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Electronic properties of diffusive three-terminal Josephson junctions and non-local quartet mechanism

Mercredi 18 décembre 2013 14:00 - Duree : 1 heure
Lieu : Salle "Nevill Mott" D420 - Institut Néel - Building D, CNRS Polygone scientifique, 25 rue des martyrs, 38000 Grenoble

Orateur : Soutenance de Thèse Andreas PFEFFER

Abstract : We have studied the electronic transport properties of three terminal superconductor (S) - normal metal (N) - superconductor (S) nano-devices using a new SQUID-based experimental set-up working at very low temperature (30 mK) and dedicated for high sensitive conductance and current noise correlations measurements [1]. In a T-shape called geometry, the superconducting Al-electrodes are connected via a common metallic, non-superconducting part of Copper. For these nanostructures, we observe features in the conductance at low voltage, which have been never observed yet experimentally. These features in conductance/resistance have a striking resemblance with a dc-Josephson effect, appearing when two applied potentials on the tri-junction compensate exactly each other. In literature, two mechanisms are proposed to explain this effect. The first mechanism, called "mode-locking", corresponds to a dynamic locking of ac-Josephson currents, which is induced by the experimental environment (circuit). This situation has been extensively studied in the 60’s on coupled microstructures, based on weak links. In order to test this explanation, we have measured a junction, which is composed of two spatially separated Josephson junctions. The anomalies do not show up in such geometry, even not with strongly reduced amplitude. This indicates that synchronization via the experimental environment cannot be at the origin of the observed features. The second theoretical mechanism is named "quartet-mode" and has been recently proposed by Freyn and Co-workers [2]. In this process, one superconducting electrode emits doublets of Cooper-pairs. Each of the two pairs splits into two quasi particles propagating toward different superconducting contacts. In such a mechanism, two quasi-particles originating of two different Cooper-pairs arrive each in the two superconducting contacts. If the applied voltage between the emitting superconducting contact and the two other contacts is exactly opposite, the phase of the electronic wave functions of the arriving quasi-particles on the same superconducting contact are such, that these two quasi-particles can recombine by forming a Cooper-pair. Due to this mechanism, the emitted doublet of Cooper-pairs is coherently distributed as two Cooper-pairs, each of them in a different superconducting contact. This mechanism is favored, since it is robust with respect to disorder and can hence also exist over a large range of voltage. Indeed, the observed features are present for applied voltage corresponding to energies well above the Thouless energy. Argumentum a contrario, the coherent effects responsible for the ac Josephson-effect have to be strongly attenuated over the same range of energy, which makes low probable the effect of mode-locking. The observed dependency with applied magnetic field and the temperature are further indications for the quartet-mode [3].

[1] A.H. Pfeffer, B. Kaviraj, O. Coupiac & F. Lefloch : "SQUID-based set-up for probing current-noise and correlations in three-terminal devices", RSI 83, 115107 (2012) [2] A. Freyn, B. Douçot, D. Feinberg & R. Mélin :" Production of Nonlocal Quartets and Phase-Sensitive Entanglement in a Superconducting Beam Splitter", PRL 106, 257005 (2011) [3] A.H. Pfeffer, J-E. Duvauchelle, H. Courtois, R. Mélin, D. Feinberg & F. Lefloch :" Sub-Gap structure in the conductance of a three-terminal Josephson junction", arXiv:1307.4862, (submitted to PRL)

Contact : pfeffer_andi@web.de

Discipline évènement : (Physique)
Entité organisatrice : (Fondation Nanosciences) -
Entité organisatrice : (INAC/SPSMS)
Nature évènement : (Soutenance de thèse)
Site de l'évènement : Polygone scientifique

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