Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M36: Superconductivity: Josephson and Nanoscale Phenomena |
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Sponsoring Units: DCMP Chair: John Clarke, University of California, Berkeley Room: 344 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M36.00001: Frequency-dependent admittance of a short superconducting weak link Filip Kos, Simon Nigg, Leonid Glazman We consider the electromagnetic response of a nanowire connecting two bulk superconductors. Andreev states appearing at a finite phase bias substantially affect the finite-frequency admittance of such wire junction. We evaluate the complex admittance analytically at arbitrary frequency and arbitrary, possibly non-equilibrium, occupation of Andreev levels. Special care is given to the limits of a single-channel contact and a disordered metallic weak link. We also evaluate the quasi-static fluctuations of admittance induced by fluctuations of the occupation factors of Andreev levels. In view of possible qubit applications, we compare properties of a weak link with those of a tunnel Josephson junction of the same normal conductance. Compared to the latter, weak link has smaller low-frequency dissipation. However, because of the deeper Andreev levels, quasi-static fluctuations of the complex admittance in a weak link are exponentially larger than in a tunnel junction. These fluctuations limit the applicability of nanowire junctions in superconducting qubits. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M36.00002: Nonlocal transport in superconducting oxide nanostructures Joshua Veazey, Guanglei Cheng, Shicheng Lu, Michelle Tomczyk, Patrick Irvin, Mengchen Huang, Chung Wung Bark, Sangwoo Ryu, Chang-beom Eom, Jeremy Levy We report nonlocal transport signatures in the superconducting state of nanostructures formed\footnote{J.P. Veazey, \textit{et al.}, arXiv:1210.3606 (2012).} at the LaAlO$_3$/SrTiO$_3$ interface using conductive AFM lithography. Nonlocal resistances (nonlocal voltage divided by current) are as large as 200 $\Omega$ when 2-10 $\mu$m separate the current-carrying segments from the voltage-sensing leads. The nonlocal resistance reverses sign at the local critical current of the superconducting state. Features observed in the nonlocal \textit{V-I} curves evolve with back gate voltage and magnetic field, and are correlated with the local four-terminal \textit{V-I} curves. We discuss how nonlocal and local transport effects in LaAlO$_3$/SrTiO$_3$ nanostructures may result from the electronic phase separation and superconducting inhomogeneity reported by others in planar structures\footnote{Ariando, \textit{et al.}, \textit{Nature Comm.} \textbf{2}, 188 (2011); J.A. Bert, \textit{et al.}, \textit{Nature Phys.} \textbf{7}, 767 (2011).}. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M36.00003: Superconductivity in Centimeter Length Indium-Gallium Nanowires Weiwei Zhao, Jesse Bischof, Meenakshi Singh, Thomas Fitzgibbons, Xin Liu, Chaoxing Liu, Lin Wang, Zhonghou Cai, Si Chen, John Hayes, Pier Sazio, John Badding, Moses Chan In-doped Ga nanowires 150 nm in diameter and 6mm in length have been formed in silica nanocapillaries. X-ray fluorescence and diffraction measurements performed at the Advanced Photon Source have been used to characterize their chemical composition and crystal structure. Investigation of the low temperature transport properties of these wires reveals a two stage superconducting transition. Magnetoresistance measurements are suggestive of vortex trapping in the wire. The X-ray fluorescence measurements suggest phase separation in the capillaries into Ga nanodroplets and In-Ga eutectic wires. A model to explain the vortex trapping consistent with this observation is being developed. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M36.00004: Investigating long-range proximity effect in ferromagnetic Ni and Ni-Fe nanowires Meenakshi Singh, James Kally, Weiwei Zhao, Moses Chan Singlet superconductors and ferromagnets entail incompatible spin orders severely limiting the range of the superconducting proximity effect in a ferromagnet ($\sim$ 1 nm). Contrary to this expectation, a very long-range proximity effect (LRPE, $\sim$ 600 nm) was found in crystalline ferromagnetic nanowires [Wang et al., Nat. Phys. 6, 389 (2010)]. Several mechanisms have been suggested to explain the LRPE, the most intriguing of which is the possibility of triplet superconductivity in the ferromagnet. We have conducted experiments to probe the mechanism of the LRPE. The LRPE persists in granular Ni nanowires, ruling out ballistic transport as a possible mechanism. Surface superconductivity in the oxide layer on the ferromagnetic nanowire is also ruled out based on critical current measurements. On changing the nature of the contacting electrodes, the range of the proximity effect is found to diminish significantly. This indicates that the nature of the interface between the superconductor and the ferromagnet is important as expected for triplet superconductivity. Tunneling measurements probing the superconducting gap in the ferromagnetic nanowire are underway. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M36.00005: Critical current oscillations in superconducting Al strips Tyler Morgan-Wall, Benjamin Leith, Nikolaus Hartman, Atikur Rahman, Nina Markovic We have studied current-voltage characteristics as a function of temperature and magnetic field in superconducting aluminum strips with varying lengths and cross sections. We find that the critical current oscillates as a function of magnetic field and suggest that the effect depends on the relative energies of vortex configurations in the strips in different transport regimes. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M36.00006: Ultralow Noise Microwave Amplifier Based on the Superconducting Low-inductance Undulatory Galvanometer Shaojiang Zhu, David Hover, Guilhem Ribeill, Robert McDermott We have developed an ultralow noise microwave linear amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The compact SLUG element is straightforward to model at microwave frequencies, allowing separate optimization of the SLUG element and the resonant input matching network. SLUG amplifiers incorporating high-Jc junctions have shown gains of order 15 dB in the frequency range from 3-10 GHz with instantaneous bandwidth up to several hundred MHz. Large-volume normal metal cooling fins have been integrated into the SLUG element to promote thermalization of hot electrons in the resistive shunts at millikelvin temperatures, and the amplifiers have achieved added system noise of one photon in the GHz frequency range. We discuss application of the SLUG amplifier to single shot dispersive readout of the transmon qubit.~ [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M36.00007: Flux noise in SQUIDs: Effects of deposited surface films S.R. O'Kelley, S.M. Anton, J.S. Birenbaum, John Clarke, G.C. Hilton, H.-M. Cho, K.D. Irwin, C.D. Nugroho, A.F. Dove, G.A. Olson, Z.R. Yoscovits, V. Orlyanchik, D.J. Van Harlingen, J.N. Eckstein Magnetic flux noise in SQUIDs and superconducting qubits with a spectral density $S_\Phi(f)$ scaling as $1/(f/1 Hz)^\alpha$ is understood to arise from the random reversal of spins localized at the surface of the superconducting film. We present experimental results showing the effects on $S_\Phi(f)$ of Au, SiNx, NbN, and Al2O3 films deposited on the upper surface of Nb and NbN dc SQUID loops. For each measurement, we fabricated six identical SQUIDs on a single chip and then capped the surface of either half or all the SQUID loops. Certain capping layers, such as Au, had no discernible effect on $S_\Phi(f)$ with regard to the magnitude, slope $\alpha$, and temperature dependence. On the other hand, some capping layers significantly reduced $S_\Phi(1 Hz)$---by a factor of about two in the case of SiNx. Furthermore, some layers significantly affected the value of $\alpha$ and the temperature dependence of both $S_\phi(1 Hz)$ and $\alpha$. These results further establish the importance of the role of the surface of the SQUID loop on its flux noise. We discuss implications for microscopic models of flux noise in light of these measurements. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M36.00008: Geometry and temperature dependence of low-frequency flux noise in dc SQUIDs S.M. Anton, J.S. Birenbaum, S.R. O'Kelley, D.S. Golubev, G.C. Hilton, H.-M. Cho, K.D. Irwin, V. Bolkhovsky, D.A. Braje, G. Fitch, M. Neeley, R.C. Johnson, W.D. Oliver, F.C. Wellstood, John Clarke Measurements on dc SQUIDs reveal a flux noise spectral density $S_\Phi(f) = A^2/(f/1~Hz)^\alpha$. An analytic model assuming non-interacting spins localized at the surface of the SQUID loop predicts that the mean square noise scales as R/W---the radius and width of the loop, respectively. However, there are no established theories for the scaling of $\alpha$ with geometry or the dependences of A and $\alpha$ on temperature T. To test the predicted geometric scaling of this model experimentally, we measured flux noise in ten SQUIDs with systematically varying geometries. We find that, at fixed T, $A^2$ scales approximately as R. From the measured values of A and $\alpha$, we estimate the mean square flux noise, which does not scale with R. As T is lowered, $\alpha$ increases significantly and in such a way that the spectra ``pivot'' about an approximately fixed frequency. This phenomenon implies that the mean square noise is temperature-dependent, an effect not predicted by the analytic model. We discuss our attempts to reconcile these discrepancies by considering the locking together of spins to form clusters. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M36.00009: Niobium Nitride Thin Films and Multilayers for Superconducting Radio Frequency Cavities William Roach, Douglas Beringer, Zhaozhu Li, Cesar Clavero, Rosa Lukaszew Niobium nitride in thin film form has been considered for a number of applications including multi-layered coatings onto superconducting radio frequency cavities which have been proposed to overcome the fundamental accelerating gradient limit of $\sim$50 MV/m in niobium based accelerators [1]. In order to fulfill the latter application, the selected superconductor's thermodynamic critical field, H$_{\mathrm{C}}$, must be larger than that of niobium and separated from the Nb surface by an insulating layer in order to shield the Nb cavity from field penetration and thus allow higher field gradients. Thus, for the successful implementation of such multilayered stack it is important to consider not just the materials inherent properties but also how these properties may be affected in thin film geometry and also by the specific deposition techniques used. Here, we show the results of our correlated study of structure and superconducting properties in niobium nitride thin films and discuss the shielding exhibited in NbN/MgO/Nb multilayer samples beyond the lower critical field of Nb for the first time.\\[4pt] [1] A. Gurevich, Appl. Phys. Lett., \textbf{88}, 012511 (2006). [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M36.00010: Induced superconducting FFLO states in patterned island systems and in topological insulators Smitha Vishveshwara, Qinglei Meng, Taylor Hughes, Nadya Mason We explore the possibility of inducing the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting phase in 2D metal films by means of proximity coupling to patterned superconducting islands. We show that as a function of externally applied magnetic field, such a system not only renders the phase stable for a large region of parameter space but can also be tuned through different spatial ordering wavevectors associated with the FFLO order. We generalize these results to the surface states of 3D topological insulators and metallic surface states with Rashba coupling. We find that these FFLO states can be mapped into BCS states in which a uniform superconductor gap occurs in momentum space and can potentially be accessed in physical systems with relative ease. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M36.00011: Evidence for synchronized Andreev reflections in NSN devices Martin P. Stehno, Dale J. Van Harlingen In mesoscopic NSN devices, in which a short superconducting region separates two metallic electrodes, the Andreev reflection process may delocalize and couple electron- and hole-states on opposite sides of the superconductor. In addition to such nonlocal (or crossed) Andreev reflections, quasiparticles may also tunnel directly between the electrodes. We have studied nonlocal transport and current correlations in Cu/Al/Cu structures. We observe that the current correlations are symmetric in applied bias and show local minima when the applied voltages at the two contacts are equal in magnitude. This behavior matches theoretical predictions for devices with intermediate interface transparency in which the nonlocal exchange of quasiparticles triggers additional synchronized Andreev reflection events at the two interfaces. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M36.00012: Josephson current and density of states in proximity circuits with s+- superconductors Stanislav Apostolov, Alex Levchenko We study the emergent proximity effect in mesoscopic circuits which involve conventional superconductor and unconventional pnictide superconductor separated by a diffusive normal or ferromagnetic wire. The focus is placed on revealing signatures of the proposed $s^{+-}$ state of pnictides from the proximity-induced density of states and Josephson current. We find analytically a universal result for the density of states which exhibits both a Thouless gap at low energies, and peculiar features near the superconducting gap edges at higher energies. The latter may be used to discriminate between $s^{+-}$ and $s^{++}$ symmetry scenarios in scanning tunneling spectroscopy experiments. We also calculate Josephson current-phase relationships for different junction configurations, which are found to display robust $0-\pi$ transitions for a wide range of parameters. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M36.00013: Results of Switching Measurements in MgB$_2$ Josephson Heterojunctions: Search for Multiple Tunneling Channels and Leggett-Mode Signatures Steve Carabello, Joseph Lambert, Daniel Cunnane, Wenqing Dai, Ke Chen, Qi Li, X. X. Xi, Roberto Ramos Josephson tunnel junctions made of multi-gap and single-gap superconducting electrodes provide a useful system for understanding multiple gap superconductivity. Peaks in the differential conductance curve have been used to characterize the energy gaps of such multi-gap materials [e.g. Chen, K. et al., Nat. Commun. 3:619 (2012)]. Superconducting-to-normal switching data can also provide useful insights. While ramping the current from zero to the critical current, the washboard potential is tilted, thereby adjusting the resonant frequency of the potential well, and altering the energy level spacing. By exciting the junction with microwaves, resonant modes may be explored. We report results of conductance and switching experiments on MgB$_2$/I/Pb and MgB$_2$/I/Sn junctions, with and without microwaves, in a helium dilution refrigerator with a base temperature ~20mK. These results exhibit tunneling modes and resonances not observed in single-gap/single-gap junctions, including a peak in the escape rate that may be consistent with coupling to the Leggett mode. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M36.00014: Evidence for Multi-photon transitions between energy levels in a large Current-Biased Magnesium Diboride Josephson Heterojunction Roberto Ramos, Steven Carabello, Joseph Lambert, Daniel Cunnane, Wenqing Dai, Ke Chen, Qi Li, Xiaoxing Xi When photons are strongly coupled to a quantum system, multiphoton transitions can be observed between two energy levels when the quantum energy of the exciting radiation, multiplied by an integer, matches the level spacing. This phenomenon can be observed in Josephson junction qubits exposed to weak microwave radiation at very low temperatures. At microwave resonance, the transition probability of a junction from superconducting to normal state is enhanced and these are used to map multiphoton transitions. We report observation of single- and multi-photon transitions between ground and first excited states in current-biased MgB2 thin film junctions by applying RF with frequencies between 0.5 and 3 Ghz. These large (up to 0.2mm x 0.3 mm) junctions consist of an MgB2 electrode insulated by native oxide from a lead (Pb) or tin (Sn) counter-electrode, and have areas at least 600 times bigger than Nb junctions previously shown to exhibit multiphoton transitions. The data is consistent with theoretical models of junctions behaving in the quantum limit and show anharmonicity of the junction potential when biased near the critical current. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M36.00015: In-gap States of Josephson Junction with Two-gap Superconductiors Ju Kim, Ladan Bahrainirad We investigate the transport property of SIS junctions with two-gap superconductors. The effects of two superconducting condensates on critical current density is estimated by studying the microscopic structure of Josephson current density in a dual-mode tunnel junction with a narrow quasi-classical tunnel barrier. Following the suggestion by Golubov and coworkers [1], we use two Bloch functions to describe the condensates in the two-band superconductors. In this junction, the in-gap states which include the interband interference effect appear at the interfaces due to the discontinuity of the superconducting phase. Also, similar to a Josephson junction [2] involving one-gap and two-gap superconductors, novel broken time-reversal symmetry states are found. We estimate the effects of interband interference and broken time-reversal symmetry on the in-gap bound states and critical Josephson current density. \\[4pt] [1] A. A. Golubov \textit{et al.}, Phys. Rev. Lett. \textbf{103}, 3398 (2009). \\[0pt] [2] T. K. Ng and N. Nagaosa, EPL \textbf{87}, 17003 (2009). [Preview Abstract] |
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