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Search Results: 1 - 10 of 219471 matches for " C. Kendziora "
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Infrared and Raman Evidence for Dimers and Polymers in Rb1C60
Michael C. Martin,Daniel Koller,A. Rosenberg,C. Kendziora,L. Mihaly
Physics , 1994, DOI: 10.1103/PhysRevB.51.3210
Abstract: The infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1C60 has an $fcc$ structure at temperatures above ~100C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled ("quenched"). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.
No Far-Infrared-Spectroscopic Gap in Clean and Dirty High-T$_C$ Superconductors
D. Mandrus,Michael C. Martin,C. Kendziora,D. Koller,L. Forro,L. Mihaly
Physics , 1993, DOI: 10.1103/PhysRevLett.70.2629
Abstract: We report far infrared transmission measurements on single crystal samples derived from Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$. The impurity scattering rate of the samples was varied by electron-beam irradiation, 50MeV $^{16}$O$^{+6}$ ion irradiation, heat treatment in vacuum, and Y doping. Although substantial changes in the infrared spectra were produced, in no case was a feature observed that could be associated with the superconducting energy gap. These results all but rule out ``clean limit'' explanations for the absence of the spectroscopic gap in this material, and provide evidence that the superconductivity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is gapless.
Shadow bands in overdoped Bi_2Sr_2CaCu_2O_{8+x}
S. La Rosa,R. J. Kelley,C. Kendziora,G. Margaritondo,M. Onellion,A. Chubukov
Physics , 1995,
Abstract: We performed angle-resolved photoemission experiments on overdoped single crystal $Bi_2Sr_2CaCu_2O_{8+x}$ samples with Tc values as low as 55K and very narrow transitions as measured by AC susceptibility. The photoemission data indicate the presence of a ``shadow'' peak in the quasiparticle spectral function $A(\omega)$ shifted by $(\pi,\pi)$ from the conventional quasiparticle peak. The ratio of the shadow/quasiparticle peak amplitudes strongly increases with the binding energy. This is consistent with the idea that the shadow peak is not a structural replica of the quasiparticle peak, but rather a maximum in a spectral function which can exist due to interaction with spin fluctuations peaked at $(\pi,\pi)$.
Persistence of Strong Electron Coupling to a Narrow Boson Spectrum in Overdoped BiSrCaCuO (Bi2212) Tunneling Data
J. F. Zasadzinski,L. Ozyuzer,L. Coffey,K. E. Gray,D. G. Hinks,C. Kendziora
Physics , 2005, DOI: 10.1103/PhysRevLett.96.017004
Abstract: A d-wave, Eliashberg analysis of break junction and STM tunneling spectra on BiSrCaCuO (Bi2212) reveals that a spectral dip feature is directly linked to strong electronic coupling to a narrow boson spectrum, evidenced by a large peak in the boson spectral weight. The tunneling dip feature remains robust in the overdoped regime of Bi2212 with bulk Tc values of 56 K-62 K. This is contrary to recent optical conductivity measurements of the self-energy that suggest the narrow boson spectrum disappears in overdoped Bi2212 and therefore cannot be essential for the pairing mechanism. The discrepancy is resolved by considering the way each technique probes the electron self-energy, in particular, the unique sensitivity of tunneling to the off-diagonal or pairing part of the self-energy.
Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors
G. Blumberg,M. Kang,M. V. Klein,K. Kadowaki,C. Kendziora
Physics , 1997, DOI: 10.1126/science.278.5342.1427
Abstract: Electronic Raman scattering from high- and low-energy excitations was studied as a function of temperature, extent of hole doping, and energy of the incident photons in Bi_2Sr_2CaCu_2O_{8 \pm \delta} superconductors. For underdoped superconductors, short range antiferromagnetic (AF) correlations were found to persist with hole doping, and doped single holes were found to be incoherent in the AF environment. Above the superconducting (SC) transition temperature T_c, the system exhibits a sharp Raman resonance of B_{1g} symmetry and energy of 75 meV and a pseudogap for electron-hole excitations below 75 meV, a manifestation of a partially coherent state forming from doped incoherent quasi particles. The occupancy of the coherent state increases with cooling until phase ordering at T_c produces a global SC state.
