Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P10: Superconducting Nanostructures I |
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Sponsoring Units: DCMP Chair: Michael Bleiweiss, Naval Academy Prep School Room: Morial Convention Center RO8 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P10.00001: Microwave Response in Short Superconducting Nanowires Robert Dinsmore, Myung-Ho Bae, Alexey Bezryadin Short superconducting nanowires, of length L$\sim $100 nm, have been exposed to microwave radiation (MWR) giving rise to phase slip centers that do not appear in DC measurements, without MWR. For frequencies in the 100-1000 MHz range multiple voltage jumps are observed indicating multiple phase slip centers. Each such voltage plateau is characterized by a small differential resistance, of the order of 10-20 Ohms. In this frequency range zero crossing plateaus were also observed. For frequencies from 2 to 9 GHz, MWR had a synchronization effect on the superconducting phase rotation. Resonances are observed for integer and half integer orders, similar to the original observation of Anderson and Dayem on much larger superconducting bridges [1]. [1] Anderson and Dayem, Phys. Rev. Lett. V.13, p.195 (1964) [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P10.00002: Resistance in One-Dimensional Superconducting Epitaxial Niobium Nanowires Timothy McArdle, Kevin Inderhees, Paul Welander, Allison Dove, James Eckstein Thermally activated phase slips cause resistance in one-dimensional superconducting wires near the critical temperature. However, this description of a thermally activated process is not able to explain additional resistance observed in extremely narrow nanowires well below Tc. We fabricate nanowires using electron beam lithography from single-crystal niobium films grown by ultra-high vacuum molecular beam epitaxy. Since the films are single crystal, the role of disorder is reduced and neither weak links nor grains are present. The films are 10 to 20 nm thick, have transition temperatures ranging from 7.2 to 9.2 K, and residual resistance ratios of 5 to 10, typical for ultra-thin single-crystal niobium films. The wires are 10 $\mu $m long and range in width from 35 to 200 nm. Transport measurements on the nanowires show two distinct regions of temperature dependence below Tc. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P10.00003: Magnetoresistance and Critical Current Oscillations in Superconducting NbSe$_{2}$ and NbN Nanowires U. Patel, Z.L. Xiao, J. Hua, R. Divan, U. Welp, W.K. Kwok Magnetoresistances and critical currents of superconducting NbSe$_{2}$ and NbN nanowires with cross-section dimensions from 300 nm to 2 um were studied as a function of magnetic field. Sample specific oscillations were found with respect to applied magnetic field. The oscillations were completely reproducible, symmetric with respect to the direction of the field and independent of the field sweep direction. They disappeared at high fields, temperatures or currents. The oscillations were periodic in nature with superposition of more than one frequency as revealed in Fast Fourier Transform of the oscillations. The frequencies of oscillations were independent of temperature and current. Details about the transport measurements of resistance and critical current and origin of such oscillations will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P10.00004: ``Giant'' strengthening of superconducting pairing in small metallic nanoparticles: high T$_{c}$ state Vladimir Kresin, Yurii Ovchinnikov, Baopeng Cao, Martin Jarrold The study focuses on metallic nanoclusters containing N$\sim $10$^{2}$-10$^{3}$ free carriers (e.g., Ga$_{56}$, Al$_{45}^{-})$. The delocalized cluster electrons form energy shells similar to those in atoms or nuclei. Under special conditions, superconducting pairing in such nanoclusters can become very strong, and they form a new family of high temperature superconductors. For realistic sets of parameters one can expect a high value of T$_{c}$ (150 K) as well as strong modification of the energy spectrum. In principle, it is possible to raise T$_{c}$ up to room temperature. Specific experiments aimed at detecting the phenomenon of pair correlation in nanoclusters can be proposed: spectroscopy, magnetic, and thermodynamic properties.Transition to the superconducting state of the cluster is accompanied by the peak in its heat capacity. The phenomenon is promising for the creation of high T$_{c}$ superconducting tunneling networks, and hence macroscopic superconductivity. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P10.00005: Evidence for High T$_{c}$ Superconducting Transitions in Isolated Al$_{45}^{-}$ and Al$_{47}^{-}$ Nanoclusters Martin Jarrold, Baopeng Cao, Colleen Neal, Anne Starace, Yurii Ovchinnikov, Vladimir Kresin Heat capacities measured for Al$_{45}^{-}$ and Al$_{47}^{-}$ nanoclusters have reproducible peaks at $\sim $ 200 K. The data were obtained using a multi-collision dissociation method [1] allowing us to perform measurements for isolated nanoclusters. The peaks are observed for selected Al clusters only. These peaks are consistent with theoretical predictions that some clusters with highly degenerate electronic states near the Fermi level will undergo a transition into a high T$_{c}$ superconducting state [2]. An analysis based on a theoretical treatment of pairing in Al$_{45}^{-}$ and Al$_{47}^{-}$ agrees well with the experimental data in both the value of the critical temperature and in the size and width of the peaks in the heat capacity. The observed value of T$_{c}$ exceeds those found in bulk systems. [1] G.Breaux, C.Neal, B.Cao, M.Jarrold, Phys. Rev. Lett. \textbf{94}, 173401 (2005) [2] V.Z.Kresin, Yu.Ovchinnikov, Phys. Rev. B \textbf{74}, 024514 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P10.00006: Quantum confinement effects on superconducting properties of Lead nanocrystals Herve Aubin, Helena