Microscopic properties of low-energy spin dynamics in diluted magnetic semiconductor are addressed in a framework of the Kondo lattice model including random distribution of magnetic dopants. Based on the fluctuation-dissipation theorem, we derive an explicit dependence of the spin diffusion coefficient on the single-particle Green function which is directly evaluated by dynamical mean-field theory. In the paramagnetic state, the magnetic scattering has been manifested to suppress spin diffusion. In agreement with other ferromagnet systems, we also point out that the spin diffusion in diluted magnetic semiconductors at small carrier concentration displays a monotonic $1/T$-like temperature dependence. By investigating the spin diffusion coefficient on a wide range of the model parameters, the obtained results have provided a significant scenario to understand the spin dynamics in the paramagnetic diluted magnetic semiconductors.

In this paper, we show that HgCdTe heterostructures may support, within the envelope function approximation, bound electronic states embedded in the continuum, such that the discrete energy spectrum overlaps the continuous spectrum. Although the proposed heterostructures are generally penetrable by an incoming electron wave, it is shown that they may support spatially localized trapped stationary states with an infinite lifetime. We discuss the possibility of a free electron being captured by the proposed open resonator and present a detailed study of the trapping lifetime in the case of a detuned resonator.

We study the spontaneous entanglement generation between two qubits chirally coupled to a waveguide. The maximum achievable concurrence is demonstrated to increase by a factor of $4/e\sim 1.5$ as compared to the non-chiral coupling situation. The proposed entanglement scheme is shown to be robust against variation of the qubit properties such as detuning and separation, which are critical in the nonchiral case. This result relaxes the restrictive requirements of the nonchiral situation, paving the way toward a realistic implementation. Our results demonstrate the potential of chiral waveguides for quantum entanglement protocols.

The structure of silicene multilayers grown on $\mathrm{Ag}\left(111\right)$ and the nature of their $\sqrt{3}\times \sqrt{3}R{30}^{\circ }$ reconstruction were studied by low-energy electron diffraction, Auger spectroscopy, and optical reflectance methods. It is shown that the surface reconstruction is induced by a surfactant layer of silver atoms during the growth of the silicon film and is not due to pristine Si. The Si film displays the optical characteristics of bulk cubic diamondlike Si, which indicates that it is not formed by any other allotropic phase of Si, which does not exist in nature, whether it is stacking of silicene layers, or “silicite.”

We have studied the transport properties of a large graphene double quantum dot under the influence of a background disorder potential and a magnetic field. At low temperatures, the evolution of the charge-stability diagram as a function of the $B$ field is investigated up to 10 T. Our results indicate that the charging energy of the quantum dot is reduced, and hence the effective size of the dot increases at a high magnetic field. We provide an explanation of our results using a tight-binding model, which describes the charge redistribution in a disordered graphene quantum dot via the formation of Landau levels and edge states. Our model suggests that the tunnel barriers separating different electron/hole puddles in a dot become transparent at high $B$ fields, resulting in the charge delocalization and reduced charging energy observed experimentally.

We obtain in a direct and rigorous manner a transition from a stable molecular-hydrogen $n{\text{H}}_2$ single chain to the quasiatomic two-chain $2n\text{H}$ state. We devise a method composed of an exact diagonalization in the Fock space combined with an ab initio adjustment of the single-particle wave function in the correlated state. In this approach the well-known problem of double counting the interparticle interaction does not arise at all. The transition is strongly discontinuous, and appears even for relatively short chains that are possible to tackle, $n=3–6$. The signature of the transition as a function of applied force is a discontinuous change of the equilibrium intramolecular distance. The corresponding change of the Hubbard ratio $U/W$ reflects the Mott-Hubbard transition aspect of atomization. A universal feature of the transition relation to the Mott criterion for the insulator-metal transition is also noted. The role of electron correlations is thus shown to be of fundamental significance in this case. The long-range nature of Coulomb interactions is included.

