ORAL SESSION 2B3: Raman and IR Spectroscopy

ORAL SESSION 2B3: Raman and IR Spectroscopy

Tuesday, Feb. 22, 3:30 p.m. – 6:00 p.m., General Assembly Hall A (GRB)

Chairs: A. Bansil (Northeastern U), C. Bernhard (Max-Planck-Institute)

2B3.1 Systematics of Raman Results on the Superconducting Gap: Interaction Effects

M.V. Klein, Department of Physics and Science and Technology Center for Superconductivity, University of Illinois at Urbana-Champaign, Urbana, IL 61801

Presenting Author: M.V. Klein

The features of electronic Raman spectra from HTS track other spectroscopic features. The 2D gap peak in B_1g Raman symmetry is believed to access the maximum value of the d-wave gap, but its energy is found to be consistently lower than twice the gap measured by single-particle spectroscopies. The 2D gap that is determined by (cc) polarized Raman scattering lies between these two values. These energy shifts are caused by particle-particle interactions in the final state. The two particles are created in the same CuO₂ plane in B1g geometry, but in adjacent planes in (cc) geometry. The stronger in-plane interaction causes the stronger reduction in 2D . Above the 2D Raman peak there is a dip at the same energy as a dip in the SIS tunneling spectra from the same material. Its energy equals 3 times the value of D from tunneling or ARPES experiments. This energy has been tracked from optimum doping to under doping. The energy of the B_1g-symmetry two-magnon Raman peak allows us to measure the zone boundary magnon energy, 2J. We find that 3D =2J over this doping range, evidence that the energy scale of the superconducting gap is set by the antiferromagnetic superexchange parameter, J.

2B3.2 Impurity-Induced Gap Renormalization in Anisotropic Superconductors: Mixed-State Specific Heat of La_2–xSr_xCuO₄ and Y(Ni_1–xPt_x)₂B₂C

M. Nohara 1,2, H. Suzuki 1, N. Mangkorntong 1, and H. Takagi 1,2. 1 School of Frontier Sciences, University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan. 2 CREST, Japan Science and Technology Corporation, Japan.

Presenting Author: M. Nohara

We have measured the temperature (T) and the magnetic field (H) dependence of the quasiparticle specific heat C_QP(T,H) of a d-wave superconductor La_2–xSr_xCuO₄ (LSCO) and a highly anisotropic s-wave superconductor YNi₂B₂C, and compared the effects of nonmagnetic impurities on the superconducting gap. C_QP(T,H) of overdoped LSCO (x = 0.18) shows H^1/2 dependence at low temperatures and obeys the scaling law predicted for "clean" d-wave systems. With substituting Zn, T-linear term appears in specific heat, indicating the appearance of a finite density of states at the Fermi level with introducing impurities, and C_QP(T,H) shows HlnH dependence as recently predicted for "dirty" d-wave systems. For pure YNi₀B₂C, C_QP(T,H=0) shows a power-low behavior and C_QP(T,H) exhibits almost H^1/2 dependence at low temperatures, indicating the presence of a highly anisotropic gap. In contrast, for dirty-limit Y(Ni_0.8Pt_0.2)₂B₂C, C_QP(T,H=0) shows thermally activated behavior and C_QP(T,H) exhibits H-linear dependence as expected for conventional s-wave superconductors. This indicates the opening of an isotropic gap with introducing impurities. These observations demonstrate qualitatively different impurity response of the superconducting gap between the d-wave and the highly anisotropic s-wave systems.

2B3.3 Electronic Raman Scattering in Bi-2212 Excited with c-Axis Polarized Light

Hsiang-Lin Liu 1, G. Blumberg 2, M.V. Klein 3, P. Guptasarma 4, and D.G. Hinks 5. 1 National Taiwan Normal University, Taipei 11718, Taiwan. 2 Lucent Technologies, Bell Laboratories, Murray Hill, NJ 07974. 3 Department of Physics and Science and Technology Center for Superconductivity (STCS), University of Illinois at Urbana-Champaign, Urbana, IL 61801. 4 Department of Physics, University of Wisconsin, Milwaukee, WI 53211. 5 Materials Science Division and STCS, Argonne National Laboratory, Argonne, IL 60439.

