We consider $N$-particle generalizations of $\eta$-pairing states in a chain of $N$-component fermions and show that these states are exact (high-energy) eigenstates of an extended $\mathrm{SU}\left(N\right)$ Hubbard model. We compute the singlet correlation function of the states and find that its behavior is qualitatively different for even and odd $N$. When $N$ is even, these states exhibit off-diagonal long-range order in $N$-particle reduced density matrix. On the other hand, when $N$ is odd, the correlations decay exponentially with distance in the bulk, but end-to-end correlations do not vanish in the thermodynamic limit. Finally, we prove that these states are the unique ground states of suitably tailored Hamiltonians.

${\mathrm{V}}_2{\mathrm{O}}_3$ is an archetypical Mott material which exhibits Cr doping and/or temperature-induced paramagnetic metal to paramagnetic insulator/antiferromagnetic insulator (PM-PI/AFI) transitions. However, the strain effects on the metal-insulator transition (MIT) in thin films and the mechanisms behind them are not yet fully understood. In this study, we investigate the strain effects on the MIT in $({\mathrm{V}}_{0.99}{\mathrm{Cr}}_{0.01}{)}_2{\mathrm{O}}_3$ (VC1) thin films growing on $c$-plane (001), $r$-plane (012), and $a$-plane (110) ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates. The strains can be tuned in a wide range over 1% in $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films by changing the substrate temperature. Remarkably, the influence of strain on the MIT differs dramatically from (001)-oriented and non-(001)-oriented $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films. The PM-PI transition can be effectively modulated by strain tuning in the (001) $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films accompanied by gradual transformation from the PM/PI phase into the AFI phase upon cooling. On the contrary, both (012) and (110) $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films exhibit sharp PM-AFI transitions irrespective of the strain states. The orientation-dependent strain effects on the MIT in the $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films could be explained by the trigonal distortion within electronic correlation scenario. The trigonal distortion of $\mathrm{V}\left(\mathrm{Cr}\right){\mathrm{O}}_6$ octahedra along (within) the $c$ axis ($ab$ plane) in the (001) $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films can be directly modulated by the strains. However, the trigonal distortion in the (012) and (110) $\mathrm{VC}1\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ thin films would hardly be affected due to the deviation of the film orientation from the $c$ axis. This explanation is experimentally confirmed by Raman spectra as a sensitive probe of the trigonal distortion in VC1 thin films. These results highlight the importance of trigonal distortion to understand the MIT in the ${\mathrm{V}}_2{\mathrm{O}}_3$ compound.

Type-II Weyl fermions characterized by tilted energy dispersions have attracted significant interest because such quasiparticles exhibit unique properties due to their open Fermi surfaces. Herein, we propose type-II quadratic (cubic) Weyl fermions by combining tilted energy dispersions with higher Chern numbers $\mathcal{C}=\pm 2$ ($\pm 3$), making the classification of chiral fermions more complete. Unlike type-II Weyl fermions with tilted energy dispersions along any direction, type-II quadratic (cubic) Weyl fermions can only be tilted along a rotational axis. Our analytical solutions of Landau levels and magneto-optical spectra show that type-II quadratic (cubic) Weyl fermions have unique absorption properties, such as different locations and widths of the absorption peaks compared with known chiral fermions, which can be used as a reference to detect these Weyl fermions in experiments. Using first-principles calculations, we identify the type-II quadratic Weyl fermions in cubic ${\mathrm{CaGe}}_2$, in which the electron and hole pockets touch at the nodes with $\mathcal{C}=+2$ and two visible Fermi arcs come out from the type-II quadratic Weyl points.

We investigate the orientation dependence of the intrinsic orbital Hall effect (OHE) and spin Hall effect (SHE) in hexagonal close-packed (hcp) structure materials. The symmetry constraints of the hcp structure restrict the orbital (spin) current to the orbital (spin) Hall current. It also shows that there exist three symmetry-wise independent orientations in the hcp structure. We calculate the orbital Hall conductivity (OHC) and the spin Hall conductivity (SHC) of three hcp materials (Sc, Y, and Zr). We find that all these materials have sizable OHC, while the SHC is orders of magnitude smaller. Especially, the OHCs of Sc and Zr have significant orientation dependence, which may be employed to experimentally probe the OHE.

