The edge states of a quantum spin Hall insulator exhibit helical properties, which have generated significant interest in the field of spintronics. Although it is predicted theoretically that Rashba spin-orbit coupling can effectively regulate the edge state spin, experimental characterization of this spin control remains challenging. Here, we propose utilizing crossed Andreev reflection to probe the efficiency of gate voltage control on the edge state spin. We calculate the transport properties of the quantum spin Hall insulator-superconductor heterojunction by using nonequilibrium Green's function method. We find that the probabilities of crossed Andreev reflection, electron transmission, and nonlocal conductance all include the relative spin rotation angle $\theta$ caused by the Rashba spin-orbit coupling in the helical edge states. Notably, when the incident energy is close to the superconducting gap, the differential conductance $G_2\propto -cos\theta$. Thus, the influence of Rashba spin-orbit coupling on the edge state spin can be quantitatively characterized by crossed Andreev reflection, which provides a feasible scheme for experimental testing.

The study of Chern insulators (CIs) has been extended into two-dimensional hyperbolic systems which possess constant negative curvature without any singularities. These hyperbolic CIs can be characterized by quantized Chern numbers, and the bulk-edge correspondence still exists. In this work, we investigate the interplay between conical singularities and electronic properties of hyperbolic CIs. We establish these singular hyperbolic systems based on the hyperbolic kagome tilings by “cutting and gluing” a defined unit sector. To achieve CI states, we introduce staggered magnetic fluxes along the nearest-neighbor bonds. Their topological properties can be identified by edge states and real-space Chern number. More importantly, charge fractionalization around conical singularities emerges which reveals the topological response of singular hyperbolic CIs.

Fermions and hardcore bosons share the same restriction: no more than one particle can occupy a single site in a lattice system. Specifically, in one dimension, two systems can share the same matrix representation. In this paper, we investigate both the fermion and hardcore-boson models with nearest-neighbor (NN) interaction in a ring lattice. We construct the exact eigenstates of the hardcore-boson model with resonant NN interaction and show that they possess off-diagonal long-range order in the thermodynamic limit. In comparison, the fermionic counterpart does not support such a feature due to the different particle statistics, although they share an identical energy spectrum. In addition, we examine the effect of the periodic boundary condition on the dynamics of the condensate states through numerical simulations.

The Mpemba effect means a phenomenon where hotter matter can cool down faster than colder matter in the same refrigerated environment. We consider a system consisting of two capacitively coupled quantum dots, with each of them connected through tunneling to a finite reservoir on the left or the right, respectively. Moreover, the right finite reservoir is further in contact with an infinite reservoir. Based on the assumption that the system is in a quasiequilibrium state, we investigate the temperature evolution of electrons in the two finite reservoirs using a master equation and standard linear-response expressions for transport. We demonstrate the Mpemba effect during electron cooling in the left finite reservoir. When the left finite reservoir is at a higher initial temperature, it releases heat energy to increase the temperature of the right finite reservoir. The moderate temperature increase in the right finite reservoir can promote the temperature decreasing in the left reservoir. This is an anomalous heat conduction in the heat flow between the two finite reservoirs, which causes the emergence of the Mpemba effect. We present relevant phase diagrams to show that the existence or not of the Mpemba effect depends on the parameter choice. Our research proposes another mechanism for understanding the Mpemba effect.

We show that interference-induced flat bands (IFBs) are commonplace in reticular materials. For example, several metal-organic frameworks, including the superconductor Cu-BHT, form kagome lattices with metals at the vertices and ligands along the bonds. Similar bipartite motifs are common in reticular materials. A tight-binding model on this lattice yields partially occupied IFBs at half filling with large gaps between them and all other bands. Long-range hopping induces curvature in the bands but leaves them flatter and more isolated than those in twisted bilayer graphene. The slave boson theory of the $t-J$ model on this lattice shows unconventional superconductivity. Thus, crystal engineering of reticular materials provides a lattice-driven route to flat bands with high electronic densities, which promote exotic, strongly correlated phenomena.

