ORAL SESSION 4E6: Pseudo Gap II

Thursday, Feb. 24, 3:45 p.m. – 6:15 p.m., Room 301D (GRB)

Chairs: M. Randeria (Tata Inst. of Fund. Res.), H. Yasuoka (Japan Atomic Energy Res.Inst.)

4E6.2 Effects of the Pseudogap on the Anisotropic Penetration Depth of High Temperature Superconductors

C. Panagopoulos 1, J.R. Cooper 1, B.D. Rainford 2, T. Xiang 3, Y.S. Wang 4, and C.W. Chu 4. 1 IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom. 2 Department of Physics and Astronomy, University of Southampton, Southampton S017 1BJ, United Kingdom. 3 Institute of Theoretical Physics, Academia Sinica, P.O. Box 2735, Beijing 100080, P.R. China. 4 Department of Physics and Texas Center for Superconductivity at the University of Houston, Houston, TX 77204-5932.

Presenting Author: C. Panagopoulos

The absolute values and temperature dependences of the in-plane and out-of-plane magnetic penetration depth have been measured as a function of carrier concentration for several high temperature superconductors. We find that the superfluid density changes substantially and systematically with doping. The results are discussed in the context of a possible relationship between the superfluid density and the normal state (or pseudo) gap.

4E6.3 Critical doping and the pseudogap in HTS cuprates: sudden crossover to Fermi liquid behaviour

Jeffery L. Tallon, Industrial Research, P.O. Box 31310, Lower Hutt, New Zealand

Presenting Author: J.L. Tallon

A critical doping state in the lightly overdoped region is identified from a wide range of physical properties where the pseudogap and AF correlations abruptly disappear and the cuprates suddenly cross over to Fermi liquid behaviour. Superconductivity is found to be most robust at this point, as indicated by condensation energy, superfluid density, impurity scattering and critical currents. All of these properties pass through sharp extrema at this point in a manner suggestive of a quantum critical point. This scenario provides a possible explanation for the generic phase behaviour and the doping- and temperature-dependent transport properties.

4E6.4 Novel relation between T_c and low-temperature (T<<T_c) energy gap 2D _o in Bi2212 and La214: an STM/STS study

M. Oda, N. Momono and M. Ido, Department of Physics, Hokkaido University, Sapporo 060-0810, Japan

Presenting Author: M. Oda

We report that the low-temperature (T<<T_c) gap amplitude 2D _o of Bi2212 and La214, measured by STM/STS together with clear atomic images, continues to increase as a function of decreasing carrier-doping level p even in the underdoped region, where T_c is largely suppressed. On the basis of STM/STS and electronic specific heat data, we suggest that a pseudogap with an energy scale of ~2D ₀ will exist above T_c in La214 as in Bi2212. It is also found over a wide p range in the two systems that pseudogap temperature T* is nearly given by T*=2D _o/A; A=4~5 and T_c is proportional to pD _o (T_c~1.5pD _o) as proposed by Lee and Wen. The latter finding indicates that the energy scale in determining T_c is ~pD _o in high-T_c cuprates, differing from the expectation (D _o) in the BCS theory. We will discuss some scenarios for the transition from pseudogap to superconducting states, where T_c~1.5pD _o can be expected.

4E6.5 Short Coherence Length Superconductivity: Thermodynamics and Transport Properties in Underdoped Cuprates

K. Levin, Qijin Chen, and Ioan Kosztin, The University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637

Presenting Author: K. Levin

We describe a theory of the superconducting state in short coherence length superconductors, based on an approach which smoothly interpolates between the BCS and Bose Einstein condensation (BEC) pictures. [1] While there has been much related work on the T = 0 and T > T_c descriptions, here we address general T £ T_c. Our goal is to investigate how transport and thermodynamic properties evolve as one goes from the BCS (large x , with small attractive coupling constant, g) to the BEC regime (small x , large g). While fermionic quasi-particles dominate at small g, at larger coupling the superconducting excitations acquire a bosonic nature. The implications for the superfluid density, Knight shift and NMR, as well as specific heat are discussed and compared with experiments on the underdoped cuprates and some organic superconductors. Comparison with an alternative phase fluctuation picture of the pseudogap state is presented.

[1] Q.J. Chen, I. Kosztin, B. Janko, and K. Levin, "Pairing fluctuation theory of superconducting properties in underdoped to overdoped cuprates", Phys. Rev. Lett. 81, 4708 (1998).

4E6.7 Mean Free Path of Low-Energy Excitations in Bi 2212

Tonica Valla, Brookhaven National Laboratory, Physics 510 B, Upton, NY 11973

Presenting Author: T. Valla

The momentum width of the Bi 2212 Fermi surface has been measured in angle-resolved photoemission in a direction perpendicular to the Fermi surface. This quantity reflects the mean free path of low-energy excitations. Since it already contains all the relevant renormalizations in itself, it can be more tightly bounded to transport properties than a lifetime. It is shown that there is a significant degree of anisotropy in the mean free path along the Fermi surface. The temperature dependent measurements indicate that the temperature induced broadening exists along the most of the Fermi surface.

4E6.8 Normal State Transport Properties of Single- and Double-Layered BiSrCaCuO Thin Films and Pseudogap Effect

Z. Konstantinovic, Z.Z. Li, and H. Raffy, Laboratoire de Physique des Solides, Bât 510, Université Paris-Sud, 91405 Orsay Cédex, France

Presenting Author: H. Raffy

The temperature dependence of the in-plane resistivity r (T) of epitaxial c-axis oriented single-layered Bi₂Sr_1.6La_0.4CuO_y and double-layered Bi₂Sr₂CaCu₂O_y thin films, was investigated at different oxygen contents. The deposits are prepared by RF magnetron sputtering. The same film of a given phase is brought, by successive annealing treatments, from maximally overdoped (T_c=5 K and 50 K for one and two layers respectively) to the strongly underdoped non-superconducting state passing through its optimal state (T_cmax=30 K and 80 K respectively). Hall effect measurements confirm that the described annealing procedure produces a change in carrier concentration in the sample.

For overdoped states r (T) can be described by a phenomenological law of the form: a+bT^m with 1<m<1.5. For both phases, in the underdoped region we observe a downward deviation of the resistivity from T-linear behavior found at high-temperatures: pseudogap behavior. The temperature where this downturn occurs, T*, shows strong doping dependence and will be presented in the phase diagram (temperature vs doping) for both cases. The similarities and differences observed for single- and double-layer behavior will be discussed.