Influence of electromagnetic environment in single tunnel junctions

Abstract

In this thesis, electron transport through single tunnel junction embedded in a dissipative environment was studied both in normal and superconducting state. High-voltage asymptotics of the current voltage (IV) characteristics for small normal tunnel junctions and carbon nanotubes were studied experimentally. Power-law voltage tails of IV curve were detected when approaching the linear law V = IR + e/2C at large voltages with the offset depending of the junction capacitance C.

The effect of environment is in fact a result of Johnson-Nyquist noise in the electric circuit. In our low temperature measurements, the power-law tails at high voltages are connected only with the quantum part of Johnson-Nyquist noise. Thus, detection of these tails in a good agreement with the quantum theory of environment is a rather unique verification of quantum zero-point fluctuations in macroscopic systems.

Our experimental conditions included the case of strong tunneling when the junction resistance RT was less than the quantum resistance RK. The strong-tunneling corrections to the environmental modes at high voltages can be simply incorporated by including the junction resistance RT into the effective electric circuit for calculation of the quantum noise.

In the superconducting state, our experiments for the first time clearly confirm the existence of the dissipative phase transition in a single Josephson junction. In the transition, dissipation destroys the quantum mechanical band structure and restores the classical Josephson behaviour of dynamics governed by the classical phase difference φ.

The observed phase diagram differed from that expected originally. The agreement with theory was achieved by taking into account that the position of the measured phase boundary is governed not only by intrinsic junction parameters, but also by the accuracy of voltage measurement. Our work is a strong demonstration of quantum effects in a single Josephson junction, especially, of the Josephson phase delocalization and the band picture of phase motion.