Kathryn Moler (SIMES Friday Seminar)

Date(s) - Jan 17 2014
11:00 AM - 12:30 PM

Shasta Room, Bldg. 40, Room 361


Scanning SQUID Microscopy of Quantum Materials

Professor Kathryn A. Moler

Department of Applied Physics, Stanford University

Complex quantum materials present great opportunities but also great challenges. Magnetic imaging measurements are a valuable complement to other types of measurements such as those based on transport, thermodynamics, or interactions with photons. In this talk, I will first describe how our present and planned Superconducting QUantum Interference Devices (SQUIDs) compare to other candidate low-temperature magnetic probes. I will then describe the application of this technique to two types of quantum materials. First, the LaAlO3/SrTiO3 interface is one of the most studied examples of an engineered complex oxide heterostructure. Although both LaAlO3 and SrTiO3 are band insulators, our SIMES colleague Harold Y. Hwang showed that the interface is conducting, and later work found that it is both superconducting and oddly ferromagnetic.  Our recent results show that conductivity is enhanced by the twin structure. This interplay between structural domains and the interface provides a new mechanism for understanding and controlling the behaviour of heterostructures. Second, HgTe/HgCdTe and InAs/GaSb quantum wells are the first two quantum spin hall insulators. Until now, the quantum spin hall state has been inferred from transport measurements. I will show images of the magnetic field produced by transport current as the systems are tuned through the quantum spin hall state, showing that the current really does flow in the edge. We find that the edge resistance has very little temperature dependencce, suggesting that whatever interactions or defects are responsible for scattering are inelastic. These results make it unlikely that various proposed inelastic scattering mechanisms with strong temperature dependence could be the dominant scattering mechanism in the edge states, and bring us one step closer to achieving the “dissipationless” transport hoped for in ideal quantum spin insulators.


Kalisky et al, “Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces” Nature Materials 12, 1091–1095 (2013)

Nowack et al, “Imaging currents in HgTe quantum wells in the quantum spin Hall regime” Nature Materials 12, 787–791 (2013)