We study the dynamics of inorganic antiferroelectric nanoclusters formed during an order-disorder transition and demonstrate the coexistence of the two phases in a region of 2–3 K around the transition temperature $T_N\sim 215$ K. Single crystals of ${\mathrm{NH}}_4{\mathrm{H}}_2{\mathrm{AsO}}_4$, a model hydrogen-bonded compound, show an antiferroelectric-paraelectric transition studied by means of highly sensitive magic angle spinning ${}^{15}\mathrm{N}$ NMR at 21.1 T. The phase coexistence is demonstrated by a double-peak structure of the chemical shift. Two-dimensional chemical exchange spectroscopy and spin-lattice relaxation time $\left(T_1\right)$ measurements show that the clusters are dynamic with sizes $\sim 50$ nm and lifetimes approaching seconds as $T\to T_N$. Their occupancy increases rapidly to fill the crystal volume below $T_N$. This study provides evidence for the commonality of the phase transitions in systems with electric properties and provides an improved spectroscopic method for such studies.

The orientation-dependent equilibrium ferroelectric domain structures and dielectric properties of polydomain ${\mathrm{PbZr}}_1{}_{-}{}_x{\mathrm{Ti}}_x{\mathrm{O}}_3$ thin films are investigated using a phenomenological Ginzburg-Landau-Devonshire thermodynamic model. We develop and describe three-dimensional polydomain models for (001)-, (101)-, and (111)-oriented films and explore the evolution of the structure and dielectric permittivity of the system as a function of epitaxial strain across the composition range $0.5\le x\le 1.0$. Our studies reveal that the film orientation, epitaxial strain, and composition can combine in unexpected ways to drive exotic phase stability and transformations which have intriguing implications for the properties. In particular, in (101)- and (111)-oriented films, the application of epitaxial strains along non-$\langle 001\rangle$-type crystallographic directions significantly reduces the stability range of the parent tetragonal phase [which is dominant in (001)-oriented films] and results in a variety of new symmetries. We also observe that the film orientation can be used to tune the relative fraction of intrinsic (i.e., within a domain) and extrinsic (i.e., from domain wall motion) contributions to the dielectric permittivity. Ultimately these studies reveal how composition, epitaxial strain, and film orientation provide for comprehensive control of the structure and properties of ferroelectrics.

We employ the quasiharmonic approximation to study the temperature-dependent lattice dynamics of the four different phases of cesium tin iodide $\left({\mathrm{CsSnI}}_3\right)$. Within this framework, we obtain the temperature dependence of a number of structural properties, including the cell volume, bulk modulus, and Grüneisen parameter. The Gibbs free energy of each phase is compared against the temperature-dependent Helmholtz energy obtained from the equilibrium structure within the harmonic approximation. We find that the black tetragonal perovskite phase is not dynamically stable up to at least 500 K, with the phonon dispersion displaying negative optic modes, which pass through all of the high-symmetry wave vectors in the Brillouin zone. The main contributions to the negative modes are found to be motions of the Cs atom inside the perovskite cage. The black cubic perovskite structure shows a zone-boundary instability, indicated by soft modes at the special $\mathbf{q}$ points $M$ and $R$. These modes are present in calculations at the equilibrium (0 K) lattice constant, while at finite temperature additional negative modes develop at the zone center, indicating a ferroelectric instability. The yellow crystal, composed of one-dimensional $\left({\mathrm{SnI}}_6\right){}_n$ double chains, has the same heat of formation as the orthorhombic perovskite phase at 0 K, but becomes less energetically favorable at higher temperatures, due to its higher free energy.

Effects of interchain couplings and anisotropy on a Haldane chain have been investigated by single-crystal inelastic neutron scattering and density functional theory (DFT) calculations on the model compound ${\mathrm{SrNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$. Significant effects on low-energy excitation spectra are found where the Haldane gap (${\Delta }_0\approx 0.41J$, where $J$ is the intrachain exchange interaction) is replaced by three energy minima at different antiferromagnetic zone centers due to the complex interchain couplings. Further, the triplet states are split into two branches by single-ion anisotropy. Quantitative information on the intrachain and interchain interactions as well as on the single-ion anisotropy is obtained from the analyses of the neutron scattering spectra by the random-phase approximation method. The presence of multiple competing interchain interactions is found from the analysis of the experimental spectra and is also confirmed by the DFT calculations. The interchain interactions are two orders of magnitude weaker than the nearest-neighbor intrachain interaction $J=8.7$ meV. The DFT calculations reveal that the dominant intrachain nearest-neighbor interaction occurs via nontrivial extended superexchange pathways Ni-O-V-O-Ni involving the empty $d$ orbital of V ions. The present single-crystal study also allows us to correctly position ${\mathrm{SrNi}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in the theoretical $D\text{−}{J}_{\perp }$ phase diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)], showing where it lies within the spin-liquid phase.