Moiré lattices are a general feature of bilayer structures, where an additional periodic superstructure is generated by either lattice mismatch or from a twist angle. They have been shown to stabilize exotic ground states, including unconventional superconductivity and Mott insulating phases, attributed to strong electron correlations. However, controlling these interactions requires a detailed understanding of the low-energy electronic structure, which is lacking so far. Probing the electronic structure is challenging due to sample inhomogeneity, the low characteristic energy scales involved, and small sample sizes. Through quasiparticle interference (QPI) imaging, scanning tunneling microscopy (STM) can overcome many of these challenges but requires detailed modeling to extract the $k$-space electronic structure. Here, we present realistic calculations of QPI in twisted bilayer structures, which accounts for the effect of the long-range moiré lattice on the electronic structure as well as its interaction at a defect. These calculations reveal that, while the moiré supercell significantly reduces the size of the Brillouin zone, the QPI scattering vectors retain characteristics of the individual monolayers with distinct perturbations from the twisted geometry that can be directly linked back to the electronic structure. The procedure introduced here provides a general framework to use QPI to determine the low-energy electronic structure in moiré lattice systems.

Design of photonic crystals having large band gaps above a prescribed band is a well-known physical problem with many applications. A connection to an interesting mathematical construction was pointed out some time ago: It had been conjectured that optimal structures for gaps between bands $n$ and $n$ $+$ 1 correspond, in the case of transverse magnetic polarization, to rods located at the generators of centroidal Voronoi tessellation (CVT) and, in the case of transverse electric polarization, to the walls of this tessellation. We discover another mathematical recipe which produces even better solutions: optimal packing of disks in square and triangular tori. It provides solutions qualitatively different from CVT, sometimes increasing the resulting band gap size several times. We therefore introduce two classes of periodic structures with remarkable properties which may find applications in many other areas of modern solid state physics: arrays of particles located at the centers of optimally packed disks on tori and nets corresponding to the walls of their Voronoi tessellations.

${\mathrm{AgBiSe}}_2$ is a potentially important thermoelectric compound in which a partial substitution of Se by S and Te can stabilize the cubic phase at room temperature. The resulting compound ${\mathrm{AgBiSe}}_{2-2x}{\mathrm{Te}}_x{\mathrm{S}}_x$ is found to show small thermal conductivity in the cubic phase with highest thermoelectric performance for $x\ge 0.6$. Here the effect of this substitution on the local displacements and the electronic structure of ${\mathrm{AgBiSe}}_{2-2x}{\mathrm{Te}}_x{\mathrm{S}}_x$ has been investigated by x-ray absorption fine structure and x-ray photoelectron spectroscopy in the wide substitution range of $x=0.0$ to 0.8. The Ag K-edge extended x-ray absorption fine structure measurements reveal substantial change in the local structure across the average structural phase transition from hexagonal to cubic at $x\sim 0.5$. The x-ray absorption near edge structure spectra show a gradual change in the local geometry with the substitution. We do not find any evidence of multiple valence states of either Ag or Bi in the x-ray photoelectron spectroscopy (XPS). The core-level XPS spectra show that the cubic phase of ${\mathrm{AgBiSe}}_{2-2x}{\mathrm{Te}}_x{\mathrm{S}}_x$ is characterized by significant relative S deficiency and a possible local order. The core-level binding energies change anomalously with the substitution reflecting the behavior of changing chemical potential. The valence band spectral weight near the Fermi level is found to correlate with the chemical potential and the room-temperature resistivity. The effect of substitution on the local structure and disorder has been discussed in the light of enhanced thermoelectric properties of ${\mathrm{AgBiSe}}_{2-2x}{\mathrm{Te}}_x{\mathrm{S}}_x$.

