Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D8: Superconductivity: STM of Cuprates |
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Sponsoring Units: DMP Chair: Allen Goldman, University of Minnesota Room: Colorado Convention Center Korbel 1C |
Monday, March 5, 2007 2:30PM - 2:42PM |
D8.00001: Tunneling Spectroscopy on $c$-axis Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ Thin Films: Evidence for Multiband Superconductivity J.H. Ngai, W.A. Atkinson, J.Y.T. Wei We report scanning tunneling spectroscopy measurements on \{001\} oriented Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ thin films at \emph{x}= 0, 0.05, 0.15 and 0.20 Ca-doping at 4.2K. The tunneling spectra exhibit main-gap, sub-gap and satellite features which we analyze using a generic multiband tunneling model that accounts for the separate contributions of the plane and chain bands to the tunneling conductance spectrum. Our analysis indicates the sub-gap features could arise from the chain band density of states, while the satellite features could come from the plane band for a $d_{x^2-y^2}+s$ pairing symmetry. The doping dependent evolution indicates that all three spectral features are set by a single parameter $\Delta_0$, which monotonically decreases with Ca-doping, suggesting that superconductivity in Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$ involves multiple bands. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D8.00002: Direct evidence for predominantly phonon-mediated pairing in high-temperature superconductors Guo-meng Zhao The spectra of the second derivative of tunneling current d$^{2}I$/d$V^{2}$ in the high-temperature superconductors YBa$_{2}$Cu$_{3}$O$_{7-delta}$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ show clear dip and peak features due to strong coupling to the bosonic modes mediating electron pairing. The energies of all the peaks in $-$d$^{2}I$/d$V^{2}$-like spectra match precisely with the energies of the peaks in the phonon density of states obtained by inelastic neutron scattering. The results clearly demonstrate that the bosonic modes mediating the electron pairing are phonons and that high-temperature superconductivity arises primarily from strong coupling to multiple phonon modes. [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D8.00003: Inelastic tunneling spectroscopic imaging study of electron-lattice interactions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. Kazuhiro Fujita, J. Lee, K. McElroy, J. Slezak, M. Wang, Y. Aiura, H. Bando, M. Ishikado, T. Masui, J. -X. Zhu, A. Balatsky, H. Eisaki, S. Uchida, J. C. Davis We investigated impact of oxygen isotope effect on d$^{2}$I/dV$^{2}$ spectroscopy on $^{16}$O and $^{18}$O substituted Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta ,}$respectively, with the same doping level (nearly optimally doping. $^{16}$O$\to ^{18}$O: Tc=89K$\to $88K), using the newly developed inelastic spectroscopic imaging technique (Jinho Lee \textit{et al.}, \textit{Nature} \textbf{422},546 (2006)) . We found that oxygen isotope effect ($^{16}$O$\to ^{18}$O) leads to reduction of mode energy from 52meV to 49meV, while superconducting gap remained unchanged. Oxygen isotope re-substitution shifted mode energy back to the original energy ($^{18}$O$\to ^{16}$O) as well as $T_{c}$ back to 89K, completing the series of isotope effect probed by STM/S. We concluded that renormalization effect seen in dI/dV spectra is caused by strong electron-lattice interaction from a mode near 52meV. We will also discuss about relevance of this lattice vibration mode as a pairing glue in the talk. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D8.00004: Atomic Scale Investigation of the Effects of Pb doping in Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ W. D. Wise, Kamalesh Chatterjee, M. C. Boyer, Ming Yi, Takeshi Kondo, E. W. Hudson In the study of the Bi-based high temperature superconducting cuprates, an incommensurate structural ``supermodulation'' that runs throughout the crystal can often lead to experimental complications, especially for scattering studies. In order to eliminate this problem, a fraction of the Bi atoms are occasionally substituted by Pb, leading to a suppression of the supermodulation. As even a relatively large substitution has only a modest effect on transition temperature, it is widely believed that this suppression has little or no direct effect on superconductivity in the crystal. We will present the results of temperature dependent scanning tunneling microscopy studies of Pb doped Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ (Bi-2201) which demonstrate local spectroscopic variations associated with Pb doping. Although the exact cause of these variations is still unknown, we hypothesize that a modulated strain field due to the presence of Pb may play a role. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D8.00005: Spectroscopic imaging STM study of Bi$_{2}$Sr$_{2-x}$Ln$_{x}$CuO$_{6+\delta }$ Jhinhwan Lee, K. Fujita, K. McElroy, Jinho Lee, J.W. Alldredge, M. Wang, S. Illani, H. Eisaki, S. Uchida, J.C. Davis We will present the spectroscopic imaging STM data of near optimally doped, Ln-substituted Bi2201. The Ln (Lanthanide) atoms substituting the Sr atoms are known to cause out-of-CuO$_{2}$-plane lattice disorder and large variation of Tc (Tc(La)$\sim $34K, Tc(Gd)$\sim $13K). We will demonstrate how the coherence peaks, the gap maps, the quasi-particle interference, and the checkerboard electronic structures are affected by lattice disorder and varied Tc in these materials. We will also show, for Gd-Bi2201, how the above atomic scale spectroscopic features change at temperatures below and above Tc. