Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B41: Tunneling Spectroscopy of Superconductors |
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Sponsoring Units: DCMP Chair: Laura Greene, University of Illinois Room: F152 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B41.00001: A Model For Point Contacts Between A Normal Metal And A Superconductor In An Applied Magnetic Field Charles W. Smith, Paul J. Dolan, Jr. The BTK model for charge transport in a normal metal/superconductor point contact, with elastic and inelastic quasiparticle scattering, is modified to include the effect of an applied magnetic field. A two channel model of the form G$_{S}$(h, T) = h$^{a}$G$_{NN}$ + (1 - h$^{a})$ G$_{NS}$(T) is assumed, where h is the reduced magnetic field, G$_{S}$(h, T) is the contact conductance, G$_{NN}$ is the normal channel contribution and G$_{NS}$(T) is the superconducting channel contribution. The value of the field exponent, a, is obtained by first extracting the BTK parameters for the contact at zero applied field. Using these parameters, a is fit to the transport data as a function of applied field, at fixed temperature. Several case studies over a wide range of BTK parameters will be discussed. Best fits to the data indicate that within the assumptions of this model, a = 2. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B41.00002: Measurement of Conduction Electron Polarization Via the Pairing Resonance Philip Adams, Yimin Xiong, Gianluigi Catelani We show that the pairing resonance in the Pauli-limited normal state of ultra-thin superconducting Al films provides a spin-resolved probe of conduction electron polarization in thin magnetic films. A superconductor-insulator-ferromagnet tunneling junction is used to measure the density of states in supercritical parallel magnetic fields that are well beyond the Clogston-Chandresekhar limit, thus greatly extending the field range of tunneling the density of states technique. The applicability and limitations of using the pairing resonance as a spin probe will be discussed. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B41.00003: 2D Superconductivity of Ultra-thin Pb Films on Si(111) Jungdae Kim, Shengyong Qin, Hyungdo Nam, C. K. Shih The thickness dependence of superconductivity for globally flat ultra-thin Pb films is studied by using in-situ low temperature Scanning Tunneling Microscope / Spectroscopy (STM/S). Superconducting transition temperature (Tc) of each thickness is calibrated by fitting STS with BCS-like density of states. Superconductivity is found to be quite robust down to 5 monolayer films. In addition to the thickness dependence, this work focuses on how lateral size influences the superconducting gap measurements. Moreover, we investigate the lateral proximity effect between local superconducting regions. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B41.00004: Cryomagnetic STM Study of Multi-Gap Spectra in Superconducting 2H-NbSe$_{2}$ Igor Fridman, John Y.T. Wei, Christian Kloc The interplay between superconductivity and density wave order has been a subject of great interest, especially in materials with complex band structures. For the dichalcolgenide 2H-NbSe$_{2}$, it is believed that multiband superconductivity co-exists with charge density waves (CDW), but little is known about how they co-evolve in a magnetic field. In this study, we use a cryomagnetic scanning tunneling microscope (STM) at 300 mK and in a field of up to 5 Tesla to measure the quasiparticle tunneling spectra of 2H-NbSe$_{2}$ single crystals. Our STM data are analyzed in light of other recent experiments which have probed the multiband features of this material. Our analysis suggests that the appearance of sub-gap spectral peaks and their non-trivial field evolution are distinct signatures of a second superconducting gap, and are likely related to the CDW order in 2H-NbSe$_{2}$. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B41.00005: Recovering band structure from quasiparticle interference patterns Jacob Alldredge, E. M. Calleja, G. Gu, Kyle McElroy Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) has opened up a new window into the world of local electronic structures. This is particularly true in the realm of High Temperature Superconductors where imaging of quasiparticle interferance patterns (QPI) has allowed the underlying electronic structure in k-space to be inferred. This has led to information about the termination of the coherent states, as well as interesting revelations about the gap structure above Tc. However, current analysis techniques are based off a simplistic model which only works in a limited energy and doping range, and cannot be extended to other materials. Here we present a more general method for analyzing the QPI that allows us to infer the momentum space origin of the scattered states. This method requires fewer assumptions and allows us to image the momentum space states to higher energies and from lower signal to noise data sets. New insights into the momentum space origins of the scattered states gathered from this method will be discussed. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B41.00006: Exploring the effect of the Tunneling Matrix Element on Scanning Tunneling Spectroscopy Eduardo Calleja, Jacob Alldredge, Genda Gu, Kyle McElroy The results from scanning tunneling spectroscopy (STS) are usually interpreted with the assumption of a simple energy independent matrix tunneling element. This implies that the STS spectrum is a pure representation of the local density of state (LDOS). Recent work by A. Bansil \textit{et al.