We theoretically study the excitation spectrum of a two-dimensional Dirac semimetal in the presence of an incommensurate potential. Such models have been shown to possess magic-angle critical points in the single-particle wave functions, signaled by a momentum space delocalization of plane-wave eigenstates and flat bands due to a vanishing Dirac cone velocity. Using the kernel polynomial method, we compute the single-particle Green's function to extract the nature of the single-particle excitation energy, damping rate, and quasiparticle residue. As a result, we are able to clearly demonstrate the redistribution of spectral weight due to quasiperiodicity-induced downfolding of the Brillouin zone creating minibands with effective mini-Brillouin zones that correspond to emergent superlattices. By computing the damping rate, we show that the vanishing of the velocity and generation of a finite density of states at the magic-angle transition coincide with the development of an imaginary part in the self-energy and a suppression of the quasiparticle residue that vanishes in a power-law-like fashion. Observing these effects with ultracold atoms using momentum-resolved radiofrequency spectroscopy is discussed.

In this work we explore the effects of nonlinearity on three-dimensional topological phases. Of particular interest are the so-called Weyl semimetals, known for their Weyl nodes, i.e., pointlike topological charges which always exist in pairs and demonstrate remarkable robustness against general perturbations. It is found that the presence of onsite nonlinearity causes each of these Weyl nodes to break down into nodal lines and nodal surfaces at two different energies while preserving its topological charge. Depending on the system considered, additional nodal lines may further emerge at high nonlinearity strength. We propose two different ways to probe the observed nodal structures. First, the use of an adiabatic pumping process allows the detection of the nodal lines and surfaces arising from the original Weyl nodes. Second, an Aharonov-Bohm interference experiment is particularly fruitful to capture additional nodal lines that emerge at high nonlinearity.

We explore absorbing open-boundary modes in non-Hermitian photonic systems. The modes have a continuum spectrum in the infinite-system-size limit and can exhibit the non-Hermitian skin effect. In contrast to the conventional non-Hermitian skin modes under the fixed-end open-boundary condition, the modes concerned exhibit a strongly size-dependent spectrum that gradually converges to the non-Bloch-band dispersion. The modes correspond to the poles of the $S$ matrix and are closely related to the lasing. We demonstrate these properties in a two-dimensional non-Hermitian photonic crystal with gain having a point-(pseudo)gap topology.

In graphene, a Kekulé-Y bond texture modifies the electronic band structure, generating two concentric Dirac cones with different Fermi velocities lying in the $\mathrm{\Gamma }$ point in reciprocal space. The energy dispersion results in different group velocities for each isospin component at a given energy. This energy spectrum combined with the negative refraction index in $\mathit{p}-\mathit{n}$ junctions allows the emergence of an electronic analog of optical birefringence in graphene. We characterize the valley birefringence produced by a circularly symmetric Kekulé patterned and gated region using the scattering approach. We found caustics with two cusps separated in space by a distance dependent on the Kekulé interaction and that provides a measure of its strength. Then at low carrier concentration, we find a nonvanishing skew cross-section, showing the asymmetry in the scattering of electrons around the axis of the incoming flux. This effect is associated with the appearance of the valley Hall effect as electrons with opposite valley polarization are deflected toward opposite directions.

We present a quantum Monte Carlo investigation of the finite-temperature phase diagram of the quantum dimer model on the square lattice. We use the sweeping cluster algorithm, which allows the exact implementation of the dimer constraint, supplemented with an equal-time directed loop move that allows sampling the winding sectors. We find a high-temperature critical phase with power-law correlations that extends down to the Rokhsar-Kivelson point, in the vicinity of which a reentrance effect in the lines of constant exponent is found. For small values of the kinetic energy strength, we find finite-temperature transitions to ordered states (columnar and staggered) which match those of interacting classical dimer models.

