We report the structural, magnetic, specific heat, and electronic structure studies of the material ${\mathrm{K}}_3\mathrm{Yb}{\left({\mathrm{VO}}_4\right)}_2$, which has two-dimensional triangular layers constituted by rare-earth magnetic ${\mathrm{Yb}}^{3+}$ ions. Magnetic susceptibility data show the absence of magnetic long-range order down to $0.5\mathrm{K}$. No bifurcation is observed between zero-field-cooled and field-cooled magnetic susceptibility data, ruling out the possibility of spin-glassiness down to $0.5\mathrm{K}$. From the fit to magnetic susceptibility data with Curie-Weiss law in the low-temperature region, the observed Curie-Weiss temperature $\left(\theta {}_{\mathrm{CW}}\right)$ is about $-1\mathrm{K}$, implying an antiferromagnetic coupling between the ${\mathrm{Yb}}^{3+}$ ions. Magnetic field-dependent specific heat fits well with two-level Schottky behavior. The analysis of magnetization and specific heat data confirms that the ${\mathrm{Yb}}^{3+}$ ion hosts the effective spin $J_{\mathrm{eff}}=1/2$ state. To provide a microscopic understanding of the ground state nature of the titled material, we carried out state-of-the-art first-principles calculations based on density functional theory $+$ Hubbard U and density functional theory $+$ dynamical mean-field theory approaches. Our calculations reveal that the system belongs to the novel class of spin-orbit driven Mott Hubbard insulators and possesses large in-plane magnetocrystalline anisotropy.

The study of charge-density wave (CDW) distortions in Weyl semimetals has recently returned to the forefront, inspired by experimental interest in materials such as ${\text{(TaSe}}_4{\text{)}}_2\text{I}$. However, the interplay between collective phonon excitations and charge transport in Weyl-CDW systems has not been systematically studied. In this paper, we examine the longitudinal electromagnetic response due to collective modes in a Weyl semimetal gapped by a quasi-one-dimensional CDW order, using both continuum and lattice-regularized models. We systematically compute the contributions of the collective modes to the linear and nonlinear optical conductivity of our models, both with and without tilting of the Weyl cones. We discover that, unlike in a single-band CDW, the gapless CDW collective mode does not contribute to the conductivity unless the Weyl cones are tilted. Going further, we show that the lowest nontrivial collective mode contribution to charge transport with untilted Weyl cones comes in the third-order conductivity for a single collective mode. This third-order process is mediated by the gapped amplitude mode. We show that this leads to a sharply peaked third-harmonic response at frequencies below the single-particle energy gap. We discuss the implications of our findings for transport experiments in Weyl-CDW systems.

Recent research on disorder effects in topological phases in quasicrystalline systems has received much attention. In this work, by numerically computing the (spin) Bott index and the thermal conductance, we reveal the effects of disorder on a class D chiral topological superconductor and a class DIII time-reversal-invariant topological superconductor in a two-dimensional Ammann-Beenker tiling quasicrystalline lattice. We demonstrate that both the topologically protected chiral and helical Majorana edge modes are robust against weak disorder in the quasicrystalline lattice. More fascinating is the discovery of disorder-induced topologically nontrivial phases exhibiting chiral and helical Majorana edge modes in class D and DIII topological superconductor systems, respectively. Our findings open the door for the research on disorder-induced Majorana edge modes in quasicrystalline systems.

In a magnetic field, few-layer graphene supports, at the lowest Landau level, a multiplet of zero-mode levels nearly degenerate in orbitals as well as in spins and valleys. Those pseudo-zero-mode (PZM) levels are generally sensitive to interactions and external perturbations and have a crossing among themselves or with other higher Landau levels when an external field is swept over a certain range. A close study is made of how such PZM levels evolve when they are gradually brought from empty to filled levels under many-body interactions. It is pointed out that the level spectra generally avoid a crossing via orbital level mixing and that orbital mixing is governed by a non-Abelian Berry phase that derives from an approximate degeneracy and interactions. A look is also taken into evolution/crossing of many-body ground states with increasing external bias in bilayer graphene.