We use time- and angle-resolved photoemission to measure quasiparticle relaxation dynamics across a laser-induced superconducting phase transition in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. Whereas low-fluence measurements reveal picosecond dynamics, sharp femtosecond dynamics emerge at higher fluence. Analyses of data as a function of energy, momentum, and doping indicate that the closure of the near-nodal gap and disruption of macroscopic coherence are primary mechanisms driving this onset. The results demonstrate the important influence of transient electronic structure on relaxation dynamics, which is relevant for developing an understanding of nonequilibrium phase transitions.

We investigate the wave vector and frequency-dependent screening of the electric field in atomically thin (quasi-two-dimensional) crystals. For graphene and hexagonal boron nitride we find that, above a critical wave vector $q_c$, the static permittivity $\varepsilon (q>q_c,\omega =0)$ becomes negative and the Kramers-Kronig relations do not hold for $\varepsilon (q>q_c,\omega )$. Thus, in quasi-two-dimensional crystals, we reveal the physical confirmation of a proposition put forward decades ago [D. A. Kirzhnits, Sov. Phys. Usp. 19, 530 (1976)], allowing for the breakdown of Kramers-Kronig relations and for negative static permittivity. In the vicinity of the critical wave vector, we find a giant growth of the permittivity. Our results, obtained in the ab initio calculations using both the random-phase approximation and the adiabatic time-dependent local-density approximation, and further confirmed with a simple slab model, allow us to argue that the above properties, being exceptional in the three-dimensional case, are common to quasi-two-dimensional systems.

The hyperscaling property implies that spatially isotropic critical quantum states in $d$ spatial dimensions have a specific heat, which scales with temperature as $T^{d/z}$, and an optical conductivity, which scales with frequency as ${\omega }^{(d-2)/z}$ for $\omega \gg T$, where $z$ is the dynamic critical exponent. We examine the spin density wave critical fixed point of metals in $d=2$ found by Sur and Lee [Phys. Rev. B 91, 125136 (2015)] in an expansion in $\epsilon =3-d$. We find that the contributions of the “hot spots” on the Fermi surface to the optical conductivity and specific heat obey hyperscaling (up to logarithms), and agree with the results of the large $N$ analysis of the optical conductivity by Hartnoll et al. [ Phys. Rev. B 84, 125115 (2011)]. With a small bare velocity of the boson associated with the spin density wave order, there is an intermediate energy regime where hyperscaling is violated with $d\to d_t$, where $d_t=1$ is the number of dimensions transverse to the Fermi surface. We also present a Boltzmann equation analysis which indicates that the hot-spot contribution to the dc conductivity has the same scaling as the optical conductivity, with $T$ replacing $\omega$.

The temporal dependence of the resistance of MnAs nanocluster arrangements grown by selective-area metal-organic vapor-phase epitaxy is investigated at different temperatures. The resistance of such arrangements exhibits random telegraph noise with jumps between discrete resistance levels. The effect is attributed to thermally activated switching of the magnetic domain structure resulting in alterations of spin-dependent scattering between the MnAs clusters of the arrangements. The behavior can be qualitatively understood by a simple model in which it is assumed that the nanocluster arrangement consists of three domains in accordance with investigations by magnetic force microscopy. The magnetizations of the outer larger domains remain fixed, whereas the magnetization of a smaller intermediate domain (or domain wall) exhibits thermally activated switching between local minima of its energy landscape. The results of the model indicate that the time scale of an actual switching event of the entire intermediate domain comprises the nucleation of a seed domain consisting of a few thousand Mn spins followed by the transformation of the entire domain by domain-wall motion in order to reorient its magnetization.

Using the Green-function formalism, we calculate a current-induced spin polarization of weakly magnetized graphene with Rashba spin-orbit interaction. In a general case, all components of the current-induced spin polarization are nonzero, contrary to the nonmagnetic limit, where the only nonvanishing component of spin polarization is that in the graphene plane and normal to the electric field. When the induced spin polarization is exchange coupled to the magnetization, it exerts a spin-orbit torque on the latter. Using the Green-function method, we have derived some analytical formulas for the spin polarization and also determined the corresponding spin-orbit torque components. The analytical results are compared with those obtained numerically. Vertex corrections due to scattering on randomly distributed impurities are also calculated and shown to enhance the spin polarization calculated in the bare bubble approximation.