The generation of spin-orbital current is crucial for advancing energy-efficient spintronic devices. Here, the intricate process involved in the generation and conversion of spin and orbital to charge currents in Zr($\mathrm{t}=2$, 3, 4.5, 6, 10, 15, 20 nm)/$\mathrm{C}{\mathrm{o}}_{60}\mathrm{F}{\mathrm{e}}_{20}{\mathrm{B}}_{20}$(CFB), Zr/Pt/CFB, and Zr(t)/Pt/CFB/Pt, and Zr/Pt/CFB(t)/Pt heterostructures are investigated using spin-orbital pumping ferromagnetic resonance and longitudinal spin-orbital Seebeck effect measurements. The moderate spin-orbit coupling (SOC) in the CFB layer facilitates the simultaneous generation of spin and orbital currents, which are transferred into adjacent Zr in Zr/CFB and through Pt in Zr/Pt/CFB/Pt layers. Contributions to the spin to charge current or orbital to charge current conversion originating from different physical mechanisms, namely, inverse spin Hall effect (ISHE), inverse orbital Hall effect, and inverse orbital Rashba-Edelstein effect (IOREE) are determined. Notably, introducing a single Pt layer increases the spin-orbital to pure orbital current, in which the charge current conversion via combined effects: ISHE in Pt and IOREE in Zr/Pt interface. In the heterostructure Zr/Pt/CFB/Pt, the effective ISHE contribution is canceled out due to opposite sign of generating ISHE voltage in top and bottom layer of Pt, and only orbital contribution is remaining which arises due to Zr layer. Compared to the value of effective Rashba-Edelstein length ${\lambda }_{\mathrm{IORE}}^{Zr-\mathrm{eff}}$ ∼0.29 nm for Zr(4.5)/CFB(20) heterostructures, significantly enhanced 500% in values of ${\lambda }_{\mathrm{IORE}}^{Zr-\mathrm{eff}}$ are observed after Pt-layer insertion at the Zr/CFB interface. The ${\lambda }_{\mathrm{IORE}}^{Zr-\mathrm{eff}}$ values for Zr(4.5)/Pt(2)/CFB(20), and Zr(4.5)/Pt(2)/CFB(20)/Pt(2) heterostructures are $\sim 1.78\mathrm{nm}$ and $\sim 1.70\mathrm{nm}$, respectively. The results are further substantiated by an alternative experimental technique, i.e., longitudinal spin-orbital Seeback effect measurements. These findings provide new insights into orbital-moment dependent phenomena and offer promising avenues for developing advanced spintronic devices exploiting both spin and orbital degrees of freedom, even in materials having lower SOC.

Time-reversal symmetry is thought to be a necessary condition for realizing the valley Hall effect. However, if time-reversal symmetry is broken, whether the valley Hall effect can be realized has not been explored. In this paper, based on symmetry analysis and first-principles electronic structure calculations, we demonstrate that the valley Hall effect without time-reversal symmetry can be realized in two-dimensional altermagnetic materials ${\mathrm{Fe}}_2{\mathrm{WSe}}_4$ and ${\mathrm{Fe}}_2{\mathrm{WS}}_4$. Due to crystal symmetry being required, the valley Hall effect without time-reversal symmetry is termed the crystal valley Hall effect. In addition, under uniaxial strain, both monolayer ${\mathrm{Fe}}_2{\mathrm{WSe}}_4$ and ${\mathrm{Fe}}_2{\mathrm{WS}}_4$ can realize the piezomagnetic effect. Under biaxial compressive stress, both monolayer ${\mathrm{Fe}}_2{\mathrm{WSe}}_4$ and ${\mathrm{Fe}}_2{\mathrm{WS}}_4$ will transform from the altermagnetic semiconductor phase to the bipolarized topological Weyl semimetal phase. Our paper not only provides another direction for exploring the valley Hall effect but also provides a good platform for exploring altermagnetic semiconductors and altermagnetic topological phase transitions.

