Deformed aerogels induce a global anisotropy in the superfluid ${}^3\mathrm{He}$ and orient the orbital part of its order parameter. Here a phenomenological theory of the orientational effect of elastic deformations of aerogels on the superfluid phases of ${}^3\mathrm{He}$ in the spirit of conventional theory of elasticity is formulated. Phenomenological coefficients, entering basic relations, depend on properties of given aerogel in the nondeformed state. Examples of originally isotropic silica aerogel and axially symmetric “nematically ordered” aerogel are considered. Values of phenomenological coefficients are estimated with the aid of a simple microscopic model. An example of a nonuniform deformation of nematically ordered aerogel is discussed.

The hyperfine structure of the interstitial muonium (Mu) in rutile (${\mathrm{TiO}}_2$, weakly $n$-type) has been identified by means of a muon-spin-rotation technique. The angle-resolved hyperfine parameters exhibit a tetragonal anisotropy within the $ab$ plane and axial anisotropy with respect to the $\langle 001\rangle \left(\widehat{c}\right)$ axis. This strongly suggests that the Mu is bound to O (forming an OH bond) at an off-center site within a channel along the $\widehat{c}$ axis, while the unpaired Mu electron is localized around the neighboring Ti site. The hyperfine parameters are quantitatively explained by a model that considers spin polarization of the unpaired electron at both the Ti and O sites, providing evidence for the formation of Mu as a Ti-O-Mu-complex paramagnetic state. The disappearance of the Mu signal above $\sim 10\mathrm{K}$ suggests that the energy necessary for the promotion of the unpaired electron to the conduction band by thermal activation is of the order of ${10}^1$ meV. These observations suggest that, while the electronic structure of Mu (and hence H) differs considerably from that of the conventional shallow level donor described by the effective mass model, Mu supplies a loosely bound electron, and thus, serves as a donor in rutile.

We present a detailed time-resolved differential reflectivity study of the electronic and the coherent phonon generation response of a GaAs optical microcavity after resonant picosecond laser pulse excitation. A complex behavior is observed as a function of laser–cavity-mode detuning and incident power. The observed response is explained in terms of the large dynamical variations of the optical cavity-mode frequency induced by the ultrafast laser excitation, related to the optical modulation of the GaAs-spacer index of refraction due to photoexcited carriers. It is demonstrated that this effect leads to a strong optical dynamical tuning of the coherent phonon detection sensitivity of the device.

We perform full-potential screened-hybrid density-functional theory calculations to compare the thermodynamic stability of neutral and charged states of the surface oxygen vacancy at the rutile ${\mathrm{TiO}}_2$(110) surface. Solid-state (QM/MM) embedded-cluster calculations are employed to account for the strong ${\mathrm{TiO}}_2$ polarization response to the charged defect states. Similarly to the situation for the bulk O vacancy, the +2 charge state ${\mathrm{V}}_{\mathrm{O}}^{2+}$ is found to be energetically by far the most stable. Only for Fermi-level positions very close to the conduction band, small polarons may at best be trapped by the charged vacancy. The large decrease in the ${\mathrm{V}}_{\mathrm{O}}^{2+}$ formation energy with decreasing Fermi-level position indicates strongly enhanced surface O vacancy concentrations for $p$-doped samples.

Under which conditions do the electrical transport properties of one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene become equivalent? We have performed atomistic calculations of the phonon-limited electrical mobility in graphene and in a wide range of CNTs of different types to address this issue. The theoretical study is based on a tight-binding method and a force-constant model from which all possible electron-phonon couplings are computed. The electrical resistivity of graphene is found in very good agreement with experiments performed at high carrier density. A common methodology is applied to study the transition from one to two dimensions by considering CNTs with diameter up to 16 nm. It is found that the mobility in CNTs of increasing diameter converges to the same value, i.e., the mobility in graphene. This convergence is much faster at high temperature and high carrier density. For small-diameter CNTs, the mobility depends strongly on chirality, diameter, and the existence of a band gap.

We develop an extension of the Gutzwiller approximation to finite temperatures based on the Dirac-Frenkel variational principle. Our method does not rely on any entropy inequality, and is substantially more accurate than the approaches proposed in previous works. We apply our theory to the single-band Hubbard model at different fillings, and show that our results compare quantitatively well with dynamical mean field theory in the metallic phase. We discuss potential applications of our technique within the framework of first-principle calculations.

We report ab initio calculations for the electronic structure of organic charge transfer salts $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}, \kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{I}, {\kappa }^{″}-{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Cl}$, and $\kappa -{\left(\mathrm{ET}\right)}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$. These materials show an ordering of the relative orientation of terminal ethylene groups in the bis-ethylenedithio-tetrathiafulvalene molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene end group relative orientations can be used to reversibly tune $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}$ through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene end groups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place $\kappa -{\left(\mathrm{ET}\right)}_2\mathrm{Cu}\left[\text{N}{\left(\mathrm{CN}\right)}_2\right]\text{Br}$ in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.

