Skyrmion mass and a new kind of the cyclotron resonance for 2DEG

The skyrmionic mass was calculated in the main order of the gradient expansion in derivatives of the rotation matrix. The mass is proportinal to the topological invariant which is the absolut value of the degree of the mapping. The coefficient is defined by the exchange interaction. The charged skyrmions in magnetic field give rise to a special branch of cyclotron resonance with the frequency defined by the exchange interaction and have the corresponding term in their minimal energy. The possibility of an extra bound electron and neutral skyrmions is discussed.Comment: 4 pages, Latex (submitted to JETP lett.

Scanning Superfluid-Turbulence Cascade by Its Low-Temperature Cutoff

On the basis of recently proposed scenario of the transformation of the Kolmogorov cascade into the Kelvin-wave cascade, we develop a theory of low-temperature cutoff. The theory predicts a specific behavior of the quantized vortex line density, $L$, controlled by the frictional coefficient, $\alpha(T) \ll 1$, responsible for the cutoff. The curve $\ln L(\ln \alpha)$ is found to directly reflect the structure of the cascade, revealing four qualitatively distinct wavenumber regions. Excellent agreement with recent experiment by Walmsley {\it et al.} [arXiv:0710.1033]--in which $L(T)$ has been measured down to $T \sim 0.08$K--implies that the scenario of low-temperature superfluid turbulence is now experimentally validated, and allows to quantify the Kelvin-wave cascade spectrum.Comment: 4 pages, 2 figures, v2: extended introduction, the controversy with the scenario by L'vov et al. [13] is discussed in conclusio

Nonequilibrium effective vector potential due to pseudospin exchange in graphene

We show that exchange interactions in two-dimensional electron gases out of equilibrium can generate a fictitious vector potential with intriguing signatures in interference and Hall measurements. Detailed predictions are made for graphene, where the effect is enhanced by pseudospin exchange.Comment: 4 pages, 2 figure

Doublon relaxation in the Bose-Hubbard model

Decay of a high-energy double occupancy state, doublon, in a narrow-band lattice requires creation of a coherent many-particle excitation. This leads to an exponentially long relaxation time of such a state. We show that, if the average occupation number is sufficiently small, the corresponding exponent may be evaluated exactly. To this end we develop the quasiclassical approach to calculation of the high-order tree-level decay amplitudes.Comment: 4 pages, 1 figur

Phase-slip avalanches in the superflow of 4^4He through arrays of nanopores

Recent experiments by Sato et al. [1] have explored the dynamics of $^4$He superflow through an array of nanopores. These experiments have found that, as the temperature is lowered, phase-slippage in the pores changes its character, from synchronous to asynchronous. Inspired by these experiments, we construct a model to address the characteristics of phase-slippage in superflow through nanopore arrays. We focus on the low-temperature regime, in which the current-phase relation for a single pore is linear, and thermal fluctuations may be neglected. Our model incorporates two basic ingredients: (1) each pore has its own random value of critical velocity (due, e.g., to atomic-scale imperfections), and (2) an effective inter-pore coupling, mediated through the bulk superfluid. The inter-pore coupling tends to cause neighbours of a pore that has already phase-slipped also to phase-slip; this process may cascade, creating an avalanche of synchronously slipping phases. As the temperature is lowered, the distribution of critical velocities is expected to effectively broaden, owing to the reduction in the superfluid healing length, leading to a loss of synchronicity in phase-slippage. Furthermore, we find that competition between the strength of the disorder in the critical velocities and the strength of the inter-pore interaction leads to a phase transition between non-avalanching and avalanching regimes of phase-slippage. [1] Sato, Y., Hoskinson, E. Packard, R. E. cond-mat/0605660.Comment: 8 pages, 5 figure

