Many complex crystals show a flat or even increasing lattice thermal conductivity at high temperatures, which deviates from the traditional 1/$T$ decay trend given by conventional phonon theory. In this paper, we predict the thermal conductivity of ${\mathrm{Al}}_2{\mathrm{O}}_3$ that matches with experimental data from room temperature to near melting point (2200 K). The lattice thermal conductivity is found to be composed of contributions of phonon, diffuson, and radiation. Phonon particle thermal conductivity decays approximately as $\sim T^{-1.14}$ after considering four-phonon (4ph) scattering as well as finite-temperature corrections to the lattice constant and harmonic and anharmonic force constants (AFCs). Diffuson (interband tunneling) thermal conductivity increases roughly as $\sim T^{0.43}$. Radiation thermal conductivity increases as $\sim T^{2.51}$, being slightly smaller than $\sim T^3$ due to the increase of phonon linewidth with temperature, which increases photon extinction coefficient and reduces photon mean free path (MFP). At room temperature, phonon, diffuson, and radiation contribute ∼99, 1, and 0%, respectively. At 2200 K, their contributions change to 61, 20, and 19%, respectively. 4ph scattering is important at ultrahigh temperatures, decreasing the phonon thermal conductivity by a maximum of 24%. The finite-temperature softening effects of the harmonic and AFCs increase the phonon thermal conductivity by a maximum of 36% at ultrahigh temperatures. We also verify that Green-Kubo molecular dynamics can capture both the particle and wave nature of phonons, like the Wigner formalism. At ultrahigh temperatures, the photon MFP is found to be on the order of 100 nm, which should be considered for experimental measurement of thin films. In this paper, we aim to enhance the understanding of lattice thermal conductivity in complex crystals at ultrahigh temperatures, potentially spurring further exploration of materials suitable for such extreme conditions.

In the past few decades, two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as ${\mathrm{MoS}}_2$ and ${\mathrm{WTe}}_2$, have attracted intensive research due to their unique crystal structure, interesting physical properties, and potential applications in various fields. However, 2D transition metal dioxides with the same crystal structure as TMDCs are much less studied both theoretically and experimentally. Here, we present a systematic theoretical study on the 2D ${\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$ by using the first-principles calculations. It is found that the monolayer ${\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$ both have a 1${\mathrm{T}}^{\prime }$ crystal structure and their stabilities are confirmed by the phonon dispersions and molecular dynamics simulations. The hybrid functional calculations show that both materials are nonmagnetic semiconductors with moderate bandgaps of 0.64 and 0.69 eV for monolayer $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$, respectively. When stacking monolayer $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$ up into the three-dimensional structures, two stable ${\mathrm{T}}_{\mathrm{d}}$ and 2M phases can be formed, which are also nonmagnetic semiconductors. Interestingly, the ${\mathrm{T}}_{\mathrm{d}}$ phase lacks the inversion symmetry, which possesses sliding ferroelectricity. The calculated spontaneous out-of-plane polarization is 0.16 and 0.33 $\mu \mathrm{C}/{\mathrm{cm}}^2$ for ${\mathrm{T}}_{\mathrm{d}}\text{−}{\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$, respectively. Our work clarifies the crystal structure and electronic properties of 2D ${\mathrm{RuO}}_2$ and ${\mathrm{OsO}}_2$ and could be helpful for more experimental studies on both materials.

In this paper, we study the near-field radiative energy, linear-momentum, and angular-momentum transfer from a current-biased graphene to nanoparticles. The electric current through the graphene sheet induces nonequilibrium fluctuations, causing energy and momentum transfer even in the absence of a temperature difference. The inherent spin-momentum locking of graphene surface plasmons leads to an in-plane torque perpendicular to the direction of the electric current. In the presence of a temperature difference, the energy transfer is greatly enhanced while the lateral force and torque remain within the same order. Our work explores the potential of utilizing current-biased graphene to manipulate nanoparticles.

Conventional photoemission orbital tomography based on the Fourier iterative method enables us to extract a projected two-dimensional (2D) molecular orbital from a 2D photoelectron momentum map (PMM) of planar $\pi$-conjugated molecules in a single-orientation system but not in a multiorientation system. In this work, we demonstrate photoemission orbital tomography for $\pi$-conjugated molecules with a tight-binding ansatz using an atomic orbital basis. We analyze 2D PMMs of single-orientation pentacene/Ag(110) and multiorientation 3,4,9,10-perylenetetracarboxylic dianhydride/Ag(110) and reproduce their three-dimensional highest occupied molecular orbitals. We demonstrate that the PhaseLift algorithm can be used to analyze PMMs including experimental or theoretical uncertainties. With the 2D PMM for pentacene, we simultaneously optimize the structure and the molecular orbital. The present approach enables us to extract the three-dimensional orbitals and structures of existing materials.

