37 research outputs found
Inelastic Neutron Scattering Signal from Deconfined Spinons in a Fractionalized Antiferromagnet
We calculate the contribution of deconfined spinons to inelastic neutron scattering (INS) in the fractionalized antiferromagnet (AF*), introduced elsewhere. We find that the presence of free spin-1/2 charge-less excitations leads to a continuum INS signal above the NĂ©el gap. This signal is found above and in addition to the usual spin-1 magnon signal, which to lowest order is the same as in the more conventional confined antiferromagnet. We calculate the relative weights of these two signals and find that the spinons contribute to the longitudinal response, where the magnon signal is absent to lowest order. Possible higher-order effects of interactions between magnons and spinons in the AF* phase are also discussed
Critical Dynamics of Superconductors in the Charged Regime
The charged regime of the superconductor-metal transition was analyzed by applying a finite temperature critical dynamics. A transverse gage field coupling was applied to the superconducting order parameter. A new dynamic universality class characeterized by a finite fixed point ratio between the transport coefficients associated with the order parameter and gage fields was found by assuming relaxational dynamics for both the order parameter and gage fields within a renormalization group scheme. It was found that various features of the dynamic universality class of the charged superconductor appeared in measurable quantities
Quasiparticle density of states in dirty high-T_c superconductors
We study the density of quasiparticle states of dirty d-wave superconductors.
We show the existence of singular corrections to the density of states due to
quantum interference effects. We then argue that the density of states actually
vanishes in the localized phase as or depending on whether time
reversal is a good symmetry or not. We verify this result for systems without
time reversal symmetry in one dimension using supersymmetry techniques. This
simple, instructive calculation also provides the exact universal scaling
function for the density of states for the crossover from ballistic to
localized behaviour in one dimension. Above two dimensions, we argue that in
contrast to the conventional Anderson localization transition, the density of
states has critical singularities which we calculate in a
expansion. We discuss consequences of our results for various experiments on
dirty high- materials
Interplay between lattice-scale physics and the quantum Hall effect in graphene
Graphene's honeycomb lattice structure underlies much of the remarkable
physics inherent in this material, most strikingly through the formation of two
``flavors'' of Dirac cones for each spin. In the quantum Hall regime, the
resulting flavor degree of freedom leads to an interesting problem when a
Landau level is partially occupied. Namely, while Zeeman splitting clearly
favors polarizing spins along the field, precisely how the states for each
flavor are occupied can become quite delicate. Here we focus on clean graphene
sheets in the regime of quantum Hall ferromagnetism, and discuss how subtler
lattice-scale physics, arising either from interactions or disorder, resolves
this ambiguity to measurable consequence. Interestingly, such lattice-scale
physics favors microscopic symmetry-breaking order coexisting with the usual
liquid-like quantum Hall physics emerging on long length scales. The current
experimental situation is briefly reviewed in light of our discussion.Comment: 6 pages, 2 figures; short revie
Ring exchange, the Bose metal, and bosonization in two dimensions
Motivated by the high-T_c cuprates, we consider a model of bosonic Cooper
pairs moving on a square lattice via ring exchange. We show that this model
offers a natural middle ground between a conventional antiferromagnetic Mott
insulator and the fully deconfined fractionalized phase which underlies the
spin-charge separation scenario for high-T_c superconductivity. We show that
such ring models sustain a stable critical phase in two dimensions, the *Bose
metal*. The Bose metal is a compressible state, with gapless but uncondensed
boson and ``vortex'' excitations, power-law superconducting and charge-ordering
correlations, and broad spectral functions. We characterize the Bose metal with
the aid of an exact plaquette duality transformation, which motivates a
universal low energy description of the Bose metal. This description is in
terms of a pair of dual bosonic phase fields, and is a direct analog of the
well-known one-dimensional bosonization approach. We verify the validity of the
low energy description by numerical simulations of the ring model in its exact
dual form. The relevance to the high-T_c superconductors and a variety of
extensions to other systems are discussed, including the bosonization of a two
dimensional fermionic ring model
Superconducting ``metals'' and ``insulators''
We propose a characterization of zero temperature phases in disordered
superconductors on the basis of the nature of quasiparticle transport. In three
