ORAL SESSION 4A6: Spectroscopy and Phase Separation

Thursday, Feb. 24, 3:45 p.m. – 6:15 p.m., Room 303A (GRB)

Chairs: Y. Endoh (Tohoku U), J.H. Ranninger (CRTBT-CNRS, Grenoble)

4A6.1 Spatial Modulation of the NMR properties of the Cuprates

Jurgen Haase, Charles P. Slichter, Craig T. Milling, and Raivo Stern, Department of Physics, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801

Presenting Author: C.P. Slichter

We find that the magnetic shifts and electric quadrupole interactions of the Cu and O nuclei exhibit spatial modulation in the cuprates. The size of the modulation increases as the temperature is lowered from room temperature. By Cu Spin Echo Double Resonance,we show that the characteristic length associated with the modulation is only a few lattice distances. Our most extensive data is for optimally doped LSCO. Analysis of the O data shows that the shift and electric field gradient modulations are coherent with each other.

4A6.2 Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds

D. Mihailovic 1, J. Demsar 1, R. Hudej 1, V.V. Kabanov 1, Th. Wolf 2, J.E. Evetts 3, Florence Rullier-Albenque 4, L. Forro 5, and J. Karpinski 6. 1 Jozef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia. 2 Forschungszentrum Karlsruhe, Institut fur Technische Physik, D-76021,Karlsruhe, Germany. 3 Dept. of Materials, Cambridge University, Cambridge, United Kingdom. 4 Laboratoire des Solides Irradies, CEA, Ecole Polytechnique, 91128 Palaiseau, Cedex, France. 5 IGA, EPFL, CH-1015 Lausanne, Switzerland. 6 Festkorperphysik, ETH CH-8093 Zurich, Switzerland.

Presenting Author: J. Demsar

We show that low-energy spectral features in the cuprates can be separated into different components by the measurement of the recombination dynamics of different low-energy excitations in real-time using femtosecond laser spectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap localized state relaxation processes exhibit qualitatively different time- and temperature-dependences. A systematic study of QP recombination as a function of doping and temperature in YBCO and other materials suggests that the ground state of the cuprates is highly anisotropic and spatially inhomogeneous and can be thought of as a mixed Boson-Fermion glass with localised states present over the entire region of the phase diagram. Experiments on electron-irradiated samples show that the localised states are intrinsic and not due to defects. The relaxation measurements also show the existence of two distinct coexisting energy gaps near optimum doping and in the overdoped region, one more or less temperature independent (which exists above and below T_c and one which closes at T_c in a mean-field like fashion. The latter is suggested to arise due to collective interactions between carriers across the inhomogeneous regions (stripes).

4A6.3 NMR Study of "Stripe" Phase in (La,Nd,Eu)_2-xSr_xCuO₄

Takashi Imai, Allen W. Hunt, Philip M. Singer, and Agneta F. Cederstrom, Department of Physics and Center for Materials Science and Engineering, M.I.T., Cambridge, MA 02139, USA

Presenting Author: T. Imai

The so-called stripe phase is believed to exhibit one-dimensional spin and charge density modulation in the CuO₂ planes. Since nuclear spins couple with local electron spins and charges in the CuO₂ planes through magnetic and quadrupole hyperfine interactions, NMR is an ideal probe to investigate the nature of the stripe phase. In fact, our recent NMR investigation into the stripe phase has demonstrated that various NMR properties (e.g. spin echo intensity, quadrupole resonance frequency, and spin echo decay rate 1/T₂) are sensitive to the stripe instabilities [1,2]. In this talk, we will describe our latest efforts to elucidate the physical properties of the stripe phase of the underdoped La_2-xSr_xCuO₄ with and without Nd and Eu co-doping. We will establish a new phase diagram, and discuss the relation with supercoductivity and the possibility of phase separation.

[1] A.W. Hunt et al., Phys. Rev. Lett. 82 (1999) 4300.

