Correlated materials with open-shell $d$ and $f$ ions having degenerate band-edge states show a rich variety of interesting properties ranging from metal-insulator transition to unconventional superconductivity. The textbook view for the electronic structure of these materials is that mean-field approaches are inappropriate, as the interelectronic interaction U is required to open a band gap between the occupied and unoccupied degenerate states while retaining symmetry. We show that the latter scenario often defining what Mott insulators are is in fact not needed for the $3d$ binary oxides MnO, FeO, CoO, and NiO. The mean-field-like band theory can indeed lift such degeneracies in the binaries when nontrivial unit-cell representations (polymorphous networks) are allowed to break symmetries, in conjunction with a recently developed nonempirical exchange and correlation density functional without an on-site interelectronic interaction U. We explain how density-functional theory in the polymorphous representation achieves band-gap opening in correlated materials through a separate mechanism from the Mott-Hubbard approach. We show the method predicts magnetic moments and gaps for the four binary monoxides in both the antiferromagnetic and paramagnetic phases, offering an effective alternative to symmetry-conserving approaches for studying a range of functionalities in open $d$- and $f$-shell complex materials.

The propagation of Dirac fermions in graphene through a long-period periodic potential would result in a band folding together with the emergence of a series of cloned Dirac points (DPs) [C.-H. Park et al., Nat. Phys. 4, 213 (2008); C.-H. Park et al.Phys. Rev. Lett. 101, 126804 (2008)]. In highly aligned graphene/hexagonal boron nitride (G/hBN) heterostructures, the lattice mismatch between the two atomic crystals generates a unique kind of periodic structure known as a moiré superlattice. Of particular interest is the emergent phenomena related to the reconstructed band-structure of graphene, such as the Hofstadter butterfly [L. A. Ponomarenko et al., Nature (London) 497, 594 (2013); B. Hunt et al., Science 340, 1427 (2013); C. R. Dean et al., Nature(London) 497, 598 (2013)], topological currents [R. V Gorbachev et al., Science 346, 448 (2014)], gate-dependent pseudospin mixing [Z. Shi et al., Nat. Phys. 10, 743 (2014)], and ballistic miniband conduction [M. Lee et al., Science 353, 1526 (2016)]. However, most studies so far have been limited to the lower-order minibands, e.g., the first and second minibands counted from charge neutrality, and consequently the fundamental nature of the reconstructed higher-order miniband spectra still remains largely unknown. Here we report on probing the higher-order minibands of precisely aligned graphene moiré superlattices by transport spectroscopy. Using dual electrostatic gating, the edges of these high-order minibands, i.e., the third and fourth minibands, can be reached. Interestingly, we have observed interband Landau level (LL) crossing inducing gap closures in a multiband magnetotransport regime, which originates from band overlap between the second and third minibands. As observed, high-order minibands and LL reconstruction qualitatively match our simulated results.

We present the theory and the application of a first-principle transport model employing a basis set obtained directly from the ab initio Bloch functions. We use a plane-wave density functional theory Hamiltonian and show that a judicious choice of the reduced basis set can effectively suppress the potentially thorny problem of the unphysical solutions. Our methodology enables ab initio transport simulations with a huge reduction of the size of the problem compared to the original ab initio formulation. Moreover, the approach can also be used for local and nonlocal empirical pseudopotential Hamiltonians, thus promising a wide range of possible applications. We report results for ab initio simulations of ${\mathrm{MoS}}_2$ field effect transistors, where the transport and electrostatics equations are solved self-consistently for channel lengths up to about 20 nanometers. The simulation results rapidly converge with the size of the basis set, so that the blocks of the Hamiltonian matrix can be reduced to a size below 100. Our methodology is a viable approach for ab initio and semiempirical quantum transport simulations and, in particular, it offers an alternative to the use of maximally localized Wannier functions.

