Using newly developed quantum-classical hybrid framework, we investigate interaction between spin-polarized conduction electrons and a single spin wave (SW) coherently excited within a metallic ferromagnetic nanowire. The SW is described by classical atomistic spin dynamics as a collection of precessing localized magnetic moments on each atom. The conduction electrons are described quantum mechanically using time-dependent nonequilibrium Green functions. When the nanowire hosting SW is attached to two normal metal (NM) leads, with no dc bias voltage applied between them, the SW pumps chiral electronic charge and spin currents into the leads. Their direction is tied to the direction of SW propagation and they scale linearly with the frequency of the precession. This is in contrast to standard pumping by the uniform precession mode with identical spin currents flowing in both directions and $no$ accompanying charge current or experimentally observed [C. Ciccarelli et al., Nat. Nanotechnol. 10, 50 (2014); M. Evelt et al., Phys. Rev. B 95, 024408 (2017)] magnonic charge pumping which was interpreted by requiring spin-orbit (SO) coupling effects. Conversely, the mechanism behind our prediction is nonadiabaticity due to time-retardation effects—motion of localized magnetic moment affects conduction electron spin in a retarded way, so that it takes a finite time until the electron spin reacts to the motion of the classical vector. This makes the time-dependent nonequilibrium spin density misaligned with ‘adiabatic direction,’ even for zero SO coupling and in the absence of magnetic or spin-orbit impurities. We visualize retardation effects by computing the spatial profile of nonadiabaticity angle between the electronic nonequilibrium spin density and ‘adiabatic direction.’ Upon injecting dc spin-polarized charge current, electrons interact with SW where outflowing electronic charge and spin current are modified due to both scattering off time-dependent potential generated by the SW and superposition with the currents pumped by the SW itself. Using Lorentzian voltage pulse to excite leviton out of the Fermi sea, which carries one electron charge with no accompanying electron-hole pairs and behaves as solitonlike quasiparticle, we describe how a single electron interacts with a single SW.

A high-quality ${\mathrm{Cr}}_4{\mathrm{Te}}_5$ single crystal was prepared by the chemical vapor transport method. The crystal structure, magnetic properties, and magnetocaloric effect have been investigated in detail. Its critical behavior was methodically studied by measuring isothermal magnetization curves around the ferromagnetic to paramagnetic phase transition temperature. The critical exponents $\beta$ = 0.388, $\gamma$ = 1.29, $\delta$ = 3.93, and Curie temperature $T{}_C$ = 318.7 K were determined with modified Arrott plots, the Kouvel-Fisher method, critical isotherm analysis, and wisdom scaling law, respectively. The obtained critical exponents were self-consistent and follow the scaling equation around $T{}_C$, indicating the reliability of these values. Further analysis revealed that the spin coupling inside ${\mathrm{Cr}}_4{\mathrm{Te}}_5$ exhibits three-dimensional Heisenberg-like behavior with long-range magnetic interaction and that the exchange interaction decays as $J\left(r\right)\approx r^{-4.85}$.

Determining the exact pairing symmetry of the superconducting order parameter in candidate unconventional superconductors remains an important challenge. Recently, a new method, based on phase sensitive quasiparticle interference measurements, was developed to identify gap sign changes in isotropic multiband systems. Here we extend this approach to the single-band and multiband nodal $d$-wave superconducting cases relevant, respectively, for the cuprates and likely for the infinite-layer nickelate superconductors. Combining analytical and numerical calculations, we show that the antisymmetrized correction to the tunneling density of states due to nonmagnetic impurities in the Born limit and at intermediate-scattering strength shows characteristic features for sign-changing and sign-preserving scattering wave vectors, as well as for the momentum-integrated quantity. Furthermore, using a realistic approach accounting for the Wannier orbitals, we model scanning tunneling microscopy data of ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_{8+\delta }$, which should allow the comparison of our theory with experimental data.

We report the discovery of superconductivity at about 4.3 K in samarium-doped 1,3,5-triphenylbenzene. Using a solid-state reaction method, samarium-doped 1,3,5-triphenylbenzene samples are successfully synthesized. These samples are characterized by magnetization, x-ray diffraction, scanning electron microscope, and energy-dispersive spectroscopy measurements. The x-ray diffractions reveal that the sample crystallizes in the space group $P2/m$. The magnetization measurements reveal a superconducting transition at about 4.3 K. However, the superconducting shielding fraction is only about 1%, which is similar to previous reports of the superconductivity in other aromatic hydrocarbons. Magnetization hysteresis loops of the sample show that it is a typical type II superconductor. Our results indicate possible superconductivity in this organic material.

