Measurement of the Femtosecond Optical Absorption of LaAlO_3/SrTiO_3 Heterostructures:Evidence for an Extremely Slow Electron Relaxation at the Interface

摘要

The photocarrier relaxation dynamics of an n-type LaAlO_3/SrTiO_3 heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both LaAlO_3/SrTiO_3 heterostructures and electron-doped SrTiO_3 bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the LaAlO_3/SrTiO_3 heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the t_(2g) subbands due to the interface potential.