The strength of the interlayer Josephson tunneling in
layered superconductors is a useful phenomenological
parameter as well as a test of the Interlayer Tunneling
(ILT) Model as a mechanism for high-temperature
superconductivity. A Superconducting QUantum Interference
Device (SQUID) microscope was used to image vortices
oriented parallel to the planes in layered superconductors,
called interlayer Josephson vortices. The extent of such
vortices along the planes, the interlayer penetration depth
\lambda_\perp, is a measure of the interlayer coupling
in the superconducting state. We find the following values
for the interlayer penetration depth in various optimally
doped materials: Tl-2201, \lambda_\perp \approx 17
\mum; Hg-1201, \lambda_\perp \approx 8 \mum; LSCO,
\lambda_\perp \approx 5 \mum;
\kappa-(BEDT-TTF)_2Cu(NCS)_2, \lambda_\perp \approx
50-60 \mum. In Tl-2201, Hg-1201, and
\kappa-(BEDT-TTF)_2Cu(NCS)_2, \lambda_\perp is
much longer than predicted by Anderson from the condensation
energy in the context of the ILT model. These results
indicate that present versions of the ILT model are not
sufficient to explain the high critical temperature of these
materials.
[VC37.02] C-Axis Electrodynamics of YBCO
Walter Hardy (University of British Columbia)
This abstract not available.
[VC37.03] Sum Rules and Interlayer Conductivity of High-T_c Cuprates.
D.N. Basov (Department of Physics, University of California San Diego)
Analysis of the interlayer infrared conductivity of cuprate high-T_c superconductors reveals an anomalously large energy scale extending up to mid-infrared frequencies that can be attributed to the formation of the superconducting condensate. This unusual effect is observed in a variety of materials including Tl_2Ba_2CuO_6+x, La_2-xSr_xCuO_4,HgBa_2CuO_4+x and YBa_2Cu_3O_x which all show an incoherent interlayer response in the normal state. Mid-infrared range condensation is examined in the context of sum rules that can be formulated for the complex conductivity. One possible interpretation of these experiments is in terms of a kinetic energy change associated with the superconducting transition.
[VC37.04] C-Axis Plasmons in High Temperature Superconductors
Euyheon Hwang (Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, Maryland 20742-4111)
We present a microscopic theoretical calculation(S. Das Sarma and E. H. Hwang, Phys. Rev. Lett. 80), (1998). for the c-axis optical reflectivity of the cuprates,(K. Tamasaku et al.), Phys. Rev. Lett. 69, 1455 (1992); K. Tamasaku et al., ibid 72, 3088 (1994); K. A. Moler et. al., Science 279, 1193 (1998). concentrating on the optimum doping (corresponding to the highest critical temperature T_c) single layer (e.g., LSCO, Tl-2201) cuprate systems which have only one CuO layer per unit cell. Using a conventional BCS -- Fermi liquid model we calculate the c-axis optical reflectivity of the layered high temperature cuprate superconductors by obtaining the finite temperature dynamical dielectric function. In calculating the long wavelength frequency dependent dielectric function we use the Nambu-Gorkov formalism and carry out a fully gauge invariant self-consistent finite temperature linear response calculation(H. A. Fertig and S. Das Sarma, Phys. Rev. Lett. 65), 1482 (1990); E. H. Hwang and S. Das Sarma, Phys. Rev. B 52, R7010 (1995). including impurity scattering induced level broadening effects in the theory. We obtain semi-quantitative agreement with the existing experimental data by using the measured normal state dc resistivities as the input parameters in obtaining the c-axis hopping amplitude and the normal state level broadening in our microscopic calculation. Our theory provides a unified microscopic understanding of the dynamics of the c-axis plasmons in high temperature superconductors.
Work done in collaboration with S. Das Sarma, and supported by the US-ARO and the US-ONR.
[VC37.05] The conductivity sum rule and the mechanism of superconductivity
Sudip Chakravarty (Department of Physics, UCLA, Los Angeles, CA 90095-1547)
The theory that the change of the electronic kinetic energy in a direction perpendicular to the CuO-planes in high-temperature superconductors is a substantial fraction of the condensation energy is examined. The aim is to partly resolve a number of issues related to a theory of high-temperature superconductivity known as the interlayer tunneling theory (ILT) and to propose the efficacy of a conductivity sum rule. Within a simple version of ILT, one relates the zero-temperature c-axis penetration depth \lambda_c to the superconducting condensation energy. Here, I point out that the realization of ILT and the interpretation of recent measurements of \lambda_c, necessarily require more careful analysis and that the two can be brought into agreement. In addition, I argue that ILT accounts for two features of c-axis optical measurements: (1) the observation that the c-axis (perpendicular to the CuO-planes) kinetic energy is substantially reduced in the superconducting state and (2) the correlation (``Basov correlation") between \lambda_c and the c-axis conductivity in the normal state.