We report the observation of anomalous peak structures induced by hybridized magnon-phonon excitation (magnon polarons) in the magnetic field dependence of the spin Peltier effect (SPE) in a ${\mathrm{Lu}}_2{\mathrm{Bi}}_1{\mathrm{Fe}}_4{\mathrm{Ga}}_1{\mathrm{O}}_{12}$ (BiGa:LuIG) with Pt contact. The SPE peaks coincide with magnetic fields tuned to the threshold of magnon-polaron formation, consistent with the previous observation in the spin Seebeck effect. The enhancement of SPE is attributed to the lifetime increase in spin current caused by magnon-phonon hybridization in BiGa:LuIG.

We make testable predictions for the local two-particle spectral function of a disordered $s$-wave superconductor, probed by scanning Josephson spectroscopy (SJS), providing complementary information to scanning tunneling spectroscopy (STS). We show that SJS provides a direct map of the local superconducting order parameter that is found to be anticorrelated with the gap map obtained by STS. Furthermore, this anticorrelation increases with disorder. For the momentum-resolved spectral function, we find the Higgs mode separates from the continuum at arbitrarily small disorder and appears as a nondispersive subgap feature at low momenta, spectrally separated from phase modes for all disorder strengths. The amplitude-phase mixing remains small at low momenta even when disorder is large. Remarkably, even for large disorder and high momenta, the amplitude-phase mixing oscillates rapidly in frequency and hence does not significantly affect the purity of the Higgs and phase-dominated response functions.

We derive an $S=1$ spin polaron model which describes the motion of a single hole introduced into the $S=1$ spin antiferromagnetic ground state of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. We solve the model using the self-consistent Born approximation and show that its hole spectral function qualitatively agrees with the experimentally observed high-binding energy part of the ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ photoemission spectrum. We explain the observed peculiarities of the photoemission spectrum by linking them to two anisotropies present in the employed model: The spin anisotropy and the hopping anisotropy. We verify that these anisotropies, and not the possible differences between the ruthenate ($S=1$) and the cuprate ($S=\frac{1}{2}$) spin polaron models, are responsible for the strong qualitative differences between the photoemission spectrum of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ and of the undoped cuprates.

The study of (quantum) phase transitions in heavy fermion compounds relies on a detailed understanding of the microscopic control parameters that induce them. While the influence of external pressure is rather straightforward, atomic substitutions are more involved. Nonetheless, replacing an elemental constituent of a compound with an isovalent atom is, effects of disorder aside, often viewed as merely affecting the lattice constant. Based on this picture of chemical pressure, the unit-cell volume is identified as an empirical proxy for the Kondo coupling. Here, instead, we propose an “orbital scenario” in which the coupling in complex systems can be tuned by isoelectronic substitutions with little or no effect onto cohesive properties. Starting with the Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$, we consider, within band theory, isoelectronic substitutions of the pnictogen ($\mathrm{Bi}\to \mathrm{Sb}$) and/or the precious metal ($\mathrm{Pt}\to \mathrm{Pd}$). We show for the isovolume series ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_3$ that the Kondo coupling is in fact substantially modified by the different radial extent of the $5d$ (Pt) and $4d$ (Pd) orbitals, while spin-orbit coupling mediated changes are minute. Combining experimental Kondo temperatures with simulated hybridization functions, we also predict effective masses $m^{*}$, finding excellent agreement with many-body results for ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pt}}_3$. Our analysis motivates studying the so-far unknown Kondo insulator ${\mathrm{Ce}}_3{\mathrm{Sb}}_4{\mathrm{Pd}}_3$, for which we predict $m^{*}/m_{\mathrm{band}}=O\left(10\right)$.

