Thermal FlucTuaTions in superconducTor/FerromagneT nanosTripes

LLE Review, Volume 142 97 Introduction The problem of fluctuations in two-dimensional (2-D) superconducting stripes with a thickness d that is much smaller than the London penetration depth m and a width w that is much smaller than the Pearl length K = 2m2/d & w has been extensively discussed in the context of the Berezinsky– Kosterlitz–Thouless (BKT) transition.1,2 The interest in this topic was revived recently3,4 to try to better understand the physics of the operation of superconducting single-photon detectors (SSPD’s) consisting of nanostripes that are densely packed into a meander-type geometry. The first SSPD’s introduced in 2001 (Ref. 5) have since received great attention because of their excellent performance as ultrafast, highly efficient counters for both infrared and visible light photons and are now regarded as the devices of choice in such high-performance applications such as quantum optics and quantum communications.6,7 The SSPD basic model of operation principle is based on a supercurrent-to-resistive-state transition of a 2-D nanostripe maintained at a temperature far below the critical temperature Tc and biased sufficiently close to its critical current Ic. The energy of one or several optical photons absorbed in the nanostripe is sufficient to trigger the transition, producing a transient resistive state and resulting in a detection event.