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électronique et photonique quantiques
laboratoire pierre aigrain

Séminaire du 17/11/2008

P. Roche (CEA Saclay, Groupe Nanoelectronique)
Determination of the coherence length in the Integer Quantum Hall Regime

In the physics of quantum conductors, one of the very basic length scales that gives a limitation to the manifestation of quantum effects is the so-called quantum coherence length L. It characterizes the length on which an excitation exchanges information with other degrees of freedom and hence looses its phase coherence. Lhas been extensively studied in quasi-1D diffusive wires in the last decade. It has been shown to result from electron electron interaction as predicted by Altshuler-Aronov-Khmelnitsky [1], leading to a T-1/3 temperature dependence of L in quasi-1D diffusive wires. Surprisingly, very little has been known about the actual coherence length in the Integer Quantum Hall Regime (IQHE), where transport occurs through one dimensional chiral wires localized on the edge of the sample (the edge states) ; the number of these edge states being equal to the filling factor (the number of electron per quantum of flux). In principle, for such ballistic wires, one expects the chirality to prevent momentum conserving energy exchange processes and lead to a very long coherence length.

Here, we present an experiment where we have determined L in the quantum Hall regime, by measuring the visibility of quantum interferences in an electronic Mach-Zhender Interferometer [2]. L presents a T-1 dependence which is shown to result from the coupling between the two neighboring edge states and thermal noise [3,4] : the thermal charge noise in one edge state blur the phase on the other edge state, and hence leads to a finite coherence length proportional to T-1.

[1] B. L. Altshuler, A. G. Aronov, and D.E. Khmelnitsky, J. Phys. C 15, 7367 (1982).

[2] P. Roulleau et al., Phys. Rev. Lett. 100, 126802 (2008).

[3] G. Seelig and M. Buttiker, Phys. Rev. B 64, 245313 (2001).

[4] P. Roulleau et al., Phys. Rev. Lett. 101, 186803 (2008).