2PO5-40 Unconventional electronic Raman spectra of borocarbide superconductors

In-Sang Yang 1,2, M.V. Klein 2, T.P. Devereaux 3, and P.C. Canfield 4. 1 Department of Physics, Ewha Womans University, Seoul 120-750, Korea. 2 Department of Physics and Science and Technology Center for Superconductivity, University of Illinois, Urbana, IL 61801, USA. 3 Department of Physics, University of Waterloo, Canada. 4 Ames Laboratory, Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.

Presenting Author: I.-S. Yang

Borocarbide superconductors, which are thought to be conventional BCS-type superconductors, are not so conventional in several ways as observed in electronic Raman studies.

Anisotropic gap-like features and finite scattering strength below the gap was observed for the RNi₂B₂C (R = Lu, Y) systems. [1] The origin of the finite scattering strength below the gap and the normal state continua, as well as the 2-d peak features is studied by applying magnetic fields, by doping the magnetic/non-magnetic impurities, and by electron-irradiation.

Raman cross-section calculation which includes inelastic scattering shows a relatively good fit to the B_1g spectra; while it does not fully account for the sharpness of the B_2g spectra and the sub-gap features in both B_1g and B_2g spectra.

[1] In-Sang Yang et al., preprint (1999).

2PO5-41 First-Principles Raman Studies on Rare-Earth Carbide Halides

Peter Puschnig and Claudia Ambrosch-Draxl, Institute of Theoretical Physics, University Graz, Universitätsplatz 5, 8010 Graz, Austria

Presenting Author: P. Puschnig

Y₂C₂Hal₂ (Hal = Br, I) are metallic layered compounds, where superconductivity up to 11.6 K was found [1]. We have investigated the totally symmetric Raman modes of Y₂C₂Br₂ and Y₂C₂I₂ by first-principles calculations based on density functional theory. The phonon frequencies and Raman intensities are obtained by carrying out total-energy and atomic-force calculations using the full-potential LAPW method. All frequency values are in very good agreement with previous Raman measurements by Henn et al. [2]. Their assignment of modes which was achieved by substitution of Y by Gd is confirmed by our computed eigenvectors. Moreover, the frequency changes in the C stretching and tilting vibrations introduced by replacing C by its ¹³C isotope are excellently reproduced by our results.

[1] A. Simon, Hj. Mattausch, R. Eger, and R.K. Kremer, in Handbook of the Physics and Chemistry of Rare Earths, editeds by K.A. Gschneidner, Jr. and L. Eyring (Elsevier Science, Amsterdam, 1991), p. 191.

[2] R.W. Henn, T. Strach, R.K. Kremer, and A. Simon, Phys. Rev. B 58, 14364 (1998).

2PO5-42 Raman behavior of two Ag modes in the fullerides with t1g states

X.H. Chen 1 and Y. Iwasa 2. 1 Structure Research Laboratory and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China. 2 Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan.

Presenting Author: X.H. Chen

Raman spectra have been studied on the highly doped fullerides Ba_xC₆₀ (x=3,4, and 6), A₃Ba₃C₆₀ (A=K, Rb), and K_xSm_2.75C₆₀ (x=0, 3.25). It was found that the Raman spectra are essentially indentical to each other for all of these ternary fullerides. The results show a crossover point at the boundary of t_1u and t_1g bands for the charge transfer dependence of Raman shift. The tangential A_g(2) mode, which is known as a sensitive probe for the degree of charge transfer on the C₆₀ molecules, follows a characteristic shift of ~ 6.3cm^-1 per elementary charge for the t_1u fullerides Sm_2.75C₆₀ and Ba₃C₆₀. While the redshift of A_g(2) mode per elementary charge is about 9 cm^-1 relative to Ba₃C₆₀ for the t_1g fullerides Ba₄C₆₀, and Ba₆C₆₀ if charge transfer from Ba to C₆₀ molecules is complete, being much larger than ~6.3cm^-1. Particularly, the totally symmetric A_g(2) mode in the t1g tenary fullerides has an anomalous small downshift. It indicates that the Raman shift of the A_g(2) mode in the t_1g fullerides cannot be understood by a simple extrapolation from the low doped t_1u fullerides. An apparent different behaviour for the radial A_g(1) mode is observed between the t_1u and t_1g fullerides. The present result shows that the Raman spectra of t1g states cannot be explained only by charge transfer as in t_1u fullerides.