[2] P.M. Singer et al., to appear in Phys. Rev. B, December 1, 1999.

4A6.5 Macroscopic phase separation and superconductivity in Bi₂Sr_2-xLa_xCuO₆ single crystals

H.H. Wen, W.L. Yang, and Z.X. Zhao, National Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, P.R. China

Presenting Author: H.H. Wen

By substituting the Sr with La in Bi₂Sr_2-xLa_xCuO₆ single crystals, we obtained the electronic phase diagram with a pronounced "1/8" anomaly. Detailed investigation shows that for each overdoped sample, there are two distinct transitions versus temperature: One is corresponding to the intrinsic upper critical field, and another to the Josephson coupling between the preformed superconducting clusters, leading to a new understanding to the anolalous positive curvature of H_c2(T) in high-T_c superconductors as observed by many other authors.

Furthermore, all samples (roughly 40 samples) with rich oxygen show a magic step-like behavior of T_c versus doping level, although the general parabolic shape remains unchanged. Our discoveries here strongly suggest a cooperative influence of the charge carriers given by the cation doping and the mobile neutral doping (such as intercalated oxygen). The effect of mobile doping is the macroscopic phase separation which in turn causes the step-like behavior of transition temperatures.

Related papers:

1. Josephson-Coupling Origin for the Upward Curvature of the Pseudo-Upper-Critical Field in Bi₂Sr_2-xLa_xCuO₆ Crystals. H.H. Wen, W.L. Yang, Z.X. Zhao and Y.M. Ni, Phys. Rev. Letts. 82, 410 (1999).

2. Crystal growth and superconductivity on heavily La-doped Bi-2201 single crystals. W.L. Yang, H.H. Wen, Y.M. Ni, J.W. Xiong, H. Chen, F. Wu, Y.L. Qin, Z.X. Zhao, Physica C 308, 294 (1998).

3. Step-Like T_c Behavior Induced by Mobile Dopants in High Temperature Superconductors. W.L. Yang, H.H. Wen, Z.X. Zhao, submitted.

4A6.6 Tunneling spectroscopy and surface states of YBCO and NCCO

Satoshi Kashiwaya 1,2 and Yukio Tanaka 2,3. 1 Electrotechnical Laboratory, Tsukuba, Ibaraki 305-8568, Japan. 2 CREST, Japan Science and Technology Corporation (JST). 3 Department of Applied Physics, Nagoya University, 464-8063, Nagoya Japan.

Presenting Author: S. Kashiwaya

Tunneling spectroscopy has been accepted as one of the highest energy resolution probe of the electronic states. The essential concept of this method is based on the equivalence of tunneling spectrum with the density of states of the sample, then it has believed to be insensitive to the phase [1]. However, recent studies on high-T_c superconductors revealed another aspect of tunneling spectroscopy when the samples have anisotropy in k-space. The possibility of zero-energy state formation on the surfaces of d_xy superconductors is presented theoretically by Hu [2]. By developing this idea, a novel formula of tunneling spectroscopy has been presented. The formula suggests that the tunneling spectroscopy is phase sensitive [3-5]. The background of the formula originates from the concept that the surface states of anisotropic superconductors are completely different from those in the bulk. Moreover, various new surface effects have been theoretically discussed by recent papers [6-9].

The consistency between theory and experiments has been tested through the observation of high-T_c superconductors by scanning tunneling microscopy. Using highly oriented YBCO thin films, the zero-bias conductance peaks (ZBCP) are observed on [110] surfaces as spatially resolved profiles [10]. Detailed analysis of the film quality and the spatial dependence of ZBCP reject the possibility of the magnetic impurity effect as the origin of ZBCP. The ratio of ZBCP are relatively small for [100] and [001].

These results are consistent with d_x2-y2 wave symmetry of this material. On the other hand, no ZBCP have been observed on NCCO independent on the orientation even at rough surfaces. This results suggest that NCCO has s-wave symmetry [11].

