  Microscopic imaging based on nitrogen-vacancy (NV) centres in diamond, a tool
increasingly used for room-temperature studies of condensed matter systems, has
recently been extended to cryogenic conditions. However, it remains unclear
whether the technique is viable for imaging temperature-sensitive phenomena
below 10 K given the inherent laser illumination requirements, especially in a
widefield configuration. Here we realise a widefield NV microscope with a field
of view of 100 $\mu$m and a base temperature of 4 K, and use it to image
Abrikosov vortices and transport currents in a superconducting Nb film. We
observe the disappearance of vortices upon increase of laser power and their
clustering about hot spots upon decrease, indicating that laser powers as low
as 1 mW (4 orders of magnitude below the NV saturation) are sufficient to
locally quench the superconductivity of the film ($T_c = 9$ K). This
significant local heating is confirmed by resistance measurements, which reveal
the presence of large temperature gradients (several K) across the film. We
then investigate the effect of such gradients on transport currents, where the
current path is seen to correlate with the temperature profile even in the
fully superconducting phase. In addition to highlighting the role of
temperature inhomogeneities in superconductivity phenomena, this work
establishes that, under sufficiently low laser power conditions, widefield NV
microscopy enables imaging over mesoscopic scales down to 4 K with a
submicrometer spatial resolution, providing a new platform for real-space
investigations of a range of systems from topological insulators to van der
Waals ferromagnets.