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound ${\mathrm{EuNi}}_2{\mathrm{P}}_2$. Both the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ components arising from the $4f^6$ and $4f^5$ final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4$f$ reference compounds ${\mathrm{SrNi}}_2{\mathrm{P}}_2$ and ${\mathrm{YNi}}_2{\mathrm{P}}_2$. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of ${\mathrm{SrNi}}_2{\mathrm{P}}_2$ rather than that of ${\mathrm{YNi}}_2{\mathrm{P}}_2$. The heavy-fermion bands in ${\mathrm{EuNi}}_2{\mathrm{P}}_2$ are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for ${\mathrm{SrNi}}_2{\mathrm{P}}_2$, and the ${\mathrm{Eu}}^{2+}$ components located at the very vicinity of the Fermi level.

The bulk-boundary correspondence in one dimension asserts that the physical quantities defined in the bulk and at the edge are connected, as well established in the argument for electric polarization. Recently, a spectral bulk-boundary correspondence (SBBC), an extended version of the conventional bulk-boundary correspondence to energy-dependent spectral functions, such as Green's functions, has been proposed in chiral-symmetric systems, in which the chiral operator anticommutes with the Hamiltonian. In this study, we extend the SBBC to a system with impurity scattering and dynamical self-energies, regardless of the presence or absence of a gap in the energy spectrum. Moreover, the SBBC is observed to hold even in a system without chiral symmetry, which substantially generalizes its concept. The SBBC is demonstrated with concrete models, such as superconducting nanowires and a Su-Schrieffer-Heeger model. Its potential applications and certain remaining issues are also discussed.

We consider two concentric rings formed by bosonic condensates of exciton-polaritons. A circular superfluid flow of polaritons in one of the rings can be manipulated by acting upon the second annular polariton condensate. The complex coupling between the rings with different topological charges triggers nucleation of stable Josephson vortices (JVs) which are revealed as topological defects of the angular dependence of the relative phase between rings. Being dependent on the coupling strength, the structure of the JV governs the difference of the mean angular momenta of the inner and the outer rings. At the vanishing coupling the condensates rotate independently demonstrating no correlations of their winding numbers. At the moderate coupling, the interaction between two condensates tends to equalize their mean angular momenta despite the mismatch of the winding numbers demonstrating the phenomenology of a drag effect. Above the critical coupling strength the synchronous rotation is established via the phase slip events.

We show that low-lying excitations of a 2D Bardeen–Cooper–Schrieffer superconductor are significantly altered when coupled to an externally driven cavity, which induces controllable long-range attractive interactions between the electrons. We find that they combine nonlinearly with intrinsic local interactions to increase the Bogoliubov quasiparticle excitation energies, thus enlarging the superconducting gap. The long-range nature of the driven-cavity-induced attraction qualitatively changes the collective excitations of the superconductor. Specifically, they lead to the appearance of additional collective excitations of the excitonic modes. Furthermore, the Higgs mode is pushed into the gap and now lies below the Bogoliubov quasiparticle continuum such that it cannot decay into quasiparticles. This way, the Higgs mode's lifetime is greatly enhanced.

Using an end-to-end differentiable implementation of the Kohn-Sham self-consistent field equations, we obtain a highly accurate neural network–based exchange and correlation (XC) functional of the electronic density. The functional is optimized using information on both energy and density while exact constraints are enforced through an appropriate neural network architecture. We evaluate our model against different families of XC approximations and show that at the meta-GGA level our functional exhibits unprecedented accuracy for both energy and density predictions. For nonempirical functionals, there is a strong linear correlation between energy and density errors. We use this correlation to define an XC functional quality metric that includes both energy and density errors, leading to an improved way to rank different approximations.

Clarifying the origin of nematic state in FeSe is one of urgent problems in the field of iron-based superconductivity. Motivated by the discovery of a nematic solution in the density-functional theory implemented by on-site Coulomb interaction ($\mathrm{DFT}+U$) [npj Quantum Mater. 5, 50 (2020)], we reexamine the $U$ dependence of electronic states in the nonmagnetic normal state of FeSe and perform full multipolar analyses for the nematic state. We find that with increasing $U$ the normal state experiences a topological change in the Fermi surfaces before the emergence of a nematic ground state. The resulting nematic ground state is a multipolar state having both antiferrohexadecapoles in the $E$ representation and ferromultipoles in the $B_2$ representation on each Fe site. Cooperative coupling between the $E$ and the $B_2$ multipoles in the local coordinate with the $D_{2d}$ point group will play an important role in the formation of the $d_{xz},d_{yz}$ orbital-splitting nematic state not only in FeSe, but also in other iron pnictides.

This year, Liu et al. [Phys. Rev. B 104, L041405 (2021)] proposed another class of topological phonons [TPs, i.e., one-nodal surface (NS) phonons], which provides an effective route for realizing one-NS in phonon systems. In this work, based on first-principles calculations and symmetry analysis, we extended the types of NS phonons from one- to three-NS phonons. The existence of three-NS phonons [with three pairs of NS states on the $k_i=\pm \pi$ ($i=x, y, z$) planes in the three-dimensional Brillouin zone (BZ)] is enforced by the combination of twofold screw symmetry and time reversal symmetry. We screened all 230 space groups (SGs) and found nine candidate groups [with the SG numbers (Nos.) 19, 61, 62, 92, 96, 198, 205, 212, and 213] hosting three-NS phonons. Interestingly, with the help of first-principles calculations, we identified $P{2}_1{2}_1{2}_1$-type ${\mathrm{YCuS}}_2$ (SG No. 19), $Pbca$-type ${\mathrm{NiAs}}_2$ (SG No. 61), $Pnma$-type ${\mathrm{SrZrO}}_3$ (SG No. 62), $P{4}_1{2}_{1}2$-type ${\mathrm{LiAlO}}_2$ (SG No. 92), $P{4}_3{2}_{1}2$-type ${\mathrm{ZnP}}_2$ (SG No. 96), $P{2}_{1}3$-type NiSbSe (SG No. 198), $Pa\overline{3}$-type ${\mathrm{As}}_2\mathrm{Pt}$ (SG No. 205), $P{4}_{3}32$-type ${\mathrm{BaSi}}_2$ (SG No. 212), and $P{4}_{1}32$-type ${\mathrm{CsBe}}_2{\mathrm{F}}_5$ (SG No. 213) as realistic materials hosting three-NS phonons. The results of our presented study enrich the class of NS states in phonon systems and provide concrete guidance for searching for three-NS phonons and singular Weyl point phonons in realistic materials.

We report the theoretical prediction of a class of spintronic materials, namely, bipolar magnetic semiconductors (BMSs), also supported by our experimental data. BMSs possess a unique band structure with unequal band gaps for spin-up and spin-down channels and thus are useful for tunable spin-transport-based applications such as spin filters. The valence band and conduction band in BMSs approach the Fermi level through opposite spin channels and hence facilitate reversible spin polarization that is controllable via applied gate voltage. We report the quaternary Heusler alloy VNbRuAl to exactly possess the band structure of a BMS. A rigorous normal x-ray diffraction (XRD) fitting along with synchrotron XRD data confirms that this alloy crystallizes in the LiMgPdSn structure with partial $\mathrm{B}2$ disorder. Transport measurements show a two-channel semiconducting behavior and a quasilinear dependence of negative magnetoresistance, indicating the possible semiconducting nature. The thermoelectric power data not only confirm the semiconducting nature but also give a strong indication of the BMS nature. Interestingly, VNbRuAl also appears to show features of a fully compensated ferrimagnetic (FCF) behavior with vanishing magnetization and significantly high ordering temperature ($>900 \mathrm{K}$). Theoretical simulations of the special quasirandom structure predict partial B2 disorder to be mainly responsible for the coexistent BMS and FCF-like behavior. This study opens up the possibility of finding another class of materials for antiferromagnetic spintronics, with great significance for both fundamental and applied fronts.

