Condensed-Matter Physics & Materials Science Seminar

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 surprising results from two thermodynamic measurements of this electronic system - the electronic compressibility and the magnetization. First, the electronic compressibility is measured with capacitance spectroscopy. For some devices, we observed a greater than 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 applied electric fields. Both measurements imply a negative electronic compressibility of the oxide interface system. 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 the observed magnetic moment arising 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. Finally, 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.