808 research outputs found
Spin-charge gauge approach to metal-insulator crossover and transport properties in High-T cuprates
The spin-charge gauge approach to consider the metal-insulator crossover
(MIC) and other anomalous transport properties in High-T cuprates is
briefly reviewed. A U(1) field gauging the global charge symmetry and an SU(2)
field gauging the global spin-rotational symmetry are introduced to study the
two-dimensional model in the limit . The MIC as a clue to the
understanding of the ``pseudogap'' (PG) phase, is attributed to the competition
between the short-range antiferromagnetic order and dissipative motion of
charge carriers coupled to the slave-particle gauge field. The composite
particle formed by binding the charge carrier (holon) and spin excitation
(spinon) via the slave particle gauge field exhibits a number of peculiar
properties, and the calculated results are in good agreement with experimental
data for both PG and ``strange metal'' phases. Connections to other gauge field
approaches in studying the strong correlation problem are also briefly
outlined.Comment: 32 pages, to appear in the special issue on "Correlated Electrons" of
J. Phys.: Condens. Mat
Plastic energies in layered superconductors
We estimate the energy cost associated with two pancake vortices colliding in
a layered superconductor. It is argued that this energy sets the plastics
energy scale and is the analogue of the crossing energy for vortices in the
continuum case. The starting point of the calculation is the Lawrence-Doniach
version of the Ginzburg-Landau free energy for type-II superconductors. The
magnetic fields considered are along the c-direction and assumed to be
sufficiently high that the lowest Landau level approximation is valid. For
Bi-2212, where it is know that layering is very important, the results are
radically different from what would have been obtained using a
three-dimensional anisotropic continuum model. We then use the plastic energy
for Bi-2212 to successfully explain recent results from Hellerqvist {\em et
al.}\ on its longitudinal resistance.Comment: 5 Pages Revtex, 4 uuencoded postscript figure
Weak point disorder in strongly fluctuating flux-line liquids
We consider the effect of weak uncorrelated quenched disorder (point defects)
on a strongly fluctuating flux-line liquid. We use a hydrodynamic model which
is based on mapping the flux-line system onto a quantum liquid of relativistic
charged bosons in 2+1 dimensions [P. Benetatos and M. C. Marchetti, Phys. Rev.
B 64, 054518, (2001)]. In this model, flux lines are allowed to be arbitrarily
curved and can even form closed loops. Point defects can be scalar or polar. In
the latter case, the direction of their dipole moments can be random or
correlated. Within the Gaussian approximation of our hydrodynamic model, we
calculate disorder-induced corrections to the correlation functions of the
flux-line fields and the elastic moduli of the flux-line liquid. We find that
scalar disorder enhances loop nucleation, and polar (magnetic) defects decrease
the tilt modulus.Comment: 15 pages, submitted to Pramana-Journal of Physics for the special
volume on Vortex State Studie
Phase Transitions in a Model Anisotropic High Tc Superconductor
We carry out simulations of the anisotropic uniformly frustrated 3D XY model,
as a model for vortex line fluctuations in high Tc superconductors. We compute
the phase diagram as a function of temperature and anisotropy, for a fixed
applied magnetic field. We find that superconducting coherence parallel to the
field persists into the vortex line liquid state, and that this transition lies
well below the "mean-field" cross-over from the vortex line liquid to the
normal state.Comment: 23 pages + 19 ps figure
Helicity Modulus and Fluctuating Type II Superconductors: Elastic Approximation and Numerical Simulations
We develop the helicity modulus as a criterion for superconducting order in
the mixed phase of a fluctuating type II superconductor. We show that there is
a duality relation between this helicity modulus and the superfluid density of
a system of analog 2D bosons. We show that the vortex line lattice exhibits a
perfect Meissner effect with respect to a shearing perturbation of the applied
magnetic field, and this becomes our creterion for "longitudinal
superconductivity" parallel to the applied field. We present arguments based on
the 2D boson analogy, as well as the results of numerical simulations, that
suggest that longitudinal superconductivity can persist into the vortex line
liquid state for systems of finite thickness, comparable to those commonly
found in experiments.Comment: 63 pages, 22 postscript figure
Field-driven topological glass transition in a model flux line lattice
We show that the flux line lattice in a model layered HTSC becomes unstable
above a critical magnetic field with respect to a plastic deformation via
penetration of pairs of point-like disclination defects. The instability is
characterized by the competition between the elastic and the pinning energies
and is essentially assisted by softening of the lattice induced by a
dimensional crossover of the fluctuations as field increases. We confirm
through a computer simulation that this indeed may lead to a phase transition
from crystalline order at low fields to a topologically disordered phase at
higher fields. We propose that this mechanism provides a model of the low
temperature field--driven disordering transition observed in neutron
diffraction experiments on single crystals.Comment: 11 pages, 4 figures available upon request via snail mail from
[email protected]
Energy cost associated with vortex crossing in superconductors
Starting from the Ginzburg-Landau free energy of a type II superconductor in
a magnetic field we estimate the energy associated with two vortices crossing.
The calculations are performed by assuming that we are in a part of the phase
diagram where the lowest Landau level approximation is valid. We consider only
two vortices but with two markedly different sets of boundary conditions: on a
sphere and on a plane with quasi-periodic boundary conditions. We find that the
answers are very similar suggesting that the energy is localised to the
crossing point. The crossing energy is found to be field and temperature
dependent -- with a value at the experimentally measured melting line of
, where is the Lindemann
melting criterion parameter. The crossing energy is then used with an extension
of the Marchetti, Nelson and Cates hydrodynamic theory to suggest an
explanation of the recent transport experiments of Safar {{\em et al.}\ }.Comment: 15 pages, RevTex v3.0, followed by 5 postscript figure
U(1)xSU(2) Chern--Simons gauge theory of underdoped cuprate superconductors
The Chern-Simons bosonization with U(1)xSU(2) gauge field is applied to 2-D
t-J model in the limit t >> J, to study the normal state properties of
underdoped cuprate superconductors. We prove the existence of an upper bound on
the partition function for holons in a spinon background, and we find the
optimal spinon configuration saturating the upper bound on average--a
coexisting flux phase and s+id-like RVB state. After neglecting the feedback of
holon fluctuations on the U(1) field B and spinon fluctuations on the SU(2)
field V, the holon field is a fermion and the spinon field is a hard--core
boson. We show that the B field produces a \pi flux phase for holons,
converting them into Dirac--like fermions, while the V field, taking into
account the feedback of holons produces a gap for spinons vanishing in zero
doping limit. The nonlinear sigma-model with a mass term describes the
crossover from short-ranged antiferromagnetic (AF) state in doped samples to
long range AF order in reference compounds. Moreover, we derive a low--energy
effective action in terms of spinons, holons and a self-generated U(1) gauge
field. The gauge fluctuations are not confining due to coupling to holons, but
yield an attractive interaction between spinons and holons leading to a bound
state with electron quantum numbers. The renormalisation effects due to gauge
fluctuations give rise to non--Fermi liquid behaviour for the composite
electron.This formalism provides a new interpretation of the spin gap in
underdoped superconductors (due to short-ranged AF order) and predicts the
minimal gap for the physical electron is proportional to the square root of the
doping concentration.Comment: 31 pages, REVTEX, to be published in Phys. Rev.
Analysis of recycled poly (styrene-co-butadiene) sulfonation: a new approach in solid catalysts for biodiesel production
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c Superconductors : Thermal Fluctuations and Disorder Effects
We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations.Comment: 50 pages Latex file, including 10 postscript figure
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