Condensed Matter Seminar

Suhas Gangadharaiah
University of Utah

Tuesday, August 26, 2008; 4:00 pm in JFB 334

Novel effects due to relativistic-like spectrum in solid state problems

In the first half of the talk I will discuss spin-spin interaction between electrons in spatially separated quantum dots. Typically spin interactions between electrons in such an arrangement are of the exchange type and are anti-ferromagnetic in nature. We show that in the presence of the relativistic spin-orbit interaction (of Rashba type) in the dots and Coulomb interactions between electrons, a new anisotropic coupling of the van der Waals type between spin emerges. Unlike the standard exchange, the van der Waals type interaction is ferromagnetic with power law decay and does not require the wave functions to overlap. We argue that this ferromagnetic interaction is important in the Wigner crystal state where the exchange processes are severely suppressed.

In the second half, I will discuss electron-electron interaction effects in graphene. Many properties of interacting electrons, in graphene, with a linear Dirac spectrum differ sharply from those with the parabolic spectrum present in conventional semi-conductor heterostructures. It has previously been argued, using random phase approximation (RPA), that plasmons in undoped graphene are absent. We show that one needs to consider vertex terms for an appropriate description of electron-hole excitations. These vertex terms describe excitonic effect and are responsible for reducing the energy required to create an electron-hole pair. The response function acquires a new non-perturbative regime where plasmonic excitations can take place.