Inspired by recent experiments for hybrid organic-ferromagnet interfaces, we propose a spin-relaxation mechanism which does not depend on either the spin-orbit or the hyperfine interaction. This takes place when a molecule with initial spin imbalance is weakly coupled to a metal surface and can be excited in various vibrational states. In such a situation the electron-vibron interaction promotes the exchange of spin-polarized electrons between the molecule and the surface, serving as an energy and angular momentum reservoir. This process leads to an effective spin relaxation of the electron population in the molecule. We suggest that this nonequilibrium mechanism can be investigated through time-resolved spin-polarized scanning tunneling microscopy experiments.

An exponential interaction is constructed so that one-dimensional atoms and chains of atoms mimic the general behavior of their three-dimensional counterparts. Relative to the more commonly used soft-Coulomb interaction, the exponential greatly diminishes the computational time needed for calculating highly accurate quantities with the density matrix renormalization group. This is due to the use of a small matrix product operator and to exponentially vanishing tails. Furthermore, its more rapid decay closely mimics the screened Coulomb interaction in three dimensions. Choosing parameters to best match earlier calculations, we report results for the one-dimensional hydrogen atom, uniform gas, and small atoms and molecules both exactly and in the local density approximation.

We describe a variational approach to solving Anderson impurity models by means of exact diagonalization. Optimized parameters of a discretized auxiliary model are obtained on the basis of the Peierls-Feynman-Bogoliubov principle. Thereby, the variational approach resolves ambiguities related to the bath discretization, which is generally necessary to make Anderson impurity models tractable by exact diagonalization. The choice of variational degrees of freedom made here allows systematic improvements of total energies over mean-field decouplings like Hartree-Fock. Furthermore, our approach allows us to embed arbitrary bath discretization schemes in total-energy calculations and to systematically optimize and improve on traditional routes to the discretization problem such as fitting of hybridization functions on Matsubara frequencies. Benchmarks in terms of a single orbital Anderson model demonstrate that the variational exact diagonalization method accurately reproduces free energies as well as several single- and two-particle observables obtained from an exact solution. Finally, we demonstrate the applicability of the variational exact diagonalization approach to realistic five-orbital problems with the example system of Co impurities in bulk Cu and compare it to continuous-time Monte Carlo calculations. The accuracy of established bath discretization schemes is assessed in the framework of the variational approach introduced here.

Heterostructures like $\mathrm{InAs}\text{/}\mathrm{GaSb}$ superlattices (SLs) are distinctly different from well-studied ones like $\mathrm{GaAs}\text{/}\mathrm{AlAs}$ SLs in terms of band alignment, common interface atom, and phonon spectrum overlapping of the constituents, which manifests as stark differences in their electronic and vibrational properties. This paper reports a comprehensive examination of all four types of phonon modes (confined, quasiconfined, extended, and interface) that have long been predicted for the $\mathrm{InAs}\text{/}\mathrm{GaSb}$ SL, with the observation and interpretation of a set of phonon modes by performing cleaved edge $\mu$-Raman study with polarization analysis. Furthermore, we show a signature of symmetry reduction from ${\mathrm{D}}_{2\mathrm{d}}$ for $\mathrm{GaAs}\text{/}\mathrm{AlAs}$ SL to ${\mathrm{C}}_{2\mathrm{v}}$ for InAs/GaSb SL revealed as a phonon-polariton effect.

Deposition on a vicinal surface with alternating rough and smooth steps is described by a solid-on-solid model with anisotropic interactions. Kinetic Monte Carlo (KMC) simulations of the model reveal step pairing in the absence of any additional step attachment barriers. We explore the description of this behavior within an analytic Burton-Cabrera-Frank (BCF)-type step dynamics treatment. Without attachment barriers, conventional kinetic coefficients for the rough and smooth steps are identical, as are the predicted step velocities for a vicinal surface with equal terrace widths. However, we determine refined kinetic coefficients from a two-dimensional discrete deposition-diffusion equation formalism which accounts for step structure. These coefficients are generally higher for rough steps than for smooth steps, reflecting a higher propensity for capture of diffusing terrace adatoms due to a higher kink density. Such refined coefficients also depend on the local environment of the step and can even become negative (corresponding to net detachment despite an excess adatom density) for a smooth step in close proximity to a rough step. Our key observation is that incorporation of these refined kinetic coefficients into a BCF-type step dynamics treatment recovers quantitatively the mesoscale step-pairing behavior observed in the KMC simulations.

