2PO5-20 Processing and Characterisation of High T_c Glass Superconductors

A. Memon 1 and D.B. Tanner 2. 1 Bahrain Training Institute, Dept. of Mech. Engineering, P.O. Box. 33090 Bahrain. 2 Department of Physics, University of Florida, Gainesville, FL 32611.

Presenting Author: D. Tanner

The influence of partial substitution of lead at bismuth site in Bi_1-xPb_xSr₂Ca₂Cu₃O₁₀ (here x=0, 0.3, 0.5) glass ceramic superconductors has been investigated for the first time at various temperatures in the far and near infrared regions. Four probe technique reveals that lead doped compound namely Bi_1.7Pb_.3Sr₂Ca₂Cu₃O₁₀ exhibits a T_c of 105K whereas by increasing the lead concentration to 0.5 the T_c of the sample drops to 80K, as compared to the T_c of undoped sample of 85K.

The features of the infrared spectra for Bi(Pb)2223 are same as that of Bi(222). The temperature dependent studies shows that the reflectance of Bi_1.7Pb_0.3 compound increases as the temperature of the sample is reduced below the transition temperature. The optical conductivity calculated from the reflectance of these samples shows variation from normal state to a superconducting state. All phonon modes observed in the normal state are shifted when both the samples are cooled below the transition temperature.

2PO5-21 Interference in macroscopic state in cuprate oxide caused by large //c zero-point oscillation

M. Sugahara, S. Ogi, K. Araki, Y. Ueda, R. Shimoda, and R. Sugiura, Yokohama National Univerisity, Hodogaya, Yokohama, 240-8501, Japan

Presenting Author: M. Sugahara

HTS superconductivity is fragile for localization in contrast with tough metal supercondcutor. The fragility suggests the stability of a number-dfinite macroscopic quantum state (N-MQS) in thin HTS with localization. Anomalous dielectric behavior [1] and electromagnetic interference [2] were observed in high-temperature superconductor (HTS) with localization. Following are noted [1] concerning La_2-xSr_xCuO_4-y films with localization where x determines doping and y localization wiht slight doping £ 0.01. (i) Being confined in CuO₂ layer with D x(//c)<0.66nm, a carrier has ground energy ~ D p(//c)²/2m ~ (0.1-1eV) ~ E_F(Fermi energy)>>300k. (ii) Singlet pairing is favorable even at 300K by D p²/2m reduction in mass replacement (m® 2m). (iii) The zero-point state in each CuO₂ layer equals the (lowest level) Landau-guage solution in mangnetic field B~10^3-4 T which is strong enough to cause a N-MQS Y N of fractional quantum hall effect (FQHE) even at 300K. (iv) Space charge works to unify the individually different "Landau gauge" in each CuO₂ layer. (v) 2D FQHE appears with ± charged regions with Coulomb-energy reduction. (vi) Pairing provides boson type ground FQHE states at "filling factor" x=1/2n (n=1,2,3,...). (vii) Moderate localization stabilizes Y N, facilitating the observation of FQHE by fixing quasiparticles.

We will show the following experimental evidences for the existence of N-MQS at room temperature (and at low temperature) using La_2-xSr_xCuO₄ film with localization.

(i) Negative dielectric constant and enhancement of the negativity at x=1/2n. [1]

(ii) Double-layered charge-domain structure found in the film-thicknowss dependence of the dielctric property. [1]

(iii) dc dielectric interference at room temperature appeared as dielectric Fraunhofer patterns in the voltage dependence of the capacitance of La_2-xSr_xCuO₄ film with the double layer charging.

(iv) Existence of fractional charge respectively observed near x=1/6, 1/4 and 1/2 as the superposed Fraunhofer patterns with period 1:1/6, 1:1/4, and 1:1/2.

As the origin of the interference, Josephson effect and SET (single electron tunneling) are excluded by the room-temperature and 30THz operation in mm size devices. The experimental results are well explicable by FQHE model [1] in HTS film with localization.

[1] M. Sugahara et. al. Symmetry and Pairing in Superconductors (Kluwer, 1999), pp. 395-403; M. Sugahara et al. Science and Engineering of HTC Superconductivity (P. Vincenzini ed., Technina, Faenza, 1999), pp. 467-474.

[2] M. Sugahara et al., Science and Engineering of HTC Superconductivity (P. Vincenzini ed., Technina, Faenza, 1999), pp. 507-514.

2PO5-22 Infrared and Optical Properties of Single-Crystal YNi₂B₂C Superconductor

Jae Hoon Kim 1, Mi-Ock Mun 1, Hanhee Paik 1, B.K. Cho 2, H.B. Kim 3, and Sung-Ik Lee 3. 1 Department of Physics, Yonsei University, Seoul 120-749 Korea. 2 Department of Materials Science and Engineering, K-JIST, Kwangju 500-712 Korea. 3 National Creative Research Initiative Center for Superconductivity and Department of Physics, Postech, Pohang 790-784 Korea.