Weak localization of bulk channels in topological insulator thin film

Weak antilocalization (WAL) is expected whenever strong spin-orbit coupling or scattering comes into play. Spin-orbit coupling in the bulk states of a topological insulator is very strong, enough to result in the topological phase transition. However, the recently observed WAL in topological insulators seems to have an ambiguous origin from the bulk states. Starting from the effective model for three-dimensional topological insulators, we find that the lowest two-dimensional (2D) bulk subbands of a topological insulator thin film can be described by the modified massive Dirac model. We derive the magnetoconductivity formula for both the 2D bulk subbands and surface bands. Because with Relatively large gap, the 2D bulk subbands may lie in the regimes where the unitary behavior or even weak localization (WL) is also expected, instead of always WAL. As a result, the bulk states may contribute small magnetoconductivity or even compensate the WAL from the surface states. Inflection in magnetoconductivity curves may appear when the bulk WL channels outnumber the surface WAL channels, providing a signature of the weak localization from the bulk states.Comment: 9 pages, 5 figure

Interaction-driven topological insulator states in strained graphene

The electronic properties of graphene can be manipulated via mechanical deformations, which opens prospects for studying the Dirac fermions in new regimes and for new device applications. Certain natural configurations of strain generate large nearly uniform pseudo-magnetic fields, which have opposite signs in the two valleys, and give rise to flat spin- and valley-degenerate pseudo Landau levels (PLLs). Here we consider the effect of the Coulomb interactions in strained graphene with uniform pseudo-magnetic field. We show that the spin/valley degeneracies of the PLLs get lifted by the interactions, giving rise to topological insulator-like states. In particular, when a nonzero PLL is quarter- or three-quarter filled, an anomalous quantum Hall state spontaneously breaking time-reversal symmetry emerges. At half-filled PLL, weak spin-orbital interaction stabilizes time-reversal-symmetric quantum spin-Hall state. These many-body states are characterized by the quantized conductance and persist to a high temperature scale set by the Coulomb interactions, which we estimate to be a few hundreds Kelvin at moderate strain values. At fractional fillings, fractional quantum Hall states breaking valley symmetry emerge. These results suggest a new route to realizing robust topological insulator states in mesoscopic graphene.Comment: 5 page

Superflow in Solid 4He

Kim and Chan have recently observed Non-Classical Rotational Inertia (NCRI) for solid $^4$He in Vycor glass, gold film, and bulk. Their low $T$ value of the superfluid fraction, $\rho_{s}/\rho\approx0.015$, is consistent with what is known of the atomic delocalization in this quantum solid. By including a lattice mass density $\rho_{L}$ distinct from the normal fluid density $\rho_{n}$, we argue that $\rho_{s}(T)\approx\rho_{s}(0)-\rho_{n}(T)$, and we develop a model for the normal fluid density $\rho_{n}$ with contributions from longitudinal phonons and ``defectons'' (which dominate). The Bose-Einstein Condensation (BEC) and macroscopic phase inferred from NCRI implies quantum vortex lines and quantum vortex rings, which may explain the unusually low critical velocity and certain hysteretic phenomena.Comment: 4 page pdf, 1 figur

Weak antilocalization in HgTe quantum wells and topological surface states: Massive versus massless Dirac fermions

HgTe quantum wells and surfaces of three-dimensional topological insulators support Dirac fermions with a single-valley band dispersion. In the presence of disorder they experience weak antilocalization, which has been observed in recent transport experiments. In this work we conduct a comparative theoretical study of the weak antilocalization in HgTe quantum wells and topological surface states. The difference between these two single-valley systems comes from a finite band gap (effective Dirac mass) in HgTe quantum wells in contrast to gapless (massless) surface states in topological insulators. The finite effective Dirac mass implies a broken internal symmetry, leading to suppression of the weak antilocalization in HgTe quantum wells at times larger than certain t_M, inversely proportional to the Dirac mass. This corresponds to the opening of a relaxation gap 1/t_M in the Cooperon diffusion mode which we obtain from the Bethe-Salpeter equation including relevant spin degrees of freedom. We demonstrate that the relaxation gap exhibits an interesting nonmonotonic dependence on both carrier density and band gap, vanishing at a certain combination of these parameters. The weak-antilocalization conductivity reflects this nonmonotonic behavior which is unique to HgTe QWs and absent for topological surface states. On the other hand, the topological surface states exhibit specific weak-antilocalization magnetoconductivity in a parallel magnetic field due to their exponential decay in the bulk.Comment: 14 pages, 10 figures, version as publishe