We report quantum oscillation studies on the $\mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_{3-x}{\mathrm{S}}_x$ topological insulator single crystals in pulsed magnetic fields up to 91 T. For the $x=0.4$ sample with the lowest bulk carrier density, the surface and bulk quantum oscillations can be disentangled by combined Shubnikov–de Haas and de Hass–van Alphen oscillations, as well as quantum oscillations in nanometer-thick peeled crystals. At high magnetic fields beyond the bulk quantum limit, our results suggest that the zeroth Landau level of topological surface states is shifted due to the Zeeman effect. The $g$ factor of the topological surface states is estimated to be between 1.8 and 4.5. These observations shed new light on the quantum transport phenomena of topological insulators in ultrahigh magnetic fields.

We present experimental and theoretical results on monolayer colloidal cadmium selenide quantum dot films embedded with tiny gold nanoparticles. By varying the density of the embedded gold nanoparticles, we were able to engineer a plasmon-mediated crossover from emission quenching to enhancement regime at interparticle distances for which only quenching of emission is expected. This crossover and a nonmonotonic variation of photoluminescence intensity and decay rate, in experiments, is explained in terms of a model for plasmon-mediated collective emission of quantum emitters which points to the emergence of a new regime in plasmon-exciton interactions. The presented methodology to achieve enhancement in optical quantum efficiency for optimal doping of gold nanoparticles in such ultrathin high-density quantum dot films can be beneficial for new-generation displays and photodetectors.

We present a methodology to calculate frequency and momentum dependent all-electron response functions determined within Kohn-Sham density functional theory. It overcomes the main obstacle in calculating response functions in practice, which is the slow convergence with respect to the number of unoccupied states and the basis-set size. In this approach, the usual sum-over-states expression of perturbation theory is complemented by the response of the orbital basis functions, explicitly constructed by radial integrations of frequency-dependent Sternheimer equations. To an essential extent an infinite number of unoccupied states are included in this way. Furthermore, the response of the core electrons is treated virtually exactly, which is out of reach otherwise. The method is an extension of the recently introduced incomplete-basis-set correction (IBC) [Betzinger et al., Phys. Rev. B 85, 245124 (2012); Phys. Rev. B 88, 075130 (2013)] to the frequency and momentum domain. We have implemented the generalized IBC within the all-electron full-potential linearized augmented-plane-wave method and demonstrate for rocksalt BaO the improved convergence of the dynamical Kohn-Sham polarizability. We apply this technique to compute (a) quasiparticle energies employing the COHSEX approximation for the self-energy of many-body perturbation theory and (b) all-electron RPA correlation energies. It is shown that the favorable convergence of the polarizability is passed over to the COHSEX and RPA calculation.

We report on the study of Mn transversal spin relaxation mechanisms in ferromagnetic (Ga,Mn)As. The spins of valence band holes experience exchange interactions with ferromagnetically ordered Mn spins and strong spin relaxation due to spin-orbit coupling. The hole subsystem provides a relaxation channel for the Mn spin subsystem. The Mn spin relaxation mechanisms were studied by means of spin flip Raman scattering. Two contributions to the spin flip Raman linewidth were found. The first one dominates below the Curie temperature and is related to the fast hole spin relaxation that damps the collective motion of the coupled Mn and hole spin subsystems. The other contribution to the Mn spin relaxation is related to spin fluctuations of the hole ensemble, which grow with temperature and become most important in the paramagnetic phase.

$\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ structures epitaxially grown on $a$-plane or $m$-plane GaN exhibit in-plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for $m$-plane than $a$-plane $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$. Using density functional theory, we study the response of $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ random alloys to finite biaxial strains on both nonpolar planes. The calculated $m$-plane $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ valence band splitting is larger than that of the $a$ plane, due to a greater degree of structural relaxation in $a$-plane $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$. We provide a parametrization of the valence band splitting of $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ strained to $a$-plane and $m$-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between $a$-plane and $m$-plane polarization.

We investigate numerically different phases that can occur at half filling in the lowest and the first excited Landau levels in wide-well two-dimensional electron systems exposed to a perpendicular magnetic field. Within a two-component model that takes into account only the two lowest electronic subbands of the quantum well, we derive a phase diagram that compares favorably with an experimental one by Shabani et al. [Phys. Rev. B 88, 245413 (2013)]. In addition to the compressible composite-fermion Fermi liquid in narrow wells with a substantial subband gap and the incompressible two-component $\left(331\right)$ Halperin state, we identify in the lowest Landau level a rectangular Wigner crystal occupying the second subband. This crystal may be the origin of the experimentally observed insulating phase in the limit of wide wells and high electronic densities. In the second Landau level, the incompressible Pfaffian state, which occurs in narrow wells and large subband gaps, is also separated by an intermediate region from a large-well limit in which a similar rectangular Wigner crystal in the excited subband is the ground state, as for the lowest Landau level. However, the intermediate region is characterized by an incompressible state that consists of two four-flux Pfaffians in each of the components.

