The recent discovery of high-$T_c$ superconductivity in Sr-doped ${\mathrm{NdNiO}}_2$ has sparked a renewed interest in investigating nickelates as cuprate counterparts. Parent cuprates $[{\mathrm{Cu}}^{2+}:d^9]$ are antiferromagnetic charge transfer insulators with the involvement of a single $d_{x^2-y^2}$ band around the Fermi level and strong $p\text{−}d$ hybridization. In contrast, isoelectronic ${\mathrm{NdNiO}}_2$ $[{\mathrm{Ni}}^{+}:d^9]$ is metallic with a $d_{x^2-y^2}$ band self-doped by Nd-$d$ states. Using first principles calculations, we study the effect of Sr doping in the electronic and magnetic properties of infinite-layer nickelates as well as the nature of holes upon doping. We find that hole doping tends to make the material more cupratelike as it minimizes the self-doping effect, it enhances the $p\text{−}d$ hybridization, and it produces low-spin ($S=0$, nonmagnetic) ${\mathrm{Ni}}^{2+}$ dopants.

We recently proposed quantum simulators of “twistroniclike” physics based on ultracold atoms and synthetic dimensions, T. Salamon et al. [Phys. Rev. Lett. 125, 030504 (2020).]. In this scheme, Moiré-like patterns can be directly imprinted on the lattice by spatially modulating the interlayer coupling via laser induced Raman transitions, without the need of a physical twist of the layers. As a result, certain “magic” configurations host Dirac cones and quasiflat bands with tunable bandwidths. In this paper we extend these ideas and demonstrate that our system exhibits topological band structures under appropriate conditions. To achieve nontrivial band topology we consider imaginary next-to-nearest neighbor tunnelings that drive the system into a quantum anomalous Hall phase. In particular, we focus on three groups of bands, whose Chern numbers triplet can be associated to a trivial insulator (0,0,0), a standard nontrivial $(-1,0,1)$, and a nonstandard nontrivial $(-1,1,0)$. We identify regimes of parameters where these three situations occur. We show the presence of an anomalous Hall phase and the appearance of topological edge states. Our work opens the path for experiments on topological effects in twistronics without a twist.

Spin splitting of energy bands can be induced by relativistic spin-orbit interactions in materials without inversion symmetry. Whereas polar space-group symmetries permit Rashba (R-1) spin splitting with helical spin textures in momentum space, which could be reversed upon switching a ferroelectric polarization via applied electric fields, the ordinary Dresselhaus effect $\left(\mathrm{D}\text{−}{1}_{\mathrm{A}}\right)$ is active in materials exhibiting nonpolar noncentrosymmetric crystal classes with atoms occupying exclusively nonpolar lattice sites. Consequently, the spin-momentum locking induced by $\mathrm{D}\text{−}{1}_{\mathrm{A}}$ is not electric field switchable. Here we find a type of ferri-chiral materials with an alternative type of Dresselhaus symmetry, referred to as $\mathrm{D}\text{−}{1}_{\mathrm{B}}$, exhibiting crystal class constraints similar to $\mathrm{D}\text{−}{1}_{\mathrm{A}}$ (all dipoles add up to zero), but unlike $\mathrm{D}\text{−}{1}_{\mathrm{A}}$, at least one polar site is occupied. The spin splitting is associated with the crystalline chirality, which in principle could be reversed upon a change in chirality. Focusing on alkali metal chalcogenides, we identify ${\mathrm{NaCu}}_5{\mathrm{S}}_3$ in the nonenantiomorphic ferri-chiral structure, which exhibits ${\mathrm{CuS}}_3$ chiral units differing in the magnitude of their Cu displacements. We then synthesize ${\mathrm{NaCu}}_5{\mathrm{S}}_3$ (space group $P{6}_{3}22$) and confirm its ferri-chiral structure with powder x-ray diffraction. Our electronic structure calculations demonstrate it exhibits $\mathrm{D}\text{−}{1}_{\mathrm{B}}$ spin splitting as well as a ferri-chiral phase transition, revealing spin splitting interdependent on chirality. Our electronic structure calculations show that a few percent biaxial tensile strain can reduce (or nearly quench) the switching barrier separating the monodomain ferri-chiral $P{6}_{3}22$ states. We compute the circular dichroism absorption spectrum of the two ferri-chiral orientations and discuss what type of external stimuli might switch the chirality so as to reverse the (nonhelical) Dresselhaus $\mathrm{D}\text{−}{1}_{\mathrm{B}}$ spin texture. Our study suggests the design of ferri-chiral crystals as potential spintronic and optoelectronic materials.

The ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ spinel exhibits a metal-insulator transition on cooling below $T_s\approx 250$ K, accompanied by Ti $t_{2g}^1$ orbital ordering and spin-singlet dimerization with associated average symmetry reduction to tetragonal. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the transition we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions are observed up to at least 500 K, the highest temperature assessed in this study. Significantly, the high-temperature local state is not continuously connected to the orbitally ordered band insulator ground state and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin-singlet dimer bond lengths expand abruptly on warming across the transition, but remain shorter than those seen in the cubic average structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital $t_{2g}$ orbital degeneracy lifted (ODL) precursor state derived from electron filling, Ti substructure topology, and point symmetry considerations. The ODL state in ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ has a correlation length of about 1 nm at high temperature. We discuss that this extended character of the local distortions is consistent with the two-orbital nature of this state imposed by the charge filling and the bond charge repulsion. The ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ spinel exemplifies multiorbital ODL state and presents the possibility of a widespread presence of such precursor states in scarcely studied high-temperature regimes of transition-metal-based quantum materials.

A recently proposed curvature renormalization group scheme for topological phase transitions defines a generic “curvature function” as a function of the parameters of the theory and shows that topological phase transitions are signaled by the divergence of this function at certain parameter values, called critical points, in analogy with usual phase transitions. A renormalization group procedure was also introduced as a way of flowing away from the critical point toward a fixed point, where an appropriately defined correlation function goes to zero and topological quantum numbers characterizing the phase are easy to compute. In this paper, using two independent models, a model in the AIII symmetry class and a model in the BDI symmetry class, in one dimension as examples, we show that there are cases where the fixed-point curve and the critical-point curve appear to intersect, which turn out to be multicritical points, and focus on understanding its implications.

The electronic density of states (DOS) quantifies the distribution of the energy levels that can be occupied by electrons in a quasiparticle picture and is central to modern electronic structure theory. It also underpins the computation and interpretation of experimentally observable material properties such as optical absorption and electrical conductivity. We discuss the challenges inherent in the construction of a machine-learning (ML) framework aimed at predicting the DOS as a combination of local contributions that depend in turn on the geometric configuration of neighbors around each atom, using quasiparticle energy levels from density functional theory as training data. We present a challenging case study that includes configurations of silicon spanning a broad set of thermodynamic conditions, ranging from bulk structures to clusters and from semiconducting to metallic behavior. We compare different approaches to represent the DOS, and the accuracy of predicting quantities such as the Fermi level, the electron density at the Fermi level, or the band energy, either directly or as a side product of the evaluation of the DOS. We find that the performance of the model depends crucially on the resolution chosen to smooth the DOS and that there is a tradeoff to be made between the systematic error associated with the smoothing and the error in the ML model for a specific structure. We find however that the errors are not strongly correlated among similar structures, and so the average DOS over an ensemble of configurations is in very good agreement with the reference electronic structure calculations, despite the large nominal error on individual configurations. We demonstrate the usefulness of this approach by computing the density of states of a large amorphous silicon sample, for which it would be prohibitively expensive to compute the DOS by direct electronic structure calculations and show how the atom-centered decomposition of the DOS that is obtained through our model can be used to extract physical insights into the connections between structural and electronic features.

The voltage dependences of conductivity, $\sigma \left(V\right)$, of the monoclinic phase of ${\mathrm{NbS}}_3$ are reported. The extremely high coherence of the room-temperature charge density wave (CDW) allows its synchronization by a rf field up to $T_{\mathrm{P}1}\approx 365\mathrm{K}$, the temperature of the Peierls transition, and above it by about 10 K. The ratio of nonlinear current, $I_{\mathrm{nl}}=I-V\sigma \left(0\right)$, to the fundamental frequency of the CDW sliding, $f_{\mathrm{f}}$, falls as T increases across $T_{\mathrm{P}1}$. This result is considered in terms of spontaneous phase slippage near $T_{\mathrm{P}1}$, which could give both single-particle and collective contributions to linear conductivity. The two contributions are considered, and the former is found to dominate. The temperature-frequency dependence of $I_{\mathrm{nl}}/f_f$ is consistent with the model of thermally activated phase slippage.