${\mathrm{Sr}}_{1-x}{X}_x{\mathrm{FBiS}}_2$ $(X:\text{rare earth})$ is considered to show a pressure-induced structural transition around 1 GPa where the superconductivity changes from unconventional in the low-pressure phase to conventional in the high-pressure phase with increasing the superconducting transition temperature ($T_c$). We clarified the monoclinic crystal structure ($P{2}_1/m$) in the high-pressure phase which is the same as the high-pressure phase of $X{\mathrm{O}}_{1-x}{\mathrm{F}}_x\mathrm{Bi}{Y}_2$ ($Y$: chalcogen) systems. We performed high-resolution x-ray absorption spectroscopy (XAS) to study the electronic structure of ${\mathrm{Sr}}_{1-x}{X}_x{\mathrm{FBiS}}_2$ systematically, indicating the Bi charge state of nearly $3+$. The pressure dependence of the XAS spectra of ${\mathrm{Sr}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_2$ and ${\mathrm{Sr}}_{0.5}{\mathrm{Nd}}_{0.5}{\mathrm{FBiS}}_2$ indicate a large change in the electronic structure around the structural transition pressure, which corresponded to the transition from semiconductorlike to more metallic states. The increase of the empty states of Bi $s$ and $d$ ($t_{2g}$) bands above the Fermi level was observed in the high-pressure phase. Density functional theory calculations showed that the density of the states at the Fermi level did not show a significant change in the high-pressure phase, where higher $T_c$ was observed.

The room-temperature (300 K) electronic structure of pulsed laser deposited $\mathrm{La}{\mathrm{Ni}}_x{\mathrm{V}}_{1-x}{\mathrm{O}}_3$ thin films has been demonstrated. The substitution of early-transition metal (TM) V in $\mathrm{LaV}{\mathrm{O}}_3$ thin films with late-TM Ni leads to the decreasing in out-of-plane lattice parameter. Doping of Ni does not alter the formal valence state of Ni and V in $\mathrm{La}{\mathrm{Ni}}_x{\mathrm{V}}_{1-x}{\mathrm{O}}_3$ thin films, divulging the absence of carrier doping into the system. The valence-band spectrum is observed to comprise incoherent structure owing to the localized V $3d$ band along with the coherent structure at Fermi level. With increase in Ni concentration, the weight of the coherent feature increases, which divulges its origin to the Ni $3d\text{−}\mathrm{O} 2p$ hybridized band. The shift of Ni $3d\text{−}\mathrm{O} 2p$ hybridized band towards higher energy in Ni-doped $\mathrm{LaV}{\mathrm{O}}_3$ films compared to the $\mathrm{LaNi}{\mathrm{O}}_3$ film endorses the modification in ligand to metal charge transfer (CT) energy. The Ni doping in Mott-Hubbard insulator $\mathrm{LaV}{\mathrm{O}}_3$ leads to the closure of the Mott-Hubbard gap by building of spectral weight that provides the delocalized electrons for conduction. A transition from bandwidth control Mott-Hubbard insulator $\mathrm{LaV}{\mathrm{O}}_3$ to negative CT metallic character in $\mathrm{LaNi}{\mathrm{O}}_3$ film is observed. The study reveals that unlike in Mott-Hubbard insulators, where the strong Coulomb interaction between the $3d$ electrons decides the electronic structure of the system, CT energy can deliver an additional degree of freedom to optimize material properties in Ni-doped $\mathrm{LaV}{\mathrm{O}}_3$ films.

We calculate the lattice thermal conductivity of cubic group-III phosphides by solving the linearized Boltzmann transport equation for phonons and discuss how there is a large disagreement between the literature values for GaP. We find the presence of comparably large third-order interatomic force constants at large interaction distances that influence the thermal conductivity of GaP. When both four-phonon scattering and the contributions from the long-range interactions are included in the calculation, a thermal conductivity of $76{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ is obtained at room temperature, in good agreement with a reported experimental value of $77{\mathrm{W}\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$. Our results show the importance of carrying out convergence tests with respect to all the calculation parameters and could help explain the thermal conductivity in other systems where similar ambiguities are found.

