Slow two-level systems in point contacts

TL;DR

This paper reviews the impact of slow two-level systems on the electrical properties of point contacts, analyzing theoretical models, scattering processes, and experimental implications for disordered low-temperature systems.

Contribution

It provides a comprehensive theoretical analysis of slow TLSs in point contacts, including scattering effects and their influence on current-voltage characteristics, with comparison to experimental data.

Findings

Scattering on slow TLSs causes nonlinearity in I-V characteristics.

Elastic and inelastic scattering contributions are quantified.

Differences between slow TLSs and fast centers are discussed.

Abstract

A great variety of experiments, like heat release measurements, acoustic measurements, and transport measurements on mesoscopic samples have proved that two level systems (TLSs) have a crucial role in the low temperature thermal and electric properties of disordered systems. This paper is aimed at reviewing the role of slow TLSs in point contacts. First the theory of point contacts is summarized, concentrating on the discussion of different point contact models, and on the different regimes of electron flow in the contact, mainly focusing on the ballistic and diffusive limit. The Boltzmann equation is solved in both regimes, and the position dependence of the electrical potential is determined. Then the scattering processes in point contacts are investigated, particularly concentrating on the scattering on slow TLSs. If the the electron assisted transitions between the two states are negligible the electron-two level system interaction can be treated with a simplified Hamiltonian. The scattering on such slow TLSs causes nonlinearity in the current-voltage characteristics of the point contact, which can be determined using Fermi's golden role. These calculations are presented showing both the contribution of elastic and inelastic scattering, and including the dependence on the position of the TLS, and on the effect of high frequency irradiation. These results are used to discuss the differences between these slow TLSs and the fast centers which may be described by the two channel Kondo model. The available experimental results are analyzed, distinguishing between the effects due to the different types of TLSs.