Local constraint is closely related to the gauge field, so constrained models are usually effective low energy descriptions and important in condensed matter physics. On the other hand, local restriction hinders the application of numerical algorithms. In addition to the computational difficulties of the constraints, the various topological sectors which cannot be connected through local operators are also one of the key computational difficulties. Taking a quantum dimer model as an example in this paper, we construct a global scheme based on a sweeping cluster Monte Carlo method, which can sample among different topological sectors. In principle, this method can be generalized to other models.

The cubic antifluorite ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ has recently garnered renewed attention due to its relevance to Kitaev magnetism. Combining Raman spectroscopy with numerical calculations, we investigate its electronic structure and ensuing low-lying excitations as well as lattice instabilities. For temperatures below $T^{*}\approx 180$ K, we observe several lattice anomalies: (i) a gradual appearance of the symmetry-forbidden phonons; (ii) a central-mode-like excitation; and (iii) a soft-mode-like behavior of the ${\mathrm{\Gamma }}^{5+}$ mode involving vibrations of the ${\mathrm{K}}^{+}$ ion relative to the ${\mathrm{Cl}}^{-}$ ion. All these features indicate the occurrence of local noncubic distortions. At high energies, we observe spin-orbit (SO) excitons made of five peaks at $\omega =0.62–0.79$ eV as well as a weak electronic excitation at $\omega =0.12–0.86$ eV. Our numerical calculations reproduce their spectral energy and shape with the electronic parameters: SO coupling $\lambda =465$ meV; Hund's coupling $J_H=300$ meV; tetragonal distortion strength ${\mathrm{\Delta }}_t=30$ meV and on-site Coulomb interaction $U=2.2$ eV. The multiple SO excitons are interpreted in terms of bounded SO excitons of the $|j_{\mathrm{eff}}=\frac{3}{2},\pm \frac{1}{2}\rangle$ states arising from the coupling between the SO excitons and electron-hole excitations. Our results showcase that ${\mathrm{K}}_2{\mathrm{IrCl}}_6$ is on the brink of a structural phase transition.

We study a bilayer structure consisting of an antiferromagnetic insulator and a normal metal. An electron current is driven in the normal metal with direction parallel to the interface between the materials. Due to interfacial exchange coupling between the localized spins in the antiferromagnet and the itinerant electrons in the normal metal, a magnon current can then be induced in the antiferromagnet. Using an uncompensated antiferromagnetic interface, creating an asymmetry in the interfacial coupling to the two degenerate magnon modes, we find that it is possible to generate a magnon spin current. The magnon spin current can be enhanced by increasing the temperature or by spin-splitting the magnon modes.

We investigate how geometry influences spin dynamics in polygonal magnetic nanotubes. We find that lowering the rotational symmetry of nanotubes by decreasing the number of planar facets splits an increasing number of spin-wave modes, which are doubly degenerate in cylindrical tubes. This symmetry-governed splitting is distinct form the topological split recently observed in cylindrical nanotubes. Doublet modes where the azimuthal period is a half-integer or integer multiple of the number of facets, split to singlet pairs with lateral-standing-wave profiles of opposing mirror-plane symmetries. Moreover, the polygonal geometry facilitates the hybridization of modes with different azimuthal periods but the same symmetry, manifested in avoided level crossings. These phenomena, unimaginable in cylindrical geometry, provide new tools to control spin dynamics on the nanoscale. Our concepts can be generalized to nano-objects of versatile geometries and order parameters, offering new routes to understand and engineer dynamic responses in mesoscale physics.

We report magnetotransport measurements in the NbN-magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT)-NbN junction at low temperature. At 10 mK, the nonlinear current-voltage characteristic of the junction shows a tunneling behavior, indicating the existence of interfacial potential barriers within the heterostructure. Under an out-of-plane perpendicular magnetic field, a transition from negative to positive magnetoresistance (MR) is found when increasing the bias voltage. A proximity-induced superconducting gap ${\mathrm{\Delta }}_{\mathrm{ind}}$ is estimated to be $\sim 0.1\mathrm{meV}$ by a pair of differential resistance dips. Moreover, the induced gap is enhanced by gradually tuning the Fermi level toward the charge neutral point by a back-gate voltage, which is ascribed to the increased transport contribution of the topological surface states in MBT. Intriguingly, the induced gap exhibits an anomalous magnetic-field-assisted enhancement, which may originate from the spin-orbit coupling and magnetic order of MBT. Our results reveal the interplay between magnetism and superconductivity in MBT, paving the way for further studies on topological superconductivity and chiral Majorana edge modes in quantum anomalous Hall insulator–superconductor hybrid systems.