AB-stacked bilayer graphene has massive electron and holelike excitations with zero gap in the nearest-neighbor hopping approximation. In equilibrium, the quasiparticle occupation approximately follows the usual Fermi-Dirac distribution. In this paper we consider perturbing this equilibrium distribution so as to determine DC transport coefficients near charge neutrality. We consider the regime $\beta \left|\mu \right|\lesssim 1$ (with $\beta$ the inverse temperature and $\mu$ the chemical potential) where there is not a well-formed Fermi surface. Starting from the Kadanoff-Baym equations, we obtain the quantum Boltzmann equation of the electron and hole distribution functions when the system is weakly perturbed out of equilibrium. The effects of phonons, disorder, and boundary scattering for finite-sized systems are incorporated through a generalized collision integral. The transport coefficients, including the electrical and thermal conductivity, thermopower, and shear viscosity, are calculated in the linear response regime. We also extend the formalism to include an external magnetic field. We present results from numerical solutions of the quantum Boltzmann equation. Finally, we derive a simplified two-fluid hydrodynamic model appropriate for this system, which reproduces the salient results of the full numerical calculations.

We explore generic “unnecessary” quantum critical points with minimal degrees of freedom. These quantum critical points can be avoided with strong enough symmetry-allowed deformations of the Hamiltonian, but these deformations are irrelevant perturbations below certain threshold at the quantum critical point. These quantum critical points are hence unnecessary, but also unfine-tuned (generic). The previously known examples of such generic unnecessary quantum critical points involve at least eight Dirac fermions in both two and three spatial dimensions. In this work we seek for examples of generic unnecessary quantum critical points with minimal degrees of freedom. In particular, in three-dimensional space, we identify two examples of such generic unnecessary quantum critical points. The first example occurs in a $3d$ interacting topological insulator, and it is described by two $(3+1)d$ massless Dirac fermions in the infrared limit; the second example occurs in a $3d$ topological superconductor, and it is formally described by only one $(3+1)d$ massless Dirac fermion.

The orbital degree of freedom plays a critical role in contemporary condensed-matter physics, with recent discoveries of orbital-dependent many-body effects including electron correlation and superconductivity. Nevertheless, these orbital-dependent many-body effects have mainly been observed in bulk materials, wherein the electrons' orbital attribute is challenging to tailor selectively. Here, by planar tunneling into the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface, we observed a Kondo resonance, a model many-body phenomenon. The observation of the Kondo resonance provides an ideal opportunity to quantitatively exploit the emerging interfacial Kondo physics. Furthermore, it was found that the Kondo coupling strength and resonance line shape can be effectively regulated by electrostatic gating and show a sharp transition at the Lifshitz point, where the orbital occupancy of the 2DES changes from only $d_{\mathrm{xy}}$ to both $d_{\mathrm{xy}}$ and $d_{\mathrm{xz}/\mathrm{yz}}$. This unusual behavior is attributed to the orbital selectivity effect in this unique multiple-$d$-band system, wherein the itinerant $d_{\mathrm{xy}}$ and $d_{\mathrm{xz}/\mathrm{yz}}$ electrons have different orbital symmetries and therefore different Kondo exchange coupling with the localized $d_{\mathrm{xy}}$ electrons. The present study may pave the way for manipulating many-body effects in 2D multiband materials via orbital engineering of itinerant electrons.

We study the effective symmetry profiles of superconducting pair correlations and the flow of charge supercurrent in ballistic Weyl semimetal systems with a tilted dispersion relation. Utilizing a microscopic method in the ballistic regime and starting from both opposite-pseudospin (the band degree of freedom) and equal-pseudospin phonon-mediated opposite-spin electron-electron couplings, we calculate the anomalous Green's function to study various superconducting pair correlations that Weyl semimetal systems may develop. The momentum-space profile reveals that by properly manipulating the parameters of Weyl semimetal systems, including the tilting parameter, the effective symmetry class of even-parity $s$-wave (odd-parity $p$-wave) superconducting correlations can be converted into a $d$-wave ($f$-wave) symmetry class that consists of equal-pseudospin and opposite-pseudospin channels. We also find that the supercurrent in a ballistic Weyl Josephson junction can be made to vanish or switch directions, depending on the tilt of the Weyl cones, in addition to the relevant parameters characterizing the Weyl semimetal and junction. We show that inversion-symmetry-breaking terms introduce transitions that result in the appearance of self-biased current at zero difference between the macroscopic phases of the superconducting segments, creating a ${\phi }_0$ Josephson state. Weyl semimetal systems are shown to offer several experimentally tunable parameters to control the induction of higher harmonics into the current phase relations.