Hybrid improper ferroelectrics (HIFs) have been studied intensively over the past few years to gain an understanding of their temperature-induced phase transitions and ferroelectric switching pathways. Here we report a switching from a first- to a second-order phase transition pathway for HIFs ${\mathrm{Ca}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Ti}}_2{\mathrm{O}}_7$, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions—such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others—is a crucial step in creating novel functional materials by design.

In this paper we report on adiabatic pumping in quasiperiodic stiffness-modulated beams. We show that distinct topological states populating nontrivial gaps can nucleate avoided crossings characterized by edge-to-edge transitions. Such states are inherently coupled when a smooth variation of the modulation phase is induced along a synthetic dimension, resulting in topological edge-to-edge transport stemming from distinct polarizations of the crossing states. We first present a general framework to estimate the required modulation speed for a given transition probability in time. Then this analysis tool is exploited to tailor topological pumping in a stiffness-modulated beam.

We discuss spin-$\frac{1}{2}$ Heisenberg antiferromagnet on simple square lattice in magnetic field $H$ using a recently proposed bond-operator technique. It is well known that magnetically ordered phases of quantum magnets are well described at least qualitatively by the conventional spin-wave theory that only introduces quantum corrections into the classical solution of the problem. We observe that quantum fluctuations change drastically dynamical properties of the considered model at $H$ close to its saturation value: the dynamical structure factor shows anomalies corresponding to Green's function poles which have no counterparts in the spin-wave theory. That is, quantum fluctuations produce multiple short-wavelength magnon modes not changing qualitatively the long-wavelength spin dynamics. Our results are in agreement with previous quantum Monte Carlo simulations and exact diagonalization of finite clusters.

We have carried out neutron diffraction and small-angle neutron scattering measurements on a high-quality single crystal of the cubic lacunar spinel multiferroic, ${\mathrm{GaV}}_4{\mathrm{S}}_8$, as a function of magnetic field and temperature to determine the magnetic properties for the single electron that is located on the tetrahedrally coordinated ${\mathrm{V}}_4$ molecular unit. Our results are in good agreement with the structural transition at 44 K from cubic to rhombohedral symmetry where the system becomes a robust ferroelectric, while long-range magnetic order develops below 13 K in the form of an incommensurate cycloidal magnetic structure, which can transform into a Néel-type skyrmion phase in a modest applied magnetic field. Below 5.9(3) K, the crystal enters a ferromagnetic phase, and we find the magnetic order parameter indicates a long-range-ordered ground state with an ordered moment of 0.23(1) ${\mu }_{\mathrm{B}}$ per V ion. Both polarized and unpolarized neutron data in the ferroelectric-paramagnetic phase have been measured to determine the magnetic form factor. The data are consistent with a model of the single spin being uniformly distributed across the ${\mathrm{V}}_4$ molecular unit, rather than residing on the single apical V ion, in substantial agreement with the results of first-principles theory. In the magnetically ordered state, polarized neutron measurements are important since both the cycloidal and ferromagnetic order parameters are clearly coupled to the ferroelectricity, causing the structural peaks to be temperature and field dependent. For the ferromagnetic ground state, the spins are locked along the [1,1,1] direction by a surprisingly large anisotropy.

