Informal Physics Seminar: Pseudospins and TimeReversal Invariance in the Trigonal Band Structure of Graphene
CFM Seminars
 Speaker

Roland Winkler, Argonne National Laboratory and Northern Illinois University, USA. Currently Ikerbasque sabbatical fellow at UPV, Bilbao
 When

2011/11/29
13:00  Place
 Auditorium of the Centro de Fisica de Materiales, Paseo Manuel de Lardizabal 5, Donostia
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Pseudospins and TimeReversal Invariance in the Trigonal Band Structure of
Graphene
R. Winkler 1,2,3,4
currently
1The University of the Basque Country, Bilbao
2IKERBASQUE, Basque Foundation for Science
3Argonne National Laboratory, Argonne, IL 60439, USA
4Northern Illinois University, DeKalb, IL 60115, USA
In recent years the unusual band structure of graphene (a single layer of
graphite) has raised significant interest. While for most materials the band
dispersion E(k) near a band extremum is a quadratic function of the wave
vector k, the band dispersion of graphene is linear, thus resembling the
spectrum of massless Dirac fermions. We will present a systematic symmetry
analysis of the band structure in graphene near the Fermi edge. We will show
that the klinear dispersion arises as an unusual consequence of the interplay
of timereversal invariance and the trigonal symmetry of the electron states.
The symmetry analysis allows us to incorporate perturbations such as external
electric and magnetic fields, strain, and spin orbit coupling in a systematic
way, so that our findings are directly relevant for comparison with
experiments.
(Pseudo) spin degrees of freedom which occur in the Diraclike Hamiltonian of
graphene have raised significant interest due to possible applications in
spintronics and quantum com puting. In general we need to distinguish between
systems incorporating the electrons’ proper spin degree of freedom and
systems utilizing pseudospin degrees of freedom showing spinlike properties.
Here we compare in detail the properties of spin and pseudospin degrees of
freedom. We show that time reversal θ allows one to distinguish the proper
spin degree of freedom from common examples of pseudospin degrees of freedom.
We demonstrate that halfinteger pseudospin degrees of freedom can be either
fermionic (characterized by theta^2 = −1) or bosonic (theta^2 = +1). Spin
dependent transport provides a means to distinguish these two cases. If the
system is fermionic, it can show weak antilocalization whereas a bosonic
pseudospin degree of freedom gives rise to weak localization.
Work done in collaboration with Ulrich Zulicke (Victoria University, New
Zealand).