4,101 research outputs found
Absence of conventional quantum phase transitions in itinerant systems with disorder
Effects of disorder are examined in itinerant systems close to quantum
critical points. We argue that spin fluctuations associated with the long-range
part of the RKKY interactions generically induce non-Ohmic dissipation due to
rare disorder configurations. This dissipative mechanism is found to
destabilize quantum Griffiths phase behavior in itinerant systems with
arbitrary symmetry of the order parameter, leading to the formation of a
"cluster glass" phase preceding uniform ordering.Comment: 4+epsilon pages, 1 figure. Phys. Rev. Lett., in press (2005
Renormalization group theory of nematic ordering in d-wave superconductors
We examine the quantum theory of the spontaneous breaking of lattice rotation
symmetry in d-wave superconductors on the square lattice. This is described by
a field theory of an Ising nematic order parameter coupled to the gapless
fermionic quasiparticles. We determine the structure of the renormalization
group to all orders in a 1/N_f expansion, where N_f is the number of fermion
spin components. Asymptotically exact results are obtained for the quantum
critical theory in which, as in the large N_f theory, the nematic order has a
large anomalous dimension, and the fermion spectral functions are highly
anisotropic.Comment: 17 pages, 8 figure
Quench dynamics across quantum critical points
We study the quantum dynamics of a number of model systems as their coupling
constants are changed rapidly across a quantum critical point. The primary
motivation is provided by the recent experiments of Greiner et al. (Nature 415,
39 (2002)) who studied the response of a Mott insulator of ultracold atoms in
an optical lattice to a strong potential gradient. In a previous work
(cond-mat/0205169), it had been argued that the resonant response observed at a
critical potential gradient could be understood by proximity to an Ising
quantum critical point describing the onset of density wave order. Here we
obtain numerical results on the evolution of the density wave order as the
potential gradient is scanned across the quantum critical point. This is
supplemented by studies of the integrable quantum Ising spin chain in a
transverse field, where we obtain exact results for the evolution of the Ising
order correlations under a time-dependent transverse field. We also study the
evolution of transverse superfluid order in the three dimensional case. In all
cases, the order parameter is best enhanced in the vicinity of the quantum
critical point.Comment: 10 pages, 6 figure
Holographic Quantum Critical Transport without Self-Duality
We describe general features of frequency-dependent charge transport near
strongly interacting quantum critical points in 2+1 dimensions. The simplest
description using the AdS/CFT correspondence leads to a self-dual
Einstein-Maxwell theory on AdS_4, which fixes the conductivity at a
frequency-independent self-dual value. We describe the general structure of
higher-derivative corrections to the Einstein-Maxwell theory, and compute their
implications for the frequency dependence of the quantum-critical conductivity.
We show that physical consistency conditions on the higher-derivative terms
allow only a limited frequency dependence in the conductivity. The frequency
dependence is amenable to a physical interpretation using transport of either
particle-like or vortex-like excitations.Comment: 42 pages, 7 figures. A new figure showing the frequency dependence of
EM dual conductivity and few references added. Abstract, introduction,
section 5 and discussion extended. To appear in Phys.Rev.
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
A model of a Fermi liquid using gauge-gravity duality
We use gauge-gravity duality to model the crossover from a conformal critical
point to a confining Fermi liquid, driven by a change in fermion density. The
short-distance conformal physics is represented by an anti-de Sitter geometry,
which terminates into a confining state along the emergent spatial direction.
The Luttinger relation, relating the area enclosed by the Fermi surfaces to the
fermion density, is shown to follow from Gauss's Law for the bulk electric
field. We argue that all low energy modes are consistent with Landau's Fermi
liquid theory. An explicit solution is obtained for the Fermi liquid for the
case of hard-wall boundary conditions in the infrared.Comment: 14 pages, 3 figures; (v2) added clarification
Hall conductivity from dyonic black holes
A class of strongly interacting 2+1 dimensional conformal field theories in a
transverse magnetic field can be studied using the AdS/CFT duality. We compute
zero momentum hydrodynamic response functions of maximally supersymmetric 2+1
dimensional SU(N) Yang-Mills theory at the conformal fixed point, in the large
N limit. With background magnetic field B and electric charge density rho, the
Hall conductivity is found to be rho/B. The result, anticipated on kinematic
grounds in field theory, is obtained from perturbations of a four dimensional
AdS black hole with both electric and magnetic charges.Comment: 1+13 pages. TT correlator corrected. Typos corrected and added ref
Compressible quantum phases from conformal field theories in 2+1 dimensions
Conformal field theories (CFTs) with a globally conserved U(1) charge Q can
be deformed into compressible phases by modifying their Hamiltonian, H, by a
chemical potential H -> H - \mu Q. We study 2+1 dimensional CFTs upon which an
explicit S duality mapping can be performed. We find that this construction
leads naturally to compressible phases which are superfluids, solids, or
non-Fermi liquids which are more appropriately called `Bose metals' in the
present context. The Bose metal preserves all symmetries and has Fermi surfaces
of gauge-charged fermions, even in cases where the parent CFT can be expressed
solely by bosonic degrees of freedom. Monopole operators are identified as
order parameters of the solid, and the product of their magnetic charge and Q
determines the area of the unit cell. We discuss implications for holographic
theories on asymptotically AdS4 spacetimes: S duality and monopole/dyon fields
play important roles in this connection.Comment: 30 pages, 2 figures; (v2) small corrections and more ref
Decoherence in the dynamical quantum phase transition of the transverse Ising chain
For the prototypical example of the Ising chain in a transverse field, we
study the impact of decoherence on the sweep through a second-order quantum
phase transition. Apart from the advance in the general understanding of the
dynamics of quantum phase transitions, these findings are relevant for
adiabatic quantum algorithms due to the similarities between them. It turns out
that (in contrast to first-order transitions studied previously) the impact of
decoherence caused by a weak coupling to a rather general environment increases
with system size (i.e., number of spins/qubits), which might limit the
scalability of the system.Comment: 4 pages, 1 figure, minor clarification
Decay of Correlations in Fermi Systems at Non-zero Temperature
The locality of correlation functions is considered for Fermi systems at
non-zero temperature. We show that for all short-range, lattice Hamiltonians,
the correlation function of any two fermionic operators decays exponentially
with a correlation length which is of order the inverse temperature for small
temperature. We discuss applications to numerical simulation of quantum systems
at non-zero temperature.Comment: 3 pages, 0 figure
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