Tunable Octdong and Spindle-Torus Fermi Surfaces in Kramers Nodal Line Metals

It has recently been proposed that all achiral non-centrosymmetric crystals host so-called Kramers nodal lines, which are doubly degenerate band crossings connecting time-reversal invariant momenta in the Brillouin zone that arise due to spin-orbit coupling.
When Kramers nodal lines intersect the Fermi level, they form exotic three-dimensional Fermi surfaces which, depending on the configuration, can be fully described by two-dimensional massless Dirac fermions. These Fermi surfaces are predicted realize an enhanced form of graphene-like physics, including a quantized optical conductivity with multiple quantized levels in the thin-film limit and a zero onset frequency in the bulk, as well as a giant light- and field-induced anomalous Hall effect. However, until now, no Kramers nodal line metal with such unconventional Fermi surfaces has been experimentally observed.
Here, we extend the search for Kramers nodal line metals beyond the previously considered case in which the Fermi surfaces enclose a single time-reversal invariant momentum. Using angle-resolved photoelectron spectroscopy measurements and ab-initio calculations, we present evidence that the 3R polytypes of TaS$_2$ and NbS$_2$ are Kramers nodal line metals with open Octdong and Spindle-torus Fermi surfaces, respectively. We show that by reducing the band filling, a transition between these two configurations can be observed. Moreover, our data suggests a naturally occurring size quantization effect of inclusions of 3R-TaS$_2$ in commercially available 2H-TaS$_2$ crystals, which could enable the observation of quantized optical conductivity. Finally, since the open Fermi-surfaces encircle two time-reversal invariant momenta each, we predict a phase transition from a Kramers nodal line metal to a conventional metal by strain or uniaxial pressure. Our work establishes the 3R phase of metallic transition metal dichalcogenides as tunable materials platform to explore new phenomena expected from exotic Fermi surfaces in Kramers nodal line metals.