*2PO5-10 Far-infrared Optical Conductivity of YBa₂Cu₃O_7-x Thin Films

Hajime Shibata 1, Shinji Kimura 1, Satoshi Kashiwaya 1, Shigehiro Ueno 1, Masao Koyanagi 1, Norio Terada 1, Etsuo Kawate 2, and Nagoya Tanaka 3. 1 Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8568, Japan. 2 National Research Laboratory of Metrology, 1-1-4 Umezono, Tsukuba, Ibaraki 305-8563, Japan. 3 Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan.

Presenting Author: H. Shibata

A new method to characterize the optical constants of thin films has been developed, and has been applied to YBa₂Cu₃O_7-x epitaxial single crystal thin films (T_c =90K) grown on MgO substrates to determine the far-infrared optical conductivity s _ab(w ) in the superconducting state. In this method, both the transmittance and reflectance spectra are measured and the results are substituted into a set of coupled equations which describe exactly the transmittance and reflectance of thin films on substrates, where the complex refractive indices n and x of the thin films are unknown parameters. The coupled equations are numerically solved, and the values of n and x are determined as functions of wavenumber omega. The major benefits of the current method are (i) the possibility of application to thin films on substrates, and (ii) the elimination of the need to use Kramers-Kroning analysis which requires extrapolation of the reflectance spectra beyond the measured region to the low- and high-frequency sides. We have determined s _ab(w ) for YBa₂Cu₃O_7-x thin films between w = 50 and 250 cm^-1 at T = 34 and 75K. Analysis of the temperature dependence of the s ₁(w ) based on the two-fluid model suggests that the symmetry of the superconducting paring state of the specimen is d-wave.

2PO5-11 The far-infared c-axis dielectric function of Y_1-xCa_xBa₂Cu₃O_7-d single crystals studied by ellipsometry

C. Bernhard, D. Munzar, A. Golnik, and M. Cardona, Max-Planck-Institut fuer Festkoerperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany

Presenting Author: C. Bernhard

We have performed far-infrared ellipsometric measurements of the c-axis dielectric function of underdoped to overdoped Y_1-xCa_xBa₂Cu₃O_7-d single crystals. The technique of ellipsometry allows us to obtain the complex dielectric function directly with very high accuracy and reproducibility. We present data which show in detail how the spectral gap in the electronic c-axis conductivity evolves as a function of temperature and hole doping. In the underdoped side the spectral gap is almost temperature independent and merely fills in with thermally excited quasi-particles. The superconducting spectral gap undergoes only a modest change in the vicinity of T_c and merges into a normal state pseudo-gap of similar size and shape. As a function of hole doping the size of the spectral gap decreases continuously. The normal state pseudo-gap disappears past optimum doping. On the overdoped side the normal state electronic c-axis conductivity develops a metallic temperature- and frequency dependence. The superconducting energy gap follows a BCS-like temperature dependence and closes at T_c.

2PO5-12 Phonon anomalies in the far-infrared c-axis conductivity of underdoped bilayer compounds: influence of the spacing layer

Dominik Munzar 1, Christian Bernhard 2, Andrzej Golnik 2, Josef Humlicek 1, and Manuel Cardona 2. 1 Dept. of Solid State Physics, Faculty of Science, Masaryk University, Kotlarska 2, CZ - 611 37 Brno, Czech Republic. 2 MPI/FKF, Heisenbergstrasse 1, D -70569, Germany.

Presenting Author: D. Munzar

The far-infrared spectra of the c-axis conductivity of underdoped bilayer compounds exhibit two anomalous features. Firstly, at low temperatures a new broad absorption peak appears in the spectra around 450 (1/cm). Secondly, at the same time as the peak forms, some of the infrared-active phonons are strongly renormalized. We have recently demonstrated [1] that the data for underdoped YBCO can be naturally and probably uniquely explained within a model assuming weak (Josephson) coupling between the copper-oxygen planes, even between the closely-spaced ones. The model involves local electric fields acting on the ions participating in the lattice modes. Here we present an extension of the model which allows us to account for differences between the spectra of severalbilayer compounds. We have found that the ionic contribution to the dielectric constant of the spacing layer separating the bilayers is the most important parameter. The model also explains the observed splitting of the spectral features corresponding to some of the phonons.

