ORAL SESSION 5E7: c-Axis Coherence

Friday, Feb. 25, 9:00 a.m. – 11:00 a.m., Arboretum 5 (Hyatt)

Chairs: B. Batlogg (Bell Labs/Lucent Tech.), T. Timusk (McMaster U)

5E7.1 "Cuprate superconductivity: how far can we get without a 'model'?"*

A.J. Leggett, Loomis Laboratory of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL 61801-3080

Presenting Author: A.J. Leggett

Starting from two very generic and qualitative assumptions, namely (1) that superconductivity in the cuprates is a consequence of the interactions of the electrons in the CuO₂ planes with one another and with the static lattice, and (2) that c-axis transport between unit cells (though not necessarily between different planes within a unit cell) is too slow to be relevant to superconductivity, and - without assuming any microscopic "model" either of the normal state or of the pairing mechanism -, I enquire what constraints are placed on the latter by general considerations such as sum rules, the magnitude of the observed condensation energy etc. I conclude that in a two-dimensional layered system, in strong contrast to the 3D bulk case, it is by no means excluded that a very substantial contribution to the condensation energy comes from saving, via the improved screening which results from pair formation, of the repulsive Coulomb energy in the "overscreened" (small-q) regime, and I demonstrate that such a hypothesis provides a natural explanation of the trends of T_c with the c-axis layering structure. The consequences for the changes in the EELS and optical spectra in the superconducting state are briefly discussed.

*This work was supported by the National Science Foundation through the Science and Technology Center for Superconductivity (grant DMR 91-20000).

5E7.2 Frustrated Kinetic Energy and The Optical Sum Rule in High Temperature Superconductors

Sudip Chakravarty, Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

Presenting Author: S. Chakravarty

We explore the possibility that the superconductivity is due to the saving of the kinetic energy in the superconducting state. This idea is critically examined, and its consequences originating from a rigorous conductivity sum rule are discussed and are compared with recent optical and penetration depth measurements.

5E7.3 C-axis optical properties of high T_c superconductors

D. van der Marel, M. Grueninger, D. Dulic, H.J. Molegraaf, A.A. Tsvetkov, W.N. Hardy, B. Willems, Z.F. Ren, and J.H. Wang, Laboratory of Solid State Physics, Materials Science Centre, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Presenting Author: D. van der Marel

We investigated the Josephson plasma resonances and optical conductivity for optimally doped single-layer Tl₂Ba₂CuO₆ and La_2-xSr_xCuO₄ and the bi-layer high T_c cuprates Tl₂Ba₂CuO₆ and YBa₂Cu₃O_x (x = 6.93 and 7) as a function of temperature and -in the case of TBCCO- of magnetic field. No maximum in the temperature dependence of the c-axis conductivity was observed below T_c, indicating that even in the superconducting state a coherent quasi-particle contribution to the c-axis conductivity is absent or very weak. In YBCO we observe below T_c a transfer of spectral weight from the FIR not only to the condensate at zero frequency, but also to a new peak in the MIR. This peak is naturally explained as a transverse out-of-phase bilayer plasmon by a model for the optical conductivity which takes the layered crystal structure into account. With decreasing doping the plasmon shifts to lower frequencies and can be identified with the surprising and so far not understood FIR feature reported in underdoped bilayer cuprates.

5E7.4 Magnetotransport Study of the Charged Stripes in High-T_c Cuprates

Yoichi Ando, A.N. Lavrov, and Kouji Segawa, CRIEPI, 2-11-1 Iwato-kita, Komae, Tokyo 201-8511, Japan

Presenting Author: Y. Ando

We study detailed magnetotransport properties in heavily-underdoped antiferromagnetic YBa₂Cu₃O_6+x. Several novel features are found in the in-plane magnetoresistance (MR), which unveils the presence of a developed "charged stripe" structure. One of the most striking findings is that the charged stripes are directionally ordered by the application of an in-plane magnetic field. The direction of the stripes can be rotated with the in-plane magnetic field, which gives rise to an intriguing "d-wave"-like anisotropy in the MR as a function of the angle between current and magnetic field. At low temperatures, the directionally-ordered stripes are "frozen", and thus the field-induced resistivity anisotropy becomes persistent. These observations clearly show that the charged stripes are actually acting as "rivers of charge" and that the in-plane magnetic field couples strongly to the stripes, which bears a significant impact on our understanding of the microscopic structure of the stripes. It is interesting to note that our observation is very similar to the field-induced anisotropy in a quantum Hall system [M.P. Lilly et al., PRL 83, 824 (1999)], where a spontaneous formation of the rivers of charges is discussed.

5E7.5 Anisotropy of the Microwave Properties of YBCO

D.A. Bonn, S. Kamal, A. Hosseini, R. Harris, P. Dosanjh, Ruixing Liang, and W.N. Hardy, Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada

Presenting Author: D.A. Bonn

New data on the microwave conductivity over a wide range of frequencies indicate that the quasiparticles in the ab-plane of high purity YBCO develop extremely long transport lifetimes in the superconducting state. Conversely, the c-axis microwave conductivity remains very small and exhibits completely different temperature dependence from that seen in the ab-plane. These results suggest a very anisotropic scattering rate for the quasiparticle excitations in the superconducting state.