Presenting Author: H.L. Liu

We have studied the electronic Raman spectra excited and detected with photon polarization directions along the c-axis in a thick BSCCO 2212 crystal. In-plane polarized spectra from the edge essentially agreed with the same spectra obtained from the top surface, normal to the c-axis. By annealing in oxygen-rich and oxygen-poor atmospheres, we were able to change the doping from overdoped through underdoped. There are two remarkable experimental results: (1) There is a normal state electronic Raman continuum in the (cc) geometry. (2) In the superconducting state, this continuum redistributes, giving a gap-like peak having a value of 2D that is larger than that from the in-plane Raman geometry (B_1g) that accesses the maximum value of the gap, but smaller than twice the value of delta from single-particle spectroscopies (ARPES or tunneling). The continuum is interpreted as arising from incoherent coupling of electronic states from one CuO₂ plane to the next. The experimental Raman gap values are interpreted as evidence of final state interactions between the quasi-particles created in the Raman scattering processes.

2B3.4 Raman studies of local structure, superstructure, and phases in high temperature superconductors

Efthymios Liarokapis, Department of Physics, National Technical University of Athens, GR-15780 Athens, Greece

Presenting Author: E. Liarokapis

Raman spectroscopy is ideal for probing the local structure of a compound of the order of a few unit cells, providing important information for small structural modifications, superstructures, and phase transitions. The high T_c materials show a characteristic variety of phases, which may play an important role in the mechanism of superconductivity. Among the high T_c materials the YBCO and the LSCO compounds, which can be prepared in high purity, show a variety of doping induced phase transitions. In YBCO, the replacement of Y by Ca in a very controllable way of oxygenation can extend the doping level well above the optimum value allowing the comparison with other superconducting materials. From the careful examination of the changes induced in the Raman spectra at low temperatures, high hydrostatic pressures, and various oxygen concentrations and doping levels, the formation of phases and superstructures in the YBCO series will be examined. The Raman studies of the yttrium series will be compared with similar investigations on LSCO and the other superconducting materials. The information obtained from this analysis about the formation of phases and the slight modifications in the local structure of the high temperature superconductors will be compared with other measurements.

2B3.5 Collective excitations in two-dimensional and quasi-one-dimensional high T_c cuprate superconductors by electronic Raman spectroscopy

Girsh Blumberg, Bell Laboratories, Lucent Technologies, 700 Mountain Ave., Murray Hill, NJ 07974

Presenting Author: G. Blumberg

We study relationship between antiferromagnetism and superconductivity by resonant Raman scattering on magnetic and charge excitations in 2D (with CuO₂ planes) and quasi-1D (with Cu₂O₃ planes ladder type) cuprate antiferromagnets and high T_c superconductors. Raman scattering probes short wavelength magnetic excitations and provides microscopic information about the copper spin superexchange mechanism. Such short-range magnetic excitations without long-range antiferromagnetic order exist in underdoped cuprates. Low-frequency Raman scattering by quasi-particle excitations across the superconducting gap reveals symmetry and magnitude of the gap and the size of the Fermi surface.

For underdoped superconductors we report observation of novel Raman active resonance excitation in a collective Bardasis-Schrieffer type bound state that is below the superconducting gap edge – a manifestation of strong coupling and finite state interaction. This resonance persist at temperatures above T_c as an evidence of local particle-particle correlation in the pseudogap state that acquire global coherence below T_c.

We study quasi-1D superconducting ladder structured cuprates with the Cu₂O₃ planes. The spectroscopic signatures of decoupled collective modes of spin and charge excitations will be discussed. We also will discuss signatures of microscopic spin and charge separation in the underdoped superconductors with CuO2 planes.

