We discuss the nature of leaky modes with large phase velocity and an effective refractive index and group velocity near zero in planar arrays of subwavelength dielectric or metallic spheres far from their resonances. Large field enhancements with $Q$ factors exceeding $10{}^{10}$ are theoretically predicted for these modes. Those theoretical findings may have important applications in optical switching and memories, quantum information processing, optical storage, and enhancement of optical nonlinearities. Preliminary experimental results at microwave frequencies qualitatively confirm these predictions.

The electronic structure and numerous magnetic properties of MnBi magnetic systems are investigated using local spin density approximation (LSDA) with on-cite Coulomb correlations $(\mathrm{LSDA}+U)$ included. We show that the inclusion of Coulomb correlations provides a much better description of equilibrium magnetic moments on Mn atoms as well as the magnetic anisotropy energy behavior with temperature and magneto-optical effects. We found that the inversion of the anisotropic pairwise exchange interaction between Bi atoms is responsible for the observed spin reorientation transition at 90 K. This interaction appears as a result of strong spin orbit coupling on Bi atoms, large magnetic moments on Mn atoms, significant $p\text{−}d$ hybridization between Mn and Bi atoms, and it depends strongly on lattice constants (anisotropic Bi-Bi exchange striction). A better agreement with the magneto-optical Kerr measurements at higher energies is obtained. We also present the detailed investigation of the Fermi surface, the de Haas–van Alphen effect, and the x-ray magnetic circular dichroism in MnBi.

The prospect of combining the dissipationless nature of superconducting currents with the spin polarization of magnetic materials is interesting with respect to exploring superconducting analogs of topics in spintronics. In order to accomplish this aim, it is pivotal to understand not only how such spin supercurrents can be created, but also how they interact dynamically with magnetization textures. In this paper, we investigate the appearance of a spin supercurrent and the resulting magnetization dynamics in a textured magnetic Josephson current by using three experimentally relevant models: (i) a superconductor$\mid$ferromagnet$\mid$superconductor ($\mathrm{S}\mid \mathrm{F}\mid \mathrm{S}$) junction with spin-active interfaces, (ii) a $\mathrm{S}\mid {\mathrm{F}}_1\mid {\mathrm{F}}_2\mid {\mathrm{F}}_3\mid \mathrm{S}$ Josephson junction with a ferromagnetic trilayer, and (iii) a Josephson junction containing a domain wall. In all of these cases, the supercurrent is spin polarized and exerts a spin-transfer torque on the ferromagnetic interlayers which causes magnetization dynamics. Using a scattering matrix formalism in the clean limit, we compute the Andreev bound states and resulting free energy of the system which in turn is used to solve the Landau-Lifshiftz-Gilbert equation. We compute both how the inhomogeneous magnetism influences the phase dependence of the charge supercurrent and the magnetization dynamics caused by the spin polarization of the supercurrent. Using a realistic experimental parameter set, we find that the spin supercurrent can induce magnetization switching that is controlled by the superconducting phase difference. Moreover, we demonstrate that the combined effect of chiral spin symmetry breaking of the system as a whole with interface scattering causes the systems above to act as phase batteries that may supply any superconducting phase difference $\phi$ in the ground state. Such a $\phi$-junction is accompanied by an anomalous supercurrent appearing even at zero phase difference, and we demonstrate that the flow direction of this current is controlled by the chirality of the magnetization configuration.

We find signatures of superconductivity at $T_c=12$ K in a molybdenum arsenide sample containing ∼4% of ${\mathrm{MoO}}_{2-\delta }$ impurity by weight. Below is a report on the electronic structure calculations, structures analyses, and thermodynamic and transport properties on ${\mathrm{Mo}}_2{\mathrm{As}}_3$, ${\mathrm{Mo}}_5{\mathrm{As}}_4$, and ${\mathrm{MoO}}_2$, in order to uncover the reasons for superconductivity for the sample that contains a mixture of these phases. The experimental and theoretical evidence suggest that superconductivity in the ${\mathrm{MoO}}_{2-\delta }$ phase is likely and may be caused by nesting-type fluctuations.

