Results of in situ high-pressure x-ray powder diffraction on the mixed-valence compound Cs${}_2$Au${}^{\mathrm{I}}$Au${}^{\mathrm{III}}$I${}_6$ (CsAuI${}_3$) are reported for pressures up to 21 GPa in a diamond-anvil cell under hydrostatic conditions. We find a reversible pressure-induced tetragonal-to-orthorhombic structural transition at 5.5–6 GPa and reversible amorphization at 12–14 GPa. Two alternative structures are proposed for the high-pressure orthorhombic phase and are discussed in the context of a possible Au valence transition.

We directly measure by femtosecond time-resolved x-ray diffraction the $E$${}_g$ symmetry coherent phonon excited in bismuth by a strong optical pulse. The magnitude of the $E$${}_g$ mode observed is 0.2 pm peak-to-peak, compared against the $2.7$-pm initial displacement of the fully symmetric $A_{1g}$ mode. The much smaller motion of the $E$${}_g$ mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. The experimentally measured magnitude of the $E$${}_g$ motion allows us to rule out a previously suggested scenario for explaining the dynamics in bismuth that relies on strong coupling between these modes.

A procedure is developed to study the evolution of high anisotropy magnetic recording media due to thermally activated grain reversal. It is assumed that the system is composed of single domain grains that evolves by passing through a sequence of relatively long-lived metastable states punctuated by abrupt reversals of individual grains. Solutions to the rate equations describing the sequence of metastable states are calculated using kinetic Monte Carlo. Transition rates are formulated from the Arrhenius-Néel expression in terms of the material parameters, temperature, and applied field. Results obtained from this method are shown to be in good agreement with those calculated from finite-temperature micromagnetics. The method is applied to study the rate dependence of finite-temperature $MH$ loops and the thermal degradation of a recorded bit pattern in perpendicular recording media. A significant advantage of the procedure is its ability to extend simulations over time intervals many orders of magnitude greater than is feasible using standard finite-temperature micromagnetics with relatively modest computational effort.

We report the structural, magnetic, and electronic transport properties of SrFe${}_{2-x}$Cu${}_x$As${}_2$ single crystals grown by a self-flux technique. Both SrCu${}_2$As${}_2$ and SrFe${}_2$As${}_2$ crystallize in a ThCr${}_2$Si${}_2$-type (122-type) structure at room temperature, but exhibit distinct magnetic and electronic transport properties. According to the x-ray photoelectron spectroscopy Cu $2p$ core line position, resistivity, susceptibility, and positive Hall coefficient, SrCu${}_2$As${}_2$ is an $\mathit{sp}$-band metal with Cu in the 3d${}^{10}$ electronic configuration corresponding to the valence state Cu${}^{1+}$. Compared with SrCu${}_2$As${}_2$, the almost unchanged Cu $2p$ core line position in SrFe${}_{2-x}$Cu${}_x$As${}_2$ indicates that partial Cu substitutions for Fe in SrFe${}_2$As${}_2$ may result in hole doping rather than the expected electron doping. No superconductivity is induced by Cu substitution on Fe sites, even though the structural/spin density wave transition is gradually suppressed with increasing Cu doping.

We present a method suitable for the measurement of a high-energy resolved absorption spectrum in the emission mode upon target excitation with an intense monochromatic pulse of x-ray light. The approach is based on a measurement of a single resonant Raman x-ray spectrum at a fixed off-resonant excitation energy by a dispersive-type high-resolution emission spectrometer. Using a simple Lorentzian tail correction function and an energy-scale transformation, we reconstructed the $L_3$ absorption edge of Xe from the $3d$-$2p_{3/2}$ resonant Raman x-ray spectrum recorded at excitation energy 25 eV below the $L_3$ edge. The energy resolution is well below the $L_3$ core-hole lifetime broadening and allows for a straightforward extraction of the $\left[2p_{3/2}\right]n\ell$ resonance energies and emission strengths. The reconstructed spectrum is nearly equivalent to the high-energy resolution fluorescence detected absorption spectrum, which is recorded by scanning the photon excitation energy across the absorption edge.

