic classification of emergent particles in all 528 magnetic layer groups and 394 magnetic rod groups, which describe two-dimensional and one-dimensional crystals, respectively. Our approach is via constructing a correspondence between a given magnetic layer/rod group and one of the magnetic space groups, such that all irreducible representations of the layer/rod group can be derived from those of the corresponding space group. Based on these group representations, we explicitly construct the effective models for possible band degeneracies and identify all emergent particles, including both spinless and spinful cases. We find that there are six kinds of particles protected by magnetic layer groups and three kinds by magnetic rod groups. Our work provides a useful reference for the search and design of emergent particles in lower dimensional crystals.

${\mathrm{LaRh}}_3{\mathrm{B}}_2$ crystallizes in a layered structure where Rh atoms form a perfect kagome lattice. The material shows superconductivity at $T_c\approx 2.6$ K and no signature for density wave instabilities. We report our measurements of electronic transport, magnetization, and heat capacity in the normal and superconducting state, and we derive normal and superconducting parameters. From first-principles calculations of the electronic band structure, we identify all features of kagome bands predominantly formed by the Rh $d$ orbitals: a flat band, Dirac cones, and van Hove singularities. The calculation of the phonon dispersions and electron-phonon coupling suggests a strong similarity between ${\mathrm{LaRh}}_3{\mathrm{B}}_2$ and ${A\text{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K,Cs,Rb}$). For ${\mathrm{LaRh}}_3{\mathrm{B}}_2$, it matches quantitatively with the observed $T_c$, supporting a conventional phonon-mediated pairing mechanism. By comparison to the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ family, we conjecture that there is a reduced importance of electron correlations in ${\mathrm{LaRh}}_3{\mathrm{B}}_2$.

Excitonic bound states are characterized by a binding energy ${\epsilon }_b$ and a single-particle band gap ${\mathrm{\Delta }}_b$. This work provides a theoretical description for both strong (${\epsilon }_b\sim {\mathrm{\Delta }}_b$) and weak (${\epsilon }_b\ll {\mathrm{\Delta }}_b$) excitonic bound states, with particular application to biased bilayer graphene. Standard description of excitons is based on a wave function that is determined by a Schrödinger-like equation with screened attractive potential. The wave function approach is valid only in the weak-binding regime ${\epsilon }_b\ll {\mathrm{\Delta }}_b$. The screening depends on frequency, i.e., dynamical screening, and this implies retardation. In the case of strong binding, ${\epsilon }_b\sim {\mathrm{\Delta }}_b$, a wave function description is not possible due to the retardation. Instead we appeal to the Bethe-Salpeter equation, written in terms of the electron-hole Green's function, to solve the problem. So far only the weak-binding regime has been achieved experimentally. Our analysis demonstrates that the strong-binding regime is also possible and we specify conditions in which it can be achieved for the prototypical example of biased bilayer graphene. The conditions concern the bias, the configuration of gates, and the substrate material. To verify the accuracy of our analysis we compare with available data for the weak-binding regime. We anticipate applying the developed dynamical screening Bethe-Salpeter techniques to various 2D materials with strong binding.

The layered honeycomb lattice iridate ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ is the closest realization of the Kitaev quantum spin liquid, primarily due to the enhanced interlayer separation and nearly ideal honeycomb lattice. We report pressure-induced structural evolution of ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ by powder x-ray diffraction (PXRD) up to $\sim 17$ GPa and Raman scattering measurements up to $\sim 25$ GPa. A structural phase transition (monoclinic $C2/c\to$ triclinic $P\overline{1}$) is observed with a broad mixed phase pressure range ($\sim 4$ to 15 GPa). The triclinic phase consists of heavily distorted honeycomb lattice with Ir-Ir dimer formation and a collapsed interlayer separation. In the stability range of the low-pressure monoclinic phase, structural evolution maintains the Kitaev configuration up to 4 GPa. This is supported by the observed enhanced magnetic frustration in dc susceptibility without emergence of any magnetic ordering and an enhanced dynamic Raman susceptibility. High-pressure resistance measurements up to 25 GPa in the temperature range 1.4–300 K show resilient nonmetallic $R\left(T\right)$ behavior with significantly reduced resistivity in the high-pressure phase. The Mott 3D variable-range-hopping conduction with much reduced characteristic energy scale $T_0$ suggests that the high-pressure phase is at the boundary of localized-itinerant crossover. First-principles density functional theoretical (DFT) analysis shows that monoclinic $P{2}_1/c$ phase of ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ is energetically lower than its $C2/c$ phase at ambient pressure and both the structures are consistent with experimental XRD pattern. Our analysis reveals structural transition from $P{2}_1/c$ to $P\overline{1}$ structure at 7 GPa in agreement with experiment and uncovers the interplay between oxidation states, spin, Ir bond dimerization and their relevance to electronic band gap.

