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Spin imbalance is predicted to lead to suppression of superconductivity. We report phenomena manifesting this effect under spin-polarized quasiparticle currents in ferromagnetsuperconductor-ferromagnet single electron transistors. The measured superconducting gap as a function of magnetic field reveals a dramatic decrease when the magnetizations of the two leads are in opposite orientations. The effect of suppression increases with increasing voltage but decreases at elevated temperatures. The possible explanations for these dependences are given. This method may render it applicable to control superconductivity at low temperatures and low fields.PACS numbers: 74.25. Ha, 75.75.+a, 85.35.Gv The interplay between superconductivity and magnetism has been a topic of interest for many years. Of particular interest are recent experiments on ferromagnet/superconductor (FM/SC) junctions with high T C [1] in which a decrease of the supercurrent by nonequilibrium spin density was demonstrated. Furthermore, theoretical studies [2,3] also indicate that spin imbalance in a superconductor can lead to suppression of superconductivity. In a double tunnel junction containing a normal metal or a superconductor sandwiched between two ferromagnets, both injection of polarized current and spin accumulation are possible, and thus provide an ideal test ground for the theory. When the magnetic moments of the two ferromagnetic leads are in opposite orientations (referred to as the antiferromagnetic (AF) alignment), the difference in the number of majority and minority spins in the central electrode brings in a chemical potential difference. In a normal metal, this potential difference is superconductors δµ = P eV /2, where P is the polarization of the ferromagnetic leads and V is the voltage across the sample. In a superconductor, this difference gives rise to pair breaking, same as the Zeeman effect does to superconductivity in the paramagnetic limit. In this paper, we report the first direct observation of the superconducting gap suppression using Co/Al/Co (FM/SC/FM) double tunnel junctions. The effect can be turned on and off by manipulating the mutual orientations of the magnetic moment of the two Co leads. We further discuss some relevant time scales, including the spin relaxation time and the tunneling time, which set the criterion for spin accumulation in the superconductor.The inset of Fig. 1 shows an SEM picture of a measured sample and its biasing circuit. The samples were fabricated by standard electron-beam lithography techniques and by the shadow evaporation method. A thin native Al 2 O 3 layer between the Al island and the Co electrodes acted as a tunnel barrier. Aluminum is a good candidate to serve as the superconductor for this purpose, not just for its long spin lifetime [4] allowing the full range of spin effects to be studied, but also for its high quality native Al 2 O 3 barrier which was shown to have no spin-flip tunneling processes [5]. Our electron-gun deposited Al islands have a superconducting transition te...

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Shot noise, which accompanies the emergence of the quantization nature (discreteness) of electrons, indicates dynamic fluctuations in electrical current and further reveals underlying operations in mesoscopic applications. This work presents a multichannel junction device with which to study the shot noise of coupled-channel transports as in normal nanocircuits, and it describes the theoretical algorithms. The numerical results show that, in contrast to the indistinguishable results of average current on direct-current measurements, the shot noise related to a dynamic current definitely exhibits significant differences among devices with diverse multichannel set-ups and presents super-Poissonian behaviors associated with the splitting of tunneling states.

We present tunneling measurements on an InN nanobelt which shows signatures of superconductivity. Superconducting transition takes place at temperature of 1.3K and the critical magnetic field is measured to be about 5.5kGs. The energy gap extrapolated to absolute temperature is about 110 mu eV. As the magnetic field is decreased to cross the critical magnetic field, the device shows a huge zero-bias magnetoresistance ratio of about 400%. This is attributed to the suppression of quasiparticle subgap tunneling in the presence of superconductivity. The measured magnetic-field and temperature dependence of the superconducting gap agree well with the reported dependences for conventional metallic superconductors. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.3691830