We have studied nonequilibrium dynamics of the one-dimensional Hubbard model using the generalized hydrodynamic theory. We mainly investigated the spatiotemporal profile of particle density (equivalent to charge density), energy density, and their currents using the partitioning protocol; the initial state consists of two semi-infinite different thermal equilibrium states joined at the origin. In this protocol, a transient region where currents flow appears around the origin, and this region expands its size linearly with time. We examined how density and current profiles depend on initial conditions. Inside the transient region, we have found a clogged region where charge current is zero but nonvanishing energy current flows. This phenomenon is similar to spin-charge separation in Tomonaga-Luttinger liquids, in which spin and charge excitations propagate with different velocities. This region appears when one of the initial states has half-filled electron density, and it is located adjacent to this initial state. We have proved analytically the existence of the clogged region in the infinite temperature case of the half-filled initial state. The existence is confirmed also for finite temperatures by numerical calculations of generalized hydrodynamics. A similar analytical proof is also given for a clogged region of spin current when magnetic field is applied to one and only one of the two initial states. A universal proportionality of charge and spin currents is also proved for a special region, for general initial conditions of electron density and magnetic field. To examine the clogged region of charge current, we have calculated the current contributions of different types of quasiparticles in the Hubbard model. It is found that the charge current component carried by scattering states (i.e., Fermi liquid type) is canceled completely in the clogged region by the counter flows carried by bound-state quasiparticles (called $k\text{−}\mathrm{\Lambda }$ strings). Their contributions are not canceled for energy current, which is also proved analytically for special cases, and these different behaviors are the origin of the clogged region. Except for the clogged region, charge and energy densities are in good proportion to each other, and their ratio depends on the initial conditions. This proportionality, in nonequilibrium dynamics, is reminiscent of Wiedemann-Franz law in thermal equilibrium, which states current proportionality determined by temperature. The long-time stationary values of charge and energy currents were also studied with varying initial conditions. We have compared the results with the values of noninteracting systems and discussed the effects of electron correlations. The ratio of these stationary currents is also analyzed. We have found that the temperature dependence of the ratio is strongly suppressed by electron correlations and even reversed.

Motivated by the realization of spin-valley Hubbard model on a triangular moiré superlattice in ABC trilayer graphene aligned with hexagon boron nitride (hBN) and possibly also in twisted transition metal dichalcogenide homobilayers, we study possible Mott insulating phases and pseudogap metals based on symmetry constraint and parton mean field theories. First, we show that the Luttinger constraint allows two distinct symmetric and featureless Fermi liquids when there is an intervalley Hund's term breaking $\text{SU}\left(4\right)$ spin rotation. Especially, there exists a symmetric and featureless “pseudogap metal” with small Fermi surfaces. Then, we suggest to search for such an unconventional metallic state by doping the Mott insulator at ${\nu }_T=2$. For this purpose, we study the ${\nu }_T=2$ Mott insulator using the $\text{SO}\left(6\right)$ Schwinger boson or Schwinger fermion parton. At the $\text{SU}\left(4\right)$-symmetric point, we find two symmetric $Z_2$ spin liquids. With a large anti-intervalley Hund's term, a featureless Mott insulator is natural. Next, we show that doping the featureless Mott insulator or a $Z_2$ spin liquid can lead to featureless or orthogonal “pseudogap metal” with small Fermi surfaces proportional to the doping. Besides, we also provide one scenario for the evolution from pseudogap metal to the conventional Fermi liquid through an intermediate exotic “deconfined metal” phase. Last, we give brief comments on the possibility of $\text{U}\left(1\right)$ spinon Fermi surface state or $Z_4$ spin liquid at ${\nu }_T=1$.

