Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A35: Superconductivity: Tunneling Phenomena |
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Sponsoring Units: DCMP Chair: John Zasadzinski, Illinois Institute of Technology Room: 343 |
Monday, March 18, 2013 8:00AM - 8:12AM |
A35.00001: Scanning Tunneling Microscopy of Fe Impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ Michael Boyer, Brian Koopman, Ling Fu, W.D. Wise, Kamalesh Chatterjee, Genda Gu, E.W. Hudson We utilize scanning tunneling microscopy measurements to probe the effects of intentionally doped magnetic Fe impurities in the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$. Our spectroscopy measurements indicate an absence of particle-hole symmetry in impurity affected regions. In addition, we find evidence that the Fe impurities which substitute for Cu atoms in the CuO$_{2}$ plane are shifted from their expected locations. Both of these findings are in contrast to previous STM measurements on magnetic Ni impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ which find spectra which are overall particle-hole symmetric and centered at Cu sites.[1] Interpretations of our measurements may help us understand on a local scale why introduced Fe impurities are more detrimental to superconductivity than Ni impurities as determined by bulk measurements.[2] [1] E.W. Hudson et al., Nature 411, 920 -- 924 (2001). [2] T.D. Hien et al., J. Magn. Magn. Mater. 262, 508 -- 513 (2003). [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A35.00002: Can STM detect nematic ordering in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8+x}}$ or other correlated systems? Eduardo da Silva Neto, Pegor Aynajian, Shimpei Ono, Ryan Baumbach, Eric Bauer, John Mydosh, Ali Yazdani Electronic nematic phases, where, for example, the electronic states undergo a spontaneous four-fold (C$_{4})$ to two-fold (C$_{2})$ symmetry breaking, have recently gained vast interest as a possible candidate for various hidden order states in several correlated electron systems such as cuprates, pnictides, and heavy fermions. Such states are difficult to detect using non-local probes because of possible twin domain structures in macroscopic samples. STM spectroscopy has been proposed as a possible approach to detect such nematic orders, with several recent experiments reporting signals in the cuprates and iron-based superconductors. We specifically investigate the situation in which STM topographic data shows C$_{4}$ symmetry while energy-resolved spectroscopic maps signal C$_{2}$ symmetry. We find that such behavior can in fact occur for asymmetric tip geometries and discuss both model calculations and experimental results that provide evidence for this false nematic signature. We discuss possible future STM experiments that could unambiguously detect electronic nematic order. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A35.00003: STM investigation of incipient order in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Pegor Aynajian, Eduardo H. da Silva Neto, Shimpei Ono, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani We investigate the spatial and momentum structure of electronic excitations in underdoped samples of the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ using spectroscopic mapping with the scanning tunneling microscope. A defining feature of the electronic states in these samples is a strong Cu-O bond oriented modulation of the local density of states (Q*). Characterizing Q* as a function of temperature and doping we have established that it appears at the onset of the pseudogap phase at T*, above the regime attributed to fluctuating superconductivity [1]. Model calculations that include both the effects of impurity-induced quasiparticle scattering and incipient order reproduce the energy-dispersion of the measured Q* below and above Tc near optimal doping --- where incipient order effects are weak [2]. To extend our understanding to the underdoped samples, we have carried out new high-resolution spectroscopic mapping measurements as a function of doping which more clearly identify the low-energy signatures of the incipient order. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A35.00004: Studies of Magnetic Impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Eduardo Calleja, Jixia Dai, Genda Gu, Kyle McElroy Impurities in high temperatures superconductors, studied with spectroscopic imaging scanning tunneling spectroscopy (SI-STS) have served as a valuable tool to investigate the electronic structure of these materials (E.W. Hudson \textit{et al}., Nature \textbf{411}, 920 (2001), S.H.Pan \textit{et al}., Nature \textbf{403},746 (2000)). These experiments revealed the appearance of a quasi-localized bound state near the impurity site whose structure is sensitive to the superconducting gap symmetry and the band structure and originates from the charge scattering nature of these impurities. We studied the effects of Fe impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ and discovered that the impurities have a different behavior than those previously observed. In particular the quasi bound state near the impurity seems to be behaving as that predicted for a magnetic impurity. The superconducting gap and local electronic density of states was studied in the vicinity of the impurities using SI-STS and will be presented. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A35.00005: Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures Holger Motzkau, Thorsten Jacobs, Sven-Olof Katterwe, Andreas Rydh, Vladimir M. Krasnov We study resistive switching phenomena in small $\mathrm{Bi_2Sr_2CaCu_2O}_{8+x}$ (Bi-2212) mesa structures. Applying a significantly large bias voltage or short current pulses, we are able to controllably and reversibly manipulate the normal state resistance and doping state of the same single crystal from an underdoped to the overdoped state without changing its chemical composition. We employ this effect for an analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. It is observed that such a physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the c-axis critical current rapidly increases, while the critical temperature, the superconducting gap and the pseudogap decrease, indicating the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A35.00006: Tunneling Spectroscopy of Heavily Underdoped Bi2212 Films Nickolas Groll, Chaoyue Cao, Mike Hinton, Thomas Lemberger, Thomas Proslier, John Zasadzinski SIS break junctions exhibiting quasiparticle and Josephson tunneling were obtained on heavily underdoped Bi2212 films grown by sputtering, with Tc values as low as 5K. Well defined, but extraordinarily large gap values ($\Delta$) were reproducibly observed, even as the Josephons $I_{c}R_{n}$ $\ll$ $\Delta$. The largest values of $\Delta$ were close to $J$, the antiferromagnetic exchange energy. When the new data are combined with previous break junction data on Bi2212 crystals an abrupt change of slope of $\Delta$ vs. doping is found, suggesting a second, magnetic energy scale is being observed in the underdoped region. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A35.00007: Imaging chemical disorder in cuprates using scanning tunneling microscopy Ilija Zeljkovic, Dennis Huang, Can-Li Song, Tay-Rong Chang, Horng-Tay Jeng, Zhijun Xu, Jinsheng Wen, Genda Gu, Jouko Nieminen, Arun Bansil, Robert Markiewicz, Jennifer Hoffman High-T$_{c}$ cuprate superconductors are chemically, electronically and structurally inhomogeneous at the nanoscale. Although a body of theoretical work has predicted that local and global superconductivity may be dramatically impacted by particular dopant configurations, the exact positions of dopants introduced into cuprates to induce superconductivity are generally unknown. Here we use scanning tunneling microscopy to reveal the intra-unit-cell location of two different types of oxygen dopants in Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$. Furthermore, we show the relationship between these interstitial oxygen dopants, oxygen vacancies, and a global structural buckling known as the supermodulation. We compare our findings to theoretical simulations. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A35.00008: Cryomagnetic STM spectroscopy study of multiband pairing in layered superconductors Igor Fridman, Vladimir Lukic, Christian Kloc, Cedomir Petrovic, Pengcheng Dai, J.Y.T. Wei Cooper pairing in layered superconductors can involve multiple bands and give rise to complex gap structures in momentum space. Using scanning tunneling microscopy (STM) with a magnetic field applied parallel to the $ab$-plane, we investigate multiband pairing under diamagnetically-induced superfluid momentum. STM spectroscopy and conductance imaging were performed down to 300 mK and up to 9 T, on single-crystals of the Nb-chalcogenide $2H$-NbSe$_2$ and the Fe-pnictides LiFeAs and electron-doped BaFe$_2$As$_2$. Spectroscopy data taken on $2H$-NbSe$_2$ at 300 mK showed a distinctly two-sloped field evolution of the zero-bias conductance, consistent with Doppler-induced depairing on parts of the Fermi surface [1]. Spatial conductance maps revealed stripe patterns that originate from in-plane vortices whose cores are buried in the bulk [2] and which undergo a transition as pairing on one of the bands is suppressed. Our results demonstrate a general method for probing multiband superconductors, especially ones whose band structures host coexisting orders and also play a direct role in the pairing mechanism. \\[4pt] [1] I. Fridman \emph{et al.}, arXiv:1110.6490 (2011) \\[0pt] [2] I. Fridman \emph{et al.}, Appl. Phys. Lett. \textbf{99}, 192505 (2011) [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A35.00009: Doping-dependent vortex-state scanning tunneling spectroscopic (STS) studies of cuprate superconductors C.-C. Chen, M. L. Teague, Z.-J. Feng, R.T.-P. Wu, N.-C. Yeh We report STS studies of YBa2Cu3O7$-\delta $(Y-123) and Ca-doped Y-123 superconductors as a function of magnetic field ($H)$ and hole doping level ($p)$. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition $T_{c}$ is associated with a CO energy $\Delta_{CO}$ larger (smaller) than the SC gap $\Delta_{SC}$. We derive $\Delta_{SC}$ and $\Delta_{CO}$ by two approaches. For zero-field STS we apply Green function techniques to fit the ``peak'' features for $\Delta_{SC}$ and the ``kink'' features for $\Delta_{eff} \quad \equiv $ [($\Delta_{SC})^{2}+(\Delta_{CO})^{2}$]$^{1/2}$. For $H$ \textgreater\ 0 we analyze the PG features in the intra-vortex STS for $\Delta_{CO}$ and the peak features in the inter-vortex STS for $\Delta _{SC}$. Both approaches yield consistent results. For optimally and underdoped Y-123, we find that $\Delta_{SC}$ \textless\ $\Delta_{CO}$ with dominant $d_{x^{2}-y^{2}}$-wave pairing, and that $\Delta_{SC}$ decreases with decreasing $p$ while $\Delta_{CO}$ increases. Both $\Delta _{SC}$ and $\Delta_{CO}$ exhibit long-range spatial homogeneity. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder. For $p$ \textgreater\ 0.16, both $\Delta_{SC}$ and $\Delta_{CO}$ decrease with increasing $p$, $\Delta_{CO}$ \textless\ $\Delta_{SC}$ for $p$ \textgreater\ 0.23, and the pairing symmetry becomes ($d_{x^{2}-y^{2}}+s$) with increasing $s$-wave component, implying the diminishing Mott nature in overdoped cuprates. This work was supported by NSF through IQIM at Caltech. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A35.00010: Spatial Complexity Due to Locally Oriented Charge Modulations in a Cuprate Superconductor Erica Carlson, Elizabeth Main, Benjamin Phillabaum, Hiroshi Ikuta, Karin Dahmen, Eric Hudson, Jennifer Hoffman Surface probes such as scanning tunneling microscopy (STM) have detected complex electronic patterns at the nanoscale in many high temperature superconductors. We use scanning tunneling microscopy to image the local orientation of the static charge modulations in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$, for samples spanning a wide range of doping. For each sample, we compute the universal cluster properties arising from the locally $x$-oriented and locally $y$-oriented clusters in order to identify the fundamental physics controlling the complex pattern formation. By comparing these quantitative measures to known universality classes for rotational symmetry breaking, we find that the charge modulations are not confined only to the surface, but they also extend throughout the bulk of the material. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A35.00011: Tunneling Spectroscopy of SRF Cavity Grade Niobium Chaoyue Cao, Nick Groll, Thomas Proslier, John Zasadzinski Mechanical contact tunneling measurements are presented on high purity Nb pieces from the starting plate for superconducting radio frequency (SRF) cavity construction as well as from hot spot and cold spot regions of a tested cavity. A varying scattering rate, gamma, is found which broadens the BCS density of states. Detailed fits using Shiba theory indicate that this scattering may be due to magnetic pairbreaking. Hot spot samples reveal a zero bias conductance peak that splits in magnetic field and can be fit using Appelbaum-Anderson theory of spin flip scattering. Together these measurements indicate that the native oxide of Nb can contain varying amounts of localized magnetic moment defects, possibly due to oxygen vacancies in niobium pentoxide. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A35.00012: Andreev Reflection Spectra of $d$-wave Superconductors Charles Snider, Jessica Gifford, Jonny Martinez, Tingyong Chen At a normal metal/superconductor interface Andreev reflection occurs, which can be utilized to measure spin polarization of the normal metal and also the superconducting gap of the superconductor. An $s$-wave superconductor has an isotropic gap and for an unpolarized current the Andreev reflection spectrum within the gap is twice that of outside the gap. A fully spin polarized current suppresses the Andreev reflection therefore causes zero conductance within the gap. The scenario is quite different in a $d$-wave superconductor because the order parameter has anisotropy and phase. In this work, we calculate Andreev Reelection spectra of an interface between a normal metal and a $d$-wave superconductor for a current with any polarization, based on the recent Chen-Tesanovic-Chien (CTC) model. It is shown that the point angle of the interface can drastically change the Andreev spectra and a zero bias anomaly (ZBA) is observed in the tunneling regime only if the point angle is large. The spin polarization can also drastically affect the spectra and can completely suppress the ZBA. Our calculation shows that one can use both the spin polarization and the point angle to verify the ZBA in unconventional superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A35.00013: Direct Probe of Interplay between Local Structure and Superconductivity in FeTe0.55Se0.45 Wenzhi Lin, Qing Li, Brian Sales, Stephen Jesse, Athena Safa-Sefat, Sergei Kalinin, Minghu Pan We explore the interplay between local crystallographic structure, composition and local electronic and superconductive properties. Direct structural analysis of scanning tunneling microscopy (STM) data allows local lattice distortions and structural defects across a FeTe0.55Se0.45 surface to be explored on a single unit-cell level. Concurrent superconducting gap (SG) mapping reveals suppression of the SG at well-defined structural defects, identified as a local structural distortion (Guinier-Preston zone). The strong structural distortion is related to the vanishing of the superconducting state. This study provides insight into the origins of superconductivity in iron chalcogenides by providing an example of atomic-level studies of the structure-property relationship. Research was supported (WL, BCS, AS, SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted (MP, QL) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. (Wenzhi Lin and Qing Li, these authors contributed equally to this work) [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A35.00014: ABSTRACT WITHDRAWN |
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