In graphene nanoconstrictions, charge puddles due to substrate-induced potential inhomogeneities can form quantum dots. We use a scanning gate microscope to probe a short, wide graphene constriction and observe a set of ring-shaped resonances in maps of the graphene conductance as a function of the local potential perturbation, a signature of quantum dot-mediated transport. By varying the scanning tip height and applied tip voltage, we observe the screening effect of the nearby graphene. Introducing a simple capacitive model to account for screening, we model the conduction of the graphene constriction and how the tip gates the quantum dots. We identify the locations of some of the quantum dots and extract their parameters.

We present temperature-dependent magnetotransport experiments around the charge neutrality point in graphene and determine the amplitude of potential fluctuations $s$ responsible for the formation of electron-hole puddles. The experimental value $s\approx 20$ meV is considerably larger than in conventional semiconductors, which implies a strong localization of charge carriers observable up to room temperature. Surprisingly, in the quantized regime, the Hall resistivity overshoots the highest plateau values at high temperatures. We demonstrate by model calculations that such a peculiar behavior is expected in any system with coexisting electrons and holes when the energy spectrum is quantized and the carriers are partially localized.

We investigate the effect of free electrons on the quality factor ($Q$) of a metallic nanomechanical resonator in the form of a thin elastic beam. The flexural and longitudinal modes of the beam are modeled using thin beam elasticity theory, and simple perturbation theory is used to calculate the rate at which an externally excited vibration mode decays due to its interaction with free electrons. We find that electron-phonon interaction significantly affects the $Q$ of longitudinal modes, and may also be of significance to the damping of flexural modes in otherwise high-$Q$ beams. The finite geometry of the beam is manifested in two important ways. Its finite length breaks translation invariance along the beam and introduces an imperfect momentum conservation law in place of the exact law. Its finite width imposes a quantization of the electronic states that introduces a temperature scale for which there exists a crossover from a high-temperature macroscopic regime, where electron-phonon damping behaves as if the electrons were in the bulk, to a low-temperature mesoscopic regime, where damping is dominated by just a few dissipation channels and exhibits sharp nonmonotonic changes as parameters are varied. This suggests a scheme for probing the electronic spectrum of a nanoscale device by measuring the $Q$ of its mechanical vibrations.

An exact expression for the temperature-dependent interface stress tensor (tension) and energy is derived within a phase field approach. The key problem, of which part of the thermal energy should contribute to the surface tension, is resolved with the help of an analytical solution for a nonequilibrium interface. Thus, for a propagating interface at any temperature, the interface stress tensor represents biaxial tension with magnitude equal to the temperature-dependent interface energy. Explicit expressions for the distributions of interface stresses are obtained for a nonequilibrium interface and a critical nucleus. The results obtained are applicable for various phase transformations (solid-solid, melting-solidification, sublimation, etc.) and structural changes (twinning, grain evolution), and can be generalized for anisotropic interface energy, for dislocations, fracture, and diffusive phase transformations described by Cahn-Hilliard theory.

We report a comparative investigation of the magnetostrictive and magnetoconductivity effects in the low-temperature ferrimagnetic phases of the spinel vanadate structure $A$V${}_2$O${}_4$ ($A$ $=$ Fe, Mn, and Co). In FeV${}_2$O${}_4$, both the Fe and V sites have orbital degrees of freedom, the V-V distance places the compound intermediate between localized and itinerant charge carrier behavior, and there are three structural and two magnetic transitions versus temperature. For MnV${}_2$O${}_4$ and CoV${}_2$O${}_4$, only the V sites have orbital degrees of freedom. MnV${}_2$O${}_4$ has the largest V-V distance and the most localized conductivity, with one structural and one magnetic transition. CoV${}_2$O${}_4$ has the shortest V-V distance and approaches the itinerant charge carrier threshold. CoV${}_2$O${}_4$ has one ferrimagnetic transition and no long-range ordered structural transition. Working with single crystals, ac dielectric and resistance measurements versus temperature and magnetic field exhibit distinctive physical properties in each case. These include the magnetocaloric signatures of magnetization reversal (Fe and Co), separable isotropic and anisotropic magnetostrictive effects (Mn), glasslike dynamics (Co), and evidence in the field-dependent dielectric response for the alteration of the ferrimagnetic spin structure with increasing magnetic field (Fe). These findings can guide future dielectric- and magnetoconductance-based spinel studies with a focus on the low-temperature and high magnetic field properties of canted ferrimagnetic spin configurations and orbital-lattice ordering effects.