Mode conversion of transverse electromagnetic waves into longitudinal oscillations and the associated resonant absorption of wave energy in inhomogeneous plasmas is a phenomenon that has been studied extensively in plasma physics. We show that precisely analogous phenomena occur generically in electronic and photonic systems where dispersionless flat bands and dispersive bands coexist in the presence of an inhomogeneous potential or medium parameter. We demonstrate that the systems described by the pseudospin-1 Dirac equation with two dispersive bands and one flat band display mode conversion and resonant absorption in a very similar manner to $p$-polarized electromagnetic waves in an unmagnetized plasma by calculating the mode conversion coefficient explicitly using the invariant imbedding method. We also show that a similar mode conversion process takes place in many other systems with a flat band such as pseudospin-2 Dirac systems, continuum models obtained for one-dimensional stub and sawtooth lattices, and two-dimensional electron systems with a quadratic band and a nearly flat band. We discuss some experimental implications of mode conversion in flat-band materials and metamaterials.

We consider a three-terminal system consisting of a quantum dot strongly coupled to two superconducting reservoirs in the infinite-gap limit and weakly coupled to a normal metal. Using a real-time diagrammatic approach, we calculate the dynamics of the proximity-induced pair amplitude on the quantum dot. We find that after a quench the pair amplitude shows pronounced oscillations with a frequency determined by the coupling to the superconductors. In addition, it decays exponentially on a timescale set by the coupling to the normal metal. Strong oscillations of the pair amplitude occur also when the system is periodically driven both in the adiabatic and fast-driving limit. We relate the dynamics of the pair amplitude to the Josephson and Andreev current through the dot to demonstrate that it is an experimentally accessible quantity.

We report the neutron scattering results for magnetic orderings and excitations in ${\mathrm{Ba}}_2{\mathrm{CoTeO}}_6$ composed of two subsystems, A and B, which are described as an $S=1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. We found that the excitation spectra of both subsystems are well separated and independent of each other. Stripy ordering of Ising-like subsystem B was confirmed below $T_{\mathrm{N}1}=12.0\mathrm{K}$, whereas the elastic scattering for subsystem A displays sharp streaks along $(1/3,1/3,L)$ and $(2/3,2/3,L)$ at 0.30 K ($\ll T_{\mathrm{N}2}=3.0\mathrm{K}$). These findings reveal that subsystems A and B are almost perfectly decoupled. The excitation spectrum of subsystem A is composed of two single-magnon branches with rotonlike minima at the $M$ point and a clearly structured intense continuum, which is similarly observed in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.

Moving vortices in thin superconducting films are considered within the time-dependent London description. The dissipation due to out-of-core normal excitations for two vortices moving together turns out to have a minimum for the separation vector $\mathit{a}$ parallel to the velocity and equal to $a_m\approx 2.2\mathrm{\Lambda }$, where $\mathrm{\Lambda }$ is the Pearl length. The minimum entropy production suggests that moving vortices should have a tendency to form chains along the velocity with a period of the order $a_m$.

Using first-principles density functional theory, we investigate the dynamical properties of the room-temperature $P{2}_1/n$ and ground-state $P{2}_1/c$ phases of ${\mathrm{WO}}_3$. As a preliminary step, we assess the validity of various standard and hybrid functionals, concluding that the best description is achieved with the B1-WC hybrid functional while a reliable description can also be provided using the standard LDA functional. We also carefully rediscuss the structure and energetics of all experimentally observed and a few hypothetical metastable phases in order to provide deeper insight into the unusual sequence of phase transition of ${\mathrm{WO}}_3$ with temperature. Then, we provide a comprehensive theoretical study of the lattice dynamical properties of the $P{2}_1/n$ and $P{2}_1/c$ phases, reporting zone-center phonons, infrared and Raman spectra, as well as the full phonon dispersion curves, which attest to the dynamical stability of both phases. We carefully discuss the spectra, explaining the physical origin of their main features and evolution from one phase to another. We reveal a systematic connection between the dynamical and structural properties of ${\mathrm{WO}}_3$, highlighting that the number of peaks in the high-frequency range of the Raman spectrum appears as a fingerprint of the number of antipolar distortions that are present in the structure and a practical way to discriminate between the different phases.

