We examined the temperature ($T$) evolution of the optical conductivity spectra of ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$ over a wide range of 10–400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\le$0.1 eV. The low-energy features of the optical d-d excitation ($\hslash \omega$ < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O $K$-edge x-ray-absorption spectra. This suggests that the $T$ evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating ${\mathrm{Sr}}_3$${\mathrm{Ir}}_2$${\mathrm{O}}_7$.

Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy measurements, we identify the helical spin textures for three different Dirac cone states in the interfaced systems of a two-dimensional (2D) topological insulator (TI) of a Bi(111) bilayer and a three-dimensional (3D) TI Bi${}_2$Se${}_3$ or Bi${}_2$Te${}_3$. The spin texture is found to be the same for the intrinsic Dirac cone of Bi${}_2$Se${}_3$ or Bi${}_2$Te${}_3$ surface states, the extrinsic Dirac cone of Bi bilayer induced by the Rashba effect, and the hybridized Dirac cone between the former two states. Further orbit- and atom-resolved analysis shows that $s$ and $p_z$ orbits have a clockwise (counterclockwise) spin rotation tangent to the iso-energy contour of the upper (lower) Dirac cone, while $p_x$ and $p_y$ orbits have radial spin components. The Dirac cone states may reside on different atomic layers, but have the same spin texture. Our results suggest that the unique spin texture of Dirac cone states is a signature property of spin-orbit coupling, independent of topology.

We have studied conductance ($g$) of 1,4-butanedithiol-linked Au nanoparticle films (NPFs) through a percolation Mott-Hubbard insulator-to-metal transition (MH-MIT) from a Mott insulating phase with significant nanostructure-induced Coulomb charging barriers to a metallic phase. Near the transition, the NPFs traverse an exotic metallic phase exhibiting a pseudogaplike behavior and a zero-bias conductance peak. The peak decays rapidly with increasing temperature, yielding power-law fits with an average exponent of $3.8\pm 2.9$ and surviving in some samples up to remarkably high temperatures of $\sim$20 K. We propose that the peak is an Abrikosov-Suhl resonance previously observed in nonnanoengineered, correlated materials near MH-MITs. Since nanostructures can be controllably synthesized over a wide range of parameters, the study highlights an opportunity to use nanoengineered correlated materials as a powerful platform to study this exotic metallic phase and electron correlations.

We present an effective-medium model for calculating the frequency-dependent effective permittivity $\epsilon \left(\omega \right)$ and permeability $\mu \left(\omega \right)$ of metamaterial composites containing spherical particles with arbitrary permittivity and permeability. The model is derived from the zero-scattering condition within the dipole approximation, but does not invoke any additional long-wavelength approximations. As a result, it captures the effects of spatial dispersion and predicts a finite effective refractive index and antiresonances in $\epsilon \left(\omega \right)$ and $\mu \left(\omega \right)$, in agreement with numerical finite-element calculations.

We report an optical spectroscopy study on single crystals of ${\mathrm{Na}}_2$${\mathrm{Ti}}_2$${\mathrm{As}}_2\mathrm{O}$, a sister compound of the superconductor Ba${\mathrm{Ti}}_2$Sb${}_2\mathrm{O}$. The study unexpectedly reveals two density wave phase transitions. The first transition, at 320 K, results in the formation of a large energy gap and removes the majority of the Fermi surfaces. But the compound remains metallic with residual itinerant carriers. Below 42 K, another density wave phase transition with a smaller energy gap scale occurs and drives the compound into the semiconducting ground state. These experiments thus enable us to shed light on the complex electronic structure in titanium oxypnictides.

The Si(111)2$\times$2-In reconstruction can be considered as a precursor phase for the formation of the metallic $\sqrt{7}\times \sqrt{3}$ phases of In overlayers on a Si(111) surface. Using the ab initio random structure searching method, comparison of simulated and experimental scanning tunneling microscopy images, and resemblance of the calculated band structure to the experimental angle-resolved photoelectron spectra, we examined various 2$\times$2 structure models with 0.5, 0.75, 1.0, and 1.25 monolayer In coverage. The only model which fits well all the requirements is the one-monolayer model, where three In atoms in the $T_4$ sites form a trimer centered in the $H_3$ site and the fourth In atom occupies the on-top ($T_1$) site.

