Title: Reentrant Phase Coherence in Superconducting Nanowire Composites,
Abstract: The short coherence lengths characteristic of low-dimensional superconductors
are associated with usefully high critical fields or temperatures.
Unfortunately, such materials are often sensitive to disorder and suffer from
phase fluctuations in the superconducting order parameter which diverge with
temperature $T$, magnetic field $H$ or current $I$. We propose an approach to
overcome synthesis and fluctuation problems: building superconductors from
inhomogeneous composites of nanofilaments. Macroscopic crystals of
quasi-one-dimensional Na$_{2-\delta}$Mo$_6$Se$_6$ featuring Na vacancy disorder
($\delta\approx$~0.2) are shown to behave as percolative networks of
superconducting nanowires. Long range order is established via transverse
coupling between individual one-dimensional filaments, yet phase coherence
remains unstable to fluctuations and localization in the zero-($T$,$H$,$I$)
limit. However, a region of reentrant phase coherence develops upon raising
($T$,$H$,$I$). We attribute this phenomenon to an enhancement of the transverse
coupling due to electron delocalization. Our observations of reentrant phase
coherence coincide with a peak in the Josephson energy $E_J$ at non-zero
($T$,$H$,$I$), which we estimate using a simple analytical model for a
disordered anisotropic superconductor. Na$_{2-\delta}$Mo$_6$Se$_6$ is therefore
a blueprint for a future generation of nanofilamentary superconductors with
inbuilt resilience to phase fluctuations at elevated ($T$,$H$,$I$).