87 research outputs found
Scaling of the superfluid density in high-temperature superconductors
A scaling relation N{sub c} {approx} 4.4{sigma}{sub dc}T{sub c} has been observed parallel and perpendicular to the copper-oxygen planes in the high-temperature superconductors; N{sub c} is the spectral weight and {sigma}{sub dc} is the dc conductivity just above the critical temperature T{sub c}. In addition, Nb and Pb also fall close to the this scaling line. The application of the Ferrell-Glover-Tinkham sum rule to the BCS optical properties of Nb above and below T{sub c} yields N{sub c} {approx} 8.1{sigma}{sub dc}T{sub c} when the normal-state scattering rate is much greater than the superconducting energy gap (1/{tau} > 2{Delta}, the ''dirty'' limit). This result implies that the high-temperature superconductors may be in the dirty limit. The superconductivity perpendicular to the planes is explained by the Josephson effect, which again yields N{sub c} {approx} 8.1{sigma}{sub dc}T{sub c} in the BCS formalism. The similar forms for the scaling relation in these two directions suggests that in some regime the dirty limit and the Josephson effect may be viewed as equivalent
Sum rules and energy scales in the high-temperature superconductor YBa2Cu3O6+x
The Ferrell-Glover-Tinkham (FGT) sum rule has been applied to the temperature
dependence of the in-plane optical conductivity of optimally-doped
YBa_2Cu_3O_{6.95} and underdoped YBa_2Cu_3O_{6.60}. Within the accuracy of the
experiment, the sum rule is obeyed in both materials. However, the energy scale
\omega_c required to recover the full strength of the superfluid \rho_s in the
two materials is dramatically different; \omega_c \simeq 800 cm^{-1} in the
optimally doped system (close to twice the maximum of the superconducting gap,
2\Delta_0), but \omega_c \gtrsim 5000 cm^{-1} in the underdoped system. In both
materials, the normal-state scattering rate close to the critical temperature
is small, \Gamma < 2\Delta_0, so that the materials are not in the dirty limit
and the relevant energy scale for \rho_s in a BCS material should be twice the
energy gap. The FGT sum rule in the optimally-doped material suggests that the
majority of the spectral weight of the condensate comes from energies below
2\Delta_0, which is consistent with a BCS material in which the condensate
originates from a Fermi liquid normal state. In the underdoped material the
larger energy scale may be a result of the non-Fermi liquid nature of the
normal state. The dramatically different energy scales suggest that the nature
of the normal state creates specific conditions for observing the different
aspects of what is presumably a central mechanism for superconductivity in
these materials.Comment: RevTeX 4 file, 9 pages with 7 embedded eps figure
Doping-dependent studies of the Anderson-Mott localization in polyaniline at the metal-insulator boundary
Temperature-dependent dc conductivity measurements and infrared reflectivity measurements (20–9000 cm−1) were performed on a series of polyaniline samples with two different dopant acids at various doping levels. The typical fingerprints of a disordered metal such as a positive temperature coefficient of resistivity at high temperatures, a very high reflectivity in the far infrared, and a plasma resonance have been observed. The results were analyzed in the framework of the Anderson-Mott localization model and considerable consistency between transport studies and optical measurements was obtained. Various parameters enabling a comparative classification of the materials are also reported
An inhomogeneous Josephson phase in thin-film and High-Tc superconductors
In many cases inhomogeneities are known to exist near the metal (or
superconductor)-insulator transition, as follows from well-known domain-wall
arguments. If the conducting regions are large enough (i.e. when the T=0
superconducting gap is much larger than the single-electron level spacing), and
if they have superconducting correlations, it becomes energetically favorable
for the system to go into a Josephson-coupled zero-resistance state before
(i.e. at higher resistance than) becoming a "real" metal. We show that this is
plausible by a simple comparison of the relevant coupling constants. For small
grains in the above sense, the electronic grain structure is washed out by
delocalization and thus becomes irrelevant. When the proposed "Josephson state"
is quenched by a magnetic field, an insulating, rather then a metallic, state
should appear. This has been shown to be consistent with the existing data on
oxide materials as well as ultra-thin films. We discuss the Uemura correlations