2PO5-43 Collective Modes and Electronic Raman Scattering in the Cuprates

Francesca Venturinei 1, Umberto Michelucci 2, T.P. Devereaux 3, and A.P. Kampf 2. 1 Walther-Meissner-Institut, Walther-Meissner-Str. 8, 85748 Garching, Germany. 2 Theoretische Physik III, Elektronische Korrelationen und Magnetismus, Institut fuer Physik, Universitaet Augsburg, 86135 Augsburg, Germany. 3 Department of Physics, University of Waterloo, Waterloo Canada, N2L 3G1.

Presenting Author: U. Michelucci

While the electronic Raman response at low frequencies in the superconducting state in the cuprates can be largely understood in terms of a d-wave energy gap, a long standing problem has been an explanation for the spectra observed in A1g polarization orientations. We present calculations which suggest that the observed A1g peak is due to a collective spin fluctuation mode. We investigated several models which link our results to the 41 meV neutron resonance.

2PO5-44 Raman Intensity in HTSC: Role of the States Near the Fermi Surface

Eugene Ya. Sherman and Claudia Ambrosch-Draxl, Institute of Theoretical Physics, University Graz, Universitätsplatz 5, 8010 Graz, Austria

Presenting Author: E.Ya. Sherman

We have studied role of the states in the vicinity of the Fermi surface for intensity of phonon Raman scattering in high-T_c superconductors in different models of electron-phonon coupling. The analysis shows that these states can be crucially important when the frozen-phonon approximation is applied. However, this effect does not arise within a perturbative approach. The discrepancy is enhanced in the case of resonant Raman scattering. The difference of these methods which is due to the infinitely slow atomic movement within the frozen-phonon approximation is important for comparison of theory and experiment. We investigated redistribution of electrons near the Fermi surface between the CuO chains and the CuO₂ planes and within the planes. The phonon-driven redistribution accompanied by a possible instability of the Fermi surface is important for understanding of properties of the normal and superconducting state.

2PO5-45 IR and Raman Scattering Measurements on Single Crystal SrTiO₃

T.R. Yang and C.C. Lu, Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan

Presenting Author: T.R. Yang

Single crystal SrTiO₃ is an important substrate for superconducting thin film. We studied and analyzed the optical properties of this crystal in the longwave range at 10 - 300K. A number of strong absorption dips at 171 and 485 cm^-1 had been measured in the far-infrared region. An extra absorption dip in 439 cm^-1 at low temperature had been measured and discussed. We characterized and fitted these vibration modes and optical parameters by a set of dielectric response model. The dielectric value in the longwave limit was also reported.

2PO5-46 Far-infrared Dynamics of Underdoped La_1.985Sr_0.015CuO_4+d

Young Hoon Kim 1 and Pei-Herng Hor 2, 1 Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011. 2 Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5932.

Presenting Author: Young Hoon Kim

d = 0.032 sample exhibits much more enhanced background conductivity and an extremely narrow Drude-like conductivity peaked at below 20 cm^-1 which is completely suppressed for temperatures below T_c = 30 K. Also, in d = 0.032 sample there exists substantial spectral weight transfer from the electronic excitation to the phonons and the far-infrared background conductivity for T < T_c. We propose that charge ordering plays a significant role in this peculiar charge dynamics.

2PO5-47 Infrared in-plane Conductivity of Y_1-xCa_xBa₂Cu₃ O_7-d films and single crystals studied by spectral Ellipsometry

Todd Holden 1, C. Bernhard 1, A. Golnik 1,2, G. Cristiani 1, H.-U. Habermeier 1, and M. Cardona 1. 1 Max-Planck Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany. 2 Institute of Experimental Physics, Warsaw University, Hoza 69, PL-00-681, Warsaw, Poland.

Presenting Author: Todd Holden

[4] J.P. Carbotte, E. Schachinger, and D.N. Basov, Nature 401, 354 (1999).