567 research outputs found
Theory for Decoupling in High-T_c Superconductors from an Analysis of the Layered XY Model with Frustration
The nature of decoupling in the mixed phase of extremely type-II layered
superconductors is studied theoretically through a duality transformation of
the layered XY model with frustration. In the limit of weak coupling, we
generally find that the Josephson effect is absent if and only if the phase
correlations within isolated layers are short range. In the case specific to
uniform frustration, we notably identify a decoupled pancake vortex liquid
phase that is bounded by first-order and second-order decoupling lines in the
magnetic field vs. temperature plane. These transitions potentially account for
the flux-lattice melting and for the flux-lattice depinning that is observed in
clean high-temperature superconductors.Comment: 11 pgs. of Plain TeX, 1 postscript fig., based on a talk given at the
VORTEX Euroconference held in Heraklion, Crete, Sept. 199
P-glycoprotein (ABCB1) - weak dipolar interactions provide the key to understanding allocrite recognition, binding, and transport
P-glycoprotein (ABCB1) is the first discovered mammalian member of the large family of ATP binding cassette (ABC) transporters. It facilitates the movement of compounds (called allocrites) across membranes, using the energy of ATP binding and hydrolysis. Here, we review the thermodynamics of allocrite binding and the kinetics of ATP hydrolysis by ABCB1. In combination with our previous molecular dynamics simulations, these data lead to a new model for allocrite transport by ABCB1. In contrast to previous models, we take into account that the transporter was evolutionarily optimized to operate within a membrane, which dictates the nature of interactions. Hydrophobic interactions drive lipid-water partitioning of allocrites, the transport process's first step. Weak dipolar interactions (including hydrogen bonding, π-π stacking, and π-cation interactions) drive allocrite recognition, binding, and transport by ABCB1 within the membrane. Increasing the lateral membrane packing density reduces allocrite partitioning but enhances dipolar interactions between allocrites and ABCB1. Allocrite flopping (or reorientation of the polar part towards the extracellular aqueous phase) occurs after hydrolysis of one ATP molecule and opening of ABCB1 at the extracellular side. Rebinding of ATP re-closes the transporter at the extracellular side and expels the potentially remaining allocrite into the membrane. The high sensitivity of the steady-state ATP hydrolysis rate to the nature and number of dipolar interactions, as well as to the dielectric constant of the membrane, points to a flopping process, which occurs to a large extent at the membrane-transporter interface. The proposed unidirectional ABCB1 transport cycle, driven by weak dipolar interactions, is consistent with membrane biophysics
Flux dynamics and vortex phase diagram of the new superconductor
Magnetic critical current density and relaxation rate have been measured on
bulks from 1.6 K to at magnetic fields up to 8 Tesla. A vortex
phase diagram is depicted based on these measurement. Two phase boundaries
and characterizing different irreversible
flux motions are found. The is characterized by the
appearance of the linear resistivity and is attributed to quantum vortex
melting induced by quantum fluctuation of vortices in the rather clean system.
