Energy-gap driven low-temperature magnetic and transport properties in Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta)

The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soltion lattice (CSL) stabilized in Cr$_{1/3}$NbS$_2$ in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations demonstrating that Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) are half-metals whose low-temperature electronic and magnetic behavior can be explained by the presence of a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase.