Evidence for Thermally Activated Spontaneous Fluxoid Formation in Superconducting Thin-Film Rings

We describe measurements of the thermally-activated transitions between fluxoid states of a single isolated superconducting ring. We compare these measurements with theoretical predictions in which all of the relevant parameters are determined via independent characterization of the same ring. This no-free-parameters comparison shows qualitative agreement over a wide range of temperatures. We discuss possible origins for the remaining discrepancies between the data and theory...

We present a scanning probe technique for measuring the dynamics of individual fluxoid transitions in multiply connected superconducting structures. In these measurements, a small magnetic particle attached to the tip of a silicon cantilever is scanned over a micron-size superconducting ring fabricated from a thin aluminum film. We find that near the superconducting transition temperature of the aluminum, the dissipation and frequency of the cantilever changes significantly a...

We report the discovery of a new mechanism of spontaneous generation of a magnetic flux in a superconductor cooled through $T_c$. The sign of the spontaneous flux changes randomly from one cooldown to the next, and follows a Gaussian distribution. The width of the distribution increases with the size of the temperature gradient in the sample. Our observations appear inconsistent with the well known mechanisms of flux generation. The dependence on the temperature gradient sugg...

This article addresses the question whether the magnetic flux of stationary vortices or of half flux quanta generated by frustrated superconducting rings is noisy. It is found that the flux noise generated intrinsically by a superconductor is, in good approximation, not enhanced by stationary vortices. Half flux quanta generated by $\pi$-rings are characterized by considerably larger noise.

We carry out numerical simulations to investigate spontaneous vortex formation during a temperature quench of a superconductor film from the normal to the superconducting phase in the absence of an external magnetic field. Our results agree roughly with quantitative predictions of the flux trapping scenario: In fast quenches the agreement is almost perfect, but there appears to be some discrepancy in slower ones. In particular, our simulations demonstrate the crucial role the...

We evaluate the influence of thermal fluctuations on superconducting rings that enclose a magnetic flux, using the time-dependent Ginzburg--Landau (TDGL) or the Kramer--Watts-Tobin (KWT) model, while thermal fluctuations are accounted for by means of Langevin terms. This method is applicable in situations where previous methods are not, such as nonuniform loops, rings with large width to radius ratio and loops with large coherence length to perimeter ratio. We evaluate persis...

Fluctuations are important near phase transitions, where they can be difficult to describe quantitatively. Superconductivity in mesoscopic rings is particularly intriguing because the critical temperature is an oscillatory function of magnetic field. There is an exact theory for thermal fluctuations in one-dimensional superconducting rings, which are therefore expected to be an excellent model system. We measure the susceptibility of many rings, one ring at a time, using a sc...

We use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image individual vortices in amorphous superconducting MoSi thin films. Spatially resolved measurements of the magnetic field generated by both vortices and Meissner screening satisfy the Pearl model for vortices in thin films and yield values for the Pearl length and bulk penetration depth at 4.2 K. Flux pinning is observed and quantified through measurements of vortex motion driven by b...

We report magneto-transport measurements on a superconducting molybdenum-germanium (MoGe) film of thickness $d$=50 nm in parallel magnetic fields and show evidence of a transition from a Meissner state to a resistive state of spontaneous perpendicular vortices generated by thermal fluctuations above a certain temperature $T>T_v(B)$. Here $T_v$ appears to match the vortex core explosion condition $d\approx 4.4\xi(T_v)$, where $\xi$ is the coherence length. For $T>T_v$, we obse...

A recent experiment that sees the spontaneous creation of magnetic flux on quenching high-$T_c$ films has shown that earlier null results were a consequence of the lack of saturation of the Zurek-Kibble causal bounds against which flux density was measured. In this letter we estimate how efficient the production of topological charge is in planar systems, both for the aforementioned experiment (when flux measures topological charge) and for an earlier experiment on planar liq...