Saturation of dephasing time in mesoscopic devices produced by a ferromagnetic state

TL;DR

This paper models the decoherence mechanisms in mesoscopic ferromagnetic devices using a boson-fermion Hamiltonian, explaining the saturation of dephasing time through electron-spin wave interactions.

Contribution

It introduces a boson-fermion Hamiltonian derived from a fully polarized ferromagnet model to explain dephasing saturation in mesoscopic systems.

Findings

Decoherence arises from spin wave interactions with itinerant electrons.

The model explains experimental results in quantum dots and nanowires.

Dephasing rate is proportional to system size.

Abstract

We consider an exchange model of itinerant electrons in a Heisenberg ferromagnet and we assume that the ferromagnet is in a fully polarized state. Using the Holstein-Primakoff transformation we are able to obtain a boson-fermion Hamiltonian that is well-known in the interaction between light and matter. This model describes the spontaneous emission in two-level atoms that is the proper decoherence mechanism when the number of modes of the radiation field is taken increasingly large, the vacuum acting as a reservoir. In the same way one can see that the interaction between the bosonic modes of spin waves and an itinerant electron produces decoherence by spin flipping with a rate proportional to the size of the system. In this way we are able to show that the experiments on quantum dots, described in D. K. Ferry et al. [Phys. Rev. Lett. {\bf 82}, 4687 (1999)], and nanowires, described in D. Natelson et al. [Phys. Rev. Lett. {\bf 86}, 1821 (2001)], can be understood as the interaction of itinerant electrons and an electron gas in a fully polarized state.