Magnetoelectric Coupling in Elemental Selenium: A Consequence of Electronic Topology and Chirality

ABSTRACT: 
Materials exhibiting a static linear magnetoelectric coupling are relatively rare because (a) the symmetry restriction that such a material must have broken time reversal and spatial inversion symmetries, and (b) magnetic moment arises from the electrons occupying d-orbitals which chemically favor maintaining the inversion symmetry. Multiferroic materials with simultaneous ferroelectric and ferromagnetic ordering are promising candidates to exhibit a large magnetoelectric effect. However, these are typically ternary oxides with a relatively complex crystal and electronic structure. We show how a simple crystalline form of elemental Selenium exhibits multiferroic properties as a consequence of its electronic topology and quasi-one dimensional chiral structure. Using first-principles quantum mechanical density functional theory calculations, we show that bulk Se is a weak topological insulator with a half-integer quantum polarization, and the spin polarization of its surface electronic states gives the observed magnetism. We use a Landau-like theory for the free energy of the Selenium surface based on the coupling of phonons with spin and electric field, and explain the experimental observations of magnetoelectric coupling and magnetic field dependent Raman spectra . We find that the chiral structure and the non-trivial electronic topology are unique to selenium among the elemental solids. Further, we predict the bulk elemental Selenium to show Vibrational Circular Birefringence (VCB), i.e. it rotates the plane of polarization of electromagnetic (IR) radiation passing through it. The origin of such optical rotatory property of trigonal Selenium is in the non-zero magnetoelectric coupling associated with its infrared active phonons. 

In the beginning of the colloquium, I will give a relatively long introduction to multiferroic materials and topological insulators.

Work done in collaboration with Sharmila N. Shirodkar, Anirban Pal, Smita Gohil, Shankar Ghosh and Pushan Ayyub.