The layered semiconductor InSe has a wide range of attractive electronic and optoelectronic properties, in which the effective mass of the charge carriers plays a key role. Here, we study from first principles the many-body renormalization of the electron effective mass in $\gamma$-InSe, taking into account the effects of both electron-electron and electron-phonon interactions. Electron-electron interaction, treated within the many-body $GW$ approximation, leads to around 15% of the increase in the in-plane effective mass over the result from density functional theory, and a more than threefold increase in the out-of-plane electron effective mass. The surprisingly large directional anisotropy in the mass renormalization is explained in terms of the symmetries of band-edge wave functions. The mass enhancement induced by electron-phonon interactions, which we find to mainly originate from Fröhlich electron-phonon coupling, is less than 10% at room temperature, indicating weak polaronic effect. After including the many-body renormalization effects, the calculated electron effective masses of InSe are 0.12 and 0.09 in the in-plane and out-of-plane directions, respectively.

We report on a study of the nontrivial Berry phase in superconducting multiterminal quantum dots biased at commensurate voltages. Starting with the time-periodic Bogoliubov–de Gennes equations, we obtain a tight-binding model in Floquet space, and we solve these equations in the semiclassical limit. We observe that the parameter space defined by the contact transparencies and quartet phase splits into two components with a nontrivial Berry phase. We use the Bohr-Sommerfeld quantization to calculate the Berry phase. We find that if the quantum dot level sits at zero energy, then the Berry phase takes the values ${\phi }_B=0$ or ${\phi }_B=\pi$. We demonstrate that this nontrivial Berry phase can be observed by tunneling spectroscopy in the Floquet spectra. Consequently, the Floquet-Wannier-Stark ladder spectra of superconducting multiterminal quantum dots are shifted by half-a-period if ${\phi }_B=\pi$. Our numerical calculations based on the Keldysh Green's functions show that this Berry phase spectral shift can be observed from the quantum dot tunneling density of states.

We show that in a model of a metal photoexcited by a transient pump pulse resonant with a phonon mode, the absolute dc conductivity may become negative, depending on the interplay between the electronic structure, the phonon frequency, and the pump intensity. The analysis includes the effects of inelastic scattering and thermal relaxation. Results for the time evolution of the negative conductivity state are presented; the associated nonequilibrium physics may persist for long times after the pulse. Our findings provide a theoretical justification for previously proposed phenomenology and indicate new routes to the generation and exploration of intrinsically nonequilibrium states.

The traditional atomistic Green's function (AGF) was formulated in the harmonic regime, preventing it from capturing the role of anharmonicity in interfacial thermal transport. Incorporating anharmonicity into the AGF has long been desired but remains challenging. In this Rapid Communication, we developed an anharmonic AGF model to incorporate anharmonicity at interfaces in three-dimensional structures rigorously. Without any fitting parameters, we can obtain the thermal interface conductance using first-principles force constants as the only input. This Rapid Communication represents a significant step forward for the AGF.

In this work, we predict a hinged quantum spin Hall (HQSH) effect featured by a pair of counterpropagating chiral hinge modes in antiferromagnetic (AFM) topological insulator (TI) multilayers. This pair of chiral hinge modes are localized on the hinges of the top and bottom surfaces of the AFM TI multilayers. Unlike the conventional QSH effect, the HQSH effect survives the breaking of time-reversal symmetry (TRS) and thus represents a different kind of topological phenomenon. The pair of counterpropagating chiral hinge modes are sustainable to inelastic scattering and TRS-breaking disorder, which can be observed in macroscopic samples. We show that this HQSH effect can be understood as a three-dimensional generalization of the Su-Schrieffer-Heeger model. At last, we propose that the HQSH effect can be realized in newly found intrinsic AFM TI materials $({\mathrm{MnBi}}_2{\mathrm{Te}}_4{)}_m{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ or magnetic-doped TI multilayers by current experimental setups.

