A quantum kinetic theory of the spin transfer between carriers and Mn atoms in a Mn-doped diluted magnetic semiconductor is presented. It turns out that the typical memory time associated with these processes is orders of magnitude shorter than the time scale of the spin transfer. Nevertheless, Markovian rate equations, which are obtained by neglecting the memory, work well only for bulk systems. For quantum wells and wires the quantum kinetic results qualitatively deviate from the Markovian limit under certain conditions. Instead of a monotonic decay of an initially prepared excess electron spin, an overshoot or even coherent oscillations are found. It is demonstrated that these features are caused by energetic redistributions of the carriers due to the energy-time uncertainty.

We study the diffusive electron-electron interaction correction to conductivity by analyzing simultaneously ${\rho }_{xx}$ and ${\rho }_{xy}$ for disordered 2D electron systems in Si in a tilted magnetic field. Tilting the field is shown to be a straightforward tool to disentangle spin and orbital effects. In particular, by changing the tilt angle we prove experimentally that in the field range $g{\mu }_{B}B>k_{B}T$ the correction depends on the modulus of the magnetic field rather than on its direction, which is expected for a system with isotropic $g$ factor. In the high-field limit, the correction behaves as $\ln \left(B\right)$, as expected theoretically [Lee and Ramakrishnan, Phys. Rev. B 26, 4009 (1982)]. Our data prove that the diffusive electron-electron interaction correction to conductivity is not solely responsible for the huge and temperature-dependent magnetoresistance in a parallel field, typically observed in Si-MOSFETs.

Electronic screening renormalizes the linear bands of graphene and in the vicinity of the Dirac point, creates a diamond shaped structure in the quasiparticle spectral density. This is a result of electron-plasmon scattering processes which produce a finite momentum feature referred to as the “plasmaron ring.” In this work we explore the effects of uniaxial strain on these spectral features with the aim of understanding how strain modifies correlations. We derive and calculate the spectral density to the G${}_0$W-RPA level which allows us to identify the dispersive behavior of the diamond geometry, and thus electron-plasmon scattering, for variation in electron-electron coupling strength and magnitude of applied strain. We find that the application of strain changes the geometry (in momentum) of the electron-plasmon scattering and that renormalizations beyond simple geometrical scalings further enhance this effect. These results suggest that the properties of the plasmaron ring can be tuned through the application of uniaxial strain, effectively producing a larger fine structure constant without the need to change the sample substrate.

We advance a theory of ripple formation on the surface of an initially elemental material that is subjected to concurrent oblique-incidence impurity deposition and normal-incidence ion bombardment. The theory applies when the target atoms and impurities react to produce a chemical compound, as in many experiments that have been carried out in which the target material is silicon and the impurities are metallic. We show that the formation of a compound can destabilize a surface that would otherwise be stable. If ripples do form, they are oriented with their wave vector parallel to the projected impurity deposition direction and they propagate counter to the deposited flux. Our theory also predicts how the ripple wavelength behaves for impurity fluxes just above the threshold value necessary for ripple formation.

We examine the effect of the applied magnetic field amplitude on zero field-cooled/field-cooled (ZFC/FC) curves, through the nonlinear susceptibility of superparamagnetic particles (i.e., at thermondynamic equilibrium, but taking into account the magnetic anisotropy). This nonlinear effect is shown to be the first to manifest itself when going away from the linear response regime (i.e., when the magnetic moment is simply proportional to the applied field), largely before the modification of the macrospin switching energy due to the external field. We demonstrate that it has a significant impact on ZFC/FC curves, especially for the low-temperature behavior of the FC curve, even in usual experimental conditions. We then show how this nonlinearity can be taken into account, in an easy way, to obtain a better modeling of the susceptibility curves and consequently a more reliable determination of the nanoparticles’ magnetic properties. The theoretical considerations are confronted in a series of experimental measurements on Co nanoparticles.

We use a generalized spin-wave approach and large-scale quantum Monte Carlo (QMC) simulations to study the quantum phase diagram and quasiparticle excitations of the $S=1$ Heisenberg model with an easy-plane single-ion anisotropy in dimensions $d=2$ and 3. We consider two alternative approximations for describing the quantum paramagnetic state: the standard Holstein-Primakoff approximation and a modified treatment in which the local constraint (finite dimension of the local Hilbert space) is enforced by introducing a Lagrange multiplier. While both approximations produce qualitatively similar results, the latter approach is the only one that is in good quantitative agreement with the quantum phase diagram and the quasiparticle dispersions obtained with QMC. This result is very important for low-temperature studies of quantum paramagnets in magnetic fields because it shows that a simple modification of the standard analytical approach should produce much better quantitative agreement between theory and experiment.