MnSi has been extensively studied for five decades; nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevents a comprehensive understanding of the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at $T_C\sim 40\mathrm{K}$, along with the appearance of sharp quasiparticle emission below $T_C$. The shape of the resulting FS is found to fulfill robust nesting effects. These effects can be at the origin of strong magnetic fluctuations not accounted for by the state-of-the-art quasiparticle self-consistent $\mathit{GW}$ approximation. From this perspective, the unforeseen quasiparticle damping detected in the paramagnetic phase and relaxing only below $T_C$, along with the persistence of the $d$-band splitting well above $T_C$, at odds with a simple Stoner model for itinerant magnetism, opens the search for exotic magnetic interactions favored by FS nesting and affecting the quasiparticle lifetime.

A century-long argument made by Lord Kelvin that all swimming objects have an effective Mach number of 3, corresponding to a Kelvin angle of 19.5° for ship waves, has been challenged recently with the conclusion that the Kelvin angle should gradually transit to the Mach angle as the ship's velocity increases. Here we show that a similar phenomenon can happen for graphene plasmons. By analyzing the caustic wave pattern of graphene plasmons stimulated by a swift charged particle moving uniformly above graphene, we show that at low velocities of the charged particle, the caustics of graphene plasmons form the Kelvin angle. At large velocities of the particle, the caustics disappear and the effective semiangle of the wave pattern approaches the Mach angle. Our study introduces caustic wave theory to the field of graphene plasmonics, and reveals a physical picture of graphene plasmon excitation during electron energy-loss spectroscopy measurements.

We perform a critical assessment of the accuracy of density functional theory (DFT) based methods in predicting stable phases within the Co-Pt binary alloy. Statistical mechanical analysis applied to zero kelvin DFT predictions yields finite-temperature results that can be directly compared with experimental measurements. The predicted temperature-composition phase diagram is qualitatively incompatible with experimental observations, indicating that the predicted stability of long-period superstructures as ground states in the Co-Pt binary is incorrect. We also show that recently suggested methods to better align DFT and experiment via the hybrid functional HSE06 are unable to resolve the discrepancies in this system. Our results indicate a need for better verification of DFT based phase stability predictions, and highlight fundamental flaws in the ability of DFT to treat late $3d-5d$ binary alloys.

Using hybrid density functional calculations, we study the electronic and structural properties of ${\mathrm{SrZrO}}_3$ and ordered $\mathrm{Sr}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_3$ alloys. Calculations were performed for the ground-state orthorhombic $\left(Pnma\right)$ and high-temperature cubic $\left(Pm3m\right)$ phases of ${\mathrm{SrZrO}}_3$. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered ${\mathrm{SrTi}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_3$ structures with $x=0$, 0.25, 0.5, 0.75, and 1. As Ti is added to ${\mathrm{SrZrO}}_3$, the lattice parameter is reduced and closely follows Vegard's law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For $x=0.5$, we find that arranging the Ti and Zr atoms into a $1\times 1{\mathrm{SrZrO}}_3/{\mathrm{SrTiO}}_3$ superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure ${\mathrm{SrTiO}}_3$. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti $3d_{xy}$ orbitals. The lifting of the $t_{2g}$ orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period ${\mathrm{SrZrO}}_3/{\mathrm{SrTiO}}_3$ superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk ${\mathrm{SrTiO}}_3$.

An electric field applied to a semiconductor reduces its crystal symmetry and modifies its electronic structure which is expected to result in changes of the linear and nonlinear response to optical excitation. In GaAs, we observe experimentally strong electric field effects on the optical second (SHG) and third (THG) harmonic generation. The SHG signal for the laser-light $k$ vector parallel to the [001] crystal axis is symmetry forbidden in the electric-dipole approximation, but can be induced by an applied electric field in the vicinity of the $1s$ exciton energy. Surprisingly, the THG signal, which is allowed in this geometry, is considerably reduced by the electric field. We develop a theory which provides good agreement with the experimental data. In particular, it shows that the optical nonlinearities for the $1s$ exciton resonance are modified in an electric field by the Stark effect, which mixes the $1s$ and $2p$ exciton states of opposite parity. This mixing acts in opposite way on the SHG and THG processes, as it leads to the appearance of forbidden SHG in (001)-oriented GaAs and decreases the crystallographic THG.

We present a reconstruction of the transverse acoustic phonon dispersion of germanium from femtosecond time-resolved x-ray diffuse scattering measurements at the Linac Coherent Light Source. We demonstrate an energy resolution of 0.3 meV with a momentum resolution of $0.01{\mathrm{nm}}^{-1}$ using 10-keV x rays with a bandwidth of $\sim 1\mathrm{eV}$. This high resolution was achieved simultaneously for a large section of reciprocal space including regions closely following three of the principal symmetry directions. The phonon dispersion was reconstructed with less than 3 h of measurement time, during which neither the x-ray energy, the sample orientation, nor the detector position were scanned. These results demonstrate how time-domain measurements can complement conventional frequency domain inelastic-scattering techniques.

