Bruit de grenaille dans les conducteurs mésoscopiques
François Lefloch

SQUID based experimental set-up. The intrinsic noise is as low as few pA^2 / Hz and corresponds to the thermal current noise of a 1 ohm resistor at 100 mK.



The electronic transport in diffusive conductors shows specific properties at mesoscopic scale due to the conservation of quantum coherence of the electronic wave functions. The current flow can be described using the Landauer - Büttiker formalism that considers the transmission probabilities of electronic waves through the sample. For each mode, there is a transmission coefficient and the conductance is given by the trace of the transmission matrix. Another approach is the semi-classical description where inelastic collisions, that break the quantum coherence, can be included. When the electrons have probability different from 0 or 1 to be transferred through a mesoscopic sample, the transmitted current fluctuates around its mean value. The spectrum density of the fluctuations is the shot noise also called partition noise. One very important feature of the shot noise in electronic conductors, is its sensitivity to interactions. Indeed, any type of correlations that regulates the flux of electrons, tends to decrease the fluctuations and therefore the noise. Another property of shot noise is its proportionality to the charge of the current carriers. It can therefore be used to experimentally measure such a charge [1].
At LaTEQS, we measure the current fluctuations of small, sub-micron, conductors using an experimental set-up developed in the group at the CEA/Grenoble, using a SQUID (Superconductive Quantum Interference Device)  [2].
The samples are mainly diffusive conductors or semiconductors in contact with one or two superconductors. In such contacts, the conductor acquires some superconducting properties due to proximity effect. In that case, the charge of the current carriers is greater than in the normal case with no superconductivity and the shot noise can be very much enhanced [3,4,5].

[1] Y. Blanter and M. Büttiker, “Shot Noise in Mesoscopic Conductors”, Physics Report, 336 (2000)
[2] X. Jehl, P. Payet-Burin, C. Baraduc, R. Calemczuk and M. Sanquer, “Superconducting Quantum Interference Device based resistance Bridge for Shot Noise Measurements on Low Impedance Samples”, Review of Scientific Instruments, 70, 2711 (1999)
[3] X. Jehl, M. Sanquer, R. Calemczuk and D. Mailly, “Detection of Doubled Shot Noise in Short Normal-Metal/Superconductor Junctions” Nature, 405, 50 (2000)
[4] F. Lefloch, C. Hoffmann, M. Sanquer and D. Quirion, “Doubled Full Shot Noise in Quantum Coherent Semiconductor-Superconductor Junctions”, Physical Review Letters, 90, 067002 (2003)
[5] C. Hoffmann, F. Lefloch, M. Sanquer and B. Pannetier, “Mesoscopic Transition in the Shot Noise of Diffusive S/N/S Junctions”, Physical Review B, 70, 180503(R) (2004)

Contact : François Lefloch


Shot noise of a 0.8 µm long diffusive Superconductor / Normal metal / Superconductor (Al/Cu/AL) junctions. The shot noise is very much enhanced compared to the normal state due to Multiple Andreev Reflections. A clear minimum is revealed at the Thouless energy.

Maj : 17/10/2013 (147)


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