4,217 research outputs found
Are direct search experiments sensitive to all spin-independent WIMP candidates?
The common analysis of direct searches for spin-independent Weakly
Interacting Massive Particles (WIMPs) assumes that a spin-independent WIMP
couples with the same strength with both nucleons, \textit{i.e.} that the
spin-independent interaction is also fully isospin-independent. Though in a
fully isospin-dependent interaction scenario the spin-independent WIMP-nucleus
cross section is strongly quenched, the leading experiments are still sensitive
enough to set limits 1-2 orders of magnitude less stringent than those
traditionally presented. In the isospin-dependent scenario the difference
between the limits of CDMS-II and ZEPLIN-I is significantly reduced. Here, a
model-independent framework is discussed and applied to obtain the current
general model-independent limits.Comment: 4 pages, 4 figures, revtex4.0, submitted to Phys. Rev. Let
Rigorous construction of ground state correlations in graphene: renormalization of the velocities and Ward Identities
We consider the 2D Hubbard model on the honeycomb lattice, as a model for
single layer graphene with screened Coulomb interactions; at half filling and
weak coupling, we construct its ground state correlations by a convergent
multiscale expansion, rigorously excluding the presence of magnetic or
superconducting instabilities or the formation of a mass gap. The Fermi
velocity, which can be written in terms of a convergent series expansion,
remains close to its non-interacting value and turns out to be isotropic. On
the contrary, the interaction produces an asymmetry between the two components
of the charge velocity, in contrast with the predictions based on relativistic
or continuum approximations.Comment: 4 pages, 1 figure; version published on Phys. Rev. B; erratum adde
Fermi liquid behavior in the 2D Hubbard model at low temperatures
We prove that the weak coupling 2D Hubbard model away from half filling is a
Landau Fermi liquid up to exponentially small temperatures. In particular we
show that the wave function renormalization is an order 1 constant and
essentially temperature independent in the considered range of temperatures and
that the interacting Fermi surface is a regular convex curve. This result is
obtained by deriving a convergent expansion (which is not a power series) for
the two point Schwinger function by Renormalization Group methods and proving
at each order suitable power counting improvements due to the convexity of the
interacting Fermi surface. Convergence follows from determinant bounds for the
fermionic expectations.Comment: 66 pages, 10 figure
Spin polarization and g-factor enhancement in graphene nanoribbons in magnetic field
We provide a systematic quantitative description of spin polarization in
armchair and zigzag graphene nanoribbons in a perpendicular magnetic field. We
first address spinless electrons within the Hartree approximation studying the
evolution of the magnetoband structure and formation of the compressible
strips. We discuss the potential profile and the density distribution near the
edges and the difference and similarities between armchair and zigzag edges.
Accounting for the Zeeman interaction and describing the spin effects via the
Hubbard term we study the spin-resolved subband structure and relate the spin
polarization of the system at hand to the formation of the compressible strips
for the case of spinless electrons. At high magnetic field the calculated
effective g-factor varies around a value of ~2.25 for armchair nanoribbons
and ~3 for zigzag nanoribbons. An important finding is that in zigzag
nanoribbons the zero-energy mode remains pinned to the Fermi-energy and becomes
fully spin-polarized for all magnetic fields, which, in turn, leads to a strong
spin polarization of the electron density near the zigzag edge.Comment: 9 pages, 4 figure
Universal finite size corrections and the central charge in non solvable Ising models
We investigate a non solvable two-dimensional ferromagnetic Ising model with
nearest neighbor plus weak finite range interactions of strength \lambda. We
rigorously establish one of the predictions of Conformal Field Theory (CFT),
namely the fact that at the critical temperature the finite size corrections to
the free energy are universal, in the sense that they are exactly independent
of the interaction. The corresponding central charge, defined in terms of the
coefficient of the first subleading term to the free energy, as proposed by
Affleck and Blote-Cardy-Nightingale, is constant and equal to 1/2 for all
0<\lambda<\lambda_0 and \lambda_0 a small but finite convergence radius. This
is one of the very few cases where the predictions of CFT can be rigorously
verified starting from a microscopic non solvable statistical model. The proof
uses a combination of rigorous renormalization group methods with a novel
partition function inequality, valid for ferromagnetic interactions.Comment: 43 pages, 1 figur
Universal conductivity and dimensional crossover in multi-layer graphene
We show, by exact Renormalization Group methods, that in multi-layer graphene
the dimensional crossover energy scale is decreased by the intra-layer
interaction, and that for temperatures and frequencies greater than such scale
the conductivity is close to the one of a stack of independent layers up to
small corrections
Heavy Superheated Droplet Detectors as a Probe of Spin-independent WIMP Dark Matter Existence
At present, application of Superheated Droplet Detectors (SDDs) in WIMP dark
matter searches has been limited to the spin-dependent sector, owing to the
general use of fluorinated refrigerants which have high spin sensitivity. Given
their recent demonstration of a significant constraint capability with
relatively small exposures and the relative economy of the technique, we
consider the potential impact of heavy versions of such devices on the
spin-independent sector. Limits obtainable from a -loaded SDD
are estimated on the basis of the radiopurity levels and backgrounds already
achieved by the SIMPLE and PICASSO experiments. With 34 kgd exposure,
equivalent to the current CDMS, such a device may already probe to below
10 pb in the spin-independent cross section.Comment: 9 pages, 4 figures, accepted Phys. Rev.
Response properties of III-V dilute magnetic semiconductors: interplay of disorder, dynamical electron-electron interactions and band-structure effects
A theory of the electronic response in spin and charge disordered media is
developed with the particular aim to describe III-V dilute magnetic
semiconductors like GaMnAs. The theory combines a detailed k.p description of
the valence band, in which the itinerant carriers are assumed to reside, with
first-principles calculations of disorder contributions using an
equation-of-motion approach for the current response function. A fully dynamic
treatment of electron-electron interaction is achieved by means of
time-dependent density functional theory. It is found that collective
excitations within the valence band significantly increase the carrier
relaxation rate by providing effective channels for momentum relaxation. This
modification of the relaxation rate, however, only has a minor impact on the
infrared optical conductivity in GaMnAs, which is mostly determined by the
details of the valence band structure and found to be in agreement with
experiment.Comment: 15 pages, 9 figure
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