69 research outputs found
Band Topology and Linking Structure of Nodal Line Semimetals with Z2 Monopole Charges
We study the band topology and the associated linking structure of
topological semimetals with nodal lines carrying monopole charges,
which can be realized in three-dimensional systems invariant under the
combination of inversion and time reversal when spin-orbit coupling is
negligible. In contrast to the well-known -symmetric nodal lines protected
only by Berry phase in which a single nodal line can exist, the nodal
lines with monopole charges should always exist in pairs. We show that
a pair of nodal lines with monopole charges is created by a {\it double
band inversion} (DBI) process, and that the resulting nodal lines are always
{\it linked by another nodal line} formed between the two topmost occupied
bands. It is shown that both the linking structure and the monopole
charge are the manifestation of the nontrivial band topology characterized by
the {\it second Stiefel-Whitney class}, which can be read off from the Wilson
loop spectrum. We show that the second Stiefel-Whitney class can serve as a
well-defined topological invariant of a -invariant two-dimensional (2D)
insulator in the absence of Berry phase. Based on this, we propose that pair
creation and annihilation of nodal lines with monopole charges can
mediate a topological phase transition between a normal insulator and a
three-dimensional weak Stiefel-Whitney insulator (3D weak SWI). Moreover, using
first-principles calculations, we predict ABC-stacked graphdiyne as a nodal
line semimetal (NLSM) with monopole charges having the linking
structure. Finally, we develop a formula for computing the second
Stiefel-Whitney class based on parity eigenvalues at inversion invariant
momenta, which is used to prove the quantized bulk magnetoelectric response of
NLSMs with monopole charges under a -breaking perturbation.Comment: 4+28 pages, 3+17 figure
Substantial optical dielectric enhancement by volume compression in LiAsSe
Based on first-principles calculations, we predict a substantial increase in
the optical dielectric function of LiAsSe under pressure. We find that the
optical dielectric constant is enhanced threefold under volume compression.
This enhancement is mainly due to the dimerization strength reduction of the
one-dimensional (1D) As--Se chains in LiAsSe, which significantly alters
the wavefunction phase mismatch between two neighboring chains and changes the
transition intensity. By developing a tight-binding model of the interacting 1D
chains, the essential features of the low-energy electronic structure of
LiAsSe are captured. Our findings are important for understanding the
fundamental physics of LiAsSe and provide a feasible way to enhance the
material optical response that can be applied to light harvesting for energy
applications.Comment: 13 pages, 6 figure
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