Electron exchange and correlations emerging from the coupling between ionic vibrations and electrons are addressed. Spin-dependent electron-phonon coupling originates from the spin-orbit interaction, and it is shown that such electron-phonon coupling introduces exchange splitting between the spin channels in the structure. By application of these results to a model for a chiral molecular structure mounted between metallic leads, the chiral-induced spin selectivity is found to become several tens of percent using experimentally feasible parameters.

We performed imaging and manipulation of epitaxially grown Pb adatom structures on the Si(100) surface by noncontact atomic force microscopy (NC-AFM) in ultra-high-vacuum (UHV) and at cryogenic temperatures. We observe several distinct contrast modes during imaging, which we assign to termination of the scanning probe tip by either a single Si, or single Pb, atom, via quantitative comparison of atomic resolution force spectroscopy experiments with ab initio density functional theory (DFT) simulations. We show that the Pb adatom structures can be controllably manipulated via mechanochemical means and identify a novel semideterministic manipulation strategy that arises from the combination of low temperature operation and the asymmetric diffusion barriers present on the Si(100) surface.

In the emerging world of twisted bilayer structures, the possible configurations are limitless, which enables a rich landscape of electronic properties. In this paper, we focus on twisted bilayer transition metal dichalcogenides (TMDCs) and study their properties by means of an accurate tight-binding model. We build structures with different angles and find that the so-called flat bands emerge when the twist angle is sufficiently tiny (smaller than $7.3^{\circ }$). Interestingly, the band gap can be tuned up to 5% (107 meV) when the twist angle in the relaxed sample varies from $21.8^{\circ }$ to $0^{\circ }$. Furthermore, when looking at the local density of states we find that the band gap varies locally along the moiré pattern due to the change in the coupling between layers at different sites. Finally, we also find that the system can suffer a transition from a semiconductor to a metal when a sufficiently strong electric field is applied. Our study can serve as a guide for the practical engineering of TMDC-based optoelectronic devices.

We study the lattice dynamics of iron superconductor FeSe and address the fundamental question of how important is a proper description of fluctuating magnetic moments in metallic systems for phonon dispersion and phonon density of states. We show that density functional theory (DFT)$+$ embedded dynamical mean-field theory (eDMFT) functional approach, which truly captures the fluctuating local moments, largely eliminates the deficiency of DFT for the description of lattice dynamics in correlated metallic systems, and predicts phonon dispersion and phonon density of states in very good agreement with available x-ray data and nuclear inelastic scattering. This benchmark between eDMFT and experiment will be important for data science driven material design, in which DFT is being replaced by beyond DFT methods.

LaAlGe and PrAlGe are members of the $R\mathrm{AlGe}$ series of compounds (with $R$ a light rare-earth element), which have been shown to host Weyl fermions. By exchanging the rare-earth cation, $R\mathrm{AlGe}$ offers a remarkable tunability of its electronic properties thereby allowing to identify the electronic response of Weyl semimetals under varying temperature- and magnetic-field conditions. Through a comparative ${}^{27}\mathrm{Al}$ NMR study of LaAlGe and PrAlGe, we show that the rare-earth element can also be used to tune the NMR response of Weyl fermions in the host. In both cases, we observe unusual NMR shifts that deviate significantly from those in ordinary metals, such as Al. In LaAlGe, we report the observation of a small negative shift, strongly varying with temperature, which can be explained by a recent theory of NMR shifts invoking Weyl fermions. In PrAlGe, instead, we report a giant positive NMR shift of up to 20% at 6 K, most likely due to the transferred hyperfine field from the ${\mathrm{Pr}}^{3+}4f$-electron local moments. We discuss the implications of our findings on future studies of Weyl-fermion systems.

