Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots

TL;DR

This study measures the electromagnetic response of isolated quantum dots at very low temperatures, revealing a sign reversal in magnetoconductance related to quantum level statistics and frequency, contrasting with connected systems.

Contribution

It reports the first observation of frequency-dependent sign reversals in magnetoconductance in isolated quantum dots, linking this to energy level spacing statistics.

Findings

Magnetoconductance sign reverses with frequency in isolated dots.

Negative magnetoconductance at low frequencies contrasts with connected systems.

Sign reversal correlates with quantum energy level statistics.

Abstract

We have measured the electromagnetic response of micron-size isolated mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them to an electromagnetic micro-resonator. Both dissipative and non dissipative responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale which corresponds to a flux quantum in a dot. The real (dissipative) magnetoconductance changes sign as a function of frequency for low enough density of electrons. The signal observed at frequency below the mean level spacing corresponds to a negative magnetoconductance, which is opposite to the weak localization seen in connected systems, and becomes positive at higher frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots.