Nanoscale superconductivity: Nanowires and nanofilms (original) (raw)
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Nanowires and nanofilms: Superconductivity in quantum-size regime
Physica C: Superconductivity, 2008
Quantum confinement is the major mechanism governing superconductivity in highly crystalline metallic specimens with nanoscale dimensions. Changes in the single-electron spectrum due to size quantization result in quantum-size variations of the superconducting properties (e.g., critical temperature and critical magnetic field) with profound enhancements at the points of the superconducting resonances. Our investigation is based on a self-consistent numerical solution of the Bogoliubov-de Gennes equations for clean metallic nanofilms and nanowires.
New Andreev-Type States in Superconducting Nanowires
Physical Review Letters, 2007
Superconducting nanowires can exhibit a spatially inhomogeneous pair condensate that leads to the formation of new Andreev-type states. Such states are mainly located beyond the regions where the order parameter is enhanced, and no normal-superconducting contact or external magnetic field is needed for their formation. Our numerical self-consistent solutions of the Bogoliubov-de Gennes equations for cylindrical nanowires, in the clean limit, demonstrate that these new Andreev-type states decrease the ratio of the energy gap to the critical temperature as compared to its bulk value. The low-lying excitations in a clean superconducting nanowire are these new Andreev-type states induced by quantum confinement of the electrons in the transverse direction.
International Journal of Modern Physics B, 2009
We study the effect of electron confinement on the superconducting-to-normal phase transition driven by a magnetic field and/or on the current-carrying state of the superconducting condensate in nanowires. Our investigation is based on a self-consistent numerical solution of the Bogoliubov-de Gennes equations. We show that in a parallel magnetic field and/or in the presence of supercurrent the transition from superconducting to normal phase occurs as a cascade of discontinuous jumps in the superconducting order parameter for diameters D < 10 ÷ 15 nm at T = 0. The critical magnetic field exhibits quantum-size oscillations with pronounced resonant enhancements.
Physical Review B, 2007
We analyze the dependence of the basic superconducting quantities-the order parameter, energy gap, and critical temperature-on the size and shape of a nanoscale superconductor in the clean limit. The Bogoliubov-de Gennes equations are solved numerically for a metallic nanowire with rectangular cross section. This makes it possible to vary the confining geometry, passing from a nanowire to a nanofilm and testing the sensitivity of the basic superconductive characteristics on the transverse size and shape of the nanowire. Strong size and shape superconducting resonances are found that depend on the geometry of the sample.
Magnetic-field induced quantum-size cascades in superconducting nanowires
Physical Review B, 2008
In high-quality nanowires, quantum confinement of the transverse electron motion splits the band of single-electron states in a series of subbands. This changes in a qualitative way the scenario of the magnetic-field induced superconductor-to-normal transition. We numerically solve the Bogoliubovde Gennes equations for a clean metallic cylindrical nanowire at zero temperature in a parallel magnetic field and find that for diameters D 10 ÷ 15 nm, this transition occurs as a cascade of subsequent jumps in the order parameter (this is opposed to the smooth second-order phase transition in the mesoscopic regime). Each jump is associated with the depairing of electrons in one of the single-electron subbands. As a set of subbands contribute to the order parameter, the depairing process occurs as a cascade of jumps. We find pronounced quantum-size oscillations of the critical magnetic field with giant resonant enhancements. In addition to these orbital effects, the paramagnetic breakdown of Cooper pairing also contributes but only for smaller diameters, i. e., D 5 nm.
Superconducting nanofilms: Andreev-type states induced by quantum confinement
Physical Review B, 2008
Quantum confinement of the transverse electron motion is the major effect governing the superconducting properties of high-quality metallic nanofilms, leading to a nonuniform transverse distribution of the superconducting condensate. In this case the order parameter can exhibit significant local enhancements due to these quantum-size effects and, consequently, quasiparticles have lower energies when they avoid the local enhancements of the pair condensate. Such excitations can be considered as new Andreev-type quasiparticles but now induced by quantum confinement. By numerically solving the Bogoliubov-de Gennes equations and using Anderson's approximate solution to these equations, we: ͑a͒ formulate a criterion for such new Andreev-type states ͑NATS͒ and ͑b͒ study their effect on the superconducting characteristics in metallic nanofilms. We also argue that nanofilms made of low-carrier-density materials, e.g., of superconducting semiconductors, can be a more optimal choice for the observations of NATS and other quantum-size superconducting effects.
Size-dependent enhancement of superconductivity in Al and Sn nanowires: Shape-resonance effect
Physical Review B, 2006
A shape-dependent superconducting resonance can be expected when an energy level associated with the transverse motion in a wire passes through the Fermi surface. We show that the recently observed width-dependent increase of Tc in Al and Sn nanowires is a consequence of this shape resonance effect. 74.78.Na Increasing the critical temperature (T c ) of a superconductor (SC) has been a major challenge. On the one hand one can look for different materials which exhibit a higher T c . Such a search has been very successful over the last 20 years. On the other hand microstructuring of a superconductor is a different and new road which is able to modify T c (i.e. increase and/or decrease) and may also give us further insight in the basic mechanism of superconductivity.
Size-Resonance Effect in Cylindrical Superconducting Nanowire
We investigate the dependence of the basic superconducting quantities - the order pa- rameter, energy gap and critical temperature - on the quantum confinement of a nanoscale superconductor in the clean limit. The Bogoliubov-de Gennes equations are solved numeri- cally for a Pb nanowire in the clean limit with cylindrical cross section. Strong size supercon- ducting resonances are found in the critical temperature of a Pb nanowire.
Quantum transport in superconducting, ferromagnetic and normal nanowires
Electrons in superconductors condense to form pairs known as Cooper pairs [1]. The characteristic decay length of the wavefunction of this pair (or the "size" of a Cooper pair) is the superconducting coherence length !. When any of the 3 physical dimensions of a superconducting system become comparable to this characteristic length the effect of fluctuations become important and superconductivity is predicted to be destroyed [2]. In 1D systems (nanowires with diameter < !) the limit at which superconductivity is quenched and the mechanism by which it is quenched is an active field of study. Fluctuations and changes in boundary conditions lead to many novel phenomena in 1D nanowires. Experimental exploration of these novel effects forms the basis of this dissertation. Metallic 1D nanowires have been fabricated using template-based electrodeposition and evaporation. Availability of nanowires of different morphologies helps in performing comparative experiments to isolate effects due to disorder from true 1D physics. The electronic transport properties of superconducting, ferromagnetic and normal nanowires with superconducting and normal electrodes have been studied in various measurement geometries. Experiments on aluminum nanowires studying the counterintuitive anti-proximity effect (APE) [3] have been performed. The results of these experiments appear to bring a complete understanding of the phenomenon and have resolved a number of puzzles in the early experiments. In addition, measurements of a single resistance reading found switching from the superconducting to the normal state close to T c of the wire and at low temperatures in the APE regime. The switching at low temperature is triggered by individual quantum phase slips. These results indicate that the low temperature APE regime offers a clean platform for the observation of individual quantum phase slips, a goal eluded in numerous experiments. iv Systematic studies on crystalline and granular ferromagnetic cobalt and nickel nanowires sandwiched between superconducting electrodes have been performed. A very long-range proximity effect (~ 600 nm) was found. This range is two orders of magnitude larger than that measured for bulk superconductor-ferromagnet systems. The superconducting transition was foreshadowed by a large peak in resistance dubbed the 'critical peak'. Possible explanations of these counterintuitive effects have been discussed. In earlier experiments on crystalline gold nanowires contacted with superconducting tungsten electrodes, a mini-gap state along with magnetoresistance oscillations indicating individual vortex trapping were found [4]. The experiment has been repeated here with different electrodes and different nanowire morphologies. The mini-gap state persists in these samples demonstrating it is a robust state independent of nanowire and electrode morphology. v Contents LIST OF FIGURES ix