J Steven Dodge, Department of Physics, Simon Fraser University, Burnaby, B.C, Canada
Experiments yield contradictory results on the degree to which charge carriers are localized in doped copper oxides. Angle-resolved photoemission spectroscopy suggests that a single hole is localized as a small polaron. Optical measurements also indicate that bound holes form small polarons, but that free holes are best described as large polarons. Finally, DC transport measurements on lightly doped compounds give surprisingly high carrier mobility, suggesting that the charge excitations become more extended with increasing carrier density. As an alternative to DC transport measurements on chemically doped samples, we use visible-pump, terahertz-probe spectroscopy to determine the mobility of carriers photoexcited in the insulating layered cuprates Sr2CuO2Cl2, YBa2Cu3O6, and La2CuO4. We find that the initial mobility approaches that of lightly doped cuprates, then decays on picosecond timescales to values more consistent with hopping transport.
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