Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B37: High-Tc: STM and Pseudogap |
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Sponsoring Units: DCMP Chair: J. Y. T. Wei, University of Toronto Room: Baltimore Convention Center 340 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B37.00001: Signatures of strong correlations in the tunneling spectra of superconducting doped Mott insulators Tiago C. Ribeiro, Xiao-Gang Wen We compare the tunneling spectra from BCS mean-field theory and from a mean-field theory of superconducting doped Mott insulators to address the signatures of strong correlations in tunneling experiments, namely the bias sign asymmetry, the generic lack of evidence for the Van Hove singularity and, in some cases, the absence of coherence peaks. We also establish the connection between tunneling spectroscopy and ARPES observations in the cuprate superconductors. Thus, we provide a consistent description of these two experimental techniques which probe the single electron microscopic physics of the cuprates. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B37.00002: Controlling hole concentration in superconducting YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films for spectroscopic studies$^{\ast }$ Matthew Brinkley, Wan Kyu Park, Xiaofang Zhai, James Eckstein, Laura Greene One of the remaining controversial issues in the high-$T_{c}$ cuprate superconductors is whether the $d$-wave order parameter is robust over the whole doping range. As an essential first step to address this, we optimize a growth procedure for high-quality sputter-deposited YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ thin films with various hole concentrations by controlling the oxygen content (and by cation doping in the future). Two different approaches have been attempted successfully: annealing in an oxygen-controlled environment and ozonization. We have developed an \textit{in-situ} annealing procedure employing two-step post-deposition anneals in vacuum and O$_{2}$. The oxygen content is estimated from high-resolution x-ray diffraction data. This procedure produces high-quality thin films, optimally doped (e.g., 7-$\delta \approx $6.88, $T_{c,on}$=91.8 K, and $\Delta T_{c}<$0.9 K) and underdoped (e.g., 7-$\delta \approx $6.43, $T_{c,on}$=55.1 K, and $\Delta T_{c}<$2.9 K). Preliminary ozonization experiments show that varying the oxygen content in a controlled manner is feasible, especially in the overdoped regime. We will present detailed transport measurements, materials characterizations, and some initial conductance spectra from planar tunneling and Andreev reflection spectroscopies. *Supported by the U.S. DoE Award No. DEFG02-91ER45439 through the FSMRL and the CMM at UIUC. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B37.00003: Point-contact spectroscopy of the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ in magnetic fields up to 32 tesla Sunghee Yun, Tara Dhakal, Jacob Tosado, Amlan Biswas, Bing Liang, Richard Greene Tunneling spectra of the normal state of electron-doped cuprates at low temperatures and in magnetic fields higher than the upper critical field ($H_{c2}$) have shown evidence of a “pseudogap”. However, to understand if the origin of this pseudogap is the same as the pseudogap observed in hole-doped cuprates and to determine the theoretical model it supports, it is necessary to study the effect of high magnetic fields on this pseudogap. Point contact spectroscopy (PCS) was carried out on junctions between electron doped cuprate Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO, $0.13 < x < 0.17$) single crystals and normal metal (Pt-Rh). To probe the normal state at low temperatures ($T \sim$ 1.5 K), the superconductivity was suppressed by applying high magnetic fields (up to 32 T). These experiments showed that the normal state gap is present for all the dopings, including the overdoped one. These results at high magnetic fields can be interpreted as a combined effect of precursor superconductivity and the presence of disorder in these materials. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B37.00004: Density of state correlations in pseudogap systems Marianna Maltseva, Piers Coleman Developments in the scanning tunneling microscopy and spectroscopy (STM/STS), make it possible to probe the detailed position and energy- dependent density of states in electronically inhomogenious media [1,2]. Theoretical analysis to date has focused on the spatial autocorrelations of the density of states as a probe of the quasiparticle spectrum. In this work we discuss how correlations between the density of states above and below the Fermi energy can be used to discern the underlying nature of the pseudogap order. We can divide the density of states fluctuations into components that are symmetric and antisymmetric about the Fermi energy. Density waves produce a signal in the antisymmetric channel whereas superconductors produce a signal in the symmetric channel. Moreover, a careful analysis of the momentum dependence can be used to discern between off-diagonal and diagonal disorder [3]. We will present our theory of these topics. [1] H. C. Fu, J. C. Davis, and D.-H. Lee, cond-mat/0403001. [2] J.