21,672 research outputs found
Hard thermal loops in static background fields
We discuss the high temperature behavior of retarded thermal loops in static
external fields. We employ an analytic continuation of the imaginary time
formalism and use a spectral representation of the thermal amplitudes. We show
that, to all orders, the leading contributions of static hard thermal loops can
be directly obtained by evaluating them at zero external energies and momenta.Comment: 5 pages, to be published in The European Physical Journal
Charge distribution and screening in layered graphene systems
The charge distribution induced by external fields in finite stacks of
graphene planes, or in semiinfinite graphite is considered. The interlayer
electronic hybridization is described by a nearest neighbor hopping term, and
the charge induced by the self consistent electrostatic potential is calculated
within linear response (RPA). The screening properties are determined by
contributions from inter- and intraband electronic transitions. In neutral
systems, only interband transitions contribute to the charge polarizability,
leading to insulating-like screening properties, and to oscillations in the
induced charge, with a period equal to the interlayer spacing. In doped
systems, we find a screening length equivalent to 2-3 graphene layers,
superimposed to significant charge oscillations.Comment: 8 page
Quantum Spin Hall Effect in Graphene
We study the effects of spin orbit interactions on the low energy electronic
structure of a single plane of graphene. We find that in an experimentally
accessible low temperature regime the symmetry allowed spin orbit potential
converts graphene from an ideal two dimensional semimetallic state to a quantum
spin Hall insulator. This novel electronic state of matter is gapped in the
bulk and supports the quantized transport of spin and charge in gapless edge
states that propagate at the sample boundaries. The edge states are non chiral,
but they are insensitive to disorder because their directionality is correlated
with spin. The spin and charge conductances in these edge states are calculated
and the effects of temperature, chemical potential, Rashba coupling, disorder
and symmetry breaking fields are discussed.Comment: 4 pages, published versio
Supermetallic conductivity in bromine-intercalated graphite
Exposure of highly oriented pyrolytic graphite to bromine vapor gives rise to
in-plane charge conductivities which increase monotonically with intercalation
time toward values (for ~6 at% Br) that are significantly higher than Cu at
temperatures down to 5 K. Magnetotransport, optical reflectivity and magnetic
susceptibility measurements confirm that the Br dopes the graphene sheets with
holes while simultaneously increasing the interplanar separation. The increase
of mobility (~ 5E4 cm^2/Vs at T=300 K) and resistance anisotropy together with
the reduced diamagnetic susceptibility of the intercalated samples suggests
that the observed supermetallic conductivity derives from a parallel
combination of weakly-coupled hole-doped graphene sheets.Comment: 5 pages, 4 figure
Modern Observational Techniques for Comets
Techniques are discussed in the following areas: astrometry, photometry, infrared observations, radio observations, spectroscopy, imaging of coma and tail, image processing of observation. The determination of the chemical composition and physical structure of comets is highlighted
Analytic Solution for the Critical State in Superconducting Elliptic Films
A thin superconductor platelet with elliptic shape in a perpendicular
magnetic field is considered. Using a method originally applied to circular
disks, we obtain an approximate analytic solution for the two-dimensional
critical state of this ellipse. In the limits of the circular disk and the long
strip this solution is exact, i.e. the current density is constant in the
region penetrated by flux. For ellipses with arbitrary axis ratio the obtained
current density is constant to typically 0.001, and the magnetic moment
deviates by less than 0.001 from the exact value. This analytic solution is
thus very accurate. In increasing applied magnetic field, the penetrating flux
fronts are approximately concentric ellipses whose axis ratio b/a < 1 decreases
and shrinks to zero when the flux front reaches the center, the long axis
staying finite in the fully penetrated state. Analytic expressions for these
axes, the sheet current, the magnetic moment, and the perpendicular magnetic
field are presented and discussed. This solution applies also to
superconductors with anisotropic critical current if the anisotropy has a
particular, rather realistic form.Comment: Revtex file and 13 postscript figures, gives 10 pages of text with
figures built i
Resummation of infrared divergences in the free-energy of spin-two fields
We derive a closed form expression for the sum of all the infrared divergent
contributions to the free-energy of a gas of gravitons. An important ingredient
of our calculation is the use of a gauge fixing procedure such that the
graviton propagator becomes both traceless and transverse. This has been shown
to be possible, in a previous work, using a general gauge fixing procedure, in
the context of the lowest order expansion of the Einstein-Hilbert action,
describing non-interacting spin two fields. In order to encompass the problems
involving thermal loops, such as the resummation of the free-energy, in the
present work, we have extended this procedure to the situations when the
interactions are taken into account.Comment: 12 pages, 25 figure
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