Doubled Full Shot Noise in Quantum Coherent Superconductor-Semiconductor Junctions

Abstract

We performed low temperature shot noise measurements in superconductor (TiN) -strongly disordered normal metal (heavily doped Si) weakly transparent junctions. We show that the conductance has a maximum due to coherent multiple Andreev reflections at low energy and that the shot noise is then twice the Poisson noise (S = 4eI). When the subgap conductance reaches its minimum at finite voltage the shot noise changes to the normal value (S = 2eI) due to a large quasiparticle contribution.PACS numbers: 74.40.+k, 74.50.+r, 73.23.-b We know, from early measurements, that the full shot noise in an electronic device S P oisson = 2qI (first measured by W. Schottky in a vacuum diode) is proportional to the mean value of the current I and to the charge q of the carriers [1]. This result holds for N-I-N tunnel junctions where N is a normal metal and I an insulating barrier [2] with q = e the electronic charge. In S-I-N, due to electron pairing in the superconductor (S), the shot noise is expected to be twice the full shot noise : S = 4eI. However, in such junctions the subgap current is very small and the shot noise is not measurable. The subgap current can be restored if the quasiparticles of the normal metal are coherently backscattered towards the interface. This reflectionless tunneling regime can be achieved by adding a second barrier in the normal part of the junction (S-I-N-I-N) or when the normal metal is disordered enough [3,4,5,6,7,8,9,10,11]. The enhancement of the subgap current is only seen at low energy when the electron-hole coherence time in the normal metal is longer than the time it takes for a quasiparticles to return to the interface. Then, the coherent addition of two (or more) Andreev reflections, each of them with a very small probability Γ 2 (Γ is the transparency of the barrier), yields to an increase of the Andreev current through the interface. This effect can be large, leading to an Andreev current proportional to Γ instead of Γ 2 ≪ Γ ≪ 1 and can be comparable to the normal current above the gap (also proportional to Γ). Another way to increase the subgap current is to use highly transparent S-N junctions. In this case doubled shot noise is predicted and has been observed experimentally at various temperatures [12,13]. However, the noise level was always three times smaller than the doubled full shot noise because of the diffusive nature of the normal metal used in these experiments. Moreover, as shown experimentally and reproduced theoretically [15], the doubling of the shot noise occurs at any energies below the superconducting gap and electronhole coherence is not required.In this letter, we report shot noise measurements in a junction where a superconductor (TiN) is in contact with heavily doped silicon. We show that the shot noise is twice the full shot noise at low energy and equals the Poisson value at bias much smaller than the superconducting gap. This behavior evidences a crossover from a low bias Andreev dominated to a large bias quasiparticle dominated subgap conductance.The sample...

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