The lanthanum family of high-temperature cuprate superconductors is known to exhibit both spin and charge electronic modulations around a doping level of 1/8. We assume that these modulations have the character of a two-dimensional spin-vortex checkerboard and investigate whether this assumption is consistent with the Fermi surface and the pseudogap measured by angle-resolved photoemission spectroscopy. We also explore the possibility of observing quantum oscillations of transport coefficients in such a background. These investigations are based on a model of noninteracting spin-1/2 fermions hopping on a square lattice and coupled through spins to a magnetic field imitating a spin-vortex checkerboard. The main results of this paper include (i) a calculation of the Fermi surface containing Fermi arcs at the positions in the Brillouin zone largely consistent with experiments, (ii) identification of factors complicating the observations of quantum oscillations in the presence of spin modulations, and (iii) an investigation of the symmetries of the resulting electronic energy bands, which, in particular, indicates that each band is doubly degenerate and has at least one conical point, where it touches another doubly degenerate band. We discuss possible implications these cones may have for the transport properties and the pseudogap.

We show that the negative electronic compressibility of two-dimensional electronic systems at sufficiently low density enables the generation of charge-density waves through the application of a uniform force field, provided no current is allowed to flow. The wavelength of the density oscillations is controlled by the magnitude of the (negative) screening length, and their amplitude is proportional to the applied force. Both are electrically tunable.

We study the effect of off-resonant plasmon modes on spaser threshold in nanoparticle-based spasers. We develop an analytical semiclassical model and derive spaser threshold condition accounting for gain coupling to higher-order plasmons. We show that such a coupling originates from inhomogeneity of gain distribution near the metal surface and leads to an upward shift of spaser frequency and population inversion threshold. This effect is similar, albeit significantly weaker, to quenching of plasmon-enhanced fluorescence near metal nanostructures due to excitation of off-resonant modes with wide spectral band. We also show that spaser quenching is suppressed for high gain concentrations and establish a simple criterion for quenching onset, which we support by numerical calculations for spherical geometry.

The monotonous increasing with temperature behavior of the neutralization of $\mathrm{Ga}{}^{+}$ scattered by a gold surface plus the nonmagnetic character of the ion induced to disregard the important electronic correlation effects. In this work, we show that contrary to this assumption, the electron-electron interaction in the Ga site is crucial for describing the experimental results. We extend the formalism previously used in the $\mathrm{Sr}{}^{+}$/Au system to include more than one valence orbital in the projectile, which is the case of $\mathrm{Ga}{}^{+}$, where the neutralization occurs to a $p$-type shell. We consider the six possible orbital-spin neutral configurations in a correlated way within the Anderson model. The comparison with the results obtained from a noninteracting electron model allows one to infer how important the many-body effects in the charge exchange between $\mathrm{Ga}{}^{+}$ and the gold surface are. We also found that the proximity of the projectile one-electron energy levels to the substrate Fermi level determines the influence of electronic correlation effects on the neutral fraction and its temperature and velocity dependences.

It is known that in two-dimensional relativistic Dirac systems, the Landau levels can collapse in the presence of a critical in-plane electric field. We extend this mechanism to three-dimensional Weyl semimetals, and we analyze the physical consequences for the cases of both real and pseudo Landau levels arising from strain-induced elastic magnetic fields.

An efficient first-principles approach to calculate x-ray magnetic circular dichroism (XMCD) and x-ray natural circular dichroism (XNCD) is developed and applied in the near-edge region at the $K$ and $L_1$ edges in solids. Computation of circular dichroism requires precise calculations of x-ray absorption spectra (XAS) for circularly polarized light. For the derivation of the XAS cross section, we used a relativistic description of the photon-electron interaction that results in an additional term in the cross section that couples the electric dipole operator with an operator $\mathbf{\sigma }·(\mathbf{\epsilon }\times \mathbf{r})$ that we call the spin position operator. The numerical method relies on pseudopotentials, on the gauge including projected augmented-wave method, and on a collinear spin relativistic description of the electronic structure. We apply the method to calculations of $K$-edge XMCD spectra of ferromagnetic iron, cobalt, and nickel and of I $L_1$-edge XNCD spectra of $\alpha \text{−}{\mathrm{LiIO}}_3$, a compound with broken inversion symmetry. For XMCD spectra we find that, even if the electric dipole term is the dominant one, the electric quadrupole term is not negligible (8% in amplitude in the case of iron). The term coupling the electric dipole operator with the spin-position operator is significant (28% in amplitude in the case of iron). We obtain a sum rule relating this term to the spin magnetic moment of the $p$ states. In $\alpha \text{−}{\mathrm{LiIO}}_3$ we recover the expected angular dependence of the XNCD spectra.

