6,040 research outputs found
Quantum Hall to Insulator Transition in the Bilayer Quantum Hall Ferromagnet
We describe a new phase transition of the bilayer quantum Hall ferromagnet at
filling fraction . In the presence of static disorder (modeled by a
periodic potential), bosonic spinons can undergo a superfluid-insulator
transition while preserving the ferromagnetic order. The Mott insulating phase
has an emergent U(1) photon, and the transition is between Higgs and Coulomb
phases of this photon. Physical consequences for charge and counterflow
conductivity, and for interlayer tunneling conductance in the presence of
quenched disorder are discussed.Comment: 4 pages, no figure
Metallic spin glasses
Recent work on the zero temperature phases and phase transitions of strongly
random electronic system is reviewed. The transition between the spin glass and
quantum paramagnet is examined, for both metallic and insulating systems.
Insight gained from the solution of infinite range models leads to a quantum
field theory for the transition between a metallic quantum paramagnetic and a
metallic spin glass. The finite temperature phase diagram is described and
crossover functions are computed in mean field theory. A study of fluctuations
about mean field leads to the formulation of scaling hypotheses.Comment: Contribution to the Proceedings of the ITP Santa Barbara conference
on Non-Fermi liquids, 25 pages, requires IOP style file
Putting competing orders in their place near the Mott transition
We describe the localization transition of superfluids on two-dimensional
lattices into commensurate Mott insulators with average particle density p/q
(p, q relatively prime integers) per lattice site. For bosons on the square
lattice, we argue that the superfluid has at least q degenerate species of
vortices which transform under a projective representation of the square
lattice space group (a PSG). The formation of a single vortex condensate
produces the Mott insulator, which is required by the PSG to have density wave
order at wavelengths of q/n lattice sites (n integer) along the principle axes;
such a second-order transition is forbidden in the Landau-Ginzburg-Wilson
framework. We also discuss the superfluid-insulator transition in the direct
boson representation, and find that an interpretation of the quantum
criticality in terms of deconfined fractionalized bosons is only permitted at
special values of q for which a permutative representation of the PSG exists.
We argue (and demonstrate in detail in a companion paper: L. Balents et al.,
cond-mat/0409470) that our results apply essentially unchanged to electronic
systems with short-range pairing, with the PSG determined by the particle
density of Cooper pairs. We also describe the effect of static impurities in
the superfluid: the impurities locally break the degeneracy between the q
vortex species, and this induces density wave order near each vortex. We
suggest that such a theory offers an appealing rationale for the local density
of states modulations observed by Hoffman et al. (cond-mat/0201348) in STM
studies of the vortex lattice of BSCCO, and allows a unified description of the
nucleation of density wave order in zero and finite magnetic fields. We note
signatures of our theory that may be tested by future STM experiments.Comment: 35 pages, 16 figures; (v2) part II is cond-mat/0409470; (v3) added
new appendix and clarifying remarks; (v4) corrected typo
Quantum criticality of U(1) gauge theories with fermionic and bosonic matter in two spatial dimensions
We consider relativistic U(1) gauge theories in 2+1 dimensions, with N_b
species of complex bosons and N_f species of Dirac fermions at finite
temperature. The quantum phase transition between the Higgs and Coulomb phases
is described by a conformal field theory (CFT). At large N_b and N_f, but for
arbitrary values of the ratio N_b/N_f, we present computations of various
critical exponents and universal amplitudes for these CFTs. We make contact
with the different spin-liquids, charge-liquids and deconfined critical points
of quantum magnets that these field theories describe. We compute physical
observables that may be measured in experiments or numerical simulations of
insulating and doped quantum magnets.Comment: 30 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
Thermodynamic Properties of XXZ model in a Transverse Field
We have numerically studied the thermodynamic properties of the spin 1/2 XXZ
chain in the presence of a transverse (non commuting) magnetic field. The
thermal, field dependence of specific heat and correlation functions for chains
up to 20 sites have been calculated. The area where the specific heat decays
exponentially is considered as a measure of the energy gap. We have also
obtained the exchange interaction between chains in a bulk material using the
random phase approximation and derived the phase diagram of the three
dimensional material with this approximation. The behavior of the structure
factor at different momenta verifies the antiferromagnetic long range order in
y-direction for the three dimensional case. Moreover, we have concluded that
the Low Temperature Lanczos results [M. Aichhorn et al., Phys. Rev. B 67,
161103(R) (2003)] are more accurate for low temperatures and closer to the full
diagonalization ones than the results of Finite Temperature Lanczos Method [J.
Jaklic and P. Prelovsek, Phys. Rev. B 49, 5065 (1994)].Comment: 7 pages, 10 eps figure
Bridging the Testing Speed Gap: Design for Delay Testability
The economic testing of high-speed digital ICs is becoming increasingly problematic. Even advanced, expensive testers are not always capable of testing these ICs because of their high-speed limitations. This paper focuses on a design for delay testability technique such that high-speed ICs can be tested using inexpensive, low-speed ATE. Also extensions for possible full BIST of delay faults are addresse
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.
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
A low-speed BIST framework for high-performance circuit testing
Testing of high performance integrated circuits is becoming increasingly a challenging task owing to high clock frequencies. Often testers are not able to test such devices due to their limited high frequency capabilities. In this article we outline a design-for-test methodology such that high performance devices can be tested on relatively low performance testers. In addition, a BIST framework is discussed based on this methodology. Various implementation aspects of this technique are also addresse
- …