Condensed Matter Physics Seminar

Spin Thermopower and the Electronic Phase Diagram of Na_xCoO₂

Dr. Yayu Wang
UC Berkeley
Tuesday, October 24, 2006
Higgins 235, 3pm

The layered cobaltate Na_xCoO₂ has emerged as a new model system to study
the behavior of electrons in strongly correlated materials. A rich
variety of intriguing phenomena, such as anomalous charge transport
properties and water-intercalated superconductivity, have been discovered
in this system. To elucidate the origin of the strongly enhanced
thermopower in the x ~ 0.70 phase, we have studied the variation of
thermopower in an applied longitudinal magnetic field. We found that at
low temperatures, a large enough field can suppress the thermopower all
the way to zero, a behavior incompatible with conventional metal theory.
Both the field-dependence of thermopower and the scaling property can be
explained remarkably well by ascribing the transport entropy to the spin
degree of freedom. This finding not only confirms a classical quantum
mechanical prediction, but also provides an important new clue for finding
novel thermoelectric materials. To better understand the mechanism of the
“wet” superconductivity in the water-intercalated compound, we first
focused on the electronic and magnetic properties of the undoped phase.
Na_xCoO₂ single crystals with a wide range of Na contents have been
synthesized using a series of chemical reactions. We have established an
experimental phase diagram of the Na_xCoO₂ system which involves two
distinct metallic states separated by a charge-ordered insulator in the
middle. The implications of these results to the exotic superconductivity
will be discussed.