25,988 research outputs found
Observability of counterpropagating modes at fractional-quantum-Hall edges
When the bulk filling factor is equal to 1 - 1/m with m odd, at least one
counterpropagating chiral collective mode occurs simultaneously with
magnetoplasmons at the edge of fractional-quantum-Hall samples. Initial
experimental searches for an additional mode were unsuccessful. In this paper,
we address conditions under which its observation should be expected in
experiments where the electronic system is excited and probed by capacitive
coupling. We derive realistic expressions for the velocity of the slow
counterpropagating mode, starting from a microscopic calculation which is
simplified by a Landau-Silin-like separation between long-range Hartree and
residual interactions. The microscopic calculation determines the stiffness of
the edge to long-wavelength neutral excitations, which fixes the slow-mode
velocity, and the effective width of the edge region, which influences the
magnetoplasmon dispersion.Comment: 18 pages, RevTex, 6 figures, final version to be published in
Physical Review
Numerical Tests of the Chiral Luttinger Liquid Theory for Fractional Hall Edges
We report on microscopic numerical studies which support the chiral Luttinger
liquid theory of the fractional Hall edge proposed by Wen. Our calculations are
based in part on newly proposed and accurate many-body trial wavefunctions for
the low-energy edge excitations of fractional incompressible states.Comment: 12 pages + 1 figure, Revte
Nernst and Seebeck effect in a graphene nanoribbon
The thermoelectric power, including the Nernst and Seebeck effects, in
graphene nanoribbon is studied. By using the non-equilibrium Green function
combining with the tight-binding Hamiltonian, the Nernst and Seebeck
coefficients are obtained. Due to the electron-hole symmetry, the Nernst
coefficient is an even function of the Fermi energy while the Seebeck
coefficient is an odd function regardless of the magnetic field. In the
presence of a strong magnetic field, the Nernst and Seebeck coefficients are
almost independent of the chirality and width of the nanoribbon, and they show
peaks when the Fermi energy crosses the Landau levels. The height of -th
(excluding ) peak is for the Nernst effect and is
for the Seebeck effect. For the zeroth peak, it is abnormal with height
for the Nernst effect and the peak disappears for the Seebeck effect.
When the magnetic field is turned off, however, the Nernst effect is absent and
only Seebeck effect exists. In this case, the Seebeck coefficient strongly
depends on the chirality of the nanoribbon. The peaks are equidistant for the
nanoribbons with zigzag edge but are irregularly distributed for the armchair
edge. In particular, for the insulating armchair ribbon, the Seebeck
coefficient can be very large near the Dirac point. When the magnetic field
varies from zero to large values, the differences among the Seebeck
coefficients for different chiral ribbons gradually vanish and the nonzero
value of Nernst coefficient appears first near the Dirac point then gradually
extents to the whole energy region.Comment: 8 pages, 7 figure
Effect of Edge Roughness on Electronic Transport in Graphene Nanoribbon Channel Metal Oxide Semiconductor Field-Effect Transistors
Results of quantum mechanical simulations of the influence of edge disorder
on transport in graphene nanoribbon metal oxide semiconductor field-effect
transistors (MOSFETs) are reported. The addition of edge disorder significantly
reduces ON-state currents and increases OFF-state currents, and introduces wide
variability across devices. These effects decrease as ribbon widths increase
and as edges become smoother. However the bandgap decreases with increasing
width, thereby increasing the band-to-band tunneling mediated subthreshold
leakage current even with perfect nanoribbons. These results suggest that
without atomically precise edge control during fabrication, MOSFET performance
gains through use of graphene will be difficult to achieve.Comment: 8 pages, 5 figure
Seasonal Variation in 25(OH)D at Aberdeen (57°N) and Bone Health Indicators- Could Holidays in the Sun and Cod Liver Oil Supplements Alleviate Deficiency?
