Graphene is a convenient material for nanomechanical applications since high-frequency oscillations are easily accessible. In this article, we consider graphene on a rough substrate attached to imperfections at random locations. We explore the statistics of low-lying phonon modes, which exert most influence on the conductivity of graphene. Our numerics suggest the hypothesis that the nearest-neighbor spacings of low-lying eigenfrequencies have the Wigner-Dyson probability distribution after averaging over the random configurations of disorder. Due to interaction of electrons with the oscillations of the membrane, an electron can be transferred to higher or lower energies, which is a manifestation of the phonon-assisted Tien-Gordon effect. The Tien-Gordon effect suppresses the conductivity of graphene. In the regime of low Fermi energies and small sizes of the sample an increase of conductivity is observed which we refer to as Klein tunneling and electron-hole pair creation. Eventually, when the increase of the transmission becomes too prominent, the pair creation changes the ground state of the system, signalizing the limit of applicability of the single-particle Dirac equation used in this article.

The ab initio prediction of Raman intensities for bulk solids usually relies on the hypothesis that the frequency of the incident laser light is much smaller than the band gap. However, when the photon frequency is a sizable fraction of the energy gap, or higher, resonance effects appear. In the case of silicon, when excitonic effects are neglected, the response of the solid to light increases by nearly three orders of magnitude in the range of frequencies between the static limit and the gap. When excitonic effects are taken into account, an additional tenfold increase in the intensity is observed. We include these effects using a finite-difference scheme applied on the dielectric function obtained by solving the Bethe-Salpeter equation. Our results for the Raman susceptibility of silicon show stronger agreement with experimental data compared with previous theoretical studies. For the sampling of the Brillouin zone, a double-grid technique is proposed, resulting in a significant reduction in computational effort.

We investigate how the Kondo screening cloud builds up as a function of space and time. Starting from an impurity spin decoupled from the conduction band, the Kondo coupling is switched on at time $t=0$. We work at the Toulouse point where one can obtain exact analytical results for the ensuing spin dynamics at both zero and nonzero temperature $T$. For $t>0$ the Kondo screening cloud starts building up in the wake of the impurity spin being transported to infinity. In this buildup process the impurity spin–conduction band spin susceptibility shows a sharp light cone due to causality, while the corresponding correlation function has a tail outside the light cone. At $T=0$ this tail has a power-law decay as a function of distance from the impurity, which we interpret as due to initial entanglement in the Fermi sea.

Despite a number of previous nonequilibrium molecular dynamics (MD) studies into plasticity in face-centered-cubic metals, and phase transitions in body-centered-cubic (bcc) metals, the plastic response to rapid compression of bcc metals remains largely unexplored. Key questions remain as to the relative importance of dislocation motion and twinning in shear stress release and consequent strength. We present here large scale MD simulations of shock compressed bcc metal, using an extended Finnis-Sinclair potential for tantalum, and demonstrate the presence of significant deformation twinning for pressures above the Hugoniot elastic limit for shock waves propagating along the $\left[001\right]$ direction. The twinned variants are separately identified by a per atom order parameter, allowing the strain and stress states of the rotated material to be studied. The atomic motion during twinning, and thus its mechanism, for this potential, is identified by use of a three-dimensional pair-correlation function.

We have studied the magnetic structure of a Cr-doped iron-arsenide compound CaFe${}_4$As${}_3$ by means of single crystal neutron diffraction. The neutron data reveal that below 90 K, an antiferromagnetic structure with propagation vector $k=0$ is adopted. Refinement of the magnetic structure using one of the modes allowed by symmetry analysis indicates that two of the four Fe sites, including the one where the selective substitution Fe/Cr happens, bear reduced magnetic moments. Density functional theory calculations confirm the stability of such a magnetic arrangement.