Presenting Author: J.H. Kim

We have determined the optical conductivity and other key spectral functions of single-crystal YNi₂B₂C for the spectral range of 5 meV - 4.1 eV via infrared reflectivity measurements and spectroscopic ellipsometry. As-grown crystals, polished crystals, and polished-and-annealed crystals were compared to understand the effect of polishing and/or annealing on the optical response. The conductivity spectrum of YNi₂B₂C exhibits metallic electronic response with the screened plasma frequency of 3.76 eV. The free-electron response is not well described by the Drude model alone. A number of interband transitions also appear beyond 1 eV. Their energies and JDOS critical-point types are analyzed on the grounds of selection rules and results of band-structure calculations.

2PO5-24 Infrared absorption of lanthanum manganites

F. Gao, R.A. Lewis, X.L. Wang, and S.X. Dou, Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong NSW 2522, Australia

Presenting Author: F. Gao

Phonons and the electron-phonon interaction play an important role in the phenomenon of colossal magneto-resistance (CMR) observed in lanthanum perovskites. The phonon energies fall in the range 20-100 meV and so are ideally suited to investigation by infrared spectroscopy.

We have reported on the reflectivity of La_0.7Ca_0.3MnO₃ and La_0.8Li_0.2MnO₃ and observed a shift in the "external" phonon mode arising from (La, Ca/Li) vibration against the MnO₃ octahedra between the samples which is explained on the basis of the composition [1].

Most infrared studies on CMR materials have employed reflectivity measurements. Recently, Li et al. [2] reported transmission measurements using KBr as a carrier. We have extended such measurements, also employing CsI carrier. We have studied La_0.7Ca_0.3MnO₃ and La_0.5Ca_0.5Mn_0.9Li_0.1O₃ over the temperature range 10-300 K, i.e. through the Curie temperature. In contrast ot the earlier reflectivity data of Kim et al. [3] but in agreement with the more recent reflectivity data of Franck et al. [4], our transmission measurements reveal no distinct shift in the energies of the "bending" and "stretching" phonon modes at T_c.

[1] R.A. Lewis et al. Aust. J. Phys. 52, 197 (1999).

[2] K. Li et al. J. Phys.: Condens. Matter 10, 4315 (1998).

[3] K.H. Kim et al. Phys. Rev. Lett. 77, 1877 (1996).

[4] J.P. Franck et al. J. Supercond. 12, 263 (1999).

2PO5-25 Optical Properties of Magnetoresistive La_0.7Pb_0.3MnO₃ Single Crystals

A.P. Litvinchuk 1, M.N. Iliev 1, M. Gospodinov 2, and C.W. Chu 1. 1 Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932. 2 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, BG-1784 Sofia, Bulgaria.

Presenting Author: A.P. Litvinchuk

Optical properties of La_0.7Pb_0.3MnO₃ single crystals, which exhibit paramagnetic-to-ferromagnetic transition at T_c=333 K, are studied by reflection spectroscopy over broad frequency (100-18,000 cm^-1) and temperature (80-400 K) range. By analyzing the optical conductivity and its temperature dependence we gain an insight into the interplay of double exchange and localization. Experimental data unambiguously show an abrupt increase of the free charge carrier density upon entering the magnetically ordered ferromagnetic state due to delocalization of charge carriers (electrons). An important experimental finding is that free carriers scattering rate remains only weakly dependent on temperature over broad range. The obtained results are in excellent agreement with theoretical predictions of A. Millis et al. (Phys. Rev. B54, 5405 (1996)) for electron-phonon coupling strength slightly above 1.

*2PO5-26 Raman Scattering Study of Heavily Oxygenated YSr₂Cu₃O_7+d and AuBa₂YCu₂O_7+d Superconductors

A.P. Litvinchuk 1, M.N. Iliev 1, H.-G. Lee 1, M.V. Abrashev 2, L.M. Dezaneti 1, B.R. Hickey 1, Y.Y. Xue 1, and C.W. Chu 1. 1 Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932. 2 Faculty of Physics, Sofia University, BG-1164 Sofia, Bulgaria.

Presenting Author: A.P. Litvinchuk

Raman scattering spectroscopy was used to monitor structural pecularities and charge distribution in highly oxygenated YSr₂Cu₃O_7+d and AuBa₂YCu₂O_7+d superconductors. Both materials, unlike stoichiometric RBa₂Cu₃O₇, possess zig-zag chains. The oxidation state of the chain metal ions (either Cu or Au) was assessed by comparison of experimentally determined frequencies of the c-axis apex oxygen vibrations and results of model calculations which account for the nearest-neighbor interactions and modification of interatomic bond lengths. The enrichment of Raman spectra with respect to the optimally doped YBa₂Cu₃O_6.95 superconductors was interpreted as being due to the lower symmetry of atomic sites, which are occupied by excess oxygen ions in the basal plane.