Altermagnets, a distinct class of antiferromagnets with electronic structures resembling those of $d$-wave superconductors, exhibit intriguing properties that have gained significant attention in recent research. In this paper, we propose synthetic altermagnets, composed of two anisotropic ferromagnetic layers arranged such that the total net magnetization is zero. We investigate the properties of these synthetic altermagnets, focusing on their electronic band structures, spin current, Berry curvature, and anomalous Hall conductivity. By developing a minimal model for our synthetic altermagnets, we examine the influence of factors such as anisotropic coupling strengths and spin-orbit coupling on the physical phenomena altermagnets manifest. Our findings open a another path for the realization of altermagnetic materials experimentally and highlight their potential applications in magnetoelectronics, magneto-optics, and spintronics devices.

Prompted by the recent discovery of high-temperature superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under pressure, this study delves into a theoretical investigation of spin excitations within this intriguing material. Through self-consistent mean-field calculations, we propose that superconductivity in this compound is primarily driven by interlayer pairing mechanisms. This interlayer pairing results in an effective $s_{\pm }$ pairing gap, with the sign of the pairing gap varying across different Fermi pockets. In the superconducting state, our analysis reveals a striking absence of a spin resonance mode. Moreover, we reveal a spectrum of energy-dependent incommensurate spin excitations. The observed incommensurate structures are elegantly explained by the nesting effect of energy contours, providing a coherent and comprehensive account of experimental observations. The implications of these spin excitations in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ are profound, offering critical insights into the superconducting mechanism at play. Our results not only contribute to the understanding of this novel superconductor but also pave the way for further research into the interplay between spin dynamics and unconventional superconductivity in layered materials.

Pair density wave (PDW) is an intriguing quantum matter proposed in the frontier of condensed-matter physics. However, the existence of PDW in microscopic models has been rare. In this work, we obtain, by Ginzburg-Landau arguments and self-consistent mean-field theory, novel $2a_0\times 2a_0$ PDW on the kagome lattice arising from attractive on-bond pairing interactions and the distinct Bloch wave functions near the $p$-type van Hove singularity. The PDW state carrying three independent wave vectors, the so-called 3Q PDW, is nodeless and falls into two topological classes characterized by the Chern number $C=0$ or $C=\pm 2$. The chiral ($C=\pm 2$) PDW state presents a rare case of interaction-driven topological quantum state without the requirement of spin-orbit coupling. Finally, we analyze the stabilities and properties of these PDWs intertwining with charge orders, and discuss the relevance of our minimal model to recent experimental observations in kagome superconductors. Our theory not only elucidates the driving force of the chiral PDW, but also predicts strongly anisotropic superconducting gap structure in the momentum space and quantized transverse thermal conductivity that can be tested in future experiments.

In this paper, O-vacancy-related ${\mathrm{Ti}}^{3+}$ centers in potassium titanyl phosphate (${\mathrm{KTiOPO}}_4$, KTP), which are commonly discussed along the formation of its characteristic photochromic damage (i.e., so-called gray tracking), are systematically investigated within density functional theory. Notably, centers localized at ${\mathrm{TiO}}_6$ moieties close the vacancy, provide the best agreement with experimental hyperfine tensors, and are also energetically more favorable compared to the ones located at undercoordinated ${\mathrm{TiO}}_5$ moieties, whereby displaced K ions are found to stabilize them. ${\mathrm{Ti}}^{3+}$ centers embedded in Rb-doped KTP share the same properties as those in stoichiometric KTP. The calculated absorption spectra for K-deficient KTP suggest that gray tracking can be attributed to a vacancy pair composed of a positively charged O vacancy and a negatively charged K vacancy. The displaced K ions could contribute to the local formation of overpopulated K channels, which may also be associated with gray tracking.