} (\textit{Phys. Rev. Lett.} \textbf{102}, 037001 (2009)) suggests, at least in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$, that the matrix element strongly modifies the STS spectrum making it different from the LDOS. We present experimental studies exploring the dependence of the STS spectrum on the tunneling matrix element. By studying the tunneling current as a function of the available parameters we can explore the matrix element's effect on the STS. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B41.00007: Off-diagonal disorder and the nodal-antinodal dichotomy of the STM experiments in high-T$_c$ superconductors Dimitris Galanakis, Stefanos Papanikolaou We present an explicit model for describing the basic features of the local density of states~(LDOS) observed in STM experiments on high-T$_c$ d-wave superconductors. The model includes a d-wave superconductor which has {\it weak} modulate off-diagonal disorder. We show that the low and high energy features of the LDOS are consistent with the observed experimental patterns and in particular, the anisotropic local domain features at high energies. At low energies, we show that the scattering peaks remain in accordance with the octet model, but in addition there are weak features that should be distinguishable in future experiments. Finally, we show that the emerging features of the LDOS lose their correspondence with the features of the imposed disorder, as its complexity increases spatially. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B41.00008: Local electronic liquid crystallinity and topological defects, application to high Tc cuprate STM data Andrej Mesaros, Jan Zaanen, Subir Sachdev, J.C. Davis, Michael J. Lawler, Eun-Ah Kim The patterns observed in STM measurements on underdoped cuprates motivate a theoretical approach based on electronic liquid crystalline ordering. Here, nematic and smectic order parameters describe $C_4$ point group symmetry breaking and the stripe-like $4a/3$ wavelength modulations. We study the interplay of these two order parameter fields and their topological defects guided by local microscopic considerations. In this light, we analyze published STM data on BSCCO to extract specific information including phase of the $4a/3$ modulations (copper or oxygen centered) and relations between the two types of local ordering. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B41.00009: New insights into cuprate superconductivity and pseudogap by STM of BSCCO E.W. Hudson, Gregory Lau, M.C. Boyer, W.D. Wise, Kamalesh Chatterjee, Michelle Tomasik, Laura Popa, Takeshi Kondo, Tsunehiro Takeuchi, Hiroshi Ikuta Using a new fitting method which enables us to extract k-space information from STM spectroscopy, we have obtained spatially dependent measurements of the superconducting gap and pseudogap structure as a function of temperature and doping in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212) and Bi$_{2}$Sr$_{2}$CuO$_{6+x}$ (Bi-2201). These results confirm our previous picture of spatially homogeneous superconductivity with a gap that closes at T$_{C}$ coexisting with an inhomogeneous pseudogap [1]. They also highlight the particle-hole asymmetry of the pseudogap. This, when combined with local measurements of the checkerboard charge order, which we have attributed to a charge density wave [2], provides new insight into the nature of the pseudogap state. [1] Boyer \textit{et al}, \textit{Nat. Phys.} \textbf{3}, 802 (2007) [2] Wise \textit{et al}, \textit{Nat. Phys.} \textbf{4}, 696 (2008)~; \textit{Nat. Phys.} \textbf{5}, 213 (2009)~ [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B41.00010: Spatial modulations of electronic states in the pseudogap phase of cuprates: ordering or interference? Colin Parker, Aakash Pushp, Abhay Pasupathy, Kenjiro Gomes, Shimpei Ono, Yoichi Ando, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani Spatial modulations in the local density of states of the high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$0$_{8+\delta }$ have now been reported both above and below T$_{c}$. At low energies and low temperatures, these modulations have been attributed to the so-called octet model of Bogoliubov Quasiparticle Interference (BQPI). At temperatures above T$_{c}$, non-dispersive modulations were found near optimal doping, suggestive of a translation symmetry breaking ordered state[1]. Recent observations of low energy, dispersive modulations above T$_{c}$ in a highly underdoped sample have been taken as evidence for Bogoliubov excitations above T$_{c}$[2]. We extend these measurements to a regime where we know from other measurements, such as anomalous diamagnetism, that there are no superconducting correlations in order to clarify whether any aspects of these modulations are related to superconducting correlations or to potential ordering at T*. [1] M. Vershinin, et al. Science 305, 1993 (2004) [2] J. Lee, et al. Science 325, 1099 (2009) [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B41.00011: The role of the Z-map in observations of quasiparticle interference in the cuprates Elizabeth Nowadnick, Brian Moritz, Thomas Devereaux Scanning tunneling spectroscopy (STS) measurements have observed that the local density of states in the cuprates is spatially inhomogeneous. When the Fourier transform of the STS data is computed, this modulation shows up as a set of