We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.

Topological quantum state described by the global invariant has been extensively studied in theory and experiment. In this Letter, we investigate the relationship between Zitterbewegung and the topology of systems that reflects the properties of the local and whole energy bands, respectively. We generalize the usual two-band effective Hamiltonian to characterize the topological phase transition of the spin-$J$ topological insulator. By studying Zitterbewegung dynamics before and after topological phase transition, we find that the direction of quasiparticles' oscillation can well reflect topological properties. Furthermore, we develop a quantitative calculation formula for the topological invariant in the spin-$J$ Chern insulator and give the selection rule of the corresponding dynamics. Finally, we demonstrate that our theory is valid in different topological systems. The topological invariant can be represented by local dynamical properties of the high-symmetry points in the first Brillouin zone, which provides a measurement method from the dynamical perspective.

Topologically protected localized states are commonly existent inside bulk band gaps with topological features, which, however, can be shifted to coexist with a continuous spectrum under particular coupling modulations, forming the so-called topological bound states in the continuum (BICs). These embedded topological states have so far been derived under the Hermitian assumptions that neglect the influence of nonconservative characteristics of classical wave systems on the topological properties. In this study, with a one-dimensional trimerized elastic lattice, we report the experimental demonstration of topological edge states in the continuum produced solely by non-Hermiticity, without additional coupling modulation. The trimerized elastic lattice is a chain of plates connected through thin beams, with the non-Hermiticity introduced by attaching constrained damping layers to particular sites in the chain assembly. We prove that appropriately tailored non-Hermitian modulation can induce topological edge states that appear in the bulk spectrum rather than exist in the band gap. Besides, the existence of such topological edge states is observed to be closely linked to the configurations of the lattice boundaries. This study affirms that non-Hermiticity can play an important role in creating topological BICs, gaining insight into the non-Hermitian topological physics and offering interesting avenues for exploring sophisticated non-Hermitian topological phenomena including higher-order topology in the continuum.

Twisted trilayer graphene is a particularly promising moiré superlattice system, due to its tunability, strong superconductivity, and complex electronic symmetry breaking. Motivated by these properties, we study lattice relaxation and the long-wavelength phonon modes of this system. We show that mirror-symmetric trilayer graphene hosts, aside from the conventional acoustic phonon modes, two classes of shear modes, which are even and odd under mirror reflection. The mirror-even modes are found to be gapless and equivalent to the “phason'' modes of twisted bilayer graphene, with appropriately rescaled parameters. The modes odd under mirror symmetry have no analog in twisted bilayer graphene and exhibit a finite gap which we show is directly proportional to the degree of lattice relaxation. We also discuss the impact of mirror-symmetry breaking, which can be tuned by a displacement field or result from a stacking shift, and of rotational- as well as time-reversal-symmetry breaking, resulting from spontaneous electronic order. We demonstrate that this can induce finite angular momentum to the phonon branches. Our findings are important to the interpretation of recent experiments concerning the origin of superconductivity and of linear-in-$T$ resistivity.

We investigate the replica symmetry broken (RSB) phase of spin glass (SG) models in a random field defined on Bethe lattices at zero temperature. From the properties of the RSB solution, we deduce a closed equation for the extreme values of the cavity fields. This equation turns out not to depend on the parameters defining the RSB, and it predicts that the spontaneous RSB does not take place homogeneously on the whole system. Indeed, there exist spins having the same effective local field in all local ground states, exactly as in the replica symmetric phase, while the spontaneous RSB manifests only on the remaining spins, whose fraction vanishes at criticality. The characterization in terms of spins having fixed or fluctuating local fields can be extended also to the random field Ising model (RFIM), in which case the fluctuating spins are the only responsible for the spontaneous magnetization in the ferromagnetic phase. Close to criticality, we are able to connect the statistics of the local fields acting on the spins in the RSB phase with the correlation functions measured in the paramagnetic phase. Identifying the two types of spins on given instances of SG and RFIM, we show that they participate very differently to avalanches produced by flipping a single spin. From the scaling of the number of spins inducing RSB effects close to the critical point and using the $M$-layer expansion, we estimate the upper critical dimension $D_{\text{U}}\ge 8$ for SG.

