POSTER SESSION 2PO5: Raman and IR Spectroscopy

Tuesday, Feb. 22, 1:30 p.m. – 3:30 p.m., Hall D (GRB)

2PO5-1 Anisotropic penetration depth and optical sum rule violation in La_2-xSr_xCuO₄

F. Marsiglio, Department of Physics, University of Alberta, Edmonton, Alberta, Canada and J.E. Hirsch, Department of Physics, University of California, San Diego, La Jolla, CA 92093-0319

Presenting Author: F. Marsiglio

We calculate the doping dependence of the penetration depths and optical sum rule violation in La_2-xSr_xCuO₄ within the model of hole superconductivity. In the clean limit the predicted in-plane sum rule violation is larger than the c-axis one, however in the presence of disorder the latter one becomes very substantially enhanced. The sum rule violation is found to decrease rapidly with doping.

2PO5-2 A Consistent Picture of Electronic Raman Scattering and Infrared Conductivity in the Cuprates

Thomas P. Devereaux, Dept. of Physics, Univ. of Waterloo, Waterloo ON N2L 3G1, Canada, and Arno P. Kampf, Theoretische Physik III, Elektronische Korrelationen und Magnetismus, Institut fuer Physik, Universitaet Augsburg, 86135 Augsburg, Germany

Presenting Author: T.P. Devereaux

Calculations are presented for electronic Raman scattering and infrared conductivity in a d_x2-y2 superconductor including the effects of elastic scattering via anisotropic impurities and inelastic spin-fluctuation scattering. A consistent description of experiments on optimally doped Bi-2212 is made possible by considering the effects of correlations on both inelastic and elastic scattering.

2PO5-3 Electron-Boson Effects in the FIR Properties of Metals

Sergej V. Shulga, Theor. Dept.,Institute of Spectroscopy, RAS, Troitsk, 142092, Russia

Presenting Author: S.V. Shulga

I present the exact inversion of the normal state Nambu-Nam formalism (Nam S.B., Phys. Rev., v. 156, 470-486 (1967). Is is also shown that the superconducting stateNam formalism could be inverted numericaly. The solutions are applied to the analysis of the experimental data of conventional and HTSC materials. It is shown that the bosons responsible for the superconductivity in the optimally doped YBaCuO have the spectral functionwith the upper frequency bound less then 60 meV and averaged boson frequency near 35 meV.

2PO5-4 Possible Mechanism for Persistent Photoconductivity in YBCO: Experiment and Theory

Daniel M. Bubb 1, J.F. Federici 1, S.C. Tidrow 2, W. Wilber 3, J. Kim 4 and A. Pique 4. 1 New Jersey Institute of Technology, Department of Applied Physics, University Heights, Newark, NJ 07102. 2 Sensors and Electronic Devices Directorate, Army Research Laboratory, Adelphi, MD 20783. 3 U.S. Army CECOM, Fort Monmouth, NJ 07703. 4 Naval Research Laboratory, Surface Modification Branch, Washington, DC 20783.

Presenting Author: D.M. Bubb

Recent experimental work [1,2] has focused on apprehending the underlying mechanism for Persistent Photoconductivity (PPC) in YBCO. Photoconductive and low frequency electrical measurements on thin films form the backbone of the experimental work. These results will be interpreted in the light of self-organizing principles embodied by a filamentary model of High Temperature Superconductivity [3,4].

[1] Physical Review B, Vol. 60, 6827 (Authors above).

[2] D.M. Bubb, J.F. Federici, to be published.

[3] J.C. Phillips, Proc. Nat. Acad. Sci., USA, 94, 12 771.

[4] J.C. Phillips, Phil. Mag. B, In Press.

2PO5-5 Anisotropy of Non-Equilibrium Quasi-Particle Dynamics in Single Crystal YBa₂Cu₃O_7-d

P. Gay, C.J. Stevens, D.C. Smith, C. Chen, and J.F. Ryan, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom

Presenting Author: J.F. Ryan

We report the first measurements of non-equilibrium quasi-particle dynamics in de-twinned single crystals of YBa₂Cu₃O_7-d . Femtosecond time-resolved polarised pump-probe reflectance studies of de-twinned crystals were made which, unlike all previous thin film studies, measure the full anisotropy of ultrafast optical response in the ab-plane. The a-axis signal shows induced reflectance at all temperatures and displays two-component relaxation behaviour: there is an initial subpicosecond decay which is largely temperature independent, followed by a slower component which has a relaxation rate t ^-1=0.2ps^-1 at 4K. The amplitude of the latter component drops quickly at T_c, and t ^-1 increases with increasing temperature up to T_c, consistent with gap closure. The b-axis behaviour is markedly different: the initial induced reflectance recovers within 2ps, overshoots, and thereafter a time-independent induced transmission is observed. The amplitude of the initial response is temperature independent below T_c, but it decreases systematically at higher temperatures over the range T_c® T_c+30K, reminiscent of the reported pseudogap behaviour in YBa₂Cu₃O_7-d . Thus these measurements clearly distinguish the pseudogap and superconducting phase response which could not be resolved in previous measurements of films. These results indicate the existence of a coherence-sensitive interband absorption which may provide a new perspective on the ultrafast quasiparticle dynamics in superconductors.

2PO5-6 Observation of Geometrical Resonances and Tunneling SNS and SIS Edge Junctions with 60K YBCO Films

Gad Koran and Ofer Nesher, Physics Dept., TECHNION - Israel Institute of Technology, Haifa 32000, Israel

Presenting Author: G. Koren

Properties of oxygen-deficient edge junctions made of thin YBCO films with T_c=60K as the superconductor, and iron doped YBCO (YBFe_0.45Cu_2.55O) as the barrier are reported. Weak-link SNS behavior in the dynamic conductance of these junctions is observed when the edge angle is large (~50 degrees), while SIS tunneling behavior is found when the edge angle is small (~36 degrees). Two types of SIS junctions were prepared in which transport is either along the (100) or the (110) direction in the a-b plane. The dynamic conductance shows a gap-like structure in the (100) junctions, while a zero-bias peak is observed in the (110) junctions. These results indicate that the order parameter in YBCO has a dominant d_x2-y2-wave symmetry. In the SNS type junctions series of peaks are observed in the dynamic resistance. Below the gap, these peaks were identified as due to multiple Andreev reflections in the barrier, and from this data we found an energy gap value of about 16 meV in the a-b plane. Above the gap, the peaks are attributed to McMillan-Rowell oscillations, and from these oscillations we found that the renormalized Fermi velocity of the quasiparticles in the barrier is 1.5x10⁷ cm/sec. Above the gap, but in the c direction, resonances were identified as due to Tomasch oscillations in the cover electrode. These oscillations yield a renormalized Fermi velocity of 4.5x10⁷ cm/sec and a gap value in the c direction of ~1.4 meV.

2PO5-7 Time-domain Terahertz spectroscopy as a diagnostic tool for the electrodynamic properties of high temperature superconductors

Ingrid Wilke 1, Maxim Khazan 1, Carsten T. Rieck 1, Petr Kuzel 2, Christian Jaekel 3, and Heinrich Kurz 3. 1 Universitaet Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany. 2 Institute of Physics, Academy of the Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic. 3 Institut fuer Halbleitertechnik, RWTH Aachen, Sommerfeldstrasse 24, D-52056 Aachen, Germany.