The ideal black body fully absorbs all incident rays, that is, all propagating waves created by arbitrary sources. A known idealized realization of the black body is the perfectly matched layer (PML), widely used in numerical electromagnetics. However, ideal black bodies and PMLs do not interact with evanescent fields that exists near any finite-size source, and the energy stored in these fields cannot be harvested. Here, we introduce the concept of the ideal conjugate matched layer (CML), which fully absorbs the energy of both propagating and evanescent fields of sources acting as an ideal sink for electromagnetic energy. Conjugate matched absorbers have exciting application potentials, as resonant attractors of electromagnetic energy into the absorber volume. We derive the conditions on the constitutive parameters of media which can serve as CML materials, numerically study the performance of planar and cylindrical CML and discuss possible realizations of such materials as metal-dielectric composites.

The phase diagram and the relationship between the crystal coherence length and electrical response of ${\mathrm{Pb}[\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right)}_{1-x}{\mathrm{Ti}}_x]{\mathrm{O}}_3$ (PMN-$x\mathrm{PT}$) near the morphotropic phase boundary (MPB) have been precisely investigated using a single crystal with a Ti composition gradient by synchrotron x-ray diffraction and inelastic light scattering at room temperature. The crystal has two boundaries at Ti compositions of 29.0 and 34.7 mol% which correspond to the phase boundaries between the monoclinic $B \left(M_B\right)$ and $C \left(M_C\right)$ phases and between the $M_C$ and tetragonal $\left(T\right)$ phases, respectively. The crystal coherence length determined from the widths of Bragg peaks varies by over two orders of magnitude in a 10 mol% range of the Ti composition. It is shown that there is a strong negative correlation between the electrical response and the crystal coherence length at the sub-$\mu \mathrm{m}$ scale. The results are explained by the size effects of domains near the MPB.

The thermoelectric figure of merit $\left(ZT\right)$ of semiconductors such as PbTe can be improved by forming nanostructures within the bulk of these materials. Alloying PbTe with PbS causes PbS-rich nanostructures to precipitate from the solid solution, scattering phonons and increasing $ZT$. Understanding the thermodynamics of this process is crucial to optimizing the efficiency gains of this technique. Previous calculations of the thermodynamics of PbS-PbTe alloys [(J. W. Doak and C. Wolverton, Phys. Rev. B 86, 144202 (2012)] found that mixing energetics alone were not sufficient to quantitatively explain the thermodynamic driving force for phase separation in these materials: first-principles calculations of the thermodynamics of phase separation overestimate the thermodynamic driving force for precipitation of PbS-rich nanostructures from PbS-PbTe alloys. In this work, we re-examine the thermodynamics of PbS-PbTe, including the effects of vibrational entropy in the free energy through frozen-phonon calculations of special quasirandom structures (SQS) to explain this discrepancy between first-principles and experimental phase stability. We find that vibrational entropy of mixing reduces the calculated maximum miscibility gap temperature $T_G$ of PbS-PbTe by 470 K, bringing the error between calculated and experimental $T_G$ down from 700 to 230 K. Our calculated vibrational spectra of PbS-PbTe SQS exhibit dynamic instabilities of S ions that corroborate reports of low-$T$ ferroelectriclike phase transitions in solid solutions of PbS and PbTe, which are not present in either of the constituent compounds. We use our calculated vibrational spectra to obtain phase transition temperatures, which are in qualitative agreement with experimental results for PbTe-rich alloys, as well as to predict the existence of a low-$T$ displacive phase transition in PbS-rich PbS-PbTe, which has not yet been experimentally investigated.

We investigate dynamic effective anisotropy in photonic crystals (PCs) through a combination of an effective medium theory, which is a high-frequency homogenization (HFH) method explicitly developed to operate for short waves, as well as through numerical simulations and microwave experiments. The HFH yields accurate predictions of the effective anisotropic properties of periodic structures when the wavelength is of comparable order to the pitch of the array; specifically, we investigate a square array of pitch 2 cm consisting of dielectric rods of radius $0.5$ cm and refractive index $n=\sqrt{6}$ within an air matrix. This behaves as an effective medium, with strong artificial anisotropy, at a frequency corresponding to a flat band emerging from a Dirac-like point in transverse magnetic (TM) polarization. At this frequency, highly directive emission is predicted for an electric source placed inside this PC, and this artificial anisotropy can be shown to coincide with a change of character of the underlying effective equation from isotropic to unidirective, with coefficients of markedly different magnitudes appearing in the effective equation tensor. In transverse electric (TE) polarization, we note a second radical change of character of the underlying effective equation, this time from elliptic to hyperbolic, near a frequency at which a saddle point occurs in the corresponding dispersion curves. Delicate microwave experiments are performed in both polarizations for such a PC consisting of 80 rods, and we demonstrate that a directive emission in the form of a + (respectively, an $X$) is indeed seen experimentally at the predicted frequency $9.5$ GHz in TM polarization (respectively, $5.9$ GHz in TE polarization). These are clearly dynamic effects since in the quasistatic regime the PC just behaves as an isotropic medium.

