ORAL SESSION 2E2: Pseudo Gap I

Tuesday, Feb. 22, 10:15 a.m. – 12:30 p.m., Room 301D (GRB)

Chairs: N.P. Ong (Princeton U), S. Tajima (ISTEC)

2E2.1 Discovery of the Spin Gap and its Development

Hiroshi Yasuoka, Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki-ken, 319-1195, Japan

Presenting Author: H. Yasuoka

The existence of the Spin-Gap in high-T_c Cu oxides with low doping is well established by now, and anomalously large numbers of experimental and theoretical investigations have been pushed forward to understand the microscopic origin of it. The Spin-Gap phenomenon was first reported in 1989 by analyzing the unusual temperature dependence of the nuclear relaxation rate 1/T₁T [1]. The prominent features may be summarized as, 1 jthe characteristic energy of the Spin-Gap is comparable with the superconducting gap energy, namely ~10meV, 2) the Spin-Gap is associated with the antiferromagnetic spin excitations in the single layer CuO2 plane, and 3) the Spin-Gap temperature decreases with increasing the doping level and reaches near the same temperature of T_c at the optimal doping. Since its discovery, the Spin-Gap behavior manifests itself to the variety of physical quantities with different way, depending on the energy and wave vectors.

In this talk, we will summarize the basic feature of the spin-gap seen in variety of NMR experiments in High-T_c Cu oxides and metallic Vanadium oxides. In the latter case, we show the spin-gap phenomena may be a general characteristic of the correlated electron system near the metal-insulator transition.

[1] H. Yasuoka, T. Imai and T. Shimizu, in Springer Series in Solid State Sciences; Vol. 89 (1989) 254-26

2E2.2 Pseudogap vs Stripe Fluctuations in High-T_c Cuprates?

Shin-ichi Uchida, Department of Superconductivity, University of Tokyo, Hongo, Tokyo 113-8656, Japan

Presenting Author: Shin-ichi Uchida

However, in La-based cuprates, such as La_2-xSr_xCuO₄ and La_2-x-yNd_ySr_xCuO ₄, the stripe fluctuations appear to dominate. There is no spin gap in the underdoped regime, and the pseudogap, e.g. in the optical spectrum, appears as only a weak feature. In this regard, the stripe fluctuations are competing with the pseudogap and hence with superconducting pair formation.

2E2-3 Underdoped Cuprates Above and Below T_c

Mohit Randeria, Tata Institute of Fundamental Research, Mumbai 400005, India

Presenting Author: Mohit Randeria

In this talk I will summarize our current understanding of the unusual aspects of the normal and superconducting states of underdoped cuprates, focusing on the results of photoemission experiments, their connection with various bulk probes, and some recent theoretical developments. I will discuss the (low energy) pseudogap above T_c, its anisotropy and unusual T-dependence, emphasizing its connection with the superconducting gap below T_c. All of this evidence strongly supports the hypothesis that the pseudogap arises from pairing correlations in a state without long range phase coherence. I will then discuss the appearance of sharp quasiparticle peaks below T_c, and the relative importance of nodal quasiparticles and phase fluctuations in determining low T properties. I will conclude with a discussion of open problems.

2E2.4 The Condensation Energy and Pseudogap Energy Scale of Bi:2212 from the Electronic Specific Heat.

John W. Loram 1, John R. Cooper 1, W. Yao Liang 1, Jianlin Luo 2, and Jeff L. Tallon 3. IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 OHE,United Kingdom. 2 Institute of Physics, Chinese Academy of Sciences, Beijing 100080,China. 3 New Zealand IRL, P.O. Box 31310, Lower Hutt, New Zealand.

