Nanoscale physics

Anomalously enhanced thermal noise in quantum rings
E. Zhitlukhina1, M. Belogolovskii2, P. Seidel3
1 Department of Phase Transitions, Donetsk Institute for Physics and Engineering of the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03028, Ukraine.
E-mail: elena_zhitlukhina@ukr.net
2 Faculty of Physics and Technology, Donetsk National University, 600-richchia Str., 21, 21021 Vinnytsia-21021, Ukraine.
3 Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Helmholtzweg, 5, Jena-07743, Germany. 

Thermal agitation of the charge carriers in any circuit causes a small, yet detectable, current to flow. Quantitative analysis of thermal fluctuations showed that it is unaffected by the circuit geometry and material. Due to its proportionality to the electron temperature T, thermal (Johnson-Nyquist) noise SV = 4kBTR with R, the sample resistance has been employed as primary thermometry measuring the electron temperature. The relation that follows from the fundamental fluctuation-dissipation theorem has been verified many times for different systems. In the work [1], it was checked again for a nanoscale ring with ballistic electron transport through the quantum device. When one arm of the device was depleted of electrons, the agreement with the expected value of the Johnson-Nyquist noise was excellent while for the whole loop the authors detected an excess noise decreasing as T increases. 
Motivated by this finding, we have theoretically considered a nanoscale metallic loop coupled to two current-carrying leads. It is shown that in the presence of a transport current and related interference effects, persistent currents are flowing in the device, even in the absence of a magnetic field. Usually, clockwise and counterclockwise persistent currents compensate each other and do not reveal themselves in the device resistance while exhibit some inductance. In the case of partly reactive response, e.g., for resistive samples containing inductors, thermal noise surpasses that for a purely resistive component. We argue that it can be the origin of the excess thermal noise in the quantum nanorings.
This work has been supported by the joint German-Ukrainian project “Controllable quantum-information transfer in superconducting networks” (DFG project number SE 664/21). 

1.	Riha C., Buchholz S. S., Chiatti O., Wieck A. D., Reuter D., Fischer S. F. Excess noise in AlxGa1-xAs/GaAs based quantum rings // arXiv 1912.04397, preprint (unpublished).