  Today solid-state cooling technologies below liquid nitrogen boiling
temperature (77 K), crucial to quantum information technology and probing
quantum state of matter, are greatly limited due to the lack of good
thermoelectric and/or thermomagnetic materials. Here, we report the discovery
of colossal Nernst power factor of 3800$\times$10$^{-4}$ W m$^{-1}$ K$^{-2}$
under 5 T at 25 K and high Nernst figure-of-merit of 71$\times$10$^{-4}$
K$^{-1}$ under 5 T at 20 K in topological semimetal NbSb$_2$ single crystals.
The observed high thermomagnetic performance is attributed to large Nernst
thermopower and longitudinal electrical conductivity, and relatively low
transverse thermal conductivity. The large and unsaturated Nernst thermopower
is the result of the combination of highly desirable electronic structures of
NbSb$_2$ having compensated high mobility electrons and holes near Fermi level
and strong phonon-drag effect. This discovery opens an avenue for exploring
material option for the solid-state heat pumping below liquid nitrogen
temperature.