We present elastic and inelastic neutron scattering data on LaSrFeO${}_4$. We confirm the known magnetic structure with the magnetic moments lying in the tetragonal basal plane, but our macroscopic and neutron diffraction data do not reveal any additional magnetic phase transition connected to a spin reorientation or to a redistribution of two irreducible representations. Our inelastic neutron scattering data reveals the magnon dispersion along the main-symmetry directions [0 $\xi$ 0] and [$\xi$ -$\xi$ 0]. The dispersion can be explained within linear spin-wave theory yielding an antiferromagnetic nearest-neighbor interaction parameter $J_1=7.4\left(3\right)$ meV and a next-nearest neighbor interaction parameter $J_2=0.4\left(3\right)$ meV. The dispersion is gapped with the out-of-plane anisotropy gap found at ${\Delta }_{\mathrm{out}}=5.26\left(2\right)$ meV, while evidence is present that the in-plane anisotropy gap lies at lower energies, where it cannot be determined due to limited instrument resolution.

Using the Lagrangian formalism, we solve analytically the equations of motion for current-induced domain wall dynamics in a ferromagnet with Rashba spin-orbit coupling. An exact solution for the domain wall velocity is provided, including the effect of nonequilibrium conduction electron spin density, Gilbert damping, and the Rashba interaction parameter. We demonstrate explicitly that the influence of spin-orbit interaction can be qualitatively different from the role of nonadiabatic spin torque in the sense that the former is sensitive to the chirality of the domain wall whereas the latter is not: the domain wall velocity shows a reentrant behavior upon changing the chirality of the domain wall. This could be used to experimentally distinguish between the spin-orbit and nonadiabatic contribution to the wall speed. A quantitative estimate for the attainable domain wall velocity is given, based on an experimentally relevant set of parameters for the system.

Ghosh and Hirsch [Phys. Rev. B 86, 054511 (2012)] claimed that many micrometer-size particles in liquid nitrogen, as large as 25 to 32 μm, could be aggregated into balls by shaking. Ghosh and Hirsch performed their experiments with liquid nitrogen in open air; therefore, moisture condensed on the particle surface, leading to ball aggregation by shaking. This phenomenon has nothing to do with the electric field-induced formation of superconducting balls. In addition, their claim that a large electric field still exists in the interior of the capacitor when the electrodes are insulated is flawed. The electric field–induced superconducting ball formation reveals that the area of interaction between electric field and superconductors requires more investigation. However, the phenomenon can be explained within the BCS theory.

We argue on experimental and theoretical grounds that the spherical agglomeration of microparticles that we found is not due to moisture as claimed in the Comment by Tao et al. Rather it is the same phenomenon as the ball formation originally reported by Tao, except that our results demonstrate that it is not electric field induced and it is not related to superconductivity, contrary to the claims of Tao et al. We also argue on theoretical and experimental grounds that the electric field is not reduced when the electrodes in the capacitor are insulating rather than conducting, contrary to the claim in the Comment. Our new results give further evidence that Tao's original experiments were flawed.

Measurements on solid ${}^4$He show large softening of the shear modulus due to dislocations, behavior which has been described as giant plasticity. Dislocation networks may also be responsible for the unusual behavior seen in torsional oscillator and flow experiments. However, previous estimates of dislocation densities vary by many orders of magnitude, even in single crystals grown under similar conditions. By measuring the temperature and frequency dependencies of the elastic dissipation, we have determined dislocation densities and network lengths in ${}^4$He single crystals, both in coexistence with liquid and at higher pressures, and in polycrystals grown at constant density. In all cases, dislocation lengths are much longer and the networks are less connected than previous estimates. Even in polycrystals, the dislocation network is far too sparse to explain the torsional oscillator results in terms of superfluidity in a dislocation network.

