Fig. 1. Scanning SQUID measurements near the quantum phase transition (QPT) show fluctuations in space and time.
(a) A schematic diagram of a QPT as a function of a non thermal tuning parameter g (here, the resistance per square at a temperature just above Tc). (b) An illustration showing many ξpair grains. Red arrows represent the phase degree of freedom. Phase fluctuations weaken the Josephson coupling between the grains and create weak superconducting regions on a micron scale which we note as λdia, marked in red. (c) An optical image of the SQUID susceptometer, showing the sensing area (the pick-up loop) and the field coil. (d) A scanning SQUID susceptibility image in sample S1, showing weaker and stronger areas of superconductivity. The weaker areas appear as streaks in the scan direction marked by arrows. The scan is taken at 0.6Tc, far below the superconducting transition (scale bar 5 μm). The image contains 100k pixels with a pixel size of 120×120 nm2. Data acquisition per pixel is 40 ms. (e) The streaks are dynamic. The SQUID parked in a specific location records the susceptibility as a function of time. The streaks manifest themselves as telegraph-like noise in the susceptibility signal with discrete jumps over a wide range of time scales.