We consider a two-dimensional structure with spin-orbit-related splitting of the electron (hole) spectrum and calculate the edge spin density which appears due to the intrinsic mechanism of spin-orbit interaction in the presence of a charge current through the structure. We concentrate on the quasiballistic case when a mean-free path, being much smaller than the sample size, is larger than the spin precession length determined by the value of the spin-orbit splitting. We show that regardless of the presence or absence of the bulk spin current, the main source of the edge spin density is the boundary scattering itself. The character of the edge spin density depends on the smoothness of the bulk impurity potential. We have calculated the edge spin density profile for an arbitrary smoothness of the scattering potential in the bulk, and discussed relation to the existing experiments for two-dimensional holes.

We study the energy and temperature relaxation of electrons in graphene on a piezoelectric substrate. Scattering from the combined potential of extrinsic piezoelectric surface acoustical (PA) phonons of the substrate and intrinsic deformation acoustical phonons of graphene is considered for a (non)degenerate gas of Dirac fermions. It is shown that in the regime of low energies or temperatures the PA phonons dominate the relaxation and change qualitatively its character. This prediction is relevant for quantum metrology and electronic applications using graphene devices and suggests an experimental setup for probing electron-phonon coupling in graphene.

A variational ground state for insulating bilayer graphene (BLG), subject to quantizing magnetic fields, is proposed. Due to the Zeeman coupling, the layer antiferromagnet (LAF) order parameter in fully gapped BLG gets projected onto the spin easy plane, and simultaneously a ferromagnet order, which can further be enhanced by exchange interaction, develops in the direction of the magnetic field. The activation gap for the $\nu =0$ Hall state then displays a crossover from quadratic to linear scaling with the magnetic field, as it gets stronger, and I obtain excellent agreement with a number of recent experiments with realistic strengths for the ferromagnetic interaction. A component of the LAF order, parallel to the external magnetic field, gives birth to additional incompressible Hall states at filling $\nu =\pm 2$, whereas the remote hopping in BLG yields $\nu =\pm 1$ Hall states. Evolution of the LAF order in tilted magnetic fields, scaling of the gap at $\nu =2$, the effect of external electric fields on various Hall plateaus, and different possible hierarchies of fractional quantum Hall states are highlighted.

In quantum mechanics, string-net condensed states—a family of prototypical states exhibiting nontrivial topological order—can be classified via their long-range entanglement properties, in particular, topological corrections to the prevalent area law of the entanglement entropy. Here we consider classical analogs of such string-net models whose partition function is given by an equal-weight superposition of classical string-net configurations. Our analysis of the Shannon and Renyi entropies for a bipartition of a given system reveals that the prevalent volume law for these classical entropies is augmented by subleading topological corrections that are intimately linked to the anyonic theories underlying the construction of the classical models. We determine the universal values of these topological corrections for a number of underlying anyonic theories including SU(2)${}_k$, SU${\left(N\right)}_1$, and SU${\left(N\right)}_2$ theories.

We report a simulation of the metal-insulator transition in a model of a doped semiconductor that treats disorder and interactions on an equal footing. The model is analyzed using density functional theory. From a multifractal analysis of the Kohn-Sham eigenfunctions, we find $\nu \approx 1.3$ for the critical exponent of the correlation length. This differs from that of Anderson's model of localization and suggests that the Coulomb interaction changes the universality class of the transition.

The lattice constant, bulk modulus, and shear constant of TbN are calculated by means of density functional theory (DFT) in the local density approximation (LDA) and generalized gradient approximation (GGA), with $4f$ states treated as valence electrons or core electrons. In addition, local Coulomb repulsions $U$ are treated both statically as in the LDA$+U$ approach and dynamically as in the dynamical mean-field theory in the Hubbard-$I$ approximation. It is shown that all methods, except DFT-LDA with $4f$ electrons treated as either valence states, produce lattice constants and bulk moduli in good agreement with experiment. In the LDA$+U$ approach multiple minima are found, and we focus on the competition between a state with cubic symmetry and a state obtained from atomic Hund's rules. We find the state with cubic symmetry to be 0.59 eV lower in energy than the Hund's rules state, while the opposite was obtained in previous literature. The shear constant is shown to be rather sensitive to the theoretical method used, and the Hund's rules state obtained in LDA$+U$ is found to be unstable towards tetragonal shear. As to the magnetism, we find that the calculation based on the Hubbard-$I$ approximation reproduces observations with the best accuracy. Finally, the spectral properties of TbN are discussed, together with the general applicability of the different methods in describing rare-earth elements and compounds.