Ultraflat bands have already been detected in twisted bilayer graphene and twisted bilayer transition-metal dichalcogenides, which provide a platform to investigate strong correlations. In this paper, the electronic properties of twisted trilayer molybdenum disulfide (TTM) are investigated via an accurate tight-binding Hamiltonian. We find that the highest valence bands are derived from the $\mathrm{\Gamma }$-point of the constituent monolayer, and they exhibit a graphenelike dispersion or become isolated flat bands that are dependent on the starting stacking arrangements. The lattice relaxation, local deformation, and external fields can significantly tune the electronic structures of TTM. After introducing the spin-orbital coupling effect, we find a spin-valley-layer locking effect at the minimum of the conduction band at the $K$- and $K^{\prime }$-point of the Brillouin zone, which may provide a platform to study optical properties and magnetoelectric effects.

In an attempt to understand the density-density response of the cuprate superconductors, we study plasmons in a layered strange metal using the Gubser-Rocha model. The latter is a well-known bottom-up holographic model for a strange metal that is used here to describe the strongly repulsive on-site interactions between the electrons in each copper-oxide $\left({\mathrm{CuO}}_2\right)$ layer, whereas the long-range Coulomb interactions are incorporated by a so-called double-trace deformation. To be able to model the bilayer cuprates more realistically, we consider in particular the case of two closely spaced ${\mathrm{CuO}}_2$ layers per unit cell. In the response we then obtain for vanishing out-of-plane momentum both an optical and an acoustic plasmon, whereas for nonvanishing out-of-plane momentum there are two acoustic plasmon modes. We present the full density-density spectral functions with parameters typical for cuprates and discuss both the dispersion and the lifetime of these plasmon excitations. Moreover, we compute the conductivity after introducing disorder into the system. Finally, we also compute the loss function to facilitate a comparison with experimental results from electron energy-loss spectroscopy.

Néel spin current is defined as the staggered spin current across different magnetic sublattices and can manipulate the Néel vector of the antiferromagnet by its associated spin-transfer torques. Currently, it is believed that Néel spin currents are only generated through electrical driving in $\mathcal{PT}$-symmetric antiferromagnetic metals, which is a linear effect. In this paper, we propose that Néel spin currents can be induced through the nonlinear optical effect (spin photogalvanic effect) in $\mathcal{PT}$-symmetric antiferromagnetic semiconductors. Using CrSBr and ${\mathrm{CrI}}_3$ as representatives, we predict that light can generate Néel spin current in antiferromagnetic semiconductors, with both staggered and antiparallel spin currents distributed across different magnetic sublattices, depending on the $\mathcal{PT}$ symmetries of their corresponding spin photogalvanic coefficients. We show that the origin of the diverse manifestations of light-assisted Néel spin currents lies in the nonlinearity of the spin photogalvanic effect, resulting in the current not necessarily being the staggered spin current. Our work proposes a nonlinear effect that may have tremendous application potential in future antiferromagnetic devices.

In the field of materials research, machine learning (ML) techniques have emerged as indispensable tools. However, the opaqueness in decision making by models can compromise the trustworthiness of results, underscoring the crucial need for model interpretability. Explainable machine learning (XML) strives to augment researchers’ comprehension of material properties and performance. Yet, reliance on high-quality datasets and scarcity of prior knowledge pose challenges for XML research, particularly when dealing with smaller datasets. In this study, using spinel as a representative example, we successfully addressed the data challenges in XML through a cationic perturbation strategy. We demonstrate an effective approach for handling information scarcity in small datasets, thus offering a feasible method for material research and broadening the scope of XML applications in materials science. Furthermore, our investigation successfully uncovered potential causal relationships underlying material properties and validated their consistency with physical cognition. These causal relationships can serve as experimental guides, facilitating the design and optimization of new materials. Consequently, this research holds significant scientific merit in advancing XML in the realm of materials science, while providing profound insights into material properties and fostering the development of reliable ML-based materials research.