Degeneracy of discrete energy levels of finite-length, metallic single-wall carbon nanotubes depends on the type of nanotubes, boundary condition, length of nanotubes, and spin-orbit interaction. Metal-1 nanotubes, in which two nonequivalent valleys in the Brillouin zone have different orbital angular momenta with respect to the tube axis, exhibit nearly fourfold degeneracy and small lift of the degeneracy by the spin-orbit interaction reflecting the decoupling of two valleys in the eigenfunctions. In metal-2 nanotubes, in which the two valleys have the same orbital angular momentum, vernier-scale-like spectra appear for boundaries of orthogonal-shaped edge or cap termination reflecting the strong valley coupling and the asymmetric velocities of the Dirac states. Lift of the fourfold degeneracy by parity splitting overcomes the spin-orbit interaction in shorter nanotubes with a so-called minimal boundary. Slowly decaying evanescent modes appear in the energy gap induced by the curvature of nanotube surface. Effective one-dimensional lattice model reveals the role of boundary on the valley coupling in the eigenfunctions.

In this work, silicon-rich silicon oxide films of different stoichiometry were annealed at high temperatures in order to obtain silicon nanocrystals embedded in silica. The low-frequency Raman scattering has been observed and related to acoustic phonons confined in these nanocrystals. It has been found that this scattering consists of two modes: one at a lower frequency and one at a higher frequency. The depolarization ratios for these modes were determined, showing that the lower frequency mode is depolarized and the higher frequency mode is polarized. It has been also found that under specific conditions of film preparation the product of mode frequency and nanocrystal diameter is scale invariant. Finally, it has been shown that the confined acoustic phonon frequencies do not simply depend on the nanocrystal size alone, but also on the Si concentration in the film itself. This effect has been ascribed to the accelerated nucleation and enhanced crystallization occurring in the films deposited with higher Si content.

Recently, a crystalline-symmetry-protected three-dimensional (3D) bulk Dirac semimetal phase has been experimentally identified in a stoichiometric high-mobility compound, ${\mathrm{Cd}}_3{\mathrm{As}}_2$. The Dirac state observed in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ has been attributed to originate mostly from the bulk state while calculations show that the bulk and surface states overlap over the entire Dirac dispersion energy range. In this study we unambiguously reveal doping induced evolution of the ground state of surface and bulk electron dynamics in a 3D Dirac semimetal. The technique demonstrated in this study by simultaneously using angle-resolved photoemission spectroscopy (ARPES) and in situ surface deposition isolates the surface and bulk states in ${\mathrm{Cd}}_3{\mathrm{As}}_2$. Our experimental results provide a method for tuning the chemical potential as well as to observe surface states degenerate with bulk states, which will be useful for future applications of a 3D Dirac semimetal.

Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. ${\mathrm{Sr}}_2{\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{\mathrm{O}}_2{\mathrm{Cl}}_2$, a prototype of a diluted spin $S=1/2$ antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length $\xi$ estimated from ${}^{35}\mathrm{Cl}$ NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on $\xi$. Moreover, the temperature dependence of that spectral weight is found to scale with the $x$-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.

We revisit the problem of dynamical response in spin-charge separated one-dimensional quantum fluids. In the framework of Luttinger liquid theory, the dynamical response is formulated in terms of noninteracting bosonic collective excitations carrying either charge or spin. We argue that, as a result of spectral nonlinearity, long-lived excitations are best understood in terms of generally strongly interacting fermionic holons and spinons. This has far reaching ramifications for the construction of mobile impurity models used to determine threshold singularities in response functions. We formulate and solve the appropriate mobile impurity model describing the spinon threshold in the single-particle Green's function. Our formulation further raises the question whether it is possible to realize a model of noninteracting fermionic holons and spinons in microscopic lattice models of interacting spinful fermions. We investigate this issue in some detail by means of density matrix renormalization group (DMRG) computations.