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator ${\mathrm{SmB}}_6$ using magnetization and muon-spin relaxation/rotation ($\mu$SR) measurements. The $\mu$SR measurements exhibit magnetic field fluctuations in ${\mathrm{SmB}}_6$ below $\sim 15$ K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in ${\mathrm{SmB}}_6$ is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between $\mu$SR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in ${\mathrm{SmB}}_6$ at low temperature.

An enhanced susceptibility is a natural consequence of the “heavy fermion” (HF) state, rendering the possibility of measurably large nonlinear susceptibilities. In recent work a universal behavior of the peaks observed in the linear (${\chi }_1$) and the third-order (${\chi }_3$) susceptibility in HF metamagnets has been identified. This universality is well accounted for by a single energy scale model considering on-site correlations only. A prediction of this model is a peak in the fifth-order susceptibility ${\chi }_5$ as well. In measurements on a HF metamagnet, ${\mathrm{UPt}}_3$, reported herein, we find that ${\chi }_5$, rather than attaining a peak, saturates at low temperatures and is positive. The thermodynamic implications of these towards the stability of the metamagnetic HF state are discussed.

We present results of combined density functional theory plus dynamical mean-field theory (DFT + DMFT) calculations, which show that the Mott insulator LaTiO${}_3$ undergoes an insulator-to-metal transition under compressive epitaxial strain of about $-2$%. This transition is driven by strain-induced changes in the crystal-field splitting between the Ti $t_{2g}$ orbitals, which in turn are intimately related to the collective tilts and rotations of the oxygen octahedra in the orthorhombically distorted $Pbnm$ perovskite structure. An accurate treatment of the underlying crystal structure is therefore crucial for a correct description of the observed metal-insulator transition. Our theoretical results are consistent with recent experimental observations and demonstrate that metallic behavior in heterostructures of otherwise insulating materials can emerge also from mechanisms other than genuine interface effects.

A time and polarization resolved reflective interferometry measurement on femtosecond laser heated gold is presented. We deduced the electron momentum damping frequency, the conduction electron density, and finally the electron temperature in the $0.6–5\mathrm{eV}$ range in out of equilibrium, solid density warm dense gold. This allows the experimental determination of the electron-electron collision frequency variation with the electron temperature in warm dense matter conditions. The comparison with several models shows the importance of properly taking into account the $d$-band electrons in noble metals.

Recent experiments have demonstrated that for a quantum dot in an optical resonator, off-resonant cavity-mode emission can occur even for detunings of the order of 10 meV. We show that Coulomb-mediated Auger processes based on additional carriers in delocalized states can facilitate this far off-resonant emission. A theoretical approach is developed for the nonperturbative treatment of the Auger-assisted quantum-dot carrier recombination. Using this method we present numerical calculations of the far off-resonant cavity feeding rate and cavity mean photon number, confirming efficient coupling at higher densities of carriers in the delocalized states. In comparison to fast Auger-like intraband scattering processes, we find a reduced overall efficiency of Coulomb-mediated interband transitions due the required electron-hole correlations for the recombination processes.

To examine the insulating mechanism of 5$d$ transition metal oxide ${\text{Sr}}_2$${\text{IrO}}_4$, we study the ground state properties of a three-orbital Hubbard model with a large relativistic spin-orbit coupling on a square lattice. Using a variational Monte Carlo method, we find that the insulating state appearing in the ground state phase diagram for one hole per site varies from a weakly correlated to a strongly correlated antiferromagnetic (AF) state with increasing Coulomb interactions. This crossover is characterized by the different energy gain mechanisms of the AF insulating state, i.e., from an interaction-energy-driven Slater-type insulator to a band-energy-driven Mott-type insulator with increasing Coulomb interactions. Our calculations reveal that ${\text{Sr}}_2$${\text{IrO}}_4$ is a “moderately correlated” AF insulator located in the intermediate coupling region between a Slater-type and a Mott-type insulator.

