Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z52: Superconductivity: Tunneling Phenomena |
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Sponsoring Units: DCMP Chair: Michael Osofsky, Navel Research Laboratory Room: Mile High Ballroom 1F |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z52.00001: Non-equilibrium theory of tunneling into localized states in superconductors Ivar Martin, Dima Mozyrsky A single static magnetic impurity in a fully-gapped superconductor leads to formation of an intragap quasiparticle bound state. At temperatures much lower than the superconducting transition temperature, the energy relaxation and spin dephasing of the state are expected to be exponentially suppressed. The presence of such a state can be detected in electron tunneling experiments. Here we show, that even for an arbitrarily weak tunneling strength, the differential tunneling conductance is symmetric with respect to the sign of applied bias. This is in contrast to the standard expectation that the conductance is proportional to the local density of states, which may be particle-hole asymmetric. The standard result can be recovered is one assumes either a finite density of impurity states, or that impurities are coupled to another, non-superconducting equilibrium bath. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z52.00002: Conductance Measurements of Magnesium Diboride-based Josephson Junctions Below 1 Kelvin: Beyond the 2-Gap Model Steven Carabello, Joseph Lambert, Jerome Mlack, Wenqing Dai, Qi Li, Ke Chen, Daniel Cunnane, C.G. Zhuang, X.X. Xi, Roberto Ramos Theoretical and experimental studies have probed the nature of magnesium diboride's two superconducting energy gaps $\Delta_\pi$ and $\Delta_\sigma$. Several theoretical analyses have predicted fine structures within each energy gap, with recent experiments revealing similar structures. We have performed high-resolution tunneling measurements of low-transparency Josephson junctions using ``terraced,'' ``columnar,'' and c-axis MgB$_2$ films separated by its native oxide from either lead (Pb) or tin (Sn) counter-electrodes. Using high-resolution I-V data at $T$ as low as 23mK, we observe sub-structures within both energy gaps. We also observe sharp peaks in the subgap that identify, to high precision, the energy gap values of the junction counter-electrodes (Pb and Sn). These lead us to conclude that the substructures seen in the gaps are due to MgB$_2$. We then fit the data using simplified two-gap and four-gap models with variable weights and broadening factors. By demonstrating the inadequacy of a simple two-gap model in fitting the data, we illustrate that some distinctions between theoretical models of energy gap substructures are experimentally observable. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z52.00003: Current oscillations in ultra-small superconducting Nb-Nb junctions formed by STM at mK temperatures Michael Dreyer, Anita Roychowdhury, Rami Dana Using etched Nb STM tips we formed ultra-small tunnel junctions on a Nb crystal at an effective temperature of $\sim$ 200 mK using an Oxford dilution refrigerator. The Nb crystal was prepared in UHV and then transferred into the mK STM. The resulting superconductor-insulator-superconductor (SIS) junction displayed several sub-gap features from multiple Andreev reflections to a zero bias conductance peak. The latter showed features of a Josephson junction in the phase diffusion limit [2] with side structures due to the electrical environment [3]. Upon microwave irradiation the peak split into multiple peaks in accordance with theory [4], verifying Josephson tunneling. In addition we observed bias dependent oscillations of the tunneling current. The oscillations where recorded at a rate of 10 kS/s while acquiring conventional dI/dV or I(V) spectroscopic curves. Histograms of the current for each bias voltage step then reveal the nature of the oscillation. It ranges from multiple states in certain bias regions through pure oscillations to supercurrent-normal switching. Fourier transform of the current show in some cases a bias dependence of the main frequencies. Possible causes will be discussed. \\[4pt] [1] A. Roychowdhury, et al., arXiv:1311.1855 (2013).\\[0pt] [2] M. Ivanchenko and L.A Zil'berman, Sov. Phys. JETP, \textbf{28}, 1272 (1969).\\[0pt] [3] G. Ingold and H. Grabert, Phys. Rev. B., \textbf{50}, 395 (1994).