peaks in the momentum space density of states. The modulation can be understood as arising from quasiparticle interference, where the scattering of quasiparticles from impurities mixes the momentum space eigenstates. Researchers have recently found that the quasiparticle interference pattern in the Fourier transform density of states is enhanced if the Z-map, defined as the ratio of the density of states at positive and negative bias, is examined, rather than the density of states itself. To investigate the reasons for this enhancement, we present calculations of momentum dependent quasiparticle scattering from impurities. Using a self-consistent T-matrix formalism, we model impurities as regions of the lattice that modulate the hopping, superconducting gap, and site energy parameters. By varying the impurity size, shape, and modulation parameters in our calculation, we perform a systematic study of the effect of the Z-map on the quasiparticle interference signal. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B41.00012: Evidence from scanning tunneling spectroscopy for magnetic-field-enhanced collective modes in the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ A.D. Beyer, M.S. Grinolds, M.L. Teague, N.-C. Yeh, S. Tajima We present scanning tunneling spectroscopic evidence for field-enhanced collective modes in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$. The observed spectra inside vortices exhibit two characteristic features: a pseudogap (V$_{CO }$= 31.5\underline {+}2.0 meV) larger than the superconducting gap ($\Delta _{SC }$= 20.0\underline {+}1.0 meV) and a subgap ($\Delta $' $\approx $ 7-10meV) smaller than $\Delta _{SC}$. Outside vortices, the spectra display a gap of $\Delta _{SC}$. As magnetic field increases, spectral weight rapidly shifts from $\Delta _{SC }$to V$_{CO}$ and $\Delta $'. The vortex state also reveals energy-independent conductance modulations with peridocities of 3.6 and 7.1 lattice constants along the Cu-O bonding direction and 9.5 lattice constants along the nodal direction. The energy-independent modulations differ fundamentally from energy-dispersive modes due to Bogoliubov quasiparticle scattering interferences and originate from field-enhanced collective modes of pair-, charge- and spin-density waves. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B41.00013: Proposal for electronic nematic order parameter sensitive to intra unit cell structures Michael Lawler, K. Fujita, A. Schmidt, Jhinhwan Lee, Chung Koo Kim, H. Eisaki, S. Uchida, J. C. Davis, J. P. Sethna, Eun-Ah Kim We propose an order parameter for detecting signatures of electronic nematic ordering using local probes such as scanning tunneling microscopy(STM). The order parameter is designed to measure rotational symmetry breaking without prejudice towards translational symmetry breaking -- achieved by focusing on intra unit cell structures. This order parameter utilizes Fourier space information much the same way as in diffraction measurements, opening the possibility for a comparative study of nematicity between different probes. Our study is primarily motivated by the patterns observed in STM measurements on underdoped cuprates. We discuss theoretical implications of our results in this light. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B41.00014: Properties for the superconducting state from a one-particle derivation of the BCS gap equation. Thomas Jarlborg The BCS results for the superconducting gap $\Delta$ and $T_C$ are obtained from a one-particle model and free electron bands. The gap becomes stable when the electronic energy gain of the band structure overcomes the energy needed for atomic vibrations or magnetic moment oscillations. The vibration/oscillation amplitudes determine the size of the superconducting gap, and the Fermi surface is important for the q-dependence. This permits for complementary interpretations of the parameters for superconductivity and modeling of density-of-state effects, where the mechanism for having a gap is less exotic. Thermal excitations of phonons and/or spin fluctuations can produce a pseudogap with many similarities with a superconducting gap. Results for the pseudogap in the normal state for copper oxides, obtained previously from band calculations for spin-phonon coupled modes [1], can be extended into the superconducting state. Possible extensions and limitations of the model for different situations are discussed. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B41.00015: Search for a nematic symmetry in the intra-unit-cell pseudogap states of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ K. Fujita, M.J. Lawler, A.R. Schmidt, Jhinhwan Lee, Chung Koo Kim, H. Eisaki, S. Uchida, J.C. Davis, J.P. Sethna, Eun-Ah Kim We measure the intra-unit-cell pseudogap states (Y. Kohsaka \textit{et al}. \textit{Science} \textbf{315}, 1380 (2007)) in underdoped Bi$_{2}$Sr$_{2}$Ca$_{0.8}$Dy$_{0.2}$Cu$_{2}$O$_{8+\delta }$ ($T_{c}$=50K) using the Spectroscopic Imaging STM, and analyze this state using a local nematic order parameter. We search for the rotational symmetry breaking (nematicity) within each CuO$_{2}$~unit-cell in~ subatomic scale STM data with a particular care in correcting for instrumental drift. We investigate the correlation between this electronic nematicity and pseudogap energy scale.We discuss the relationship between the intra-unit-cell electronic structure and a nematicity. [Preview Abstract] |
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