Presenting Author: J.W. Loram

From measurements of the electronic specific heat coefficient g º C_el/T in the temperature range 5-250 K we determine the superconducting condensation energy U(0) and normal state pseudogap energy scale E_g(p) of Bi:2212. The superfluid density rs and critical fields are estimated from the reduction in free energy in magnetic fields up to 13T. Varying the oxygen content in three ceramic samples (20% Pb doped, undoped and 15% Y doped Bi:2212) we span the hole doping range 0.10<p<0.25 (p_opt ~ 0.16). For p>0.18 (moderate to heavily overdoped) the superconducting behaviour is typical of a conventional metal (an asymmetric specific heat anomaly with magnitude D g (T_c) independent of p, and U(0) µ T_c²). For p<p^crit ~ 0.18, the jump D g (T_c), U(0) and r _s all decrease sharply due to the increasing pseudogap in the normal state DOS which opens abruptly at p=p_crit. A progressive loss of low energy spectral weight with entropy ~ k_B/hole leads to a linear increase in E_g(p) for p< p_crit. In the pseudogap region above T_cg is field independent (< 1% g _n for H=13T). We conclude that the doping dependence of the pseudogap in Bi:2212 and its influence on the condensate are similar to that in other cuprates, and is unrelated to precursor superconducting fluctuations.

2E2.5 Probing Energetics of Superconducting State in High-T_c Cuprates with Infrared Spectroscopy

Dimitri Basov, University of California, San Diego, Dept. of Physics, La Jolla, CA 92093-0319

Presenting Author: D. Basov

Recent experiments performed on a variety of high-T_c superconductors suggest that the electronic kinetic energy is lowered at T < T_c. At least in YBCO and LSCO materials this effect is connected with the development of the normal state pseudogap. We will discuss implications of these results for the understanding of the pseudogap state and of the superconducting state.

2E2.6 Superconducting Gap and Pseudogap from Tunneling Conductance on Bi2212 with Various Oxygen Concentration

N. Miyakawa 1, L. Ozyuzer 2,3, J.F. Zasadzinski 2,3, P. Guptasarma 2, C. Kendziora 4, D.G. Hinks 2, T. Kaneko 1, and K.E. Gray 2. 1 Department of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan. 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. 3 Illinois Institute of Technology, Chicago, Illinois 60616, USA. 4 Naval Research Laboratory, Washington, D.C. 20375, USA.

Presenting Author: N. Miyakawa

Tunneling Spectra have been measured on single crystals Bi2212 with various oxygen concentrations using point contact, STM and break junction methods. The superconducting gap monotonically increases with decreasing hole concentration, while T_c decreases on the underdoped regime, which is a violation of mean field theory. Generic features of the tunneling spectra including dip/hump structures measured at high bias scale with the superconducting gap over the entire doping range. In addition, the Josephson IcRn products increases with decreased doping. The results for temperature dependence of SIS tunneling conductance show that the energy gap goes to zero at T_c for overdoped Bi2212, but for underdoped crystals the gap magnitude decreases significantly as T increases near T_c but a weak depression in the conductance at zero bias remains at T>T_c. These results indicate that these energy gaps are of superconducting origin. Furthermore, since the doping dependence of superconducting gap and T* follow the same trends in Bi2212, this suggests that the pseudogap state between T_c and T* is characterized by some form of precursor superconductivity and that T_c is the temperature where long range coherence sets in.

2E2.7 Origin of the Pseudogap in High Temperature Superconductors

Richard A. Klemm, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439-4845 USA

Presenting Author: R.A. Klemm

Underdoped high temperature superconductors exhibit a 'normal' state for energies E > E_g and/or temperatures T > T*, and a pseudogap in the electronic spectrum for E < E_g and/or T* > T > T_c. Strikingly similar behavior occurs in the dichalcogenides 2H-MX₂, where M = Ta, Nb, and X= S, Se, both in the 'normal' (T > T₀) and in the charge-density wave (T₀ > T > T_c) states. Such strikingly similar behavior also occurs in the organic layered superconductors k -(ET)₂X, where ET is bis(ethylenedithio)tetrathiafulvalene, and X=Cu[N(CN)2]Cl, Cu[N(CN)2]Br, and Cu(SCN)₂, both in the 'normal' region T > T1 > T_c and in the spin-density wave regime T1 > T > T_c. Moreover, the electronic instabilities in both the high temperature superconductors and the 2H-MX₂ dichalcogenides occur on extended saddle bands lying just below the Fermi energy. In addition, in all three classes of materials, a strong magnetic field has no effect upon the behavior in the pseudogap regime, but causes the transport and thermodynamic behavior to fan out greatly below T_c. Thus, we propose that the the pseudogap in underdoped high temperature superconductors arises from spin and/or charge density waves, and not from superconducting fluctations.