Vitamin D has been linked with many health outcomes. The aim of this longitudinal study, was to assess predictors of seasonal variation of 25-hydroxy-vitamin D (25(OH)D) (including use of supplements and holidays in sunny destinations) at a northerly latitude in the UK (57°N) in relation to bone health indicators. 365 healthy postmenopausal women (mean age 62.0 y (SD 1.4)) had 25(OH)D measurements by immunoassay, serum C-telopeptide (CTX), estimates of sunlight exposure (badges of polysulphone film), information regarding holidays in sunny destinations, and diet (from food diaries, including use of supplements such as cod liver oil (CLO)) at fixed 3-monthly intervals over 15 months (subject retention 88%) with an additional 25(OH)D assessment in spring 2008. Bone mineral density (BMD) at the lumbar spine (LS) and dual hip was measured in autumn 2006 and spring 2007 (Lunar I-DXA). Deficiency prevalence (25(OH)
Small scale irrigation using collector wells pilot project - Zimbabwe. Final report October 1992 - January 1996
Spin-dependent thermoelectric transport in HgTe/CdTe quantum wells
We analyze thermally induced spin and charge transport in HgTe/CdTe quantum
wells on the basis of the numerical non-equilibrium Green's function technique
in the linear response regime. In the topologically non-trivial regime, we find
a clear signature of the gap of the edge states due to their finite overlap
from opposite sample boundaries -- both in the charge Seebeck and spin Nernst
signal. We are able to fully understand the physical origin of the
thermoelectric transport signatures of edge and bulk states based on simple
analytical models. Interestingly, we derive that the spin Nernst signal is
related to the spin Hall conductance by a Mott-like relation which is exact to
all orders in the temperature difference between the warm and the cold
reservoir.Comment: 11 pages, 13 figures, submitted to PR
Description of Pairing correlation in Many-Body finite systems with density functional theory
Different steps leading to the new functional for pairing based on natural
orbitals and occupancies proposed in ref. [D. Lacroix and G. Hupin,
arXiv:1003.2860] are carefully analyzed. Properties of quasi-particle states
projected onto good particle number are first reviewed. These properties are
used (i) to prove the existence of such a functional (ii) to provide an
explicit functional through a 1/N expansion starting from the BCS approach
(iii) to give a compact form of the functional summing up all orders in the
expansion. The functional is benchmarked in the case of the picked fence
pairing Hamiltonian where even and odd systems, using blocking technique are
studied, at various particle number and coupling strength, with uniform and
random single-particle level spacing. In all cases, a very good agreement is
found with a deviation inferior to 1% compared to the exact energy.Comment: 14 pages, 9 figure
A Toy Model for Blandford-Znajek Mechanism
A toy model for the Blandford-Znajek mechanism is investigated: a Kerr black
hole with a toroidal electric current residing in a thin disk around the black
hole. The toroidal electric current generates a poloidal magnetic field
threading the black hole and disk. Due to the interaction of the magnetic field
with remote charged particles, the rotation of the black hole and disk induces
an electromotive force, which can power an astrophysical load at remote
distance. The power of the black hole and disk is calculated. It is found that,
for a wide range of parameters specifying the rotation of the black hole and
the distribution of the electric current in the disk, the power of the disk
exceeds the power of the black hole. The torque provided by the black hole and
disk is also calculated. The torque of the disk is comparable to the torque of
the black hole. As the disk loses its angular momentum, the mass of the disk
gradually drifts towards the black hole and gets accreted. Ultimately the power
comes from the gravitational binding energy between the disk and the black
hole, as in the standard theory of accretion disk, instead of the rotational
energy of the black hole. This suggests that the Blandford-Znajek mechanism may
be less efficient in extracting energy from a rotating black hole with a thin
disk. The limitations of our simple model and possible improvements deserved
for future work are also discussed.Comment: 16 pages, 4 figures. Accepted for publication in Physical Review
Magnons and skyrmions in fractional Hall ferromagnets
Recent experiments have established a qualitative difference between the
magnetization temperature-dependences of quantum Hall ferromagnets at
integer and fractional filling factors. We explain this difference in terms of
the relative energies of collective magnon and particle-hole excitations in the
two cases. Analytic calculations for hard-core model systems are used to
demonstrate that, in the fractional case, interactions suppress the
magnetization at finite temperatures and that particle-hole excitations rather
than long-wavelength magnons control at low .Comment: 4 pages, no figure
- âŠ