4,047 research outputs found
Quantum critical transport, duality, and M-theory
We consider charge transport properties of 2+1 dimensional conformal field
theories at non-zero temperature. For theories with only Abelian U(1) charges,
we describe the action of particle-vortex duality on the
hydrodynamic-to-collisionless crossover function: this leads to powerful
functional constraints for self-dual theories. For the n=8 supersymmetric,
SU(N) Yang-Mills theory at the conformal fixed point, exact
hydrodynamic-to-collisionless crossover functions of the SO(8) R-currents can
be obtained in the large N limit by applying the AdS/CFT correspondence to
M-theory. In the gravity theory, fluctuating currents are mapped to fluctuating
gauge fields in the background of a black hole in 3+1 dimensional anti-de
Sitter space. The electromagnetic self-duality of the 3+1 dimensional theory
implies that the correlators of the R-currents obey a functional constraint
similar to that found from particle-vortex duality in 2+1 dimensional Abelian
theories. Thus the 2+1 dimensional, superconformal Yang Mills theory obeys a
"holographic self duality" in the large N limit, and perhaps more generally.Comment: 35 pages, 4 figures; (v2) New appendix on CFT2, corrected
normalization of gauge field action, added ref
Numerical evidence for the spin-Peierls state in the frustrated quantum antiferromagnet
We study the spin- Heisenberg antiferromagnet with an
antiferromagnetic (third nearest neighbor) interaction on a square
lattice. We numerically diagonalize this ``-'' model on clusters up
to 32-sites and search for novel ground state properties as the frustration
parameter changes. For ``larger'' we find enhancement of
incommensurate spin order, in agreement with spin-wave, large- expansions,
and other predictions. But for intermediate , the low lying excitation
energy spectrum suggests that this incommensurate order is short-range. In the
same region, the first excited state has the symmetries of the columnar dimer
(spin-Peierls) state. The columnar dimer order parameter suggests the presence
of long-range columnar dimer order. Hence, this spin-Peierls state is the best
candidate for the ground state of the - model in an intermediate
region.Comment: RevTeX file with five postscript figures uuencode
Quantum phase transitions in bilayer SU(N) anti-ferromagnets
We present a detailed study of the destruction of SU(N) magnetic order in
square lattice bilayer anti-ferromagnets using unbiased quantum Monte Carlo
numerical simulations and field theoretic techniques. We study phase
transitions from an SU(N) N\'eel state into two distinct quantum disordered
"valence-bond" phases: a valence-bond liquid (VBL) with no broken symmetries
and a lattice-symmetry breaking valence-bond solid (VBS) state. For finite
inter-layer coupling, the cancellation of Berry phases between the layers has
dramatic consequences on the two phase transitions: the N\'eel-VBS transition
is first order for all accesible in our model, whereas the N\'eel-VBL
transition is continuous for N=2 and first order for N>= 4; for N=3 the
N\'eel-VBL transition show no signs of first-order behavior
Evolution of the single-hole spectral function across a quantum phase transition in the anisotropic-triangular-lattice antiferromagnet
We study the evolution of the single-hole spectral function when the ground
state of the anisotropic-triangular-lattice antiferromagnet changes from the
incommensurate magnetically-ordered phase to the spin-liquid state. In order to
describe both of the ground states on equal footing, we use the large-N
approach where the transition between these two phases can be obtained by
controlling the quantum fluctuations via an 'effective' spin magnitude. Adding
a hole into these ground states is described by a t-J type model in the
slave-fermion representation. Implications of our results to possible future
ARPES experiments on insulating frustrated magnets, especially CsCuCl,
are discussed.Comment: 8 pages, 7 figure
APOE genotype and entorhinal cortex volume in non-demented community-dwelling adults in midlife and early old age
Copyright Ā© 2012 IOS PressThis article has been made available through the Brunel Open Access Publishing Fund.The apolipoprotein E (APOE) Īµ4 allele is a risk factor for the neuropathological decline accompanying Alzheimer's disease (AD) while, conversely, the Īµ2 allele offers protection. One of the brain structures exhibiting the earliest changes associated with the disease is the entorhinal cortex. We therefore investigated the volumes of the entorhinal cortex and other structures in the medial temporal lobe including the parahippocampal gyrus, temporal pole, and inferior, middle, and superior temporal cortices, in relation to APOE genotype. Our main objectives were to determine if (a) volumes systematically varied according to allele in a stepwise fashion, Īµ2 > Īµ3 > Īµ4, and (b) associations varied according to age. We investigate this association in 627 non-demented community-dwelling adults in middle age (44 to 48 years; n = 314) and older age (64 to 68 years; n = 313) who underwent structural MRI scans. We found no evidence of APOE-related variation in brain volumes in the age groups examined. We conclude that if a Īµ2 > Īµ3 > Īµ4 pattern in brain volumes does emerge in non-demented adults living in the community in old age, it is not until after the age of 68 years.This study was funded by the UK Leverhulme
Trust, the British Academy, the NHMRC
Research Fellowship No. 471501, the NHMRC Research Fellowship No.#1002560, the National Health and Medical Research Council of Australia Unit Grant No. 973302, Program Grant No. 179805, Project grant No. 157125; Program grant no. 350833, and the National Computational Infrastructure. This article is made available through the Brunel Open Access Publishing Fund
Fractionalized Fermi liquids
In spatial dimensions d >= 2, Kondo lattice models of conduction and local
moment electrons can exhibit a fractionalized, non-magnetic state (FL*) with a
Fermi surface of sharp electron-like quasiparticles, enclosing a volume
quantized by (\rho_a-1)(mod 2), with \rho_a the mean number of all electrons
per unit cell of the ground state. Such states have fractionalized excitations
linked to the deconfined phase of a gauge theory. Confinement leads to a
conventional Fermi liquid state, with a Fermi volume quantized by \rho_a (mod
2), and an intermediate superconducting state for the Z_2 gauge case. The FL*
state permits a second order metamagnetic transition in an applied magnetic
field.Comment: 4 pages, 1 figure; (v2) changed title and terminology, but content
largely unchanged; (v3) updated version to appear in PR
Transport anomalies in a simplified model for a heavy electron quantum critical point
We discuss the transport anomalies associated with the development of heavy
electrons out of a neutral spin fluid using the large-N treatment of the
Kondo-Heisenberg lattice model. At the phase transition in this model the spin
excitations suddenly acquire charge. The Higgs process by which this takes
place causes the constraint gauge field to loosely ``lock'' together with the
external, electromagnetic gauge field. From this perspective, the heavy fermion
phase is a Meissner phase in which the field representing the difference
between the electromagnetic and constraint gauge field, is excluded from the
material. We show that at the transition into the heavy fermion phase, both the
linear and the Hall conductivity jump together. However, the Drude weight of
the heavy electron fluid does not jump at the quantum critical point, but
instead grows linearly with the distance from the quantum critical point,
forming a kind of ``gossamer'' Fermi-liquid.Comment: 15 pages, 3 figures. Small change in references in v
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