We investigate the mesoscopic transport through a twisted bilayer graphene (TBG) consisting of a clean graphene nanoribbon on the bottom and a disordered graphene disk on the top. We show that, with strong top-layer disorder, the transmission through such a device shows a sequence of resonant peaks with respect to the rotation angle $\theta$, where at the resonant angles ${\theta }_c$ the disk region contains one giant hexagonal moiré supercell. A further investigation shows that the value of ${\theta }_c$ shows negligible dependence on the disorder strength, the Fermi energy, and the shape distortion, indicating the resonance is a robust commensuration feature of the moiré supercell. We explain this commensuration resonance based on the bound states formed inside TBG disk as a result of the nonuniform interlayer coupling, with their average local density of states dominating at the AA stacking region while minimizing at the AB/BA stacking regions. By increasing the interlayer distance, the peak becomes less pronounced which further confirms the role of interlayer coupling. The results presented here suggest a new mechanism to tune the quantum transport signal through the twist angle in disordered moiré systems.

As elementary particles carry energy and momentum in the Universe, quasiparticles are the elementary carriers of energy and momentum quanta in condensed matter. And, as elementary particles, under certain conditions quasiparticles can be unstable and decay, emitting pairs of less energetic ones. Pitaevskii [Sov. Phys. JETP 9, 830 (1959)] proposed that such processes exist in superfluid helium, a quantum fluid where the very concept of quasiparticles was borne by Landau and which presented the first notable success of that concept. Pitaevskii's decays have important consequences, including the possible breakdown of a quasiparticle [M. B. Stone et al., Nature (London) 440, 187 (2006)]. Here, we present neutron scattering experiments, which provide evidence that such decays explain the collapsing lifetime (strong damping) of higher-energy phonon-roton sound-wave quasiparticles in superfluid helium. This damping develops when helium is pressurized towards crystallization, or warmed towards approaching the superfluid transition. Our results resolve a number of puzzles raised by previous experiments and reveal the ubiquity of quasiparticle decays and their importance for understanding quantum matter.

We present a detailed study of the electronic and magnetic ground-state properties of ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ using a combination of macroscopic and microscopic experimental techniques. From these experiments we quantitatively characterize the metal-insulator transition and magnetic/nonmagnetic quantum phase transition occurring at low-doping levels in ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$. From our study, we find a surprising closeness of the critical composition of the metal-insulator transition at $x_{\mathrm{MIT}}=0.014$ and the quantum phase transition at $x_{\mathrm{LRO}}\sim 0.024-0.031$. This suggests that these effects are cooperative and depend on each other.

We report the effect of Ir doping on the anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) in ${\mathrm{Fe}}_3\mathrm{Co}$ single-crystal thin films. ${\left({\mathrm{Fe}}_3\mathrm{Co}\right)}_{100\text{--}x}{\mathrm{Ir}}_x$ $(x\le 12\%)$ composition-spread thin films are fabricated on MgO(100) substrates by a combinatorial sputtering technique for a high-throughput and systematic characterization. From a structural analysis using x-ray diffraction, the films are grown epitaxially on the substrates and B2-ordered phase is detected from $x=7.3\%$ to 12%. The compositional dependence of the AHE, magnetoresistance, ANE, and Seebeck effect is measured to obtain anomalous Hall resistivity $\left({\rho }_{yx}^A\right)$, longitudinal resistivity $\left({\rho }_{xx}\right)$, anomalous Nernst coefficient $\left(S_{\mathrm{ANE}}\right)$, and Seebeck coefficient $\left(S_{\mathrm{SE}}\right)$, respectively. From these transport measurements, we calculate anomalous Nernst conductivity $\left({\alpha }_{xy}^A\right)$. A large enhancement of the ${\rho }_{yx}^A$ value is observed upon Ir doping by a factor of ≈ 9.2 for $x=12\%$ at 300 K. By employing a scaling analysis to the AHE results, extrinsic contribution is found to be increased after Ir doping in the low-Ir concentration regime, while intrinsic contribution becomes more dominant in the high-Ir concentration regime. On the other hand, unlike the ${\rho }_{yx}^A$, the $S_{\mathrm{ANE}}$ does not show significant compositional dependence. We find that AHE-related contribution $(-S_{\mathrm{SE}}{\rho }_{yx}^A/{\rho }_{xx})$ is dominant to the $S_{\mathrm{ANE}}$ compared to the contribution from direct conversion of a temperature gradient to a transverse charge current by anomalous Nernst conductivity $\left({\alpha }_{xy}^A{\rho }_{xx}\right)$. We also find that the ${\alpha }_{xy}^A$ value sharply changes from a positive to negative value immediately after Ir doping, showing a negative maximum at $x\approx 1\%$, followed by a decrease in magnitude to almost zero for $x=12\%$. Based on the Mott's relation and the scaling analysis of the AHE, we suggest a possible connection between the ${\alpha }_{xy}^A$ and the extrinsic contribution of the AHE. These experimental results and analysis will provide insights into the relationship between the AHE and ANE in ferromagnetic alloys upon heavy metal doping.

