We study strong coupling between plasmons in monolayer charge-doped graphene and excitons in a narrow gap semiconductor quantum well separated from graphene by a potential barrier. We show that the Coulomb interaction between excitons and plasmons results in mixed states described by a Hamiltonian similar to that for exciton-polaritons and derive the exciton-plasmon coupling constant that depends on system parameters. We calculate numerically the Rabi splitting of exciton-plasmariton dispersion branches for several semiconductor materials and find that it can reach values of up to 50–100 meV.

We consider the application of a small in-plane magnetic field to electrons on a helium surface in a perpendicular magnetic field. Certain states that were bound to the helium surface then dissolve into the continuum, turning into long-lived resonances. As a result microwave absorption lines acquire an asymmetric Fano lineshape that is tunable by varying the microwave polarization or the in-plane magnetic field. Electrons trapped in a formerly bound state will tunnel off the surface of helium; we show that under suitable circumstances this “radioactive decay” can show damped oscillations rather than a simple exponential decay. The mechanism for oscillatory exponential decay is not specific to electrons on helium and this effect may also be relevant elsewhere in physics.

Terahertz spectroscopy is employed for the noncontact measurement of transversal mobility in InP nanowires, wherein photonic effects (waveguiding of excitation beam and propagation of terahertz beam in a complex gradient environment) were successfully deconvoluted. Monte Carlo calculations accounting for electron localization and heavy doping were used to determine electron momentum relaxation time corresponding to electron mobility $\ge 3000\hspace{0.17em}{\text{cm}}^2/\mathrm{V}\hspace{0.17em}\mathrm{s}$, which is similar to that in bulk InP. The developed approach paves a way for noncontact determination of charge mobility in advanced semiconductor nanostructures.

We have grown single crystals of ${\mathrm{Na}}_x{\mathrm{Ca}}_y\mathrm{Co}{\mathrm{O}}_2$ and determined their superstructures as a function of composition using neutron and x-ray diffraction. Inclusion of ${\mathrm{Ca}}^{2+}$ stabilizes a single superstructure across a wide range of temperatures and concentrations. The superstructure in the ${\mathrm{Na}}^{+}$ layers is based on arrays of divacancy clusters with ${\mathrm{Ca}}^{2+}$ ions occupying the central site, and it has an ideal concentration ${\mathrm{Na}}_{4/7}{\mathrm{Ca}}_{1/7}\mathrm{Co}{\mathrm{O}}_2$. Previous measurements of the thermoelectric properties on this system are discussed in light of this superstructure. ${\mathrm{Na}}_{4/7}{\mathrm{Ca}}_{1/7}\mathrm{Co}{\mathrm{O}}_2$ corresponds to the maximum in thermoelectric performance of this system.

We study the dynamics of spin valves consisting of two layers of magnetic insulators separated by a normal metal in the macrospin model. A current through the spacer generates a spin Hall current that can actuate the magnetization via the spin-transfer torque. We derive expressions for the effective Gilbert damping and the critical currents for the onset of magnetization dynamics including the effects of spin pumping that can be tested by ferromagnetic resonance experiments. The current generates an amplitude asymmetry between the in-phase and out-of-phase modes. We discuss superlattices of stacked films of metals and magnetic insulators.

$\mathrm{Rb}\mathrm{Mn}{\mathrm{F}}_3$ is known to have a very small magnetic anisotropy and to be a nearly ideal realization of a three-dimensional Heisenberg antiferromagnet. Although its critical behavior has been studied in detail, the origin of the residual magnetic anisotropy has not had sufficient attention. Here we present an experimental investigation of the temperature dependence of the anisotropy in $\mathrm{Rb}\mathrm{Mn}{\mathrm{F}}_3$ and show that it can be explained by a model for the crystal field. The experimental data are obtained with magnetization and susceptibility measurements from which we can accurately obtain the critical magnetic field for the transition from the antiferromagnetic to the spin-flop phase. The fit of the theoretical model to the data yields information on some microscopic parameters.

We report on small-angle neutron scattering (SANS) measurements of the vortex lattice (VL) in twin-free $\mathrm{YBa}{}_2\mathrm{Cu}{}_3\mathrm{O}{}_7$, extending the previously investigated maximum magnetic field of <11 T up to 16.7 T with the field applied parallel to the c axis. This is a microscopic study of vortex matter in this region of the superconducting phase. We find that the high magnetic field VL displays a rhombic structure, with a field-dependent coordination that passes through a square configuration with no lock-in to a field-independent structure. The VL pinning reduces with increasing temperature but is seen to affect the VL correlation length even above the irreversibility temperature of the lattice structure. At high field and temperature we observe a VL melting transition, which appears to be first order. The vortex liquid phase above the transition does not give a detectable SANS signal.

