ORAL SESSION 1E1: Pseudo Gap and Symmetry

Monday, Feb. 21, 4:30 p.m. – 6:30 p.m., Room 301D (GRB)

Chairs: C.C. Tsuei (IBM Yorktown), K. Maki (U Southern California)

1E1.1 Midgap States Provide Many Stringent Tests on the Pairing Symmetry of High-Temperature and Other Superconductors.*

Chia-Ren Hu, Department of Physics, Texas A&M University, College Station, TX 77843-4242,

Presenting Author: C.-R. Hu

Midgap states (MS's) are a type of Andreev bound (quasiparticle-)state which exist at defects in unconventional (e.g., d-wave) superconductors, such as surfaces/interfaces. They have a topological origin, and depend for their existence on the sign variation of the pair-potential order-parameter on the Fermi surface only. A sizable number of these MS's can occur in a system, all with essentially the same sero energy relative to the Fermi energy. [1] they can thus lead to many novel properties which can serve as solid tests of the pairing symmetry. some of these properties, observed before they were predicted, appeared as mysteries, but they can be unraveled by assuming unconventional-pairing, so that MS's can occur. One such mystery is the zero-bias conductance peak (ZBCP) in single-particle tunneling, whose explanation in terms of the MS's has now been widely confirmed. A stronger confirmation is to observe the ZBCP with local c-axis tunneling near some surfaces/interfaces using lot-T STM/S. Another observed mystery, which has been explained recently [2] in terms of the MS's, is a non-Fraunhofer magnetic-field dependence of the Josephson critical current of a {100}½ {001} high-T_c grain-boundary junction with an atomically flat interface. Various researchers have also predicted many other such novel properties, and most of them have not yet been tested experimentally. One that we suggested years ago, [1] but have analyzed in detail only recently, [3] is a phenomenon we called a "giant magnetic moment".

* Work supported by the Texas Higher Education Coordinating Board and by the Texas Center for Superconductivity at the University of Houston.

[1] C.-R. Hu, Phys. Rev. Lett. 72, 1526 (1994); Phys. Rev. B57, 1266 (1998).

[2] X.-Z. Yan and C.-R. Hu, Phys. Rev. Lett. 83, 1656 (1999).

[3] C.-R. Hu and X.-Z. Yan, to be published in Phys. Rev. B., rapid commun. (1999).

1E1.2 Marginal Stability of High-T_c Superconductors and Time Reversal Symmetry Violation

A.V. Balatsky , T-Div, LANL, Los Alamos, NM 87545, USA

Presenting Author: A.V. Balatsky

Recent experiments indicate that the symmetry of the gap in high-T_c materials can be lower than the generally accepted d-wave. I will argue that the presence of the low lying excitations in the nodes of the d-wave gap allows the secondary gap component ( is or idxy) to be generated once the d-wave state is subjected to the perturbations, thus producing fully gapped spectrum. The symmetry of the order parameter is lowered and often time reversal invariance is violated. This 'softness' of the pure d-wave gap to developing secondary components is reflection of the marginal stability of d-wave superconductor. I will consider specific examples where the complex d xy gap is generated in the bulk near magnetic impurities and in the external magnetic field. I will discuss the implications of this phenomena for the physics in cuprates, such as novel collective modes, and estimate the magnitude of the induced secondary components.

Work done in collaboration with M.Graf, P. Kumar, R. Movshovich and J.R. Schrieffer.

1E1.3 Normal State Gap and High Temperature Superconductivity

W.Y. Liang, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom

Presenting Author: W.Y. Liang

Pseudogap phenomenon in the form of a normal state gap has been widely reported in cuprate superconductors and yet its characteristics and its relationship with superconductivity remain unclear. Theoretical progress has also been slow. This paper reports two recent specific heat results on the Ca doped YBCO and Bi-2212 superconductors, both allowing investigations to be made through the full under- to over-doped range. The Bi-2212 is of particular interest because the results can be directly compared with the previously reported tunnelling and ARPES data, and the similarity of these results and those of YBCO and LSCO provides further evidence of the general nature of the pseudogap in the cuprates. In addition to the information on the distribution of low lying states our data also allow us to deduce other important thermodynamic quantities such as the superconducting condensation energy, which in particular, has given further insights into the irreversibility behaviour of these materials. Thus the variation of the pseudogap size and the condensation energy with carrier density has provided a good basis for understanding T_c values and the strength of critical current density. At the same time, it remains a formidable challenge to fully grasp the origin of the pseudogap and its impact on high T_c superconductivity.

1E1.4 Detection of Phase Fluctuations in BSCCO

Joseph Orenstein 1, John Corson 1, and James N. Eckstein 2. 1 UC Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94729. 2 University of Illinois, Urbana, IL 61801.

