Vortex flow and shot noise in the quantum-liquid phase of thick amorphous superconducting films

Abstract

We measure the fluctuations ͑noise͒ of the vortex-flow voltage in the low-temperature liquid phase of thick amorphous ͑a-͒Mg x B 1−x and a-Mo x Si 1−x films with different transition temperatures and widths of the quantum-vortex-liquid ͑QVL͒ phase. An anomalous vortex flow with an asymmetric distribution of ͑real-time͒ voltage fluctuations is commonly observed in the QVL phase, independent of material as well as strength of quantum fluctuations. In the QVL phase we observe the Lorentzian-type noise spectra indicative of shot noise, whose origin is attributed to the vortex bundles that are depinned and pinned randomly in the stationary flow of QVL.

Vortex Phase Diagram and Dynamics near T = 0 Probed by Voltage Noise in Amorphous Superconducting Films

“…In contrast, at 0.10 K («7Q) in 12.62 T (i.e., in the QVL phase or the crossover regime between the TVL and QVL phases) the contribution of 8V(t) from the vortex motion is clearly visible, consistent with the results for the thick a-Mo x Sii_ x and a-MgcBi-x films with moderately strong pinning [9,10]. In contrast, at 0.10 K («7Q) in 12.62 T (i.e., in the QVL phase or the crossover regime between the TVL and QVL phases) the contribution of 8V(t) from the vortex motion is clearly visible, consistent with the results for the thick a-Mo x Sii_ x and a-MgcBi-x films with moderately strong pinning [9,10].…”

“…Following the same procedure as performed in our previous work [9,10], we calculate the skewness A to quantify the degree of the asymmetry of the probability distribution P(8V). Below 7Q quantum tunneling may become remarkable in the presence of /, which assists the vortex bundles to depin from their pinning sites [10,12]. In Figs.…”

“…Even at zero temperature (T = 0), where thermal fluctuations are absent, the vortex solid is able to melt into the liquid in the presence of strong quantum fluctuations. Recently, we have found the anomalous (noisy) vortex flow in the QVL phase [9,10], whose origin is attributed to the vortex bundles that are depinned and pinned randomly in the stationary flow of QVL. We have provided evidence for the QVL phase at low T using thick amorphous films with moderately strong pinning (a-MoxSii-x [7,8] and a-MgxBi-x).…”

Order–disorder transition and flow noise in an film

“…Our work clearly shows that S V is a very powerful tool to detect the phase transition(s) in the vortex-solid phase [14], where the linear resistance R is always zero, as well as in the liquid phase [16,17], which cannot be detected by ordinary static measurements (e.g., dc voltage V or R). From an experimental point of view, in order to detect such transitions clearly, we must select an appropriate value of V in measuring S V .…”

Broadband Noise Across the Mode-Locking Freezing in Driven Vortex Matter

“…In measuring Sv{f) in a broad frequency range / =1 Hz-40 kHz, the voltage enhanced with a preamplifier was analyzed with a fast-Fourier transform spectrum analyzer. 27,28 To reduce external noise coming from the current source, in case of SV(/) measurements, in particular, we used batteries as the dc current source. 18 We obtained the excess noise spectra Sv{f) by subtracting the background contribution, which was measured with / = I r f = 0.…”