1,160 research outputs found
Vortex Glass is a Metal: Unified Theory of the Magnetic Field and Disorder-Tuned Bose Metals
We consider the disordered quantum rotor model in the presence of a magnetic
field. We analyze the transport properties in the vicinity of the multicritical
point between the superconductor, phase glass and paramagnetic phases. We find
that the magnetic field leaves metallic transport of bosons in the glassy phase
in tact. In the vicinity of the vicinity of the superconductivity-to-Bose metal
transition, the resistitivy turns on as with . This
functional form is in excellent agreement with the experimentally observed
turn-on of the resistivity in the metallic state in MoGe, namely , . The metallic state is also shown to presist in
three spatial dimensions. In addition, we also show that the metallic state
remains intact in the presence of Ohmic dissipation in spite of recent claims
to the contrary. As the phase glass in is identical to the vortex glass,
we conclude that the vortex glass is, in actuality, a metal rather than a
superconductor at T=0. Our analysis unifies the recent experiments on vortex
glass systems in which the linear resistivity remained non-zero below the
putative vortex glass transition and the experiments on thin films in which a
metallic phase has been observed to disrupt the direct transition from a
superconductor to an insulator.Comment: Published version with an appendix showing that the claim in
cond-mat/0510380 (and cond-mat/0606522) that Ohmic dissipation in the phase
glass leads to a superconducting state is false. A metal persists in this
case as wel
On the sign of kurtosis near the QCD critical point
We point out that the quartic cumulant (and kurtosis) of the order parameter
fluctuations is universally negative when the critical point is approached on
the crossover side of the phase separation line. As a consequence, the kurtosis
of a fluctuating observable, such as, e.g., proton multiplicity, may become
smaller than the value given by independent Poisson statistics. We discuss
implications for the Beam Energy Scan program at RHIC.Comment: 4 pages, 2 figure
Effective Field Theory for the Quantum Electrodynamics of a Graphene Wire
We study the low-energy quantum electrodynamics of electrons and holes, in a
thin graphene wire. We develop an effective field theory (EFT) based on an
expansion in p/p_T, where p_T is the typical momentum of electrons and holes in
the transverse direction, while p are the momenta in the longitudinal
direction. We show that, to the lowest-order in (p/p_T), our EFT theory is
formally equivalent to the exactly solvable Schwinger model. By exploiting such
an analogy, we find that the ground state of the quantum wire contains a
condensate of electron-hole pairs. The excitation spectrum is saturated by
electron-hole collective bound-states, and we calculate the dispersion law of
such modes. We also compute the DC conductivity per unit length at zero
chemical potential and find g_s =e^2/h, where g_s=4 is the degeneracy factor.Comment: 7 pages, 2 figures. Definitive version, accepted for publication on
Phys. Rev.
The Renormalization Group and the Superconducting Susceptibility of a Fermi Liquid
A free Fermi gas has, famously, a superconducting susceptibility that
diverges logarithmically at zero temperature. In this paper we ask whether this
is still true for a Fermi liquid and find that the answer is that it does {\it
not}. From the perspective of the renormalization group for interacting
fermions, the question arises because a repulsive interaction in the Cooper
channel is a marginally irrelevant operator at the Fermi liquid fixed point and
thus is also expected to infect various physical quantities with logarithms.