We report a combined theoretical and simulation study of collective excitations in the three-component metallic glass ${\mathrm{Zr}}_{46}{\mathrm{Cu}}_{46}{\mathrm{Al}}_8$. It is shown in that case how one can combine partial current time correlation functions to represent them via total mass and mass-concentration currents, which for the long-wavelength region separately describe hydrodynamic acoustic and nonhydrodynamic opticlike modes. We present dispersions of the longitudinal and transverse acoustic and optic modes over a wide range of wave numbers. Our theoretical analysis of dispersion and damping of three transverse short-wavelength modes sets the basis for the interpretation of already existing experimental data, where the presence of three modes has been disregarded leading to an overestimation of the alleged transverse mode damping.

We study the pairing properties of the two-dimensional $t\text{−}{t}^{\prime }\text{−}{t}^{″}\text{−}J$ model, where $t^{\prime }$ and $t^{″}$ are second and third neighbor hoppings, at a doping level $x\approx 0.1$. Recent studies of the $t\text{−}{t}^{\prime }\text{−}J$ model find strong pairing for $t^{\prime }>0$, associated with electron doping, but an absence of pairing for $t^{\prime }<0$, associated with hole doping. This is in contrast to the cuprates, where the highest transition temperatures appear for hole doping. Model parametrizations for the cuprates estimate a $t^{″}$ comparable to $t^{\prime }$, which, in principle, might fix this discrepancy. However, we find that it does not; we observe a suppression of pairing for the hole-doped system $(t^{\prime }<0,t^{″}>0)$, while for the electron-doped system $(t^{\prime }>0,t^{″}<0)$ $d$-wave pairing is robust. Extended hoppings appear to be insufficient to make the one-band $t\text{−}{t}^{\prime }\text{−}J$ model capable of describing the pairing in the hole-doped system.

The properties of systems with exact $n$-fold screw symmetry $(n=2,3,4,6)$ have been well-studied because they can be understood in terms of space groups. On the other hand, the existence of materials with approximate screw symmetries, such as sevenfold and tenfold screw symmetries, has been predicted. In this paper, we study the properties of phonons in crystals with approximate screw symmetries, which will lead to unique and new physical phenomena. In a crystal with an approximate screw symmetry, we propose a method to extract information on the pseudoangular momentum of phonons, which is a quantum number characterizing the properties of phonon modes under screw symmetry, based on the fact that the information of the quantum numbers defined under exact screw symmetry partially remains in the eigenvectors of approximate screw symmetric systems. As a preparation, we study a one-dimensional crystal with partially broken translation symmetry that has an enlarged unit cell, and we show how to extract information on a quantum number corresponding to the pseudoangular momentum by studying a relative phase between neighboring atoms. We also extend this method to systems with an approximate screw symmetry, and we discuss the properties of the pseudoangular momentum. We apply this method to the results of our first-principles calculations on candidate materials with an approximate translational symmetry or with an approximate screw symmetry, and we show how this approximate symmetry is reflected in the phonon wave functions.

In this paper, we report the measurements of specific heat of an amorphous ${\mathrm{Ti}}_{9.5}{\mathrm{Si}}_{90.5}$ alloy located very close to the critical point of the metal-insulator transition. In the presence of a magnetic field, the specific heat is dominated by the Schottky anomaly caused by magnetic moments associated with the dangling bonds in the matrix of amorphous Si. Subtraction of this contribution exposes the behavior of the electronic specific heat coefficient $\gamma$. The coefficient is temperature independent above 2 K and is, in order of magnitude, close to the value expected in the absence of electron-electron interactions. In the temperature range 0.4–1.5 K, the coefficient $\gamma$ shows an anomalous downturn, which can be approximated by the dependence $\gamma \left(T\right)={\gamma }_{0}ln(T/T_0)$, with $T_0\approx 0.2\mathrm{K}$. In a companion paper, we found that the Hall coefficient in Ti-Si alloys is affected by the electron-electron interaction up to much higher temperature of 150 K and also varies critically across the metal-insulator transition. We compare our results with theoretical predictions for three models, which can potentially explain the anomalous behavior of the specific heat: generalized nonlinear $\sigma$ model, Coulomb glass, and many-body localization.

