Thesis presented December 18, 2013

Abstract:
During this PhD, I have first finished the development of a unique experimental set-up, dedicated for studies of electronic transport of low impedance multi-terminal nanostructures. This set-up allows conductance and noise measurements at very low temperature (30mK), with a resolution of a few pico-ampere by using SQUIDs as current amplifiers. In chapter 5, I give some explanation of the measurement working principle. Furthermore, I explain the calibration of the experimental set-up as well as how to extract physical quantities from the measurements. In chapter 6, I explain transport measurements on diffusive tri-terminal junctions (tri-junction). 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 does not show up in such a geometry, even not with strongly reduced amplitude. This indicates, that synchronization via the experimental environment can't be the origin of the observed features. The second theoretical mechanism is named "quartet-mode" and has been recently proposed by Freyn and Co-workers. 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 quasiparticles 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. During this PhD, I have shown that these anomalies are indeed 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. Unfortunately, for the moment, no microscopic complete theory of such a tri-junction exists in order to provide an inevitable argumentation. Nevertheless, a phenomenological approach has been developed based on a extended RSJ model. This allows an estimation of the influence of the particularities of the experimental set-up (voltage-/current-bias). The first part of the manuscript (chapter 1-4) consists of an introduction, followed by a quite detailed description of mesoscopic physics, on which the further manuscript is based as well as a state-of the art about the studies of coupled Josephson junctions. Chapter 5-7 describes the performed experimental work. In chapter 8, I give a conclusion about the effected work and propose complementary experiments as well as long term perspectives.

Keywords:
Mesoscopic quantum physics, superconductivity, shot noise

On-line thesis.