In this work, we have studied the chemical and magnetic interactions of ${\mathrm{Fe}}_n$ ($n=1–6$) clusters with vacancy defects (monovacancy to correlated vacancies with six missing C atoms) in a graphene sheet by ab initio density functional calculations combined with Hubbard $U$ corrections for correlated Fe-$d$ electrons. It is found that the vacancy formation energies are lowered in the presence of Fe, indicating an easier destruction of the graphene sheet. Due to strong chemical interactions between Fe clusters and vacancies, a complex distribution of magnetic moments appear on the distorted Fe clusters which results in reduced averaged magnetic moments compared to the free clusters. In addition to that, we have calculated spin-dipole moments and magnetic anisotropy energies. The calculated spin-dipole moments arising from anisotropic spin density distributions vary between positive and negative values, yielding increased or decreased effective moments. Depending on the cluster geometry, the easy axis of magnetization of the Fe clusters shows in-plane or out-of-plane behavior.

A complete numerical description of the charge and spin dynamics of a two-electron system confined in narrow nanowire quantum dots under oscillating electric field is presented in the context of recent electric dipole spin resonance experiments. We find that the spin-orbit coupling results in lifting the spin blockade by phonon mediated relaxation provided that the initially occupied state is close in energy to the ground state. This leads to suppression of the blockade from the triplet state with spins polarized parallel to the external magnetic field $B$. At higher $B$, after singlet-triplet ground-state transition a new channel for lifting the Pauli blockade opens which results in an appearance of additional resonance lines. The calculated signatures of this transition are consistent with recent experimental results [S. M. Frolov et al., Phys. Rev. Lett. 109, 236805 (2012)].

We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterized in both hydrogenated (NH${}_3$) and deuterated (ND${}_3$) ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy, and x-ray diffraction. The presence of a new strong infrared absorption band centered at 2500 cm${}^{-1}$ in NH${}_3$ (1900 cm${}^{-1}$ in ND${}_3$) is in line with previous theoretical predictions regarding the ionization of ammonia molecules into ${{\mathrm{NH}}_2}^{-}$ and ${{\mathrm{NH}}_4}^{+}$ ions. The experimental data suggest the coexistence of two crystalline ionic forms, which our ab initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia appears stable at low temperatures, which contrasts with the behavior of water in which no equivalent crystalline ionic phase has been found.

We report comprehensive Raman and infrared investigations of charge-order (CO) fluctuations in the organic metal ${\beta }^{\prime \prime }$-(BEDT-TTF)${}_2$SF${}_5$CHFSO${}_3$ and superconductor ${\beta }^{\prime \prime }$-(BEDT-TTF)${}_2$SF${}_5$CH${}_2$CF${}_2$SO${}_3$. The charge-sensitive vibrational bands are analyzed through an extension of the well-known Kubo model for the spectral signatures of an equilibrium between two states. At room temperature, both salts exhibit charge fluctuations between two differently charged molecular states with an exchange frequency of about $6\times {10}^{11}{\mathrm{s}}^{-1}$. The exchange rate of the metallic salt remains roughly constant down to 10 K, while in the superconductor, the exchange velocity starts to decrease below 200 K, and a “frozen” charge-ordered state emerges and coexists with the charge-order fluctuation state down to the superconducting temperature. These findings are confronted with other spectroscopic experiments, and a tentative phase diagram is proposed for the ${\beta }^{\prime \prime }$ BEDT-TTF quarter-filled salts.

From the spatial structure of vortex lattice state calculated by Eilenberger theory, we study the resonance line shape of Knight shift of the paramagnetic moments in $s$- and $d$-wave superconductors, comparing with the Redfield pattern of the internal field distribution. We discuss the deviation from the temperature dependence of the Yosida function and the magnetic field dependence of the paramagnetic susceptibility. In addition to the calculation in the clean limit, influences of the impurity scattering are estimated in the Born limit and in the unitary limit. These results are helpful for the analysis of NMR experiments to know properties of the superconductors.