We study a system of dipolar molecules confined in a two-dimensional trap and subject to an optical square lattice. The repulsive long-range dipolar interaction $D/r^3$ favors an equilateral triangular arrangement of the molecules, which competes against the square symmetry of the underlying optical lattice with lattice constant $b$ and amplitude $V$. We find the minimal-energy states at the commensurate density $n=1/b^2$ and establish the complete square-to-triangular transformation pathway of the lattice with decreasing $V$ involving period-doubled, solitonic, and distorted-triangular configurations.

Microstructural changes during mechanical shear of a ferroelastic or martensitic material and their signature in acoustic emission (AE) spectroscopy during strain-induced yield and detwinning are investigated by computer simulation. Complex domain patterns are generated during the main yield event, which leads to large displacements of surface atoms and emission of acoustic waves. Loading beyond the yield point leads, eventually, to a simplification of the domain patterns by local movements of needle domains, the nucleation and movement of kinks in domain walls, and the collapse of domains spanning the entire sample (from surface to surface). These microstructural changes lead to much weaker acoustic emissions than those near the yield point. Nucleation/collapse during a yield event involves an energy drop of some 3.7 meV/atom; the collapse of spanning domains releases 0.56 meV/atom, a kink crashing into the surface changes the energy by 0.017 meV/atom, and the collapsing vertical needle changes the energy by 0.017 meV/atom. All these energy bursts can, in principle, be seen by AE. The large energy spread means that AE spectroscopy measures a mixture of events whereby weak and strong signals may signify smaller and bigger events of the same kind or different microstructural changes with intrinsically different signal strengths. In order to disentangle the various contributions, other observables are needed, such as the time-dependent strain matrix of the deformed sample.

We studied the processes of hole and electron trapping in yttrium orthosilicate ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ single crystals using continuous wave and pulse electron spin resonance methods. We show that holes created by x-ray irradiation at low temperatures ($T<80$ K) are preferably self-trapped at Si-unbound oxygen ions in the form of $\mathrm{O}{}^{-}$ centers. Under irradiation at higher temperatures (200–290 K), the holes are trapped at the Si-unbound oxygen ions in the vicinity of perturbing defects such as yttrium vacancies and impurity ions forming a variety of $\mathrm{O}{}^{-}$ centers with thermal stability up to room and higher temperatures. We have also found that under x-ray irradiation at $T$ < 60 K, electrons are preferably trapped in the vicinity of Si-unbound oxygen ion vacancies and partly trapped also at Mo impurity ions in the form of $\mathrm{F}{}^{+}$-type and $\mathrm{Mo}{}^{5+}$ centers, respectively. The trapped electrons are thermally released from the $\mathrm{F}{}^{+}$ centers at 75–90 K, thus giving rise to a thermally stimulated luminescence peak at these temperatures. We assume that this process is realized without excitation of the electrons to the conduction band. The spectroscopic parameters ($g$ and hyperfine tensors) of all the investigated centers have been determined as well. Electron spin resonance measurements of electron and hole traps in the related compound lutetium orthosilicate $\left({\mathrm{Lu}}_2{\mathrm{SiO}}_5\right)$ are discussed as well.

Dynamic magnetoelectric coupling in the improper ferroelectric ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x\mathrm{O} (x=0,x=0.05)$ was investigated using terahertz time-domain spectroscopy to probe electromagnon and magnon modes. Zinc substitution was found to reduce the antiferromagnetic ordering temperature and widen the multiferroic phase, under the dual influences of spin dilution and a reduction in unit-cell volume. The impact of Zn substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, and shell-model calculations. Pronounced softenings of the $A_u$ phonons, active along the direction of ferroelectric polarization, occur in the multiferroic state of ${\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x\mathrm{O}$, and indicate strong spin-phonon coupling. The commensurate antiferromagnetic phase also exhibits spin-phonon coupling, as evidenced by a Raman-active zone-folded acoustic phonon, and spin dilution reduces the spin-phonon coupling coefficient. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon is relatively insensitive to Zn substitution, increasing in energy without widening. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained even in disordered spin systems.

