Nodal quasiparticles and their quantum interference effects in superconductors and magnets AIP Conf. Proc. 918, 308 (2007); 10.1063/1.2752000Quasiparticle charge imbalance, first-order phase transition and quantum criticality in ferromagnet∕superconductor∕ferromagnet double-tunnel junctions
Conventional electron beam lithography has been used to fabricate mesoscopic Nb wires with a superconducting transition temperature above 7.0 K. The typical line width and the thickness were 200 nm and 45 nm respectively. Nb was deposited in an ultra-high vacuum evaporation chamber using electron gun heating. All samples exhibited a normal-superconducting transition. The transition temperature decreased with thickness and line width. To demonstrate the feasibility of two angle evaporation techniques we also fabricated small Nb/(Al-)AlOx/Nb tunnel junctions. 85.25.K, 73.63, 73.23.H. A number of nanometer-scale devices such as singleelectron transistors [1], superconducting quantum bits [2], and other mesoscopic superconducting devices [3] have been realized using the self-alignment technique which provides submicron accuracy [4]. Self alignment is achieved by the shadow evaporation technique, commonly used with the resist polymethylmethacrylate (PMMA) and co-polymer [P(MMA-MAA)] as a double layer stencil mask patterned by electron-beam (e-beam) lithography. Until now this conventional shadow evaporation technique has been applied successfully for the soft metals such as Al, Cu and Pb. For the refractory metals such as Nb, W or Ta, however, this technique is known to be difficult to apply. In particular Nb is a promising alternative to the soft metals for superconducting nano devices such as single-electron transistors and quantum bits, because of its large superconducting gap and high stability under thermal cycling. A demonstration that it is possible to fabricate nanostructures by conventional e-beam lithography would therefore be highly significant.The problems with the application of the conventional technique have been ascribed to the partial decomposition of the PMMA-co-polymer double layer during the evaporation of the refractory metals [5]. The resulting outgassing from the resist [6] and consequent contamination of the deposited Nb would then explain the changes in the electronic properties of the deposited Nb. Contrary to this common experience, we were able to fabricate mesoscopic Nb wires with zero-field critical temperatures T c higher than 7.0 K by using the conventional shadow evaporation technique.The results presented here were obtained with an ebeam lithography process which did not have any special features but followed the most common procedures with conventional recipes. We deposited a double layer of PMMA-P(MMA-MAA) on the oxidized Si substrate with the thickness of the oxidization layer of about 250 nm. The spinning rates for PMMA and P(MMA-MAA) were 3000 rpm and 6000 rpm, respectively, and the spinning time was 30 seconds for both. The thickness of the PMMA and P(MMA-MAA) were measured to be about 270 nm and 300 nm, respectively. The resist was baked at 160• C for about 60 minutes. We then drew a pattern of wire (see Fig. 1) by using a scanning electron microscope (JEOL, JSM 840A). The line width was about 200 nm. To develop the upper layer of the PMMA resist we immersed the sample in a...
Cooling by normal-insulator–superconductor junctions in external magnetic field has been studied experimentally. For all orientations of magnetic field the cooling performance correlates with the magnetic field dependent superconducting energy gap Δ(H). In perpendicular orientation of magnetic field with respect to the sample plane, additional degradation of the cooling power originates from scattering of nonequilibrium quasiparticles in the superconductor on magnetic vortices. The effect is hysteretic and its magnitude depends on the shape of the superconducting probes.
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