Thermal fluctuations in superconductor/ferromagnet nanostripes

作     者：U. Nasti L. Parlato M. Ejrnaes R. Cristiano T. Taino H. Myoren Roman Sobolewski G. Pepe 

作者机构：Dipartimento di Fisica Università degli Studi di Napoli Federico II I-80125 Napoli Italy CNR-SPIN Institute of Superconductors Innovative Materials and Devices UOS-Napoli I-80125 Napoli Italy Graduate School of Science and Engineering Saitama University 338-8570 Saitama Japan Institute of Electron Technology PL-02668 Warszawa Poland Departments of Electrical and Computer Engineering and Physics and Astronomy and Laboratory for Laser Energetics University of Rochester New York 14627-0231 USA 

出 版 物：《Physical Review B》 (物理学评论B辑：凝聚态物质与材料物理学)

年 卷 期：2015年第92卷第1期

页      面：014501-014501页

核心收录：

学科分类：07[理学] 0702[理学-物理学] 

主　　题：TEMPERATURE RANGE Bilayers Theoretical Models Superconductivity Vortex motion Bias currents vortices Critical current density fluctuations Thermal 

摘      要：Thermal fluctuations in hybrid superconductor/ferromagnetic NbN/NiCu bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from 4.2to8K. In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a 6−nm−thick NiCu film grown on top of 8−nm−thick NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in NbN/NiCu bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.