More detailed experimental studies imply several new effects on YBCO surfaces [12,13]. They are: i) broken time-reversal symmetry states with spatially resolved profile, ii) surface charging up, iii) proximity effect from YBCO to Au overlayer. The consistency between these new effects with theories are discussed.

[1] E.L. Wolf 1985, "Principles of Electron Tunneling Spectroscopy", (Oxford University Press, 1985, New York).

[2] C.R. Hu: Phys. Rev. Lett. 72 (1994) 1526.

[3] Y. Tanaka and S. Kashiwaya: Phys. Rev. Lett. 74 (1995) 3451; Phys. Rev. B 53 (1996) 9371.

[4] S. Kashiwaya, Y. Tanaka, M. Koyanagi, H. Takashima and K. Kajimura: Phys. Rev. B 51 (1995) 1350, ibid, 53 (1996) 2667.

[5] S. Kashiwaya and Y. Tanaka, submitted to Review of Modern Physics.

[6] M. Fogelstrom, D. Rainer and J.A. Sauls: Phys. Rev. Lett. 79 (1997)281.

[7] M. Matsumoto and H. Shiba: J. Phys. Soc. Jpn. 65 (1995) 3384; ibid(1995) 4867.

[8] Y. Tanauma, Y. Tanaka, M. Ogata and S. Kashiwaya, J. Phys. Soc. Jpn., Vol. 67, No. 4, pp.1118-1121, 1998.

[9] Y. Tanaka and S. Kashiwaya, Phys. Rev. B, 53, (1996), 11957, ibid 56, (1997) 892.

[10] L. Alff, H. Takashima, S. Kashiwaya, N. Terada, H. Ihara, Y. Tanaka, M. Koyanagi and K. Kajimura, 1997, Phys. Rev. B, 55, (1997) 14757.

[11] S. Kashiwaya, T. Ito, K. Oka, S. Ueno, H. Takashima, M. Koyanagi, Y. Tanaka and K. Kajimura, Phys. Rev.B 57, (1998) 8680.

[12] M. Covington, M. Aprili, L.H. Greene, F. Xu and C.A. Mirkin: Phys. Rev. Lett. 79 (1997) 277.

[13] S. Kashiwaya, Y. Tanaka, N. Terada, M. Koyanagi, S. Ueno, L. Alff, H. Takashima, Y. Tanuma and K. Kajimura, to be published in J. Phys. Chem. Solid (1998).

4A6.7 Scanning Tunneling Gap Spectroscopy of Cuprate Superconductors

Christophe Renner 1, Ivan Maggio-Aprile 2, Martin Kugler 2, Bart Hoogenboom 2, and Øystein Fischer 2. 1 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA. 2 Department of Condensed Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland.

Presenting Author: C. Renner

The temperature dependence of the superconducting gap (D ) and the normal-state gap, and the question whether these two features have a common origin or not, remain controversial issues. Scanning tunneling spectroscopy (STS) is an ideal, very high resolution technique to address these questions experimentally. We shall review our STS investigations of the quasiparticle excitation spectrum in Bi₂Sr₂CaCu₂O_8+d (Bi-2212) single crystals as a function of doping and as a function of temperature. We find that the generic shape of the tunneling conductance in the gap region is very similar in underdoped and overdoped Bi-2212. The main effect of increasing the hole concentration is to shift the spectroscopic features (coherence peaks at ± D dip at -2D ) continuously closer to the Fermi level. A striking result is that delta does not scale with the transition temperature T_c, but rather with the doping level. The superconducting gap is essentially temperature independent, and instead of closing at T_c, it smoothly evolves into another gap structure above T_c. This normal state gap has a similar amplitude and carrier concentration dependence as D . These results strongly suggest that they are related. We shall also present more recent STS experiments performed on Bi₂Sr₂CuO₆ and YBa₂Cu₃O_7-d single crystals.