We investigate the anomalous Hall effect in perpendicularly magnetized ferrimagnet $\mathrm{Ni}{\mathrm{Co}}_2{\mathrm{O}}_4$ epitaxial films 3 to 30 nm thick. We find that the anomalous Hall conductivity ${\sigma }_{\mathrm{AHE}}$ has complex relations with the longitudinal electrical conductivity ${\sigma }_{\mathrm{xx}}$. When the ${\sigma }_{\mathrm{xx}}$ is relatively large (larger than ${10}^3{\mathrm{\Omega }}^{–1}\mathrm{c}{\mathrm{m}}^{–1}$), the ${\sigma }_{\mathrm{AHE}}$ is almost independent of the ${\sigma }_{\mathrm{xx}}$, but scales with the magnetization. This unique scaling behavior of ${\sigma }_{\mathrm{AHE}}$ indicates that the anomalous Hall effect is of intrinsic origin dominated by the Berry curvature of occupied electronic states. On the other hand, when the ${\sigma }_{\mathrm{xx}}$ is lowered, the scaling relation is changed to ${\sigma }_{\mathrm{AHE}}\sim {\sigma }_{\mathrm{xx}}^{1.6}$, indicating a crossover of the dominant contribution to ${\sigma }_{\mathrm{AHE}}$, from the intrinsic Berry phase to the impurity scatterings. Our results show the intrinsic contribution from Berry curvature plays a crucial role in the anomalous Hall effect in $\mathrm{Ni}{\mathrm{Co}}_2{\mathrm{O}}_4$ films.

We theoretically study the superconducting properties of multiband two-dimensional transition metal oxide superconductors by analyzing not only the role played by conventional singlet pairings, but also by the triplet order parameters, favored by the spin-orbit couplings present in these materials. In particular, we focus on the two-dimensional electron gas at the (001) interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ band insulators where the low electron densities and the sizable spin-orbit couplings affect the superconducting features. Our theoretical study is based on an extended superconducting mean-field analysis of the typical multiband tight-binding Hamiltonian, as well as on a parallel analysis of the effective electronic bands in the low-momentum limit, including static on-site and intersite intraband attractive potentials under applied magnetic fields. The presence of triplet pairings is able to strongly reduce the singlet order parameters which, as a result, are no longer a monotonic function of the charge density. The interplay between the singlet and the triplet pairings affects the dispersion of quasiparticle excitations in the Brillouin zone and also induces anisotropy in the superconducting behavior under the action of an in-plane and of an out-of-plane magnetic fields. Finally, nontrivial topological superconducting states become stable as a function of the charge density, as well as of the magnitude and of the orientation of the magnetic field. In addition to the chiral, time-reversal breaking, topological superconducting phase, favored by the linear Rashba couplings and by the on-site attractive potentials in the presence of an out-of-plane magnetic field, we find that a time-reversal invariant topological helical superconducting phase is promoted by nonlinear spin-orbit couplings and by the intersite attractive interactions in the absence of magnetic field.

Néel-type magnetic domain walls locally reduce the symmetry of the crystal and give rise to electric polarization. This type of localized multiferroicity is particularly promising due to the sensibility of the domain wall's internal structure to the magnetic field. In this paper, the magnetic-field-modified electric polarization of the domain wall is considered and compared to the room-temperature giant magnetoelectric effect observed on domain walls in iron garnet films.

Searching for Kitaev quantum spin liquids (QSLs) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in $5d$-electron iridates and $4d$-electron ${\mathrm{RuCl}}_3$. Recently, theoretical studies suggested that $3d^7$ Co-based honeycomb materials with a high spin state $S=3/2$ and effective orbital angular momentum $L=1$ could also be promising candidates of a Kitaev QSL. One of the candidates, ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, was revisited recently. The long-range magnetic order in ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ can be suppressed by a very weak in-plane magnetic field, suggesting its proximity to a Kitaev QSL. Here, we perform time-domain terahertz spectroscopy measurements to study the magnetic excitations on ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long-range magnetic order is suppressed by a weak field $H_{c2}$. The spectra beyond $H_{c2}$ are dominated by single-magnon and two-magnon excitations. Therefore, in the measured low-temperature region, the field-induced state is not a Kitaev QSL. We also compared the excitation spectra of ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$ with that of the widely studied $4d$-electron Kitaev candidate ${\mathrm{RuCl}}_3$ and addressed their similarities in magnetic excitations.