Optical and dynamic nuclear polarization of ${}^{29}$Si nuclei under saturation of the electron paramagnetic resonance (EPR) transitions of the photoexcited spin $S$ = 1 states (Si-SL1 centers) of the oxygen+vacancy complexes ($A$-centers) have been investigated using silicon single crystals with various ${}^{29}$Si nuclear abundances. The optical nuclear polarization alone led to the ${}^{29}$Si nuclear polarization enhancement by a factor of ∼500 in the magnetic fields below 50 mT, and such enhancement was found to decrease significantly in the stronger fields up to 300 mT. On the other hand, saturation of the EPR lines of Si-SL1 centers having the electron spin polarization of 70–80$\%$ was transferred successfully to surrounding ${}^{29}$Si nuclear spins to achieve the overall ${}^{29}$Si nuclear spin polarization of ∼6.4$\%$, and a method to increase up to 13–15$\%$ is proposed. The optimum condition to achieve such a high degree of nuclear polarization was found by systematic investigations of the dynamic nuclear polarization mechanisms.

Spin quantum beat spectroscopy is employed to investigate the in-plane anisotropy of the spin dynamics in (001) GaAs/AlGaAs quantum wells induced by an external electric field. This technique allows the anisotropy of the spin relaxation rate ${\Gamma }^s$ and the electron Landé $g$ factor $g^{*}$ to be measured simultaneously. The measurements are compared to similar data from (001) GaAs/AlGaAs quantum wells with applied shear strain and asymmetric barrier growth. All of these operations act to reduce the symmetry compared to that of a symmetric (001) quantum well in an identical manner (D${}_{2d}$ $\to$ C${}_{2v}$). However, by looking at the anisotropy of both ${\Gamma }^s$ and $g^{*}$ simultaneously we show that the microscopic actions of these symmetry breaking operations are very different. The experiments attest that although symmetry arguments are a very useful tool to identify the allowed spin dependent properties of a material system, only a microscopic approach reveals if allowed anisotropies will manifest.

We study adiabatic pumping through a two-level quantum dot with spin-orbit coupling. Using a diagrammatic real-time approach, we calculate both the pumped charge and spin for a periodic variation of the dot's energy levels in the limit of weak tunnel coupling. Thereby, we compare the two limits of vanishing and infinitely large charging energy on the quantum dot. We discuss the dependence of the pumped charge and pumped spin on gate voltages, the symmetry in the tunnel-matrix elements, and spin-orbit coupling strength. We identify the possibility to generate pure spin currents in the absence of charge currents.

The electronic structure of cobalt-phthalocyanine (CoPc) molecules adsorbed on Ag(100) is investigated by photoemission spectroscopy. The results are compared to first-principles electronic structure calculations, based on many-body perturbation theory in the GW approximation. The photoemission data, obtained from both multilayer and monolayer films of CoPc, show that charge transfer occurs between the first molecular layer and the metal surface. Varying the photon energy, to tune the photoionization cross sections, reveals that the charge-transfer-related interface states mainly involve the Co $3d$ atomic orbitals of the Co central atom. GW calculations for the neutral CoPc molecule and its anion compare well with the experimental observations for a multilayer and a monolayer CoPc film, respectively. They confirm the major role played by the Co atom in the charge-transfer process and elucidate the complex energy rearrangement of the molecular electronic levels upon metal adsorption.

We study the interplay between strong electron-electron and electron-phonon interactions within a two-orbital molecule coupled to metallic leads, taking into account Holstein-like coupling of a local phonon mode to the molecular charge as well as phonon-mediated interorbital tunneling. By combining canonical transformations with numerical renormalization-group calculations to address the interactions nonperturbatively and on equal footing, we obtain a comprehensive description of the system's many-body physics in the antiadiabatic regime where the phonons adjust rapidly to changes in the orbital occupancies and are thereby able to strongly affect the Kondo physics. The electron-phonon interactions strongly modify the bare orbital energies and the Coulomb repulsion between electrons in the molecule and tend to inhibit tunneling of electrons between the molecule and the leads. The consequences of these effects are considerably more pronounced when both molecular orbitals lie near the Fermi energy of the leads than when only one orbital is active. In situations where a local moment forms on the molecule, there is a crossover with increasing electron-phonon coupling from a regime of collective Kondo screening of the moment to a limit of local phonon quenching. At low temperatures, this crossover is associated with a rapid increase in the electronic occupancy of the molecule as well as a marked drop in the linear electrical conductance through the single-molecule junction.