Since any nontrivial infrared dynamics in strongly correlated electron matter must be controlled by a critical fixed point, we argue that the form of the single-particle propagator can be deduced simply by imposing scale invariance. As a consequence, the unparticle picture proposed by Georgi14 is the natural candidate to describe such dynamics. Unparticle stuff is scale-invariant matter with no particular mass. Scale invariance dictates that the propagator has an algebraic form which can admit zeros and hence is a candidate to explain the ubiquitous pseudogap state of the cuprates. We refer to the nonperturbative electronic state formed out of unparticles as an un-Fermi liquid. We show that the underlying action of the continuous mass formulation of unparticles can be recast as an action in anti–de Sitter space which serves as the generating functional for the propagator. We find that this mapping fixes the scaling dimension of the unparticle to be $d_U=d/2+\sqrt{d^2+4}/2$ and ensures that the corresponding propagator has zeros with $d$ the space-time dimension of the unparticle field. Should $d=2+1$, unparticles acquire the nontrivial phase $2\pi d_U$ upon interchange. Because $d_U$ is noninteger and in general not half integer, clockwise and counterclockwise interchange of unparticles do not lead to the same phase and time-reversal symmetry is broken spontaneously as reported in numerous experiments in the pseudogap phase of the cuprates. The possible relevance of this mechanism to such experiments is discussed. We then formulate the analogous BCS gap using unparticles and find that in contrast to the Fermi-liquid case, the transition temperature increases as the attractive interaction strength decreases, indicating that unparticles are highly susceptible to a superconducting instability.

The in-plane resistivity anisotropy has been studied with the Montgomery method on the detwinned parent compound of the iron-based superconductor FeTe. The observed resistivity in the antiferromagnetic (AFM) direction is larger than that in the ferromagnetic (FM) direction, which is different from that observed in BaFe${}_2$As${}_2$ before. We show that the opposite resistivity anisotropy behavior in FeTe could be attributed to the strong Hund's rule coupling effects, which should be understood in a localized picture: Hund's rule coupling makes hopping along the FM direction easier than along the AFM direction in FeTe, similar to the colossal magnetoresistance observed in some manganites.

The question of the correlation between magnetization, band splittings, and magnetic circular dichroism (MCD) in the fundamental gap region of dilute magnetic semiconductors is examined experimentally and through model calculations, taking the case of wurtzite Ga${}_{1-x}$Fe${}_x$N as an example. Magnetization and polarization-resolved reflectivity measurements have been performed down to 2 K and up to 7 T for $x=0.2$%. Optical transitions originating from all three free excitons $A$, $B$, and $C$, specific to the wurtzite structure, have been observed and their evolution with the magnetic field determined. It is demonstrated that the magnitude of the exciton splittings evaluated from reflectivity-MCD data can be overestimated by more than a factor of 2, as compared to the values obtained by describing the polarization-resolved reflectivity spectra with appropriate dielectric functions. A series of model calculations shows that the quantitative inaccuracy of MCD originates from a substantial influence of the magnetization-dependent exchange interactions not only on the spin splittings of excitons but also upon their linewidth and oscillator strength. At the same time, a method is proposed that allows us to evaluate the field and temperature dependencies of the magnetization from MCD spectra. The accurate values of the excitonic splittings and of the magnetization reported here substantiate the magnitudes of the apparent $sp-d$ exchange integrals in (Ga,Fe)N previously determined.

We study the edge and surface theories of topological insulators from the perspective of anomalies and identify a ${\mathbb{Z}}_2$ anomaly associated with charge conservation. The anomaly is manifested through a two-point correlation function involving creation and annihilation operators on two decoupled boundaries. Although charge conservation on each boundary requires this quantity to vanish, we find that it diverges. A corollary result is that under an insertion of a flux quantum, the ground state evolves to an exactly orthogonal state independent of the rate at which the flux is inserted. The anomaly persists in the presence of disorder and imposes sharp restrictions on possible low-energy theories. Being formulated in a many-body, field-theoretical language, the anomaly allows one to test the robustness of topological insulators to interactions in a concise way.

We study a model for the observed phenomenon of electron spin resonance (ESR) at the Zeeman frequency as seen by a scanning tunneling microscope (STM) via its current noise. The model for this ESR-STM phenomenon allows the STM current to flow in two arms of a nanoscopic interferometer, one arm has direct tunneling from the tip to the substrate while the second arm has tunneling through two spin states. We evaluate analytically the noise spectrum for nonpolarized leads, as relevant to the experimental setup. We show that spin-orbit interactions allow for an interference of two tunneling paths resulting in a resonance effect.