Fundamental equalities, such as the Ward-Takahashi identity (WTI) and the fluctuation-dissipation theorem (FDT), are important in the calculation of the response functions, which are defined as the variations of physical quantities with respect to the external sources. In this paper, the formalism of calculating the response functions according to their original definitions is presented, based on the generalized $GW$ (GGW) method which was developed for the electronic systems including spin-dependent interaction. This formalism automatically ensures the FDT, and is theorectically proved to respect the WTI. By contrast, the commonly used random phase approximation (RPA) within the GGW method violates both the WTI and the FDT, and the Bethe-Salpeter equation (BSE) satisfies the WTI but does not fulfill the FDT. The validity of this methodology is demonstrated on the two-dimensional one-band Hubbard model, and the results show that our formalism makes significant improvements over the RPA formula. Due to the similar computational cost to the BSE, our formalism is expected to be applied to realistic materials.

Much attention has been focused on understanding the mechanism of low lattice thermal conductivity in Cu-based diamondlike thermoelectric compounds. For ${\mathrm{Cu}}_2\mathrm{Ge}{\mathrm{Se}}_3$, the underlying origin of low lattice thermal conductivity remains to be clarified. In this work, the first-principles calculations are employed to systematically investigate the effects of spin-orbit coupling, higher-order phonon-phonon scattering, phonon wavelike tunneling, temperature-induced renormalization of phonon and phonon-phonon interaction, and volumetric expanse on the phonon transport of ${\mathrm{Cu}}_2\mathrm{Ge}{\mathrm{Se}}_3$. We show that the spin-orbit coupling results in no detectable change on the phonon frequencies, compared with those obtained without spin-orbit coupling, while it induces slight increase in both Grüneisen parameter and lattice thermal conductivity of ${\mathrm{Cu}}_2\mathrm{Ge}{\mathrm{Se}}_3$, and the underlying mechanism is thoroughly analyzed. With the fourth-order phonon scattering and temperature-induced renormalization, the calculated lattice thermal conductivities are well consistent with the experimental results. Due to the enhanced four-phonon scattering process, there are remarkable reductions for the thermal conductivity. In ${\mathrm{Cu}}_2\mathrm{Ge}{\mathrm{Se}}_3$, the coherences term ${\kappa }_c$ contributes increasingly to the total lattice thermal conductivity when temperature arises. And after considering the effects of temperature-induced renormalization and higher-order phonon-phonon interactions, the ${\kappa }_c$ would provide $\sim 5\%$ and above 25% of total conductivity at 300 and 800 K, respectively. Our finding clarifies the mechanism of low thermal conductivity in ${\mathrm{Cu}}_2\mathrm{Ge}{\mathrm{Se}}_3$, and benefits the design of similar Cu-based diamondlike materials in thermoelectric applications.

Magnetocrystalline anisotropy is essential in the physics of antiferromagnets and commonly treated as a constant, not depending on an external magnetic field. However, we demonstrate that in CoO the anisotropy should necessarily depend on the magnetic field, which is shown by the spin Hall magnetoresistance of the CoO|Pt device. Below the Néel temperature CoO reveals a spin-flop transition at 240 K at 7.0 T, above which a hysteresis in the angular dependence of magnetoresistance unexpectedly persists up to 30 T. This behavior is shown to agree with the presence of the unquenched orbital momentum, which can play an important role in antiferromagnetic spintronics.