A parametric interatomic potential is constructed for graphene. The potential energy consists of two parts: a bond energy function and a radial interaction energy function. The bond energy function is based on the Tersoff-Brenner potential model. It includes angular terms and explicitly accounts for flexural deformation of the lattice normal to the plane of graphene. It determines the cohesive energy of graphene and its equilibrium lattice constant. The radial energy function has been chosen such that it does not contribute to the binding energy or the equilibrium lattice constant but contributes to the interatomic force constants. The range of interaction of each atom extends up to its fourth-neighbor atoms in contrast to the Tersoff-Brenner potential, which extends only up to second neighbors. The parameters of the potential are obtained by fitting the calculated values to the cohesive energy, lattice constant, elastic constants, and phonon frequencies of graphene. The values of the force constants between an atom and other atoms that are within its fourth-neighbor distance are calculated. Analytical expressions are given for the elastic constants and the flexural rigidity of graphene. The flexural rigidity of the graphene lattice is found to be 2.13 eV, which is much higher than 0.797 eV calculated earlier using the Tersoff-Brenner potential.

Nuclear spin polarization dynamics are measured in optically pumped individual $\text{GaAs}/{\text{Al}}_x{\text{Ga}}_{1-x}\text{As}$ interface quantum dots by detecting the time dependence of the Overhauser shift in photoluminescence spectra. Long nuclear polarization decay times of $\approx 1\text{min}$ have been found indicating inefficient nuclear spin diffusion from the GaAs dot into the surrounding AlGaAs matrix in externally applied magnetic field. A spin-diffusion coefficient two orders lower than that previously found in bulk GaAs is deduced.

A general method for directly measuring the low-frequency flux noise (below 10 Hz) in compound Josephson-junction superconducting flux qubits has been used to study a series of 85 devices of varying design. The variation in flux noise across sets of qubits with identical designs was observed to be small. However, the levels of flux noise systematically varied between qubit designs with strong dependence upon qubit wiring length and wiring width. Furthermore, qubits fabricated above a superconducting ground plane yielded lower noise than qubits without such a layer. These results support the hypothesis that local impurities in the vicinity of the qubit wiring are a key source of low-frequency flux noise in superconducting devices.

Activation energies for vacancy-mediated impurity diffusion in face-centered-cubic aluminum have been computed ab initio for all technologically important alloying elements, as well as for most of the lanthanides. The so-called five-frequency rate model is used to establish the limiting vacancy interchange process. Many elements were shown to be limited by Al-vacancy interchanges. For these elements we showed that the diffusion activation energy is rather close to that for Al self-diffusion, and additionally the diffusion pre-exponential factor is of the same order as that for Al self-diffusion. The diffusion activation energy is shown to exhibit a linear relation with the solute partial molar volume in Al. In contrast, transition metals are shown to deviate strongly from these generalities. Diffusion of transition-metal atoms is limited by solute-vacancy interchanges that require remarkably high activation energies. Transition-metal diffusivities in Al show strong trends with the number of $d$-valence electrons but not with partial molar volume.

Structural phase transitions and superconducting properties in three phases ($9R$, fcc, and $cI16$) of solid Li are investigated using a pseudopotential plane-wave method based on density functional perturbation theory. In particular, it is shown that phonon softening is responsible for a pressure-induced $\text{fcc}\to cI16$ transition as well as for a significant enhancement of electron-phonon coupling and superconducting transition temperature $T_c$ preceding this structural transformation. The nature of superconductivity in the fcc and $cI16$ phases is examined by solving the Eliashberg equations with the spectral function ${\alpha }^{2}F\left(\omega \right)$ obtained from first-principles calculations and by evaluating the functional derivative $\delta T_c/\delta {\alpha }^{2}F\left(\omega \right)$. The calculated $T_c$ reaches a maximum at pressure close to the $\text{fcc}\to cI16$ transition and is significantly reduced in the $cI16$ phase, in agreement with the trend observed experimentally. The variation in $T_c$ as a function of pressure is explained in terms of the functional derivative and shifts of the spectral weight.

Atomic hydrogen adsorption on the silicon-rich 3$C$-SiC(001) $3\times 2$ surface is investigated by means of first-principles molecular-dynamics calculations. Considering nearly fifty surface structures with 2–14 hydrogen atoms per unit cell, we show that up to a critical partial pressure of atomic hydrogen the equilibrium is represented by two or four hydrogen atoms adsorbed onto the two topmost Si atoms, causing complete passivation. These stable semiconducting structures are compatible with observations by scanning tunneling microscopy (STM) and core-level photoelectron spectroscopy (PES). Above the critical pressure, etching of the excess silicon sets in, but the etched surface is again semiconducting. Frequencies and infrared (IR) intensities of the vibration modes, calculated for the dihydrogenated surface, appear to be compatible with the observed IR spectrum, but none of the investigated metastable metallic structures is. It is also shown that the appearance of partially filled states near the Fermi level, as observed by PES and STM, can also be explained by a completely passivated surface, when taking into account the band bending effects due to surface dangling bonds and due to the spontaneous polarization before and after the hydrogenation, respectively.

