Fermiology and type-I superconductivity in the chiral superconductor NbGe2 with Kramers-Weyl fermions

In this paper we present a systematic study of the de Haas-van Alphen (dHvA) oscillations, magnetotransport, and superconductivity in the Kramers-Weyl semimetal candidate NbGe2. We show that NbGe2 is a type-I chiral superconductor with a Tc of 2.06 K, a full gap, a type-II to type-I crossover around 1.5 K, and an enhanced critical field from filamentary superconductivity suggested by resistivity measurements. The study of the dHvA oscillations reveals three distinct frequency branches β and the spin-split α/α′, with cyclotron effective masses of 1.21(5)me, 0.21(1)me, and 0.24(1)me, respectively. The magnetoresistance of NbGe2 exceeds 1000% at 2.4 K and 9 T and exhibits quasilinear behavior at low temperatures. The comparison between experimental results and first-principles calculations shows excellent agreement between the two and suggests that α and α′ correspond to the hole Fermi pockets centered around the Weyl M point, while β is associated with the hole pocket near the trivial H point in the Brillouin zone. Furthermore, we show that a Van Hove singularity arising from the contributions of the M and H points at the Fermi level, together with the calculated electron-phonon coupling, is strong enough to account for the observed superconductivity. Last, our calculation of the conductivity tensor within the relaxation time approximation suggests that the observed linear magnetoresistance most likely arises from the presence of open Fermi surfaces.