The development of low cost and environmentally friendly organic electronic/optoelectronic devices has attracted a lot of interest. The integration of DNA and RNA nucleobases to improve the performance of organic light-emitting diodes has been proposed recently [Gomez et al., Sci. Rep. 4, 7105 (2014)], notwithstanding limited experimental and theoretical information on the optoelectronic properties of DNA/RNA thin films. As a contribution to an improved understanding of DNA/RNA-based devices in the solid state, we have performed in this paper dispersion corrected density functional theory (DFT) and time-dependent DFT (TDDFT) calculations to obtain the optimized geometries, Kohn-Sham band structures and orbitals, charge distribution, optical absorption, Frenkel exciton binding energies, and complex dielectric functions of the five DNA/RNA nucleobase anhydrous crystals, namely cytosine, guanine, adenine, thymine, and uracil. Optical absorption measurements on DNA/RNA nucleobase powders were also performed for comparison with the simulations. An improvement on the local density approximation (LDA) description of the lattice parameter estimates was achieved considering the generalized gradient approach (GGA) with a semiempirical dispersion correction scheme in comparison with structural x-ray data found in the literature. Energy gap correction using the Δ-sol methodology provided a good agreement between theory and experimental estimates from our optical absorption data, greatly surpassing the quality of previous simulations. Effective masses for the carriers were also found, indicating that the guanine crystal as well as the cytosine one (although with some drawbacks) has potential applications in optoelectronics as a direct gap semiconductor, with the other nucleobases presenting either a semiconductor or an insulator character depending on the carrier type. The complex dielectric function exhibits a high degree of anisotropy for different states of light polarization relative to the molecular stacking planes, while the Frenkel exciton binding energy estimation for the adenine crystal is very close to the optical absorption experimental data.

An approximate description of surface waves propagating along periodically grooved surfaces is intuitively developed in the limit where the grooves are narrow relative to the period. Considering acoustic and electromagnetic waves guided by rigid and perfectly conducting gratings, respectively, the wave field is obtained by interrelating elementary approximations obtained in three overlapping spatial domains. Specifically, above the grating and on the scale of the period the grooves are effectively reduced to point resonators characterized by their dimensions as well as the geometry of their apertures. Along with this descriptive physical picture emerges an analytical dispersion relation, which agrees remarkably well with exact calculations and improves on preceding approximations. Scalings and explicit formulas are obtained by simplifying the theory in three distinguished propagation regimes, namely where the Bloch wave number is respectively smaller than, close to, or larger than that corresponding to a groove resonance. Of particular interest is the latter regime where the field within the grooves is resonantly enhanced and the field above the grating is maximally localized, attenuating on a length scale comparable with the period.

To clarify the microscopic structure of densified $\mathrm{Si}{\mathrm{O}}_2$ glass, we have conducted ab initio molecular-dynamics simulations on the decompression process of $\mathrm{Si}{\mathrm{O}}_2$ glass in its relaxed state from high pressures up to 40 GPa. When decompressed from high pressures above at least 15 GPa, the density and structure always converge to those of densified glass, while the coordination number of silicon decreases to four rapidly. This is in good agreement with previous experimental studies and strongly suggests that densified glass behaves as a high-pressure polymorph of $\mathrm{Si}{\mathrm{O}}_2$ glass. In comparison to ordinary glass, although the coordination number of densified glass is almost the same, the size of an intermediate-range network consisting of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra is smaller. Detailed analyses clarify that $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra in densified glass are deformed and the Si-O bonds are less covalent.

For the case of ${\mathrm{Mn}}_2{\mathrm{GeO}}_4$ we characterize the complex magnetic phase that exists at temperatures below 5.5 K by order parameters for both the commensurate and the incommensurate ordering. For the incommensurate ordering we are forced to consider the transformation properties which interrelate magnetic modes at different noncollinear members of the star of the incommensurate wave vector. The known transformation properties of the underlying magnetic wave functions are used to deduce the transformation properties of the order parameters. These results are applied to construct the high-order invariants in the free energy which have been used elsewhere to describe the characteristics of switching between different domains. The properties of different domains are discussed in detail.

We develop a theory of the quasiparticle interference (QPI) in multiband superconductors based on the strong-coupling Eliashberg approach within the Born approximation. In the framework of this theory, we study dependencies of the QPI response function in the multiband superconductors with the nodeless $s$-wave superconductive order parameter. We pay special attention to the difference in the quasiparticle scattering between the bands having the same and opposite signs of the order parameter. We show that at the momentum values close to the momentum transfer between two bands, the energy dependence of the quasiparticle interference response function has three singularities. Two of these correspond to the values of the gap functions and the third one depends on both the gaps and the transfer momentum. We argue that only the singularity near the smallest band gap may be used as a universal tool to distinguish between the $s_{++}$ and $s_{\pm }$ order parameters. The robustness of the sign of the response function peak near the smaller gap value, irrespective of the change in parameters, in both the symmetry cases is a promising feature that can be harnessed experimentally.