Somewhat surprisingly, at least from the renormalization group viewpoint, the
result for the superconducting susceptibility is that two logarithms are not
better than one. In the course of this investigation we derive a
Callan-Symanzik equation for the repulsive Fermi liquid using the
momentum-shell renormalization group, and use it to compute the long-wavelength
behavior of the superconducting correlation function in the emergent low-energy
theory. We expect this technique to be of broader interest.Comment: 9 pages, 2 figure
Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point
We consider a three-dimensional quantum antiferromagnet in the vicinity of a
quantum critical point separating the magnetically ordered and the magnetically
disordered phases. A specific example is TlCuCl where the quantum phase
transition can be driven by hydrostatic pressure and/or by external magnetic
field. As expected two transverse and one longitudinal magnetic excitation have
been observed in the pressure driven magnetically ordered phase. According to
the experimental data, the longitudinal magnon has a substantial width, which
has not been understood and has remained a puzzle. In the present work, we
explain the mechanism for the width, calculate the width and relate value of
the width with parameters of the Bose condensate of magnons observed in the
same compound. The method of an effective quantum field theory is employed in
the work.Comment: 6 pages, 3 figure
Quantum critical scaling behavior of deconfined spinons
We perform a renormalization group analysis of some important effective field
theoretic models for deconfined spinons. We show that deconfined spinons are
critical for an isotropic SU(N) Heisenberg antiferromagnet, if is large
enough. We argue that nonperturbatively this result should persist down to N=2
and provide further evidence for the so called deconfined quantum criticality
scenario. Deconfined spinons are also shown to be critical for the case
describing a transition between quantum spin nematic and dimerized phases. On
the other hand, the deconfined quantum criticality scenario is shown to fail
for a class of easy-plane models. For the cases where deconfined quantum
criticality occurs, we calculate the critical exponent for the decay of
the two-spin correlation function to first-order in . We also
note the scaling relation connecting the exponent
for the decay to the correlation length exponent and the crossover
exponent .Comment: 4.1 pages, no figures, references added; Version accepted for
publication in PRB (RC
Conformal invariance in three-dimensional rotating turbulence
We examine three--dimensional turbulent flows in the presence of solid-body
rotation and helical forcing in the framework of stochastic Schramm-L\"owner
evolution curves (SLE). The data stems from a run on a grid of points,
with Reynolds and Rossby numbers of respectively 5100 and 0.06. We average the
parallel component of the vorticity in the direction parallel to that of
rotation, and examine the resulting field for
scaling properties of its zero-value contours. We find for the first time for
three-dimensional fluid turbulence evidence of nodal curves being conformal
invariant, belonging to a SLE class with associated Brownian diffusivity
. SLE behavior is related to the self-similarity of the
direct cascade of energy to small scales in this flow, and to the partial
bi-dimensionalization of the flow because of rotation. We recover the value of
with a heuristic argument and show that this value is consistent with
several non-trivial SLE predictions.Comment: 4 pages, 3 figures, submitted to PR
Critical exponents of the O(N) model in the infrared limit from functional renormalization
We determined the critical exponent of the scalar O(N) model with a
strategy based on the definition of the correlation length in the infrared
limit. The functional renormalization group treatment of the model shows that
there is an infrared fixed point in the broken phase. The appearing degeneracy
induces a dynamical length scale there, which can be considered as the
correlation length. It is shown that the IR scaling behavior can account either
for the Ising type phase transition in the 3-dimensional O(N) model, or for the
Kosterlitz-Thouless type scaling of the 2-dimensional O(2) model.Comment: final version, 7 pages 7 figures, to appear in Phys. Rev.
Spin dynamics across the superfluid-insulator transition of spinful bosons
Bosons with non-zero spin exhibit a rich variety of superfluid and insulating
phases. Most phases support coherent spin oscillations, which have been the
focus of numerous recent experiments. These spin oscillations are Rabi
oscillations between discrete levels deep in the insulator, while deep in the
superfluid they can be oscillations in the orientation of a spinful condensate.
We describe the evolution of spin oscillations across the superfluid-insulator
quantum phase transition. For transitions with an order parameter carrying
spin, the damping of such oscillations is determined by the scaling dimension
of the composite spin operator. For transitions with a spinless order parameter
and gapped spin excitations, we demonstrate that the damping is determined by
an associated quantum impurity problem of a localized spin excitation
interacting with the bulk critical modes. We present a renormalization group
analysis of the quantum impurity problem, and discuss the relationship of our
results to experiments on ultracold atoms in optical lattices.Comment: 43 pages (single-column format), 8 figures; v2: corrected discussion
of fixed points in Section V
Galilean invariance and homogeneous anisotropic randomly stirred flows
The Ward-Takahashi (WT) identities for incompressible flow implied by
Galilean invariance are derived for the randomly forced Navier-Stokes equation
(NSE), in which both the mean and fluctuating velocity components are
explicitly present. The consequences of Galilean invariance for the vertex
renormalization are drawn from this identity.Comment: REVTeX 4, 4 pages, no figures. To appear as a Brief Report in the
Physical Review
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