-X. Zhu, K. McElroy, J. Lee, T. P. Devereaux, Qimiao Si, J.C. Davis, and A.V. Balatsky, cond-mat/0507621. [3] T.S. Nunner, B.M. Andersen, A. Melikyan, and P.J. Hirschfeld, Phys. Rev. Lett. 95, 177003 (2005). [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B37.00005: Observation of the pseudogap features in overdoped Bi2Sr2CaCu2O8+d by breakjunction tunneling spectroscopy Y. Xuan, H. J. Tao, Z. Z. Li, B. R. Zhao, Z. X. Zhao, C.T. Lin Tunneling measurements of Bi2Sr2CaCu2O8+d single crystals with different oxygen doping have been carried out by using break-junction technique to study the pseudogap in the overdoped regime. The normal-state pseudogap has been observed clearly in the three overdoped crystals with Tc = 90 K, 82 K, and 72 K, respectively. Furthermore, direct measurements on the pseudogap opening temperature T*, by tracing the disappearance of the tunneling conductance peak, reveal that the relationship of T* versus hole concentration is linear and its extrapolation just crosses the end of the superconducting phase boundary. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B37.00006: Gap Inhomogeneity-Induced Electronic States in Superconducting $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ Alan Fang, Luca Capriotti, Doug Scalapino, Steve Kivelson, Nobu Kaneko, Martin Greven, Aharon Kapitulnik We use STM to measure nearly optimally doped $\rm Bi_2Sr_2CaCu_2O_{8+\delta}$ in zero field. Focusing on the superconducting gap, we find patches of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. We compare these spectra with calculations of the local density of states for a simple phenomenological model in which a $2\xi_0 \times 2\xi_0$ patch with an enhanced or supressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B37.00007: Electronic modulations in the pseudogap state of $Bi_{2}Sr_{2}CaCu_{2}O_{8+d}$ and $Ca_{2-x}Na_{x}CuO_{2}Cl_{2} $ Abhay Pasupathy, Aakash Pushp, Kenjiro Gomes, Shimpei Ono, Yasushi Abe, Yoichi Ando, Hiroshi Eisaki, Kyung Hee Kim, Hye Kyong Lee, Sung-Ik Lee, Ali Yazdani The nature of the local modulations of the density of states (DOS) found in the pseudogap regime of the cuprates is still a mystery. Using variable-temperature scanning tunneling spectroscopy we observe non-dispersive spatial DOS modulations in the pseudogap state of $Bi_{2}Sr_{2}CaCu_{2}O_{8+d}$ (1) and $Ca_{2-x}Na_{x}CuO_{2}Cl_{2}$ (2). We will compare the nature of modulations found in these materials to various models such as those arising from interference effects due to impurity scattering or from local electronic organization. We will also discuss the spatial variation of the pseudogap in $Bi_{2}Sr_{2}CaCu_{2}O_{8+d}$ and $Ca_{2-x}Na_{x}CuO_{2}Cl_{2}$. \\ (1) Vershinin et al., Science 303, 1995 (2004)\\ (2) Hanaguri et al., Nature 430, 1001 (2004). [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B37.00008: Influence of vortex quantum fluctuations on the electronic spectra of superconductors Lorenz Bartosch, Subir Sachdev We compute the influence of the zero point motion of vortices on the electronic quasiparticle spectra of two-dimensional $s$- and $d$-wave superconductors. In the core region the zero point motion of the vortices leads to a shift of spectral weight away from the Fermi level and thereby reduces the zero bias conductance peak. We discuss the relationship of our results to STM measurements on the cuprates and the observed 7 meV LDOS peaks near the core of vortices. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B37.00009: Local effects of impurities on the Pseudogap State of~Bi-2212 Kenjiro Gomes, Aakash Pushp, Abhay Pasupathy, Shimpei Ono, Yasushi Abe, Yoichi Ando, Hiroshi Eisaki, Ali Yazdani Some of the most surprising properties of the cuprates are found in~the pseudogap state, above the superconducting transition temperature~Tc on underdoped samples. Modifications of the local density of states (LDOS) due to the~presence of atomic impurities can unveil clues about the nature of the~pseudogap. Using atomically resolved scanning tunneling spectroscopy~we map the LDOS near Zn and Ni impurities in Bi-2212~above Tc. We find modifications of the pseudogap LDOS near both~Ni and Zn sites. We will report on the spatial dependence of these~variations~and how they relate to the underlying pseudogap state. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B37.00010: Scanning Tunneling Spectroscopy Studies of Current-Carrying Superconductors J. Ngai, P. Morales, J. Y. T. Wei Understanding how a superconducting order parameter evolves in the current-carrying state is of fundamental physical importance. In this talk we present novel applications of scanning tunneling spectroscopy (STS) on current-carrying superconductors, with the current either directly applied [D. Zhang et al., Phys. Rev. B 70, 172508 (2004)] or induced by a magnetic field [G. Deutscher, Rev. Mod. Phys. 77, 109 (2005)]. We will discuss STS measurements on YBa2Cu3O7 thin-film strips under directly-applied current, showing suppression of the d- wave Andreev states [J. Ngai et al. Phys. Rev. B 72, 054513 (2005)]. We will also discuss more recent STS experiments on high-Tc and other superconductors, performed under magnetic field-induced currents. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B37.00011: High Precision Spectroscopic Imaging of Single Layer Cuprate: Bi$_{2}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+x}$ A.M. Wang, J. Lee, J. Slezak, K. Fujita, H. Eisaki, S. Uchida, J.C. Davis We present high spatial resolution scanning tunneling spectroscopy (STS) study of single layer cuprate Bi$_{2}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+x.