The Su-Schrieffer-Heeger model describes fermions that hop on a one-dimensional chain with staggered hopping amplitude, where the unit cell contains two sites, or two sublattices. In this work we consider the Su-Schrieffer-Heeger model with superconducting pairing and show that the sublattice index acts as an additional quantum number in the classification of Cooper pairs, giving rise to inter- and intrasublattice odd-frequency pair correlations in the bulk. Interestingly, this system behaves as a two-band superconductor where the bulk odd-frequency correlations depend solely on the intrinsic staggering properties of the model. In general, odd-frequency correlations coexist with even-frequency correlations in both the trivial and topological phases, with comparable and even larger odd-frequency amplitudes at the topological phase transition points at low frequencies, due to the closing of the energy gap at these points. Furthermore, we also discuss how bulk odd-frequency amplitudes are correlated with pseudogaps in the density of states and also with a charge density wave that appears due to the chemical potential imbalance between sublattices.

We present here a theoretical study of ordering processes in metal-hydrogen compounds based on a generalized perturbation method and on tight-binding coherent potential approximation. This approach is illustrated for zirconium hydrides, in which case we demonstrate that a cluster expansion of the ordering energy can be limited to effective pair interactions, the leading one being between hydrogen atoms in third-neighbor positions. These results are quantitatively confirmed by comparison to density functional theory calculations and qualitatively interpreted through orbital symmetry analysis. The method is then applied first to draw a preliminary Zr-H phase diagram and then to characterize the effect of lattice deformation on the ordering processes in zirconium hydrides.

A single-crystal sample of the frustrated quasi-one-dimensional quantum magnet ${\mathrm{Cs}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ is investigated by magnetic and thermodynamic measurements. A combination of specific heat and magnetic torque measurements maps out the entire $H\text{−}T$ phase diagram for three orientations. Remarkably, a new phase emerges below the saturation field, irrespective of the crystal orientation. It is suggested that the presaturation phase represents spin-nematic order or other multimagnon condensate. The phase diagrams within the long-range-ordered dome are qualitatively different for each geometry. In particular, multiple transitions are identified in the field along the chain direction.

We report results of a ${}^{57}\mathrm{Fe}$ Mössbauer study focused on Co-substituted $\mathrm{BiF}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x{\mathrm{O}}_3$ ($x=0.05$). The experimental spectra were analyzed assuming the space-modulated cycloidal magnetic structure with high values of the anharmonicity parameter $m=0.47-0.92$ (at 4.7 K). The $m$ parameter increases with increasing temperature to reach its maximum value ($m\approx 1$) for $x=0.10$, 0.15 corresponding to the transition to a collinear antiferromagnetic structure of easy-plane type (${\mu }_{\mathrm{Fe}}\perp c_h$). Such an unusual trend of the anharmonicity parameter $m$(T) increasing with temperature was well reproduced by the molecular field model in which the low-spin to intermediate-spin transition of $\mathrm{C}{\mathrm{o}}^{3+}$ is assumed. At low cobalt content, $\mathrm{BiF}{\mathrm{e}}_{0.95}\mathrm{C}{\mathrm{o}}_{0.05}{\mathrm{O}}_3$, the cycloidal structure is retained but with a very high degree of anharmonicity, corresponding to the nearly collinear spin order.

We investigate how to facilitate the injection of domain walls in chiral ferromagnetic nanowires by electrical means. We calculate the critical current density above which domain walls are injected into the nanowire depending on the material parameters and the source of interaction including spin-transfer torques as well as spin-orbit torques. We demonstrate that the Dzyaloshinskii-Moriya interaction can significantly reduce the required critical current to inject the types of domain walls favored by the Dzyaloshinskii-Moriya interaction. We find that in chiral magnets it is only possible to shed a single domain wall by means of spin-orbit torques, as they modify the ground state orientation of the system. In contrast, for spin-transfer torque-induced shedding of domain walls, we show that there exist two different critical current densities for the two different domain wall chiralities, respectively. Additionally, for the consecutive creation of domain walls by means of spin-transfer torques, we find that the interaction between the domain walls cannot be neglected and even may lead to the pairwise annihilation of consecutive domain walls with opposite chiralities.

