On Random Field Induced Ordering in the Classical XY Model

Consider the classical XY model in a weak random external field pointing along the $Y$ axis with strength $\epsilon$. We study the behavior of this model as the range of the interaction is varied. We prove that in any dimension $d \geq 2$ and for all $\epsilon$ sufficiently small, there is a range $L=L(\epsilon)$ so that whenever the inverse temperature $\beta$ is larger than some $\beta(\epsilon)$, there is strong residual ordering along the $X$ direction.Comment: 30 page

Transition to Superfluid Turbulence

Turbulence in superfluids depends crucially on the dissipative damping in vortex motion. This is observed in the B phase of superfluid 3He where the dynamics of quantized vortices changes radically in character as a function of temperature. An abrupt transition to turbulence is the most peculiar consequence. As distinct from viscous hydrodynamics, this transition to turbulence is not governed by the velocity-dependent Reynolds number, but by a velocity-independent dimensionless parameter 1/q which depends only on the temperature-dependent mutual friction -- the dissipation which sets in when vortices move with respect to the normal excitations of the liquid. At large friction and small values of 1/q < 1 the dynamics is vortex number conserving, while at low friction and large 1/q > 1 vortices are easily destabilized and proliferate in number. A new measuring technique was employed to identify this hydrodynamic transition: the injection of a tight bundle of many small vortex loops in applied vortex-free flow at relatively high velocities. These vortices are ejected from a vortex sheet covering the AB interface when a two-phase sample of 3He-A and 3He-B is set in rotation and the interface becomes unstable at a critical rotation velocity, triggered by the superfluid Kelvin-Helmholtz instability.Comment: Short review; to be published in Journal of Low Temperature Physics (2006

On Slow Light as a Black Hole Analogue

Although slow light (electromagnetically induced transparency) would seem an ideal medium in which to institute a ``dumb hole'' (black hole analog), it suffers from a number of problems. We show that the high phase velocity in the slow light regime ensures that the system cannot be used as an analog displaying Hawking radiation. Even though an appropriately designed slow-light set-up may simulate classical features of black holes -- such as horizon, mode mixing, Bogoliubov coefficients, etc. -- it does not reproduce the related quantum effects. PACS: 04.70.Dy, 04.80.-y, 42.50.Gy, 04.60.-m.Comment: 14 pages RevTeX, 5 figure

Evanescent wave transport and shot noise in graphene: ballistic regime and effect of disorder

We have investigated electrical transport and shot noise in graphene field effect devices. In large width over length ratio $W/L$ graphene strips, we have measured shot noise at low frequency ($f$ = 600--850 MHz) in the temperature range of 4.2--30 K. We observe a minimum conductivity of $\frac{4e^{2}}{\pi h}$ and a finite and gate dependent Fano factor reaching the universal value of 1/3 at the Dirac point, i.e. where the density of states vanishes. These findings are in good agreement with the theory describing that transport at the Dirac point should occur via evanescent waves in perfect graphene samples with large $W/L$. Moreover, we show and discuss how disorder and non-parallel leads affect both conductivity and shot noise.Comment: Extended version (19 pages, 10 figures) of Phys. Rev. Lett. 100, 196802 (2008). Additional data on the effect of disorder and non-parallel leads. Submitted for publication in Journal of Low Temperature Physics for the Proceedings of the International Symposium on Quantum Phenomena and Devices at Low Temperatures (ULTI 2008), Espoo, Finlan

Electronic and magnetic properties of SiC nanoribbons by F termination

By using the first-principles calculations, the electronic properties are studied for the F-terminated SiC nanoribbons (SiCNRs) with either zigzag edges (ZSiCNRs) or armchair edges (ASiCNRs). The results show that the broader F-terminated ZSiCNRs are metallic and the edge states appear at the Fermi level, while the F-terminated ASiCNRs are always semiconductors independent of their width but the edge states do not appear due to the Si-C dimer bonds at the edges. The charge density contours analyses shows that the Si-F and Si-C bonds are all ionic bonds due to the much stronger electronegativities of the F and C atoms than that of the Si atom. However, the C-F bonds display a typical non-polar covalent bonding feature because of the electronegativity difference between the F and C atoms of 1.5 is a much smaller than that of between the F and Si atoms of 2.2, as well as the tighter bounded C 2s22p2 electrons with smaller orbital radius than the Si 3s23p2 electrons. For both the F- and the H-terminated ZSiCNRs, the ground state is a ferromagnetic semiconductor

Edge states and distributions of edge currents in semi-infinite graphene

The distributions of edge currents in semi-infinite graphene under a uniform perpendicular magnetic field are investigated. We show unambiguously that the edge current is finite at the armchair edge but vanishes at the zigzag edge. It is shown that the current density oscillates with the distance away from the boundary and tends to zero deep inside the graphene. The study shows that the total current is independent of edge configurations. The interplay of the bulk and edge contributions to the total current is presented. The quantized plateaus of Hall conductivity at (4e (2)/h)(n+1/2) provide a direct evidence of the connection between the edge states and topological properties of relativistic fermions in a magnetic field.Physics, Condensed MatterSCI(E)EI0ARTICLE4431-4398