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Events at Physics

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R. G. Herb Condensed Matter Seminar
Electromagnetic coupling of spins and pseudospins in bilayer graphene
Date: Thursday, December 3rd
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Prof. Roland Winkler, Northern Illinois University
Abstract: We present a detailed theoretical study of bilayer-graphene's
electronic properties in the presence of electric and magnetic
fields. Using group-theoretical methods, we derive an invariant
expansion of the Hamiltonian for electron states near the K point of
the Brillouin zone. In contrast to known materials, including
single-layer graphene, any possible coupling of physical quantities
to components of the external electric field has a counterpart where
the analogous component of the magnetic field couples to exactly the
same combination of quantities. For example, a purely electric spin
splitting appears as the magneto-electric analogue of the familiar
magnetic Zeeman spin splitting. The measurable thermodynamic
response induced by magnetic and electric fields is thus completely
symmetric. The Pauli magnetization induced by a magnetic field
takes exactly the same functional form as the polarization induced
by an electric field. Our findings thus reveal unconventional
behavior of spin and pseudospin degrees of freedom induced by
external fields. Although they seem counterintuitive, our findings
are consistent with fundamental principles such as time reversal
symmetry. For example, only a magnetic field can give rise to a
macroscopic spin polarization, whereas only a perpendicular electric
field can induce a macroscopic polarization of the
sublattice-related pseudospin degree of freedom characterizing the
intravalley orbital motion in bilayer graphene. These rules
enforced by symmetry for the matter-field interactions clarify the
nature of spins versus pseudospins. While our theoretical arguments
use bilayer graphene as an example, they are generally valid for any
material with similar symmetries. The unusual equivalence of
magnetic and electric fields discussed here can provide the basis
for designing more versatile device architectures for creating
polarizations and manipulating the orientation of spins and
pseudospins.
Host: Joynt
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