Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A1: Quantum Properties of Superconducting Nanowires |
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Sponsoring Units: DCMP Chair: Nina Markovic, Johns Hopkins University Room: Baltimore Convention Center Ballroom IV |
Monday, March 13, 2006 8:00AM - 8:36AM |
A1.00001: Universal conductance of nanowires near the superconductor-metal quantum transition Invited Speaker: We consider wires near a zero temperature transition between superconducting and metallic states. The critical theory obeys hyperscaling, which leads to a universal frequency, temperature, and length dependence of the conductance; quantum and thermal phase slips are contained within this critical theory. Normal (NN), superconducting (SS) and mixed (SN) leads on the wire determine distinct universality classes. For the SN case, wires near the critical point have a universal d.c. conductance which is independent of the length of the wire at low temperatures. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A1.00002: Suppression of superconductivity in zinc nanowires by bulk superconductors Invited Speaker: When a superconducting nanowire of a few micrometers in length is connected to two macroscopic normal metal electrodes, a substantial fraction of the wire will become resistive due to the proximity effect. When such a wire is sandwiched between two macroscopic superconducting electrodes, the superconductivity of a nanowire is intuitively expected to become more robust through the coupling with its strong superconducting environment. This expectation is not fulfilled in our recent observation in a system consisting of zinc nanowires (ZNWs) between two bulk superconductors (Sn, In and Pb). We found evidence that the superconductivity of the ZNWs of 40 nm in diameter and 2 or 6 \textit{$\mu $}m in length is suppressed completely or partially when bulk Sn or In electrodes are superconducting. When bulk Sn or In electrodes are driven into the normal state by applying a magnetic field, the ZNWs switch back to their superconducting state. This anti-proximity effect is significantly weakened when both Sn or In leads were replaced by Pb or one of the two superconducting electrodes is replaced by a normal metal. The phenomenon is not seen in wires with diameters equal to and thicker than 70 nm. \textit{This work is in collaboration with N. Kumar, S. Y. Xu, J. G. Wang, J. Kurtz and M.H.W.Chan and supported by the Center for Nanoscale Science (Penn State MRSEC) funded by NSF under grant DMR-0213623}. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A1.00003: Effects of a strong magnetic field on superconducting nanowires. Invited Speaker: Effects of strong magnetic fields on superconducting Nb and MoGe nanowires with diameters 5-15 nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over 11 orders of magnitude. We do not observe any resistance in excess of the LAMH theory, even in wires, which are close to the critical point of the superconductor-insulator transition. This fact can be considered as an evidence of the absence of quantum phase slippage. In thicker wires the field dependence of the critical temperature agrees well with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. In the insulating-phase wires, the magnetic field has a little effect on electron transport, indicating that the superconductivity in the insulating-phase wires is completely suppressed and the Coulomb blockade is the dominant factor, which causes suppression of the charge transport. [1] A. Rogachev, A.T. Bollinger, and A. Bezryadin, Phys. Rev. Lett. 94, 017004 (2005). [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:24AM |
A1.00004: Quantum phase slips with and without disorder Invited Speaker: The rate of quantum phase slips (QPS) in one-dimensional superfluids and superconductors can be computed from first principles in two limiting cases. One [1] is the uniform (no-disorder) limit, appropriate for suitably prepared atomic gases. The other [2] is the limit when the core resistance of a QPS is effectively infinite, appropriate for a sufficiently long disordered superconducting wire. In the latter case, the calculation applies on the superconducting side of the superconductor-insulator transition, where the dilute instanton gas approximation can be used. It is essential to first compute the instanton rate for a given disorder configuration and then average over disorder, as opposed to working with instantons of a disorder-averaged effective theory. Curiously, in neither of the above cases the system is in the XY universality class: in the uniform limit, the QPS rate is suppressed exponentially at low temperatures, as a consequence of the momentum conservation, while the second case (a relatively long disordered wire) maps onto dissipative quantum mechanics, with the dissipative coefficient controlled by the plasmon impedance $Z$. The role of a finite core resistance can be understood within the picture of effective resistors, representing different dissipative effects, connected in parallel. \newline \newline [1] S. Khlebnikov, Phys. Rev. Lett. {\bf 93}, 090403 (2004); Phys. Rev. A {\bf 71}, 013602 (2005). \newline [2] S. Khlebnikov and L. P. Pryadko, Phys. Rev. Lett. {\bf 95}, 107007 (2005). [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 11:00AM |
A1.00005: Size Dependent Breakdown of Superconductivity in Ultra-Narrow Nanowires Invited Speaker: Below a certain temperature T$_{c}$ (typically cryogenic) some materials lose their electric resistance R entering a superconducting state. Following the general trend towards a large scale integration of greater number of electronic components it is desirable to use superconducting elements in order to minimize heat dissipation. It is expected that the basic property of a superconductor, i.e. dissipationless electric current, will be preserved at reduced scales required by modern nanoelectronics. Unfortunately, there are indications that for a certain critical size limit of the order of $\sim $ 10 nm below which a `superconducting' nanowire is no longer a superconductor in a sense that it acquires a finite resistance even at temperatures close to absolute zero. We developed a method of non-destructive reduction of a nanostructure dimension(s) by low-energy Ar$^{+}$ ion sputtering. The method enables study of a purely size phenomena between the sputtering sessions: \underline {\textit{same}} sample with progressively reduced characteristic dimension. We were able to trace the evolution of the shape of superconducting transition R(T) an aluminum nanowire with original effective diameter $\sim $ 70 nm down $\sim $ 8 nm. Below $\sim $ 15 nm the initially abrupt R(T) dependence suddenly broadens. With further reduction of the wire cross section finite resistance is observed down to temperatures much below the initial superconducting transition. We associate the observed phenomena with the quantum phase slippage process: destruction of superconductivity in quasi-1D channels due to quantum fluctuations of the order parameter. The effect should have a universal validity setting a fundamental size limit for utilization of superconducting elements as building blocks of nanoelectronics circuits. [Preview Abstract] |
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