In this work, we investigate the effect of disorder on the topological properties of multichannel superconductor nanowires. While the standard expectation is that the spectral gap is closed and opened at transitions that change the topological index of the wire, we show that the closing and opening of a transport gap can also cause topological transitions, even in the presence of nonzero density of states across the transition. Such transport gaps induced by disorder can change the topological index, driving a topologically trivial wire into a nontrivial state or vice versa. We focus on the Rashba spin-orbit coupled semiconductor nanowires in proximity to a conventional superconductor, which is an experimentally relevant system, and we obtain analytical formulas for topological transitions in these wires, valid for generic realizations of disorder. Full tight-binding simulations show excellent agreement with our analytical results without any fitting parameters.

The natural periodic stacking of symmetry-inequivalent planes in layered compounds can lead to the formation of natural superlattices; albeit close in total energy, (thus in their thermodynamic stability), such polytype superlattices can exhibit different structural symmetries, thus have markedly different electronic properties which can in turn be used as “structural markers”. We illustrate this general principle on the layered $\mathrm{LaOBi}{\mathrm{S}}_2$ compound where density-functional theory (DFT) calculations on the ($\mathrm{Bi}{\mathrm{S}}_2$)/(LaO)/($\mathrm{Bi}{\mathrm{S}}_2$) polytype superlattices reveal both qualitatively and quantitatively distinct electronic structure markers associated with the Rashba physics, yet the total energies are only ∼ 0.1 meV apart. This opens the exciting possibility of identifying subtle structural features via electronic markers. We show that the pattern of removal of band degeneracies in different polytypes by the different forms of symmetry breaking leads to Rashba “minigaps” with characteristic Rashba parameters that can be determined from spectroscopy, thereby narrowing down the physically possible polytypes. By identifying these distinct DFT-predicted fingerprints via angle-resolved photoemission spectroscopy (ARPES) measurements on $\mathrm{LaBiO}{\mathrm{S}}_2$ we found the dominant polytype with small amounts of mixtures of other polytypes. This conclusion, consistent with neutron scattering results, establishes ARPES detection of theoretically established electronic markers as a powerful tool to delineate energetically quasidegenerate polytypes.

We show how to compute the magneto-optical functions (absorption, reflection, and transmission) when Rydberg exciton polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field, and the polaritonic effect. Using the real density-matrix approach the analytical expressions for magneto-optical functions are obtained and numerical calculations for ${\mathrm{Cu}}_2\mathrm{O}$ crystal are performed. The influence of the strength of applied external magnetic field on the resonance displacement of excitonic spectra is discussed. We report a good agreement with recently published experimental data.

Using a combination of first-principles calculations based on density functional theory and Boltzmann semiclassical transport theory, we compute and study the properties of pristine layered calcium cobaltite ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$. We model the system with the B1WC hybrid functional. Two supercells of increasing size which approximate the incommensurate crystallographic structure of the compound are studied and we determine their structural, magnetic, and electronic properties. It is found that the B1WC hybrid functional is appropriate to reproduce the structural, electronic, and magnetic properties, which are then extensively discussed. From the electronic band structure, the Seebeck $\left(S\right)$ and electrical resistivity $\left(\rho \right)$ tensors are computed using Boltzmann transport theory within the constant relaxation-time approximation. The differences between the diagonal components are detailed and reveal a strong in-plane anisotropy of the properties. The qualitative behavior of the averaged in-plane properties, $S_{//}$ and ${\rho }_{//}$, is consistent with the measurements reported in the literature. Our calculation clarifies and provides a broad picture of the evolution of the thermoelectric properties with both carrier density and temperature, and suggests that the change in $S_{//}$ and ${\rho }_{//}$ around 100 K is not necessarily related to the magnetic transitions occurring around 100 K.

We show that all the singlet even-frequency, singlet odd-frequency, triplet even-frequency, and triplet odd-frequency pairings, together with the corresponding order parameters (gaps), can be realized in InSb (110) spin-orbit-coupled quantum well in proximity to $s$-wave superconductor in Fulde-Ferrell-Larkin-Ovchinnikov phase or with a supercurrent. It is revealed that with the singlet even-frequency order parameter induced by the proximity effect, triplet even-frequency pairing is induced due to the broken spin-rotational symmetry by the spin-orbit coupling. Since the translational symmetry is broken by the center-of-mass momentum of Cooper pair in Fulde-Ferrell-Larkin-Ovchinnikov phase or with a supercurrent, the singlet odd-frequency pairing can be induced. With the translational and spin-rotational asymmetries, the triplet odd-frequency pairing is also realized. Then, we show that the corresponding order parameters can be obtained from the self-energy of the electron-electron Coulomb interaction with the dynamic screening. The singlet and the induced triplet even-frequency order parameters are found to exhibit the conventional $s$- and $p$-wave characters in the momentum space, respectively. Whereas for the induced odd-frequency order parameters in quantum well, the singlet and triplet ones show the $p$- and $d$-wave characters, respectively. Moreover, the $p$-wave character of the singlet odd-frequency order parameter exhibits anisotropy with respect to the direction of the center-of-mass momentum. While for the triplet one, we find that $d_{x^2}$- and $d_{xy}$-wave characters can be obtained with respect to the direction of the center-of-mass momentum. We show that at proper density, the singlet even-frequency order parameter is suppressed and the induced singlet odd-frequency, triplet even-frequency, and triplet odd-frequency ones can be detected experimentally.

