Mesoscopic and Condensed Matter Physics

Biography

Biography: Gayan Prasad Hettiarachchi

Abstract

Insulator-to-metal transitions (IMTs) still remain a central theme in condensed-matter physics despite many zealous experimental and theoretical efforts. The persistence of this question for decades can be attributed to the complexities that arise due to strong correlation-effects and many degrees of freedom that exist in real systems. The seminal work of Mott introduced insulating character originating from strong on-site Coloumb repulsion energy U. The ratio of U and Hubbard bandwidth W (U/W) is a critical parameter that tips the balance of insulating and metallic states. However, in real systems, correlation effects are not limited to these two parameters, and effects from intra-atomic exchange energy, orbital degeneracy, crystal-field splitting, etc., come into the equation. For electrons in a deformable lattice, electron-phonon interaction energy S and the induced deformations become crucial parameters that govern the evolution of the trasfer integral t. Aluminosilicate zeolites (M_aAl_aSi_bO_2(a+b)) provide an ideal, but non-trivial playground for exploring such interactions. The negative charge of Al_aSi_bO_2(a+b) framework is balanced by the cations M_a and present a deformable lattice. Guest electrons can be introduced into this deformable lattice through encapsulation of guest atoms. Experimental investigations show that even with same/comparable effective U, subltle changes in the deformable lattice (in turn change in the deformation potential), and tuning of S by changing M can give rise to different ground states. The competition between U, t, and S together with the electron density govern the balance between insulating vs. metallic states and magnetic vs. nonmagentic states directing the dscussions of IMTs towards a relatively old but less discussed branch, the “polaron physics”.