Recently developed tensor network methods demonstrate great potential for addressing the quantum many-body problem, by constructing variational spaces with polynomially, instead of exponentially, scaled parameters. Constructing such an efficient tensor network, and thus the variational space, is a subtle problem and the main obstacle of the method. We demonstrate the necessity of size consistency in tensor network methods for their success in addressing the quantum many-body problem. We further demonstrate that size consistency is independent of the entanglement criterion, thus providing a general and tight constraint to construct the tensor network method. We propose a general and easy rule to construct a size-consistent tensor network.

We performed photoinduced inverse spin-Hall effect (ISHE) measurements on Si-doped bulk GaAs at room temperature. The spin current is optically injected in the sample by means of circularly polarized light and is detected through spin-dependent scattering by Si impurities, which provide an electromotive field ${\mathbf{E}}_{\mathrm{ISHE}}$ close to the sample surface. We measured the ISHE signal as a function of the incident photon energy, and we determined the corresponding averaged spin current, flowing through the sample, to experimentally determine the spin-Hall conductivity.

Discrete-time quantum walks (DTQW) have topological phases that are richer than those of time-independent lattice Hamiltonians. Even the basic symmetries, on which the standard classification of topological insulators hinges, have not yet been properly defined for quantum walks. We introduce the key tool of time frames, i.e., we describe a DTQW by the ensemble of time-shifted unitary time-step operators belonging to the walk. This gives us a way to consistently define chiral symmetry (CS) for DTQW's. We show that CS can be ensured by using an “inversion symmetric” pulse sequence. For one-dimensional DTQW's with CS, we identify the bulk $\mathbb{Z}\times \mathbb{Z}$ topological invariant that controls the number of topologically protected 0 and $\pi$ energy edge states at the interfaces between different domains, and give simple formulas for these invariants. We illustrate this bulk-boundary correspondence for DTQW's on the example of the “4-step quantum walk,” where tuning CS and particle-hole symmetry realizes edge states in various symmetry classes.

We report on the two-beam study of indirect excitons in the inner ring in the exciton emission pattern. One laser beam generates the inner ring and a second weaker beam is positioned in the inner ring. The beam positioned in the inner ring is found to locally suppress the exciton emission intensity. We also report on the inner ring fragmentation and formation of multiple rings in the inner ring region. These features are found to originate from a weak spatial modulation of the excitation beam intensity in the inner ring region. The modulation of exciton emission intensity anticorrelates with the modulation of the laser excitation intensity. The three phenomena—inner ring fragmentation, formation of multiple rings in the inner ring region, and emission suppression by a weak laser beam in the inner ring—have a common feature: a reduction of exciton emission intensity in the region of enhanced laser excitation. This effect is explained in terms of exciton transport and thermalization.

The effect of hydrogenation on the conduction bands of the Si-rich $6H$-SiC$\left(0001\right)$-(3×3) reconstruction is studied using inverse photoemission spectroscopy in order to distinguish surface from bulk states. These results are exploited for the comparative study of the interaction of C${}_{60}$ adsorbed on (3×3) and on hydrogen-terminated (3×3). For the latter, as in the case of hydrogen-terminated Si, C${}_{60}$ is electronically decoupled from the substrate. Upon annealing a C${}_{60}$ thick film deposited on hydrogenated (3×3) up to 670 K, there is a hint for a possible hydrogen transfer to the C${}_{60}$ molecules. The initially physisorbed molecules then adopt covalent bonding with Si, forming the contact layer. Part of the substrate is already found uncovered at this temperature. By further annealing up to 860 K all H atoms have desorbed. Finally, at 1100 K the remaining covalently bound C${}_{60}$ have desorbed. Unexpectedly, the structural damages caused by H and C${}_{60}$ deposition and by the successive annealing steps do not prevent a final restoration of the initial (3×3) reconstruction at about 1100 K.

