Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H1: Spin-Triplet Supercurrents in Superconductor/Ferromagnet/Superconductor Josephson Junctions |
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Sponsoring Units: DCMP Chair: James Sauls, Northwestern University Room: Ballroom A1 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H1.00001: Odd Triplet superconductivity in Superconductor-Ferromagnet hybrid structures Invited Speaker: Since the prediction of a long-range triplet component of the superconducting condensate in ferromagnetic-superconductor (S/F) proximity structures in 2001 [1], the activity in the field has increased considerably, both theoretically and experimentally The coexistence of conventional singlet superconductivity and ferromagnetism in S/F structures is closely related with the appearence of a triplet component of the condensate, which is odd in frequency and even in momentum and therefore insensitive to nonmagnetic impurities. The presence of the triplet component leads to new effects as for example, long-range Josephson coupling in SFS junctions [1], flow of a supercurrent through a half-metallic link [2] and screening of the magnetic moment of ferromagnetic particles embbeded in a superconductor [3]. In this talk I will review the main issues of the odd-triplet superconductivity, its manifestation in physical properties, and briefly discuss the relevant experiments in the field [4]. \\[4pt] [1] F.S. Bergeret, A. F. Volkov and K. B. Efetov, Phys. Rev. Lett. 86, 4096 (2001); see also by the same authors Rev. Mod. Phys. 77, 1321 (2005).\\[0pt] [2] M. Eschrig and T. L\"ofwander, Nature Phys. 4, 138 (2008).\\[0pt] [3] F. S. Bergeret, A. F. Volkov and K. B. Efetov, Phys. Rev. B 69, 174504 (2004).\\[0pt] [4] R. S. Keizer et al, Nature 439, 825 (2006); T. S. Khaire et al., Phys. Rev. Lett. 104, 137002 (2010); J. W. A. Robinson, J. D. S. Witt and M. G. Blamire, Science 329, 59 (2010), Jian Wang et al, Nature Phys. 6, 389 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H1.00002: Proximity effect-induced superconductivity in crystalline metallic and ferromagnetic nanowires Invited Speaker: In a single crystal gold nanowire of 1.2 microns contacted by superconducting contacts, the proximity effect induced superconductivity was found to appear in two distinct steps. The superconducting and normal regions are separated by a mini-gap state of low critical field. We suggest that a superposition of two distinct magnetic-flux states, which correspond to quantum flux 0 and 1 trapped in the nanowire, can explain the mini-gap state. Furthermore, we observed clear periodic differential magnetoresistance oscillations in the superconducting to normal transition region, which corresponds to the generation or annihilation of one vortex. In crystalline ferromagnetic Co and Ni nanowires, unexpected long-range proximity effect was observed. Josephson current associated with weakly damped singlet superconducting correlations or triplet correlations produced by the contact regions may lead to the observed long ranged proximity effect. In addition, a large and sharp resistance peak around the transition temperature was observed in the wires exhibiting incomplete superconductivity. Further theoretical model needs to be developed to reveal the physics behind the ``peak effect.'' [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:48AM |
H1.00003: Observation of spin-triplet supercurrent in Co-based Josephson junctions Invited Speaker: When a superconductor (S) and a ferromagnet (F) are put into contact with each other, the combined S/F system may exhibit altogether new properties. There is a proximity effect where pair correlations from S penetrate into F, but these correlations decay over a very short distance due to the large exchange splitting between the spin-up and spin-down electron bands in F. Theory predicts that, under certain conditions, electron pair correlations can be generated with spin-triplet rather than spin-singlet symmetry [1]. The two electrons in such a spin-triplet pair have parallel spins and are not subject to the exchange splitting in F; hence they propagate long distances. We have measured a long-range supercurrent in Josephson junctions of the form S/X/N/SAF/N/X/S, where S is a superconductor (Nb), N is a normal metal (Cu), SAF is a synthetic antiferromagnet of the form Co/Ru/Co, and X is a thin ferromagnetic layer necessary to induce spin-triplet correlations in the structure [2]. Spin-triplet correlations are generated due to non-collinearity of the magnetizations in each X layer and the nearest Co layer. Using X = PdNi, CuNi, and Ni, we observe enhancements of the critical current of up to 300 times relative to similar samples lacking the X layers. We also observe a large additional enhancement of the spin-triplet supercurrent after the samples are magnetized in a large field. This result is counter-intuitive, since one would expect magnetizing the samples to suppress the occurrence of non-collinear magnetization. We will present a model of the SAF magnetization structure that explains these intriguing results. \\[4pt] [1] F.S. Bergeret, A.F. Volkov, and K.B. Efetov, Phys. Rev. Lett., 86, 4096 (2001).\\[0pt] [2] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:24AM |
H1.00004: Triplet supercurrents in ferromagnets Invited Speaker: In almost all superconductors the pairs of electrons which carry the charge are in the so-called singlet state in which the quantum spin of the two electrons is antiparallel. During the past five years there has been increasing evidence that proximity coupling between singlet superconductors and ferromagnets can sometimes generate triplet pairs within the ferromagnet in which the spins of the electrons are parallel rather than antiparallel -- the evidence being that supercurrents can be passed through thicknesses of ferromagnetic material which are simply too large for singlet pairs to survive. The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. For strong ferromagnets such as Co, a thickness of only a few nanometres is sufficient to almost completely suppress the critical current of a Nb/Co/Nb Josephson junction. Here we report experiments in which a conical magnet (holmium) is placed at the interface between the superconductor and ferromagnet. The results showed that a long-ranged supercurrent can occur through the ferromagnetic Co layer but only for certain critical thicknesses of the Ho [1]. These thicknesses correspond to maximum magnetic inhomogeneity on the Ho and are therefore consistent with models which predict that a spin-mixing interface between the superconductor and ferromagnet can generate triplet pairs which are long-ranged in the ferromagnet. This paper reports recent experiments which aim to understand further the behaviour of triplet pairs in superconductor / ferromagnet heterostructures. \\[4pt] [1] J. W. A. Robinson, J. D. S. Witt, and M. G. Blamire, ``Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet'' Science \textbf{329}, 59-61 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 11:00AM |
H1.00005: Long-ranged supercurrents through half-metallic ferromagnetic CrO$_2$ Invited Speaker: In the last few years, the scenery in the physics of superconductor/ ferromagnet hybrids has changed considerably with the realization that spin triplets may be induced in the ferromagnet through the mechanism of odd-frequency pairing. Since the equal-spin component of the triplet is not susceptible to pair breaking by the exchange field, such correlations can sustain supercurrents over long lengths, in particular in fully spin polarized materials where only one spin band is available. In halfmetallic ferromagnetic CrO$_2$ for instance, where superconducting contacts were deposited on top of the ferromagnetic films, we observed the current to flow over 700~nm at 4.2~K [1]. Still, we also have fabricated devices where the supercurrent is absent, which indicates that the mechanism of triplet generation is not yet well in hand. The presence of non-homogeneous magnetization is important, and here the grain structure of the film appears to play a key role, as can be illustrated with data for films grown on different substrates (TiO$_2$ and Al$_2$O$_3$). Moreover, recent data will be presented which suggest that triplet generation can be improved by using an additional ferromagnetic layer in the contact area. \\ \newline [1] M. S. Anwar, F. Czeschka, M. Hesselberth, M. Porcu, and J. Aarts, Phys. Rev. B {\bf 82}, 100501(R) (2010). [Preview Abstract] |
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