Disorder and homogeneity are two concepts that refer to spatial variation of the system potential. In condensed-matter systems disorder is typically divided into two types: with local parameters varying from site to site (diagonal disorder) and those characterized by random transfer-integral values (off-diagonal disorder). Amorphous systems in particular exhibit off-diagonal disorder due to random positions of their constituents. In real systems diagonal and off-diagonal disorder may be interconnected. The formal depiction of disorder as local deviations from a common value focuses attention on the short-range components of the potential landscape. However, long-range potential fluctuation are quite common in real systems. In this paper we seek to find a correlation between disorder and homogeneity using amorphous indium-oxide (${\mathrm{In}}_x\mathrm{O}$) films with different carrier concentrations and with different degree of disorder. Thermal treatment is used as a means of fine tuning the system disorder. In this process the resistance of the sample decreases while its amorphous structure and chemical composition is preserved. The reduced resistivity affects the Ioffe-Regel parameter ${\mathrm{k}}_{\text{F}}\ell$ that is taken as a relative measure of disorder in a given sample. The homogeneity of the system was monitored using inelastic light-scattering. This is based on collecting the Raman signal from micron-size spots across the sample. The statistics of these low-energy data are compared with the sample disorder independently estimated from transport measurements. The analysis establishes that heterogeneity and disorder are correlated.

Temperature dependent x-ray absorption spectroscopy and x-ray magnetic circular dichroism experiments were performed at the Fe $L_{2,3}$ edges to unveil the magnetic orbital moment and spin-orbit coupling (SOC) behavior in thin films of the multiferroic ${\mathrm{Ga}}_{0.6}{\mathrm{Fe}}_{1.4}{\mathrm{O}}_3$ (GFO) compound. A thorough analysis of the data evidenced a nonmonotonous behavior for both magnetic orbital moment and SOC with a minimum at approximately 120 K, accompanied with indications of disturbances of the magnetic exchange and electric field dipole. Resonant photoemission spectroscopy experiments performed at the Fe $L_{2,3}$ edges rendered an important modification of the crystal field configuration around this temperature value, pointing to a temperature-driven structural trigonal distortion. These instabilities can be compared to those observed in other iron oxides, in particular, during transitions such as the Verwey transition, with the difference that on GFO they do not have any impact on the macroscopic magnetic properties, such as the coercive field and the global magnetization. This work demonstrates that the absence of instabilities in magnetism at the macroscopic scale does not guarantee the absence of fluctuations at the atomic scale. Such atomic-scale fluctuations of the SOC are undoubtedly shown here and have an impact on the spin Hall magnetoresistance response of the films. This point is of importance for prospective applications of GFO in spin-orbitronics.

We demonstrate that the current-density functional theory for superconductors (sc-CDFT) can describe the magnetic-field-induced first-order phase transition of aluminum from a superconducting state to a normal state. This is accomplished by introducing a model for the magnetic-field dependence of the attractive interaction between superconducting electrons. This model states that the surface potential well produced by a penetrating magnetic field leads to the magnetic-field dependence of the attractive interaction. Specifically, the electron density near the surface increases with the magnetic field owing to the surface potential well, which causes a reduction in the attractive interaction because of the screening effect. We also develop a calculation scheme to solve the gap equation of the sc-CDFT by considering the magnetic-field dependence of the attractive interaction. The calculation results for the magnetic-field dependence of the superconducting gap are in good agreement with the experimental results of the first-order phase transition.