The second boundary reflects the irreversible flux motion in
some local regions due to either very strong pinning or the surface barrier on
the tiny grains.Comment: 4 pages, 5 figure
Dynamics of disordered quantum Hall crystals
Charge density waves are thought to be common in two-dimensional electron
systems in quantizing magnetic fields. Such phases are formed by the
quasiparticles of the topmost occupied Landau level when it is partially
filled. One class of charge density wave phases can be described as electron
solids. In weak magnetic fields (at high Landau levels) solids with many
particles per unit cell - bubble phases - predominate. In strong magnetic
fields (at the lowest Landau level) only crystals with one particle per unit
cell - Wigner crystals - can form. Experimental identification of these phases
is facilitated by the fact that even a weak disorder influences their dc and ac
magnetotransport in a very specific way. In the ac domain, a range of
frequencies appears where the electromagnetic response is dominated by
magnetophonon collective modes. The effect of disorder is to localize the
collective modes and to create an inhomogeneously broadened absorption line,
the pinning mode. In recent microwave experiments pinning modes have been
discovered both at the lowest and at high Landau levels. We present the theory
of the pinning mode for a classical two-dimensional electron crystal
collectively pinned by weak impurities. We show that long-range Coulomb
interaction causes a dramatic line narrowing, in qualitative agreement with the
experiments.Comment: 6 pages, 3 figures. To be presented at EP2DS-15, Nara, Japan. One
typo correcte
Supersymmetric Model of a 2D Long-Range Bose Liquid
The model Hamiltonian of a two-dimensional Bose liquid (proposed earlier by
Kane, Kivelson, Lee and Zhang as the Hamiltonian which has Jastrow-type
wavefunctions as the ground-state solution), is shown to possess
nonrelativistic supersymmetry. For the special value of the coupling constant
the quantum mechanics described by this Hamiltonian is shown to be
equivalent to the dynamics of (complex) eigenvalues of random Gaussian ensemble
of normal complex matrices. For general , an exact relation between the
equal-time current-current and density-density correlation functions is
obtained, and used to derive an asymptotically exact (at low wavevectors q)
spectrum of single-particle excitations beyond the superfluid ground-state
(realized at low 's). The ground-state at very large is shown
to be of ``Quantum Hexatic" type, possessing long-range orientational order and
quasi-long-range translational order but with zero shear modulus. Possible
scenaria of the ground-state phase transitions as function of are
discussed.Comment: Revtex; 12 pages, 1 Postscript figur
Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals
The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined
from magnetization curves for different field sweep rates dH/dt and
temperatures. The obtained results are consistent with theoretical reports in
the literature about vortex creep over surface and geometrical barriers. The
frequently observed low-temperature upturn of H_p is shown to be related to
metastable configurations due to barriers for vortex entry. Data of the true
lower critical field H_c1 are presented. The low-temperature dependence of H_c1
is consistent with a superconducting state with nodes in the gap function.
[PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs
Object orientation and visualization of physics in two dimensions
We present a generalized framework for cellular/lattice based visualizations
in two dimensions based on state of the art computing abstractions. Our
implementation takes the form of a library of reusable functions written in C++
which hides complex graphical programming issues from the user and mimics the
algebraic structure of physics at the Hamiltonian level. Our toolkit is not
just a graphics library but an object analysis of physical systems which
disentangles separate concepts in a faithful analytical way. It could be
rewritten in other languages such as Java and extended to three dimensional
systems straightforwardly. We illustrate the usefulness of our analysis with
implementations of spin-films (the two-dimensional XY model with and without an
external magnetic field) and a model for diffusion through a triangular
lattice.Comment: 12 pages, 10 figure
Vortices and 2D bosons: A Path-Integral Monte Carlo Study
The vortex system in a high-T_c superconductor has been studied numerically
using the mapping to 2D bosons and the path-integral Monte Carlo method. We
find a single first-order transition from an Abrikosov lattice to an entangled
vortex liquid. The transition is characterized by an entropy jump dS = 0.4 k_B
per vortex and layer (parameters for YBCO) and a Lindemann number c_L = 0.25.
The increase in density at melting is given by d\rho = 6.0*10^{-4} /
\lambda(T)^2. The vortex liquid corresponds to a bosonic superfluid, with
\rho_s = \rho even in the limit \lambda -> \infty.Comment: 9 pages, RevTeX, 4 PostScript figures. The entropy jump at the
transition has been recomputed and is now in agreement with experiments on
YBCO. Some minor modifications were made in the tex
Dimensional Crossover of Vortex Dynamics Induced by Gd Substitution on Bi2212 Single Crystals
The vortex dynamics of BiSrCaGdCuO
single crystals is investigated by magnetic relaxation and hysteresis
measurements. By substituting with , it is found that the interlayer
Josephson coupling is weakened and the anisotropy is increased, which leads to
the change of vortex dynamics from 3D elastic to 2D plastic vortex creep.
Moreover, the second magnetization peak, which can be observed in samples near
the optimal doping, is absent in the strongly underdoped (with 2D vortex)
region.Comment: 16 Pages, 6 Figures, To appear in Physica
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