Materials featuring the kagome configuration hold significant research potential following their discovery. Drawing inspiration from ${\mathrm{MgB}}_2$, we introduce a kagome topological superconducting family: ${X\mathrm{B}}_3$ $(X=\mathrm{Ni}, \mathrm{Pd})$. This prediction is based on first-principles calculations and an analysis of bulk-edge correspondence. In this family, the boron atoms transition from their original honeycomb arrangement to form a stable kagome lattice, as confirmed by the phonon spectrum analysis. Through the calculations of the topological invariant and surface states, we establish that these materials belong to the ${\mathbb{Z}}_2$ Dirac metal category with nodal surface characteristics. Employing the Allen-Dynes modified McMillan formula, we anticipate superconducting critical temperatures, denoted as $T_c$, of 14.5 K for ${\mathrm{NiB}}_3$ and 19.1 K for ${\mathrm{PdB}}_3$. These values surpass most of both conventional topological superconductors' candidates and kagome superconductors due to the relatively robust electron-phonon coupling. Moreover, the spin texture analysis of the Dirac cone at the high symmetry point $K$ lends further support to their status as topological superconductors. Consequently, the ${X\mathrm{B}}_3$ family presents an exciting platform for experimental investigations into topological superconductors and their potential applications in fault-tolerant quantum computation.

A two-component quasi-two-dimensional superconductor with Dzyaloshinsky-Moriya interaction is studied based on the Ginzburg-Landau and Bogoliubov-de Gennes theories. Under external in-plane magnetic fields, the order parameter of the superconducting state is a type of the Fulde-Ferrell states with a finite momentum of Cooper pairs due to the Dzyaloshinsky-Moriya interaction. It is shown that the superconducting diode effect can emerge when a supercurrent flows parallel to the external magnetic field, characteristic of chiral crystals. In the Bogoliubov-de Gennes theory, phase diagrams associated with the transition of the Cooper-pair momentum and the Josephson phase between spin-singlet and spin-triplet Cooper pairs are derived, and a close relationship with the diode quality factor is demonstrated. Implications of critical currents in the aspect of thermodynamics are also discussed. Based on such an argument, it is argued that the first-order phase transition in terms of Cooper-pair momentum and the coexistence of phases with different Cooper-pair momentum and Josephson phase can occur. The argument also implies the issue with the definition of critical currents calculated from the extremes of the supercurrent when metastable states exist. Comments on purely two-dimensional superconductors are also given.

${\mathrm{KMnF}}_3$ is one of the first systems in which a dynamic “central peak” was observed by inelastic neutron scattering in the vicinity of a structural phase transition. The origin of this peak, however, whether intrinsic or extrinsic, was never fully resolved. In the present paper, we report combined dielectric and ultrasonic results on ${\mathrm{KMnF}}_3$, pure and lightly doped with lithium, which provide direct evidence for the presence of intrinsic “heterophase (cubic-tetragonal) oscillations” near the transition. These oscillations are seen as a “second sound” wave, akin to the density wave observed in liquid helium near the superfluidity transition.

We investigate the ground-state properties of the bipolaron coupled to quantum dispersive optical phonons in the one-dimensional Holstein-Hubbard model. We concentrate on the interplay between the phonon dispersion and the Coulomb repulsion and their mutual effect on the bipolaron effective mass, the binding energy, and the phase diagram. Most surprisingly, the sign of the curvature of the optical phonon dispersion plays a decisive role in the bipolaron binding energy in the presence of the Coulomb repulsion $U$. In particular, when the sign of the phonon dispersion curvature matches the sign of the electron dispersion curvature, the bipolaron remains bound in the strong-coupling limit even when $U\to \infty$ and the binding emanates from the exchange of phonons between two electrons residing on adjacent sites. At moderate electron-phonon coupling a light bipolaron exists up to large values of $U$. Finally, an intuitive explanation of the role of the phonon dispersion on the bipolaron binding energy is derived using the strong-coupling limit where the binding emanates from the exchange of phonons between two electrons residing on adjacent sites which leads to enhanced stability of bipolarons at elevated Coulomb repulsion.

Using first-principle calculations, we investigate the electronic, topological, and superconducting properties of ${\mathrm{Nb}}_{3}X$ $(X=\mathrm{Ge}, \mathrm{Sn}, \mathrm{Sb})$ and ${\mathrm{Ta}}_{3}Y$ $(Y=\mathrm{As}, \mathrm{Sb}, \mathrm{Bi})$ A15 compounds. We demonstrate that these compounds host Dirac surface states, which are related to a nontrivial ${\mathbb{Z}}_2$ topological value. The spin-orbit coupling (SOC) splits the highly degenerate R point close to the Fermi level enhancing the amplitude of the spin Hall conductance. Indeed, despite the moderate spin orbit of the Nb-compounds, a large spin Hall effect is also obtained in ${\mathrm{Nb}}_3\mathrm{Ge}$ and ${\mathrm{Nb}}_3\mathrm{Sn}$ compounds. We show that the Coulomb interaction opens the gap at the R point thus making the occurrence of Dirac surface states more obvious. We then investigate the superconducting properties by determining the strength of the electron-phonon BCS coupling. The evolution of the critical temperature is tracked down to the 2D limit indicating a reduction of the transition temperature, which mainly arises from the suppression of the density of states at the Fermi level. Finally, we propose a minimal tight-binding model based on three coupled Su-Schrieffer-Heeger chains with ${\mathrm{t}}_{2g}$ Ta and Nb orbitals reproducing the spin-orbit splittings at the R point among the $\pi$-bond bands in this class of compounds. We separate the kinetic parameters in $\pi$ and $\delta$ bonds, in intradimer and interdimer hoppings, and discuss their relevance for the topological electronic structure. We point out that ${\mathrm{Nb}}_3\mathrm{Ge}$ might represent a ${\mathbb{Z}}_2$ topological metal with the highest superconducting temperature ever recorded.

The $GW$ self-energy may become computationally challenging to evaluate because of frequency and momentum convolutions. These difficulties were recently addressed by the development of the multipole approximation (MPA) and the W-av methods: MPA accurately approximates full-frequency response functions using a small number of poles, while W-av improves the convergence with respect to the $\mathbf{k}$-point sampling in 2D materials. In this work, we $\left(i\right)$ present a theoretical scheme to combine them, and $\left(ii\right)$ apply the newly developed approach to the paradigmatic case of graphene. Our findings show an excellent agreement of the calculated QP band structure with angle resolved photoemission spectroscopy (ARPES) data. Furthermore, the computational efficiency of MPA and W-av allows us to explore the logarithmic renormalization of the Dirac cone. To this aim, we develop an analytical model, derived from a Dirac Hamiltonian, that we parametrize using ab initio data. The comparison of the models obtained with the plasmon pole approximation (PPA) and MPA results highlights an important role of the dynamical screening in the cone renormalization.

In a milestone paper [F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)], Haldane elaborated a model of graphene within the time-reversal symmetry breaking is achieved by next-nearest-neighbors imaginary counter-rotating hopping, hence conferring topological properties. In recent years, the time-reversal symmetry turned out to be broken also by light irradiation in so-called Floquet topological insulators (FTIs). On the other hand, Kane and Mele introduced a spin-orbit coupling (SOC) model [C. L. Kane et al., Phys. Rev. Lett. 95, 226801 (2005)] inspired by the Haldane's mechanism. In this paper, we present the topological properties of a FTI possessing SOC, using graphene as the playground. It was found that the interplay between sublattice subspace and the spin one triggers interesting topological phase transitions. Basically, in a FTI with SOC, two topological phases may be excited: charge quantum Hall effect (CQHE) and, respectively, spin quantum Hall effect (SQHE) phases. Also, it was demonstrated that the CQHE and SQHE coexistence is forbidden by the topology of the system. As well, it was identified a special driving regime of spin filter (SF), in which only one spin state is topological and, consequently, will be filtered in quantum transport.

We investigate the staggered correlation of the on-site pairs, the so-called $\eta$-pairing correlation, induced by pump electric fields in the Hubbard model on the ladder lattice. Employing the time-evolution method based on exact diagonalization, we compute the photoinduced $\eta$-pairing correlation with different strengths of the interchain hopping. When the pump field is polarized along the chain direction, the $\eta$-pairing correlation is noticeably induced in the weakly coupled double-chain region, where the pairing property is similar to the $\eta$ pairing in the single-chain Hubbard model. On the other hand, the pump field polarized along the rung direction prominently induces the $\eta$-pairing correlation in the strong interchain coupling regime, where the pair correlations developing across rungs strongly contribute to the $\eta$ pairing. Based on the local rung approximation, which is valid in the strong interchain coupling regime, we discuss the origin of the photoinduced pair.