Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W23: Superconductivity: Mesoscopic and Nanometer Scale Phenomena |
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Sponsoring Units: DCMP Chair: Myron Salamon, University of Texas at Dallas Room: D165 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W23.00001: Fluxoid Quantization in Superconducting Al Nano-Rings Stephen Snyder, Allen Goldman The Little-Parks experiment on superconducting cylinders is an important demonstration of fluxoid quantization in superconductors. The transition temperature oscillations in magnetic field have a period of $h/2e$ for the micro cylinders in their studies, which was further evidence for Cooper paring at the time {[}W. A. Little, R. D. Parks, PRL 1964, 9, 9{]}. However recent theoretical works have suggested that in superconducting loops smaller than the coherence length this period changes from $h/2e$ to $h/e$, for details see {[}F. Loder, et al. PRB, 2008, 78, 174526{]} and references therein. We present experimental work in an effort to achieve this limit with Al nano-rings prepared by electron beam lithography. The rings presented here are smaller than others reported in the literature by as much as a factor of two or three {[}H. Wang, et al. PRB, 2007, 75, 064509{]}. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W23.00002: Destructive regime in Al loops prepared by e-beam lithography Neal Staley, Ying Liu For doubly connected superconductors, cylinders, loops, or rings, the fundamental fluxoid quantization leads to oscillations in the superconducting transition temperature (T$_c$) with the applied flux. This is known as the Little-Parks effect. For sufficiently small loops, with a circumference smaller than $\pi$$\xi$(0), where $\xi$(0) is the zero temperature coherence length, superconductivity is completely destroyed near the half-flux quanta. This ``destructive regime" emerges because of the competition between the kinetic energy carried by the supercurrent, and the condensation energy of the system. Theoretically, it has been shown that adding a tail to the loop can increase the condensation energy and possibly eliminate the destructive regime. We present electrical transport measurements on Al loops defined by e-beam lithography with a size comparable to $\xi$(0). The loops were varied to have different condensation energies by adding explicit tails, and by lengthening and shortening the measurement leads. We observed strongly enhanced Little-Parks oscillations due to the reduction of the sample size, and the transition to the destructive regime when the size of sample was further reduced. These experimental results will be examined in the context of the competition between the kinetic and condensed energies. Work supported by NSF. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W23.00003: Superconducting nanowires as nonlinear inductive elements for qubits Jaseung Ku, Vladimir Manucharyan, Alexey Bezryadin We report microwave transmission measurements of superconducting Fabry-Perot resonators, having a superconducting nanowire placed at a supercurrent antinode. As the plasma oscillation is excited, the supercurrent is forced to flow through the nanowire. The microwave transmission of the resonator-nanowire device shows a nonlinear resonance behavior, significantly dependent on the amplitude of the supercurrent oscillation. We show that such amplitude-dependent response is due to the nonlinearity of the current-phase relationship of the nanowire. The results are explained within a nonlinear oscillator model of the Duffing oscillator, in which the nanowire acts as a purely inductive element, in the limit of low temperatures and low amplitudes. The low-quality factor sample exhibits a ``crater'' at the resonance peak at higher driving power, which is due to dissipation. We observe a hysteretic bifurcation behavior of the transmission response to frequency sweep in a sample with a higher quality factor. The Duffing model is used to explain the Duffing bistability diagram. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W23.00004: Electrical transport properties of topological insulator Bi2Te3 nanowires contacted with superconducting electrodes Mingliang Tian, Jian Wang, Meenakshi Singh, Moses Chan Single-crystal Bi2Te3 nanowires were fabricated by template-assisted electrochemical deposition. The electrical transport properties of the nanowires in the temperature range 1.8-300 K were investigated by connecting nonsuperconducting or superconducting electrodes. When the wire was connected to focused-ions beam deposited W-electrodes, a series of exotic quasi-periodic oscillations were found and the amplitude of the oscillations was unusually enhanced near 3.5 K below the Tc, 4.5 K, of W-electrodes. When the wire was connected to nonsuperconducting Pt electrodes, the wire showed positive magnetoresistance accompanied with random fluctuations. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W23.00005: Observation of Little-Parks Oscillations of the Kinetic Inductance at Low Temperatures Using a GHz Resonator with Two Parallel Superconducting Nanowires Andrey Belkin, Matthew Brenner, Thomas Aref, Jaseung Ku, Alexey Bezryadin Little-Parks (LP) effect manifests the phenomenon of the fluxoid quantization in doubly connected superconductors. Usually it is observed at high temperatures, i.e. slightly below the critical temperature (Tc). We demonstrated that a thin-film Fabry-Perot superconducting resonator with a pair of nanowires inserted at the point of supercurrent antinode can be used to reveal LP effect even at temperatures much lower than Tc. As magnetic field (H) is applied, the Meissner current develops, changing the kinetic inductance of the wires and, correspondingly, the resonance frequency of the resonator and its transmission S21 measured at the fixed frequency. The periodicity of the LP effect is revealed as a periodic set of distorted parabolas S21(H) corresponding to the states with different vorticities. The transition from one state to another corresponds to a Little's phase slip. We suggest a theoretical explanation to the shape of the observed parabolas. We also report a statistical analysis of the jumps between the parabolas. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W23.00006: Spin Resonance and dc Current Generation in a Quantum Wire Peng Zhou, Artem Abanov, Wayne Saslow, Valery Pokrovsky We show that in a quantum wire the spin-orbit interaction leads to a narrow spin resonance at low temperatures, even in the absence of an external magnetic field. A relatively weak dc magnetic field of a definite direction strongly increases the resonance absorption. Linearly polarized resonance radiation produces dynamic magnetization as well as electric and spin currents. The effect strongly depends on the external magnetic field. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W23.00007: Large oscillations of the magnetoresistance in nano-patterned high-temperature superconducting films Ilya Sochnikov, Avner Shaulov, Yosef Yeshurun, Gennady Logvenov, Ivan Bozovic Measurements on nano-scale structures made of high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. The size of loops made of these compounds was so far limited to the submicron scale. We report the results of measurements on loops of La$_{1.84}$Sr$_{0.16}$CuO$_{4}$, with dimensions down to tens of nanometers. We observe oscillations in the loops resistance as a function of the magnetic flux through the loops. The oscillations have a period of $h/2e$ and their amplitude is much larger than the amplitude of resistance oscillations expected from the Little-Parks effect [1-2]. Unlike the Little-Parks oscillations, caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally-excited moving vortices and the oscillating persistent current induced in the loops. Despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of $h/e$, as recently predicted for nanoscale loops of superconductors with d-wave symmetry, or with a period of $h/4e$, as predicted for superconductors that exhibit stripes. [1] I. Sochnikov \textit{et al.}, Nature Nano. \textbf{5}, 516 (2010). [2] I. Sochnikov \textit{et al}., PRB\textbf{ 82,} 094513 (2010). [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W23.00008: Penetration Depth of Superconductors with Random Superfluid Density Thomas Lippman, Kathryn Moler Electronic inhomogeneity is inevitable in doped superconductors, due simply to the random nature of the doping process. We argue that when the superconducting coherence length is not much larger than the lattice scale, this creates spatial randomness in the superconducting properties. In particular, we expect the superfluid density to be stochastic, which modifies the measured diamagnetic response. We approximate the London equation with random superfluid density as a modified equation for the disorder averaged field response, and speculate on implications for the interpretation of measurements of the penetration depth. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W23.00009: MBE Growth for Qubit Devices Anthony Megrant, Alex Kozhanov, Ludwig Feigl, Martin Weides, Jian Zhao, Yi Yin, James Wenner, Andrew Cleland, Chris Palmstrom, John Martinis A major component of the UCSB research program is to find and understand proper growth conditions and procedures for epitaxial Re and sapphire thin films. Separately, we are taking what we have learned from this research and applying it to develop growth-optimized procedures in our dedicated MBE chamber for qubit devices. I will report on important parameters that need to be adjusted to obtain optimum growth, some of which include: temperature, deposition rate and surface preparation. Measurement tools such as in-situ RHEED and ex-situ XRD and AFM are used to characterize the quality of the films, as well as the fabrication of resonators to measure the quality factor. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W23.00010: Single Electron Turnstiles: Improving Performance for a Quantum Metrological Current Standard Thomas Aref, Ville Maisi, Olli-Pentti Saira, Antti Kemppinen, Jukka Pekola A top priority in metrology is to develop measurement standards that are based on fundamental physical constants. Although such standards exist for resistance and voltage (the quantum Hall effect and the Josephson effect respectively), no such standard exists for the unit of current, the ampere, at the present time. The best candidate for a quantum metrological standard for current is a single electron turnstile. Such turnstiles allow electrons through one at a time. When operated at a specific frequency, they produce a proportional current traceable to the single electron charge. Our turnstiles are based on a normal metal copper island contacted through insulating barriers by superconducting aluminum electrodes. By engineering improvements such as on-chip filtering and increased charging energy, we have improved our turnstiles to suppress both first order leakage and second order Andreev currents thus approaching the required accuracy for a new quantum metrological current standard. This current standard would allow the closing of the quantum metrological triangle created by voltage, resistance and current. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W23.00011: Tunneling spectroscopy of the Andreev Bound States in a Carbon Nanotube Jean-Damien Pillet, Charis Quay, Cristina Bena, Alfredo Levy Yeyati, Philippe Joyez Carbon nanotubes are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes. This supercurrent is mainly transmitted by discrete entangled (electron-hole) states confined to the nanotube, called Andreev Bound States. These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (e.g. molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads. We report here the first tunneling spectroscopy of individually resolved ABS, in a nanotube-superconductor device. Analyzing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W23.00012: Quantum impurity model for microwave photons Moshe Goldstein, Michel Devoret, Manuel Houzet, Leonid Glazman We consider propagation of microwave photons along an array of superconducting grains with a set of weakly-coupled grains at its center. Quantum fluctuations of charge on the weakly- coupled grains make the process of ``photon splitting'' effective. In such a process, an incoming photon may be split into a number of photons of lower energy. The minimal number of photons created in such process depends on the symmetry properties of the corresponding quantum impurity model. As an example, we consider a specific circuit allowing quantum fluctuations between two charge configurations of two weakly- coupled grains, thus mimicking the behavior of an anisotropic Kondo impurity. Both ferromagnetic and antiferromagnetic Kondo regimes may be reached this way. We relate the rate of conversion of the incoming photons into the lower-energy ones to the dynamic spin susceptibility of the Kondo model. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W23.00013: Elliptic billiard is a nontrivial integrable system Tao Ma, Rostislav Serota We investigate the semiclassical energy spectrum of quantum elliptic billiards. The nearest neighbor spacing distribution, level number variance and spectral rigidity support the notion that the elliptic billiard is a generic integrable system. A classical simulation shows that all periodic orbits, except two, are not isolated. From Fourier analysis of the spectrum, all peaks correspond to periodic orbits. The two isolated periodic orbits have relatively small contribution to the fluctuations of the level density as compared to non-isolated periodic orbits. We argue that elliptic billiard is a nontrivial classically integrable system that enables us to gain new insights into their properties. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W23.00014: Measurement of the statistical properties of the persistent current in normal metal rings Manuel Castellanos Beltran, Will Shanks, Dustin Ngo, Ania Bleszynski-Jayich, Jack Harris A striking manifestation of quantum mechanics at the mesoscopic scale is the existence of an equilibrium persistent current in normal metal rings threaded by a magnetic flux. A theory of non-interacting diffusive electrons predicts that the amplitude of these currents is a stochastic function of the disorder profile of the specific ring. Thus the persistent current is different from sample to sample, with a Gaussian distribution. Due to the difficulty of measuring these currents, experiments to determine the form of the persistent current distribution had not yet been performed. However, our group recently developed a technique for measuring persistent currents in normal metal rings with high SNR, low measurement back-action, excellent background rejection, and over a large range of magnetic fields. We have measured a total of roughly 100 independent realizations of persistent current amplitudes in single rings. Within the statistical limits of our data, we corroborate that the first five cumulants are consistent with a Gaussian distribution. As a further test of the higher-order statistical properties of the persistent current, we also show that the quadrature amplitudes of the current's Aharonov-Bohm oscillations are uncorrelated. [Preview Abstract] |
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