Recently, the intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has attracted great attention. It has an out-of-plane antiferromagnetic order, which is believed to open a sizable energy gap in the surface states. This gap, however, was not always observable in the latest angle-resolved photoemission spectroscopy (ARPES) experiments. To address this issue, we analytically derive an effective model for the two-dimensional (2D) surface states by starting from a three-dimensional (3D) Hamiltonian for bulk ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and taking into account the spatial profile of the bulk magnetization. Our calculations suggest that the diminished surface gap may be caused by a much smaller and more localized intralayer ferromagnetic order. In addition, we calculate the spatial distribution and penetration depth of the surface states, which indicates that the surface states are mainly embedded in the first two septuple layers from the terminating surface. From our analytical results, the influence of the bulk parameters on the surface states can be found explicitly. Furthermore, we derive a $\mathbf{k}·\mathbf{p}$ model for ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ thin films and show the oscillation of the Chern number between odd and even septuple layers. Our results will be helpful for the ongoing explorations of the ${\mathrm{MnBi}}_x{\mathrm{Te}}_y$ family.

In the framework of time-dependent density-functional theory, we obtain high-order harmonics of photon energies up to $10U_p$ from bilayer crystals with an interlayer spacing $d=70\AA$. At grazing incidence, a clear double-plateau structure is observed in the harmonic spectrum. The photon energy of the second plateau far beyond atomiclike harmonics can be well explained by the inclusion of backscattering of ionized electrons. Ab initio simulations reveal that the cutoff of the second plateau is continuously extended with an increasing $d$. Our classical calculations predict that the maximum electronic kinetic energy is linearly dependent on $d$ over a wide range. Moreover, the harmonic yield in the second plateau is significantly enhanced by increases in the wavelength of the driving laser. Owing to the confined spreading of the electronic wave packet, a beneficial wavelength scaling of ${\lambda }^{2.85}$ is obtained. This study therefore establishes an efficient way of producing high-energy photon source based on layered nanostructures.

We investigate the second-order nonlinear Hall effect of Weyl semimetals using a quantum theory, paying attention to the interband process and quantum interference effect. The Drude-type nonlinear Hall conductivity due to the intraband process is found for each Weyl node in the low-frequency regime (the photon energy of the incident field is much smaller than the chemical potential). While in the high-frequency regime with the interband process dominated, the nonlinear photoconductivity shows a two-photon resonant peak or double peaks due to inversion symmetry breaking. The peak splitting is determined by the chemical potential and tilt of the Weyl node. There is a nonmonotonic dependence of the nonlinear Hall current on the tilt of the Weyl node, and an optimal tilt for the maximum Hall current has been found. Moreover, with the help of quantum interference, the nonlinear Hall response can be effectively modulated by tuning the amplitude, phase, and polarization of the incident fields. In particular, in the presence of driving fields with frequencies ${\omega }_0$ and $3{\omega }_0$, the interplay between the optical processes of second-harmonic generation and different frequency generation leads to the chirality-selective generation of nonlinear Hall current under suitable conditions.

We discuss galvanomagnetic and thermomagnetic effects in disordered electronic systems focusing on intermediate temperatures, for which electron-electron scattering and electron-impurity scattering occur at similar rates, while phonon-related effects can be neglected. In particular, we explore how electric and thermal currents driven either by an electric field or by a temperature gradient are affected by the interplay of momentum-dependent electron-impurity scattering, electron-electron scattering, and the presence of a magnetic field. We find that the electric resistance, the Seebeck coefficient and the Nernst coefficient are particularly sensitive to the momentum dependence of the electron-impurity scattering rate at intermediate temperatures. A sufficiently strong momentum dependence of the electron-impurity scattering rate can induce a sign change of the Seebeck coefficient. This sign change can be suppressed by a perpendicular magnetic field. The temperature and magnetic field dependence of the Seebeck coefficient can be used for measuring the magnitude of the electron-impurity and electron-electron scattering rates. The Nernst coefficient vanishes for momentum-independent electron-impurity scattering but displays a maximum at finite temperatures once the momentum dependence is accounted for. By contrast, the Hall coefficient and the Righi-Leduc coefficient display only a weak dependence on the momentum dependence of the electron-impurity scattering at intermediate temperatures.