2PO5-13 Ultrafast Quasiparticle Dynamics of Tl₂Ba₂CuO_6+d

D.C. Smith 1, P. Gay 1, C.J. Stevens 1, D. Wang 2, J.H. Wang 3, Z.F. Ren 2, and J.F. Ryan 1. 1 Clarendon Laboratory, University of Oxford, Oxford, OX13PU, United Kingdom. 2 Department of Physics, Boston College, Chestnut Hill, MA 02467, USA. Dept. of Chemistry, SUNY-Buffalo, Buffalo, NY 14260.

Presenting Author: D.C. Smith

Most previous studies of ultrafast dynamics in high-T_c superconductors have focused on YBCO [1], and recent work has proposed that the optical response is explained by an isotropic rather than noded superconducting energy gap [2]. However, this material has CuO chains which significantly complicate the response. We present here the first measurements of the ultrafast response of TBCO-2201 thin films over a range of oxygen dopings. In contrast to the YBCO data [1], our results fit better with a d-wave scenario. In particular, the temperature dependence of the amplitude of the induced reflectance follows the condensate density expected for d-wave symmetry. The relaxation slows with decreasing temperature to 4K, where the decay time is on the order of 30ps. This behaviour is consistent with the photoexcited quasi-particles relaxing within nodes. For T>T_c we observe a fast relaxing (t » 500fs) dynamics which is most likely due to the presence of a pseudogap. The magnitude of this component does not peak until T_c+18K. We also observe a remnant of the superconducting dynamics for T_c<T<T_c+13K. Together these observations suggest the possibility of dynamic superconducting fluctuations with a lifetime on the order of 1ps.

[1] C.J.Stevens et al. Phys. Rev. Lett. 59, 2212 (1997), and refs. there in.

[2] V.V.Kabanov et al. Phys. Rev.B59, 1497 (1999)

Supported by the EPSRC (UK) and the EC ULTRAFAST TMR Network.

2PO5-14 Femtosecond Dynamics of BSCCO-2212

D.C. Smith 1, P. Gay 1, C.J. Stevens 1, J.F. Ryan 1, G. Yang 2, and S.J. Abell 2. 1 Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom. 2 School of Metallurgy and Materials, University of Birmingham, Birmingham B15 21T, United Kingdom.

Presenting Author: D.C. Smith

There is now a considerable body of evidence that indicates the presence of a pseudogap in the excitation spectrum of cuprate superconductors, but its origin remains unclear. We report time-resolved pump-probe measurements of the photoinduced optical reflectance in BSCCO-2212 which reveal anomalous behaviour in the temperature range where the pseudogap is observed [1]. For T<T_c the induced reflectance is positive, with an amplitude that approximately follows the temperature dependence of the condensate density, and a relaxation rate that increases rapidly as T® T_c. At high temperatures, the reflectance is similar to that of normal metals, showing only a weak positive transient. However, in the pseudogap temperature range T_c<T<T_c+35K the differential reflectance shows a strong negative transient with much faster sub-picosecond relaxation. This enhanced relaxation is contrary to the reduced quasiparticle scattering inferred from transport measurements in the pseudogap phase, and suggests that photoexcitation directly modulates the pseudogap itself. We will also present the first measurements of the saturation of the induced reflection at high excitation densities which yield the threshold excitation density for destroying the entire condensate with one laser pulse, and a new regime of superconducting dynamics near this threshold.

[1] H. Ding et al., Nature 382, 51 (1996); A.G. Loeser et al., Science 273, 325 (1996).

Supported by the EPSRC (UK) and the EC ULTRAFAST TMR Network.

2PO5-15 Uncovering the Gap, D (w ): Measurement of the Re(s _S/s _N) in Superconducting Tl₂Ba₂CaCu₂O₈

Matthew J. Holcomb* and William A. Little, Department of Physics, Stanford University, Stanford, CA 94305. *Present Address: Nove Technologies, 200 42nd Street, Manhattan Beach CA 90266.

Presenting Author: M.J. Holcomb

Previously we reported the results of a series of optical experiments which yielded detailed information on the energy dependence of the superconducting gap function D (w ) for a series of high-T_c (>80K) cuprate superconductors [Phys. Rev. B 53, 6734 (1996)]. Through high-precision measurements of the superconducting- to normal-state reflectance ratio R_S/R_N obtained from the Thermal Difference Reflectance (TDR) spectra of these materials, we observed significant deviations from unity in R_S/R_N at photon energies as high as 2.0eV. The structure in these spectra could not be accounted for by assuming that the electron pairing was mediated by phonons or any other low-energy boson, alone. However, good agreement between experiment and theory was obtained by assuming that the electron-boson coupling spectrum, which plays a central role in Eliashberg theory, consisted of both an electron-phonon component and a high-energy electronic component between 1.6eV and 2.2eV for all of the different cuprates.