The spin relaxation induced by the Elliott-Yafet mechanism and the extrinsic spin Hall conductivity due to the skew scattering are investigated in $5d$ transition-metal ultrathin films with self-adatom impurities as scatterers. The values of the Elliott-Yafet parameter and of the spin-flip relaxation rate reveal a correlation with each other that is in agreement with the Elliott approximation. At 10-layer thickness, the spin-flip relaxation time in $5d$ transition-metal films is quantitatively reported about few hundred nanoseconds at atomic percent. This time scale is one and two orders of magnitude shorter than the values in Au and Cu thin films, respectively. The anisotropy effect of the Elliott-Yafet parameter and of the spin-flip relaxation rate with respect to the direction of the spin-quantization axis in relation to the crystallographic axes is also analyzed. We find that the anisotropy of the spin-flip relaxation rate is enhanced due to the Rashba surface states on the Fermi surface, reaching values as high as 97% in 10-layer Hf(0001) film or 71% in 10-layer W(110) film. Finally, the spin Hall conductivity as well as the spin Hall angle due to the skew scattering off self-adatom impurities are calculated using the Boltzmann approach. Our calculations employ a relativistic version of the first-principles full-potential Korringa-Kohn-Rostoker Green function method.

In this paper we explore the magnetocaloric effect (MCE) of chromium-doped elpasolite ${\mathrm{Cs}}_2{\mathrm{NaAl}}_{1-x}{\mathrm{Cr}}_x{\mathrm{F}}_6$ $(x=0.01$ and 0.62) single crystals. Magnetization and heat capacity data show the magnetocaloric potentials to be comparable to those of garnets, perovskites, and other fluorides, producing magnetic entropy changes of 0.5 J/kg K $(x=0.01)$ and 11 J/kg K $(x=0.62)$, and corresponding adiabatic temperature changes of 4 and 8 K, respectively. These values are for a magnetic field change of 50 kOe at a temperature around 3 K. A clear Schottky anomaly below 10 K, which becomes more apparent when an external magnetic field is applied, was observed and related to the splitting of the ${\mathrm{Cr}}^{3+}$ energy levels. These results hint at a new family of materials with potential wide use in cryorefrigeration.

The crystal structure and physical properties of the layered material ${\mathrm{EuMnBi}}_2$ have been characterized by measurements on single crystals. ${\mathrm{EuMnBi}}_2$ is isostructural with the Dirac material ${\mathrm{SrMnBi}}_2$ based on single-crystal x-ray diffraction, crystallizing in the $I4/mmm$ space group (No. 139). Magnetic susceptibility measurements suggest antiferromagnetic (AFM) ordering of moments on divalent Eu ions near $T_N=22$ K. For low fields, the ordered Eu moments are aligned along the c axis, and a spin flop is observed near 5.4 T at 5 K. The moment is not saturated in an applied field of 13 T at 5 K, which is uncommon for compounds containing ${\mathrm{Eu}}^{2+}$. The magnetic behavior suggests an anisotropy enhancement via interaction between Eu and the Mn moments that appear to be ordered antiferromagnetically below $\approx$310 K. A large increase in the magnetoresistance is observed across the spin flop, with absolute magnetoresistance reaching $\approx$650% at 5 K and 12 T. Hall effect measurements reveal a decrease in the carrier density below $T_N$, which implies a manipulation of the Fermi surface by magnetism on the sites surrounding the Bi square nets that lead to Dirac cones in this family of materials.