We present a simple criterion for solvability of lattice spin systems on the basis of graph theory and simplicial homology. The lattice systems satisfy algebras with graphical representations. It is shown that the null spaces of adjacency matrices of the graphs provide conserved quantities of the systems. Furthermore, when the graphs belong to a class of simplicial complexes, the Hamiltonians are found to be mapped to bilinear forms of Majorana fermions, from which the full spectra of the systems are obtained. In the latter situation, we find a relation between conserved quantities and the first homology group of the graph, and the relation enables us to interpret the conserved quantities as flux excitations of the systems. The validity of our theory is confirmed in several known solvable spin systems including the one-dimensional (1D) transverse-field Ising chain, the 2D Kitaev honeycomb model, and the 3D diamond lattice model. We also present new solvable models on a 1D trijunction, 2D and 3D fractal lattices, and the 3D cubic lattice.

A framework is presented for modeling and understanding magnetic excitations in localized, intermediate coupling magnets where the interplay between spin-orbit coupling, magnetic exchange, and crystal-field effects are known to create a complex landscape of unconventional magnetic behaviors and ground states. A spin-orbit exciton approach for modeling these excitations is developed based upon a Hamiltonian which explicitly incorporates single-ion crystalline electric field and spin exchange terms. This framework is then leveraged to understand a canonical Van Vleck $j{}_{\mathrm{eff}}=0$ singlet ground state whose excitations are coupled spin and crystalline electric-field levels. Specifically, the anomalous Higgs mode [Jain et al., Nat. Phys. 13, 633 (2017)], spin-waves [Kunkemöller et al., Phys. Rev. Lett. 115, 247201 (2015)], and orbital excitations [Das et al., Phys. Rev. X 8, 011048 (2018)] in the multiorbital Mott insulator ${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ are captured and good agreement is found with previous neutron and inelastic x-ray spectroscopic measurements. Furthermore, our results illustrate how a crystalline electric-field-induced singlet ground state can support coherent longitudinal, or amplitude excitations, and transverse wavelike dynamics. We use this description to discuss mechanisms for accessing a nearby critical point.

The clock transitions (CTs) or optimal working points (OWPs) of central qubits have long coherence times because their frequency fluctuations vanish in the linear order of external field noise (such as Overhauser fields from nuclear spin baths). Therefore, CTs (or OWPs) are useful for quantum technologies. Also, the quadratic dependence of frequencies on noises makes the CT decoherence an interesting physics problem. Thus we are motivated to study the decoherence of CTs. We consider noise from spin baths, which is one of the most relevant mechanisms of qubit decoherence. Various quantum many-body methods have been developed to study the decoherence of a central spin in spin baths. In particular, the cluster-correlation expansion (CCE) systematically accounts for the many-body correlations that cause the central spin decoherence. However, the CCE cannot be straightforwardly applied to CTs in spin baths, for the expansion may fail to converge due to the effective long-range interactions resulting from the quadratic term of the noise (e.g., the second-order interaction mediated by hyperfine interactions for a nuclear spin bath). In this work, we develop a modified CCE method to tackle this class of decoherence problems. By diagonalizing the central spin Hamiltonian for each bath eigenstate of the hyperfine interaction, we find that the effects of long-range interactions are absorbed as fluctuations of central spin eigenenergies in the form of single-spin correlations. We apply the method to two important systems, namely, nitrogen vacancy center electron spins in near zero magnetic field and the singlet-triplet transition of two electrons in a double quantum dot. The numerical simulation shows that the modified CCE converges rapidly for the CTs. This modified cluster correlation expansion provides a nonperturbative approach to treating the central spin mediated long-range interactions in central spin decoherence problems.

We analyze theoretically quench dynamics of a two-site system abruptly driven from its equilibrium state focusing on the time dependent spectral density function. In the presence of electron reservoirs this function reveals in time a nontrivial regular pattern of peaks corresponding to the stationary quantum chain structure. Such dynamical system with periodic structure of the spectral density stands for a new transient crystal material. We investigate here the role of the Coulomb repulsion between the sites, nontrivial substrates and different system geometries on the transient crystal pattern which can be measured in the tunneling conductance experiments. We also propose the transient crystal system between unbiased leads as an effective monoparametric pump.