2B3.6 Polarized Electronic Raman Scattering in High T_c Superconductors

C. Kendziora 1, D. Pelloquin 2, G. Vaillard 2, P. Fournier 3, Z.Y. Li, and R.L. Greene 3. 1 Materials Sciences Division, Naval Research Laboratory, Washington DC 20375. 2 Laboratoire CRISMAT-ISMRA, Caen CEDEX, France. 3 Center for Superconductivity Research and Department of Physics, University of Maryland, College Park.

Presenting Author: C. Kendziora

Electronic Raman scattering (ERS) provides a non-invasive optical technique for accessing density of states and scattering rate information as a function of frequency and temperature. In superconductors, the typical redistribution fo states below the transition temperature, is taken as evidence for the superconducting energy gap. In particular, a (2D ) peak forms in the superconducting state and the ERS intensity below this energy is decreased with respect to the normal state. For the case of single crystal samples, the polarization of the incoming and outgoing photons can be aligned and selected with respect to the crystallographic axes to measure electronic excitations as a function of symmetry. The symmetry dependence of the ERS spectra indicates k dependence on the Fermi Surface, specifically superconducting gap anisotropy.

In this talk we will present recent results of ERS from a wide variety of high- T_c single crystals at a range of doping values. In the Bi₂Sr₂CaCu₂O₈₊_dsuperconductor, the data separate into 3 distinct regimes. 1) Optimally doed samples show a large anisotropic gap consistent with a d-wave order parameter, 2) Overdoped samples show the gap value and anisotropy decrease, and 3) Underdoped samples show much weaker or no 2D peaks, but may show a small peak well above T_c indicating evidence for pre-formed Cooper pairs. Some of these trends hold for the HgBa₂CuO₄₊_dsingle layer material as a function of doping despite its very different predicted band structure. We also compare and contrast the trends observed in the hole type high-T_c superconductors with the ERS spectra from n-type (PrCe)₂CuO₄ crystals.

2B3.7 The pseudogap in the IR scattering rate of HTSC

Tom Timusk 1, Jeff McGuire 1,Tatiana Startseva 1, Dimitri Basov 2, and Anton Puchkov 3. 1 McMaster University. 2 UCSD. 3 Stanford University.

Presenting Author: T. Timusk

The frequency dependent scattering rate spectrum, obtained from the ab plane reflectance of HTSC, can be used to study the pseudogap that appears well above T_c in high temperature superconductors. Unlike other competing methods, reflectance spectroscopy is not surface sensitive, or demanding very large crystals and has therefore been applied to a large number of HTSC materials. This review summarizes recent results as well as earlier measurements of the pseudogap in one, two and three plane materials from LaSrCuO to Hg-1223, including YBCO and Bi-2212. All have a pseudogap but there are interesting trends and difference that offer some clues towards the understanding of the dynamics of carriers in HTSC.

2B3.8 Superfluid and normal-fluid densities in the high-T_c superconductors

D.B. Tanner 1, F. Gao 1, K. Kamarás 1, H.L. Liu 1, M.A. Quijada 1, D.B. Romero 1, Y-D. Yoon 1, A. Zibold 1, H. Berger 2, G. Margaritondo 2, L. Forró 2, R.J. Kelly 3, M. Onellion 3, G. Cao 4, J.E. Crow 4, O. Beom-Hoan 5, J.T. Markert 5, J.P. Rice 6, D.M. Ginsberg 6, and Th. Wolf 7. 1 University of Florida. 2 École Polytechnique Fédéral de Lausanne. 3 University of Wisconsin. 4 National High Magnetic Field Laboratory. 5 University of Texas. 6 University of Illinois. 7 Forschungszentrum Karlsruhe.

Presenting Author: D.B. Tanner

In clean metallic superconductors, 100% of the mobile carriers participate in the condensate, so that the London penetration depth (which measures the electromagnetic screening by the superconductor) indicates charge densities comparable to those inferred from the free-carrier plasma frequency. In the cuprates, this is not the case, even though penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-doped part of the phase diagram. Optical measurements, which permit independent determination of the total doping-induced spectral weight and the superfluid density, show that in optimally doped materials only about 20% of the doping-induced spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials, the superfluid fraction is even smaller. This result implies extremely strong coupling for these superconductors.