Topological spin configurations in proximity to a superconductor have recently attracted great interest due to the potential application of the former in spintronics and also as another platform for realizing nontrivial topological superconductors. Their application in these areas requires precise knowledge of the existing exchange fields and/or the stray fields, which are therefore essential for the study of these systems. Here, we determine the effective stray field ${\mathbf{H}}_{\mathrm{str}}$ and the Meissner currents ${\mathbf{j}}_{\text{S}}$ in a superconductor/ferromagnet/superconductor (S/F/S) junction produced by various nonhomogenous magnetic textures $\mathbf{M}\left(\mathbf{r}\right)$ in the F. The inhomogeneity arises either due to a periodic structure with flat domain walls (DW) or is caused by an isolated chiral magnetic skyrmion (Sk). We consider both Bloch- and Néel-type Sk and also analyze in detail the periodic structures of different types of DW's, that is, Bloch-type DW (BDW) and Néel-type DW (NDW) of finite width with in- and out-of-plane magnetization vector $\mathbf{M}\left(x\right)$. The spatial dependence of the fields ${\mathbf{H}}_{\mathrm{str}}\left(\mathbf{r}\right)$ and Meissner currents ${\mathbf{j}}_{\text{S}}\left(\mathbf{r}\right)$ are shown to be qualitatively different for the case of Bloch- and Néel-type magnetic textures. While the spatial distributions in the upper and lower S are identical for Bloch-type Sk and DW's they are asymmetric for the case of Néel-type magnetic textures. The depairing factor, which determines the critical temperature $T_c$ and which is related to the vector potential of the stray field, can have its maximum at the center of a magnetic domain but also, as we show, above the DW. For Sk's, the maximum is located at a finite distance within the Sk radius $r_{\text{Sk}}$. Based on this, we study the nucleation of superconductivity in the presence of DW's. Because of the asymmetry for Néel-type structures, the critical temperature $T_c$ in the upper and lower S is expected to be different. The obtained results can also be applied to S/F bilayers.

A crystal structure that cannot be superposed on its mirror image by any combination of rotations and translations is classified as chiral. Such crystal structures have gained importance in recent years since they are prone to host unconventional magnetic orders and to exhibit topological magnetic textures. These properties result from the Dzyaloshinskii-Moriya antisymmetric exchange interaction which is authorized when space inversion is broken. While recent reports have shown the muon spin rotation and relaxation technique to provide unique information about structural and dynamical properties which are specific to chiral magnets in their ordered phase, the question here is whether this technique is sensitive to paramagnetic chiral correlations that are observed in neutron scattering experiments above the critical temperature. In the relevant long-wavelength limit, it is shown that they do not contribute to the relaxation rate, which in turn only probes nonchiral correlations.

We systematically investigate the detailed dynamics of the phonon and local structure of rare-earth orthoferrites ${\mathrm{GdFeO}}_3$ single crystal with temperature and pressure induced structural/magnetic phase transition by Raman spectroscopy. Phonon evolution related to the motion of octahedra reveals paramagnetic to antiferromagnetic ordering transition of ${\mathrm{Fe}}^{3+}$ ions at Néel temperature $T_{N,Fe}$. By quantifying the polarized Raman spectra, especially the cross-polarized geometry, the lattice dynamics and distortion with local structure rearrangement during ferromagnetic transition has been also discovered. Particularly, we claim that the depolarization ratio could be quantified and used to precisely determine ferromagnetic phase transition of ${\mathrm{GdFeO}}_3$ and symmetry evolution simultaneously. Additionally, pressure dependence (up to 25.03 GPa) of collective phonon behavior indicates that antiphase tilt in ${\mathrm{FeO}}_6$ octahedra is more susceptible to the stress field than the in-phase one. The ${\mathrm{FeO}}_6$ octahedra presents better compressible than ${\mathrm{GdO}}_{12}$ dodecahedra in the ${\mathrm{GdFeO}}_3$ lattice with respect to pressure. This work has discovered the physical mechanism underlying variation of local structural symmetry, octahedra tilt, and phonon dynamics in ${\mathrm{GdFeO}}_3$, which can be regarded as the basic view for a series of ${\mathrm{GdFeO}}_3$-type perovskites and more R${\mathrm{FeO}}_3$ system.

Combining inelastic neutron and x-ray scattering with atomic scale simulation, we report on a comprehensive study of the lattice dynamics and its relationship with the low thermal conductivity of the o-${\mathrm{Al}}_{13}{\mathrm{Co}}_4$ phase, a periodic approximant (about 100 atoms per cell) to a decagonal quasicrystal. The obtained experimental data, phonon lifetimes, and temperature-independent lattice thermal conductivity can only be fully described by molecular dynamics simulations, using oscillating pair potential, when the disorder of 12 Al sites is properly taken into account. Our results pave the way for a detailed understanding of materials where structural complexity and local disorder are at play.

