Fig. 3. The temperature range and length scale of fluctuations grow towards the critical point.
(a) Susceptibility maps at different temperatures in S1. For T > Tc no diamagnetic response is observed and the image reflects the SQUID noise. For 0.55Tc < T < Tc the image shows the presence of darker puddles and streaks of weaker superconductivity that survive well below Tc, (see the full sequence in the supplementary movie) (b) A histogram of χ values for three images in the bottom row of (a). The wide range of χ values highlights the fluctuation activity in comparison to the narrow range of χ values above Tc. (c) Resistance and susceptibility vs. temperature for a series of NbTiN samples approaching the QPT. The vertical bars represent the span of the susceptibility values in the imaged area. These values are the limits of the color bars in e. (d) Standard deviation (STD) vs. temperature extracted from χ-maps (several are shown in panel (a)). The vertical bars show the range of susceptibility values in each image. The STD is normalized to the highest value in each curve. (e) Susceptibility maps for samples S1, S2 and S3, chosen at the peak of the STD curve (arrows in (d) mark the peak temperature). Each scan contains 13k pixels with a pixel size of 120x120 nm2. Data acquisition per pixel is 45 ms. Scale bar is 3 μm for all images.