Presenting Author: J.W. Orenstein

Cuprate superconductors display remarkable properties, collectively known as pseudogap phenomena, in the normal state below an onset temperature T*. Much speculation as to the origin of the pseudogap has centered on the formation of Cooper pairs below T* which achieve long range phase coherence at T_c. We recently reported [1] measurements which reveal the existence of short range phase coherence above T_c in underdoped BSCCO. We measured the complex conductivity in the high (terahertz) frequency regime where the photon energy is comparable to kT. From a scaling analysis of the conductivity we determined the phase fluctuation frequency and the bare superfluid density as a function of T. We discovered that while partial phase coherence indeed persists above T_c, there is a well-defined crossover to a completely incoherent state at a temperature well below T*. In this talk we present new data from an extended study which includes optimal and overdoped samples. For this set of samples we will describe the evolution of phase fluctuations in the normal state, and the superfluid density and dissipation in the superconducting state, as a function of carrier concentration.

[1] J. Corson et al., Nature 398, p. 221-223 (1999).

1E1.5 Models of the Pseudogap State in Cuprates

M.V. Sadovskii, Institute for Electrophysics, Russian Academy of Sciences, Ural Branch, Ekaterinburg 620049, Russia

Presenting Author: M.V. Sadovskii

We review a certain class of ("nearly") exactly solvable models of electronic spectrum of two-dimensional systems with fluctuations of short range order of "dielectric" (e.g. antiferromagnetic) or "superconducting" type, leading to the formation of anisotropic pseudogap state on certain parts of the Fermi surface. The models are based on recurrence procedure for one- and two-electron Green's functions which takes into account of all Feynman diagrams in higher-order terms in this series. These models can be applied to calculation of spectral density, density of states and conductivity in the normal state, as well as to calculation of some properties of superconducting state.

1E1.6 Small and large pseudogaps in high-T_c superconductors observed by ultra high-resolution photoemission spectroscopy

T. Sato 1, Y. Naitoh 1, T. Kamiyama 1, T. Takahashi 1, T. Yokoya 2, K. Yamada 3, Y. Endoh 4, and K. Kadowaki 5. 1 Department of Physics, Tohoku University, Sendai 980-8578, Japan. 2 Institute of Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan. 3 Institute for Chemical Reseach, Kyoto University, Uji 611-0011, Japan. 4 Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 5 Institute of Materials Science, University of Tsukuba, Ibaraki 305-3573, Japan.

Presenting Author: T. Takahashi

We have performed ultrahigh-resolution (DE=7meV) angle-integrated photoemission study of optimally doped La_1.85Sr_0.15CuO₄ (T_c=38K) and Bi₂Sr₂CaCu₂O_8+d (T_c=87K). We found that the density of states near the Fermi level is remarkably depleted at low temperatures in both compounds irrespective with opening of superconducting gap. The depleted energy region is larger than the size of superconducting gap and the two "pseudogap" do not connect each other. On increasing temperature, the "large pseudogap" is gradually filled-in by spectral-weight transfer from the higher binding energy, but persists well above T_c and T* (at which a "small pseudogap" is closed as observed by ARPES). The energy scale of large pseudogap is consistent with the characteristic temperatures of the magnetic susceptibility and the Hall coefficient. The present results suggest that there are at least two different pseudogaps in high-T_c superconductors.

1E1.7 Responses of the Pseudogap and d-wave Superconductivity to High Magnetic Fields in Underdoped and Overdoped High-T_c Superconductors: ⁶³Cu NMR Studies

Guo-qing Zheng 1, H. Ozaki 1, Y. Kitaoka 1, W.G. Clark 2, P.Kuhns 3, A.P. Reyes 3, W.G. Moulton 3, Y. Kodama 4, T. Kondo 5, Y. Shimakawa 5, and Y. Kubo 5. 1 Department of Physical Science, Grad. Sch. of Eng. Sci., Osaka University, Osaka 560, Japan. 2 Dept. of Phys., UCLA, Los Angeles. 3 Nat. High Magnetic Field Lab. Tallahassee. 4 IRIN, Nagoya 462, Japan. 5NEC Corp. Kawasaki, Japan.

Presenting Author: G.-Q. Zheng

We report the responses of the pseudogap and superconductivity to high magnetic fields up to 30 T in underdoped and overdoped high-T_c superconductors, based on ⁶³Cu NMR measurements. We found contrasting field response of the pseudogap in underdoped material YBa₂Cu₄O₈ (T_c=74K) and slightly overdoped material TlSr₂CaCu₂O_6.88 (T_c=68 K), but a common field-induced change in the electronic state in the superconducting state.

In YBa₂Cu₄O₈, even though a magnetic field of H=23 T reduces T_c by 20 K, no effect was found on the pseudogap, which persists in the temperature region where the superconductivity is destroyed [1]. By contrary, the pseudogap in TlSr₂CaCu₂O_6.88 shows a strong H -variation which can be accounted for quantitatively by the Cooper pair fluctuations above the mean-field transition temperature. These results suggest that one needs to treat the pseudogap in the underdoped and overdoped regimes on a different footing.

At low T in the superconducting state, both materials show a common field response, i.e., the spin susceptibility increases as H1/2 and 1/T1T increases linearly with H. These results are evidences for the H-induced gapless quasiparticle states which are extended from a vortex core of a d-wave superconductor.

[1] G.-q. Zheng et al: Phys. Rev. B60 (1999) R9947.