149 research outputs found
Observation of a Triangular to Square Flux Lattice Phase Transition in YBCO
We have used the technique of small-angle neutron scattering to observe
magnetic flux lines directly in an YBCO single crystal at fields higher than
previously reported. For field directions close to perpendicular to the CuO2
planes, we find that the flux lattice structure changes smoothly from a
distorted triangular co-ordination to nearly perfectly square as the magnetic
induction approaches 11 T. The orientation of the square flux lattice is as
expected from recent d-wave theories, but is 45 deg from that recently observed
in LSCO
Observation of non-exponential magnetic penetration profiles in the Meissner state - A manifestation of non-local effects in superconductors
Implanting fully polarized low energy muons on the nanometer scale beneath
the surface of a superconductor in the Meissner state enabled us to probe the
evanescent magnetic field profile B(z)(0<z<=200nm measured from the surface).
All the investigated samples [Nb: kappa \simeq 0.7(2), Pb: kappa \simeq 0.6(1),
Ta: kappa \simeq 0.5(2)] show clear deviations from the simple exponential B(z)
expected in the London limit, thus revealing the non-local response of these
superconductors. From a quantitative analysis within the Pippard and BCS models
the London penetration depth lambda_L is extracted. In the case of Pb also the
clean limit coherence length xi0 is obtained. Furthermore we find that the
temperature dependence of the magnetic penetration depth follows closely the
two-fluid expectation 1/lambda^2 \propto 1-(T/T_c)^4. While B(z) for Nb and Pb
are rather well described within the Pippard and BCS models, for Ta this is
only true to a lesser degree. We attribute this discrepancy to the fact that
the superfluid density is decreased by approaching the surface on a length
scale xi0. This effect, which is not taken self-consistently into account in
the mentioned models, should be more pronounced in the lowest kappa regime
consistently with our findings.Comment: accepted in PRB 14 pages, 17 figure
Screening by symmetry of long-range hydrodynamic interactions of polymers confined in sheets
Hydrodynamic forces may significantly affect the motion of polymers. In
sheet-like cavities, such as the cell's cytoplasm and microfluidic channels,
the hydrodynamic forces are long-range. It is therefore expected that that
hydrodynamic interactions will dominate the motion of polymers in sheets and
will be manifested by Zimm-like scaling. Quite the opposite, we note here that
although the hydrodynamic forces are long-range their overall effect on the
motion of polymers vanishes due to the symmetry of the two-dimensional flow. As
a result, the predicted scaling of experimental observables such as the
diffusion coefficient or the rotational diffusion time is Rouse-like, in accord
with recent experiments. The effective screening validates the use of the
non-interacting blobs picture for polymers confined in a sheet.Comment: http://www.weizmann.ac.il/complex/tlusty/papers/Macromolecules2006.pdf
http://pubs.acs.org/doi/abs/10.1021/ma060251
Supercooled vortex liquid and quantitative theory of melting of the flux line lattice in type II superconductors
A metastable homogeneous state exists down to zero temperature in systems of
repelling objects. Zero ''fluctuation temperature'' liquid state therefore
serves as a (pseudo) ''fixed point'' controlling the properties of vortex
liquid below and even around melting point. There exists Madelung constant for
the liquid in the limit of zero temperature which is higher than that of the
solid by an amount approximately equal to the latent heat of melting. This
picture is supported by an exactly solvable large Ginzburg - Landau model
in magnetic field. Based on this understanding we apply Borel - Pade
resummation technique to develop a theory of the vortex liquid in type II
superconductors. Applicability of the effective lowest Landau level model is
discussed and corrections due to higher levels is calculated. Combined with
previous quantitative description of the vortex solid the melting line is
located. Magnetization, entropy and specific heat jumps along it are
calculated. The magnetization of liquid is larger than that of solid by irrespective of the melting temperature. We compare the result with
experiments on high cuprates , , low material and with Monte Carlo simulations.Comment: 28 pages and 4 figures. Enlarged version of paper cond-mat/0107281
with many new content
Vortex State of TlBaCuO via Tl NMR at 2 Tesla
We report a Tl NMR study of vortex state for an aligned
polycrystalline sample of an overdoped high- superconductor
TlBaCuO (85 K) with magnetic field 2 T along
the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at
=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and
vortex melting between 65 and 85 K. No evidence for local antiferromagnetic
ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure
Interlayer Coupling and p-wave Pairing in Strontium Ruthenate
On the basis of a three orbital model and an effective attractive interaction
between electrons we investigate the possible superconducting states, with
and -wave internal symmetry, of SrRuO. For an orbital dependent
interaction which acts between in plane and out of plane nearest neighbour
Ruthenium atoms we find a state for which the gap in the quasi-particle spectra
has a line node on the and sheets of the Fermi Surface, but
it is complex with no nodes on the -sheet. We show that this state is
consistent with all the available experimental data. In particular, we present
the results of our calculations of the specific heat and penetration depth as
functions of the temperature.Comment: 4 pages, 5 figure
Zeeman Perturbed Cu Nuclear Quadrupole Resonance Study of the Vortex State of YBaCuO
We report a Cu nuclear quadrupole resonance (NQR) study of the vortex
state for an aligned polycrystalline sample of a slightly overdoped high-
superconductor YBaCuO (92 K) at a low magnetic
field of 96 mT along the c axis, near a lower critical field . We
observed the frequency distribution of the nuclear spin-lattice relaxation time
in the Zeeman-perturbed Cu NQR spectrum below . The
characteristic behavior of 1/, taking the minimum values with respect
to temperature and frequency, indicates the significant role of
antiferromagnetic spin fluctuations in the Doppler-shifted quasiparticle energy
spectrum inside and outside vortex cores.Comment: 4 pages, 4 figure
Ginzburg-Landau Theory for a p-Wave Sr_2RuO_4 Superconductor: Vortex Core Structure and Extended London Theory
Based on a two dimensional odd-parity superconducting order parameter for
Sr_2RuO_4 with p-wave symmetry, we investigate the single vortex and vortex
lattice structure of the mixed phase near H_{c1}. Ginzburg-Landau calculations
for a single vortex show a fourfold structure with an orientation depending on
the microscopic Fermi surface properties. The corresponding extended London
theory is developed to determine the vortex lattice structure and we find near
H_{c1} a centered rectangular vortex lattice. As the field is increased from
H_{c1} this lattice continuously deforms until a square vortex lattice is
achieved. In the centered rectangular phase the field distribution, as
measurable through \mu-SR experiments, exhibits a characteristic two peak
structure (similar to that predicted in high temperature and borocarbide
superconductors).Comment: 12 pages, 7 figure
- …