Hilary Noad

Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces.

The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO; refs 1, 2). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO3

Strontium titanate is a low-temperature, non-Bardeen-Cooper-Schrieffer superconductor that superconducts to carrier concentrations lower than in any other system and exhibits avoided ferroelectricity at low temperatures. Neither the mechanism of superconductivity in strontium titanate nor the importance of the structure and dielectric properties for the superconductivity are well understood. We studied the effects of twin structure on superconductivity in a 5.5-nm-thick layer of niobium-doped SrTiO3 embedded in undoped SrTiO3. We used a scanning superconducting quantum interference device susceptometer to image the local diamagnetic response of the sample as a function of temperature. We observed regions that exhibited a superconducting transition temperature T-c greater than or similar to 10% higher than the temperature at which the sample was fully superconducting. The pattern of these regions varied spatially in a manner characteristic of structural twin domains. Some regions are too wide to originate on twin boundaries; therefore, we propose that the orientation of the tetragonal unit cell with respect to the doped plane affects T-c. Our results suggest that the anisotropic dielectric properties of SrTiO3 are important for its superconductivity and need to be considered in any theory of the mechanism of the superconductivity.

Observation of signatures of subresolution defects in two-dimensional superconductors with a scanning SQUID

The diamagnetic susceptibility of a superconductor is directly related to its superfluid density. Mutual inductance is a highly sensitive method for characterizing thin films, however, in traditional mutual inductance measurements, the measured response is a nontrivial average over the area of the mutual inductance coils, which are typically of millimeter size. Here we measure localized, isolated features in the diamagnetic susceptibility of Nb superconducting thin films with lithographically defined through holes,

Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface

\ensuremath{\delta}

Imaging sub-resolution defects in two-dimensional superconductors with scanning SQUID

-doped

SQUID imaging of edge magnetization and weak local diamagnetism in un-doped Ba122

{\mathrm{SrTiO}}_{3}

Scanning SQUID Susceptometry of Sr 2 RuO 4 Under Uniaxial Pressure

, and the two-dimensional electron system at the interface between

Effects of SQUID Alignment on Penetration Depth Measurements

{\mathrm{LaAlO}}_{3}

Scanning SQUID Microscopy of Sr

and

_2

{\mathrm{SrTiO}}_{3}