2PO5-27 Microstructure, transport property and phonon vibration of Fe doped Bi2201 systems

Gaojie Xu 1,2, Zhiqiang Mao 2, Zejun Din 1, Qirong Pu 1, and Yuheng Zhang 2. 1 Department of Astronomy and Applied Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China. 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China.

Presenting Author: G. Xu

Polycrystalline samples of Bi_1.8Pb_0.2Sr₂Cu_1-xFe_xO_y and Bi_1.8Pb_0.2Sr_1.6+xLa_0.4-xCu_1-xFe_xO_y have been investigated by means of electron diffraction (ED), Raman scattering and resistivity measurements. Fe doping in the both systems leads to the lattice parameter a increasing and parameter c decreasing. The ED patters analyses indicate that the characteristic of the modulation structure of Bi2201 system independ on the amount of the extra oxygen, and the crystal misfit between Bi₂O₂ layer and perovskite blocks should be the origin of the superstructure modulation. The Raman scattering analyses suggest that the phonon frequencies of OSr mode around 625 cm^-1 "charge transfer" effect, i.e., with the decrease of carrier concentration, the OSr mode softens. The change of transport properties induced by Fe doping (Bi_1.8Pb_0.2Sr_n42Cu_1-xFe_n4xO_y) and double doping (Bi_1.8Pb_0.2Sr_1.6+xLa_0.4-xCu_1-xFe_xO_y) has been discussed in this paper.

2PO5-28 Resonance of "Raman-forbidden" mode in La doped Bi-2201 single crystals

Peng Li 1, Wanli Yang 1, Pingheng Tan 2, Haihu Wen 1, and Zhongxian Zhao 1. 1 National Laboratory for Superconductivity, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China. 2 National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R. China.

Presenting Author: P. Li

A "Raman-forbidden" phonon mode at about 840 cm^-1 is observed popularly in oxygen annealed pure and La doped Bi-2201 single crystals. X-ray diffraction measurements indicate that there exists only one single phase in our samples. The linear dependence of this phonon line intensity on the excitation power ruled out the possibility of nonlinear effects. Polarized Raman experiments indicate the additional 840 cm^-1 mode has almost the same perfect A_1g symmetry as other main phonon modes, such as the lines at about 460 and 620 cm^-1.

At the same time, a remarkable resonance effect is found. The additional line exhibits an extremely resonance at (or little less than) the 2.4 eV (514.5 nm). The most pronounced resonance, with the intensity ratio for the 840 and 460 cm^-1 lines as large as about 20, is found in several Bi₂Sr_2-xLa_xCuO_6+y crystals with x ~ 0.5.

We propose a model for the manifestation of the additional 840 cm^-1 in the Raman spectra of the oxygen annealed La doped Bi-2201 crystals due to the ordering of the interstitial oxygen in the Bi-O layers. As a result of the short-range ordering, the Brillouin-zone folding effect makes a few G -point phonon states from an infrared-active phonon branch, which can be seen in Raman spectra.

Basing on the study of the similarity in the appearance of the "Raman-forbidden" modes between Y123 and Bi2201 systems. Our results provide Raman evidences for confirming that the ordering of the movable oxygen exists popularly in high temperature superconductors.

2PO5-29 High-pressure Raman study of Bi₂Sr₂CaCu₂O_8+d : indications of strong bond-strength hierarchy and pressure-induced charge transfer

Minoru Osada 1, Masato Kakihana 2, Haruo Arashi 3, Mikael Kall 4, and Lars Borjesson 4. 1 The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan. 2 Materials and Structures Laboratory, Tokyo Institute of Technology., 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. 3 Department of Machine Intelligence and Systems Engineering, Tohuku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8579, Japan. 4 Department of Applied Physics, Chalmers University of Technology, S-412 96 Gothenburg, Sweden.

Presenting Author: M. Osada

Phonon Raman scattering from slightly overdoped Bi₂Sr₂CaCu₂O_8+d (Bi2212) samples have been investigated under hydrostatic pressure up to 7.2 GPa. We found that the degree of phonon hardening is lower than expected from the large c-axis compressibility of Bi2212 and that the mode-specific Grunesien parameters decrease with increasing phonon frequency. These results can be understood if the complicated structure of the Bi2212 unit-cell, which includes strong "intra-molecular" and soft "inter-molecular" bonds, is considered. We also found a pronounced nonlinear pressure dependence of the 140 cm^-1 Cu mode which correlates to the dependence of T_c versus pressure. We argue that this correlation is due to a transfer of holes to the CuO2-planes beginning at ~2 GPa which alters the Cu bond potentials and causes T_c to decrease due to overdoping.