We theoretically obtain the phase diagram of localized magnetic impurity spins arranged in a one-dimensional chain on top of a one- or two-dimensional electron gas. The interactions between the spins are mediated by the Ruderman-Kittel-Kasuya-Yosida mechanism through the electron gas. Recent work predicts that such a system may intrinsically support topological superconductivity without spin-orbit coupling when a helical spin-density wave is spontaneously formed in the spins, and superconductivity is induced in the electron gas. We analyze, using both analytical and numerical techniques, the conditions under which such a helical spin state is stable in a realistic situation in the presence of disorder. We show that (i) it appears only when the spins are coupled to a (quasi-) one-dimensional electron gas, and (ii) it becomes unstable towards the formation of (anti)ferromagnetic domains if the disorder in the impurity spin positions $\delta R$ becomes comparable with the Fermi wavelength. We also examine the stability of the helical state against Gaussian potential disorder in the electronic system using a diagrammatic approach. Our results suggest that in order to stabilize the helical spin state and thus the emergent topological superconductivity under realistic experimental conditions, a sufficiently strong Rashba spin-orbit coupling, giving rise to Dzyaloshinskii-Moriya interactions, is required.

We show, using molecular statics and dynamics simulations along with dislocation dynamics calculations, that the structure of a screw dislocation in a thin film or at a free surface for face-centered cubic Cu differs from that found in bulk. In agreement with earlier work, a screw dislocation at the surface is observed to dissociate in two different $\left\{111\right\}$ planes, forming a constriction at the site where the glide plane changes. We analyze in detail the energetics of the structure and conclude that the constricted configuration is stable due to the long-range elastic interactions. We have also performed shear stress simulations and compared to bulk stress to understand how the constriction modifies the response of the dislocation to an applied load. We found that such constriction represents a strong pinning point, substantially increasing the yield stress required for the dislocation to glide. In contrast, the configuration provides a barrierless source for the dislocation to cross slip.

We discuss the semiclassical and classical character of the dynamics of a single spin $1/2$ coupled to a bath of noninteracting spins $1/2$. On the semiclassical level, we extend our previous approach presented in D. Stanek, C. Raas, and G. S. Uhrig, Phys. Rev. B 88, 155305 (2013) by the explicit consideration of the conservation of the total spin. On the classical level, we compare the results of the classical equations of motions in absence and presence of an external field to the full quantum result obtained by density-matrix renormalization (DMRG). We show that for large bath sizes and not too low magnetic field the classical dynamics, averaged over Gaussian distributed initial spin vectors, agrees quantitatively with the quantum-mechanical one. This observation paves the way for an efficient approach for certain parameter regimes.

Motivated by recent developments in conformal field theory (CFT), we devise a quantum Monte Carlo (QMC) method to calculate the moments of the partially transposed reduced density matrix at finite temperature. These are used to construct scale invariant combinations that are related to the negativity, a true measure of entanglement for two intervals embedded in a chain. These quantities can serve as witnesses of criticality. In particular, we study several scale invariant combinations of the moments for the one-dimensional (1D) hard-core boson model. For two adjacent intervals unusual finite-size corrections are present, showing parity effects that oscillate with a filling dependent period. These are more pronounced in the presence of boundaries. For large chains we find perfect agreement with CFT. Oppositely, for disjoint intervals corrections are more severe and CFT is recovered only asymptotically. Furthermore, we provide evidence that their exponent is the same as that governing the corrections of the mutual information. Additionally we study the 1D Bose-Hubbard model in the superfluid phase. Remarkably, the finite-size effects are smaller and QMC data are already in impressive agreement with CFT at moderately large sizes.

We find a state characterized by a spontaneous loop-spin current and a single-particle gap in the Hubbard model within the variational cluster approach. This state exists for arbitrarily small interaction in a half-filled honeycomb lattice. Moreover, from the calculations of the topological invariants for the interacting system, it is shown that this gapped state has nontrivial topological characters; this state is the topological Mott insulating state. This result implies the ubiquity of topological Mott insulating phases.