In $\kappa$-(BEDT-${\mathrm{TTF})}_2\mathrm{Cu}$[N(${\mathrm{CN})}_2]X$ ($X$ = Cl, Br) salts, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene, the electronic properties can be characterized using a universal pressure-temperature phase diagram. Previous research [T. Kobayashi et al., Phys. Rev. B 100, 195115 (2019)] revealed a low-temperature insulating phase without antiferromagnetic ordering in the isostructural $X$ = I (hereafter, $\kappa$-I) salt, which cannot be understood using the universal phase diagram. In addition, the $\kappa$-I salt naturally contains electronic inhomogeneity because of the development of a superlattice structure and/or disorder of the ethylene end groups. This could lead to physical properties that are different from those described by the universal phase diagram of $\kappa$-type salts. To investigate the relationship between the electronic inhomogeneity and universal phase diagram, ${}^{13}\mathrm{C}$ NMR measurements were performed under pressure on this material. With increasing pressure, an increase in the bandwidth was revealed from a decrease in the Knight shift. The accompanying suppression of the inhomogeneity at low temperatures was evident from the linewidth. Once the inhomogeneity was suppressed, superconductivity was observed. The temperature dependence of the Knight shift and spin-lattice relaxation rate in the superconducting state is similar to that previously observed for $\kappa$-type salts, indicating that the symmetry of the superconducting gap is a $d$ wave. When pressure was applied and the electronic inhomogeneity was suppressed, the physical properties of the $\kappa \text{−}\mathrm{I}$ salt were found to be qualitatively identical to those of the high-pressure side on the universal phase diagram. These results indicate that the ratio of electronic inhomogeneity to bandwidth is an essential parameter for this system.

Stacking a graphene monolayer onto a three-dimensional topological insulator results in a van der Waals heterostructure that can serve as a versatile platform for rich emergent exotic physics. In particular, the Dirac electrons of the graphene and that of the two-dimensional surface states of the topological insulator hybridize effectively, leading to the emergence of two distinct electronic bands around the Fermi level, with respective Mexican-hat and bell shapes. In this paper, we explore the emergent collective plasmonic excitations in the representative heterostructure of graphene/${\mathrm{Sb}}_2{\mathrm{Te}}_3$, combining first-principles calculations and low-energy effective Hamiltonian descriptions. We discover two different plasmon modes of the system, and our band-resolved analysis allows us to attribute the two modes to the intraband transitions within the Mexican-hat and bell-shaped bands, respectively. We also analyze the relative strengths and damping rates of the plasmon modes upon variations of the bias potential across the heterojunction, enabling potential experimental detections of those collective excitations. These findings provide alternate avenues for regulating the electronic states and plasmonic excitations in the hybridized Dirac electron systems for potential device applications.

The optical selection rules obeyed by two-dimensional materials with spin-valley coupling enable the selective excitation of carriers. We show that several members of the monolayer ${MA}_2{Z}_4$ ($M=\mathrm{Mo}$ and $W;A=\mathrm{C}$, Si, and Ge; $Z=\mathrm{N}$, P, and As) family are direct band-gap semiconductors with protected valley states and that circularly polarized infrared light can induce valley-selective interband transitions. Therefore, they are able to generate a close to 100% valley- and spin-polarized current under an in-plane bias and circularly polarized infrared light, which can be exploited to encode, process, and store information.

Density matrix renormalization group (DMRG) algorithm has been extremely successful for computing the ground states of one-dimensional quantum many-body systems. For problems concerned with mixed quantum states, however, it is less successful in that either such an algorithm does not exist yet or that it may return unphysical solutions. Here we propose a positive matrix product ansatz for mixed quantum states which preserves positivity by construction. More importantly, it allows one to build a DMRG algorithm which, the same as the standard DMRG for ground states, iteratively reduces the global optimization problem to local ones of the same type, with the energy converging monotonically in principle. This algorithm is applied for computing both the equilibrium states and the nonequilibrium steady states and its advantages are numerically demonstrated.

Using the linear response-based constrained local density approximation (cLDA) approach we systematically computed the Hubbard $U$ parameters for series of $3d, 4d,$ and $5d$ transition metals. We compare the results with estimations by the constrained random phase approximation (cRPA) method and discuss the performance of the self-consistent density functional theory + $U$ ($\mathrm{DFT}+U$) method for prediction of lattice parameters, work functions, $d$-bandwidths and $d$-band centers. Interestingly, we found that blindly applied the standard, fully localized limit (FLL) version of the $\mathrm{DFT}+U$ approach heavily overestimates the positions of $d$-band centers with respect to the Fermi level, but much better agreement with experiment is obtained when applying a more realistic, Wannier-type representation of $d$ orbitals for projection of $d$ states occupancies. We present another, independent estimate of the Hubbard $U$ parameter based on the comparison of Hartree-Fock and DFT eigenvalues, and positions of $d$-band centers. The so-derived estimates are surprisingly well consistent with the ones derived from the above-mentioned first principles approaches, and allow for validation of cRPA or cLDA results for the disputed cases, including Cu, Ag, and Au for which large $U$ parameters are obtained from the cLDA method.