The iron-chromium alloy and its derivatives are widely used for their remarkable resistance to corrosion, which only occurs in a narrow concentration range around 9 to 13 atomic percent chromium. Although known to be due to chromium enrichment of a few atoms thick layer at the surfaces, the understanding of its complex atomistic origin has been a remaining challenge. We report an investigation of the thermodynamics of such surfaces at the atomic scale by means of Monte Carlo simulations. We use a Hamiltonian which provides a parametrization of previous ab initio results and successfully describes the alloy's unusual thermodynamics. We report a strong enrichment in Cr of the surfaces for low bulk concentrations, with a narrow optimum around 12 atomic percent chromium, beyond which the surface composition decreases drastically. This behavior is explained by a synergy between (i) the complex phase separation in the bulk alloy, (ii) local phase transitions that tune the layers closest to the surface to an iron-rich state and inhibit the bulk phase separation in this region, and (iii) its compensation by a strong and nonlinear enrichment in Cr of the next few layers. Implications with respect to the design of prospective nanomaterials are briefly discussed.

Hydrogen adsorbates in graphene are interesting as they are not only strong Coulomb scatterers, but they also induce a change in orbital hybridization of the carbon network from $sp$${}^2$ into $sp$${}^3$. This change increases the spin-orbit coupling and is expected to largely modify spin relaxation. In this work, we report the change in spin transport properties of graphene due to plasma hydrogenation. We observe an up to threefold increase of spin relaxation time ${\tau }_S$ after moderate hydrogen exposure. This increase of ${\tau }_S$ is accompanied by the decrease of charge and spin diffusion coefficients, resulting in a minor change in spin relaxation length ${\lambda }_S$. At high carrier density, we obtain ${\lambda }_S$ of 7 $\mu$m, which allows for spin detection over a distance of 11 $\mu$m. After hydrogenation, a value of ${\tau }_S$ as high as 2.7 ns is measured at room temperature.

To explore the connection between $\alpha$-(BEDT$-$TTF)${}_2$I${}_3$ and $\alpha$-(BEDT$-$TTF)${}_2$$M$Hg(SCN)${}_4$ ($M=\text{K}$, Rb, Tl and NH${}_4$) and to resolve the difference between band picture and charge fluctuation of $\alpha$-(BEDT$-$TTF)${}_2$$M$Hg(SCN)${}_4$, we utilized ${}^{13}$C-NMR to examine $\alpha$-(BEDT$-$TTF)${}_2$RbHg(SCN)${}_4$ under the conditions with a large hyperfine coupling constant at each site. The temperature dependence of site susceptibility showed the development of spin disproportionation as observed in $\alpha$-(BEDT$-$TTF)${}_2$I${}_3$. We found that the linewidth of the A site only increased as temperature decreased from 200 to 100 K, a change associated with the development of lattice modulation. These findings indicate that density-wave modulation occurs during charge disproportionation instability.

We construct models of interacting itinerant non-Abelian anyons moving along one-dimensional chains. We focus on itinerant Ising (Majorana) and Fibonacci anyons, which are, respectively, related to SU${\left(2\right)}_2$ and SU${\left(2\right)}_3$ anyons and also, respectively, describe quasiparticles of the Moore-Read and ${\mathbb{Z}}_3$-Read-Rezayi fractional quantum Hall states. Following the derivation of the electronic large-U effective Hubbard model, we derive effective anyonic $t$-$J$ models for the low-energy sectors. Solving these models by exact diagonalization, we find a fractionalization of the anyons into charge and (neutral) anyonic degrees of freedom—a generalization of spin-charge separation of electrons which occurs in Luttinger liquids. A detailed description of the excitation spectrum can be performed by combining spectra for charge and anyonic sectors. The anyonic sector is that of a squeezed chain of localized interacting anyons and, hence, is described by the same conformal field theory (CFT), with central charge $c=1/2$ for Ising anyons and $c=7/10$ or $c=4/5$ for Fibonacci anyons with antiferromagnetic or ferromagnetic coupling, respectively. The charge sector is the spectrum of a chain of hard-core bosons subject to phase shifts which coincide with the momenta of the combined anyonic eigenstates, revealing a subtle coupling between charge and anyonic excitations at the microscopic level (which we also find to be present in Luttinger liquids), despite the macroscopic fractionalization. The combined central charge extracted from the entanglement entropy between segments of the chain is shown to be $1+c$, where $c$ is the central charge of the underlying CFT of the localized anyon (squeezed) chain.