We studied a finite Fibonacci sequence of Co and Py stripes aligned side by side and in direct contact with each other. Calculations based on a continuous model, including exchange and dipole interactions, were performed for structures feasible for fabrication and characterization of the main properties of magnonic quasicrystals. We have shown the fractal structure of the magnonic spectrum with a number of magnonic gaps of different widths. Moreover, localization of spin waves in quasicrystals and the existence of surface spin waves in finite quaiscrystal structure is demonstrated.

We present measurements of the thermopower $S\left(T\right)$ on ${\mathrm{CePd}}_{1-x}{\mathrm{Rh}}_x$ between 2 K and 300 K. For low Rh content, the system behaves as a ferromagnetic Kondo system with a Curie temperature $T_{\mathrm{C}}$ of about 6 K, a Kondo scale smaller than $T_{\mathrm{C}}$, and an overall crystal electric field splitting of 210 K. As the Rh content increases $T_{\mathrm{C}}$ is suppressed, while the average Kondo scale gets larger. Simultaneously, the presence of different Ce environments leads to a broad distribution of local Kondo scales ranging from very small values to above 50 K. As a consequence, large thermopower values are observed over an extended temperature range. Close to the critical concentration we find power-law dependencies of $S/T$ vs $T$ down to 2 K. For a Rh content of $x=0.95$ we may show explicitly that the thermopower contains contributions from Ce sites with different local energy scales.

We have studied the origin of the exchange bias effect in the ${\text{Au-Fe}}_3{\mathrm{O}}_4$ dumbbell nanoparticles in two samples with different sizes of the Au seed nanoparticles (4.1 and 2.7 nm) and same size of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles (9.8 nm). The magnetization, small-angle neutron-scattering, synchrotron x-ray diffraction, and scanning transmission electron microscope measurements determined the antiferromagnetic FeO wüstite phase within ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles, originating at the interface with the Au nanoparticles. The interface between antiferromagnetic FeO and ferrimagnetic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ is giving rise to the exchange bias effect. The strength of the exchange bias fields depends on the interfacial area and lattice mismatch between both phases. We propose that the charge transfer from the Au nanoparticles is responsible for a partial reduction of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ into the FeO phase at the interface with Au nanoparticles. The Au-O bonds are formed, presumably across the interface to accommodate an excess of oxygen released during the reduction of magnetite.

A magnetic helix wire is one of the simplest magnetic systems which manifests properties of both curvature and torsion. Possible equilibrium magnetization states in the helix wire with different anisotropy directions are studied theoretically. There exist two equilibrium states in the helix wire with easy-tangential anisotropy: a quasitangential magnetization distribution in the case of relatively small curvatures and torsions, and an onion state in the opposite case. The curvature and torsion also essentially influence the spin-wave dynamics in the helix wire, acting as an effective magnetic field. Originated from a geometry-induced effective Dzyaloshinskii interaction, this magnetic field leads to a coupling between the helix chirality and the magnetochirality and breaks mirror symmetry in the spin-wave spectrum: the modification of magnon dispersion relation is linear with respect to the torsion and quadratic with respect to the curvature. All analytical predictions on magnetization statics and dynamics are well confirmed by direct spin-lattice simulations.

In this work we report the appearance of a large perpendicular magnetic anisotropy (PMA) in ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ thin films grown onto ZnSe/GaAs(001). This arising anisotropy is related to the tetragonal metastable phase in as-grown samples recently reported [M. Eddrief et al., Phys. Rev. B 84, 161410 (2011)]. By means of ferromagnetic resonance studies we measured PMA values up to $\sim 5\times {10}^5$ $\mathrm{J}/{\mathrm{m}}^3$. PMA vanishes when the cubic structure is recovered upon annealing at $300{}^{\circ }\mathrm{C}$. Despite the important values of the magnetoelastic constants measured via the cantilever method, the consequent magnetoelastic contribution to PMA is not enough to explain the observed anisotropy values in the distorted state. Ab initio calculations show that the chemical ordering plays a crucial role in the appearance of PMA. Through a phenomenological model we propose that an excess of next-nearest-neighbor Ga pairs $({\mathrm{B}}_2$-like ordering) along the perpendicular direction arises as a possible source of PMA in ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ thin films.

We report a counterintuitive finding that notches in an otherwise homogeneous magnetic nanowire can boost current-induced domain wall (DW) propagation. DW motion in notch-modulated wires can be classified into three phases: (1) A DW is pinned around a notch when the current density is below the depinning current density. (2) DW propagation velocity is boosted by notches above the depinning current density and when nonadiabatic spin-transfer torque strength $\beta$ is smaller than the Gilbert damping constant $\alpha$. The boost can be multifold. (3) DW propagation velocity is hindered when $\beta >\alpha$. The results are explained by using the Thiele equation.