We explain the recent numerical successes obtained by Tao Xiang's group, who developed and applied tensor renormalization group methods for the Ising model on square and cubic lattices, by the fact that their new truncation method sharply singles out a surprisingly small subspace of dimension two. We show that in the two-state approximation, their transformation can be handled analytically, yielding a value of 0.964 for the critical exponent $\nu$ much closer to the exact value 1 than the 1.338 value obtained in the Migdal-Kadanoff approximation. We propose two alternative blocking procedures that preserve the isotropy and improve the accuracy to $\nu =0.987$ and 0.993, respectively. We discuss applications to other classical lattice models, including models with fermions, and suggest that it could become a competitor for Monte Carlo methods suitable for accurate calculations of critical exponents, taking continuum limits, and the study of near-conformal systems in arbitrarily large volumes.

We calculate the charge transport in a flux-biased dissipative Cooper pair pump using the method of full counting statistics (FCS). This is used instead of a more traditional technique of integrating a very small expectation value of the instantaneous current over the pumping period. We show that the rotating wave approximation (RWA), which fails in the traditional technique, produces accurate results within the FCS method.

High-order ring-exchange interactions are crucial for the study of quantum fluctuations on many highly frustrated systems. A versatile and efficient quantum Monte Carlo method, which can handle finite and essentially zero temperature and canonical and grand-canonical ensembles, has long been sought. In this paper, we present an exact quantum Monte Carlo study of a model of hard-core bosons with sixth-order ring-exchange interactions on a two-dimensional kagome lattice. By using the stochastic Green function algorithm with global space-time update, we show that the system becomes unstable in the limit of large ring-exchange interactions. It undergoes a phase separation at all fillings, except at $\frac{1}{3}$ and $\frac{2}{3}$ fillings for which the superfluid density vanishes and an unusual mixed valence bond and charge density ordered solid is formed. This explains the universal features seen in previous studies on various different models, such as the transverse-field Ising models, on a kagome lattice near the classical limit.

We investigate energetics and scanning tunneling microscopy (STM) images of several Ge films on Si(001) substrates using a first-principles total-energy calculation within the density-functional theory. We calculate the film energies of various Ge films with dimer-vacancy lines (DVL) and 90${}^{\circ }$ dislocation cores (DC) deposited on Si(001) substrate as a function of Ge-layer thickness. Our energetics calculation suggests that 90${}^{\circ }$ DC structure becomes stable when sufficiently thick Ge overlayers are deposited on Si(001) substrate. We also calculate the STM images of $p$$(2\times 2)$, $2\times 8$ DVL, and 90${}^{\circ }$ DC structures, and we find that STM images of their surface structures are distinguishable from one another.

We investigate intersubband polaritons formed in the asymmetric quantum well embedded into the semiconductor microcavity and study the effects of spin-orbit interaction (SOI) acting on intersubband excitations. The spin-orbit interactions of Rashba and Dresselhaus type remove the spin degeneracy of electrons with finite value of in-plane momentum and allow four types of intersubband excitations. While optical spin-flip transitions are suppressed, the spectrum of elementary excitations shows the appearance of upper, lower, and middle polariton branches based on spin-conserving transitions. The accounting of finite photon momentum leads to nonzero average spin projection of electronic ensemble in the first excited subband under continuous wave excitation for both isotropic (Rashba) and anisotropic (Rashba and Dresselhaus) SOI. We predict the possibility of spin-current generation in the considered systems with long coherence length.

The spectra of electronic excitations in graphene are calculated using first principles time-dependent density functional theory formalism, and used to obtain $\pi$ and $\pi +\sigma$ plasmon dispersion curves. The spectra and dispersion are in excellent agreement with recent experimental results, and they are used to investigate the anisotropy and splitting of a $\pi$ plasmon, which has also been experimentally verified. The high accuracy of this calculation enabled the discovery of some different features in the spectra, especially the $\Gamma M$-$\Gamma K$ anisotropy of the two-dimensional (2D) plasmon dispersion curve, which qualitatively agrees with recent experimental results. Our ab initio 2D plasmon dispersion curves are compared with the ones obtained in some recently proposed 2D models. They show strong disagreement with the dispersion curve obtained using a simple one-band 2D theory, as well as some discrepancies with respect to the commonly used Das Sarma et al.'s dispersion curve, even in the isotropic region. Excellent agreement of the calculated spectrum in pristine graphene with the electron energy loss spectroscopy spectrum measured for lower momentum transfers is demonstrated.

