Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A13: Spectroscopic Properties of Superconductors |
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Sponsoring Units: DCMP Chair: David Tanner, University of Florida Room: LACC 402B |
Monday, March 21, 2005 8:00AM - 8:12AM |
A13.00001: Resonance properties in optimally and overdoped Y-123 - investigation of phonons and the electronic background M. Ruebhausen, S. Mueller, R. Maeser, B. Schulz, I. Mahns, D. Budelmann, M.V. Klein, D. Bonn, W. Hardy, R. Liang We have studied the variation of the 2$\Delta$-gap like features and the phonons in optimally to overdoped Y-123 with incident photon energy E$_{i}$. The optimally doped YBCO and overdoped YBCO exhibit a complex behavior in the gap feature when the incident photon energy is tuned from 1.8 eV to above 5 eV. Our results highlight the composite nature of the 2$\Delta$-gap like peaks in the HTS. Below the superconducting T$_{c}$ we see a modification of spectral weight and the appearance of the 2$\Delta$ like features. In B$_{1g}$ Raman symmetry these features change their shape, peak position, and intensity with increasing E$_{i}$. For E$_{i}$ 3.5 to 4 eV we find little redistribution, only a strong new peak at Raman frequencies well above the accepted values of twice $\Delta _{max}$. This peak is located at 620 cm$^{-1}$ in optimally doped samples, it shifts to lower energies and broadens in overdoped materials. The matrix element of this peak shifts as a function of doping by 0.2 eV to higher incident photon energies, outlining the interplay between the local electronic structure and superconductivity. Strong resonances of the chain disorder-induced phonons appear at around 4.1 eV, which are absent in the overdoped material. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A13.00002: Raman Spectroscopy of the electron-doped cuprates in magnetic field M. M. Qazilbash, R. L. Greene, A. Koitzsch, B. S. Dennis, A. Gozar, G. Blumberg We investigate the influence of magnetic field on the electronic excitations across the superconducting (SC) gap of the electron-doped cuprates R$_{2-x}$Ce$_{x}$CuO$_{4-\delta }$ (R = Pr, Nd) for 0.13$<$x$<$0.18. We report the anomalous result that the 2$\Delta $ coherence peak energy decreases rapidly with increasing field. In sharp contrast, the magnetic field has only a weak effect on the coherence peak energy in the hole-doped cuprates and conventional superconductors. We determine effective upper critical field lines H$^{\ast }_{c2}$ at which the superfluid stiffness vanishes and H$^{2\Delta }_{c2}$ at which the SC amplitude is suppressed. We find that H$^{2\Delta }_{c2}$ is larger than H$^{\ast }_{c2}$ for all dopings, especially for x\underline {$<$}0.15. Both H$^{2\Delta }_{c2}$ and H$^{\ast }_{c2}$ decrease more rapidly with doping than T$_{c}$ or 2$\Delta $ for x$>$0.15 (over-doped). The rapid increase of SC coherence length (several hundred Angstrom) with increasing doping implies a weak pair potential with less momentum dependence. This work was supported by NSF grant No. DMR 01-02350. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A13.00003: Raman Investigation of Zn-Doped YB$_{2}$Cu$_{3}$O$_{6.99}$ Single Crystals X. K. Chen, J. C. Irwin, R. Liang, D.A. Bonn, W. N. Hardy We report the results of a Raman scattering study of the effects of doping YBa$_{2}$Cu$_{3}$O$_{6.99}$ crystals with 1.0{\%} Zn, which decreased the critical temperature from 92K to 81K. We found that the strength and shape of the 340cm$^{-1}$ B1g phonon anomaly are nearly the same in both the doped and undoped crystals. This observation suggests that Zn-doping does not significantly change the hole concentration. Furthermore, we extracted the B1g electronic continuum by removing the phonon contributions using a decoupling procedure. The resulting electronic spectra from the Zn- doped and undoped crystals are again essentially identical and are consistent with the scattering of light by the superconducting quasi-particles across a d-wave gap. The equal intensities of the B1g electronic continua again indicates that the carrier concentrations are the same in both the Zn-doped and undoped crystals. This in turn implies that Zn-doping does not lead to the opening of a pseudo gap. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A13.00004: Competing CDW and SC order parameters in NbSe$_2$ Aleksej Mialitsin, B.S. Dennis, Girsh Blumberg We study collective excitations in NbSe$_2$ by low-frequency electronic Raman spectroscopy as a function of temperature, magnetic field and excitation energy. We discuss observed relations between collective modes induced by charge density waves (CDW) and superconducting (SC) order parameters. We compare Raman data to recent STM and ARPES studies. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A13.00005: Understanding the totally symmetric intramolecular vibrations in $\kappa$-phase organic superconductors J.T. Haraldsen, R. Wesolowski, J. Cao, J.L. Musfeldt, I. Olejniczak, J. Choi, Y.J. Wang, J.A. Schlueter We report magneto-infrared measurements of three quasi- isostructural $\kappa$-phase organic molecular solids: $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br (T$_c$=11.6 K), $\kappa$-(ET)$_2$Cu(SCN)$_2$ (T$_c$=10.4 K), and the non-superconducting $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Cl analog. Our results support the contributing role of electron-molecular vibrational coupling in the pairing mechanism of layered organic superconductors, and we identify the most important totally symmetric modes in $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br within the non-planar molecular building block picture. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A13.00006: Resonant Inelastic X-ray Scattering Study of the Model Superconductor HgBa$_2$CuO$_{4+\delta}$ Li Lu, Xudong Zhao, Guillaume Chabot-Couture, Nobuhisa Kaneko, Owen Vajk, Guichuan Yu, Stephane Grenier, Young-June Kim, Diego Case, Thomas Gog, Martin Greven The characteristics of the elementary excitations in high-temperature superconductors (HTSC) and their Mott-insulating parent compounds remain a controversial issue after many years of extensive study by different spectroscopic methods. Energy- and momentum-resolved resonant inelastic x-ray scattering (RIXS) is gaining in importance as a powerful tool in the study of elementary charge excitations in HTSC. We report a RIXS study of charge excitations in the 2 - 8 eV range in the structurally simple compound HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_c = 96.5 $ K) at several high-symmetry points in the Brillouin zone. The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of electron-hole excitations, including an excitation at 2 eV, which was previously observed in undoped parent compounds. The observation of the 2 eV excitation is suggestive of a remnant charge transfer gap in the superconducting phase. Our data are consistent with a relatively weak ($ < 0.5$ eV) dispersion of all excitations. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A13.00007: Momentum-Structure of Remnant Mott-gap in Prototype Doped Cuprates Yinwan Li, Zahid Hasan, Dong Qian, Yi-De Chuang, H. Kisaki, S. Uchida, Y. Kaga, T. Sasagawa, H. Takagi Momentum dependence of charge excitation of 2-D prototype cuprate classes La$_{2-x}$Sr$_x$CuO$_4$ and Nd$_{2-x}$Ce$_x$CuO$_4$ with different doping levels (x) is measured using high resolution resonant inelastic X-ray scattering. Although a low-energy continuum is built up with doping, a remnant excitation gap behavior continues to exist even in highly doped metallic phases which we have studied in some detail. The excitation of the Mott gap becomes less dispersive asymmetrically with the increase of doping level and suggests a~ many-body coupling between charge fluctuation and magnetic order of the lattice. The results can be qualitatively described within the framework of t-t$'$-t$''$-U model. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A13.00008: Direct measurement of charge transfer phenomena at ferromagnetic/superconducting oxide interfaces Maria Varela, Andrew R Lupini, Vanessa Pena, Zouhair Sefrioui, Ilke Arslan, Nigel D. Browning, Jacobo Santamaria, Stephen J. Pennycook YBa$_{2}$Cu$_{3}$O$_{7-x}$/La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ ferromagnetic/superconducting interfaces have been analyzed by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) with monolayer resolution. Extensive charge transfer occurs between the manganite and the superconductor, in a manner similar to modulation-doped semiconductors, which explains the reduced critical temperatures of heterostructures. Since CMR and HTCS oxides are extremely sensitive to doping these charge transfer processes at the interfaces will directly affect the superconducting and/or magnetic properties of the individual layers. This behavior has not been observed with insulating PrBa$_{2}$Cu$_{3}$O$_{7}$ layers in YBa$_{2}$Cu$_{3}$O$_{7-x}$/ PrBa$_{2}$Cu$_{3}$O$_{7 }$superlattices. EELS in these samples provides direct confirmation that holes in the YBa$_{2}$Cu$_{3}$O$_{7-x}$ are located on the CuO$_{2}$ planes. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A13.00009: Evidence for different spin-orbit interactions in the Ba and Cu layers of YBa$_2$Cu$_3$O$_7$ at the Ba and Cu L2,3 edges. Juana Acrivos, Hizam Sahibudeen, Maria Angeles Navacerrada The L$_{2,3}$ edges arise from the 2p$_{3/2,1/2}$ core transitions to final band states of d$_{5/2,3/2 }$character, and may give information on the uneven population of the spin-orbit split bands of d-character that lead to a material ferromagnetic or anti-ferromagnetic properties. Absorption, fluorescence and enhanced scattering data at the Ba and Cu L$_{2,3}$ edges of a single crystal, and a 50 nm film were measured respectively at SSRL (Station 2-3) and LBNL-ALS (6.3.1 Station: P. Nachimuthu chamber for fluorescence and total electron yield and J.B. Kortright chamber for enhanced scattering). The theoretical relative intensity I(L$_{3})$/I(L$_{2})$=2 is satisfied for the reference material BaBr$_{2}$ but deviations from 2 at the respective L$_{2,3}$ edges: Ba by --14{\%} and Cu by +19{\%} in a material that is neither ferromagnetic nor anti-ferromagnetic suggests the presence of magnetic interactions for the individual elements that cancel out in the material. All models for magnetic interactions should then take into account the coupling of the Cu and Ba atoms in adjacent layers of YBCO, a 2D material. DOE, NATO, NSF, Dreyfus supported. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A13.00010: Real time optical observation of precursor phases during YBCO thin film growth Gertjan Koster, Jeong-Uk Huh, Arturas Vailionis, Robert Hammond, Theodore Geballe, Malcolm Beasley We report on our findings using real-time Fourier Transform Infrared (FTIR) radiance and reflectance measurements during high rate electron beam deposited [100 angstroms/sec] YBa2Cu3O7 (YBCO) films [1]. The data can best be explained by assuming the presence of a liquid or glassy phase from which the YBCO crystal grows [2]. We have found that the optical properties of the as deposited material change dramatically upon absorption of oxygen. It is inferred from the strong thin film interference fringes which appear in the reflectance spectrum. The fringes subsequently change amplitude when YBCO precipitates from the liquid; the rate of precipitation can be controlled by oxygen pressure and substrate temperature. The nature of the liquid or glassy phase is studied by XPS and XRD on quenched films at various stages. Subsequently, YBCO nucleation and growth is monitored ex situ by real time x-ray scattering using an ambient controlled hot stage. \\[4pt] [1] G. Koster, J-U. Huh, R.H. Hammond and M.R. Beasley, in preparation.\\[0pt] [2] T. Ohnishi, J.-U. Huh, R.H. Hammond and W. Jo, 2004\textit{ J. Mater. Res.}\textbf{ 19} 977; A. Kursumovic\textit{ et al.}, Supercond. Tech. {\&} Sci. 17 (2004) 1215 [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A13.00011: Electron irradiation induced oxygen ordering in YBCO twin-free crystals. H. W. Seo, Q. Y. Chen, M. N. Iliev, C. Wang, Wei-Kan Chu, U. Welp Oxygen ordering in CuO chain plane of YBCO is interesting topic because the distribution of oxygen, or the length of chain-fragments, plays an important role in carrier-doping of the CuO2 plane; longer chain-fragments are more effective in hole doping and result in higher Tc under the same overall oxygen stoichiometry. In this presentation, we report on the low energy electron irradiation effects in twin-free oxygen deficient YBCO single crystals via comparative Raman spectroscopy studies of electron irradiated and non-irradiated areas at room temperature. We observed that low energy electrons heal the existing point defects of CuO chains and enhances oxygen ordering of twin-free but oxygen deficient YBCO. The comparison of the polarized Raman spectra from non-irradiated and irradiated areas provides clear indications of extended average length of the chains without changing the overall oxygen content. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A13.00012: Optical Characterization and Superconducting Behavior of MgB2 Thin Films Jafar Amirzadeh, Michael Bleiweiss, Ming Yin, Timir Datta MgB$_{2}$ thin films were sequentially deposited from pure metal sources on optically transparent glass. The resulting films were investigated by SCEM, optical microscopy, EDAX, and by optical transmission. As deposited multilayer films show high metallic reflectivity in 400 -- 1000nm regime with very little transmission. After \textit{ex situ} annealing (525, 550, 575 {\&} 600$^{\circ}$C) the films appeared progressively less shiny and took up darker hues. The films rendered superconducting after annealing at 550$^{\circ}$C with Tc of 38K, approaching that of the bulk material. Also, upon reaction a direct band gap opened up. For all the films absorption coefficient alpha and photon energy followed a Tauc behavior rather than a Cody type behavior. As annealing temperature was raised the Tauc plot slope was observed to decrease monotonically. However the band gap first increased to a maximum of 2.5eV (550$^{\circ}$ C) and then rapidly decreased to below 0.5 eV with 600$^{\circ}$ C anneal. Our results indicate that MgB$_{2}$ films have excellent optical properties and are commensurate with VLSI and optoelectronics processing. [Preview Abstract] |
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A13.00013: Direct Observation of Cooperative Doping Mechanisms at Grain Boundaries in Ca-doped YBa$_2$Cu$_3$O$_{7-\delta}$ Robert Klie, Maria Varela, Alberto Franceschetti, Yimei Zhu, Sokrates Pantelides, Stephen Pennycook Atomic-column resolved electron energy-loss spectroscopy (EELS) was used to study the effects of Ca-doping on the local atomic and electronic structure of grain boundaries in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$. Grain boundary doping with Ca has been shown to increase the grain boundary critical current, and it has been previously suggested that Ca$^{2+}$ substitutes for Y$^{3+}$ thus providing additional holes. We have performed atomic-column resolved EELS of the grain boundary dislocation cores and we will show that in the highly strained regions of the grain boundary plane Ca segregates to Cu and Ba sites, where it does not provide holes directly. However, due to the resulting strain relief, the oxygen deficiency in the vicinity of the grain boundary is reduced and the hole concentration increased. The results demonstrate that to improve grain boundary J$_{c}$, ionic size may be more important than the electronic nature of the impurity. [Preview Abstract] |
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A13.00014: Search for the Coexistence of Magnetism and Superconductivity in Ce$_{1-x}$Gd$_x$Ru$_2$ using the Mossbauer Effect Michael De Marco, Ryan Heary, Dermot Coffey, Pe-Chun Ho, Todd Sayles, Brian Maple, Steve Toorongian, Michael Haka We use the $^{99}$Ru Mossbauer Effect(ME) to look for evidence for the coexistence of magnetism and superconductivity in Ce$_{1-x}$Gd$_x$Ru$_2$. These compounds have superconducting and ferromagnetic phases. The T$_c$(x) and $\theta$(x) curves cross at $x=0.135$. Here $\theta$(x) is the Curie temperature found by extrapolating magnetic susceptibility data below T$_c$. A sample of Ce$_{0.93}$Gd$_{.07}$Ru$_2$ was prepared with enriched $^{99}$Ru(97\%) for which T$_c$=4.5K and $\theta$=4.19K. The ME spectrum measured at 2.2K showed a pure quadrupole with a splitting of 0.36mm/s and an isomer shift of 0.12mm/s with no hyperfine field. The absence of the magnetic field in the sample brings into question the use of extrapolated magnetic susceptibility data as evidence for the coexistence of magnetism and superconductivity. Comparing the ME spectrum with that of CeRu$_2$(T$_c$=6.1K) one sees that there is an impurity Ru phase. This suggests that previous evidence for magnetism below T$_c$ on the basis of $^{57}$Fe ME could be attributed to an impurity Fe phase. [Preview Abstract] |
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