The thesis of Wei-Li Lee, entitled
"The Anomalous Hall Effect and Related Transport Phenomena in
Ferromagnetic Spinel CuCr_2Se_{4-x}Br_x, Mn-doped Chalcopyrite
CuGaTe_2, and Ruthenate Bi_3Ru_3O_{11}", has been placed on deposit.
Any member of the University wishing to read the thesis may do
so. Any objections should be submitted to me in writing. The
principal advisor for this work was Professor N. Phuan Ong.
ABSTRACT
One of the interesting and unsettled transport phenomena is the,
so called, ``anomalous Hall Effect". It generally refers to the
phenomenon of a non-linear field dependence of the Hall
resistivity \rho_{xy}. An emerging idea relates to the ''gauge
field \Omega({\vec k})" experienced by electrons. It gives rise
to Berry phase accumulation. In ferromagnets, a non-vanishing
\Omega({\vec k}) is related to the spin-orbit coupling and the
time-reversal asymmetry. we show that, in the ferromagnetic
spinel CuCr_2Se_{4-x}Br_x, the anomalous Hall conductivity
\sigma_{xy} (normalized to per carrier, at 5K) remains unchanged
with a 1000-fold increase in resistivity. From the anomalous
Nernst effect, we uncover a simple relation for the off-diagonal
Peltier conductivity tensor \alpha_{xy}, which is a measure of a
transverse electrical current density generated per unit of
applied temperature gradient. They both strongly support the
anomalous-velocity theory based on the intrinsic spin-or bit
coupling of the electrons. An alternative way to procure
\Omega({\vec k}) is from the spin-chirality effect. In the
Mn-doped chalcopyrite semiconductor CuGaTe_2, with a few percent
doping, the orbitals of the Mn ions overlap to form an impurity
band. Therefore, the electrons will accumulate Berry phase while
hopping around the Mn ions that have a non-collinear magnetic
ground state. We observed the enhanced and non-monotonic field
dependence of \rho_{xy}, which may be understood from the
spin-chirality effect. Finally, we studied the Hall effect and
thermopower n the ruthenate Bi_3Ru_3O_{11}. From the Hall effect,
we observed field-tuning of electron and hole populations. We
also found a large field dependence of the thermopower at low
temperature. The origin of the colossal field-dependence in the
thermopower is still an open question, but it is linked to the
unusual electronic properties in the Bi_3Ru_3O_{11}.
Daniel Marlow
Chair, Dept. of Physics