The instability of a free-standing one-sided hydrogenated/fluorinated graphene nanoribbon, i.e., graphone/fluorographene, is studied using ab initio, semiempirical, and large-scale molecular dynamics simulations. Free-standing semi-infinite armchairlike hydrogenated/fluorinated graphene (AC-GH/AC-GF) and boatlike hydrogenated/fluorinated graphene (B-GH/B-GF) (nanoribbons which are periodic along the zigzag direction) are unstable and spontaneously transform into spiral structures. We find that rolled, spiral B-GH and B-GF are energetically more favorable than spiral AC-GH and AC-GF which is opposite to the double-sided flat hydrogenated/fluorinated graphene, i.e., graphane/fluorographene. We found that the packed, spiral structures exhibit an unexpected localized highest occupied molecular orbital and lowest occupied molecular orbital at the edges with increasing energy gap during rolling. These rolled hydrocarbon structures are stable beyond room temperature up to at least $T=1000$ K within our simulation time of 1 ns.

The initial oxidation of $4H$-SiC is investigated under realistic temperature and pressure conditions in order to better understand the mechanism of defect creation at the SiO${}_2$/SiC interface. The oxidation reaction commences on a clean $4H$-SiC (0001) surface with chemisorption of the on-surface oxygen atoms at high-symmetry coordinated sites. Our calculations show that oxygen adsorption on the $4H$-SiC (0001) surface occurs preferentially at the bridge site when the coverage is low of $1/9$ monolayer (ML). As oxygen coverage increases to $3/4$ ML, the first oxygen atom is incorporated into the subsurface region that actuates the formation of a surface oxide on the SiC surface. Above 1 ML, oxidation mechanisms in the subsurface layer are explored by taking into account oxygen incorporation and substitutions. To determine the stability of these O/SiC structures for real oxidation conditions, a surface phase diagram of $4H$-SiC initial oxidation is constructed over a coverage range of $1/9$–2 ML. With the increase of oxygen chemical potential, the $1/9$-, $1/4$-, $3/2$-, and 2-ML configurations become thermodynamically favorable. Moreover, a C${}_2$ dimer defect is observed in the 2-ML configuration. This can be extended to the SiO${}_2$/SiC interface as a thin SiO${}_2$ layer and may provide a possible mechanism for the creation of a C-cluster defect in the interface.

We study the dynamical dielectric function of a two-dimensional hole gas, exemplified on [001] GaAs and InAs quantum wells, within the Luttinger model extended to the two lowest subbands including bulk and structure inversion asymmetric terms. The plasmon dispersion shows a pronounced anisotropy for GaAs- and InAs-based systems. In GaAs this leads to a suppression of plasmons due to Landau damping in some orientations. Due to the large Rashba contribution in InAs, the lifetime of plasmons can be controlled by changing the electric field. This effect is potentially useful in plasmon field effect transistors as previously proposed for electron gases.

For a quantum state, or classical harmonic normal mode, of a system of spatial periodicity “$R$,” Bloch character is encoded in a wave vector “$K$.” One can ask whether this state has partial Bloch character “$k$” corresponding to a finer scale of periodicity “$r$.” Answering this is called “unfolding.” A theorem is proven that yields a mathematically clear prescription for unfolding, by examining translational properties of the state, requiring no “reference states” or basis functions with the finer periodicity ($r,k$). A question then arises: How should one assign partial Bloch character to a state of a finite system? A slab, finite in one direction, is used as the example. Perpendicular components $k_z$ of the wave vector are not explicitly defined, but may be hidden in the state (and eigenvector $|i\rangle$). A prescription for extracting $k_z$ is offered and tested. An idealized silicon (111) surface is used as the example. Slab unfolding reveals surface-localized states and resonances which were not evident from dispersion curves alone.