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D8.00006: $d^2I/dV^2$-STS measurements of isotope effect on the electron-lattice interaction(ELI) energy and the gap energy in Bi-2212. Jinho Lee, K. Fujita, M. Wang, K. McElroy, J. Slezak, J. -X. Zhu, A. V. Balatsky, H. Eisaki, S. Uchida, J. C. Davis We measured gap energy and the electron-lattice interaction(ELI) energy on the $O^{16}$ and $O^{18}$ substituted Bi-2212 single crystals using $d^2I/dV^2$-STS(Scanning Tunnelling Spectroscopy). Bi-2212 crystals were from the same batch, and nearly optimally doped after $O^{16}$ and $O^{18}$ annealing under the same conditions. $T_c$'s were around 88K in both crystals. While the ELI energy was shifted to $\sim 4mV$ lower energy in $O^{18}$ substituted Bi-2212, we found the gap energy of $\sim 38mV$ in both crystals virtually didn't change. We also extracted gap energies from the quasi-particle(QP) dispersions using QP interferences, and will determine weather this gap energy is altered by isotope substitution. The possible identity of the lattice mode will be addressed in the context of recent results from IR spectroscopy and ARPES. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 3:54PM |
D8.00007: Local pairing and the origin of pseudogaps in Bi$_{2}$Sr$_{2}$Ca$_{2}$CuO$_{8+\delta }$ probed with high-resolution STM measurements Kenjiro K. Gomes, Abhay Pasupathy, Aakash Pushp, Shimpei Ono, Yoichi Ando, Ali Yazdani The evolution of the local electronic density of states in Bi$_{2}$Sr$_{2}$Ca$_{2}$CuO$_{8+\delta }$ has been measured as function of doping (x=0.12-0.22) and temperature (20-180K) using state-of-art variable temperature scanning tunneling microscopy (STM). These measurements allow the first characterization of spatially varying energy gaps in both the superconducting and non-superconducting states with temperature. From these observations, we are able to demonstrate that pseudogaps observed at optimal doping at temperatures as high as 50K above T$_{c}$ are due to pairing correlations. We have been able to extract a remarkably universal relation that describes the strength of local pairing to a local pairing temperature, for samples which are weakly underdoped to highly overdoped. With decreasing doping, we show that the evolution of the local electronic states with temperature cannot be captured with a single pairing energy scale, signaling the presence of another phenomenon, which is unrelated or perhaps competing with superconductivity. [Preview Abstract] |
Monday, March 5, 2007 3:54PM - 4:06PM |
D8.00008: Spatially Inhomogeneous Collapse of Superconducting Gaps on the Nanoscale: Connection to Macroscopic Measurements on Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Ali Yazdani, Abhay Pasupathy, Kenjiro Gomes, Aakash Pushp, Shimpei Ono, Yoichi Ando Using spatially resolved STM spectroscopy, we have mapped the superconducting correlations in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ to show that these correlations collapse in a spatially inhomogeneous manner with increasing temperature. These experiments provide valuable insight for understanding the results of spatially averaged measurements such as angle-resolved photoemission, vortex Nernst and field-induced diamagnetism on the same material system. The connection between nanoscale measurements and bulk parameters such as T$_{c}$ and T* is clarified. Finally, high-resolution STM measurements provide a method to characterize the processes by which superconductivity is destroyed with increasing temperature in samples with various hole doping levels. [Preview Abstract] |
Monday, March 5, 2007 4:06PM - 4:18PM |
D8.00009: Atomic-Position Tracking and Spectroscopy of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ across T$_{c}$ Abhay Pasupathy, Kenjiro Gomes, Aakash Pushp, Genda Gu, Shimpei Ono, Yoichi Ando, Ali Yazdani The inhomogeneous nature of the pairing in the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ system makes the precise characterization of the changes in the density of states at the superconducting transition difficult to measure. Thermal drift in a typical STM prevents the tracking of a specific area while varying the temperature. We have used a specially designed STM, as well as a controlled electrical and acoustic environment, to track the position of a single atomic site on the surface as the temperature is varied. Using this technique, we have been able to measure, for the first time, how the onset of superconductivity changes the local tunneling spectra at a specific site. These measurements enable us to extract changes to the local density of states at T$_{c}$ independent of the normal state background. We will describe this technique and our measurements on the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ system for different doping concentrations. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D8.00010: Real Space Mapping of Electronic States in Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ as a function of Temperature. Aakash Pushp, Abhay Pasupathy, Kenjiro K. Gomes, Shimpei Ono, Yoichi Ando, Ali Yazdani We have studied the single layer Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ system using high resolution scanning tunneling microscopy and spectroscopy at various temperatures. From these measurements, we have determined the evolution of the local density of states from the superconducting to the non-superconducting state. In this talk, we will describe the evolution of the gap and other spectroscopic features as a function of doping (y=0.2 to 0.9) and temperature (20-100K). Real space maps of the electronic states that show strongly modulated patterns will also be presented. These experimental results are used to determine the connection between the superconducting gaps and the pseudo-gaps in this compound. Finally, we will discuss the similarities and differences between the