We use nonperturbative linked-cluster expansions to determine the ground-state energy per site of the spin-one Heisenberg model on the kagome lattice. To this end, a parameter is introduced allowing us to interpolate between a fully trimerized state and the isotropic model. The ground-state energy per site of the full graph decomposition up to graphs of six triangles (18 spins) displays a complex behavior as a function of this parameter close to the isotropic model which we attribute to divergencies of partial series in the graph expansion of quasi-1D unfrustrated chain graphs. More concretely, these divergencies can be traced back to a quantum critical point of the one-dimensional unfrustrated chain of coupled triangles. Interestingly, the reorganization of the nonperturbative linked-cluster expansion in terms of clusters with enhanced symmetry yields a ground-state energy per site of the isotropic two-dimensional model that is in quantitative agreement with other numerical approaches in favor of a spontaneous trimerization of the system. Our findings are of general importance for any nonperturbative linked-cluster expansion on geometrically frustrated systems.

The negatively charged nitrogen-vacancy (NV) center spin in diamond can be used to realize quantum computation and to sense magnetic fields. Its spin triplet consists of three levels labeled with its spin $z$-components of +1, 0, and −1. Without external field, the +1 and −1 states are degenerate and higher than the 0 state due to the zero-field splitting. By taking the symmetrical and antisymmetrical superpositions of the +1 and −1 states as our qubit basis, we obtain the exact evolution operator of the NV center spin under time-dependent magnetic field by mapping the three-level system on time-dependent quantum two-level systems with exact analytical solutions. With our exact evolution operator of the NV center spin including three levels, we show that arbitrary qubits can be prepared from the starting 0 state, and arbitrary rapid quantum logic gates of these qubits can be realized with magnetic fields. In addition, it is made clear that the typical quantum logic gates can be accomplished within a few nanoseconds and the fidelity can be very high because only magnetic field strength needs to be controlled in this approach. These results should be useful to realizing quantum computing with the NV center spin systems in diamond and exploring other effects and applications.

We report the observation of the Barnett effect in paramagnetic states by mechanically rotating gadolinium (Gd) metal with a rotational frequency of up to 1.5 kHz above the Curie temperature. An in situ magnetic measurement setup comprising a high-speed rotational system and a fluxgate magnetic sensor was developed for the measurement. Temperature dependence of the observed magnetization follows that of paramagnetic susceptibility, indicating that any emergent magnetic field is proportional to the rotational frequency and is independent of temperature. From the proportionality constant of the emergent field, the gyromagnetic ratio of Gd is calculated to be $-29\pm 5\mathrm{GHz}/\mathrm{T}$. This study revisits the primordial issue of magnetism with modern technologies to shed new light on the fundamental spin-rotation coupling.

The ${\mathrm{Co}}^{2+}$ ions of ${\mathrm{CoSb}}_2{\mathrm{O}}_6$ exhibit local magnetic order below $\sim 80$ K followed by long-range antiferromagnetic order below $T_N=13.45$ K. Analysis of the magnetic susceptibility above $T_N$ using an Ising model of Co-Co dimers yields a magnetic exchange coupling $J_{\parallel }/k_{\mathrm{B}}=-10.603\left(6\right)$ K ($k_{\mathrm{B}}$ is the Boltzmann constant). The transition at $T_N$ is accompanied by a spin gap in the heat capacity $[{\mathrm{\Delta }}_2/k_{\mathrm{B}}=33.92\left(9\right)$ K]. Highly anisotropic behavior of the thermal expansion is observed, and the influence of local magnetic order is evident. A critical exponent of $\alpha =0.103\left(3\right)$ is obtained from analysis of the critical behavior of the heat capacity and thermal expansion near $T_N$, similar to the value expected for the three-dimensional Ising universality class.

Magnetic droplet solitons are nonlinear dynamical modes that can be excited in a thin film with perpendicular magnetic anisotropy with a spin-transfer torque. Although droplet solitons have been proved to be stable with a hysteretic response to applied currents and magnetic fields at low temperature, measurements at room temperature indicate less stability and reduced hysteresis width. Here, we report evidence of droplet soliton drift instabilities, leading to drift resonances, at room temperature that explains their lower stability. Micromagnetic simulations show that the drift instability is produced by an effective-field asymmetry in the nanocontact region that can have different origins.