}$ The sample is optimally doped, with $T_{c}$ = 34K. We focus on two main points: (1) Quasiparticle interference pattern obtained by Fourier transform of high resolution local density of states (LDOS) map, (2) LDOS map shows even stronger nanoscale electronic disorder than double layer cuprate Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x }$ of similar doping. Inside the primary energy gap, at very low energies, small LDOS peaks emerge and exhibit short range modulations in their heights. Scattering by La substitution on Sr sites may be key to these new phenomena. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B37.00012: Atomic-resolution tunneling asymmetry mapping in Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$: charge distribution, crystal distortion and superconducting electronic structure properties of glassy electronic nanodomains Y. Kohsaka, C. Taylor, C. Lupien, T. Hanaguri, H. Takagi, M. Azuma, M. Takano, J. C. Davis We report spectroscopic imaging on the electronic state in a lightly-doped high-$T_{\rm c}$ superconductor Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$. Conductance maps at $|E|<$100~mV show 4$a_0$ correlation `checkerboard' patterns ($a_0$: in-plane lattice constant) [1]. At higher energies, the electronic state shows strong hole-electron asymmetry changing with the doping levels. The asymmetry integrated up to 600~mV changes at atomic scale and exhibits spatial patterns with 4$a_0$ correlation, consistent with spatial modulation of hole density [2]. At intermediate energies, the asymmetry structures locally break rotational symmetry of background lattice, forming uni-directional and glassy nano-domains with 4$a_0$ correlation. In-plane oxygen atoms appear electronically inequivalent, implying oxygen orbitals are key for physics in this energy range. We also discuss atomic motions and relationship of superconductivity to the glassy electronic nano-domains. [1] T. Hanaguri {\it et al.}, Nature 430, 1001 (2004). [2] M. Randeria {\it et al.}, PRL 95, 137001 (2005). [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B37.00013: Low energy spectroscopic mapping study on (Ca,Na)$_2$CuO$_2$Cl$_2$ T. Hanaguri, Y. Kohsaka, J. C. Davis, I. Yamada, M. Azuma, M. Takano, K. Ohishi, H. Takagi Spatial variations of the low energy ($<$50~meV) local density of states (LDOS) spectra of (Ca,Na)$_2$CuO$_2$Cl$_2$ ($T_c\sim$ 25~K) have been studied using scanning tunneling microscopy/spectroscopy (STM/STS) in the temperature range between 0.4~K and 20~K. Previous STM/STS studies revealed the checkerboard-like glassy electronic order which is associated with a V-shaped pseudogap $\sim150$~mV.~[1] We have found another small gap ($\sim10$~mV) with LDOS peaks at the gap edges. The gap-edge peak is spatially inhomogeneous and tends to disappear at elevated temperatures, suggesting that the small gap is related to superconductivity. [1]~T. Hanaguri {\it et al.}, Nature {\bf 430}, 1001 (2004). [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B37.00014: Many-Impurity Effects in Fourier Transform Scanning Tunneling Spectroscopy William Atkinson Fourier transform scanning tunneling spectroscopy (FTSTS) is a useful technique for extracting details of the momentum-resolved electronic band structure from inhomogeneities in the local density of states due to disorder-related quasiparticle scattering. To a large extent, current understanding of FTSTS is based on models of Friedel oscillations near isolated impurities. Here, a framework for understanding many-impurity effects is developed based on a systematic treatment of the variance $\Delta \rho^2({\bf q},\omega)$ of the Fourier transformed local density of states $\rho({\bf q},\omega)$. One important consequence of this work is a demonstration that the poor signal-to-noise ratio inherent in $\rho({\bf q},\omega)$ due to randomness in impurity positions can be eliminated by configuration averaging $\Delta \rho^2({\bf q},\omega)$. Furthermore, we develop a diagrammatic perturbation theory for $\Delta \rho^2({\bf q},\omega)$ and show that an important bulk quantity, the mean-free-path, can be extracted from FTSTS experiments. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B37.00015: Atomic-resolution Inelastic Electron Tunneling Spectroscopy studies of doping dependence of electron-boson interaction in Bi-2212 Jinho Lee, K. McElroy, J. Slezak, K. Fujita, J.-X. Zhu, A.V. Balatsky, S. Uchida, H. Eisaki, J.C. Davis Inelastic Electron Tunneling Spectroscopy(IETS) has been one of the most powerful tool to probe collective modes in the solids. We use the recently developed \textit{STM-based Atomic-resolution Inelastic Electron Tunneling Spectroscopy(IETS)} technique to study the doping dependence of the electron-boson interactions(EBI) in Bi-2212 high-Tc cuprates. According to our results, the distribution of the bosonic mode energy($\Omega$) does not change as doping level changes, while the gap distribution changes greatly. Also $\Omega$-maps show nano-scale disorder. We will discuss the possible origin of this doping independent EBI. [Preview Abstract] |
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