CM-AMO Seminar | Electronic Compressibility and Magnetization of LaAlO3/SrTiO3 Heterostructure Interface

Speaker: Dr. Lu Li, Pappalardo Fellow and U-M Faculty Candidate (M.I.T.)

The LaAlO3/SrTiO3 heterostructure is a potential candidate for a high mobility two-dimensional electron system with novel electronic and magnetic properties. Though LaAlO3 and SrTiO3 are both large-gap band insulators, the interface is conductive, and even superconducting below 200 mK. In this talk, we describe results from two thermodynamic measurements: the electronic compressibility and the magnetization. First, the electronic compressibility is measured with capacitance spectroscopy.

For some devices, we observed a 40% enhancement of the gate capacitance at low carrier densities. At the same densities, electric field penetration measurements show that the oxide interface significantly overscreens the applied electric fields. Both measurements imply a negative electronic compressibility. Second, the magnetic moment of the interface system is detected directly using torque magnetometry.  Control experiments with samples without LaAlO3 display a background signal two orders of magnitude smaller, which verifies that the observed magnetic moment arise from the deposition of LaAlO3. The measured equilibrium M-H curve resembles that of a soft ferromagnet. The observed spontaneous magnetic moment is in-plane, and exists even in the superconducting state.  To summarize, the observation of the negative compressibility supports the two-dimensionality of the electronic system, whereas the measured M-H behavior implies a magnetic ordering at the interface. These two thermodynamic measurements suggest the existence of a two-dimensional magnetic state.