dimensional systems, there are two distinct phases in close analogy to the
distinction between normal metals and insulators: the superconducting "metal"
with delocalized quasiparticle excitations and the superconducting "insulator"
with localized quasiparticles. We describe experimental realizations of either
phase, and study their general properties theoretically. We suggest experiments
where it should be possible to tune from one superconducting phase to the
other, thereby probing a novel "metal-insulator" transition inside a
superconductor. We point out various implications of our results for the phase
transitions where the superconductor is destroyed at zero temperature to form
either a normal metal or a normal insulator.Comment: 18 page
Exotic quantum phases and phase transitions in correlated matter
We present a pedagogical overview of recent theoretical work on
unconventional quantum phases and quantum phase transitions in condensed matter
systems. Strong correlations between electrons can lead to a breakdown of two
traditional paradigms of solid state physics: Landau's theories of Fermi
liquids and phase transitions. We discuss two resulting "exotic" states of
matter: topological and critical spin liquids. These two quantum phases do not
display any long-range order even at zero temperature. In each case, we show
how a gauge theory description is useful to describe the new concepts of
topological order, fractionalization and deconfinement of excitations which can
be present in such spin liquids. We make brief connections, when possible, to
experiments in which the corresponding physics can be probed. Finally, we
review recent work on deconfined quantum critical points. The tone of these
lecture notes is expository: focus is on gaining a physical picture and
understanding, with technical details kept to a minimum.Comment: 22 pages, 15 figures; Notes of the Lectures at the International
Summer School on Fundamental Problems in Statistical Physics XI, September
2005, Leuven, Belgium; High-resolution version available at
http://w3-phystheo.ups-tlse.fr/~alet/leuven.htm
Detecting fractions of electrons in the high- cuprates
We propose several tests of the idea that the electron is fractionalized in
the underdoped and undoped cuprates. These include the ac Josephson effect, and
tunneling into small superconducting grains in the Coulomb blockade regime. In
both cases, we argue that the results are qualitatively modified from the
conventional ones if the insulating tunnel barrier is fractionalized. These
experiments directly detect the possible existence of the chargon - a charge
spinless boson - in the insulator. The effects described in this paper
provide a means to probing whether the undoped cuprate (despite it's magnetism)
is fractionalized. Thus, the experiments discussed here are complementary to
the flux-trapping experiment we proposed in our earlier work(cond-mat/0006481).Comment: 7 pages, 5 figure
Fractionalization, topological order, and cuprate superconductivity
This paper is concerned with the idea that the electron is fractionalized in
the cuprate high- materials. We show how the notion of topological order
may be used to develop a precise theoretical characterization of a
fractionalized phase in spatial dimension higher than one. Apart from the
fractional particles into which the electron breaks apart, there are
non-trivial gapped topological excitations - dubbed "visons". A cylindrical
sample that is fractionalized exhibits two disconnected topological sectors
depending on whether a vison is trapped in the "hole" or not. Indeed, "vison
expulsion" is to fractionalization what the Meissner effect ("flux expulsion")
is to superconductivity. This understanding enables us to address a number of
conceptual issues that need to be confronted by any theory of the cuprates
based on fractionalization ideas. We argue that whether or not the electron
fractionalizes in the cuprates is a sharp and well-posed question with a
definite answer. We elaborate on our recent proposal for an experiment to
unambiguously settle this issue.Comment: 18 pages, 7 figure
Boundary Conformal Field Theory and Tunneling of Edge Quasiparticles in non-Abelian Topological States
We explain how (perturbed) boundary conformal field theory allows us to
understand the tunneling of edge quasiparticles in non-Abelian topological
states. The coupling between a bulk non-Abelian quasiparticle and the edge is
due to resonant tunneling to a zero mode on the quasiparticle, which causes the
zero mode to hybridize with the edge. This can be reformulated as the flow from
one conformally-invariant boundary condition to another in an associated
critical statistical mechanical model. Tunneling from one edge to another at a
point contact can split the system in two, either partially or completely. This
can be reformulated in the critical statistical mechanical model as the flow
from one type of defect line to another. We illustrate these two phenomena in
detail in the context of the nu=5/2 quantum Hall state and the critical Ising
model. We briefly discuss the case of Fibonacci anyons and conclude by
explaining the general formulation and its physical interpretation