Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W40: Mesoscopic and Nanoscopic Superconductors |
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Sponsoring Units: DCMP Chair: Mingliang Tian, Pennsylvania State University Room: F151 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W40.00001: Quantum Dot Superconducting Quantum Interference Devices with self-assembled InAs Ryosuke Ishiguro, Sunmi Kim, Eichiro Watanabe, Daiju Tsuya, Kenji Shibata, Kazuhiko Hirakawa, Hideaki Takayanagi We have fabricated quantum-dot superconducting quantum interference devices (QD-SQUID), which have two superconductor / quantum dot /superconductor junctions using superconducting aluminum electrodes with laterally contacting a self-assembled InAs quantum dot. The differential resistance as a function of bias voltage clearly shows multiple Andreev reflection peaks. That indicates a good electric contact is formed at the Al-InAs interface. dV/dI-V at the zero bias voltage is very sensitive to gate voltage and magnetic field, and oscillates with every flux quantum through the SQUID loop. This indicates that the fabricated superconducting loop embedded with two S/QD/S junctions is the coherent state. QD-SQUID will be used for the study of the phase correlation with a Kondo singlet state and the superconductor. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W40.00002: Thermodynamic properties of ultra-small metallic grains in the presence of pairing and exchange correlations Konstantin Nesterov, Yoram Alhassid We study the heat capacity and spin susceptibility of chaotic nano-size metallic grains in the ballistic regime with a large dimensionless Thouless conductance in the presence of superconducting and ferromagnetic correlations. Our analysis is based on a low-energy effective Hamiltonian, the universal Hamiltonian, that contains a sample-specific one-body part modeled by the random-matrix theory, a BCS-like pairing term and a ferromagnetic Stoner-like spin exchange term. The exchange interaction is taken into account exactly by means of the spin projection technique, and the pairing interaction is treated in the static-path approximation with the random-phase approximation correction. The exchange interaction shifts the number-parity effects induced by pairing correlations to lower temperatures, and may suppress the reentrant behavior of the spin susceptibility for an odd number of electrons in the grain. If the exchange coupling constant is sufficiently large and the pairing gap $\Delta$ is comparable to the single-particle mean level spacing $\delta$, the number-parity effects are suppressed by mesoscopic fluctuations. The fluctuations of the spin susceptibility are particularly large in the presence of exchange correlations at $\Delta/\delta <1$. This work was supported in part by U.S. DOE grant No. DE-FG-0291-ER-40608. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W40.00003: Simulation of electronic structures of nano-scaled superconductors using 3-dimensional finite element method Masaru Kato I have developed numerical method for investigating the electronic structures in the nano-scaled superconductors, effectively, using three-dimensional finite element method. Previously co-workers and I investigated the electronic structures in the nano-scaled superconducting plate using two-dimensional finite element method. In those studies, we obtained stable magnetic flux structures and the quasi-particle structures around these magnetic fluxes. Especially, we investigated stability of the giant vortex, which has a doubly quantized magnetic flux. Also we found the transition temperature enhancement for the nano-scaled superconducting plate due to the inhomogeneous superconducting structure. But because of the two-dimensionality of previous method, we cannot investigate the three dimensional structures of the quantized magnetic fluxes, for example, Josephson vortices and pancake vortices in the cuprate high-Tc superconductors. In this study, I investigate such three dimensional vortex structures using this microscopic three-dimensional numerical method. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W40.00004: Geometrically confined superconducting condensates in nanoscopic Al squares prepared by e-beam lithography Neal Staley, Xinxin Cai, Ying Liu Previous studies on mesoscopic superconductors with a sample size greater than the zero temperature coherence length, $\xi$(0), and penetration depth, $\lambda$(0), have revealed interesting features including quantized states in the H-T phase diagrams. These features are well described by a linearized Ginzburg-Landau formalism. However, work on the regime where the sample size becomes smaller than $\xi$(0) and $\lambda$(0) has been scant. Here, neither a spacially varying order parameter nor a magnetic field induced current is expected according to the Ginzburg-Landau theory. We will present electrical transport measurements on Al squares prepared by e-beam lithography with thicknesses of 20 nm and a length ranging from 130 nm to 530 nm. These devices had a $\xi$(0) of $\sim$120 nm and $\lambda$(0) of $\sim$130nm. The H-T phase diagram was constructed for each sample from the measurement on the superconducting to normal resistive transition as a function of both magnetic field and temperature. We observed evidence for the existence of quantized states in the smallest squares that were not anticipated from Ginzburg-Landau theory. Work supported by NSF. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W40.00005: Scanning SQUID investigation of the suppression of superfluid density in mesoscopic superconducting rings Julie A. Bert, Nicholas C. Koshnick, Hendrik Bluhm, Martin E. Huber, Kathryn A. Moler We use a scanning SQUID microscope to measure the suppression of the superconducting response of quasi-1D rings specifically designed to exhibit phase winding fluctuations below Tc.~ Extremely high flux sensitivity as well as a positionable sensor capable of measuring many individual rings and subtracting the background in situ make scanning SQUID ideal for this measurement.~ The physical ring parameters are carefully controlled during fabrication to reduce the ring's superconducting phase stiffness by tuning the energy spacing of states where a uniform phase winds an integer number of times around the ring.~ When the energy difference between adjacent phase winding states is approximately equal to the temperature, the superfluid density is suppressed by the contribution of multiple states to the response. ~ We present susceptibility data and a theoretical framework that demonstrate how these fluxoid fluctuations suppress the ring's diamagnetism below Tc. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W40.00006: Probing Macroscopic Quantum Tunneling in Superconducting Nanowires by Varying the Critical Current Thomas Aref, Matthew Brenner, Alexey Bezryadin Macroscopic Quantum Tunneling (MQT) is a foremost example of a ``Schr\"odinger cat'' state, which is a quantum superposition of macroscopically distinct states. Here we detect MQT by measuring the dispersion of the switching current of thin superconducting wires. Using controlled application of high bias pulses, we are able to vary the critical current of a single nanowire. The resulting behavior rules out electromagnetic noise or inhomogeneity in the superconducting nanowires as being a source for the observed dispersion of the switching current. Thus we explain the observed dispersion in terms of MQT. We compare and contrast these experiments to the measured dispersions of the switching current in double-wire samples. In such nanowire-SQUIDs, the variation of the switching current is caused by the external magnetic field. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W40.00007: Boundary Effects in Superconducting Zn Nanowires Stephen Snyder, Yu Chen, Allen Goldman We have studied Zn nanowires connected to Zn electrodes while an applied magnetic field drives the electrodes into the normal state. A finite resistance jump appears at the critical field of the electrodes. The current dependence of this resistance exhibits contrasting differences from its temperature dependence in the low current limit, and this discrepancy builds with shorter wires. The observations may unveil an unconventional breakdown of superconductivity due to the nonequilibrium quasiparticle distribution. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W40.00008: Magnetic field induced superconductivity in out-of-equilibium Zn nanowires Yu Chen, Stephen Snyder, Allen Goldman We report on electrical transport measurements of Zinc nanowires prepared using electron beam lithography. The wires, when driven resistive by current, re-enter the superconducting state upon application of small magnetic fields.[Chen et al., PRL 103, 127002] This occurs at temperatures well below the zero-magnetic field critical temperature. The parameter space of this out-of equilibrium field-induced reentrant superconductivity has been investigated by changing the magnetic field orientation as well as the length of the nanowires. The results suggests this to be a nonequilibrium effect and that the quasiparticle population in the Zn electrodes plays a central role in the re-entrance phenomenon. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W40.00009: Transport studies of individual crystalline nanowires contacted by superconducting electrodes Jian Wang, Meenakshi Singh, Mingliang Tian, Nitesh Kumar, Bangzhi Liu, Chuntai Shi, J.K. Jain, Nitin Samarth, T.E. Mallouk, M.H.W. Chan When a crystalline Au nanowire of 70 nm diameter is contacted by superconducting electrodes, the wire acquires superconductivity via the proximity effect. Instead of a single sharp drop to zero resistance as seen in a shorter wire, a two step superconducting transition was found for a wire of 1.2 microns in length. The normal and fully superconducting regions are separated by what we call the ``mini-gap'' phase. In addition, clear periodic differential magnetoresistance oscillations in the superconducting to normal transition region were observed. Our systematic study of individual single crystal ferromagnetic Co nanowires, contacted by superconducting electrodes found the surprising result that a wire of 600 nm is completely superconducting with zero resistance. For longer wires, there is a large and sharp resistance peak at the onset of superconductivity that ``anticipates'' the incomplete superconductivity at low temperatures. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W40.00010: Maintaining the local temperature below the critical value in thermally out of equilibrium superconducting wires Massilliano Di Ventra, Yonatan Dubi A generalized theory of open quantum systems combined with the mean-field theory is used to study a super-conducting wire in contact with thermal baths at different temperatures. It is shown that, depending on the temperature of the colder bath, the temperature of the hotter bath can greatly exceed the equilibrium critical temperature. The effects of contact areas and disorder are studied. Finally, an experimental setup is suggested to test our predictions, and the relevance of our results to the quest of a superconducting device that will operate at room temperatures is discussed. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W40.00011: Re-examining the critical velocity of a clean one-dimensional superconductor Paul Goldbart, Tzu-Chieh Wei We re-examine the issue of the critical velocity of a clean, one-dimensional superconductor at the level of mean-field theory. We find that the zero-temperature value of the critical velocity (i.e., the uniform velocity of the superfluid condensate at which the superconducting state becomes unstable) is a factor of $\surd $2 smaller than the Landau value. This contrasts with a prior finding, which held that these velocities equal one another. The smaller critical velocity, which our analysis yields, is the result of a pre-emptive Clogston-Chandrasekhar-type discontinuous phase transition, analogous to the Sarma and Maki-Tsuneto threshold exchange field of a superconductor. We also consider the role of temperature, explore critical currents, and examine metastability and its limits in the temperature vs.~flow-velocity phase diagram, and we comment on the effects of electron scattering by impurities. [1]~T-C. Wei and P.M. Goldbart, Phys. Rev. B 80, 134507 (2009). [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W40.00012: Switching Current Fluctuation of Superconducting Aluminum Nanowires Peng Li, Phillip Wu, Albert Chang The switching current ($I_s$) from superconducting state to normal state in Aluminum superconducting nanowires is studied. The degree of $I_s$ fluctuation first increases and then decreases with temperature rising. The result is qualitatively in agreement with the theory that the transition is triggered by a single phase slips at low temperature and by multiple consecutive phase slips at higher temperature. The fluctuation in whole temperature range shows phase slips assisted by thermal fluctuation, but the systematic deviation at low temperature indicates possible phase slips by quantum tunneling. This suggests the ultra-narrow superconducting nanowire can behaves very much like an under-damped Josephson junction. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W40.00013: Nonlocal transport in multi-terminal NS structures with transparent interfaces Martin Stehno, Dale J. Van Harlingen Several explanations have been proposed to describe nonlocal transport measurements in multi-terminal normal metal-superconductor structures. While Crossed-Andreev Refection (CAR) and Elastic Co-tunneling (EC) processes are expected to dominate in structures with low-transparency contacts, proximity effects and non-equilibrium transport effects must be taken into account in systems with opaque interfaces. We fabricated and characterized Cu-Al structures with low resistance contacts and found rich structure in their nonlocal transport behavior. We will discuss the role of non-equilibrium effects and proximity coupling in NS systems and their relevance to experiments in FS (ferromagnet-superconductor) hybrid devices. We will also report on experiments to measure current noise in these devices. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W40.00014: Andreev bound states in superconductor ferromagnet hybrids Shi-Hsin Lin, Milorad Milosevic, Francois Peeters, Boldizsar Janko We investigate the electronic structure of a superconductor in proximity of nanoscale ferromagnets (SC/FM hybrids) and find that a wide variety of superconducting order parameter landscape can be achieved by varying the parameters and the magnetic state of the nanomagnets. In particular, we propose an energy-angular momentum dispersion $\varepsilon(l)$ with the energy minimum at $l \neq 0$ in systems with superconductivity suppressed in a ring. This resembles to Landau's energy-momentum dispersion $ \varepsilon(k)$ for the roton in superfluid, a dispersion that also exhibits a local minimum at $k\neq0$. We show how such structures will emerge in specific examples SC/FM hybrids and investigate these systems via Ginzburg-Landau and Bogoliubov-de Gennes calculations. Our results show that the low-lying bound states correspond to a non-zero angular momentum. In the presence of injected current, our system becomes analogous to an inverted pendulum, which could be driven towards equilibrium with an AC magnetic field. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W40.00015: Electrical Transport Properties of Hybrid Magnet-Superconductor Nanostructures near the Transition Temperature K. Kim, A.E. Ozmetin, D.G. Naugle, W. Wu, I. Lyuksyutov Magnetic hysteretic behavior of the magnetoresistance and the critical current is observed in a hybrid magnet-superconductor nanostructure (MSN) in which vertically aligned nickel nanorods in a square lattice are partially embedded in a Pb$_{82}$Bi$_{18}$ film. From mean field theory and the measured hysteresis curve of the magnetic nanostructure unusual hysteretic behavior of the phase diagram in this MSN system is predicted. The phase diagram for the hybrid MSN obtained from resistive measurements of Hc$_{2}$ is compared with the theoretical prediction. Other observed effects with the MSN, such as the matching effect and strong flux pinning are presented. Cusps and kinks appear at specific applied magnetic fields. The values of fields are consistent with the matching fields. The critical current in the strongest pinning regime of the hybrid system is up to about three orders of magnitude higher than the critical current of the Pb$_{82}$Bi$_{18}$ control film. Work supported by DOE No. DE-FG02-07ER46450, NSF CHE-0809651, NSF No. DMR-0606529, the Robert A. Welch Foundation A-0514 and A-1688, and NHARP under grant {\#} 010366-0039-2007. [Preview Abstract] |
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