Triplet-exciton polaron quenching can lead to large changes in sample conductivity in organic devices. The application of pulsed magnetic resonance leads to changes in the quenching process by coherently driving spin populations between different eigenstates. Here, we investigate the influence of the exciton-polaron dissociation, intersystem crossing, and recombination rates on the electrical response of a device following such resonant excitation. Although these incoherent processes often occur on time scales that are orders of magnitude slower than coherent mixing, we find that they have a major influence on the electrical response. For example, as the relative recombination and dissociation rates are varied, certain resonant transitions can become electrically invisible. We demonstrate that transitions between different regimes are determined by a dimensionless parameter $\chi =\frac{r}{d}$, the ratio of the recombination to the dissociation rate.

We investigate the origin of the depolarization rates in ultrathin adsorbate-stabilized ferroelectric wires. By applying density functional theory calculations and analytic modeling, we demonstrate that the depolarization results from the leakage of charges stored at the surface adsorbates, which play an important role in the polarization stabilization. The depolarization speed varies with thickness and temperature, following several complex trends. A comprehensive physical model is presented, in which quantum tunneling, Schottky emission, and temperature-dependent electron mobility are taken into consideration. This model simulates experimental results, validating the physical mechanism. We also expect that this improved tunneling-Schottky emission model could be applied to predict the retention time of polarization and the leakage current for various ferroelectric materials with different thicknesses and temperatures.

Branded for its potential electronic ferroelectricity, charge-ordered $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ has a layered Fe triangular framework, whose topology is a source of degeneracy, both at the charge and spin levels. Here we present an in-depth characterization of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$, the high-pressure $\left(hp\right)$ polymorph of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$, using electron, x-ray, and neutron diffraction, combined with transport and magnetization measurements. We show that $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4\text{−}hp$ is characterized by a misfit-related monoclinic structure, accommodating a buckled triangular ${\left[\mathrm{Lu}\right]}_{\infty }$ layer and two shifted adjacent rectangular ${\left[\mathrm{Fe}\right]}_{\infty }$ planes belonging to a distorted rock salt–type layer. The release of the geometric frustration of the Fe magnetic lattice in the $hp$ form leads to collinear antiferromagnetic ordering at $T_N=380\mathrm{K}$. Possible coexistence of charge and magnetic orders in this material opens research pathways for the design of tunable multifunctional devices using high-pressure techniques.

We calculate the angular dependence of the x-ray linear and circular dichroism at the $L_{2,3}$ edges of $\alpha$-Fe(II) Phthalocyanine (FePc) thin films using a ligand-field model with full configuration interaction. We find the best agreement with the experimental spectra for a mixed ground state of ${}^3{E}_g\left(a_{1g}^2{e}_g^3{b}_{2g}^1\right)$ and ${}^3{B}_{2g}\left(a_{1g}^1{e}_g^4{b}_{2g}^1\right)$ with the two configurations coupled by the spin-orbit interaction. The ${}^3{E}_g\left(b\right)$ and ${}^3{B}_{2g}$ states have easy-axis and easy-plane anisotropies, respectively. Our model accounts for an easy-plane magnetic anisotropy and the measured magnitudes of the in-plane orbital and spin moments. The proximity in energy of the two configurations allows a switching of the magnetic anisotropy from easy plane to easy axis with a small change in the crystal field, as recently observed for FePc adsorbed on an oxidized Cu surface. We also discuss the possibility of a quintet ground state (${}^5{A}_{1g}$ is 250 meV above the ground state) with planar anisotropy by manipulation of the Fe-C bond length by depositing the complex on a substrate that is subjected to a mechanical strain.

Holmium, the archetypical system for spin-spiral antiferromagnetism, undergoes an in-plane spin-flop transition earlier attributed to competing symmetry-breaking and fully symmetric magnetoelastic anisotropy terms [Phys. Rev. Lett. 94, 227204 (2005)], which underlines the emergence of sixfold magnetoelastic constants in heavy rare earth metals, as otherwise later studies suggested. A model that encompasses magnetoelastic contributions to the in-plane sixfold magnetic anisotropy is laid out to elucidate the mechanism behind the spin-flop transition. The model, which is tested in a Ho-based superlattice, shows that the interplay between competing fully symmetric $\alpha$-magnetoelastic and symmetry-breaking $\gamma$-magnetoelastic anisotropy terms triggers the spin reorientation. This also unveils the dominant role played by the sixfold exchange magnetostriction constant, where $D_{\alpha 2}^{66}\simeq 0.32$ GPa against its crystal-field counterpart $M_{\alpha 2}^{66}\simeq -0.2$ GPa, in contrast to the crystal-field origin of the symmetry-breaking magnetostriction in rare earth metals.