We present a systematic investigation of two-photon excitation processes in a GaAs-based microcavity in the strong-coupling regime. We observe second harmonic generation resonant to the upper and lower polariton level, which exhibits a strong dependence on the photonic fraction of the corresponding polariton. In addition, we have performed two-photon excitation spectroscopy to identify $2p$ exciton states which are crucial for the operation as a terahertz lasing device, which was suggested recently [A. V. Kavokin et al., Phys. Rev. Lett. 108, 197401 (2012)]. However, no distinct signatures of a $2p$ exciton state could be identified, which indicates a low two-photon pumping efficiency.

We present a perturbative approach to the study of the Hofstadter model for when the amount of flux per plaquette is close to a rational fraction. Within this approximation, certain eigenstates of the system are shown to be multicomponent wave functions that connect smoothly to the Landau levels of the continuum. The perturbative corrections to these are higher Landau level contributions that break rotational invariance and allow the perturbed states to adopt the symmetry of the lattice. In the presence of interactions, this approach allows for the calculation of generalized Haldane pseudopotentials, and in turn, the many-body properties of the system. The method is sufficiently general that it can apply to a wide variety of lattices, interactions, and magnetic field strengths.

The possibility of designing core-shell nanoparticles that are “invisible” to the conduction electrons has been demonstrated recently. A total scattering cross section smaller than 0.01% of the physical cross section was demonstrated by artificially adjusting the parameters of the barrier and the well in a core-shell geometry. In this paper, we aim to extend the developed concept and find realistic material combinations that satisfy the cloaking criteria. We report designs of hollow nanoparticles that could be used to realize the cloaking concept in III–V semiconductor host matrices. Such particles could be used in advanced materials design to enhance and tune the electrical and the thermoelectric properties of a given host matrix. This paper may also contribute to defect engineering by coating defect sites with a proper cloaking layer.

Three-terminal superconductor $\left(S\right)$-normal metal $\left(N\right)$-superconductor $\left(S\right)$ Josephson junctions are investigated. In a geometry where a T-shape normal metal is connected to three superconducting reservoirs, new subgap structures appear in the differential resistance for specific combinations of the superconductor chemical potentials. Those correspond to a correlated motion of Cooper pairs within the device that persist well above the Thouless energy and is consistent with the prediction of quartets formed by two entangled Cooper pairs. A simplified nonequilibrium Keldysh-Green's function calculation is presented that supports this interpretation.

The influence of surface/interface on the lattice dynamics of spin crossover nanoparticles has been investigated by a spring-ball model solved by Monte Carlo methods. The bond cohesion energy of the model has been extracted from Mössbauer spectroscopy measurements performed on the model compound ${\mathrm{Ni}}_3\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$. We show that the coupling between bulk and surface vibrational properties, which drastically affects the mechanical properties of the whole particle below a characteristic size, has a major impact on the phase stability of the particles. In the case of free surfaces, the Debye temperature decreases with the size and the first-order nature of the spin transition disappears. On the other hand, a hardening of the surface bonds leads to increasing particle stiffness with the size reduction. In this case, a persistence of the hysteretic behavior in the spin transition curve is also predicted in good agreement with previous theoretical and experimental results.

The nature of the fractional quantum Hall state with filling factor $\nu =2/3$ and its edge modes continues to remain an open problem in low-dimensional condensed matter physics. Here, we suggest an experimental setting to probe the $\nu =2/3$ edge by tunnel-coupling it to a $\nu =1$ integer quantum Hall edge in another layer of a two-dimensional electron gas (2DEG). In this double-layer geometry, the momentum of tunneling electrons may be boosted by an auxiliary magnetic field parallel to the two planes of 2DEGs. The threshold behavior of the current as a function of bias voltage and the boosting magnetic field yields information about the spectral function of the $\nu =2/3$ edge, and in particular about the nature of the chiral edge modes. Our theoretical analysis accounts also for the effects of Coulomb interaction and disorder.

By means of finite-size exact diagonalization we theoretically study the electronic many-body effects on the nearly flat-band structure with time-reversal symmetry in a checkerboard lattice model and identify the topological nature of two quantum phases, with ninefold and threefold degeneracy, that appear, respectively, at small and large values $\lambda$ of a nearest-neighbor spin-dependent interaction. Numerical evidence from the evolution of low-lying energy spectra and Berry phases with both spin-independent and spin-dependent twisted boundary conditions reveal that these two different ground states share the same topological spin Chern number. Quantum phase transition between these two states by tuning $\lambda$ is confirmed by evaluating the closing of energy and quasispin excitation spectra. Finally, the counting rules of spin excitation spectra are demonstrated as the fingerprints of the fractionalized quantum spin Hall states.