Presenting Author: I. Wilke

Time-domain Terahertz transmission spectroscopy (TDTTS) has proved to be a unique method for the experimental study of electromagnetic properties of high temperature superconducting thin-films at THz-frequencies in basic and applied research. We have improved the analysis of TDTTS-measurements by numerically extracting the complex index of refraction from the measured complex transmission of the superconducting thin film. By dielectric conversion the complex dielectric function is obtained. This is the first measurement of the dielectric function of YBCO between 0.5-2.0THz. Based on this other important figures as surface resistance, penetration depths and complex conductivity are derived. The advantage of this approach is that it only involves basic electrodynamics and no assumption of the mechanism of high temperature superconductivity. Furthermore, analysis of superconducting thin-films by TDTTS is extended to higher frequencies and thicker films than previously.

We have determined by TDTTS the magnetic penetration depths and surface resistance of c-axis oriented YBCO-thin films up to 2THz. The temperature and frequency dependence of our data is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering. The temperature dependence of the inelastic scattering is based on spin fluctuation exchange within the Nested-Fermi-Liquid (NFL) model.

*2PO5-8 Femtosecond mid-infrared study of YBa₂Cu₃O_7-d

R.A. Kaindl 1, M. Woerner 1, T. Elsaesser 1, G.A. Garnan 2, M.P. McCurry 2, D.G. Walmsley 2, D.C. Smith 3, and J.F. Ryan 3. 1 Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany. 2 Department of Pure and Applied Physics, Queen's University of Belfast, Belfast BT7 1NN, United Kingdom. 3 Department of Physics, Clarendon Laboratory, Oxford University, Oxford OX1 3PU, United Kingdom.

Presenting Author: R.A. Kaindl

The mid-infrared optical response of high-T_c cuprate superconductors is closely connected to the low-energy electronic excitations and their correlated dynamics. The most pronounced changes of the (ab)-plane reflectivity R(w,T) upon cooling to low temperatures appear in the mid-infrared. When crossing T_c in optimally-doped cuprates, an absorption shoulder develops around 100 meV, which has been linked to the opening of a gap in the scattering rate of intraband transitions. In underdoped cuprates, however, this feature is already observed below pseudogap temperatures T* >> T_c. Here we present the first ultrafast investigation probing the mid-infrared (l = 7-20 µm) response of a high-temperature superconductor. Femtosecond nonequilibrium excitation of optimally-doped and underdoped YBa₂Cu₃O_7-d allows us to investigate the ultrafast fill-in of the conductivity gap. During the subsequent reformation of this conductivity gap, two components appear which are separated by their different decay dynamics. The slow component (~ 5 ps) is directly connected to the condensate density and disappears at T_c in both optimally and underdoped samples, while the fast component (~ 700 fs) persists up to T*. We discuss the nature of the optically-induced reflectivity changes and their close relation to the 41 meV antiferromagnetic resonance peak observed in neutron scattering.

2PO5-9 Dielectric function of YBCO in the mid-infrared: temperature and doping dependence at 3392nm.

G.A. Farnan, G.F. Cairns, P. Dawson, M.P. McCurry, S. Oprey, and D.G. Walmsley, Department of Pure and Applied Physics, Queen's University, Belfast BT7 1NN, Northern Ireland

Presenting Author: G.A. Farnan

High quality c-axis thin films of YBa₂Cu₃O_7-d have been deposited by PLD on MgO substrates and their doping has been modified by annealing in different pressures of O₂. The films have been coupled to a sapphire prism in the Otto configuration, and the attenuated total reflectance of the resulting multilayer has been measured as a function of angle at a fixed wavelength of 3392nm. The reflectance versus angle characteristic is dominated by a strong absorption feature due to the excitation of surface plasmons. This can be modelled, allowing a determination of the ab-plane dielectric function. Our results show that while e ₂, and therefore s ₁, are temperature independent, ½ e _1½ decreases by ~25% with increasing temperature. A stronger dependence on doping is observed: the magnitudes of e ₂ and ½ e _1½ are both strongly suppressed as d increases. A generalised Drude analysis shows that the plasma frequency is temperature-independent, but scales with doping. The scattering rate increases super-linearly with temperature, and also increases with decreasing doping, consistent with stronger coupling in the underdoped regime.