Specific features of orbital and spin structure of transition-metal compounds in the case of the face-sharing ${\text{MO}}_6$ octahedra are analyzed. In this geometry, we consider the form of the spin-orbital Hamiltonian for transition-metal ions with double $\left(e_g^{\sigma }\right)$ or triple $\left(t_{2g}\right)$ orbital degeneracy. Trigonal distortions typical of the structures with face-sharing octahedra lead to splitting of $t_{2g}$ orbitals into an $a_{1g}$ singlet and $e_g^{\pi }$ doublet. For both doublets ($e_g^{\sigma }$ and $e_g^{\pi }$), in the case of one electron or hole per site, we arrive at a symmetric model with the orbital and spin interaction of the Heisenberg type and the Hamiltonian of unexpectedly high symmetry: SU(4). Thus, many real materials with this geometry can serve as a testing ground for checking the prediction of this interesting theoretical model. We also compare general trends in the spin-orbital (“Kugel-Khomskii”) exchange interaction for three typical situations: those of ${\text{MO}}_6$ octahedra with common corner, common edge, and the present case of common face, which has not been considered yet.

We show that the anomalous temperature scaling of five distinct transport quantities in the strange metal regime of the cuprate superconductors can be reproduced with only two nontrivial critical exponents. The quantities are (i) the electrical resistivity, (ii) the Hall angle, (iii) the Hall-Lorenz ratio, (iv) the magnetoresistance, and (v) the thermopower. The exponents are the dynamical critical exponent $z=4/3$ and an anomalous scaling dimension $\Phi =-2/3$ for the charge density operator.

We study the nonequilibrium time evolution of a variety of one-dimensional (1D) and two-dimensional (2D) systems (including SSH model, Kitaev-chain, Haldane model, $p+ip$ superconductor, etc.) following a sudden quench. We prove analytically that topology-changing quenches are always followed by nonanalytical temporal behavior of return rates (logarithm of the Loschmidt echo), referred to as dynamical phase transitions (DPTs) in the literature. Similarly to edge states in topological insulators, DPTs can be classified as being topologically protected or not. In 1D systems the number of topologically protected nonequilibrium time scales are determined by the difference between the initial and final winding numbers, while in 2D systems no such relation exists for the Chern numbers. The singularities of dynamical free energy in the 2D case are qualitatively different from those of the 1D case; the cusps appear only in the first time derivative.

Anfuso and Rosch [Phys. Rev. B 75, 144420 (2007)] showed that the “topological” Haldane phase in a fermionic spin-$1/2$ ladder can be continuously deformed into a “trivial” phase without explicitly breaking symmetries when local charge fluctuations are taken into account. Within the framework of symmetry-protected topological phases, we revisit the model and demonstrate how the Haldane phase can be adiabatically connected to a trivial phase due to charge fluctuations. Furthermore, we show that the Haldane phase remains stable as long as the system is symmetric under particular reflection symmetries.

We report on noise-induced spin ordering in a collective quasiparticle system: spinor stochastic resonance. Synergetic interplay of a polarization-modulated signal and a polarization noise allows us to switch coherently between the two metastable states of a microcavity-polariton spin bistable system. Spinor stochastic resonance is demonstrated in a zero-dimensional GaAs based microcavity. The resonance behaviors of both the spin amplification and the signal-to-noise ratio are experimentally evidenced as a function of the noise strength for different amplitude modulations. They are theoretically reproduced using a spinor Gross-Pitaevskii equation driven by a randomly polarized laser field.

We consider resonant scatterers with large scattering cross sections in graphene that are produced by a gated disk or a vacancy, and show that a gated ring can be engineered to produce an efficient electron cloak. We also demonstrate that this same scheme can be applied to tune the direction of electron flow. Our analysis is based on a partial-wave expansion of the electronic wave functions in the continuum approximation, described by the Dirac equation. Using a symmetrized version of the massless Dirac equation, we derive a general condition for the cloaking of a scatterer by a potential with radial symmetry. We also perform tight-binding calculations to show that our findings are robust against the presence of disorder in the gate potential.

Combining scanning tunneling microscopy and first-principles calculations, we have shown that single Co atoms adsorbed on a $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ surface have eight different configurations that possess different spin magnetic moments. Despite the large adsorption energy, we have demonstrated that both the position and the spin state of single adsorbed Co atoms can be well controlled through the vertical atomic manipulation and the “tip-touch” operation that converts Co atoms among various adsorption configurations. Our approach to construct atomic-scale magnetic structures on a semiconductor surface can provide a new pathway to realize spin-based devices, including a scalable solid-state quantum computer.

We report the formation of an ordered ${\mathrm{C}}_{60}$ monolayer on the ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of ${\mathrm{C}}_{60}$ with the sample held between 573–673 K leads to a [4, $-2\mid 1,3$] phase. The growth of ${\mathrm{C}}_{60}$ proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 Å${}^2$. The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each ${\mathrm{C}}_{60}$ in the observed superstructure. Several defects (vacancies, protrusions,…) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging ${\mathrm{C}}_{60}$ molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of ${\mathrm{Al}}_4{\mathrm{C}}_3$ domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface dictates the carbide domain orientations.