The coexistence of different ferroelectric phases enables the tunability of the macroscopic properties and extensive applications from piezoelectric transducers to nonvolatile memories. Here we develop a thermodynamic model to predict the stability and low-energy orientations of boundaries between different phases in ferroelectrics. Taking lead zirconate titanate and bismuth ferrite as two examples, we demonstrate that the low-energy orientations of interphase boundaries are largely determined by minimizing the electrostatic and elastic energies. Phase-field simulations are employed to analyze the competition between the interfacial energy and the electrostatic and elastic energies. Our simulation results demonstrate that the lowering of crystal symmetry could occur due to the electrical and mechanical incompatibilities between the two phases, which can be used to explain the experimentally observed low-symmetry phases near morphotropic phase boundaries. Our work provides theoretical foundations for understanding and controlling the interphase boundaries in ferroelectric materials for multifunctional applications.

Topological spin textures with versatile configurations exhibit fascinating physical behavior as high-efficiency information units. Recent observations of nontrivial spin textures in two-dimensional (2D) van der Waals (vdW) ferromagnets have shed light on their functionality as a paradigm for spintronic devices. Here, the spin configuration of single-crystal 2D vdW ${\mathrm{Fe}}_{5\text{−}x}\mathrm{Ge}{\mathrm{Te}}_2$ is investigated by Lorentz transmission electron microscopy, where the conversion from Bloch-type bubbles to Néel-type skyrmions can be observed by tuning sample thickness. Remarkably, high-density Néel-type skyrmions can be obtained via field-cooling manipulation. We further demonstrate the underlying mechanism by varying the sample thickness and Dzyaloshinskii-Moriya interaction (DMI) parameters in a series of micromagnetic simulations, manifesting the presence of DMI at the ${\mathrm{Fe}}_{5\text{−}x}\mathrm{Ge}{\mathrm{Te}}_2$ surface by first-principles calculation. Our findings disclose the rich variety of topological spin textures in 2D vdW ${\mathrm{Fe}}_{5\text{−}x}\mathrm{Ge}{\mathrm{Te}}_2$, possessing great promise for future nonvolatile memories based on spin topology.

We report the synthesis and detailed characterization of superconducting ${\mathrm{La}}_3{\mathrm{Se}}_4$, with $T_c\sim 8.5\pm 0.1$ K, using x-ray diffraction, electrical transport, magnetization, and heat capacity measurements. ${\mathrm{La}}_3{\mathrm{Se}}_4$ crystallizes in the noncentrosymmetric cubic ${\mathrm{Th}}_3{\mathrm{P}}_4$-type structure with space group $I\overline{4}3d$. Characteristic superconducting parameters such as the lower critical field, upper critical field, thermodynamic critical field, coherence length, penetration depth, and Ginzburg-Landau parameter have been determined. The specific heat jump at $T_c$, $\mathrm{\Delta }C/\gamma T_c=2.04\pm 0.05$, exceeds the value for a weakly coupled BCS superconductor, and the electron-phonon coupling constant is found to be ${\lambda }_{ep}=0.87\pm 0.02$, suggesting superconductivity in ${\mathrm{La}}_3{\mathrm{Se}}_4$ is in the strong-coupling regime. The estimated upper critical field is well below the calculated Pauli limit, and the Maki parameter value ($\alpha <$ 1) indicates that the superconducting upper critical field is dominated by orbital pair breaking. From density functional theory based first-principles simulations we observe the number of states at the Fermi energy is dominated mainly by $d$ and $f$ electrons of La. Furthermore, we observe band crossings along the high-symmetry $k$ lines in the vicinity of the Fermi energy. These bands are observed to split due to the removal of spin degeneracy associated with spin-orbit coupling, with the splitting energy $E_{\mathrm{ASOC}}\approx$ 65 meV.