We study the many-body localization problem in the non-Abelian SU(2)-invariant random antiferromagnetic exchange model in 1D. Exact and sparse matrix diagonalization methods are used to calculate eigenvalues and eigenvectors of the Hamiltonian matrix. We investigate the behavior of the energy level gap-ratio statistic, participation ratio, entanglement entropy and the entanglement spectral parameter as a function of disorder strength. Different distributions of random couplings are considered. We find, up to $L=24$, a clear distinction between our non-Abelian model and the more often studied random field Heisenberg model: the regime of seemingly localized behavior is much less pronounced in the random exchange model than in the field model case.

Models with correlated disorder are rather common in physics. In some of them, like the Aubry-André (AA) model, the localization phase diagram can be found from the (self)duality with respect to the Fourier transform. In others, like the all-to-all translation-invariant Rosenzweig-Porter (TI RP) ensemble or the many-body localization (MBL), one needs to develop more sophisticated and usually phenomenological methods to obtain the localization transition. In addition, such models contain not only localization, but also the ergodicity-breaking transition in the Hilbert-space eigenstate structure, giving way to the nonergodic (i.e., MBL) phase of states with nontrivial fractal dimensions $D_q>0$. In this work, we suggest a method to calculate both the above transitions and a lower bound to the fractal dimensions $D_2$ and $D_{\infty }$, relevant for the physical observables. To verify this method, we apply it to the class of long-range (self-)dual models, interpolating between a tight-binding ballistic model and TI RP via both power-law dependencies of the hopping terms and the on-site disorder correlations, and, thus, being out of the validity range of the previously developed methods. We show that the interplay of the correlated disorder and the power-law decaying hopping terms leads to the emergence of the two types of nonergodic phases in the entire range of parameters, even without having any quasiperiodicity of the AA potential. The analytical results of the above method are in full agreement with the extensive numerical calculations.

We conduct a comprehensive study of anomalous charge transport in the quantum sine-Gordon model. Employing the framework of generalized hydrodynamics, we compute Drude weights and Onsager matrices across a wide range of coupling strengths to quantify ballistic and diffusive transport, respectively. We find that charge transport is predominantly diffusive at accessible timescales, indicated by the corresponding Onsager matrix significantly exceeding the Drude weight – contrary to most integrable models where transport is primarily ballistic. Reducing the Onsager matrix to a few key two-particle scattering processes enables us to efficiently examine transport in both low- and high-temperature limits. The charge transport is dictated by nondiagonal scattering of the internal charge degree of freedom: at particular values of the coupling strength with diagonal, diffusive effects amount to merely subleading corrections. However, at couplings approaching these points, the charge Onsager matrix and corresponding diffusive timescale diverge. Our findings relate to similar transport anomalies in XXZ spin chains, offering insights through their shared Bethe ansatz structures.

Topological materials, characterized by various types of band crossings, provide a fertile ground for the emergence of novel topological excitations. While topological electronic excitations have been widely studied, investigations into topological phonons remain relatively limited. Exploring fermionic and bosonic topological properties within a single material opens new avenues for advancing key concepts in condensed matter physics. In this work, we introduce BeAu as a promising candidate that hosts multiple multifold topological excitations in its electronic and phononic band spectra, each protected by specific symmetries. A detailed electronic structure analysis reveals the coexistence of three distinct topological fermions, such as Weyl, Rarita-Schwinger-Weyl, and fourfold fermions, alongside two-dimensional doubly degenerate nodal surface states. Similarly, the phonon dispersion exhibits threefold and fourfold degenerate phonons, as well as two-dimensional nodal surface states. Moreover, BeAu shows a significant spin Hall conductivity, driven by the pronounced spin Berry curvature near degenerate nodes and the spin-orbit coupling-induced band splitting in the electronic band structure. We also investigate the transverse magnetoresistance, observing intriguing behavior under varying magnetic field strengths, suggesting a potential correlation between magnetoresistance and field intensity. Our findings position this material as an exceptional platform for hosting and exploring diverse quasiparticle excitations with significant spin Hall conductivity, making it suitable for applications in spintronic devices.