We develop a systematic variational coherent-state expansion for the many-body ground state of the spin-boson model, in which a quantum two-level system is coupled to a continuum of harmonic oscillators. Energetic constraints at the heart of this technique are rationalized in terms of polarons (displacements of the bath states in agreement with classical expectations) and antipolarons (counterdisplacements due to quantum tunneling effects). We present a comprehensive study of the ground-state two-level system population and coherence as a function of tunneling amplitude, dissipation strength, and bias (akin to asymmetry of the double-well potential defining the two-state system). The entanglement among the different environmental modes is investigated by looking at spectroscopic signatures of the bipartite entanglement entropy between a given environmental mode and all the other modes. We observe a drastic change in behavior of this entropy for increasing dissipation, indicative of the entangled nature of the environmental states. In addition, the entropy spreads over a large energy range at strong dissipation, a testimony to the wide entanglement window characterizing the underlying Kondo state. Finally, comparisons to accurate numerical renormalization-group calculations and to the exact Bethe ansatz solution of the model demonstrate the rapid convergence of our variationally optimized multipolaron expansion, suggesting that it should also be a useful tool for dissipative models of greater complexity, as relevant for numerous systems of interest in quantum physics and chemistry.

Arrays of metallic particles may exhibit optical collective excitations known as surface lattice resonances (SLRs). These SLRs occur near the diffraction edge of the array and can be sharper than the plasmon resonance associated with the isolated single particle response. We have fabricated and modeled arrays of silver nanoparticles of different geometries. We show that square, hexagonal, and honeycomb arrays show similar SLRs; no one geometry shows a clear advantage over the others in terms of resonance linewidth. We investigate the nature of the coupling between the particles by looking at rectangular arrays. Our results combine experiment and modeling based on a simple coupled-dipole model.

We investigate numerically the propagation of steady-state monochromatic surface plasmon polaritons (SPPs) in curved chains of metal nanoparticles of various spheroidal shapes. We discuss the SPP propagation (decay of the amplitude), the polarization conversion due to coupling of orthogonally polarized SPPs, and the electromagnetic field localization in the near-field vicinity of a chain.

The theoretical results are presented showing that strain-induced anisotropy of graphene spectrum gives rise to the valley currents under the illumination by normally incident light. The currents of the two graphene valleys are mutually compensated providing zero net electric current. The magnitude and direction of the valley currents are determined by the parameters of strain and light polarization. For not too high photon energy strain-induced valley current exceeds that due to intrinsic warping of the graphene spectrum which suggests feasibility of strain-mediated valleytronics.

The coupled electronic-structural modulations of the ligand states in ${\mathrm{IrTe}}_2$ have been studied by x-ray absorption spectroscopy and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the Te-$M_{4,5} (3d \to 5p)$ absorption edge, indicating the presence of a Te $5p–\mathrm{Ir} 5d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te $3d \to 5p$ resonance at the $Q=(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_s\sim 280$ K. The analysis of the energy-dependent REXS line shape reveals the key role played by the spatial modulation of the covalent Te $5p–\mathrm{Ir} 5d$ bond density in driving the stripelike order in ${\mathrm{IrTe}}_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology recently observed in the high-temperature superconducting cuprates suggests that the iridates may harbor similar exotic phases.

We study the temperature dependence of the triplet and singlet exciton dynamics in the archetype small molecule fluorescent guest-host system tris(8-hydroxyquinolinato) aluminum $\left({\mathrm{Alq}}_3\right)$ doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2). We develop a comprehensive model of the exciton dynamics, and use it to fit the transient photoluminescence under different pulsed optical pumping in the temperature range of $80<T<295\mathrm{K}$. The triplet decay has a significantly different temperature dependence than that of singlets. From 295 to 80 K, the triplet-triplet annihilation (TTA) rate decreases by two orders of magnitude, whereas the singlet-triplet annihilation rate decreases by $<50\%$, primarily a result of the different energy transfer mechanisms of singlets and triplets. The temperature dependence of the TTA rate reveals two regimes separated by a transition at 180 K from Marcus to Miller-Abrahams transfer. This work deepens our understanding of exciton dynamics and energy transfer in small molecule organic materials.