The linear-response approach with the nonlocal van der Waals density functionals is presented, considering three kinds of perturbations: Atomic displacements, uniform electric fields, and strain. The formulas to compute the response with respect to strain are derived for the van der Waals density functionals as well as for the generalized gradient approximation. The linear-response method is implemented within the ultrasoft pseudopotential scheme. The method is applied to weak-coupled layered materials, graphite and ${\mathrm{MoS}}_2$. The results support the validity of the derived formulas and demonstrate the utilities of the linear-response method for weak-coupled van der Waals systems.

We have studied the local structure of ${\mathrm{La}}_{0.5-x}{\mathrm{Na}}_{0.5+x}{\mathrm{Fe}}_2{\mathrm{As}}_2$ superconductor by As $K$-edge x-ray absorption fine structure measurements as a function of temperature in a wide range of hole doping ($x$ = 0.0 to 0.35). A substantial change in the local structure around the arsenic atom has been observed with doping. The axial As-${\mathrm{As}}_a$ bond length is found to be >$\sim 3.12$ Å, indicating that the system is in its collapsed tetragonal phase. The temperature-dependent mean square relative displacements reveal that the As-Fe bond is relatively weak in the undoped sample; however, it gets stronger with hole doping. On the other hand, the axial As-${\mathrm{As}}_a$ bond tends to get weaker in the optimally doped regime ($x$ = 0.25–0.35), suggesting the possible role of increased As atom displacements in the axial direction in the superconducting phase of these materials. Consistently, the arsenic height from the Fe-Fe plane shows a gradual increase in the optimally doped regime. Apart from providing a quantitative information on the local structure of ${\mathrm{La}}_{0.5-x}{\mathrm{Na}}_{0.5+x}{\mathrm{Fe}}_2{\mathrm{As}}_2$, the results suggest a vital role of electron-lattice coupling and axial As atom displacements in the superconductivity of these iron-based materials.

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence of the formation of magnetic polarons in antiferromagnetic ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$. First, we observe a reduction of the ${\mathrm{Eu}}^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the ${\mathrm{Eu}}^{2+}$ line shape changes markedly below $T^{\prime }\sim 200$ K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provides strong evidence that magnetic polarons grow in size below $T^{\prime }$ and start influencing the macroscopic properties of the system.

By means of systematically first-principles calculations and model analysis, a complete phase diagram of the body-centered silicon (BC8-Si) via lattice constant $a$ and internal atomic coordinate $x$ is explored, which demonstrates that BC8-Si is a topological Luttinger semimetal (LSM) accompanied with topologically nontrivial surface states, and the electronic properties of BC8-Si can be further tuned to a normal insulator or topological Dirac semimetal by very tiny changing of $a$ and $x$. These results successfully explain the contradictory transport reports of BC8-Si. More importantly, the topological surface states in the LSM phase fill in the gap between the topological matters and silicon, which provide an opportunity to integrate the topological quantum devices and silicon chips together.

The electronic structures and topological properties of the orthorhombic and monoclinic phases of the quasi-one-dimensional excitonic insulator ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ are investigated based on density functional theory. In contrast to a single parity or band inversion across the Fermi level in many topological insulators studied previously, there are multiple parity and band inversions with or without spin-orbit coupling in both phases of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, resulting in more complex and topologically nontrivial electronic structures. The Dirac cone type surface states of the low-temperature monoclinic phase are also obtained. In this paper, we demonstrate that ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is a promising candidate as a three-dimensional topological excitonic insulator.

Analytic continuation (AC) from the imaginary-time Green's function to the spectral function is a crucial process for numerical studies of the dynamical properties of quantum many-body systems. This process, however, is an ill-posed problem; that is, the obtained spectrum is unstable against the noise of the Green's function. Though several numerical methods have been developed, each of them has its own advantages and disadvantages. The sparse modeling (SpM) AC method, for example, is robust against the noise of the Green's function but suffers from unphysical oscillations in the low-energy region. We propose a method that combines the SpM AC with the Padé approximation. This combination, called SpM-Padé, inherits robustness against noise from SpM and low-energy accuracy from Padé, compensating for the disadvantages of each. We demonstrate that the SpM-Padé method yields low-variance and low-biased results with almost the same computational cost as that of the SpM method.