Robust Topological Nodal-Line Semimetals from Periodic Vacancies in Two-Dimensional Materials.

A nodal-line semimetal (NLSM) is suppressed in the presence of spin-orbit coupling unless it is protected by a nonsymmorphic symmetry. We show that two-dimensional (2D) materials can realize robust NLSMs when vacancies are introduced on the lattice. As a case study we investigate borophene, a boron honeycomb-like sheet. While the Dirac cones of pristine borophene are shown to be gapped out by spin-orbit coupling and by magnetic exchange, robust nodal lines (NLs) emerge in the spectrum when selected atoms are removed. We propose an effective 2D model and a symmetry analysis to demonstrate that these NLs are topological and protected by a nonsymmorphic glide plane. Our findings offer a paradigm shift to the design of NLSMs: instead of searching for nonsymmorphic materials, robust NLSMs may be realized simply by removing atoms from ordinary symmorphic crystals.

A scaling approach for interacting quantum wires--a possible explanation for the 0.7 anomalous conductance.

We consider a weakly interacting finite wire with short and long range interactions. The long range interactions enhance the 4k(F) scattering and renormalize the wire to a strongly interacting limit. For large screening lengths, the renormalized charge stiffness Luttinger parameter K(eff) decreases to [Formula: see text], giving rise to a Wigner crystal at T=0 with an anomalous conductance at finite temperatures. For short screening lengths, the renormalized Luttinger parameter K(eff) is restricted to ½≤K(eff)≤1. As a result, at temperatures larger than the magnetic exchange energy we find an interacting metal which, for K(eff)≈½, is equivalent to the Hubbard U−>∞ model, with the anomalous conductance G≈e(2)/h.