measurements on the single-layer Bi$_{2}$Sr$_{2-y}$La$_{y}$CuO$_{6}$ and the double-layer Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D8.00011: Superconducting Scanning Tunneling Microscope -- Josephson effect and High-T$_{C}$ superconducting cuprate Hikari Kimura, Robert Dynes, Shimpei Ono, Yoichi Ando We have developed and characterized superconducting scanning tunneling microscope (STM) tips that consist of Pb coated Pt/Ir wires. We have observed the thermally fluctuated Josephson effects between a conventional superconductor and this superconducting STM. STM-based Josephson junctions formed between the superconducting-STM tip and superconducting samples can be a powerful tool to detect both superconducting quasiparticles and the phase of the superconducting condensate via the Josephson effect on a length scale of nanometers. This technique is especially powerful when we study spatially inhomogeneous electronic systems such as High-T$_{C}$ superconducting cuprates. In this talk we present data of the STM Josephson junctions formed between S-STM tips and both Pb/Ag films and NbSe$_{2}$ single crystals. The former experiments give us the effective noise temperature T$_{n}$ and the impedance of the environment around the junction, Z$_{env}$. The latter is a precursor to the Pb/I/HT$_{C}$ cuprate Josephson junctions. We have derived the I$_{C}$R$_{N}$ product of NbSe$_{2}$ junctions using T$_{n}$ and Z$_{env}$ obtained as described above. Preliminary results of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals by S-STM tips are also discussed. This work is supported by DOE Grant No. FDDE-FG02-05ER46194. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D8.00012: STM Spectroscopy of Electron Doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ Francis Niestemski, Shankar Kunwar, Vidya Madhavan We present high resolution scanning tunneling spectroscopy (STS) of electron doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$. We focus on x=0.12 doping (T$_{c} \quad \sim $ 25K ) at various temperatures at and above 2 K. Our data reveal both small and large gap behavior in the same sample. The magnitude of the smallest measured gap is consistent with superconductive gaps observed by other probes. The larger gaps are of varying magnitudes ranging from 10meV to 100meV or greater. We will discuss our observations in light of experimental data on similar PLCCO samples from ARPES and neutron scattering. [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D8.00013: Atomic Resolution Scanning Tunneling Microscopy of Electron-Doped Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ Shankar Kunwar, Francis Niestemski, Vidya Madhavan The study of electron-doped superconductors offers a new window into the still intractable problem of high temperature superconductivity. While hole-doped High T$_{C}$ superconductors have been extensively studied with scanning tunneling microscopy (STM), there has been very little atomic resolution STM data on electron doped superconductors. We present STM images of \textit{in situ} cleaved Pr$_{1-x}$LaCe$_{x}$CuO$_{4-\delta }$ (PLCCO), obtained with a low temperature (5.6K), ultra high vacuum (UHV) STM. Due to the post annealing process required for superconductivity, superconducting PLCCO is expected to contain between 0.1{\%} to 1{\%} of the oxide impurity phases, (Pr, Ce)$_{2}$O$_{3}$. Our STM data on superconducting PLCCO reveal a few different atomic scale features, some of which have a periodicity consistent with the lattice constant expected for the impurity phases. Spectroscopy performed on these areas reveal gaps in the meV energy range. Possible origins of these and the other atomic scale structures will be discussed in this talk. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D8.00014: Tunneling spectroscopy of e-doped cuprates Lei Shan, Yan Huang, Yong-Lei Wang, Shi-Liang Li, Jun Zhao, Peng-Cheng Dai, Hai-Hu Wen Point-contact tunneling spectra were measured on electron-doped high-T$_{c}$ cuprates (NCCO and PLCCO). By phenomenological analysis, we found that the superconducting gap ($\Delta _{sc})$ definitely decreases towards zero in an almost universal law with continuously increasing temperature or magnetic field. At the fields above H$_{c2}$, a clear ``pseudogap'' was opened indicated by the obvious spectral losing below a characteristic energy scale ($\Delta _{pg})$ which is much larger than $\Delta _{sc}$. All the phenomena observed here seem to be crucial to distinguish the mechanism of HTSC and need to be extensively studied on more doping levels. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D8.00015: Competing Orders: Origin of the Non-Universal Low-Energy Pseudogap Phenomena in Cuprate Superconductors Ching-Tzu Chen, A. D. Beyer, N.-C. Yeh The contrasting low-energy pseudogap phenomena and quasiparticle spectral characteristics between the electron- and hole-type cuprates remain an open issue in cuprate superconductivity. Here we review the experimental manifestation of various non-universal properties and show that a phenomenological model of coexisting density-wave orders with superconductivity can consistently explain these disparate observations. By incorporating quantum phase fluctuations and adopting realistic bandstructures, numerical simulations of the quasiparticle tunneling spectra reproduce the empirical observations for both types of cuprates. Specifically, by tuning the ratio of the density waves to superconductivity, we can account for the absence of low-energy pseudogap in electron-type cuprates and the presence of pseudogap in hole-type cuprates. We therefore conclude that competing orders play an important role in the rich phenomenology of cuprate superconductivity. [Preview Abstract] |
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