We study the classification of interacting fermionic symmetry-protected topological (SPT) phases with both rotation symmetry and Abelian internal symmetries in one, two, and three dimensions. By working out this classification, on the one hand, we demonstrate the recently proposed correspondence principle between crystalline topological phases and those with internal symmetries through explicit block-state constructions. We find that for the precise correspondence to hold it is necessary to change the central extension structure of the symmetry group by the ${\mathbb{Z}}_2$ fermion parity. On the other hand, we uncover new classes of intrinsically fermionic SPT phases that are only enabled by interactions, both in 2D and 3D. Several 3D Majorana-type fermionic SPT phases are identified. Moreover, several new instances of Lieb-Schultz-Mattis-type theorems for Majorana-type fermionic SPT phases are obtained and we discuss their interpretations from the perspective of bulk-boundary correspondence.

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both $\mathrm{n}$- and $\mathrm{p}$-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ thin film. The observed Fe $2p\text{−}3d$ RPES spectra show that the Fe $3d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe $L_{2,3}$ XMCD spectra of $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ show preedge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin (↓) $e_{\downarrow }$ states are vacant in $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$. The XMCD results suggest that the carrier-induced ferromagnetic interaction in $\left({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05}\right)\mathrm{As}$ is short ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the $e_{\downarrow }$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for $\mathrm{p}$-type as well as $\mathrm{n}$-type compounds.

We solve the problem of a magnetic impurity coupled to a $U\left(1\right)$ quantum spin liquid with a spinon Fermi surface, and we compute the impurity spectral function. Using the slave rotor mean-field approach combined with gauge field fluctuations, we find that a peak located at the top of the lower Hubbard band and one at the bottom of the upper Hubbard band can emerge in the impurity spectral function. The peaks at the Hubbard band edges arise from the gauge field fluctuation-induced spinon-chargon binding inside the spinon Kondo screening cloud of the magnetic impurity. For a magnetic impurity embedded in a Mott insulator, our findings suggest that the emergence of a pair of peaks at the Hubbard band edges in the impurity density of states spectra provides strong evidence that the host Mott insulator is a $U\left(1\right)$ quantum spin liquid with a spinon Fermi surface.

${\mathrm{Cu}}_2\mathrm{Te}$ was recently shown to be an excellent thermoelectric material with a figure of merit approaching 2. Ag alloying was found to be effective for tuning the electrical conductivity of ${\mathrm{Cu}}_2\mathrm{Te}$ toward high thermoelectric performance. The liquidlike behavior of cations is well-known to yield ultralow thermal conductivity in the cuprous chalcogenide materials. Here, we study the effect of the liquidlike behavior on the electronic and defect properties of the solid solutions ${\mathrm{Cu}}_{2-x}{\mathrm{Ag}}_x\mathrm{Te}$. Using first-principles molecular dynamics (MD) simulation, we found that the band-gap variation with increasing $x$ as obtained from MD ensemble averages is much slower than that from direct calculation using the high-symmetry ordered structures. The solid solutions within the whole range of $x$ exhibit significant band gaps. The liquidlike behavior also results in flattened top valence bands, which reduce the Seebeck coefficient and electrical conductivity. Regarding the defect properties, we found that the liquidlike behavior can fail the standard defect models employing the high-symmetry ordered structure. Again, MD simulation would be necessary for sampling the configuration space in defect calculations. However, converging the defect formation energy for the diffusive system would require much longer simulation time than converging the band gap. Our present results suggest that Ag alloying could slightly increase the formation energy of Cu vacancy.

In a Laughlin fractional quantum Hall state, one- and two-quasihole states can be obtained by diagonalizing the many-body Hamiltonian with a trapping potential or, for larger systems, from the linear combination of the edge Jack polynomials. The quasihole states live entirely in the subspace of the lowest-energy branch in the energy spectrum with a fixed number of orbits, or a hard-wall confinement. The reduction in the Hilbert space dimension facilitates the study of time evolution of the quasihole states after, say, the removal of the trapping potential. We explore the quench dynamics under a harmonic external potential, which rotates the quasiholes in the droplet, and discuss the effect of long-range interaction and more realistic confinement. Accurate evaluation of the mutual statistics phase of anyons for a wide range of anyon separation can be achieved from the Berry-phase calculation.