Evolution of Magnetic and Superconducting Fluctuations with Doping of High-Tc Superconductors (An electronic Raman scattering study)
G. Blumberg,M. V. Klein,K. Kadowaki,C. Kendziora,P. Guptasarma,D. Hinks
Physics , 1997, DOI: 10.1016/S0022-3697(98)00135-8
Abstract: For YBa_2Cu_3O_{6+\delta} and Bi_2Sr_2CaCu_2O_8 superconductors, electronic Raman scattering from high- and low-energy excitations has been studied in relation to the hole doping level, temperature, and energy of the incident photons. For underdoped superconductors, it is concluded that short range antiferromagnetic (AF) correlations persist with hole doping and doped single holes are incoherent in the AF environment. Above the superconducting (SC) transition temperature T_c the system exhibits a sharp Raman resonance of B_1g symmetry and about 75 meV energy and a pseudogap for electron-hole excitations below 75 meV, a manifestation of a partially coherent state forming from doped incoherent quasi-particles. The occupancy of the coherent state increases with cooling until phase ordering at T_c produces a global SC state.
Correlation of Tunneling Spectra in Bi2Sr2CaCu2O(8+delta) with the Resonance Spin Excitation
J. F. Zasadzinski,L. Ozyuzer,N. Miyakawa,K. E. Gray,D. G. Hinks,C. Kendziora
Physics , 2001, DOI: 10.1103/PhysRevLett.87.067005
Abstract: New break-junction tunneling data are reported in Bi2Sr2CaCu2O(8+delta) over a wide range of hole concentration from underdoped (Tc = 74 K) to optimal doped (Tc = 95 K) to overdoped (Tc = 48 K). The conductances exhibit sharp dips at a voltage, Omega/e, measured with respect to the superconducting gap. Clear trends are found such that the dip strength is maximum at optimal doping and that Omega scales as 4.9 kTc over the entire doping range. These features link the dip to the resonance spin excitation and suggest quasiparticle interactions with this mode are important for superconductivity.
Pressure Dependence of the Irreversibility Line in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$:Role of Anisotropy in Flux-Line Formation
M. P. Raphael,M. E. Reeves,E. F. Skelton,C. Kendziora
Physics , 1999, DOI: 10.1103/PhysRevLett.84.1587
Abstract: One of the important problems of high-temperature superconductivity is to understand and ultimately to control fluxoid motion. We present the results of a new technique for measuring the pressure dependence of the transition to superconductivity in a diamond anvil cell. By measuring the third harmonic of the {\it ac} susceptibility, we determine the onset of irreversible flux motion. This enables us to study the effects of pressure on flux motion. The application of pressure changes interplanar spacing, and hence the interplanar coupling, without significantly disturbing the intraplanar superconductivity. Thus we are able to separate the effects of coupling from other properties that might affect the flux motion. Our results directly show the relationship between lattice spacing, effective- mass anisotropy, and the irreversibility line in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$. Our results also demonstrate that an application of 2.5 GPa pressure causes a dramatic increase in interplanar coupling.
Evolution of Coherence and Superconductivity in Electron-Doped Cuprates
M. M. Qazilbash,B. S. Dennis,C. A. Kendziora,Hamza Balci,R. L. Greene,G. Blumberg
Physics , 2005,
Abstract: The electron-doped cuprates were studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. We determined that the magnitude of the SC order parameter varies between 4.6 and 3.5 k_BT_c, consistent with weak coupling BCS theory. Using a ``Raman conductivity" sum rule, we found that doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (pi/2a, pi/2a) regions of the Brillouin zone. The carriers doped beyond optimal doping remain incoherent. Only coherent QPs contribute to the superfluid density in the SC state.
Goldstone-Mode Phonon Dynamics in the Pyrochlore Cd2Re2O7
C. A. Kendziora,I. A. Sergienko,R. Jin,J. He,V. Keppens,B. C. Sales,D. Mandrus
Physics , 2005, DOI: 10.1103/PhysRevLett.95.125503
Abstract: We have measured the polarized Raman scattering spectra of Cd2Re2O7, the first superconducting pyrochlore, as a function of temperature. For temperatures below the cubic-to-tetragonal structural phase transition (SPT) at 200K, a peak with B1 symmetry develops at zero frequency with divergent intensity. We identify this peak as the first observation of the Goldstone phonon in a crystalline solid. The Goldstone phonon is a collective excitation that exists due to the breaking of the continuous symmetry with the SPT. Its emergence coincides with that of a Raman-active soft mode. The order parameter for both features derives from an unstable doubly-degenerate vibration (with Eu symmetry) of the O1 atoms which drives the SPT.
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