157 research outputs found
Possible Microscopic Doping Mechanism in Tl-2201
X-ray absorption spectroscopy on oxygen-annealed, self-flux-grown single
crystals of Tl-2201 suggests a microscopic doping mechanism whereby
interstitial oxygens are attracted to copper substituted on the thallium site,
contributing holes to both the planes and to these coppers, and typically
promoting only one hole to the plane rather than two. These copper substituents
would provide an intrinsic hole doping. The evidence for this is discussed,
along with an alternative interpretation.Comment: 2 pages, 1 figure, submitted as conference proceedings for M2S-IX,
Toky
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
Superconducting Order Parameter in Bi-Layer Cuprates: Occurrence of Phase Shifts in Corner Junctions
We study the order parameter symmetry in bi-layer cuprates such as YBaCuO,
where interesting phase shifts have been observed in Josephson junctions.
Taking models which represent the measured spin fluctuation spectra of this
cuprate, as well as more general models of Coulomb correlation effects, we
classify the allowed symmetries and determine their associated physical
properties. phase shifts are shown to be a general consequence of
repulsive interactions, independent of whether a magnetic mechanism is
operative. While it is known to occur in d-states, this behavior can also be
associated with (orthorhombic) s-symmetry when the two sub-band gaps have
opposite phase. Implications for the magnitude of are discussed.Comment: 5 pages, RevTeX 3.0, 9 figures (available upon request
Electrodynamics of quasi-two-dimensional BEDT-TTF charge transfer salts
We consider the millimeter-wave electrodynamics specific to
quasi-two-dimensional conductors and superconductors based on the organic donor
molecule BEDT-TTF. Using realistic physical parameters, we examine the current
polarizations that result for different oscillating (GHz) electric and magnetic
field polarizations. We show that, in general, it is possible to discriminate
between effects (dissipation and dispersion) due to in-plane and interlayer ac
currents. However, we also show that it is not possible to selectively probe
any single component of the in-plane conductivity tensor, and that excitation
of interlayer currents is strongly influenced by the sample geometry and the
electromagnetic field polarization.Comment: 5 pages including 3 figures Minor correction to figure
Binding of Holes to Magnetic Impurities in a Strongly Correlated System
The effect of a magnetic (S=1/2) impurity coupled to a 2D system of
correlated electrons (described by the t--J model) is studied by exact
diagonalisations. It is found that, if the exchange coupling of the impurity
with the neighboring spins is ferromagnetic or weakly antiferromagnetic, an
extra hole can form bound states of different spatial symmetries with the
impurity extending to a few lattice spacings. The binding energy is maximum
when the impurity is completely decoupled (vacancy) and vanishes for an
antiferromagnetic coupling exceeding . Several peaks appear in the
single hole spectral function below the lower edge of the quasiparticle band as
signatures of the d-, s- and p-wave boundstates.Comment: Latex 11 pages, postscript files in uuencoded form, report#
LPQTH-94/
Doping dependence of superconducting gap in YBa_2Cu_3O_y from universal heat transport
Thermal transport in the T -> 0 limit was measured as a function of doping in
high-quality single crystals of the cuprate superconductor YBa_2Cu_3O_y. The
residual linear term kappa_0/T is found to decrease as one moves from the
overdoped regime towards the Mott insulator region of the phase diagram. The
doping dependence of the low-energy quasiparticle gap extracted from kappa_0/T
is seen to scale closely with that of the pseudogap, arguing against a
non-superconducting origin for the pseudogap. The presence of a linear term for
all dopings is evidence against the existence of a quantum phase transition to
an order parameter with a complex (ix) component.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Resonant Impurity Scattering in a Strongly Correlated Electron Model
Scattering by a single impurity introduced in a strongly correlated
electronic system is studied by exact diagonalization of small clusters. It is
shown that an inert site which is spinless and unable to accomodate holes can
give rise to strong resonant scattering. A calculation of the local density of
state reveals that, for increasing antiferromagnetic exchange coupling, d, s
and p-wave symmetry bound states in which a mobile hole is trapped by the
impurity potential induced by a local distortion of the antiferromagnetic
background successively pull out from the continuum.Comment: 10 pages, 4 figures available on request, report LPQTH-93-2
Encapsulated Single Crystal Growth and Annealing of the High-Temperature Superconductor Tl-2201
Highly-perfect platelet single crystals of Tl_2Ba_2CuO_{6+d} (Tl-2201) were
grown by a self-flux technique. A novel encapsulation scheme allowed the
precursors to react prior to the sealing required to contain volatile thallium
oxides, and permitted the removal of melt at the conclusion of growth,
reproducibly producing high yields of clean crystals. The crystals were
annealed under well-controlled oxygen partial pressures, then characterised.
They have sharp superconducting transitions, narrow X-ray rocking curves and a
low 4% substitution of thallium by copper, all evidence of their high
perfection and homogeneity. The crystals are orthorhombic at most dopings, and
a previously unreported commensurate superlattice distortion is observed.Comment: 8 pages, 5 figures, submitted to Journal of Crystal Growt
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
Nodal Quasiparticle Dispersion in Strongly Correlated d-wave Superconductors
We analyze the effects of a momentum-dependent self-energy on the
photoemission momentum distribution curve (MDC) lineshape, dispersion and
linewidth. We illustrate this general analysis by a detailed examination of
nodal quasiparticles in high Tc cuprates. We use variational results for the
nodal quasiparticle weight Z (which varies rapidly with hole doping x) and the
low energy Fermi velocity (which is independent of x), to show that
the high energy MDC dispersion , so that it is much
larger than the bare (band structure) velocity and also increases strongly with
underdoping. We also present arguments for why the low energy Fermi velocity
and the high energy dispersion are independent of the bare band structure at
small x. All of these results are in good agreement with earlier and recent
photoemission data [Zhou et al, Nature 423, 398 (2003)].Comment: 4 pages, 3 eps fig
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