Ferrovalley and topology are two basic concepts in both fundamental research fields and emerging device applications. So far, reports are extremely scarce regarding the coupling of these two scenarios in a single material system. By first-principles calculations and Monte Carlo simulations, we predict a highly stable intrinsic ferrovalley $2H\text{−}{\mathrm{RuCl}}_{\text{2}}$ semiconductor with an easy out-of-plane magnetization and a high Curie temperature of 260 K. Due to the combination of exchange interaction and spin-orbit coupling, the system exhibits a spontaneous valley polarization of 240 meV in the top valence band, which is further confirmed by a first-order perturbation theory. Moreover, its out-of-plane ferromagnetic ordering can remain rather robust and survive up to 547 K under $-5$% compressive strain well above room temperature. On the other hand, the application of tensile strain can drive the system from the ferrovalley phase into a half-valley-metal state with 100% spin and valley polarizations, followed then by a quantum anomalous Hall (QAH) phase in support of scarce high-temperature QAH effect. Beyond around 2% tensile strain, the system is switched to the in-plane magnetization. If the magnetization is steered to the out-of-plane direction through overcoming the magnetic anisotropy energy barriers, the system is further transitioned from the QAH phase to another half-valley-metal state with the rise of tensile strain, and finally restores to the initial ferrovalley phase. Once synthesized, monolayer $2H\text{−}{\mathrm{RuCl}}_{\text{2}}$ would find many important applications in spintronic, valleytronic, and topological electronic nanodevices.

Surface engineering to tune the surface characteristics for the enhancement of a system is an emerging area of research in terms of its immense technological applications and basic scientific understanding. The ion beam sputtering technique in combination with cosputtered material deposition to form various structures is found to be a very effective bottom-up approach to modify the surface morphology of a material. In this paper, a theoretical model is developed for ripple evolution of a flat elemental surface which is subjected to erosion by ion bombardment at off-normal incidence with concurrent deposition of some impurity or foreign materials. The orientation of the ripple wave vector thus produced exhibits a directional dependence behavior on the projection of both the ion beam and the foreign elements on the surface. The compound formation at the near-surface depth is considered to be the key assumption for inducing such patterns on the surface. The theoretical prediction is realized in the experimental work demonstrated herein. Stainless steel is used as a cosputtering target during irradiation of Si at ${30}^{\circ }$ incidence by a low-energy ${\text{Ar}}^{+}$ ion beam. The ripples that so emerge possess a wave vector lying oblique to the direction of the ion beam and impurity. The development of such morphological instability is attributed to the reaction between Fe and Si, which gives rise to a compound with an erosion rate lower than that of Si. Thus, the formation of the ${\text{FeSi}}_2$ bond, which is interpreted as being a crucial criterion for pattern evolution, agrees well with the theory, making it quite efficient and widely acceptable.

The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.

Jacutingaite family, a family of naturally occurring exfoliable minerals (${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ and ${\mathrm{Pd}}_2{\mathrm{HgSe}}_3$) discovered in Brazil, is recently predicted to be the very first large-gap Kane-Mele quantum spin Hall insulators. However, yet to date, the door is locked for realizing versatile spin-valley based phenomena due to inversion symmetry in this family. By exploiting the inversion symmetry using different strategies such as placing the jacutingaite family in the vertical electric field or growing its composites with some suitable well-matched systems, we reveal several promising valley spin based phenomena in the jacutingaite family. Overall, we achieve the coupled spin valley Hall effect, valley spin-valve effect, Rashba spin splitting around the low-symmetric $M$ point, layer crossover accompanied by spin crossover, and selective-excitation of carriers from opposite valleys in this family. More interestingly, swapping of the induced spin splittings, Berry curvatures, and the spin texture between the two valleys occur upon the reversal of electric field from $E_z>0$ to $E_z<0$ without destroying the ${\mathcal{Z}}_2$ topological hallmarks. Meanwhile, considerable hexagonal trigonal warping effects around the $K_{+}$ and $K_{-}$ valleys are realized. Furthermore, by applying vertical electric field through interfacial coupling with ferroelectric ${\mathrm{III}}_2-{\mathrm{VI}}_3$ film, valley contrasting Berry curvature in the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer with a larger spin splitting in the lower conduction band $(\sim 188\mathrm{meV})$ is achieved in the $+P$ polarization state. By inverting the polarization state from $+P$ to $-P$, the Fermi level shifts down into the valence band of the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer $(\sim 82\mathrm{meV})$ revealing a hole doping in the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer. This could establish an effective mechanism to control doping levels in the ${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$ layer as desirable for achieving superconductivity at finite doping in the jacutingaite family. Our paper provides a path towards integrating valleytronics and spintronics in multivalley materials with broken inversion symmetry.