The low-energy electron dynamics of the quasi-two-dimensional electron-hole multiorbital system ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ is investigated by time-domain terahertz spectroscopy for a thin film sample. The direct observation of the real and imaginary part of the optical conductivity below 8 meV clearly demonstrates a narrowing of the coherent quasiparticle response with decreasing temperature. The optical conductivity spectra, dc conductivity, and Hall coefficient can be quantitatively well described by the multiband Drude-Lorentz model, from which band dependent scattering rates can be extracted. The resulting modest band dependent scattering rates, which are lower than 2 meV at 4 K, support a rather isotropic relaxation regime.

We study various aspects of interplay between two-dimensional helical electrons, realized on the surface of a three-dimensional topological insulator, and the magnetization of a ferromagnet coupled to them. The magnetization is assumed to be perpendicular to the surface, with small transverse fluctuations $\mathit{u}$. In the first part of this paper, we calculate spin torques that the helical electrons exert on the magnetization. Up to first orders with respect to $\mathit{u}$, space/time derivative and electric current, we have determined all torques, which include Gilbert damping, spin renormalization, current-induced spin-orbit torques, and gradient corrections to them. Thanks to the identity between the velocity and spin in this model, these torques have exact interpretation in terms of transport phenomena, namely, diagonal conductivity, (anomalous) Hall conductivity, and corrections to them due to ordinary Hall effect on top of the anomalous one. These torque (and transport) coefficients are studied in detail with particular attention to the effects of vertex corrections and type of impurities (normal and magnetic). It is shown rigorously that the conventional current-induced torques, namely, spin-transfer torque and the so-called $\beta$ term, are absent. An electromotive force generated by spin dynamics, which is the inverse to the current-induced spin-orbit torque, is also studied. In the second part, we study the feedback effects arising as combinations of current-induced spin-orbit torques and spin-dynamics-induced electromotive force. It is demonstrated that the Gilbert damping process in this system is completely understood as a feedback effect. Another feedback effect, which may be called “magnon-drag electrical conductivity,” is shown to violate the exact correspondence between spin-torque and transport phenomena demonstrated in the first part.

By quickly modifying the shape of the effective potential of a double SQUID flux qubit from a single-well to a double-well condition, we experimentally observe an anomalous behavior, namely, an alternation of resonance peaks, in the probability to find the qubit in a given flux state. The occurrence of Landau-Zener transitions as well as resonant tunneling between degenerate levels in the two wells may be invoked to partially justify the experimental results. A quantum simulation of the time evolution of the system indeed suggests that the observed anomalous behavior can be imputable to quantum coherence effects. The interplay among all these mechanisms has a practical implication for quantum computing purposes, giving a direct measurement of the limits on the sweeping rates possible for a correct manipulation of the qubit state by means of fast flux pulses, avoiding transitions to noncomputational states.

We theoretically investigate pairing states of the spin-triplet $p$-wave superfluid ${}^3$He confined in narrow cylinders. The surface-induced distortion and the multiple internal degrees of freedom of the order parameter lead to the occurrence of a stripe structure along the cylinder axis in the superfluid ${}^3$He-B phase. We show that in sufficiently small cylinders with an anisotropic surface scattering, the stripe order with broken translational symmetry may be stabilized as the lowest energy state. Periodic spin-current textures caused in this inhomogeneous superfluid phase are also discussed.

The plasticity of hexagonal titanium is reexamined based on the split slip modes phenomenon, revealing the existence of subslip modes in prismatic and pyramidal hcp slip systems. The energetics of dislocation emission and motions were described using all-dimension relaxed atomic models of crystal slip, calculated with density functional theory. The proposed computational methodology is based on the generalized stacking fault energy concept and respects all elastic effects arising within dislocation nucleation. As a result, improved accuracy has been obtained with regard to ductility prediction and a breach has been discovered in the fundamental Peierls-Nabarro rule. This approach is essential for the Rice and Peierls-Nabarro models and can be used as an effective tool for ductility predictions when designing new hexagonal alloys.

Two ferromagnetic (FM) layers magnetically decoupled by a thick, normal metal spacer layer can be dynamically coupled via spin currents emitted by the spin pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two FM resonances. In the case of nonzero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling (IC) hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and nonlocal damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the IC strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two FM layers coupled magnetically and by spin currents has been done to separate the effects of the nonlocal damping from the resonance modes hybridization. In our samples, both mechanisms bring about linewidth changes of the same order of magnitude but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.