We investigate the competition between superconductivity (SC) and charge density wave (CDW) under a time-dependent periodic field in the attractive Hubbard model. By employing the time-dependent exact diagonalization method, we show that the driving frequency and amplitude of the external field can control the enhancement of either the superconducting pair or the CDW correlation in the system, for which SC and CDW are degenerate in the ground state of the half-filled attractive Hubbard model in the absence of the field. In the strong-coupling limit of the attractive Hubbard interaction, the controllability is characterized by the anisotropic interaction of the effective model. The anisotropy is induced by the external field and lifts the degeneracy of SC and CDW. We find that the enhancement or suppression of the superconducting pair and CDW correlations in the periodically driven attractive Hubbard model can be well interpreted by the quench dynamics of the effective model derived in the strong-coupling limit.

Combining the time-dependent variational principle (TDVP) algorithm with the parallelization scheme introduced by Stoudenmire and White for the density matrix renormalization group (DMRG), we present the first parallel matrix product state (MPS) algorithm capable of time evolving one-dimensional (1D) quantum lattice systems with long-range interactions. We benchmark the accuracy and performance of the algorithm by simulating quenches in the long-range Ising and XY models. We show that our code scales well up to 32 processes, with parallel efficiencies as high as $86\%$. Finally, we calculate the dynamical correlation function of a 201-site Heisenberg XXX spin chain with $1/r^2$ interactions, which is challenging to compute sequentially. These results pave the way for the application of tensor networks to increasingly complex many-body systems.

We investigate transport through a normal-superconductor (NS) junction made from a quantum spin Hall (QSH) system with helical edge states and a two-dimensional (2D) chiral topological superconductor (TSC) having a chiral Majorana edge mode. We employ a two-dimensional extended four-band model for HgTe-based quantum wells in a magnetic (Zeeman) field and subject to $s$-wave superconductivity. We show using the Bogoliubov-de Gennes scattering formalism that this structure provides a striking transport signal of a 2D TSC. As a function of the sample width (or Fermi energy) the conductance resonances go through a sequence of $2e^2/h$ (nontrivial phase) and $4e^2/h$ plateaux (trivial phase) which fall within the region of a nonzero Chern number (2D limit) as the sample width becomes large. These signatures are a manifestation of the topological nature of the QSH effect and the TSC.

The effect of spin-orbit coupling on quantum well states (QWSs) in atomically thin Ir adlayers deposited on the Au(111) substrate is studied in the framework of the density functional theory. Varying the Ir film thickness from 1 to 3 atomic layers, we find numerous Ir-derived QWSs, which are mainly of $d$ character. The resulting band dispersion of QWSs appearing around the surface Brillouin zone center in a wide Au(111) energy gap is analyzed in the framework of the Rashba model. In all such QWSs, the fitted values of the Rashba parameter exceed $2\mathrm{eV}\AA$. The maximal value of $6.4\mathrm{eV}\AA$ was obtained for the 1-monolayer-Ir/Au(111) system. We explain such large spin splitting by hybridization between different QWSs. Strong enhancement is observed in the density of electronic states at the surface in the energy region around the Fermi level caused by these QWSs.

We insert and manipulate a single chlorine atom in chlorine monolayer on a Si(100)-$2\times 1$ surface using a scanning tunneling microscope. Two objects were created—a Cl atom in a groove between two dimer rows, and bridge-bonded Cl on a silicon dimer. Changing the voltage polarity leads to conversion of the objects into each other. Anisotropic movement of the objects at 77 K is mediated by two different diffusion mechanisms: hopping and crowdion-like motion. Insertion of a Cl atom in a groove between two dimer rows leads to the formation of a dangling bond on a third-layer Si atom. At positive sample voltage bias, the first object is positively charged while the second object can be neutral or negatively charged depending on silicon sample doping.

