arXiv subjects

Carlos Pinilla

Publications and source records attributed to Carlos Pinilla.

3 recordsLinked to original sources

arXiv2022

Axion insulators protected by C2T and their K-theory invariants and material realization

Axion insulators are generally understood as magnetic topological insulators whose Chern-Simons axion coupling term is quantized and equal to $\pi$. Inversion and time reversal, or the composition of either one with a rotation or a translation, are symmetries which protect this invariant. In this work, we focus our attention on the composition of a 2-fold rotation with time reversal, and we show that insulators with this symmetry possess a Z2 invariant arising from Atiyah's real K-theory. We call this invariant the K-theory Kane-Mele invariant due to the similarities it has with the Kane-Mele invariant for systems with time-reversal symmetry. Whenever all Chern numbers vanish, we demonstrate that this invariant is equivalent to the Chern-Simons axion coupling, and in the presence of the inversion symmetry, we show how this invariant could be obtained from the eigenvalues of the inversion operator on its fixed points in momentum space. For the general case of non-trivial Chern numbers, the Chern-Simons axion coupling term incorporates information of the K-theory Kane-Mele invariant as well as information regarding bands with non-trival Chern numbers. An explicit formula in terms of K-theory generators is presented for the Chern-Simons axion coupling term, the relation with the K-theory Kane-Mele invariant is explained, and a formula in terms of eigenvalues of the inversion operator is obtained. Using an effective Hamiltonian model and first-principles calculations, we also show that the occurrence of bulk-band inversion and nontrivial K-theory Kane-Mele invariant index can be observed in axion insulators of the pnictides family. In particular, we demonstrate that NpBi can be classified as an axion insulator due to the detection of additional topological indicators such as the quantum spin Hall effect, gapped surface states, surface quantized anomalous Hall effect, and chiral hinge modes

cond-mat.mtrl-sci↗

arXiv2022

On quasi-nodal spheres and the spin Hall effect: the case of YH3 and CaTe

Band inversion is a known feature in a wide range of topological insulators characterized by a change of orbital type around a high-symmetry point close to the Fermi level. In some cases of band inversion in topological insulators, the existence of quasinodal spheres has been detected, and the change of orbital type is shown to be concentrated along these spheres in momentum space. To understand this phenomenon, we develop a local effective fourfold Hamiltonian that models the band inversion and reproduces the quasinodal sphere. This model shows that the signal of the spin Hall conductivity, as well as the change of orbital type, are both localized on the quasinodal sphere, and moreover, that these two indicators characterize the topological nature of the material. Using K-theoretical methods, we show that the change of orbital type parametrized by an odd clutching function is equivalent to the strong Fu-Kane-Mele invariant. We corroborate these results with ab initio calculations for the materials YH3 and CaTe, where in both cases the signal of the spin Hall conductivity is localized on the quasinodal spheres in momentum space. We conclude that a nontrivial spin Hall conductivity localized on the points of change of orbital type is a good indicator for topological insulation.

cond-mat.mtrl-sci↗

arXiv2013

High Energy Density Mixed Polymeric Phase From Carbon Monoxide and Nitrogen

Carbon monoxide and nitrogen are among the potentially interesting high-energy density materials. However, in spite of the physical similarities of the molecules, they behave very differently at high pressures. Using density functional theory and structural prediction methods, we examine the ability of these systems to combine their respective properties and form novel mixed crystalline phases under pressures of up to 100 GPa. Interestingly, we find that CO catalyzes the molecular dissociation of N2, which means mixed structures are favored at a relatively low pressure (below 18 GPa), and that a three-dimensional framework with Pbam symmetry becomes the most stable phase above 52 GPa, i.e., at much milder conditions than in pure solid nitrogen. This structure is dynamically stable at ambient pressure and has an energy density of approximately 2:2 kJ/g, making it a candidate for a high-energy density material, and one that could be achieved at less prohibitive experimental conditions.

cond-mat.mtrl-sci↗