Three-dimensional Dirac semimetals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties. It has been suggested that structurally distorting a DSM can create a topological insulator but this has not yet been experimentally verified. Furthermore, Majorana fermions have been theoretically proposed to exist in materials that exhibit both superconductivity and topological surface states. Here we show that the cubic Laves phase ${\mathrm{Au}}_2\mathrm{Pb}$ has a bulk Dirac cone that is predicted to gap on cooling through a structural phase transition at 100 K. The low temperature phase can be assigned a ${\mathrm{Z}}_2=-1$ topological index, and this phase becomes superconducting below 1.2 K. These characteristics make ${\mathrm{Au}}_2\mathrm{Pb}$ a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states, combining many different properties of emergent materials—superconductivity, bulk Dirac electrons, and a topologically nontrivial ${\mathrm{Z}}_2$ invariant.

We report $\alpha -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ to be an improper multiferroic with the simultaneous development of electric polarization and magnetization below $T_C=35\mathrm{K}$. The observed spontaneous polarization of $0.55\mu \mathrm{C}{\mathrm{cm}}^{-2}$ magnitude is highest among copper-based improper multiferroic materials. Our study demonstrates a sizable amount of magnetoelectric coupling below $T_C$, even with a low magnetic field. The theoretical calculations based on density functional theory indicate magnetism in $\alpha -{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ is a consequence of ferro-orbital ordering driven by a polar lattice distortion due to the unique pyramidal (${\mathrm{CuO}}_5$) environment of Cu. Spin-orbit coupling further stabilizes orbital ordering and is crucial for magnetism. The calculations indicate that the origin of the giant ferroelectric polarization is primarily due to the symmetric exchange-striction mechanism and is corroborated by temperature-dependent x-ray studies.

The mineral haydeeite, $\alpha {\text{-MgCu}}_3{\left(\text{OD}\right)}_6{\text{Cl}}_2$, is a $S=\frac{1}{2}$ kagome ferromagnet that displays long-range magnetic order below $T_C=4.2$ K with a strongly reduced moment. Our inelastic neutron scattering data show clear spin-wave excitations that are well described by a Heisenberg Hamiltonian with a ferromagnetic nearest-neighbor exchange $J_1=-38$ K and an antiferromagnetic exchange $J_d=+11$ K across the hexagons of the kagome lattice. These values place haydeeite in the ferromagnetic part of the phase diagram, close to the cuboc2 type noncoplanar 12-sublattice antiferromagnetic order of cuboctahedral symmetry. Diffuse dynamic short-range ferromagnetic correlations observed above $T_C$ persist well into the ferromagnetically ordered phase with a behavior distinct from critical scattering.

We report measurements of the anisotropic upper critical field $H_{c2}\left(T\right)$ for ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ single crystals up to 60 T and $T>0.6\mathrm{K}$. Our results show that the upper critical field parallel to the Cr chains, $H_{c2}^{\parallel }\left(T\right)$, exhibits a paramagnetically limited behavior, whereas the shape of the $H_{c2}^{\perp }\left(T\right)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves $H_{c2}^{\perp }\left(T\right)$ and $H_{c2}^{\parallel }\left(T\right)$ cross at $T\approx 4\mathrm{K}$, so that the anisotropy parameter ${\gamma }_H\left(T\right)=H_{c2}^{\perp }/H_{c2}^{\parallel }\left(T\right)$ increases from ${\gamma }_H\left(T_c\right)\approx 0.35$ near $T_c$ to ${\gamma }_H\left(0\right)\approx 1.7$ at 0.6 K. The paramagnetically limited behavior of $H_{c2}^{\parallel }\left(T\right)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains.