Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y10: Superconducting Nanostructures II |
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Sponsoring Units: DCMP Chair: Timir Datta, University of South Carolina Room: Morial Convention Center RO8 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y10.00001: Studying pinning on the nanoscale by vortex dragging in a YBa$_2$Cu$_3$O$_{6.991}$ single crystal O. M. Auslaender, Lan Luan, E. Zeldov, K. A. Moler, D. A. Bonn, Ruixing Liang, W. N. Hardy We have used a magnetic force microscope to drag individual, well isolated vortices in a detwinned YBa$_2$Cu$_3$O$_{6.991}$ single crystal. At this slight degree of overdoping, a vortex can be described as a one-dimensional elastic string. We find an angle dependent dragging distance, implying that it is easier to drag a vortex along the Cu-O chains than across them. We understand this as a manifestation of single vortex weak collective pinning (WCP) by oxygen vacancies along the Cu-O chains, the dominant source of pinning in our sample. Single vortex WCP arises when individual pinning sites are too weak individually, but are able to compete with elasticity by cumulative effect. Using the usual single vortex WCP assumption of isotropic point pinning sites at uncorrelated positions we find that the anisotropy of superconductivity in YBa$_2$Cu$_3$O$_{6.991}$ only partially accounts for the angle dependence. Relaxing that assumption to account for the known tendency of the vacancies to cluster along the Cu-O chains, we find that single vortex WCP describes the dragging anisotropy quantitatively. This picture is further supported by the observation that vortex motion is more erratic along the chains than across them. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y10.00002: Fabrication and characterization of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Aharonov-Bohm rings and ultra-long nanowires PaiChia Kuo, Jessie Shiue, Patrick Morales, J.Y.T. Wei We report a novel technique to fabricate YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) nanodevices with characteristic length scale smaller than the YBCO penetration depth. The nanodevices presented here are Aharonov-Bohm rings $\sim $ 1.5 $\mu $m in diameter, and 200 nm nanowires $\sim $ 300 $\mu $m in length. These devices have Tc behaviors similar to that of unpatterned YBCO thin films. Fabrication of nanostructured complex oxide is a challenge even with advanced thin film growth techniques since either chemical or physical etching tends to compromise the film properties. The effective method is to epitaxially deposit thin film onto nano-patterned oxide substrate without any post-deposition treatment. Our novel technique takes advantage of the 3D micromachining capability of focused-ion-beam to nano-pattern the oxide substrate without the inherent surface damage and edge rounding problems caused by the energetic ion beam. This method is a reliable way to fabricate nanostructures of complex oxides and hence enables the studies of their properties. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y10.00003: Electrical Transport Properties of Nanostructured YBa$_2$Cu$_3$O$_{7-\delta}$ Rings and Wires P. Morales, J.Y.T. Wei, P.C. Kuo, J. Shiue, M.K. Wu The resistance and current-voltage characteristics of nanostructured high-T$_c$ superconducting YBa$_2$Cu$_3$O$_{7-\delta}$ rings and wires were studied as a function of temperature and applied magnetic field. The rings and wires were fabricated by pulsed laser deposition of YBa$_2$Cu$_3$O$_{7-\delta}$ on patterned SrTiO$_3$ substrates. The substrates were patterned using two different techniques. The first technique is based on selective epitaxial growth, and the second, using a method based on focused ion beam. Nanostructured superconducting rings were fabricated with a diameter of 1.5${\mu}$m with the width of the arms of the rings being 150nm. The low field magnetoresistance of the rings exhibit characteristics indicative of quantum interference effects. Nanostructured superconducting wires were fabricated with lengths up to 300${\mu}$m and widths as small as 200nm. The current-voltage characteristics of the wires exhibit discontinuities under current biasing and s-shaped non-linearities under voltage biasing characteristic of the formation of phase slip lines, the 2D analog of phase-slip centers. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y10.00004: Flux and bias driven superconducting to normal transition in an SNS proximity dc SQUID Jian Wei, Paul Cadden-Zimansky, Venkat Chandrasekhar We measure the magnetoresistance of a dc SNS SQUID in the form of a mesoscopic normal-metal loop in contact with two superconducting electrodes. Below the transition temperature of the superconducting leads, large $h/2e$ periodic magnetoresistance oscillations can be observed when the normal sections of the SNS junctions enter a proximity regime induced by the superconducting electrodes. As the temperature is lowered, the entire device becomes superconducting. In this regime, sharp switching from the zero-resistance state to a finite-resistance state is seen at half-integer flux quanta. With the application of a dc bias current at even lower temperatures, periodic switching from the superconducting state to the fully normal state can be produced with the external field. The observation of periodic flux-driven transitions in this device suggests that beyond the current SQUID theory for SIS junctions the development of SQUID theory for SNS junctions that incorporates the kinetic energy of the coherent electrons in the junctions is needed. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y10.00005: Charge and mass of Cooper pairs in small superconducting rings. Victor Vakaryuk It is well known that response of a neutral fermionic superfluid to rotation or a superconductor to magnetic field and/or rotation involves$\,$ such$\,$ characteristics of a Cooper pair as, stretching terms a little, its mass $2m$ and charge $2e$. Here $m$ and $e$ are essentially\footnote{ignoring tiny relativistic corrections} bare mass and charge of particles constituting the Cooper pair e.g.~electrons in case of superconductors. On a phenomenological level this is a consequence of the fact that expressions for currents are written for pairs of particles. We analyze this situation in BCS framework and show that for superfluids mesoscopically constrained in (at least) one spatial dimension the pair's mass and/or charge become smaller than their values for the bulk case (i.e.~$2m$ and/or $2e$). One of the implications of this result is the absence of $hc/2e$ harmonic in the response of small superconducting rings or tubes to external magnetic field. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y10.00006: Emergence of $h/e$-period oscillations in the critical temperature of small superconducting rings enclosing magnetic flux Tzu-Chieh Wei, Paul M. Goldbart The Little-Parks critical-temperature oscillations, with magnetic flux, of a large-radius hollow cylindrical superconductor have a period $h/2e$. This oscillation period reflects the binding of electrons into Cooper pairs. On the other hand, the single-electron Aharonov-Bohm oscillations in the resistance or persistent current in a clean metallic ring have period $h/e$. By using the Gor'kov approach to BCS theory, we investigate oscillations in the critical temperature of a superconducting ring, for radii that are comparable to the superconducting coherence length. In this regime, oscillations in the critical temperature of period $h/e$ emerge, in addition to the usual Little-Parks-period oscillations. We argue that in the clean limit there is a superconductor-normal phase transition at nonzero flux, as the ring radius becomes sufficiently small, and that this transition can be either second- or first-order, depending on the ring radius and the external flux. In the dirty limit, we argue that the transition is rendered second-order, which results in continuous quantum phase transitions tuned by flux and radius. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y10.00007: Current fluctuations in rough Josephson tunnel junctions Frank Wilhelm, Georg Heinrich The barrier material of superconducting tunnel junction has become the focus of interest as there is evidence that it limits the intrinsic quantum coherence of superconducting qubits. It is also potentially responsible for 1/f noise in SQUIDs. We study the model of a ``sieve'' junction of many opaque transport channels plus few pinholes, modeling a rough tunnel barrier. Even if the pinholes have a small effect on the subgap current, they completely dominate the shot noise at low voltages. Remarkably, even a fully open pinhole contributes shot noise because the size of the charge quantum it carries is uncertain. The full-counting statistics of charge transfer leads to a multimodal distribution. It is discussed, to what extent this distribution can be interpreted as the onset of telegraph or 1/f noise. This theoretical work is based on an extensive full counting statistics calculation using Keldysh Green's function. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y10.00008: Josephson effect through a resonant level coupled to a single oscillator mode Mikael Fogelstrom, Jonas Skoldberg, Tomas Lofwander Motivated by very recent experiments on superconducting transport through single-walled carbon nanotubes, we investigate the supercurrent through a one-level quantum dot connected to a single phonon mode. Calculations are done using non-equilibrium Green's function methods within a self-consistent Born approximation, i.e. assuming that the tunneling rate is much larger that the effective electron-phonon coupling. We calculate both the modified Andreev-bound state spectrum and the renormalization of the phonon density-of-states in situations when the phonon-mode is either in or out-off thermal equilibrium with the electron system. Based on our calculations we discuss possible Andreev-state spectroscopy. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y10.00009: Quantum Phase Slips in 1D Josephson Junction arrays Jack Lidmar One-dimensional arrays of Josephson junctions can undergo a zero temperature superconductor-insulator quantum phase transition by tuning the Josephson coupling. Quantum phase slips (QPS) play the key role in this transition: In the superconducting regime they are very rare, while in the insulating Coulomb blockade regime, they proliferate and destroy the phase coherence. We derive an expression for the QPS rate that is amenable to quantum Monte Carlo simulations and perform calculations in a realistic model of an array over a wide range of parameters including the transition region. In particular we can determine the scaling properties of the QPS rate at the transition. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y10.00010: Macroscopic Resonant Tunneling through Andreev Interferometers Jeff Weiss, Marlies Goorden, Philippe Jacquod We investigate the conductance through and the spectrum of ballistic {\it Andreev interferometers}, chaotic quantum dots attached to two $s$-wave superconductors, as a function of the phase difference $\phi$ between the two order parameters. A combination of analytical techniques -- random matrix theory, Nazarov's circuit theory and the trajectory-based semiclassical theory -- allows us to explore the quantum-to-classical crossover in detail. When the superconductors are not phase-biased, $\phi=0$, we recover known results that the spectrum of the quantum dot exhibits an excitation gap, while the conductance across two normal leads carrying $N_{\rm N}$ channels and connected to the dot via tunnel contacts of transparency $\Gamma_{\rm N}$ is $\propto \Gamma_{\rm N}^2 N_{\rm N}$. In contrast, when $\phi=\pi$, the excitation gap closes and the conductance becomes $G \propto \Gamma_{\rm N} N_{\rm N}$ in the universal regime. In the tunneling regime, $\Gamma_{\rm N} \ll 1$, resonant contributions induce an order-of-magnitude enhancement of the conductance towards $G \propto N_{\rm N}$ in the short-wavelength limit. We relate this to the emergence of a giant peak in the density of states at the Fermi level. Our predictions are corroborated by numerical simulations. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y10.00011: Measurement of the Magnetization of Mesoscopic Superconducting Rings with Cantilever Torsional Magnetometry Will Shanks, Ania Bleszynski-Jayich, Jack Harris We have measured the magnetization of micron-sized aluminum rings in the superconducting state as a function of magnetic flux threading the rings. The rings were fabricated on the ends of 400 $\mu $m long, 400 nm thick silicon cantilevers, which act as sensitive torque meters. By measuring the shift in resonant frequency of the cantilever as a function of applied magnetic field, we are able to determine the ring's magnetization. Our measurements are in qualitative agreement with previous studies of similar rings. The results are promising for other proposed measurements of closed mesoscopic electronic samples using cantilever torsional magnetometry. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y10.00012: Interplay of superconductivity and ferromagnetism in ultra-small metallic grains Sebastian Schmidt, Yoram Alhassid, Kris van Houcke We investigate the competition between superconductivity and ferromagnetism in ultra-small metallic grains in a regime where both phases can coexist. We use an effective Hamiltonian in the mesoscopic regime that combines a BCS-like pairing term and a ferromagnetic Stoner-like spin exchange term. The presence of spin jumps in the ground-state phase diagram of the grain is a unique feature of the coexistence of pairing and ferromagnetic correlations. We show that the coexistence regime can be made accessible to experiments by tuning an external Zeeman field [1]. We also study the transport properties of the grain in the Coulomb blockade regime and identify signatures of the competition between superconductivity and ferromagnetism in the mesoscopic fluctuations of the conductance [2]. \newline [1] S. Schmidt, Y. Alhassid and K. Van Houcke, Europhys. Lett. 80, 47004 (2007). \newline [2] S. Schmidt, Y. Alhassid, to be published (2007). [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y10.00013: The optimal inhomogeneity for superconductivity - finite size studies Wei-Feng Tsai, Hong Yao, Steven Kivelson, Andreas Lauchli We report the results of exact diagonalization studies of Hubbard models on
a $4\times 4$ square lattice with periodic boundary conditions and various
degrees and patterns of inhomogeneity. Inhomogeneities are represented by
different patterns of inequivalent hopping integrals ($t$ and ${t}')$, such
that for ${t}'=t$, the model is ``homogeneous'', while for ${t}'< |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y10.00014: Charge redistribution phenomena at the interfaces of the HTS, insulating and metallic oxides Vladimir Butko, Gennady Logvenov, David Reagor, Ivan Bozovic Interface and low dimensional phenomena are currently a focus of the active and broad scientific investigations. We have fabricated atomically sharp interface structures of the La2-xSrxCuO4 , SrRuO3 , SrTiO3 and LaSrAlO4 by using the atomic layer by layer molecular beam epitaxy MBE, Rf- sputtering and Ion Beam Preferential Etching (IBPE). Based on our structural, transport and inductance studies we discussed possible mechanisms of the surface metallic state in the IBPE SrTiO3 [1], and elevated superconducting temperature observed in the La2-xSrxCuO4 bi-layers [2]. [1] Reagor D.W., Butko V.Y. Highly conductive nanolayers on strontium titanate produced by preferential ion beam etching. Nature Materials 4 (8): 593-596 Aug., 2005. [2] Bozovic I, Logvenov G, Belca I, et al. Epitaxial strain and superconductivity in La2-xSrxCO4 thin films. Physical Review Letters 89 (10) Art. 107001, Sep.2, 2002. [Preview Abstract] |
Friday, March 14, 2008 2:03PM - 2:15PM |
Y10.00015: Observation of phase transition from Tomonaga-Luttinger liquid states to superconductive phase in carbon nanotubes Junji Haruyama, Masaharu Matsudaira, Naoyoshi Murata, Yuko Yagi, Erick Einarson, Shigeo Maruyama, Toshiki Sugai, Hisanori Shinohara A carbon nanotube (CNT) is a one-dimensional (1D) ballistic conductor, which has Tomonaga-Luttinger liquid (TLL) state that arises from the repulsive Coulomb interaction between electrons. In contrast, the phonon-mediated attractive Coulomb interaction leads to BCS-type superconductivity (SC) in 2D and 3D conductors. Thus, interplay of SC with the TLL states in CNTs has attracted considerable attention [1]. The experimental report, however, was only in our multi-walled CNTs (MWNTs) [2]. Here, we report the detailed observation of change in 1D Coulomb interaction in the MWNTs. The results indicate occurrence of the phase transition from the TLL states to the SC phase, as energy decreases. The small number of shells with current flow in the partially end-bonded MWNTs makes the observation possible. \newline [1] e.g., D.Loss et al., Phys.Rev.B 50, 12160 (1994-II), E.Perfetto et al., Phys.Rev.B 74, 201403(R) (2006) \newline [2] I.Takesue, J.Haruyama., et al., Phys.Rev.Lett.96, 057001 (2006) [Preview Abstract] |
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