We investigate theoretically the non-Markovian dynamics of a degenerate V-type quantum emitter in the vicinity of a metallic nanosphere, a system that exhibits quantum interference in spontaneous emission due to the anisotropic Purcell effect. We calculate numerically the electromagnetic Green's tensor and employ the effective modes differential equation method for calculating the quantum dynamics of the emitter population, with respect to the resonance frequency and the initial state of the emitter, as well as its distance from the nanosphere. We find that the emitter population evolution varies between a gradual total decay and a partial decay combined with oscillatory population dynamics, depending strongly on the specific values of the above three parameters. Under strong-coupling conditions, coherent population trapping can be observed in this system. We compare our exact results with results when the flat continuum approximation for the vacuum modified by the metallic nanosphere is applied. We conclude that the flat continuum approximation is an excellent approximation only when the spectral density of the system under study is characterized by nonoverlapping plasmonic resonances.

The temperature and thickness dependencies of the in-plane anisotropic magnetoresistance (AMR) of ${\mathrm{SmB}}_6$ thin films are reported. We find that the AMR changes sign from negative (${\rho }_{\left|\right|}<{\rho }_{\perp }$) at high temperatures to positive (${\rho }_{\left|\right|}>{\rho }_{\perp }$) at low temperatures. The temperature, $T_s$, at which this sign change occurs, decreases with increasing film thickness $t$ and $T_s$ vanishes for $t>$ 30 nm. We interpret our results in the framework of a competition between two components: a negative bulk contribution and a positive surface AMR.

We analyze the interplay of longitudinal and transverse thermal fluctuations in a $\text{U}\left(1\right)$ symmetric two-dimensional ${\varphi }^4$ theory. Toward this end, we derive coupled renormalization-group equations for both types of fluctuations obtained from a linear (Cartesian) decomposition of the order-parameter field. Discarding the longitudinal fluctuations, the expected Berezinskii-Kosterlitz-Thouless phase characterized by a finite stiffness and an algebraic decay of order-parameter correlations is recovered. Renormalized by transverse fluctuations, the longitudinal mass scales to zero so that longitudinal fluctuations become increasingly important for small momenta. Within our expansion of the effective action, they generate a logarithmic decrease of the stiffness, in agreement with previous functional renormalization-group calculations. The logarithmic terms imply a deviation from the vanishing $\beta$ function for the stiffness in the nonlinear $\sigma$ model describing the phase fluctuations at three-loop order. To gain further insight, we also compute the flow of the parameters characterizing longitudinal and transverse fluctuations from a density-phase representation of the order-parameter field, with a cutoff on phase fluctuations. The power-law flow of the longitudinal mass and other quantities is thereby confirmed, but the stiffness remains finite in this approach. We conclude that the marginal flow of the stiffness obtained in the Cartesian representation is an artifact of the truncated expansion of momentum dependences.

The formation of nanopatterns induced by low-energy (0.5–1.5 keV) ${\mathrm{Xe}}^{+}$ ion-beam sputtering of amorphous alumina thin films is investigated by atomic force microscopy and grazing incidence small-angle x-ray scattering. The observed dependence of the surface morphology on ion incidence angle, temperature, ion energy, and fluence is compared with the predictions of linear and nonlinear continuum theoretical models. The results show that ion-induced mass redistribution stabilizes the surface at near-normal and very grazing incidence angles, while curvature-dependent erosion governs the formation of periodic nanoripples in the range of incidence angles between ${50}^{\circ }$ and ${65}^{\circ }$. Surface-confined ion-induced viscous flow is shown to be the dominant relaxation mechanism during erosion. Moreover, pattern evolution with ion fluence (pattern ordering and asymmetry of the ripple profile, in particular) suggests that nonlinear effects that are ignored by the Sigmund's collision cascade theory of sputtering contribute strongly to the observed dynamics of ripple formation.