Single crystals of $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ir}}_x\right)}_{2-y}{\mathrm{As}}_2$ with $0\le x\le 0.35$ and $0.10\le y\le 0.14$ have been grown using the self-flux technique and characterized by single-crystal x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy, magnetization $M$, and magnetic susceptibility $\chi$ measurements versus temperature $T$, magnetic field $H$, and time $t$, and heat-capacity $C_{\mathrm{p}}(H,T)$ measurements. The XRD refinements reveal that all the Ir-substituted crystals crystallize in a collapsed-tetragonal structure as does the parent ${\mathrm{CaCo}}_{2-y}{\mathrm{As}}_2$ compound. A small 3.3% Ir substitution for Co in ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ drastically lowers the A-type antiferromagnetic (AFM) transition temperature $T_{\mathrm{N}}$ from 52 to 23 K with a significant enhancement of the Sommerfeld electronic heat-capacity coefficient. The A-type AFM structure consists of $ab$-plane layers of spins ferromagnetically aligned along the $c$ axis with AFM alignment of the spins in adjacent layers along this axis. The positive Weiss temperatures obtained from Curie-Weiss fits to the $\chi (T>T_{\mathrm{N}})$ data indicate that the dominant magnetic interactions are ferromagnetic (FM) for all $x$. A magnetic phase boundary is inferred to be present between $x=0.14$ and $x=0.17$ from a discontinuity in the $x$ dependencies of the effective moment and Weiss temperature in the Curie-Weiss fits. FM fluctuations that strongly increase with increasing $x$ are also revealed from the $\chi \left(T\right)$ data. The magnetic ground state for $x\ge 0.17$ is a spin glass as indicated by hysteresis in $\chi \left(T\right)$ between field-cooled and zero-field-cooled measurements and from the relaxation of $M$ in a small field that exhibits a stretched-exponential time dependence. The spin glass has a small FM component to the ordering and is hence inferred to be comprised of small FM clusters. The competing AFM and FM interactions along with crystallographic disorder associated with Ir substitution are inferred to be responsible for the development of a FM cluster-glass phase. A logarithmic $T$ dependence of $C_{\mathrm{p}}$ at low $T$ for $x=0.14$ is consistent with the presence of significant FM quantum fluctuations. This composition is near the $T=0$ boundary at $x\approx 0.16$ between the A-type AFM phase containing ferromagnetically-aligned layers of spins and the FM cluster-glass phase.

${\mathrm{Sc}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ is a diamagnetic metallic compound (space group: $I{4}_1/acd, a\approx 13.5\AA$, $c\approx 27.1\AA$) which becomes superconducting (sc) at $T_{\mathrm{c}}=5.15\mathrm{K}$. The sc state was studied on single-crystal samples by temperature- and field-dependent electrical resistivity, specific heat, and muon-spin rotation ($\mu \mathrm{SR}$) spectroscopy. No anisotropy of the upper critical field $B_{\mathrm{c}2}=7.75\mathrm{T}$ is observed. Analyses of specific heat indicate enhanced values of the jump $[\mathrm{\Delta }{c}_p/\left({\gamma }_{\mathrm{tot}}{T}_{\mathrm{c}}\right)=1.58]$ and of the energy gap ratio $[\mathrm{\Delta }\left(0\right)/\left(k_{\mathrm{B}}{T}_{\mathrm{c}}\right)=2.39]$ as well as an exponential decrease of the electronic contribution below $T_{\mathrm{c}}/2$. Together with the saturation of the superfluid density for $T<3\mathrm{K}$ observed by $\mu \mathrm{SR}$ spectroscopy these findings lead us to classify ${\mathrm{Sc}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ as a moderately strong coupled isotropic $s$-wave superconductor. However, above $T_{\mathrm{c}}$, the crystals reveal a surprisingly anisotropic and large resistivity of $2.5–4.5\mathrm{m}\mathrm{\Omega }$ m with metalliclike behavior perpendicular to the $c$ axis and by a factor of $\sim 2$ larger parallel to $c$, which decreases upon increasing temperature. These observations are discussed in connection to a characteristic intrinsic disorder in the crystal structure of ${\mathrm{Sc}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$.

