Wednesday 29 July 2015 - Nicolas Bergeal - Quantum phase transitions at oxide interface

Nicolas brings back the discussion to the superconducting oxide interfaces, talking about 

LaXO3/SrTiO3 heterostructures, where X stands for Ti or Al. Despite differences in the constituents of these interfaces (a Mott insulator facing a band insulator in the LaTiO3/SrTiO3 case vs two band insulators for the LaAlO3/SrTiO3 configuration), he announces that the physics of the 2D electron gas (2DEG), that sits in the STO side, so far, is the same.

After a short review of the relevant properties of these polar/non-polar interfaces, he starts discussing the magnetic field-driven superconductor to insulator transitions. His scaling analysis reveals two critical behaviors with two quantum critical fields: for a high temperature region (between 100 and 200 mK), the critical exponents are znu=⅔; for lower temperatures (down to 40 mK), they are equal to 3/2. This behavior, observed for positive gate voltages, persists also for negative gates, but in the latter case only one critical field is observed. These data (see publication in Nat. Mat. 2013) raise different questions in the audience: why the resistance changes just by few percent at the critical point?  Is the low T region originating from cooling issues that renormalize the T scale?

Some of the answers can be found in the model that he introduces to explain these results: it is based on dilute superconducting puddles coupled by the 2DEG, discussed yesterday in detail by Marco Grilli.

When the superconductor to insulator transition is induced by sweeping the gate voltage, the scaling analysis reveals critical exponents equal to 1.6, close to 3/2. This value, despite having being already observed for instance in high temperature superconductors, it is difficult to justify theoretically with existing models. The new scenario they consider is based on density driven fluctuations, as discussed by Marco. The picture is the one of a material where a phase separation generates regions with high electron density and low electron density and their interplay determines the physics. This scenario asks confirmation from local spectroscopy.

Blogged by Stefano Gariglio