We calculate the effective spin diffusion coefficient of weak ferromagnet and quasiequilibrium paramagnetic systems using Landau Fermi liquid theory. We find that the behavior of the diffusion coefficient of the quasiequilibrium system is determined primarily by the internal magnetic field, which, in turn, depends upon the nonequilibrium magnetization and the antisymmetric Landau parameters. We also show that this is qualitatively similar to the diffusion coefficient of the weak ferromagnetic system. We discuss its implication for the spin-polarized state created in cold atom systems.

In spin-transfer torque (STT) for technological applications, the miniaturization of the magnet may reach the stage of requiring a fully quantum-mechanical treatment. We present an STT theory which uses the quantum macrospin ground and excited (magnon) states of the nanomagnet. This allows for energy and angular momentum exchanges between the current electron and the nano-magnet. We develop a method of magnetization dynamics simulation which captures the heating effect on the magnet by the spin-polarized current and the temperature dependence in STT. We also discuss the magnetostatics effect on magnon scattering for ferromagnetic relaxation in a thin film. Our work demonstrates a realistic step towards simulation of quantum spin-transfer torque physics in nanoscale magnets.

Motivated by experiments on Josephson junction arrays in a magnetic field and ultracold interacting atoms in an optical lattice in the presence of a “synthetic” orbital magnetic field, we study the “fully frustrated” Bose-Hubbard model and quantum $\mathrm{XY}$ model with half a flux quantum per lattice plaquette. Using Monte Carlo simulations and the density matrix renormalization group method, we show that these kinetically frustrated boson models admit three phases at integer filling: a weakly interacting chiral superfluid phase with staggered loop currents which spontaneously break time-reversal symmetry, a conventional Mott insulator at strong coupling, and a remarkable “chiral Mott insulator” (CMI) with staggered loop currents sandwiched between them at intermediate correlation. We discuss how the CMI state may be viewed as an exciton condensate or a vortex supersolid, study a Jastrow variational wave function which captures its correlations, present results for the boson momentum distribution across the phase diagram, and consider various experimental implications of our phase diagram. Finally, we consider generalizations to a staggered flux Bose-Hubbard model and a two-dimensional (2D) version of the CMI in weakly coupled ladders.

Photoelectron spectroscopic measurements have been performed, with synchrotron radiation on PbSe/CdSe heteronanocrystals that initially consist of core-shell structures. The study of the chemical states of the main elements in the nanocrystals shows a reproducible and progressive change in the valence-band and core-level spectra under photon irradiation, whatever the core and shell sizes are. Such chemical modifications are explained in light of transmission electron microscopy observations and reveal a phase transformation of the nanocrystals: The core-shell nanocrystals undergo a morphological change toward a Janus structure with the formation of semidetached PbSe and CdSe clusters. Photoelectron spectroscopy gives new insight into the reorganization of the ligands anchored at the surface of the nanocrystals and the modification of the electronic structure of these heteronanocrystals.

High-pressure structural and electronic properties of CeO${}_2$ and ThO${}_2$ are investigated with density functional calculations. An isostructural phase transition with space group Pnma at about $100\mathrm{GPa}$ has been observed in both compounds. The high-pressure Pnma phase is revealed as being an orthorhombic distortion of the hexagonal Ni${}_2$In structure. A pressure-induced metallization in the high-pressure Pnma phase with a mechanism of Wilson transition is also predicted at around $250\mathrm{GPa}$ in CeO${}_2$, and at $350\mathrm{GPa}$ in ThO${}_2$, respectively. Two new metallic phases with space group P$\overline{3}$m1 and I4/mmm are found as the ground state in CeO${}_2$ when pressure is above $450\mathrm{GPa}$, whereas in ThO${}_2$ the Pnma phase is stable up to $1\mathrm{TPa}$. The equations of state are analyzed, as well as the pressure dependence of the lattice parameters, atomic fractional coordinates, and electronic band gaps.

The spin currents generated by thermal gradients through the spin Seebeck effect (SSE) are usually detected by the voltage generated in a normal metal by means of the inverse spin Hall effect. Here, we present a detailed account of an experimental investigation of the action of spin currents due to SSE on the relaxation rate of spin waves. Propagating spin-wave packets with a frequency in the range of 1–2 GHz are launched in film strips of single-crystal yttrium iron garnet, Y${}_3$Fe${}_5$O${}_{12}$ (YIG) while a thermal gradient is applied across the thickness in the so-called longitudinal SSE configuration. No change in damping is observed in bare YIG films. However, if the YIG film is covered with an ultrathin platinum layer, we observe a striking change in the amplitude of the detected spin-wave pulses. Depending on the sign of the gradient, the spin-wave relaxation rate can be increased or decreased, leading in the latter case to an apparent amplification. The change in the relaxation rate is attributed to the action of a spin current generated in the YIG film by the SSE while the role of the Pt layer is to supply or absorb the flow of spins.

In this paper, we study the symmetry protected Majorana edge states for the ${\mathbb{Z}}_2$ topological order of the Wen plaquette and toric code models and calculate the dispersion of the Majorana edge states. For the system with translational symmetry, the Majorana edge states are gapless and have the nodal points at $k=0$ and $\pi$. For the edge states of the toric code model without translational symmetry, the edge modes become gapped.

We report measurements of the temperature dependence of phonon densities of states in K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting ∼25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infrared measurements, indicating a lowering of symmetry of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ upon cooling below 250 K. Ab initio phonon calculations have been carried out for K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ and KFe${}_2$Se${}_2$. The comparison of the phonon spectra as obtained from the magnetic and nonmagnetic calculations show pronounced differences. We show that in the two calculations, the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and plays an important role in stabilizing the structure of K${}_{0.8}$Fe${}_{1.6}$Se${}_2$, as well as that of KFe${}_2$Se${}_2$. The calculations highlight the presence of low energy optical vibrational modes in K${}_{0.8}$Fe${}_{1.6}$Se${}_2$ compared to KFe${}_2$Se${}_2$.

We construct in the $K$ matrix formalism concrete examples of symmetry-enriched topological phases, namely intrinsically topological phases with global symmetries. We focus on the Abelian and nonchiral topological phases and demonstrate by our examples how the interplay between the global symmetry and the fusion algebra of the anyons of a topologically ordered system determines the existence of gapless edge modes protected by the symmetry and that a (quasi)group structure can be defined among these phases. Our examples include phases that display charge fractionalization and more exotic nonlocal anyon exchange under global symmetry that correspond to general group extensions of the global symmetry group.

We investigate the dynamics following sudden quenches across quantum critical points belonging to different universality classes. Specifically, we use matrix product state methods to study the quantum Ising chain in the presence of two additional terms which break integrability. We find that in all models the rate function for the return probability to the initial state becomes a nonanalytic function of time in the thermodynamic limit. This so-called “dynamical phase transition” was first observed in a recent work by Heyl, Polkovnikov, and Kehrein [Phys. Rev. Lett. 110, 135704 (2013)] for the exactly-solvable quantum Ising chain, which can be mapped to free fermions. Our results for “interacting theories” indicate that nonanalytic dynamics is a generic feature of sudden quenches across quantum critical points. We discuss potential connections to the dynamics of the order parameter.

The thermal conductivities of solid silicon thin films and silicon thin films with periodic pore arrays are predicted using a Monte Carlo technique to include phonon-boundary scattering and the Boltzmann transport equation. The bulk phonon properties required as input are obtained from harmonic and anharmonic lattice dynamics calculations. The force constants required for the lattice dynamics calculations are obtained from forces calculated using density functional theory. For both solid and porous films, the in-plane thermal conductivity predictions capture the magnitudes and trends of previous experimental measurements. Because the prediction methodology treats the phonons as particles with bulk properties, the results indicate that coherent phonon modes associated with the secondary periodicity of the pores do not contribute to thermal transport in porous films with feature sizes greater than 100 nm.

The interaction between graphene and various substrates plays an important and limiting role on the behavior of graphene films and devices. Here we uncover that dynamic screening of so-called remote substrate phonons (RPs) has a significant effect on the thermal coupling at the graphene-substrate interface. We calculate the thermal conductance $h_{\text{RP}}$ between graphene electrons and substrate, and its dependence on carrier density and temperature for SiO${}_2$, HfO${}_2$, h-BN, and Al${}_2$O${}_3$ substrates. The dynamic screening of RPs leads to one order of magnitude or more decrease in $h_{\text{RP}}$ and a change in its dependence on carrier density. Dynamic screening predicts a decrease of $\sim$1 MW K${}^{-1}$ m${}^{-2}$ while static screening predicts a rise of $\sim$10 MW K${}^{-1}$ m${}^{-2}$ when the carrier density in Al${}_2$O${}_3$-supported graphene is increased from 10${}^{12}$ to 10${}^{13}$ cm${}^{-2}$.

We study the thermodynamics of the three-dimensional Hubbard model at half filling on approach to the Néel transition by means of large-scale unbiased diagrammatic determinant Monte Carlo simulations. We obtain the transition temperature in the strongly correlated regime, as well as the temperature dependence of the energy, entropy, double occupancy, and nearest-neighbor spin correlation function. Our results improve the accuracy of previous unbiased studies and present accurate benchmarks in the ongoing effort to realize the antiferromagnetic state of matter with ultracold atoms in optical lattices.

We report on many-body corrections to one-electron energy spectra of bulk bismuth tellurohalides—materials that exhibit a giant Rashba-type spin splitting of the band-gap edge states. We show that the corrections obtained in the one-shot $\mathit{GW}$ approximation noticeably modify the spin-orbit-induced spin splitting evaluated within density functional theory. We demonstrate that taking into account many-body effects is crucial to interpret the available experimental data.