Magnetic properties of single crystalline ${\mathrm{SmRu}}_2{\mathrm{Al}}_{10}$ have been investigated by electrical resistivity, magnetic susceptibility, and specific heat. We have confirmed the successive magnetic phase transitions at $T_{\text{N}}=12.3$ K and $T_{\text{M}}=5.6$ K. Resonant x-ray diffraction has also been performed to study the magnetic structures. Below $T_{\text{N}}$, the ${\mathrm{Sm}}^{3+}$ moments order in an incommensurate structure with ${\mathit{q}}_1=(0,0.759,0)$. The magnetic moments are oriented along the orthorhombic $b$ axis, which coincides with the magnetization easy axis in the paramagnetic phase. A very weak third harmonic peak is also observed at ${\mathit{q}}_3=(0,0.278,0)$. The transition at $T_{\text{M}}$ is a lock-in transition to the commensurate structure described by ${\mathit{q}}_1=(0,0.75,0)$. A well-developed third harmonic peak is observed at ${\mathit{q}}_3=(0,0.25,0)$. From the discussion of the magnetic structure, we propose that the long-range RKKY interaction plays an important role, in addition to the strong nearest-neighbor antiferromagnetic interaction.

Interacting bosons with two “spin” states in a lattice show superfluid-insulator phase transitions in the presence of spin-orbit coupling. Depending on the parameter regime, bosons in the superfluid phase can condense to either a zero-momentum state or to one or multiple states with finite momentum, leading to an unconventional superfluid phase. We study the response of such a system to modulation of the optical lattice potential. We show that the change in momentum distribution after lattice modulation shows distinct patterns in the Mott and the superfluid phase and these patterns can be used to detect these phases and the quantum phase transition between them. Further, the momentum-resolved optical modulation spectroscopy can identify both the gapless (Goldstone) and gapped amplitude (Higgs) mode of the superfluid phase and clearly distinguish between the superfluid phases with a zero-momentum condensate and a twisted superfluid phase by looking at the location of these modes in the Brillouin zone. We discuss experiments which can test our theory.

We perform fully coupled two-dimensional numerical simulations of plane channel helium II counterflows with vortex-line density typical of experiments. The main features of our approach are the inclusion of the back reaction of the superfluid vortices on the normal fluid and the presence of solid boundaries. Despite the reduced dimensionality, our model is realistic enough to reproduce vortex density distributions across the channel recently calculated in three dimensions. We focus on the coarse-grained superfluid and normal fluid velocity profiles, recovering the normal fluid profile recently observed employing a technique based on laser-induced fluorescence of metastable helium molecules.

We present a multiband model for superconductivity at the metallic interface between insulating oxides ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ (001). Using a self-consistent Bogoliubov–de Gennes theory, formulated with the realistic bands at the interface, we investigate the spin-singlet and spin-triplet pairings in intraband and interband channels. We find that the Rashba and atomic spin-orbit interactions at the interface induce singlet pairing in the interband channel and triplet pairing in both the intraband and interband channels when the pairing amplitude in the singlet intraband channel is finite. The gate-voltage variation of superconductivity is resolved in different pairing channels, compared with experimental results, and found to match quite well. Interestingly, an enhancement of the superconducting transition temperature by external in-plane magnetic field is found, revealing the existence of a hidden superconducting state above the observed one. As the interface is known to possess a high level of inhomogeneity, we explore the role of nonmagnetic disorder incorporating thermal phase fluctuations by using a Monte Carlo method. We show that even after the transition to the nonsuperconducting phase, driven by temperature or magnetic field, the interface possesses localized Cooper pairs whose signature was observed in previous experiments.

Motivated by recent experiments in which a voltage-biased Josephson junction is placed in series with a resonator, the classical dynamics of the circuit is studied in various domains of parameter space. This problem can be mapped onto the dissipative motion of a single degree of freedom in a nonlinear time-dependent potential, where in contrast to conventional settings the nonlinearity appears in the driving while the static potential is purely harmonic. For long times the system approaches steady states which are analyzed in the underdamped regime over the full range of driving parameters including the fundamental resonance as well as higher harmonics and subharmonics. Observables such as the dc-Josephson current and the radiated microwave power give direct information about the underlying dynamics covering phenomena such as bifurcations, irregular motion, and up- and down-conversion. Due to their tunability, present and future setups provide versatile platforms to explore the changeover from linear response to strongly nonlinear behavior in driven dissipative systems under well defined conditions.