versus the Homes law, and derive the former for the large-grain Josephson array
(inhomogenous superconductor) model. The small-grain case behaves like a dirty
homogenous metal. It should obey the Homes law provided that the system is in
the dirty supeconductivity limit. A speculation why that is typically the case
for d-wave superconductors is presented.Comment: Conference proceeding for "Fluctuations in Superconductors" held in
Nazareth, Israel in June, 2007; 6 pages with 1 figure, to appear in Physica
Anisotropic conductivity of Nd_{1.85}Ce_{0.15}CuO_{4-\delta} films at submillimeter wavelengths
The anisotropic conductivity of thin NdCeCuO
films was measured in the frequency range 8 cm 40 cm and
for temperatures 4 K K. A tilted sample geometry allowed to extract
both, in-plane and c-axis properties. The in-plane quasiparticle scattering
rate remains unchanged as the sample becomes superconducting. The temperature
dependence of the in-plane conductivity is reasonably well described using the
Born limit for a d-wave superconductor. Below T_{{\rm C}%} the c-axis
dielectric constant changes sign at the screened c-axis plasma
frequency. The temperature dependence of the c-axis conductivity closely
follows the linear in T behavior within the plane.Comment: 4 pages, 4 figure
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
The anomaly of the oxygen bond-bending mode at 320 cm and the additional absorption peak in the c-axis infrared conductivity of underdoped YBaCuO single crystals revisited by ellipsometricmeasurements
We have performed ellipsometric measurements of the far-infrared c-axis
dielectric response of underdoped YBaCuO single
crystals. Here we report a detailed analysis of the temperature-dependent
renormalization of the oxygen bending phonon mode at 320 cm and the
formation of the additional absorption peak around 400-500 cm. For a
strongly underdoped YBaCuO crystal with T=52 K we
find that, in agreement with previous reports based on conventional reflection
measurements, the gradual onset of both features occurs well above T at
T*150 K. Contrary to some of these reports, however, our data establish
that the phonon anomaly and the formation of the additional peak exhibit very
pronounced and steep changes right at T. For a less underdoped
YBaCuO crystal with T=80 K, the onset temperature of
the phonon anomaly almost coincides with T. Also in contrast to some
previous reports, we find for both crystals that a sizeable fraction of the
spectral weight of the additional absorption peak cannot be accounted for by
the spectral-weight loss of the phonon modes but instead arises from a
redistribution of the electronic continuum. Our ellipsometric data are
consistent with a model where the bilayer cuprate compounds are treated as a
superlattice of intra- and inter-bilayer Josephson-junctions
Quasiparticles and c-axis coherent hopping in high T_c superconductors
We study the problem of the low-energy quasiparticle spectrum of the extended
t-J model and analyze the coherent hopping between weakly coupled planes
described by this model. Starting with a two-band model describing the Cu-O
planes and the unoccupied bands associated to the metallic atoms located in
between the planes, we obtain effective hopping matrix elements describing the
c-axis charge transfer. A computational study of these processes shows an
anomalously large charge anisotropy for doping concentrations around and below
the optimal doping.Comment: 4 pages, 3 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
Lattice dielectric response of CdCu{3}Ti{4}O{12} and of CaCu{3}Ti{4}O{12} from first principles
Structural, vibrational, and lattice dielectric properties of
CdCu{3}Ti{4}O{12} are studied using density-functional theory within the local
spin-density approximation, and the results are compared with those computed
previously for CaCu{3}Ti{4}O{12}. Replacing Ca with Cd is found to leave many
calculated quantities largely unaltered, although significant differences do
emerge in zone-center optical phonon frequencies and mode effective charges.
The computed phonon frequencies of CdCu{3}Ti{4}O{12} are found to be in
excellent agreement with experiment, and the computed lattice contribution to
the intrinsic static dielectric constant (~60) also agrees exceptionally well
with a recent optical absorption experiment. These results provide further
support for a picture in which the lattice dielectric response is essentially
conventional, suggesting an extrinsic origin for the anomalous low-frequency
dielectric response recently observed in both materials.Comment: 5 pages; uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/lh_cdct/index.htm
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