We describe a fluctuating volume-current formulation of electromagnetic fluctuations that extends our recent work on heat exchange and Casimir interactions between arbitrarily shaped homogeneous bodies [A. W. Rodriguez, M. T. H. Reid, and S. G. Johnson, Phys. Rev. B 88, 054305 (2013)] to situations involving incandescence and luminescence problems, including thermal radiation, heat transfer, Casimir forces, spontaneous emission, fluorescence, and Raman scattering, in inhomogeneous media. Unlike previous scattering formulations based on field and/or surface unknowns, our work exploits powerful techniques from the volume-integral equation (VIE) method, in which electromagnetic scattering is described in terms of volumetric, current unknowns throughout the bodies. The resulting trace formulas (boxed equations) involve products of well-studied VIE matrices and describe power and momentum transfer between objects with spatially varying material properties and fluctuation characteristics. We demonstrate that thanks to the low-rank properties of the associated matrices, these formulas are susceptible to fast-trace computations based on iterative methods, making practical calculations tractable. We apply our techniques to study thermal radiation, heat transfer, and fluorescence in complicated geometries, checking our method against established techniques best suited for homogeneous bodies as well as applying it to obtain predictions of radiation from complex bodies with spatially varying permittivities and/or temperature profiles.

Several static and dynamic properties of bulk liquid Fe at a thermodynamic state near its triple point have been evaluated by ab initio molecular dynamics simulations. The calculated static structure shows very good agreement with the available experimental data, including an asymmetric second peak in the structure factor which underlines a substantial local icosahedral short-range order in the liquid. The dynamical structure reveals propagating density fluctuations, with an associated dispersion relation which closely follows the experimental data. The dynamic structure factors $S(q,\omega )$ show a good agreement with their experimental counterparts which have been recently measured by an inelastic x-ray scattering experiment. The dynamical processes behind the $S(q,\omega )$ have been analyzed by using a model with two decay channels (a fast and a slow) associated with the relaxations of the collective excitations. The recent finding of transverselike excitation modes in the IXS data is analyzed by using the present ab initio simulation results. Several transport coefficients have been evaluated and the results are compared with the available experimental data.

Classical phonon scattering calculations predict the drag force acting on defects and dislocations rises linearly with temperature, in direct contradiction with molecular dynamics simulations that find the drag force to be independent of temperature for nanoscale defects. Using the Mori-Zwanzig projection technique, with no recourse to elasticity or scattering theories, we derive a general Langevin equation for highly mobile crystal defects and dislocations, with full treatment of discreteness and nonlinearity in the defect core. We obtain an analytical expression for the drag force that is evaluated in molecular statics and molecular dynamics, extracting the force on a defect directly from the interatomic forces. Our results show that a temperature-independent drag force arises because vibrations in a discrete crystal are never independent of the defect motion, an implicit assumption in any phonon-based approach. This effect remains even when the Peierls barrier is effectively zero, invalidating qualitative explanations involving the radiation of phonons. We apply our methods to an interstitial defect in tungsten and solitons in the Frenkel-Kontorova model, finding very good agreement with trajectory-based estimations of the thermal drag force.

Semiconducting GaAs is widely used in microwave and millimeter integrated circuits, infrared LEDs, lasers, and solar cells. Introducing semimetallic ErAs nanoparticles provides a way to controllably tune the optical and electronic properties of GaAs. We show that for high volume fractions $(0.5\%–10\%)$ of ErAs nanoparticles embedded in GaAs, the relaxation dynamics indicates that ErAs forms discrete states in the GaAs band gap. For specific carrier momentum conditions, the localized Schottky states may be occupied, exhibit carrier trapping, or inject carriers into the GaAs conduction band. Carrier occupation and scattering from the Schottky states has not previously been reported in optical studies of this system. The scattering mechanism is observed to be active above an occupation threshold where the excited carrier density exceeds the trap density. The array of nanoparticle densities and the characterization of the relaxation pathways at multiple carrier excitation energies represents the most complete fundamental investigation of these systems to date.

Insulating $5d^3\mathrm{L}{\mathrm{a}}_3\mathrm{Os}{\mathrm{O}}_7$ and the isostructural hole-doped analog $\mathrm{L}{\mathrm{a}}_{2.8}\mathrm{C}{\mathrm{a}}_{0.2}\mathrm{Os}{\mathrm{O}}_7$ that feature pseudo-one-dimensional zigzag chains of corner-sharing $\mathrm{Os}{\mathrm{O}}_6$ octahedra have been synthesized and their magnetic and electrical transport properties characterized. Both compounds are insulating antiferromagnets. Long range magnetic order between the antiferromagnetic chains is determined with a propagation vector $k=1/2,1/2,1$ and $T_{\mathrm{N}}=45$ and 33 K for the parent and doped materials. An $\mathrm{O}{\mathrm{s}}^{5+}$ moment of $1.7\left(1\right){\mu }_B$ for $\mathrm{L}{\mathrm{a}}_3\mathrm{Os}{\mathrm{O}}_7$ and $1.2\left(2\right){\mu }_B$ for $\mathrm{L}{\mathrm{a}}_{2.8}\mathrm{C}{\mathrm{a}}_{0.2}\mathrm{Os}{\mathrm{O}}_7$ is refined. The long range magnetic structure is compared to the isostructural compounds $\mathrm{L}{\mathrm{a}}_3\mathrm{Ru}{\mathrm{O}}_7$ and $\mathrm{L}{\mathrm{a}}_3\mathrm{Mo}{\mathrm{O}}_7$, both of which adopt different magnetic structures.

We present depth-resolved nuclear resonance reflectivity studies of the magnetization evolution in ${[{}^{57}\mathrm{Fe}\left(3\mathrm{nm}\right)/\mathrm{Cr}\left(1.2\mathrm{nm}\right)]}_{10}$ multilayer under applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (0–1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of results. We show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has a more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe how to select the true azimuth angle from the determined “effective azimuth angle.” Finally we obtain the noncollinear twisted magnetization depth profiles where the spin-flop state appears sequentially in different ${}^{57}\mathrm{Fe}$ layers at increasing applied field.

The $\mathrm{SU}\left(N\right)$ symmetric antiferromagnetic Heisenberg model with multicolumn representations on the two-dimensional square lattice is investigated by quantum Monte Carlo simulations. For the representation of a Young diagram with two columns, we confirm that a valence-bond solid (VBS) order appears as soon as the Néel order disappears at $N=10$, indicating no intermediate phase. In the case of the representation with three columns, there is no evidence for either the Néel or the VBS ordering for $N\ge 15$. This is actually consistent with the large-$N$ theory, which predicts that the VBS state immediately follows the Néel state, because the expected spontaneous order is too weak to be detected.

We argue that the relative thermal conductance between interfaces with different morphologies is controlled by crystal structure through $M_{\min }/M_c>1$, the ratio between the minimum mode count on either side $M_{\min }$, and the conserving modes $M_c$ that preserve phonon momentum transverse to the interface. Junctions with an added homogenous layer, “uniform,” and “abrupt” junctions are limited to $M_c$, while junctions with interfacial disorder, “mixed,” exploit the expansion of mode spectrum to $M_{\min }$. In our studies with cubic crystals, the largest enhancement of conductance from “abrupt” to “mixed” interfaces seems to be correlated with the emergence of voids in the conserving modes, where $M_c=0$. Such voids typically arise when the interlayer coupling is weakly dispersive, making the bands shift rigidly with momentum. Interfacial mixing also increases alloy scattering, which reduces conductance in opposition with the mode spectrum expansion. Thus the conductance across a “mixed” junction does not always increase relative to that at a “uniform” interface.

We investigate free carriers dynamics and anharmonic decay of optical phonons in ${\mathrm{LiInSe}}_2$ by using low temperature THz time-domain spectroscopy. The quantitatively determined refractive index and absorption coefficient are significantly influenced by temperature. By fitting the evaluated dielectric functions with a Drude-Lorentz model, the temperature dependencies of the plasmon frequency, scattering rates of free carriers, activation energies of carriers, and frequencies and linewidths of optical phonons are investigated. The contributions of three- and four-phonon processes to the temperature dependent optical phonon frequency and linewidth variations are quantitatively determined.