Distorted Weyl cones under perturbation & Monopole stabilization in interpenetrating pyrochlore lattices

Abstract

In Weyl semimetals the Fermi velocity associated with the approximately linear dispersion of the low-energy quasiparticles near the band touching point is much smaller than the fundamental speed of sound in vacuum. Consequently their conical spectrum is not bound by Lorentz symmetry and can develop tilts and anisotropies, as has been found in real-life materials such as WTe2. Furthermore, perturbations like disorder and Coulomb interactions can obfuscate and complicate the picture that emerges from the free, fully symmetric Weyl fermion model. Our goal is now to investigate how the interplay of these disturbances impacts the scaling behaviour of observables depending on the Density of States, which nominally vanishes at the free theory nodal point. The pyrochlore lattice is composed of corner-sharing tetrahedra that make it impossible to satisfy all the antiferromagnetic interactions between the Ising-like spins. In such spin ice models lowest energy states obey the spin ice rule, which states that every ground state tetrahedron must have two spins pointing inwards and two spins pointing outwards. This can be interpreted as a divergence-free condition of an effective field, with as many of its field lines leaving the tetrahedron as are entering it. Excitations above this extensively degenerate ground state manifold then obtain an interpretation as pairs of sources and sinks of this field. These monopoles are detectable in for example neutron scattering experiments and influence the low-temperature transport behavior of spin ice materials. We have investigated perturbations to a particular set-up in which two of these pyrochlore lattices interpenetrate and their spins interact, resulting in an effective chemical potential for the monopoles that can stabilize them.