Tunable topological semimetal states with ultraflat nodal rings in strained YN

Using first-principles calculations and effective model analysis, we demonstrate that the strained WC-type YN can host an ideal topological semimetal state with an ultraflat nodal ring in the absence of spin-orbit coupling. Distinguished from previous nodal-line semimetals, the nodal ring in YN lies in a single plane of the Brillouin zone with an energy variation less than 0.3 meV; that is, the nodal ring is flat in both momentum and energy space. Interestingly, by applying a uniaxial strain along the c axis, one can effectively tune the size of the nodal ring and even realize a topological phase transition from nodal-ring semimetals to normal insulators. The nodal ring here is topologically nontrivial, as confirmed by directly calculating the topological invariant and topological drumhead surface state. Moreover, due to an anisotropic torus Fermi surface and the nontrivial topology, exotic magnetoresponse effects, including anomalous Landau level structures and direction-dependent phase shifts of quantum oscillations with and without a Berry phase of pi, can be probed experimentally.