Heat radiation and near-field radiative heat transfer can be strongly manipulated by adjusting geometrical shapes, optical properties, or the relative positions of the objects involved. Typically, these objects are considered as embedded in vacuum. By applying the methods of fluctuational electrodynamics, we derive general closed-form expressions for heat radiation and heat transfer in a system of $N$ arbitrary objects embedded in a passive nonabsorbing background medium. Taking into account the principle of reciprocity, we explicitly prove the symmetry and positivity of transfer in any such system. Regarding applications, we find that the heat radiation of a sphere as well as the heat transfer between two parallel plates is strongly enhanced by the presence of a background medium. Regarding near- and far-field transfer through a gas like air, we show that a microscopic model (based on gas particles) and a macroscopic model (using a dielectric contrast) yield identical results. We also compare the radiative transfer through a medium like air and the energy transfer found from kinetic gas theory.

A three-terminal Josephson junction consists of three superconductors coupled coherently to a small nonsuperconducting island, such as a diffusive metal, a single or double quantum dot. A specific resonant single quantum dot three-terminal Josephson junction $(S_a,S_b,S_c)$ biased with voltages $(V,-V,0)$ is considered, but the conclusions hold more generally for resonant semiconducting quantum wire setups. A simple physical picture of the steady state is developed, using Floquet theory. It is shown that the equilibrium Andreev bound states (for $V=0$) evolve into nonequilibrium Floquet-Wannier-Stark-Andreev (FWS-Andreev) ladders of resonances (for $V\ne 0$). These resonances acquire a finite width due to multiple Andreev reflection (MAR) processes. We also consider the effect of an extrinsic linewidth broadening on the quantum dot, introduced through a Dynes phenomenological parameter. The dc-quartet current manifests a crossover between the extrinsic relaxation dominated regime at low voltage to an intrinsic relaxation due to MAR processes at higher voltage. Finally, we study the coupling between the two FWS-Andreev ladders due to Landau-Zener-Stückelberg transitions, and its effect on the crossover in the relaxation mechanism. Three important low-energy scales are identified, and a perspective is to relate those low-energy scales to a recent noise cross-correlation experiment (Y. Cohen et al., arXiv:1606.08436).

We report on joint experimental and theoretical investigations of the unoccupied surface electronic structure of W(110). The spin-resolved inverse-photoemission experiments reveal a number of bands influenced by spin-orbit interaction and an image-potential state. The bands disperse differently within the two nonequivalent mirror planes of the surface, which is explained by their origin and their localization within the surface region. Surprisingly, the image-potential state also exhibits anisotropic dispersion, although it is strongly located within the surface barrier. The experimental findings are confirmed by first-principles electronic-structure calculations.

In this paper we investigate the role of two-temperature heating dynamics for formation of periodic structures on metal surfaces exposed to single ultrashort laser pulses.The results of two-temperature model (TTM) two-dimensional simulations are presented on the irradiation of gold by a single 800-nm femtosecond laser pulse the intensity of which is modulated in order to reproduce an initial electron temperature perturbation, which can arise from incoming and scattered surface wave interference. The growing (unstable) modes of the lattice temperature distribution along the surface may be significant in the laser induced periodic surface structures formation. After the end of the laser pulse and before the complete coupling between lattice and electrons occurs, the evolution of the amplitude of the subsequent modulation in the lattice temperature reveals different tendencies depending on the spatial period of the initial modulation. This instabilitylike behavior is shown to arise due to the perturbation of the electronic temperature which relaxes slower for bigger spatial periods and thus imparts more significant modulations to the lattice temperature. Small spatial periods of the order of 100 nm and smaller experience stabilization and fast decay from the more efficient lateral heat diffusion which facilitates the relaxation of the electronic temperature amplitude due to in-depth diffusion. An analytical instability analysis of a simplified version of the TTM set of equations supports the lattice temperature modulation behavior obtained in the simulations and reveals that in-depth diffusion length is a determining parameter in the dispersion relation of unstable modes. Finally, it is discussed how the change in optical properties can intensify the modulation-related effects.

Motivated by recent experiments on the vanadium oxyfluoride material DQVOF, we examine possible spin liquid phases on a breathing kagome lattice of $S=1/2$ spins. By performing a projective symmetry group analysis, we determine the possible phases for both fermionic and bosonic ${\mathbb{Z}}_2$ spin liquids on this lattice, and establish the correspondence between the two. The nature of the ground state of the Heisenberg model on the isotropic kagome lattice is a hotly debated topic, with both ${\mathbb{Z}}_2$ and U(1) spin liquids argued to be plausible ground states. Using variational Monte Carlo techniques, we show that a gapped ${\mathbb{Z}}_2$ spin liquid emerges as the clear ground state in the presence of this breathing anisotropy. Our results suggest that the breathing anisotropy helps to stabilize this spin liquid ground state, which may aid us in understanding the results of experiments and help to direct future numerical studies on these systems.