We present a theoretical study of the dynamics of H atoms adsorbed on graphene bilayers with Bernal stacking. First, through extensive density functional theory calculations, including van der Waals interactions, we obtain the activation barriers involved in the desorption and migration processes of a single H atom. These barriers, along with attempt rates and the energetics of H pairs, are used as input parameters in kinetic Monte Carlo simulations to study the time evolution of an initial random distribution of adsorbed H atoms. The simulations reveal that, at room temperature, H atoms occupy only one sublattice before they completely desorb or form clusters. This sublattice selectivity in the distribution of H atoms may last for sufficiently long periods of time upon lowering the temperature down to $0{}^{\circ }\mathrm{C}$. The final fate of the H atoms, namely, desorption or cluster formation, depends on the actual relative values of the activation barriers which can be tuned by doping. In some cases, a sublattice selectivity can be obtained for periods of time experimentally relevant even at room temperature. This result shows the possibility for observation and applications of the ferromagnetic state associated with such distribution.

Making a metal contact to the two-dimensional semiconductor ${\mathrm{MoS}}_2$ without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function $(\gtrsim 4.7$ eV) metals, the Fermi level is pinned at 0.1–0.3 eV below the conduction band edge of ${\mathrm{MoS}}_2$ for low work function metals, due to the metal-${\mathrm{MoS}}_2$ interaction. Inserting a boron nitride (BN) monolayer between the metal and the ${\mathrm{MoS}}_2$ disrupts this interaction, and restores the ${\mathrm{MoS}}_2$ electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by $\sim 2$ eV, and enables a lineup of the Fermi level with the ${\mathrm{MoS}}_2$ conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights.

We address the stability of the surface phases that occur on the C side of $3C-\mathrm{SiC}\left(\overline{1}\overline{1}\overline{1}\right)$ at the onset of graphene formation. In this growth range, experimental reports reveal a coexistence of several surface phases. This coexistence can be explained by a Si-rich model for the unknown $(3\times 3)$ reconstruction, the known $(2\times 2{)}_{\text{C}}$ adatom phase, and the graphene-covered $(2\times 2{)}_{\text{C}}$ phase. By constructing an ab initio surface phase diagram using a van der Waals corrected density functional, we show that the formation of a well defined interface structure like the “buffer layer” on the Si side is blocked by Si-rich surface reconstructions.

We investigate theoretically the nonequilibrium transport properties of a topological insulator quantum dot (TIQD) in the Coulomb blockade and Kondo regime. An Anderson impurity model is applied to a TIQD system coupled to two external leads, and we show that the model realizes the spin-orbital Kondo effect at the Dirac point where the edge states are not split by a finite-size effect, leading to an additional $SU\left(4\right)$ symmetry because of the presence of strong mixture among four internal degrees of freedom. In a more realistic situation where the degeneracy is lifted due to the finite-size effect, we demonstrate that there is a richer structure in transport measurements. We illustrate a continuous crossover from four (spin and orbital) Coulomb peaks with large interpair spacing and small intrapair spacing to a double-peak structure in the local density of states (LDOS) as increasing the hybridization strength $\Gamma$ within the Coulomb blockade regime. When temperature falls below the Kondo temperature $T_K$, four Kondo peaks show up in the nonequilibrium LDOS. Two of them are located at the chemical potential of each lead, and the other two are shifted away from the chemical potential by an amount proportional to the TIQD's bare energy level, leading to a triple-peak structure in the differential conductance when a bias voltage is applied.

The exact-exchange (EXX) potential, which is obtained by solving the optimized-effective potential (OEP) equation, is compared to various approximate semilocal exchange potentials for a set of selected solids (C, Si, BN, MgO, ${\mathrm{Cu}}_2\mathrm{O}$, and NiO). This is done in the framework of the linearized augmented plane-wave method, which allows for a very accurate all-electron solution of electronic structure problems in solids. In order to assess the ability of the semilocal potentials to approximate the EXX-OEP, we considered the EXX total energy, electronic structure, electric-field gradient, and magnetic moment. An attempt to parametrize a semilocal exchange potential is also reported.

We use ab initio electronic structure calculations within the generalized gradient approximation (GGA+U) to density functional theory to determine the microscopic exchange interactions in the series of orthorhombic rare-earth manganites, o-$R{\mathrm{MnO}}_3$. Our motivation is to construct a model Hamiltonian (excluding effects due to spin-orbit coupling), which can provide an accurate description of the magnetism in these materials. First, we consider ${\mathrm{TbMnO}}_3$, which exhibits a spiral magnetic order at low temperatures. We map the exchange couplings in this compound onto a Heisenberg Hamiltonian and observe a clear deviation from the Heisenberg-like behavior. We consider first the coupling between magnetic and orbital degrees of freedom as a potential source of non-Heisenberg behavior in ${\mathrm{TbMnO}}_3$, but conclude that it does not explain the observed deviation. We find that higher order magnetic interactions (biquadratic and four-spin ring couplings) should be taken into account for a proper treatment of the magnetism in ${\mathrm{TbMnO}}_3$ as well as in the other representatives of the o-$R{\mathrm{MnO}}_3$ series with small radii of the $R$ cation.

In $p^{+}$ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly focused light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about $2\mu \mathrm{m}$ from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photoelectron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and band-gap renormalization are negligible due to electrostatic screening by the hole gas.