Layered transition-metal dichalcogenide $\mathrm{Hf}{\mathrm{Te}}_2$ is a topological semimetal candidate attracting increasing attention recently. The map of the Fermi surface is of interest and importance to understand its properties. Here, we present a study of Shubnikov–de Haas (SdH) oscillations and planar Hall effect (PHE) in $\mathrm{Hf}{\mathrm{Te}}_2$. The single crystals grown by flux method show the largest unsaturated magnetoresistance (MR) effect of $1.1\times {10}^4\%$ at 14 T and 2 K. The angle-resolved SdH oscillations reveal that the Fermi surface consists of three pockets with different anisotropy. In addition, we observe PHE and anisotropic MR effect in the material for a wide temperature range. The effective mass, carrier density, and quantum transport mobility are quantified in the system, and the Berry phase is discussed. Our work provides crucial insights into the electronic structure and the Fermi surface of the semimetal.

The Fermi surface as a contour of the gapless quasiparticle excitation in momentum space is studied based on a mean-field theory of the doped Mott insulator, where the underlying pseudogap phase is characterized by a two-component resonating-valence-bond order that vanishes in the overdoping at $\delta >{\delta }^{*}$. Here the quasiparticle emerges as a collective mode and a Fermi arc is naturally present in the pseudogap regime, while a full Fermi surface is recovered at $\delta >{\delta }^{*}$. The area enclosed by the gapless quasiparticle contour still satisfies the Luttinger volume in both cases, and the Fermi arc at $\delta <{\delta }^{*}$ is actually due to a significant reduction of the spectral weight caused by a quasiparticle fractionalization in the antinodal region. The endpoints of the Fermi arcs exhibit enhanced density of states or hot spots, which can further give rise to a charge-density-wave-like quasiparticle interference pattern. At the critical doping ${\delta }^{*}$, the fractionalized spin excitations become gapless and incoherent, which is signaled by a divergent specific heat. At $\delta >{\delta }^{*}$, the quasiparticle excitation restores the coherence over the full Fermi surface, but the fractionalization still persists at a higher energy/temperature which may be responsible for a strange metal behavior. Different mechanisms for the Fermi arc and experimental comparisons are briefly discussed.

We formulate and analyze in detail the ground-state quantum electrodynamical density functional theory (QEDFT) for a generalized Dicke model describing a collection of $N$ tight-binding dimers coupled to a cavity photon mode. This model is aimed at capturing essential physics of molecules in quantum cavities in polaritonic chemistry, or quantum emitters embedded in mesoscopic resonators of the circuit QED, and, because of its simplicity, is expected to provide important insights regarding the general QEDFT. We adopt the adiabatic connection formalism and the diagrammatic many-body theory to regularly derive a sequence of explicit approximations for the exchange-correlation (xc) energy in the ground-state QEDFT, and to compare their performance with the results of exact numerical diagonalization. Specifically, we analyze the earlier proposed one-photon optimized effective potential (OEP) scheme, its direct second-order extensions, and a nonperturbative xc functional based on the photon random-phase approximation (RPA). Our results demonstrate the excellent performance of RPA-QEDFT in the ultrastrong-coupling regime and for any number $N$ of Dicke molecules in the cavity. We study in detail the scaling of xc energy with $N$ and emphasize the importance for the ground-state QEDFT of collective effects in the interaction of molecules with cavity photons. Finally, we discuss implications of our results for realistic systems.

We investigate valley filtering in black phosphorus (BP) nanofilms modulated by either a pure electric barrier or a magnetic-electric barrier. The considered BP nanofilm has a negative band gap and thus hosts two degenerate Dirac points. We confirm that the system under a pure electric barrier can undergo three distinct valley states as the gate voltage varies. However, the three valley states appear only when the angle $\beta$ between the transport and the zigzag direction is very close to 0. The absence of intervalley transmission at a finite $\beta$ disallows pseudospin-selective tunneling, leading to a strictly zero (at $\beta ={90}^{\circ }$) or weak valley polarization. When a magnetic barrier is included, the three valley states under $\beta =0^{\circ }$ survive. The magnetic barrier can effectively break the reflection symmetry between the two valleys at $\beta$ near ${90}^{\circ }$. Accordingly, one can achieve a remarkable valley polarization with gate-tunable polarity when the transport direction is near the armchair direction. Our findings could be helpful for design valleytronic devices based on two-dimensional systems with merging Dirac cones.

Kagome metals are reported to exhibit remarkable properties, including superconductivity, charge density wave order, and a large anomalous Hall conductivity, which facilitate the implementation of spintronic devices. In this work, we study a kagome metal based on Mn magnetic sites in a ${\mathrm{KMn}}_3{\mathrm{Sb}}_5$ stoichiometry. By means of first-principles density functional theory calculations, we demonstrate that the studied compound is dynamically stable, locking the ferromagnetic order as the ground state configuration, thus preventing the charge-density-wave state as reported in its vanadium-based counterpart ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. Our calculations predict that ${\mathrm{KMn}}_3{\mathrm{Sb}}_5$ exhibits an out-of-plane (001) ferromagnetic response as the ground state, allowing for the emergence of topologically protected Weyl nodes near the Fermi level and nonzero anomalous Hall conductivity (${\sigma }_{\alpha \beta }$) in this centrosymmetric system. We obtain a tangible ${\sigma }_{xy}=314\mathrm{S}{\mathrm{cm}}^{-1}$ component, which is comparable to that of other kagome metals. Finally, we explore the effect of the on-site Coulomb repulsion ($+U$) on the structural and electronic properties. We observed a switching of the magnetic moment from the $z$ axis to the $x$ axis, for $U$ values larger than 1.0 eV. Thus, we find that, although the lattice parameters, magnetic moment, and ${\sigma }_{\alpha \beta }$ moderately vary with increasing $+U$, ${\mathrm{KMn}}_3{\mathrm{Sb}}_5$ stands as an ideal stable ferromagnetic kagome metal with a tangible anomalous Hall conductivity response.

This work presents a combined experimental and ab initio study investigating the relevance of applying the density-functional theory plus $U$ $\left(\text{DFT+}U\right)$ formalism to correctly describe strongly correlated electrons of diluted probe atoms useful to sense the nanoscale in any material, in particular, $5d$ orbitals of ${}^{181}\mathrm{Ta}$ probe atoms doping insulating and semiconducting oxides. Experimentally, we applied the time differential perturbed γ-γ angular correlation (TDPAC) spectroscopy using ion-implanted ${}^{181}\mathrm{Hf}(\to {}^{181}\mathrm{Ta})$ tracers in parts per million concentration in an $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ single crystal, measuring the hyperfine interactions (HFIs) for a complete key set of crystal orientations, achieving the observation of the desired axially symmetric electric-field gradient tensor (EFG) sensed by substitutional ${}^{181}\mathrm{Ta}$ atoms at Al sites free of defects. This goal allowed a direct comparison of a known physical situation with the correct first-principles modeling. The all-electron ab initio electronic structure calculations were performed in the framework of DFT and $\text{DFT+}U$ formalisms. A complete defect-formation energy study for different charge states of the doped systems was essential to determine the correct ionization degree of the Ta impurity, which is likely to be double ionized already at room temperature, showing also the necessity to apply the Hubbard $U$ parameter to correctly describe the electron density and electronic structure of the strongly localized Ta-$5d$ states as ${}^{181}\mathrm{Ta}$ acts as an isolated probe impurity in $\alpha \text{-A}{\mathrm{l}}_2{\mathrm{O}}_3$. This combined study also permitted to assign the second observed HFI to ${}^{181}\mathrm{Ta}$ atoms at substitutional Al sites with structural disorder in their distant neighborhood. We show that the inclusion of the $U$ parameter does not produce significant changes in the equilibrium atomic positions, indicating that all the changes produced in the EFG come from the electron density recombination, showing the extraordinary capability of the EFG to detect subtle changes in the electronic structure. We also show that the Ta-$d$ contribution to the EFG is dominant over the $p$ one, decreasing as the impurity level becomes ionized. When this level is empty, the Ta-$p$ contribution is the dominant one, leading to the experimentally observed EFG. The necessity to apply the $\text{DFT+}U$ formalism at Ta probe atoms in semiconductors and insulators to correctly describe the EFG tensor is discussed, proposing that the use of $\text{DFT+}U$ becomes relevant only when the probe´s impurity level is strongly localized.

The experimental work [J. Crossno et al., Science 351, 1058 (2016)] that reported the violation of the Wiedemann-Franz law in monolayer graphene characterized by a sharp peak of the Lorenz ratio at a finite temperature has not been fully explained. Our previous work [Y.-T. Tu and S. Das Sarma, Phys. Rev. B 107, 085401 (2023)] provided a possible explanation through a Boltzmann-transport model with bipolar diffusion and an energy gap possibly induced by the substrate. In this paper we extend our calculation to include a weak magnetic field perpendicular to the graphene layer, which is experimentally relevant, and may shed light on the possible violation or not of the Wiedemann-Franz law. We find that the magnetic field enhances the size of the peak of the Lorenz ratio but has little effect on its position, and that the transverse component of the Lorenz ratio can be either positive or negative, depending on the parameter regime. In addition, we do the same calculation for bilayer graphene in the presence of a magnetic field and show the qualitative similarity with monolayer graphene. Our work should motivate magnetic-field-dependent experiments elucidating the nature of the charge carriers in graphene layers.

The rare and abnormal elemental ferroelectricity has been predicted and realized in group-V monolayers in recent years [Xiao et al., Adv. Funct. Mater. 28, 1707383 (2018); Gou, Nature (London) 617, 67 (2023)]. However, the valuable out-of-plane (OOP) ferroelectricity is missing in these monolayers owing to the symmetry of the glide plane. In this article, the low-energy nanotube arrays (NTs) of group-V elements (P, As, Sb, and Bi) are carefully confirmed to be ferroelectric materials with both OOP (0.15–0.43 pC/m) and in-plane (IP, 0.52–6.63 pC/m) spontaneous polarizations. The spontaneous polarization originates from the asymmetric electron distribution caused by the noncentrosymmetric tubular crystal structure. Meanwhile, benefited by the coexistence of OOP and IP polarizations, such tubular crystal structure also exhibits unique quadruple polarization states. The lower switching barriers between the logic states (ranging from 13 to 93 meV/atom) further indicate that they are easily controlled by external electric fields, facilitating efficient data read and write operations. These characteristics lay the foundation for their application in high-speed, low-power, and large-scale data storage. More interestingly, our calculations demonstrate that these NTs of group-V elements possess not only IP piezoelectricity comparable to ${\mathrm{MoS}}_2$ (0.95–7.97 pm/V) but also nonzero OOP piezoelectric coefficients (0.04–8.01 pm/V), further expanding their potential applications in energy conversion. It is anticipated that these discoveries will undoubtedly serve to catalyze further research and applications involving single-element materials within the domains of ferroelectricity-based information storage and piezoelectricity-based energy conversion.

A dimerized chain of dipolar emitters strongly coupled to a multimode optical waveguide cavity is studied. By integrating out the photonic degrees of freedom of the cavity, the system is recast in a two-band model with an effective coupling, so that it mimics a variation of the paradigmatic Su-Schrieffer-Heeger model, which features a nontrivial topological phase and hosts topological edge states. In the strong-coupling regime, the cavity photons hybridize the bright dipolar bulk band into a polaritonic one, renormalizing the eigenspectrum and strongly breaking chiral symmetry. This leads to a formal loss of the in-gap edge states present in the topological phase while they merge into the polaritonic bulk band. Interestingly, however, we find that bulk polaritons entering in resonance with the edge states inherit part of their localization properties, so that multiple polaritonic edge states are observed. Although these states are not fully localized on the edges, they present unusual properties. In particular, due to their delocalized bulk part, owing from their polaritonic nature, such edge states exhibit efficient edge-to-edge transport characteristics. Instead of being degenerate, they occupy a large portion of the spectrum, allowing one to probe them in a wide driving frequency range. Moreover, being reminiscent of symmetry-protected topological edge states, they feature a strong tolerance to positional disorder.

The substantial amount of recent research into spin torques has been accompanied by a revival of interest in the spin-Hall effect. This effect contributes to the spin torque in many materials, including topological insulator/ferromagnet devices, Weyl semimetals, and van der Waals heterostructures. In general, the relative sizes of competing spin torque mechanisms remain poorly understood. Whereas a consensus is beginning to emerge on the evaluation of the proper spin current, the role of extrinsic disorder mechanisms in the spin-Hall effect has not been clarified. In this work, we present a comprehensive calculation of the extrinsic spin-Hall effect while focusing on the bulk states of topological insulators as a prototype system and employing a fully quantum-mechanical formalism to calculate the proper spin current. Our calculation of the proper spin current employs a $4\times 4\mathit{k}·\mathit{p}$ Hamiltonian describing the bulk states of topological insulators. At the same time, we provide a qualitative explanation of the proper spin currents calculated based on an effective $2\times 2$ Hamiltonian obtained via a Schrieffer-Wolf transformation. We find that the extrinsic contribution to the proper spin current, driven by side jump, skew scattering, and related mechanisms, is of a comparable magnitude to the intrinsic contribution, making it vital to take such disorder effects into account when seeking to understand experiments. Among the scattering effects considered, side jump scattering is the primary contributor to the extrinsic spin-Hall effect. The total spin conductivity calculated here is too small to explain experimentally measured spin torques, hence we expect the spin-Hall effect to make a negligible contribution to the spin torque in topological insulator structures.

The topological Kondo effect is a genuine manifestation of the nonlocality of Majorana modes. We investigate its out-of-equilibrium signatures in a model with a Cooper-pair box hosting four of these topological modes, each connected to a metallic lead. Through an advanced matrix-product-state approach tailored to study the dynamics of superconductors, we simulate the relaxation of the Majorana magnetization, which allows us to determine the related Kondo temperature, and we analyze the onset of electric transport after a quantum quench of a lead voltage. Our results apply to Majorana Cooper-pair boxes fabricated in double nanowire devices and provide nonperturbative evidence of the crossover from weak-coupling states to the strongly correlated topological Kondo regime. The latter dominates at the superconductor charge degeneracy points and displays the expected universal fractional zero-bias conductance.

We theoretically study the low-lying collective modes of an even-parity spin-singlet superconducting bilayer, where strong spin-orbit coupling leads to a closely competing odd-parity pairing state. We develop a gauge-invariant theory for the coupling of phase fluctuations to an external electromagnetic field and show that the competing odd-parity pairing instability gives rise to a Bardasis-Schrieffer-like phase mode within the excitation gap. Accounting for the long-range Coulomb interaction, however, we find that this mode is converted into an antisymmetric plasmon and is likely pushed into the quasiparticle continuum.

While colossal magnetoresistance (CMR) in Eu-based compounds is often associated with strong spin-carrier interactions, the underlying reconstruction of the electronic bands is much less understood from spectroscopic experiments. Here using angle-resolved photoemission, we directly observe an electronic band reconstruction across the insulator-metal (and magnetic) transition in the recently discovered CMR compound ${\mathrm{EuCd}}_2{\mathrm{P}}_2$. This transition is manifested by a large magnetic band splitting associated with the magnetic order, as well as unusual energy shifts of the valence bands: both the large ordered moment of Eu and carrier localization in the paramagnetic phase are crucial. Our results provide spectroscopic evidence for an electronic structure reconstruction underlying the enormous CMR observed in ${\mathrm{EuCd}}_2{\mathrm{P}}_2$, which could be important for understanding Eu-based CMR materials, as well as designing CMR materials based on large-moment rare-earth magnets.

Recently, the twist-angle effect on two-dimensional van der Waals (vdW) materials, such as bilayer graphene, has attracted great attention. Many novel electronic, magnetic, and even optical properties induced by such effects have been discovered. However, the twist-angle effect on a phononic property is not so remarkable. By investigating the thermal conductivity of twisted bilayer graphene (TBG), we reveal that the trivial twist-angle effect on a phononic property observed in previous studies is owing to the nonlocalization nature of phonons. This characteristic makes phonons hardly trapped by the weak interlayer potentials induced by the twist-angle dependent moiré pattern. We propose that the twist-angle effect can be effectively enhanced by increasing the interface coupling. Using a sandwich structure composed of hexagonal boron nitride and TBG, we demonstrate that the thermal conductivity of TBG can be either significantly increased or dramatically decreased under the synergistic modulation of interlayer-coupling strength and twist angle. Particularly, the twist-angle effect can lead to a nontrivial reduction of thermal conductivity by up to 78% when a strong interlayer coupling is applied. The reduction is several times larger than that observed in the freestanding TBG originating from the twist-angle dependent phonon scatterings induced by the edge phonons. The underlying mechanism for the giant twist-angle dependent thermal conductivity is further revealed based on phonon transport theory. Our findings provide a platform for achieving efficient twist-angle modulation on the phonon transport property of vdW materials.

Magnon spin transport in a metal–antiferromagnetic insulator–ferromagnetic insulator heterostructure is considered. The spin current is generated via the spin Seebeck effect and in the limit of the clean sample where the effects of interface imperfections and lattice defects are excluded. For NiO as an antiferromagnetic insulator we have a magnetic order of antiferromagnetically combined planes which are internally in ferromagnetic order. We find that the sign of the spin current depends on the magnetization direction of the plane next to the metal resulting in an even-odd effect for the spin current. Moreover, as long as damping is excluded, this even-odd effect is the only remaining dependence on the NiO thickness for high temperatures.

The recently discovered 2D magnets represent versatile building blocks for spintronic devices. In particular, the out-of-plane 2D ferromagnets ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT) are attracting strong attention, owing to their ability to host topological spin textures. Among these, CGT is particularly interesting, as its semiconducting property could facilitate electric field control over the spin textures. Here, we systematically explore the magnetic phases of an exfoliated CGT flake with a thickness on the order of $50\mathrm{n}\mathrm{m}$ by scanning transmission x-ray microscopy using three different measurement protocols. The domain periodicity was found to decrease with increasing temperature and decreasing flake thickness, the latter of which can be attributed to the relatively small magnetic anisotropy of CGT. Moreover, the magnetic phase diagram of CGT features a high-temperature skyrmion phase pocket, and skyrmionium formation occurs upon zero-field cooling. The sensitivity of the skyrmions to magnetic field changes indicates that dipolar interactions play a major role in stabilizing these magnetic spin textures.

Collective dynamics in molten alumina is studied by a combination of ab initio molecular dynamics (AIMD) simulations and theoretical analysis of time correlation functions. Two models of dynamics in molten salts are used to analyze time correlation functions: a six-variable viscoelastic model and an eight-variable thermoviscoelastic one. The corresponding sets of dynamic variables are used to construct $6\times 6$ and $8\times 8$ generalized hydrodynamic matrices, eigenmodes of which represent (wave number) $k$-dependent propagating and relaxing processes. It is shown that the thermoviscoelastic model is able to recover the AIMD-derived partial current-current time correlation functions, represented via contributions of the eight $k$-dependent collective eigenmodes. Dispersion and damping of three branches of propagating eigenmodes are compared to the numerical estimates from peak positions of corresponding current spectral functions in order to rationalize effects of acoustic, longitudinal and transverse opticlike excitations in collective dynamics of molten alumina. The role of another branch of propagating eigenmodes in recovering the frequencies and damping of long-wavelength longitudinal optic-like modes estimated from AIMD-derived spectral current functions is discussed.

The Zintl compound $A{\mathrm{In}}_2{X}_2$ $(A=\mathrm{Ca}, \mathrm{Sr}, \text{and} X=\mathrm{P}, \mathrm{As})$, as a theoretically predicted nonmagnetic topological insulator, requires experiments to understand its electronic structure and topological characteristics. In this paper, we systematically investigate the crystal structures and electronic properties of the Zintl compound $\mathrm{Sr}{\mathrm{In}}_2{\mathrm{As}}_2$ under both ambient and high-pressure conditions. By means of angle-resolved photoemission spectroscopy, we observe the overall band structures and “tails” of the topological surface states on the (001) surface that qualitatively agree well with ab initio calculations, thus suggesting $\mathrm{Sr}{\mathrm{In}}_2{\mathrm{As}}_2$ as a topological insulator. Interestingly, application of pressure effectively tunes the crystal structure and electronic properties of $\mathrm{Sr}{\mathrm{In}}_2{\mathrm{As}}_2$. Superconductivity is observed in $\mathrm{Sr}{\mathrm{In}}_2{\mathrm{As}}_2$ for pressure where the temperature dependence of the resistivity changes from a semiconductinglike behavior to that of a metal. The observation of nontrivial topological states and pressure-induced superconductivity in $\mathrm{Sr}{\mathrm{In}}_2{\mathrm{As}}_2$ provides crucial insights into the relationship between topology and superconductivity, and also stimulates further studies of superconductivity in topological materials.

We study the distinctive features of the phase-pumping effect in Josephson transport through a three-layered ferromagnet ${\mathrm{F}}_1/\mathrm{F}/{\mathrm{F}}_2$ with noncoplanar magnetization. Using Gor'kov and Bogoliubov–de Gennes formalisms we go beyond the quasiclassical approximation and analyze the dependence of the spontaneous Josephson phase $\psi$ on the exchange field $h$ in the F layer and details of magnetization profile. The pumping of the Josephson phase can be generated by the mutual rotation of magnetizations in ${\text{F}}_1$ and ${\text{F}}_2$ layers resulting in the nontrivial phase gain at the rotation period (Berry phase). The increase in $h$ is shown to cause changes in the topology of the phase evolution: the gain of the Josephson phase at the pumping period switches from 0 to $2\pi$. We study the scenario of these switchings originating from the interplay between several competing local minima of the free energy of the junction versus the superconducting phase difference. Our analysis provides the basis for the search of experimental setup realizing the phase-pumping phenomenon.