In the present work, we report measurements of both the normal-state reflectance and the superconducting- to normal-state reflectance ratio of a thin film sample of Tl₂Ba₂CaCu₂O₈ at near-normal incidence for photon energies between 0.09 and 4.5eV. From these data we have obtained the real part of the ratio of the superconducting- to normal-state optical conductivities, Re (s _S/s _N). This quantity can be related directly to structure in D (w ) without the need to invoke Eliashberg theory. We find structure in D (w ) for superconducting Tl₂Ba₂CaCu₂O₈ at low-energy, presumably due to electron-phonon coupling, and additional structure at approximately 1.2eV and 1.7eV which we attribute to an electronic component of the pairing interaction this material.

*2PO5-16 Pseudo Gap and Superconducting Gap in an Organic Superconductor k -(BEDT-TTF)₂Cu(SCN)₂

Akito Ugawa and David B. Tanner, Dept. of Physics, Univ. of Florida, Gainesville, FL 32611-8440

Presenting Author: A. Ugawa

The far-infrared polarized reflectance of k -(BEDT-TTF)₂Cu(SCN)₂ was measured over 20-5000 cm^-1 (2.5-620 meV). The optical anisotopry in the (100) crystal face is found to be quite small at all temperatures studied (6.3 K, 50 K, 100 K, and 298 K). The Kramers-Kronig-derived conductivity agrees well with the reported dc conductivity.

The reflectance change from a transition between superconducting and normal states was investigated by applying at 6.3 K a dc current larger than the critical current. No gap-like feature was found above 20 cm^-1 witin an accuracy of 0.1%D R. This suggests a clean-limit superconductivity.

A pseudogap was found in both the E½ ½ b and E½ ½ c conductivity spectra at T=50 K, indicating a strongly correlated state. The sum rule on s (w ) shows that the electronic structure at low temperatures (T<100 K) is essentially different from that at room temperature.

2PO5-17 Crossover from a small to a large polaron in k -(BEDT-TTF)₂Cu(NCS)₂ as probed by infrared spectrosocpy

Markus Bauer, Reinhard Guserle, Ralf Metzger, Kai Numssen, Paul Berberich, and Helmut Kinder, Physik Department E10, Technische Universitaet Muenchen, D-85747 Garchnig, Germany

Presenting Author: M. Bauer

On polycrystalline substrates like metal tapes YBCO thin films must be biaxially textured in order to achieve high critical current densities. This can be achieved with MgO buffer layers grown by inclined substrate deposition (ISD). YBCO thin films on ISD-MgO buffer layers are highly textured with an in plane FWHM of less than 10°. The YBCO c-axis is tilted towards the direction of vapor incidence during the deposition of the underlying MgO buffer layer. Due to this tilt, the critical current density is anisotropic, being lower along the vapor direction and larger across. In this paper we report in detail on the transport properties of the YBCO films, including inductive and resistive measurements. Samples 1 cm wide and up to 10 cm long were prepared and characterized. Additionally the magnetic field dependency of the critical current density is presented. The implication of the tilted c-axis on the transport properties of the coated conductors will be discussed.

2PO5-19 Infrared properties of superconducting Bi-2212 based crystals

L.V. Gasparov 1, D.B. Tanner 2, H. Berger 3, L. Forro 3, and G. Margaritondo 3. 1 Department of Natural Sciences, University of North Florida, 4567 St. Johns Bluff Road S., Jacksonville, FL, 32224-2661. 2 Department of Physics, University of Florida, P.O. Box 118440, Gainesville, FL 32611-8440. 3 EPF de Lausanne, CH 1015, Lausanne, Switzerland.

Presenting Author: L.V. Gasparov

We present temperature-dependent reflectance measurements for Bi-2212 based crystals in the frequency range from 100 to 40,000 wavenumbers. We compare differently doped Bi-2212 as well as Pr-doped Bi-2212 based crystals. We analyze optical conductivity in the framework of a two-fluid approach and analyze how the superfluid density correlates with the superconducting transition temperature of the crystals.

Supported by NSF grant DMR-9705108.