In recent years, the infinite time-evolving block decimation (iTEBD) method has been demonstrated to be one of the most efficient and powerful numerical schemes for time evolution in one-dimensional quantum many-body systems. However, a major shortcoming of the method, along with other state-of-the-art algorithms for many-body dynamics, has been their restriction to one spatial dimension. We present an algorithm based on a hybrid extension of iTEBD where finite blocks of a chain are first locally time evolved before an iTEBD-like method combines these processes globally. This in turn permits simulating the dynamics of many-body systems in spatial dimensions $d\ge 1$ where the thermodynamic limit is achieved along one spatial dimension and where long-range interactions can also be included. Our work paves the way for simulating the dynamics of many-body phenomena that occur exclusively in higher dimensions and whose numerical treatments have hitherto been limited to exact diagonalization of small systems, which fundamentally limits a proper investigation of dynamical criticality. We expect the algorithm presented here to be of significant importance to validating and guiding investigations in state-of-the-art ion-trap and ultracold-atom experiments.

Through model Hamiltonian studies and first-principle electronic structure calculations, we have examined the effect of inversion symmetry breaking (ISB) field and hydrostatic pressure on the band topology of halide perovskites by taking ${\mathrm{MAPbI}}_3$ as a representative. Our study shows that while hydrostatic pressure induces normal to topological insulator continuous phase transition, the ISB field makes it first order. The pressure smoothly reduces the normal band gap, and without ISB, the system achieves a gapless state before it produces a nontrivial band gap with inverted characters. The ISB field does not stabilize the gapless state, and therefore, the discontinuity in the band gap with pressure gives rise to the first-order transition. Furthermore, in the nontrivial phase, the ISB field forms an invariant surface Dirac circle in the neighborhood of TRIM, which is the first of its kind. The circle is formed due to interpenetration of Dirac cones resembling the band topology of AA-stacked bilayer graphene.

Determination of defect ionization energy in low-dimensional semiconductors has been a long-standing unsolved problem in first-principles defect calculations, because the commonly used methods based on the jellium model introduce an unphysical charge density uniformly distributed in the material and vacuum regions, causing the well-known divergence issue of charged defect formation energies. In addition, because of the unphysical jellium charge, how to determine structures of charged defects is also not clear. These two issues pose great challenges in studying defect properties of low-dimensional semiconductors. Here in this work, we combine the jellium framework together with the idea of constraining charge transfer to deal with charged defects in low-dimensional semiconductors by replacing the unphysical jellium background charge density with the band-edge charge density. By doing this, we show that not only the total energy calculation but also the structure relaxation can be self-consistently obtained in one single calculation, thus providing a simple and efficient way to determine the defect ionization energies in low-dimensional semiconductors.

Using finite-difference time-domain simulations, we theoretically show that trapezoid α-phase molybdenum trioxide ($\alpha \text{−}\mathrm{Mo}{\mathrm{O}}_3$) patch arrays can be used to achieve broadband perfect absorption due to its wide hyperbolic regions in infrared. In the merit of different reststrahlen (RS) bands for different crystal axes of $\alpha \text{−}\mathrm{Mo}{\mathrm{O}}_3$, we further demonstrate anisotropic broadband absorption behavior by aligning an $\alpha \text{−}\mathrm{Mo}{\mathrm{O}}_3$ trapezoidal patch in the in-plane $x$ and $y$ directions, respectively. Electromagnetic simulations reveal that the broadband absorption is elucidated by the combination of strong phonon resonances within the RS band, which dissipate energies at different wavelengths in different regions of the trapezoid. Using patterned $\alpha \text{−}\mathrm{Mo}{\mathrm{O}}_3$ on top of reflector and lossless dielectric layers, we report average absorption values as high as 70% from 10.78 to 11.7 µm and 12.92 to 17.19 µm for the two orthogonal in-plane crystal directions of $\alpha \text{−}\mathrm{Mo}{\mathrm{O}}_3$. More attractively, the spectral range and absorption bandwidth of the proposed structure can be easily engineered by changing the shapes and the thicknesses of the trapezoid patches.