We study the effect of inhomogeneities in Hall conductivity on the nature of the zero resistance states seen in the microwave irradiated two-dimensional electron systems in weak perpendicular magnetic fields and show that time-dependent domain patterns may emerge in some situations. For an annular Corbino geometry, with an equilibrium charge density that varies linearly with radius, we find a time-periodic nonequilibrium solution, which might be detected by a charge sensor, such as a single electron transistor. For a model on a torus, in addition to static domain patterns seen at high and low values of the equilibrium charge inhomogeneity, we find that, in the intermediate regime, a variety of nonstationary states can also exist. We catalog the possibilities we have seen in our simulations. Within a particular phenomenological model, we show that linearizing the nonlinear charge continuity equation about a particularly simple domain-wall configuration and analyzing the eigenmodes allows us to estimate the periods of the solutions to the full nonlinear equation.

The Fermi surface (FS) of ${\text{URu}}_2{\text{Si}}_2$ in the paramagnetic phase has been reinvestigated via the measurement of the two-dimensional angular correlation of the positron annihilation radiation. Unlike previous measurements our experiment provides evidence that density-functional theory gives a fair qualitative description of the FS, showing a large contribution from the $5f$ electrons, although the Fermi volume of electron and hole pockets determined experimentally is smaller than the theoretical description. Furthermore, we propose a method to investigate the nature of the hidden order responsible for the 17.5 K transition.

The electrodynamic properties of ${\text{Au}}_{1-x}{\text{Fe}}_x$ films are measured for various Fe contents $x=0$, 6, 17, and $22\text{at}\text{.}\%$. From room-temperature optical measurements over a broad spectral range $\left(10–3.3\times {10}^4{\text{cm}}^{-1}\right)$, we obtain the frequency-dependent conductivity of the films. As the iron concentration increases, a stronger decrease in the magnetic contribution to the dc conductivity compared to ac conductivity (at frequency $10{\text{cm}}^{-1}$) is detected. In addition, a broad absorption peak is discovered at frequency $1000–3000{\text{cm}}^{-1}$ for concentrations of $6–22\text{at}\text{.}\%$. Both observations are associated with ferromagnetic clusters that cause partial localization of conduction electrons.

Magnetic domain walls in patterned permalloy ring elements of different widths and thicknesses have been probed by magnetic force microscopy and micromagnetic simulations. We report the experimental observation of antivortex structures in single-layer magnetic rings, previously only theoretically predicted. Furthermore, vortex-antivortex chains are observed on occasion in wider rings that prefer a circulating flux closure state. It is possible that they are pinned transient states of the relaxation path from the magnetically saturated to the circulating flux closure state. Similar tendencies are seen in micromagnetic simulations implying that vortex-antivortex pair nucleation and annihilation are fundamental parts of the said relaxation. Finally we present a phase diagram of different domain-wall configurations as a function of ring width and thickness.

We describe experiments we have performed in which we are able to image the motion of individual electrons moving in liquid helium 4. Electrons in helium form bubbles of radius ~19 Å. We use the negative pressure produced by a sound wave to expand these bubbles to a radius of about $10\mu \text{m}$. The bubbles are then illuminated with light from a flash lamp and their position recorded. We report on several interesting phenomena that have been observed in these experiments. It appears that the majority of the electrons that we detect result from cosmic rays passing through the experimental cell. We discuss this mechanism for electron production.

We explain the origin of most of the peaks in the Nernst coefficient that were recently observed at magnetic fields directed along the trigonal axis and the bisectrix direction in bismuth. Additional experiments are discussed that enable one to verify our explanation.

We compute the electronic component $\left(\kappa \right)$ of the thermal conductivity and the thermoelectric power $\left(\alpha \right)$ of monolayer graphene within the hydrodynamic regime, taking into account the slow rate of carrier population imbalance relaxation. Interband electron-hole generation and recombination processes are inefficient due to the nondecaying nature of the relativistic energy spectrum. As a result, a population imbalance of the conduction and valence bands [i.e., a nonequilibrium state with ${\mu }_e+{\mu }_h\ne 0$, where ${\mu }_e$ $\left({\mu }_h\right)$ denotes the electron (hole) chemical potential] is generically induced upon the application of a thermal gradient. We show that the thermoelectric response of a graphene monolayer depends upon the ratio of the sample length to an intrinsic length scale $l_Q$ set by the imbalance relaxation rate. At the same time, we incorporate the crucial influence of the metallic contacts required for the thermopower measurement (under open circuit boundary conditions) since carrier exchange with the contacts also relaxes the imbalance. These effects are especially pronounced for clean graphene, where the thermoelectric transport is limited exclusively by intercarrier collisions. For specimens shorter than $l_Q$, the population imbalance extends throughout the sample; $\kappa$ and $\alpha$ asymptote toward their zero imbalance relaxation limits. In the opposite limit of a graphene slab longer than $l_Q$, at nonzero doping $\kappa$ and $\alpha$ approach intrinsic values characteristic of the infinite imbalance relaxation limit. Samples of intermediate (long) length in the doped (undoped) case are predicted to exhibit an inhomogeneous temperature profile, while $\kappa$ and $\alpha$ grow linearly with the system size. In all cases except for the shortest devices, we develop a picture of bulk electron and hole number currents that flow between thermally conductive leads, where steady-state recombination and generation processes relax the accumulating imbalance. Our analysis incorporates, in addition, the effects of (weak) quenched disorder.