\\[0pt] [4] G. Falci, V. Bubunja and G. Schon, Z. Phys. B., \textbf{85}, 451 (1991) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z52.00004: Point Contact Spectroscopy Study of the New Superconductor Nb2PdxSe5 Yeping Jiang, Xiaohang Zhang, Ichiro Takeuchi, Richard Greene, Seunghyun Khim, Bumsung Lee, Kee-Hoon Kim We have systematically investigated the temperature dependence of the energy gap structure for the new quasi-one-dimensional superconductor Nb2PdxSe5 by point contact spectroscopy (PCS). Our studies were performed on highly transparent Andreev reflection junctions evidenced by sharp and dramatic conductance enhancements at low temperatures. By applying the BTK model, we find that the energy gap scales with the transition temperature (Tc$=$5.5 K) in a BCS-like manner. Details of this and a few anomalous features of the PCS will be presented. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z52.00005: STM Spectroscopy Probe of Field Depairing and Vortex Lattice Transition in a Multiband Superconductor I. Fridman, V. Lukic, C. Kloc, C. Petrovic, J.Y.T. Wei The Cooper pairing in a variety of superconductors involves carriers from multiple bands, which can optimize the pairing phase space and provide novel pairing interactions. We have developed a novel technique to probe multiband pairing, using a directional diamagnetic supercurrent to perturb the quasiparticle density-of-states spectrum, and measuring the spectral evolution due to pair breaking by finite superfluid momentum. This technique is demonstrated on the layered superconductor 2H-NbSe2, using a scanning tunneling microscope (STM) at 300 mK with an in-plane magnetic field up to 9 T [1]. The STM spectroscopy measurements revealed unambiguous evidence for multiband pairing [2], as well as a novel reorientation transition of the in-plane vortex lattice [3]. We will discuss the first-order and quantum-critical characteristics of this transition, in terms of the geometric frustration of a distorted hexagonal vortex lattice, and show that this transition is intimately related to the multiband pairing. \\[4pt] [1] I. Fridman et al., Applied Physics Letters 99, 192505 (2011).\\[0pt] [2] I. Fridman et al., arXiv:1110.6490.\\[0pt] [3] I. Fridman et al., arXiv:1303.3559. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z52.00006: Low-Bias Anomaly and Tunnel-Fluctuoscopy Andreas Glatz, Andrey Varlamov, Valerii Vinokur Electron tunneling spectroscopy pioneered by Esaki and Giaever offers a powerful tool for studying electronic spectra in superconductors. The phenomenological theory by Giaever and Megerle related the tunneling current to the electronic densities of states and to the difference of their equilibrium distribution functions in electrodes. This led to impressive discoveries having revealed, in particular, of the wide, $eV_{pg}\sim\Delta_{BCS}$, pseudogap in the tunneling spectrum of superconductors above their critical temperatures. However, it turns that this standard approach is insufficient to reveal the nontrivial, related to Andreev reflection of the tunneling electrons from superconducting fluctuation domains in the biased electrode, zero-bias anomaly carrying important information about the scattering, interactions, and decoherence. Here, operating in frameworks of the microscopic theory of tunneling, we report the existence of a such low-energy singularity in a tunneling conductivity of N-I-N(S) junction directly indicating on the presence of fluctuating Cooper pairs. Our findings mark a radical departure from the conventional picture of the ZBA and open new horizons for quantitative analysis of electronic spectra of superconductors in fluctuation regime. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z52.00007: Tunneling Spectroscopy of MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N Thin Films Grown by Atomic Layer Deposition Chaoyue Cao, Nickolas Groll, Jeffrey Klug, Nicholas Becker, Serdar Altin, Thomas Proslier, John Zasadzinski Tunneling I(V) and dI/dV vs. V are reported on superconducting thin films of MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N using a point contact method with a Au tip. The films are grown by the chemical process of atomic layer deposition (ALD) onto various substrates (Si, quartz, sapphire) held at 450 C. Resistively measured superconducting Tc values up to 12K and 13K are found for the MoN and Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N respectively. Artificial tunnel barriers (1-3 nm thick) of Al$_{\mathrm{2}}$O$_{\mathrm{3}}$, also grown by ALD, are shown to provide much improved tunneling characteristics compared to the native oxides. Relatively high quality gap features are observed with zero-bias conductance values as low as $\sim$ 10{\%} of the high bias values. Gap parameters $\Delta$ $\sim$ 2.0meV are found for the MoN and $\Delta$ $\sim$ 2.0-2.4 meV for the Nb$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$N which follow the BCS temperature dependence and close near the measured film Tc indicating bulk superconductivity at the surface. The suitability of such conformal ALD grown films for potential superconducting devices is discussed. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z52.00008: Observing tunnel magnetoresistance in junctions comprising of superconductors with Zeeman-split energy bands Bin Li, Guo-Xing Miao, Jagadeesh S. Moodera The spin-splitting of the quasiparticle density of states (DOS) in a superconductor due to Zeeman energy can lead to a highly field responsive spintronic device. We present our magnetotunneling studies in superconductor/insulator/ferromagnet tunnel junctions in which the superconducting quasiparticle DOS is energy split by an internal exchange field at the interface from an adjacent ferromagnetic insulator EuS layer. A tunnel magnetoresistance (TMR) as large as 36\% is observed, and that only occurs in the superconducting state. Tunnel conductance simulation suggests that the TMR originates from the conductance variation resulting from spin selective quasiparticle tunneling. Our results show that in addition to the naturally existent spin imbalance at Fermi level in ferromagnets that gives rise to conventional TMR in standard magnetic junction (MTJs), we can manipulate tunnel conductance by tailoring spin dependent density of states with interfacial exchange fields. Furthermore, a similar TMR is also observed even with a tunnel junction with both superconducting electrodes that have exchange split DOS. [B. Li, G.-X. Miao, and J. S. Moodera, \textit{Phys. Rev. B} \textbf{88}, 161105(R) (2013)] [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z52.00009: Macroscopic Refrigeration Using Superconducting Tunnel Junctions Peter Lowell, Galen O'Neil, Jason Underwood, Xiaohang Zhang, Joel Ullom Sub-kelvin temperatures are often a prerequisite for modern scientific experiments, such as quantum information processing, astrophysical missions looking for dark energy signatures and tabletop time resolved x-ray spectroscopy. Existing methods of reaching these temperatures, such as dilution refrigerators, are bulky and costly. In order to increase the accessibility of sub-Kelvin temperatures, we have developed a new method of refrigeration using normal-metal/insulator/superconductor (NIS) tunnel junctions. NIS junctions cool the electrons in the normal metal since the hottest electrons selectively tunnel from the normal metal into the superconductor. By extending the normal metal onto a thermally isolated membrane, the cold electrons can cool the phonons through the electron-phonon coupling. When these junctions are combined with a pumped $^{3}$He system, they provide a potentially inexpensive method of reaching these temperatures. Using only three devices, each with a junction area of approximately 3,500 $\mu$m$^{2}$, we have cooled a 2 cm$^{3}$ Cu plate from 290 mK to 256 mK. We will present these experimental results along with recent modeling predictions that strongly suggest that further refinements will allow cooling from 300 mK to 120 mK. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z52.00010: Superconductivity of lanthanum revisited Peter Loeptien, Lihui Zhou, Jens Wiebe, Alexander Ako Khajetoorians, Roland Wiesendanger The thickness dependence of the superconductivity in clean hexagonal lanthanum films grown on tungsten (110) is studied by means of scanning tunneling microscopy (STM) and spectroscopy (STS). Fitting of the measured spectra to BCS theory yields the superconducting energy gaps from which the critical temperatures are determined. For the case of thick, bulk-like films, the bulk energy gap and critical temperature of dhcp lanthanum turn out to be considerably higher as compared to values from the literature measured by other techniques. In thin films the superconductivity is quenched by the boundary condition for the superconducting wavefunction imposed by the substrate and surface, leading to a linear decrease of the superconducting transition temperature as a function of the inverse film thickness. This opens up the possibility to grow lanthanum films with defined superconducting properties. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z52.00011: Ultra-high vacuum fabrication of metal/insulator/superconductor junctions for spin polarization measurements Zachary Barcikowski, Joshua Pomeroy Using a unique ultra-high vacuum (UHV) deposition chamber equipped with electron gun deposition sources, sputter deposition and plasma oxidation, we are depositing shadow mask defined tunnel junctions. These unique capabilities allow us to assess the importance of creating high quality tunneling materials in the ultra-thin regime where abrupt chemical interfaces and near-ideal stoichiometries are important. In this talk, I will present experimental details about this unique system and discuss devices being fabricated, including spectroscopy measurement techniques using the superconductor quasi-particle DOS as an analyzer. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z52.00012: The Effect of Inelastic Scattering of Charge Carriers on the Reliability of the Value of the Spin Polarization as Determined from Superconductor/Ferromagnet Point Contact Conductance Data Paul J. Dolan, Jr., Charles W. Smith An extended BTK model for charge transport in a superconductor/ferromagnet point contact can be used to determine the value of the spin polarization of the ferromagnet. We estimate the effect of inelastic scattering of charge carriers in the active region of the contact on the reliability of the value of the polarization as determined from conductance data. The effect can be substantial and depends upon contact transparency. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z52.00013: Cooper pair splitting in isolated NSN island Pochen Tai, Chia-Heng Sun, Jheng-An Jiang, Cen-Shawn Wu, Jeng-Chung Chen, Yung-Fu Chen Cooper pair is in a maximally entangled two-particle state, and may have applications on solid state version of quantum teleportation. We investigate Cooper pair splitting, a charge-transfer process by crossed Andreev reflection (CAR), in a three-island system. The system consists of one superconducting island (S) and two normal-metal islands (N), while S connects to two N via two tunnel junctions. When the charging energy of the system is the dominant energy factor, this system is suitable to study charge transfers down to single-electron regime. Two single-electron transistors as charge sensors are capacitively coupled to two N, respectively, to observe charge tunneling events. Several competing charge-transfer processes as long with CAR also occur in two S/N interfaces, such as quasi-particle tunneling, cotunneling, and ordinary Andreev reflection. Correlation measurements of charge fluctuation at two S/N interfaces should help to tell CAR process apart from other competing processes. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z52.00014: Temperature Dependence of Quasiparticle Spectral Weight and Coherence in High $T_c$ Superconductors Yang He, Jessie Zhang, Jennifer Hoffman Superconductivity arises from the Cooper pairing of quasiparticles on the Fermi surface. Understanding the formation of Cooper pairs is an essential step towards unveiling the mechanism of high $T_c$ superconductivity. We compare scanning tunneling microscope investigations of the temperature dependence of quasiparticle spectral weight and quasiparticle interference in several families of high $T_c$ materials. We calculate the coherent spectral weight related to superconductivity, despite the coexistence of competing orders. The relation between pairing temperature and coherent spectral weight is discussed. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z52.00015: Coulomb Blockade of Shot Noise Denis Vion, Carles Altimiras, Olivier Parlavecchio, Philippe Joyez, Patrice Roche, Daniel Esteve, Fabien Portier We observe the suppression of the finite frequency shot-noise produced by a voltage biased tunnel junction due to its interaction with a single electromagnetic mode of high impedance. The junction is embedded in a superconducting lambda/4 resonator containing a dense SQUID array yielding a resonator characteristic impedance in the kOhm range and a resonant frequency tunable in the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb blockade on the tunnel junction (about 30{\%} reduction in the differential conductance) and allows an efficient measurement of the spectral density of the current fluctuations at the resonator frequency. The observed blockade of shot-noise is found in agreement with an extension of the dynamical Coulomb blockade theory. [Preview Abstract] |
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