The orbital Hall effect (OHE) is the phenomenon of transverse flow of orbital moment in the presence of an applied electric field. Solids with broken inversion symmetry are expected to exhibit a strong OHE due to the presence of an intrinsic orbital moment at individual momentum points in the Brillouin zone, which in the presence of an applied electric field flows in different directions causing a net orbital Hall current. Here we provide a comprehensive understanding of the effect and its tunability in the monolayer two-dimensional (2D) transition metal dichalcogenides (TMDCs). Both metallic and insulating TMDCs are investigated from full density-functional calculations and effective $d$-band tight-binding models, as well as a minimal four-band model for the valley points that captures the key physics of the system. For the tuning of the OHE, we examine the role of hole doping as well as the change in the band parameters, which, e.g., can be controlled by strain. We demonstrate that the OHE is a more fundamental effect than the spin Hall effect (SHE), with the momentum-space orbital moments inducing a spin moment in the presence of the spin-orbit coupling, leading to the SHE. The physics of the OHE, described here, is relevant for 2D materials with broken inversion symmetry in general, even beyond the TMDCs, providing a broad platform for future research.

A Dirac fermion emerges as a result of interplay between symmetry and topology in condensed matter. Current research moves towards investigating the Dirac fermions in the presence of many-body effects in correlated systems. Here, we demonstrate the emergence of a correlation-induced symmetry-protected Dirac semimetal state in the lightly doped spin-orbit-coupled Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. We find that the nonsymmorphic crystalline symmetry stabilizes a Dirac line-node semimetal and that the correlation-induced symmetry-breaking electronic order further leads to a phase transition from the Dirac line-node to a Dirac point-node semimetal. The latter state is experimentally confirmed by angle-resolved photoemission spectroscopy and terahertz spectroscopy on ${\mathrm{Sr}}_2(\text{Ir},\mathrm{Tb}){\mathrm{O}}_4$ and ${(\text{Sr},\mathrm{La})}_2{\mathrm{IrO}}_4$. Remarkably, the electrodynamics of the massless Dirac carriers is governed by the extremely small scattering rate of about 6 ${\mathrm{cm}}^{-1}$ even at room temperature, which is iconic behavior of relativistic quasiparticles. Temperature-dependent changes in electrodynamic parameters are also consistently explained based on the Dirac point-node semimetal state.

The diversity of various manganese types and its complexes in the Mn-doped ${\mathrm{A}}^{\mathrm{III}}{\mathrm{B}}^{\mathrm{V}}$ semiconductor structures leads to a number of intriguing phenomena. Here we show that the interplay between the ordinary substitutional Mn acceptors and interstitial Mn donors as well as donor-acceptor dimers could result in a reversal of electron magnetization. In our all-optical scheme the impurity-to-band excitation via the Mn dimers results in direct orientation of the ionized Mn-donor $d$ shell. A photoexcited electron is then captured by the interstitial Mn and the electron spin becomes parallel to the optically oriented $d$ shell. That produces, in the low excitation regime, the spin-reversal electron magnetization. As the excitation intensity increases the capture by donors is saturated and the polarization of delocalized electrons restores the normal average spin in accordance with the selection rules. A possibility of the experimental observation of the electron spin reversal by means of polarized photoluminescence is discussed.