40 research outputs found
Two-dimensional vortex behavior in highly underdoped YBa_2Cu_3O_{6+x} observed by scanning Hall probe microscopy
We report scanning Hall probe microscopy of highly underdoped superconducting
YBa_2Cu_3O_{6+x} with T_c ranging from 5 to 15 K which showed distinct flux
bundles with less than one superconducting flux quantum (Phi_0) through the
sample surface. The sub-Phi_0 features occurred more frequently for lower T_c,
were more mobile than conventional vortices, and occurred more readily when the
sample was cooled with an in-plane field component. We show that these features
are consistent with kinked stacks of pancake vortices.Comment: 11 pages, 8 figures, accepted for publication in Physical Review
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Noise Characteristics of 100nm-scaleGaAs/Al_xGa_{1-x}As Scanning Hall Probes
The authors have fabricated and characterized GaAs/Al{sub x}Ga{sub 1-x}As two-dimensional electron gas scanning Hall probes for imaging perpendicular magnetic fields at surfaces. The Hall crosses range from 85 x 85 to 1000 x 1000 nm{sup 2}. They study low-frequency noise in these probes, especially random telegraph noise, and show that low-frequency noise can be significantly reduced by optimizing the voltage on a gate over the Hall cross. The authors demonstrate a 100 nm Hall probe with a sensitivity of 0.5 G/{radical}Hz (flux sensitivity of 0.25m {Phi}{sub 0}/{radical}Hz; spin sensitivity of 1.2 x 10{sup 4} {mu}{sub B}/{radical}Hz) at 3 Hz and 9 K
Magnetic field effects on the density of states of orthorhombic superconductors
The quasiparticle density of states in a two-dimensional d-wave
superconductor depends on the orientation of the in-plane external magnetic
field H. This is because. in the region of the gap nodes, the Doppler shift due
to the circulating supercurrents around a vortex depend on the direction of H.
For a tetragonal system the induced pattern is four-fold symmetric and, at zero
energy, the density of states exhibits minima along the node directions. But
YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes
two-fold symmetric with the position of the minima occuring when H is oriented
along the Fermi velocity at a node on the Fermi surface. The effect of impurity
scattering in the Born and unitary limit is discussed.Comment: 24 pages, 11 Figure
Preparation and Characterization of Homogeneous YBCO Single Crystals with Doping Level near the SC-AFM Boundary
High-purity and homogeneous YBa2Cu3Oy single crystals with carrier doping
level near the AFM-SC boundary have been obtained in the oxygen content range
between y = 6.340 and 6.370. The crystals are ortho-II phase at room
temperature and undergo the orthorhombic to tetragonal transition at about
140_Degree_C. They show sharp superconducting transitions, with Tc between 4
and 20 K. Tc changes by 0.8 K when the oxygen content y is changed by 0.001,
and is also sensitive to annealing conditions near room temperature, due to the
dependence of doping on oxygen ordering correlation lengths. Crystals with
oxygen content y lower than 6.345 are non-superconducting.Comment: 6 page
Influence of Fermi surface topology on the quasiparticle spectrum in the vortex state
We study the influence of Fermi surface topology on the quasiparticle density
of states in the vortex state of type II superconductors. We observe that the
field dependence and the shape of the momentum and spatially averaged density
of states is affected significantly by the topology of the Fermi surface. We
show that this behavior can be understood in terms of characteristic Fermi
surface functions and that an important role is played by the number of points
on the Fermi surface at which the Fermi velocity is directed parallel to the
magnetic field. A critical comparison is made with a broadened BCS type density
of states, that has been used frequently in analysis of tunneling data. We
suggest a new formula as a replacement for the broadened BCS model for the
special case of a cylindrical Fermi surface. We apply our results to the two
gap superconductor MgB and show that in this particular case the field
dependence of the partial densities of states of the two gaps behaves very
differently due to the different topologies of the corresponding Fermi
surfaces, in qualitative agreement with recent tunneling experiments.Comment: 12 pages 12 figure
Approximate tight-binding sum rule for the superconductivity related change of c-axis kinetic energy in multilayer cuprate superconductors
We present an extension of the c-axis tight-binding sum rule discussed by
Chakravarty, Kee, and Abrahams [Phys. Rev. Lett. 82, 2366 (1999)] that applies
to multilayer high-Tc cuprate superconductors (HTCS) and use it to
estimate--from available infrared data--the change below Tc of the c-axis
kinetic energy, Hc, in YBa2Cu3O(7-delta) (delta=0.45,0.25,0.07), Bi2Sr2CaCu2O8,
and Bi2Sr2Ca2Cu3O10. In all these compounds Hc decreases below Tc and except
for Bi2Sr2CaCu2O8 the change of Hc is of the same order of magnitude as the
condensation energy. This observation supports the hypothesis that in
multilayer HTCS superconductivity is considerably amplified by the interlayer
tunnelling mechanism.Comment: 6 pages, 2 figure
Phase transition from a to superconductor
We study the phase transition from a to
superconductor using the tight-binding model of two-dimensional cuprates. As
the temperature is lowered past the critical temperature , first a superconducting phase is created. With further reduction of
temperature, the phase is created at temperature
. We study the temperature dependencies of the order parameter,
specific heat and spin susceptibility in these mixed-angular-momentum states on
square lattice and on a lattice with orthorhombic distortion. The
above-mentioned phase transitions are identified by two jumps in specific heat
at and .Comment: Latex file, 5 pages, 6 postscript figures, Accepted in Physical
Review
Condensation Energy and High Tc Superconductivity
From an analysis of the specific heat of one of the cuprate superconductors
it is shown, that even if a large part of the experimental specific heat
associated with the superconducting phase transition is due to fluctuations,
this part must be counted when one tries to extract the condensation energy
from the data. Previous work by Chakravarty, Kee and Abrahams, where the
fluctuation part was subtracted, has resulted in an incorrect estimation of the
condensation energy.Comment: 4 pages, 5 encapsulated Postscript figures, uses ReVTeX.st
Quasiparticle and Cooper Pair Tunneling in the Vortex State of Bi-2212
From measurements of the c-axis I-V characteristics of intrinsic Josephson
junctions in Bi_2Sr_2CaCu_2O_{8+delta} (Bi-2212) mesas we obtain the field
dependence (H || c) of the quasiparticle (QP) conductivity, sigma_q(H,T), and
of the Josephson critical current density, J_c(H,T). The quasiparticle
conductivity sigma_q(H) increases sharply with H and reaches a plateau at 0.05
T <H< 0.3 T. We explain such behavior by the dual effect of supercurrents
around vortices. First, they enhance the QP DOS, leading to an increase of
sigma_q with H at low H and, second, they enhance the scattering rate for
specular tunneling as pancakes become disordered along the c-axis at higher H,
leading to a plateau at moderate H.Comment: 4 pages, 4 figure
Effect of controlled disorder on quasiparticle thermal transport in BiSrCaCuO
Low temperature thermal conductivity, , of optimally-doped Bi2212 was
studied before and after the introduction of point defects by electron
irradiation. The amplitude of the linear component of remains
unchanged, confirming the universal nature of heat transport by zero-energy
quasiparticles. The induced decrease in the absolute value of at
finite temperatures allows us to resolve a nonuniversal term in due to
conduction by finite-energy quasiparticles. The magnitude of this term provides
an estimate of the quasiparticle lifetime at subkelvin temperatures.Comment: 5 pages including 2 .eps figuer