We study domain wall propagation in systems with long-range interactions caused by lattice distortion. When the local bistable states of the molecular unit have different sizes, e.g., high-spin and low-spin states in spin-crossover systems, the elastic distortion causes an effective long-range interaction. In switching processes between the ordered states in such systems, there exist two degrees of freedom for the domain wall, that is, one with respect to lattice structure and the other to a bistable spin (electronic) state. The interface width of the lattice domain wall is proportional to the system size, which reflects the macroscopic structure due to the long-range interaction, while the interface width of the spin state shows different characteristics depending on the relative time scales of dynamics of the spin state and the lattice. When the change of the spin state is relatively fast and the lattice relaxation is insufficient, the roughness exponent $\alpha$ is $1/2$, which has been commonly found in growth models of short-range interaction systems in two dimensions. In contrast, when the lattice relaxes sufficiently, $\alpha$ of the spin interface is the same as that of the lattice interface, i.e., $\alpha =1$. We analyze the pressure field of the systems and find that the equilibration of the field via the long-range interaction plays a key role in the mechanism of the crossover of $\alpha$ between 1/2 and 1.

Since the commercialization of rechargeable lithium-ion energy storage systems in the early 1990s, graphite intercalation compounds (GICs) have served as the number one negative electrode material in most of today's batteries. During charging the performance of a battery is closely tied with facile Li insertion into the graphite host structure. So far, only occasionally time-domain nuclear magnetic resonance (NMR) measurements have been reported to study Li self-diffusion parameters in GICs. Here, we used several NMR techniques to enlighten Li hopping motions from an atomic-scale point of view. Li self-diffusion in the stage-1 GIC LiC${}_6$ has been studied comparatively by temperature-variable spin-spin relaxation NMR as well as (rotating frame) spin-lattice relaxation NMR. The data collected yield information on both the relevant activation energies and jump rates, which can directly be transformed into Li self-diffusion coefficients. At room temperature the Li self-diffusion coefficient turns out to be ${10}^{-15}{\mathrm{m}}^2{\mathrm{s}}^{-1}$, thus, slightly lower than that for layer-structured cathode materials such as Li${}_{x\approx 0.7}$TiS${}_2$.

High-resolution thermal-expansion measurements of single-crystalline CuO (tenorite) are reported for the temperature range $5<T<350$ K. The data reveal three transitions ($T_{N1}$ $=$ 213 K, $T_{N2}$ $=$ 229.2 K, and $T_{N3}$ $=$ 229.8 K), which corroborate the recently proposed magnetic phase diagram [Villarreal, Quirion, Plumer, Poirier, Usui, and Kimura, Phys. Rev. Lett. 109, 167206 (2012)] revealing three distinct antiferromagnetic (AFM) phases. Analysis of the region surrounding $T_{N2}$ and $T_{N3}$ suggests that these phase transitions are continuous and yields estimates for the heat-capacity critical exponents of ${\alpha }_{T_{N2}}$ $=$ 0.033(2) and ${\alpha }_{T_{N3}}$ $=$ 0.040(9). Magnetic susceptibility measurements reveal spin-flop transitions at temperatures below $T_{N1}$, confirming that the $b$ axis is the easy AFM axis.

The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co concentration, in contradiction to previous calculations but in agreement with recent experiments. This dependence is explained by the bulk properties of the alloys. By using a realistic description of the disorder in the alloys we show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides an explanation of the difference between calculated and measured values. The spin-transfer torque shows a linear voltage dependence for the in-plane component and a quadratic one for the out-of-plane component for all concentrations at low bias voltages. In particular, the linear slope of the in-plane torque is independent of the concentration. For high bias voltages the in-plane torque shows a strong nonlinear deviation from the linear slope for high Co concentrations. This is explained by the same effects which govern the tunneling magnetoresistance.

We present a theoretical description of spin-transfer torque in a spin valve with perpendicularly magnetized polarizer. The polarizer consisting of several ultrathin layers is considered as a single interfacial magnetic scatterer between two nonmagnetic layers, and is included in the theory based on diffusive transport via appropriate boundary conditions. The model has been used to study systematically the spin-transfer torque and current-induced switching in a spin valve with both perpendicular and in-plane polarizers and with in-plane magnetized free layer. The wave-function matching ab initio calculations have been used to determine transport parameters of the perpendicular polarizer. Additionally, the effect of disorder on the spin-transfer torque has been examined.