Session 22aB

Concepts in High Temperature Superconductivity

Steven A. Kivelson

Physics Department,University of California, Los Angeles, CA 90095-9547 USA

The fact that the repulsive interactions between electrons are large compared to the Fermi energy is the defining feature of highly correlated electronic fluids. It has long been known that this can give rise to high temperature magnetic phenomena, and more recently it has become clear that this leads to various forms of local charge inhomogeneity and charge order. I will explore the relation between these phenomena and high temperature superconductivity. In particular, I will discuss the theoretical reasons to believe that magnetism opposes superconductivity, and that charge inhomogeneity on appropriate scales has the potential to enhance high temperature superconducting pairing, although it tends to suppress superconducting phase coherence.

Gap Inhomogeneity, Phase Separation and Pseudogap in Bi₂Sr₂CaCu₂O_8+d

Azusa Matsuda^a, Takenori Fujii^b, Takao Watanabe^c

^aNTT Basic Research Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi Kanagawa 243-0198, Japan

^bDept. of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjyuku-ku, Tokyo 169-8555, Japan

^cNTT Photonics Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi Kanagawa 243-0198, Japan

Recent STM study showed that the electronic system of Bi₂Sr₂CaCu₂O₈ (Bi-2212) is inhomogeneous. We have studied the detailed doping and temperature dependence of gap inhomogeneity. The temperature dependence showed clear evidence of phase separation into the ßuperconducting" and pseudogapped regions as a double peaked structure in the gap distribution function. The doping dependence supports this view. It also suggests the monotonic increase in the gap energy with reducing doping is due to diverging inhomogeneity. The gap energy dependence of the peak-dip-hump structure suggests gap inhomogeneity accompanies the Fermi level shift or in-plane charge inhomogeneity.

⁶³Cu NQR Observation of Spatial Variation of Electronic Properties in La_2-xSr_xCuO₄

Takashi Imai^a, Philip M. Singer^b

^aDepartment of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada

^bDepartment of Physics, M.I.T., Cambridge, MA 02139, U.S.A.

The common assumption made in the debate over the mechanism of high T_c cuprates is that electronic properties are spatially uniform in the doped CuO₂ planes. In this talk, we will present our detailed NQR and NMR studies of La_2-xSr_xCuO₄ that reveal evidence for the presence of short length-scales in the local electronic properties [P.M. Singer, A.W. Hunt and T. Imai, PRL88 (2002) 047602]. We will also compare the results with the Ca substitution effects in Y_1-xCa_xBa₂Cu₃O_6+d [P.M. Singer and T. Imai, PRL88 (2002) 187601].

STM Observation of Inhomogeneity as a Function of Doping in the High Temperature Superconductor BSCCO-2212

K. M. Lang^a, V. Madhavan^a, J.E. Hoffman^a, E.W. Hudson^a, K. McElroy^a, H. Eisaki^b, S. Uchida^b, J.C. Davis^a

^aDepartment of Physics, University of California, Berkeley, CA 94720-7300

^bDepartment of Superconductivity, University of Tokyo, Japan

We present results from low temperature scanning tunneling microscopy of the high temperature superconductor Bi₂Sr₂CaCu₂O_8+x, focusing on atomic scale spatial variations in the local density of states. By using maps of the local variation of the energy gap magnitude and gap-edge peak amplitude, and by studying the local properties of impurity atom resonances, we examine the local density of states of this material at a variety of oxygen doping-levels. In particular, I will discuss both the granular nature of the electronic structure as observed in underdoped crystals, and its evolution with increasing oxygen doping.