Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L4: Bosonic Modes in HTSC |
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Sponsoring Units: DCMP Chair: Mike Norman, Argonne National Laboratory Room: Morial Convention Center 206 |
Tuesday, March 11, 2008 2:30PM - 3:06PM |
L4.00001: STM Observation of a Bosonic Mode in the Electron-Doped Superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ Invited Speaker: Information on bosonic excitations in high temperature superconductors is part of a critical dataset that is necessary to decipher the puzzle of the pairing mechanism in these materials. In this talk, I will discuss our recent STM investigations of the electron-doped cuprate superconductor Pr$_{0.88}$LaCe$_{0.12 }$CuO$_{4-\delta }$ (PLCCO) ($T_{c}$ = 24 K). Our spectra reveal superconducting gaps with coherence peaks that disappear above $T_{c}$. In addition, multiple step/peak-like features are observed outside the gap. Such features in STM spectra are suggestive of bosonic excitations that couple strongly to the electrons. Analysis of the data indicates that the observed (bosonic) mode energy in PLCCO lies at 10.5$\pm $2.5 meV which is much lower than the bosonic mode observed in hole-doped BSCCO. The energy scale of our mode is the same as the magnetic resonance mode (spin-excitations) in PLCCO$^{ }$measured by inelastic neutron scattering but is also consistent with a low energy acoustic mode. Additionally, I will show that both the local mode energy and the intensity reveal correlations with the local gap energy scale. The sensitivity of the mode intensity to the energy scale of the onset of the continuum of excitations (2$\Delta )$ may indicate an electronic origin rather than phonons. This work was done in collaboration with F. C. Niestemski, S. Kunwar, S. Zhou, Shiliang Li, H. Ding, Ziqiang Wang, and Pengcheng Dai. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L4.00002: Bosonic self energy spectrum of high temperature superconductors from optical spectroscopy Invited Speaker: We address the problem of extracting the bosonic spectral function in high temperature superconductors using optical spectroscopy. Last year, we succeeded in inverting the optical spectra of the cuprates and extract the quantity, analogous to the electron-phonon spectral density $\alpha^2F(\Omega)$ in the conventional superconductors, for YBCO Ortho II system. We used the highly ordered crystals grown by Hardy, Bonn and Liang and compared our results with magnetic neutron spectra on samples from the same source measured by Stock {\it et al}. There was excellent agreement between the results of the two sets of spectroscopies. Since then we have refined our inversion technique and have been able to make a similar comparison for the LSCO system with new neutron scattering data from Vignolle {\it et al}. This magnetic spectrum, together with our Eliashberg inversion, accounts in a straightforward way for the lower $T_c$ of LSCO as compared to other cuprates. We also offer a detailed prediction of the evolution of the magnetic excitation spectrum with temperature and doping for the highly studied system Bi-2212, a material were neutron scattering data are very hard to get. Our data show that the bosonic self energy function evolves continuously from the broad background. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 4:18PM |
L4.00003: Visualizing pair formation in $Bi_2Sr_2CaCu_2O_{8+x}$ Invited Speaker: Unlike traditional superconductors, the density of states (DOS)
of the high-Tc superconductor $Bi_2Sr_2CaCu_2O_{8+x}$ shows
large nanoscale variations that have been detected using scanning
tunneling microscopy (STM) [1,2]. Such variations are seen in
the low temperature superconducting gap [1] and in features
associated with the coupling of pairs to boson modes [2]. In
order to understand these variations in the spectra, we perform
atomic resolution STM measurements of $Bi_2Sr_2CaCu_2O_{8+x}$ as
a function of temperature [3]. Using newly developed experimental
techniques, we measure the evolution of the DOS from low
temperature $(T< |
Tuesday, March 11, 2008 4:18PM - 4:54PM |
L4.00004: Evolution of the gaps through the cuprate phase-diagram Invited Speaker: The actual physical origin of the gap at the antinodes, and a clear identification of the superconducting gap are fundamental open issues in the physics of high-Tc superconductors. Here, we present an electronic Raman scattering study of single layer cuprates, as a function of both doping level and temperature. We examine both the evolution of the gaps close to the nodes and at the antinodes in the normal and superconducting sates. On the deeply over-doped side, we show that the anti-nodal gap is a true superconducting gap. In contrast, on the under-doped side, our results reveal the existence of a break point close to optimal doping below which the anti-nodal gap is gradually disconnected from superconductivity. The nature of both the superconducting and normal state is distinctly different on each side of this breakpoint and will be discussed. \newline References: M. Le Tacon, A. Sacuto, A. Georges, G. Kotliar, Y. Gallais, D. Colson, A. Forget Two Energy Scales and two Quasiparticle Dynamics in the Superconducting State of Underdoped Cuprates, Nature Physics 2, 537, August 2006; W. Guyard, M. Le Tacon, M. Cazayous, A. Sacuto, A. Georges, D. Colson, A. Forget, Breakpoint in the evolution of the gap through the cuprate phase diagram, Cond Mat 0708.3732 [Preview Abstract] |
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