We present a temperature-dependent study of the thermal excitation of a magnon accumulation in two-sublattice magnetic materials. Using atomistic spin model simulations, we study the local magnetization profiles sublattice-wise in the vicinity of a temperature step in antiferromagnets, as well as in ferrimagnets. It is shown that the strength of the magnon accumulation in these systems scales with the derivative of the magnetization with respect to the temperature. These results give an insight into the complex temperature dependence of the magnon accumulation by making a direct link to the macroscopic behavior of the magnetization.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for studying the structural and electronic properties of paramagnetic solids. However, the interpretation of paramagnetic NMR spectra is often challenging as a result of the interactions of unpaired electrons with the nuclear spins of interest. In this work, we extend the formalism of the paramagnetic NMR shielding in the presence of spin-orbit coupling towards solid systems with multiple paramagnetic centers. We demonstrate how the single-ion electron paramagnetic resonance $g$ tensor is defined and calculated in periodic paramagnetic solids. We then calculate the hyperfine tensor and the $g$ tensor with density functional theory to show the validity of the presented model and we further demonstrate how these interactions can be combined to give the overall paramagnetic shielding tensor, ${\mathbf{\sigma }}^s$. The method is applied to a series of olivine-type $\text{Li}\mathit{\text{TM}}{\text{PO}}_4$ materials (with $\mathit{\text{TM}}=\text{Mn}$, Fe, Co, and Ni) and the corresponding ${}^7\mathrm{Li}$ and ${}^{31}\mathrm{P}$ NMR spectra are simulated. We analyze the effects of spin-orbit coupling and of the electron-nuclear magnetic interactions on the calculated NMR parameters. A detailed comparison is presented between contact and dipolar interactions across the $\text{Li}\mathit{\text{TM}}{\text{PO}}_4$ series, in which the magnitudes and signs of the nonrelativistic and relativistic components of the overall isotropic shift and shift anisotropy are computed and rationalized.

We consider charge transport properties in realistic, fabricable, ferromagnet/superconductor spin valves having a layered structure $F_1/N/F_2/S$, where $F_1$ and $F_2$ denote the ferromagnets, $S$ the superconductor, and $N$ the normal-metal spacer usually inserted in actual devices. Our calculation is fully self-consistent, as required to ensure that conservation laws are satisfied. We include the effects of scattering at all the interfaces. We obtain results for the device conductance $G$, as a function of bias voltage, for all values of the angle $\varphi$ between the magnetizations of the $F_1$ and $F_2$ layers and a range of realistic values for the material and geometrical parameters in the sample. We discuss, in the context of our results for $G$, the relative influence of all parameters on the spin valve properties. We also study the spin current and the corresponding spin-transfer torque in $F_1/F_2/S$ structures.

Through three-dimensional discrete dislocation dynamics simulations, we show that by tuning the mode of external loading, the collective dynamics of dislocations undergo a transition from driven avalanches under stress control to quasiperiodic oscillations under strain control. We directly correlate measured intermittent plastic events with internal dislocation activities and collective dynamics. Under different loading modes, the roles of the weakest dislocation source and the defect population trend are significantly different. This finding raises new possibilities of controlling correlated dislocation activities and obtaining a low defect density in nanostructured devices by tuning external constraints. In addition, the effect of machine stiffness comes to light. The statistical analysis of the burst magnitude is revisited and carefully discussed. Self-organized criticality and scale-free statistics of strain bursts are obeyed under stress control. However, this behavior is shown to break down under strain control. Rapid stress drops under pure strain control force truncation of dislocation avalanches, leading to a dynamical transition to quasiperiodic oscillations.

We investigate entanglement dynamics and correlations in the quantum Ising model in arbitrary dimensions using a large-coordination-number expansion. We start from the pure paramagnetic regime obtained through zero spin-spin coupling and subsequently turn on the interspin interaction in a time-dependent fashion. We investigate analytically and compare results for both the slow adiabatic onset of the interactions and the fast instantaneous switching. We find that in the latter case of an initial excitation mode a quantum correlation wave spreads through the system, propagating with twice the group velocity of the linearized equilibrium modes. This wave establishes the spatiotemporal regime of entangled quantum properties of the system for time scales shorter than the decoherence time and thus provides an indicator for the “quantumness” of the physical system that the specific system models.