Moreira, Benoit Mahler, Benoit Dubertret We developed a new chemical synthesis method for producing large quantities of monodispersed lead (Pb) nanocrystals. They are obtained from the alcohol reduction of a mixture of two lead carboxylates with alkyl chains of different lengths, dissolved in a high temperature solvent. The nanocrystals obtained are protected from oxydation and aggregation by long chain fatty acids and their diameter can be tuned to reach values as low as 10 nm. Our results suggest that monodispersed particules are obtained when nucleation and growth occur at distincts temperatures, possibly as a consequence of different reactivities of the two lead carboxylates used in the solution. Owing to the large quantities of monodispersed particles produced, thermodynamics studies as function of particles diameter become possible. In particular, we will present a study of the effect of quantum confinement on superconducting properties of these Pb particles through SQUID magnetometry measurements. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P10.00007: Superconductivity of nano-size Pb Islands studied by low-temperature scanning tunneling microscopy / spectroscopy Takahiro Nishio, Toshu An, Atsushi Nomura, Kousuke Miyachi, Toyoaki Eguchi, Yukio Hasegawa, Hideaki Sakata Nano-size superconducting materials, whose dimensions are comparable with or smaller than their coherent length / penetration depth, behave differently from bulk superconductors. By forming structures using lithographic methods various unique properties of mesoscopic superconductors have been elucidated. Since these studies, however, measure electrical conductance and / or magnetization, details inside the superconductors cannot be directly probed. In this study we investigated superconductivity inside superconductors by directly measuring the superconducting gaps over ultra thin Pb island structures using a LT-STM at 2.0 K [1]. The obtained tunneling spectra exhibit an increment of zero bias conductance (ZBC) with a magnetic field and its dependence on the lateral size of the islands. Moreover, from spatial mappings of ZBC, the island size dependence and spatial variation of superconductivity inside of each island are visualized. We found that the number of vortices piercing the islands before breakdown of superconductivity depends on the lateral size of the islands. Details of the size-dependent critical fields are discussed at the presentation. [1] Nishio et al., APL 88, 113115 '06, JJAP 46, L880 '07. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P10.00008: Flux jump in superconducting Pb networks at fractional numbers of the matching fields Takekazu Ishida, Yoshiaki Matsushima, Makoto Shimizu, Masahiko Hayashi, Hiromichi Ebisawa, Masaru Kato, Osamu Sato, Kazuo Sato, Tsutomu Yotsuya The extended Little-Parks effect of superconducting network is known as a periodic Tc variation as a function of magnetic field. Superconducting Pb honeycomb networks of matching field 0.106 G and triangular microhole lattice of Pb of matching field 0.425 G have been fabricated by the combined techniques of the electron beam lithography and a lift-off process of evaporated Pb films. The application of magnetic field corresponds to the vortex doping into networks. We measured the magnetization of the networks systematically by using a SQUID magnetometer. We found that flux jump appears rather periodically as a function of magnetic field. Flux jumps may be induced by a periodic decrease in the critical current density of the network. To the authors' knowledge, this is for the first time to observe the extended Little-Parks effect appeared in flux jumps. We also discuss the anomalous matching effect. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P10.00009: Switching Current Distributions of Superconducting Nanowires: Evidence for Individual Quantum Phase Slips Mitrabhanu Sahu, Myung-Ho Bae, Andrey Rogachev, David Pekker , Nayana Shah, Tzu-Chieh Wei, Paul Goldbart, Alexey Bezryadin Phase slip fluctuations cause premature stochastic switching of the state of a current biased quasi-one-dimensional nanowire from superconducting to normal at sub-critical currents. Here, we report on measurement of the distributions of switching currents performed on amorphous superconducting Mo$_{79}$Ge$_{21}$ nanowires over a range of temperatures. The measured widths of the switching current distributions are observed to increase with decreasing temperature. We explain this counterintuitive result by considering a microscopic stochastic model of heating caused by each phase slip event. The measured rates of escape from the superconducting state agree well with the predictions of the stochastic model under the assumption of phase slippage by thermal activation at relatively high temperatures and macroscopic quantum tunneling at sufficiently low temperatures. We identify and explore a region in which a \textit{single} quantum phase slip is all that is necessary to trigger switching from the superconducting to the normal state. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P10.00010: Theory of superconductive-resistive switching in nanowires due to heating by stochastic phase slips Nayana Shah, David Pekker, Paul Goldbart We study the stochastic dynamics of superconductive-to-resistive switching in hysteretic current-biased superconducting nanowires undergoing phase-slip fluctuations. We assume that the hysteresis is thermal in nature, and postulate that the mechanism for the switching is thermal runaway, i.e. rare sequences of stochastic phase slips, closely spaced in time, that heat the nanowire. Thus, via the master-equation formalism, we obtain the distribution of currents at which switching occurs. If switching were caused by single, thermally-activated phase-slip events then this distribution would narrow as the temperature is reduced. However, at higher temperatures we find that several phase-slip events are typically necessary for inducing switching, and this results in an initial broadening of the distribution upon cooling. Quite generally, we predict that at low temperatures thermal runaway is caused by a single phase-slip event. Thus, measurements of switching-current distributions in this regime are a direct probe of this basic collective process. In particular, this regime could yield observations of individual quantum phase slips in nanowires. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P10.00011: Anisotropic Superconductivity in MoGe - Permalloy Bilayers Goran Karapetrov, A. Belkin, V. Novosad, M. Iavarone, J. Fedor, A. Troncalli, J. Pearson, W.K. Kwok We studied the magneto-transport properties of superconductor-ferromagnet MoGe/Permalloy bilayers. The ferromagnet with stripe domain structure induces in-plane anisotropy in superconducting order parameter. Superconducting phase diagram shows that near the S-N phase boundary the superconductivity is localized in narrow mesoscopic channels just above the magnetic domain walls. By changing the in-plane direction of magnetic stripe domains it is possible to re-configure the direction of the superconducting channels and enable one to control the direction of the anisotropy axis in the superconductor. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P10.00012: Superconductor-Ferromagnet Bilayers: Influence of Magnetic Domain Structure on Vortex Dynamics Andrey Belkin, V. Novosad, M. Iavarone, J. Pearson, W.K. Kwok, G. Karapetrov We investigate the influence of orientation of stripe-like domain structure in ferromagnetic films on vortex dynamics in superconductor-ferromagnet bilayers. We measure transport properties in different external magnetic fields applied perpendicular to the surface of the bilayers. Parameters of superconductor-ferromagnet bilayers are such that domain period is much bigger than the superconducting coherence length but much smaller than the effective penetration depth. Prominent dissimilarity of critical currents of two studied configurations as well as pronounced commensurability effects are found. Diverse behavior of superconductor-ferromagnet bilayers with mutually orthogonal orientations of stripe domains is demonstrated by dependence of critical temperature on external magnetic field. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P10.00013: Topological Excitations in Superconducting Nanostripes: Resistive States and Noise Matt Bell, Andrei Sergeev, Vladimir Mitin, Aleksandr Verevkin We investigate competition between one- and two-dimensional topological excitations - phase slips and vortices - in formation of resistive states and noise generation in ultrathin superconducting NbN nanostripes in a wide temperature range below the mean-field transition temperature $T_{C0}$. The widths $w$ = 100 nm of our ultrathin NbN samples is substantially larger than the Ginzburg-Landau coherence length $\xi$ = 4nm and the fluctuation resistivity above $T_{C0}$ has a two-dimensional character. However, our data shows that the resistivity below $T_{C0}$ is produced by one-dimensional excitations, - thermally activated phase slip strips (PSSs) overlapping the sample cross-section. We determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current/temperature variations [1]. The above fluctuations generated by topological excitations also provide a noise limit to superconducting detectors operating in a resistive state, e.g. for dark counts in single-photon detectors. [1] M. Bell et al., Phys. Rev. B 76, 094521 (2007). [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P10.00014: Nonlinearities and Parametric Amplification of Superconducting Coplanar Waveguide Resonators David Haviland, Erik Thol\'en, Adem Ergul We have experimentally studied the nonlinear properties of superconducting coplanar stripline resonators fabricated from Al and Nb films with small transverse dimensions (gap size 1$\mu$m). Magnetic field penetration into the superconductor causes a current-dependant kinetic inductance, which gives an ideal Kerr nonlinearity. When the nonlinear oscillator is pumped very near its dynamic instability, it can be used to realize parametric amplification. We have achieved a gain of +22.4dB in a 5.8 GHz resonator cooled to 450 mK [E. Thol\'{e}n et. al. Appl. Phys. Lett. 90, 253509 (2007)]. Parametric deamplification or squeezing of a signal has also been verified with squeezing of 30 dB. The later effect is interesting because it can be used to generate squeezed vacuum states of the electromagnetic field. We have modeled the data using a theory developed by Yurke and Buks [J. Lightwave Technol. \textbf{24}, 5054 (2006)]. Excellent fit of the model to the measured data can be achieved over a wide range of pump power, and the strength of the nonlinear terms can be obtained with high accuracy. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P10.00015: Observation of Nonlocal Coherence Between Normal Metals Coupled by a Superconductor Paul Cadden-Zimansky, Jian Wei, Venkat Chandrasekhar In conventional superconductors the effective size of the constituent Cooper pairs can approach several hundred nanometers, a length scale accessible by nanolithographic techniques. By placing two normal metals on a superconductor within a coherence length of each other, it has been predicted that the quasiparticles in the separate metals can be coherently coupled by the Cooper pairs through the nonlocal processes of Elastic Cotunneling and Crossed Andreev Reflection. We present experimental observations of coherent, nonlocal thermoelectric effects between normal metals coupled to a superconductor by using a hybrid normal-superconductor interferometer. The sign of the observed thermoelectric voltage can be switched using an external field, indicating that the voltage is dynamically driven by a persistent current induced in the interferometer. [Preview Abstract] |
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