We derive effective Floquet Hamiltonians for twisted bilayer graphene driven by circularly polarized light in two different regimes beyond the weak-drive, high-frequency regime. First, we consider a driving protocol relevant for experiments with frequencies smaller than the bandwidth and weak amplitudes and derive an effective Hamiltonian, which through a symmetry analysis, provides analytical insight into the rich effects of the drive. We find that circularly polarized light at low frequencies can selectively decrease the strength of AA-type interlayer hopping while leaving the AB-type unaffected. Then, we consider the intermediate frequency and intermediate-strength drive regime. We provide a compact and accurate effective Hamiltonian which we compare with the Van Vleck expansion and demonstrate that it provides a significantly improved representation of the exact quasienergies. Finally, we discuss the effect of the drive on the symmetries, Fermi velocity, and the gap of the Floquet flat bands.

Higher-order topological phases of matter have been extensively studied in various areas of physics. While the Aubry-André-Harper model provides a paradigmatic example to study topological phases, it has not been explored whether a generalized Aubry-André-Harper model can exhibit a higher-order topological phenomenon. Here, we construct a two-dimensional higher-order topological insulator with chiral symmetry based on the Aubry-André-Harper model. We find the coexistence of zero-energy and nonzero-energy corner-localized modes. The former is protected by the quantized quadrupole moment, while the latter by the first Chern number of the Wannier band. The nonzero-energy mode can also be viewed as the consequence of a Chern insulator localized on a surface. More interestingly, the nonzero-energy corner mode can lie in the continuum of extended bulk states and form a bound state in the continuum of higher-order topological systems. We finally propose an experimental scheme to realize our model in electric circuits. Our study opens a door to further study higher-order topological phases based on the Aubry-André-Harper model.

The construction of good effective models is an essential part of understanding and simulating complex systems in many areas of science. It is a particular challenge for correlated many-body quantum systems displaying emergent physics. We propose a machine learning approach that optimizes an effective model based on an estimation of its partition function. The success of the method is demonstrated by application to the single impurity Anderson model and double quantum dots, where nonperturbative results are obtained for the old problem of mapping to effective Kondo models. We also show that an alternative approach based on learning minimal models from observables may yield the wrong low-energy physics. On the other hand, learning minimal models from the partition function recovers the correct low-energy physics but may not reproduce all observables.

We introduce an approach that allows one complete control over the modulation of the effective twist angle change in few-layer van der Waals heterostructures by irradiating them with longitudinal waves of light at the end of a waveguide. As a specific application, we consider twisted bilayer graphene and show that one can tune the magic angles to be either larger or smaller, allowing in situ experimental control of the phase diagram of this and other related materials. A waveguide allows one to circumvent the free-space constraints on the absence of longitudinal electric field components of light. We propose to place twisted bilayer graphene at a specific location at the exit of a waveguide, such that it is subjected to purely longitudinal components of a transverse magnetic mode wave.

To obtain accurate independent-particle descriptions for ferromagnetic two-dimensional van der Waals materials, we apply the quasiparticle self-consistent $GW$ (QSGW) method to ${\mathrm{VI}}_3, {\mathrm{CrI}}_3, {\mathrm{CrGeTe}}_3$, and ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. QSGW provides a description of the nonlocal exchange-correlation term in the one-particle Hamiltonian. The nonlocal term is important not only as the $U$ of density functional theory $\left(\mathrm{DFT}\right)+U$ but also for differentiating occupied and unoccupied states in semiconductors. We show the limitations of $\mathrm{DFT}+U$ in mimicking QSGW.

$\left(\mathrm{ET}\right){\mathrm{Ag}}_4{\left(\mathrm{CN}\right)}_5$ [ET = BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] is a rare example of organic conductors that crystallizes in a diamond structure due to the tetrahedral arrangement of neighboring ET molecules. Because each ET has a $+1$ site charge, the band is half filled, which in combination with strong Coulomb interactions, turn $\left(\mathrm{ET}\right){\mathrm{Ag}}_4{\left(\mathrm{CN}\right)}_5$ into a Mott insulator. Recently, it was realized that this material may realize a nodal Dirac semimetal with a diamondlike lattice, whose state can be induced by suppressing the strong electron correlations. In this report, we present high pressure transport and optical studies on $\left(\mathrm{ET}\right){\mathrm{Ag}}_4{\left(\mathrm{CN}\right)}_5$. Our transport study indicates that the activation energy is highly suppressed by the application of hydrostatic pressure. At high pressures, the system varies from activated behavior into Coulomb gap variable range hopping.