The electrodynamic behavior of isotropic superconducting Nb films cooled below their critical temperature in the presence of in-plane applied magnetic fields is investigated using magneto-optical imaging. A specially designed local flux injector is used to show that the frozen-in in-plane vortices strongly guide and enhance the penetration of perpendicular vortices, whereas their penetration across the array of in-plane vortices is essentially unchanged. This result provides the key to understanding why field-cooled square superconducting films show anisotropic nucleation of flux avalanches (jumps) along the four edges. The explanation is based on an analytical model for thermomagnetic avalanche nucleation in type-II superconducting films, and allows one to understand the entire scenario of different flux dynamics observed experimentally.

The crystal structure, magnetic ordering, and electrical resistivity of $\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_2$ were studied at high pressures. Below $\sim 7\mathrm{GPa}$, $\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_2$ is an antiferromagnetically ordered semiconductor with a $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$-type structure. The insulator-to-metal transformation observed at a pressure of $\sim 7\mathrm{GPa}$ is accompanied by a loss of magnetic ordering and an isostructural phase transition. In the pressure range $\sim 7.5\text{–}11\mathrm{GPa}$ a remarkable downturn in resistivity, which resembles a superconducting transition, is observed below 15 K. We discuss this feature as the possible onset of superconductivity originating from a phase separation in a small fraction of the sample in the vicinity of the magnetic transition.

We study vortex domain wall dynamics in wide permalloy strips driven by applied magnetic fields and spin-polarized electric currents. As recently reported [V. Estévez and L. Laurson, Phys. Rev. B 93, 064403 (2016)], for sufficiently wide strips and above a threshold field, periodic dynamics of the vortex core are localized in the vicinity of one of the strip edges, and the velocity drop typically observed for narrow strips is replaced by a high-velocity plateau. Here, we analyze this behavior in more detail by means of micromagnetic simulations. We show that the high-velocity plateau originates from a repeated double switching of the magnetic vortex core, underlying the periodic vortex core dynamics in the vicinity of the strip edge, i.e., the “attraction-repulsion” effect. We also discuss the corresponding dynamics driven by spin-polarized currents, as well as the effect of including quenched random structural disorder to the system.

With a combination of simple analytical arguments and extensive numerical simulations, we theoretically propose a Josephson junction with $n+1$ superconductors where the current-phase relation can be toggled in situ between a $sin\left(\delta \phi \right)$ and $sin(\delta \phi /n)$ shape using an applied magnetic field. Focusing in particular on the case $n=2$, we show that by using realistic system parameters such as unequal interface transparencies, the $sin(\delta \phi /2)$-shaped solution retains its $2\pi$ periodicity due to discontinuities at $\delta \phi =\pm \pi$. Moreover, we demonstrate that as one toggles between the $sin\left(\delta \phi \right)$- and $sin(\delta \phi /2)$-shaped solutions, the system acts as an on-off switch, and can achieve more than two orders of magnitude difference between the supercurrent in the on and off states. Finally, we argue that the same approach can be generalized to switchable $sin(\delta \phi /n)$ junctions for arbitrary integers $n$, which we motivate by analytically solving the Josephson equations for double- and triple-barrier junctions.

The high pressure phases of HBr are explored with an ab initio crystal structure search. By taking into account the contribution of zero-point energy (ZPE), we find that the $P4/nmm$ phase of HBr is thermodynamically stable in the pressure range from 150 to 200 GPa. The superconducting critical temperature ($T_c$) of $P4/nmm$ HBr is evaluated to be around 73 K at 170 GPa, which is the highest record so far among binary halogen hydrides. Its $T_c$ can be further raised to around 95K under 170 GPa if half of the bromine atoms in the $P4/nmm$ HBr are substituted by the lighter chlorine atoms. Our study shows that, in addition to lower mass, higher coordination number, shorter bonds, and more highly symmetric environment for the hydrogen atoms are important factors to enhance the superconductivity in hydrides.

We present a systematic study of the spectral functions of a time-periodically driven Falicov-Kimball Hamiltonian. In the high-frequency limit, this system can be effectively described as a Harper-Hofstadter-Falicov-Kimball model. Using real-space Floquet dynamical mean-field theory (DMFT), we take into account the interaction effects and contributions from higher Floquet bands in a nonperturbative way. Our calculations show a high degree of similarity between the interacting driven system and its effective static counterpart with respect to spectral properties. However, as also illustrated by our results, one should bear in mind that Floquet DMFT describes a nonequilibrium steady state, while an effective static Hamiltonian describes an equilibrium state. We further demonstrate the possibility of using real-space Floquet DMFT to study edge states on a cylinder geometry.