To understand the role of local sublattice imbalance in low-energy spectra of $s=\frac{1}{2}$ quantum antiferromagnets, we study the $s=\frac{1}{2}$ quantum nearest neighbor Heisenberg antiferromagnet on the coordination 3 Cayley tree. We perform many-body calculations using an implementation of the density matrix renormalization group (DMRG) technique for generic tree graphs. We discover that the bond-centered Cayley tree has a quasidegenerate set of a low-lying tower of states and an “anomalous” singlet-triplet finite-size gap scaling. For understanding the construction of the first excited state from the many-body ground state, we consider a wave function ansatz given by the single-mode approximation, which yields a high overlap with the DMRG wave function. Observing the ground-state entanglement spectrum leads us to a picture of the low-energy degrees of freedom being “giant spins” arising out of sublattice imbalance, which helps us analytically understand the scaling of the finite-size spin gap. The Schwinger-boson mean-field theory has been generalized to nonuniform lattices, and ground states have been found which are spatially inhomogeneous in the mean-field parameters.

We present a systematic numerical simulation of the scanning gate microscopy experiment as performed on a quantum ring etched out of InGaAs/InAlAs heterostructure containing two-dimensional electron gas (2DEG). The present simulation accounts for 2DEG deformation under the presence of the tip, for modification of the electron charge trapped at the semiconductor/vacuum interface, for polarization effects at the interface, for the image charges related to the presence of the metal tip and for the three-dimensional character of the confinement. Once the potential is established by self-consistence of the Schrödinger-Poisson scheme, the linear conductance is evaluated by solution of the quantum scattering problem with several subbands at the Fermi level. We find that the conductance is a slowly varying function of the tip position in accordance with the experimental data. We study the high-pass-filtered conductance maps, which generally exhibit concentric patterns outside the ring area, and radial features inside the ring for negative charge at the tip. Influence of the surface charge and potential imperfections on conductance maps are also discussed.

The adsorption of pentacene (Pn) on the fivefold surface of an $\mathit{\text{i}}$-Al-Pd-Mn quasicrystal and on a one-dimensionally aperiodic Cu multilayer formed thereon is observed with scanning tunneling microscopy. The molecule has a strong interaction with the clean quasicrystal surface, leading to the formation of a disordered layer. On the Cu film, a molecular layer assembles, with the Cu rows acting as a template. At lower coverages, there is a repulsive interaction between the molecules, leading to a dispersed homogeneous arrangement. At higher coverages, the steric interaction of the Pn molecules counterbalances the aperiodic template, which results in a short-range periodic “checkerboard” arrangement of molecules. Density functional theory calculations using lower order acenes are used to probe the details of the interactions with the aperiodic Cu surface and it is found that adsorption with molecules parallel to Cu rows is preferred, in agreement with experimental results.

Conducting steady states of doped bilayer graphene have a nonzero sublattice pseudospin polarization. Electron-electron interactions renormalize this polarization even at zero temperature, when the phase space for electron-electron scattering vanishes. We show that, because of the strength of interlayer tunneling, electron-electron interactions nevertheless have a negligible influence on the conductivity, which vanishes as the carrier number density goes to zero. The influence of interactions is qualitatively weaker than in the comparable cases of single-layer graphene or topological insulators, because the momentum-space layer pseudospin vorticity is 2 rather than 1. Our study relies on the quantum Liouville equation in the first Born approximation with respect to the scattering potential, with electron-electron interactions taken into account self-consistently in the Hartree-Fock approximation and screening in the random phase approximation. Within this framework the result we obtain is exact.

Atomic resolution imaging using secondary electrons (emitted as a result of the interaction of incident fast electrons with a specimen) was only achieved recently. There has been considerable speculation as to the physical mechanisms underpinning the imaging. In this paper we use a quantum mechanical model to show that the image contrast is due to electrons ejected in inner-shell ionization events initiated by the primary beam, an atomic scale, and focused coherent electron probe. The angular probability distribution of the ejected electrons is key in understanding the (relative) contrast from different atomic species within the specimen. For a given species of atom, this angular probability distribution is predominantly determined by the angular momentum quantum number of the bound electron prior to ionization. The model is compared to experiment and reproduces the essential features in the data.