We have analyzed the magnetic configuration for highly ordered ${\mathrm{Sr}}_2$${\mathrm{CrReO}}_6$ films as a function of epitaxial strain using magnetometry and x-ray magnetic circular dichroism (XMCD) measurements of Cr, Re, and O sites. The in-plane magnetic moments change significantly when tensile strain is applied. O $K$-edge XMCD indicates that O sites carry at least a portion of the bulk magnetization. Spin moment values measured for Cr match calculations incorporating spin-orbit effects, while both spin and orbital moments measured for Re sites are slightly higher than previously predicted. Finally, we discuss large changes in the x-ray absorption near-edge structure that are observed at the Cr and Re $L$ edges due to epitaxial strain.

We report results from neutron scattering experiments on single crystals of YbBiPt that demonstrate antiferromagnetic order characterized by a propagation vector, ${\mathit{\tau }}_{\mathrm{AFM}}$ = ($\frac{1}{2}\frac{1}{2}\frac{1}{2}$), and ordered moments that align along the [1 1 1] direction of the cubic unit cell. We describe the scattering in terms of a two-Gaussian peak fit, which consists of a narrower component that appears below $T_{\mathrm{N}}\approx 0.4$ K and corresponds to a magnetic correlation length of ${\xi }_{\mathrm{n}}\approx$ 80 $\AA$, and a broad component that persists up to $T^{*}\approx$ 0.7 K and corresponds to antiferromagnetic correlations extending over ${\xi }_{\mathrm{b}}\approx$ 20 $\AA$. Our results illustrate the fragile magnetic order present in YbBiPt and provide a path forward for microscopic investigations of the ground states and fluctuations associated with the purported quantum critical point in this heavy-fermion compound.

We report measurements of the superconducting transition temperature, $T_c$, in CoO/Co/Cu/Co/Nb multilayers as a function of the angle $\alpha$ between the magnetic moments of the Co layers. Our measurements reveal that $T_c\left(\alpha \right)$ is a nonmonotonic function, with a minimum near $\alpha =\pi /2$. Numerical self-consistent solutions of the Bogoliubov–de Gennes equations quantitatively and accurately describe the behavior of $T_c$ as a function of $\alpha$ and layer thicknesses in these superconductor/spin-valve heterostructures. We show that experimental data and theoretical evidence agree in relating $T_c\left(\alpha \right)$ to enhanced penetration of the triplet component of the condensate into the Co/Cu/Co spin valve in the maximally noncollinear magnetic configuration.

The field distribution in the vortex lattice of a pure niobium single crystal with an external field applied along a three-fold axis has been investigated by the transverse-field muon-spin-rotation (TF-$\mu$SR) technique over a wide range of temperatures and fields. The experimental data have been analyzed with Delrieu's solution for the form factor supplemented by phenomenological formulas for the parameters. This has enabled us to experimentally establish the temperatures and fields for Delrieu's, Ginzburg and Landau's, and Klein's regions of the vortex lattice. Using the numerical solution of the quasiclassical Eilenberger equation, the experimental results have been reasonably understood. They should apply to all clean BCS superconductors. Delrieu's analytical model supplemented by phenomenological formulas for its parameters is found to be reliable for analyzing TF-$\mu$SR experimental data for a substantial part of the mixed phase. Abrikosov's limit is contained in it.

We analyze a system composed of a qubit coupled to the electromagnetic fields in two high quality quantum oscillators. A particular realization of such a system is the superconducting qubit coupled to a transmission-line resonator driven by two signals with frequencies close to the resonator's harmonics. This doubly driven system can be described in terms of the doubly dressed qubit states. Our calculations demonstrate the possibility to change the number of photons in the resonator and the transmission of the fundamental-mode signal over a wide parameter range exploiting resonances with the dressed qubit. Experiments show that in the case of high quality resonators the dressed energy levels and corresponding resonance conditions can be probed, even for high driving amplitudes. The interaction of the qubit with photons of two harmonics can be used for the creation of quantum amplifiers or attenuators.