A Weyl semimetal (WSM) is a three-dimensional topological phase of matter where pairs of nondegenerate bands cross at isolated points in the Brillouin zone called Weyl nodes. Near these points the electronic dispersion is gapless and linear. A magnetic field $B$ changes this dispersion into a set of positive and negative energy Landau levels which are dispersive along the direction of the magnetic field only. In this set, the $n=0$ Landau level is special since its dispersion is linear and unidirectional. The presence of this chiral level distinguishes Weyl from Schrödinger fermions. In this paper we study the quantum oscillations of the orbital magnetization and magnetic susceptibility in Weyl semimetals. We generalize earlier works on these de Haas–van Alphen oscillations by considering the effect of a tilt of the Weyl nodes. We study how the fundamental period of the oscillations in the small $B$ limit and the strength of the magnetic field $B_1$ required to reach the quantum limit (i.e., where the Fermi level is lying in the chiral level) are modified by the magnitude and orientation of the tilt vector $\mathbf{t}$. We show that the magnetization from a single node is finite in the $B\to 0$ limit. Its sign depends on the product of the chirality and sign of the tilt component along the magnetic field direction. We also study the magnetic oscillations from a pair of Weyl nodes with opposite chirality and with opposite or identical tilt. Our calculation shows that these two cases lead to a very different behavior of the magnetization in the small and large $B$ limits. We finally consider the effect of an energy shift $\pm {\mathrm{\Delta }}_0$ of a pair of Weyl nodes on the magnetic oscillations. We assume a constant density of carriers so that both nodes share a common Fermi level and the density of carriers is constantly redistributed between the two nodes as the magnetic field is varied. Our calculation can easily be extended to a WSM with an arbitrary number of pairs of Weyl nodes.

Here, we report the observation of pressure-induced superconductivity in type-II Weyl semimetal (WSM) candidate $\mathrm{NbIr}{\mathrm{Te}}_4$ and the evolution of its Hall coefficient ($R_{\mathrm{H}}$), magnetoresistance (MR), and lattice with increasing pressure to ∼63 GPa. These results provide a significant opportunity to investigate the universal high-pressure (HP) behavior of ternary WSMs, including the sister compound $\mathrm{TaIr}{\mathrm{Te}}_4$ that has been known through our previous studies. We find that the pressure-tuned evolution from the WSM to the superconducting (SC) state in these two compounds exhibits the same trend, i.e., a pressure-induced SC state emerges from the matrix of the non-SC WSM state at ∼27 GPa, and then the WSM and SC states coexist up to 40 GPa. Above this pressure, an identical HP behavior, characterized by almost the same value of $R_{\mathrm{H}}$ and MR in its normal state and the same value of $T_c$ in its SC state, appears in both compounds. Our results not only reveal the evolution from the WSM state to the SC state but also demonstrate that $\mathrm{NbIr}{\mathrm{Te}}_4$ and $\mathrm{TaIr}{\mathrm{Te}}_4$ can make the same contribution to the normal and SC states that inhabit the HP phase, although these two compounds have dramatically different band structure at ambient pressure.

Many Mott systems feature a first-order metal-insulator transition at finite temperatures, with an associated phase coexistence region displaying inhomogeneities and local phase separation. Here one typically finds “bubbles” or domains of the respective phases, which are separated by surprisingly thick, or fat, domain walls, as revealed both by imaging experiments and recent theoretical modeling. To gain insight into this unexpected behavior, we perform a systematic model study of the structure of such metal-insulator domain walls around the Mott point, within the dynamical mean-field theory framework. Our study reveals that a mechanism producing such “fat” domain walls can be traced to strong magnetic frustration, which is expected to be a robust feature of “spin-liquid” Mott systems.