We present QMD simulations of water in the ultra-high-pressure regime up to conditions typical for the deep interior of Jupiter and Saturn. We calculate the equation of state and the Hugoniot curve and study the structural properties via pair correlation functions and self-diffusion coefficients. In the ultradense superionic phase, we find a continuous transition in the protonic structure. With rising density, the mobile protons stay with increasing probability at the octahedral sites while leaving the ice X positions to the same degree unoccupied. Water forms a fluid dense plasma at the conditions of Jupiter’s core (i.e., $20000\text{K}$, 50 Mbar, $11\text{g}/{\text{cm}}^3$), while it may be superionic in the core of Saturn. We expect a substantial amount of superionic water inside Neptune.

Recently, it has been shown that spin nematic (quadrupolar) or higher multipolar correlation functions exhibit a quasi-long-range order in the wide region of the field-induced Tomonaga-Luttinger-liquid (TLL) phase in spin-$\frac{1}{2}$ zigzag chains. In this Rapid Communication, we point out that the temperature dependence of the NMR relaxation rate $1/T_1$ in these multipolar TLLs is qualitatively different from that in more conventional TLLs of one-dimensional quantum magnets (e.g., the spin-$\frac{1}{2}$ Heisenberg chain); $1/T_1$ decreases with lowering temperature in multipolar TLL. We also discuss low-energy features in spin dynamical structure factors which are characteristic of the multipolar TLL phases.

We define a distinct phase of matter, a pair-density wave (PDW), in which the superconducting order parameter $\varphi \left(\vec{r},{\vec{r}}^{\prime }\right)$ varies periodically as a function of position such that when averaged over the center of mass position, $\left(\vec{r}+{\vec{r}}^{\prime }\right)/2$, all components of $\varphi$ vanish identically. Specifically, we study the simplest unidirectional PDW, the “striped superconductor,” which we argue may be at the heart of a number of spectacular experimental anomalies that have been observed in the failed high-temperature superconductor ${\text{La}}_{2-x}{\text{Ba}}_x{\text{CuO}}_4$. We present a solvable microscopic model with strong electron-electron interactions which supports a PDW ground state. We also discuss, at the level of Landau theory, the nature of the coupling between the PDW and other order parameters and the origins and some consequences of the unusual sensitivity of this state to quenched disorder.

The Earth’s core consists of a solid ball with a radius of 1221 Km, surrounded by a liquid shell which extends up to 3480 km from the center of the planet, roughly half way toward the surface (the mean radius of the Earth is 6373 km). The main constituent of the core is iron, and therefore the melting temperature of iron at the pressure encountered at the boundary between the solid and the liquid [the inner-core boundary (ICB)] provides an estimate of the temperature of the core. Here I report the melting temperature of Fe at pressures near that of the ICB, obtained with first-principles techniques based on density-functional theory. The calculations have been performed by directly simulating solid and liquid iron in coexistence and show that and at a pressure of $\sim 328\text{GPa}$ iron melts at $\sim 6370\pm 100\text{K}$. These findings are in good agreement with earlier simulations, which used exactly the same quantum-mechanics techniques but obtained melting properties from the calculation of the free energies of solid and liquid Fe.

Neutron scattering is used to study magnetic-field-induced ordering in the quasi-one-dimensional quantum spin-tube compound ${\text{Sul-Cu}}_2{\text{Cl}}_4$ that in zero field has a nonmagnetic spin-liquid ground state. The experiments reveal an incommensurate chiral high-field phase stabilized by a geometric frustration of the magnetic interactions. The measured critical exponents $\beta \approx 0.235$ and $\nu \approx 0.34$ at $H_c\approx 3.7\text{T}$ point to an unusual subcritical scaling regime and may reflect the chiral nature of the quantum critical point.

Nonlinear effects of two-dimensional electron-gas systems at high magnetic fields are studied, and a current-driven instability is found to take place in magnetic fields corresponding to integer filling factors of Landau levels. The current instability is ascribed to the occurrence of a negative differential conductivity, predicted theoretically more than three decades ago. Physical origin of the negative differential conductivity is discussed and suggested to originate from an electric-field-induced reduction in the average scattering probability of electrons.

We present evidence for hole injection into ${\text{LaAlO}}_3/{\text{LaVO}}_3/{\text{LaAlO}}_3$ quantum wells near a polar surface of ${\text{LaAlO}}_3$ (001). As the surface is brought in proximity to the ${\text{LaVO}}_3$ layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient are observed below a characteristic coupling length of 10–15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the ${\text{AlO}}_2$-terminated ${\text{LaAlO}}_3$ surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole doping in oxide thin-film heterostructures.