We study the Néel to fourfold columnar valence bond solid (cVBS) quantum phase transition in a sign-free $S=1$ square-lattice model. This is the same kind of transition that for $S=1/2$ has been argued to realize the prototypical deconfined critical point. Extensive numerical simulations of the square-lattice $S=1/2$ Néel-VBS transition have found consistency with the deconfined critical point scenario with no direct evidence for first-order behavior. In contrast to the $S=1/2$ case, in our quantum Monte Carlo simulations for the $S=1$ model, we present unambiguous evidence for a direct conventional first-order quantum phase transition. Classic signs of a first-order transition demonstrating coexistence including double-peaked histograms and switching behavior are observed. The sharp contrast from the $S=1/2$ case is remarkable; we hypothesize that this is a striking demonstration of the role of the size of the quantum spin in the phase diagram of two-dimensional lattice models.

The existence of the Nagaoka ferromagnetism is examined in the context of the one-dimensional $U=\infty$ Hubbard model. We construct the exact quantum partition function to describe the physics of such a regime. Our calculation reveals that, while the ground state in an open chain is always spin degenerate, in a finite size closed chain with at least one vacancy, the ground state can only be ferromagnetic when the number of electrons is less than or equal to three. Our results shed more light on a very recent experimental verification of Nagaoka ferromagnetism in a quantum dot setup.

In this research, we demonstrate the large spin and valley splittings in Janus monolayer WSSe brought by the interaction with a magnetic substrate. The MnO(111) substrate plays the role of ferromagnet since the importance lies in the magnetism of the surface Mn atoms. The responses of both in-plane and out-of-plane spin splittings with the magnetic direction of the substrate are investigated. It is indicated that a large valley splitting of about 410 meV at the $K$ and $K^{\prime }$ points can be produced by the magnetic substrate, which is positively correlated with the vertical magnetic moments of the surface Mn atoms. In addition, a helical gap is opened and the Rashba spin-orbit coupling induced dispersion relation is also significantly modified under the action of proximity coupling with the substrate. Through the effective Hamiltonian method, the derived effective Zeeman fields from the $K/K^{\prime }$ point and the $\mathrm{\Gamma }$ point are greatly different, due to the combination with other factors, such as the difference of the $g$ factor, the existence of charge transfer, or electrostatic interaction with the substrate. The greatly manipulated asymmetric electronic structure and spin/valley splittings can be realized in the spin- and valley-polarized transmission, which are of some significance for the realization of spintronic and valleytronic devices using magnetic substrates and Janus two-dimensional materials.

We study the electronic and structural properties of the low-temperature ordered phase of hydrogen-bonded molecular conductors, $\kappa -{\mathrm{D}}_3{\left(\text{Cat-EDT-TTF}\right)}_2$ and its selenium-substituted analog $\kappa -{\mathrm{D}}_3{\left(\text{Cat-EDT-ST}\right)}_2$, by means of first-principles density functional theory (DFT) calculations. In these compounds, the charge ordering in the $\pi$-electron system is coupled with the ordering of the displacements in the deuteriums forming the hydrogen bond, equally shared by two oxygens in the high-temperature phase. While the structural optimization within the standard DFT method based on the generalized gradient approximation fails to reproduce the structural stability of the charge-ordered phase, we show that a hybrid functional of Heyd, Scuseria, and Ernzerhof can reproduce structural characters of the charge-ordered phase owing to the more localized nature of the wave functions. Furthermore, using the ability of the hybrid functional to predict the electronic and structural properties, we find a stable noncentrosymmetric charge-ordered phase with another pattern of deuterium ordering.