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Magnetoresistance in the superconducting state at the (111) LaAlO/SrTiO interface
Condensed matter systems that simultaneously exhibit superconductivity and
ferromagnetism are rare due the antagonistic relationship between conventional
spin-singlet superconductivity and ferromagnetic order. In materials in which
superconductivity and magnetic order is known to coexist (such as some
heavy-fermion materials), the superconductivity is thought to be of an
unconventional nature. Recently, the conducting gas that lives at the interface
between the perovskite band insulators LaAlO (LAO) and SrTiO (STO) has
also been shown to host both superconductivity and magnetism. Most previous
research has focused on LAO/STO samples in which the interface is in the (001)
crystal plane. Relatively little work has focused on the (111) crystal
orientation, which has hexagonal symmetry at the interface, and has been
predicted to have potentially interesting topological properties, including
unconventional superconducting pairing states. Here we report measurements of
the magnetoresistance of (111) LAO/STO heterostructures at temperatures at
which they are also superconducting. As with the (001) structures, the
magnetoresistance is hysteretic, indicating the coexistence of magnetism and
superconductivity, but in addition, we find that this magnetoresistance is
anisotropic. Such an anisotropic response is completely unexpected in the
superconducting state, and suggests that (111) LAO/STO heterostructures may
support unconventional superconductivity.Comment: 6 Pages 4 figure
Anisotropic, multi-carrier transport at the (111) LaAlO/SrTiO interface
The conducting gas that forms at the interface between LaAlO and
SrTiO has proven to be a fertile playground for a wide variety of physical
phenomena. The bulk of previous research has focused on the (001) and (110)
crystal orientations. Here we report detailed measurements of the
low-temperature electrical properties of (111) LAO/STO interface samples. We
find that the low-temperature electrical transport properties are highly
anisotropic, in that they differ significantly along two mutually orthogonal
crystal orientations at the interface. While anisotropy in the resistivity has
been reported in some (001) samples and in (110) samples, the anisotropy in the
(111) samples reported here is much stronger, and also manifests itself in the
Hall coefficient as well as the capacitance. In addition, the anisotropy is not
present at room temperature and at liquid nitrogen temperatures, but only at
liquid helium temperatures and below. The anisotropy is accentuated by exposure
to ultraviolet light, which disproportionately affects transport along one
surface crystal direction. Furthermore, analysis of the low-temperature Hall
coefficient and the capacitance as a function of back gate voltage indicates
that in addition to electrons, holes contribute to the electrical transport.Comment: 11 pages, 9 figure
Superconductivity and Frozen Electronic States at the (111) LaAlO/SrTiO Interface
In spite of Anderson's theorem, disorder is known to affect superconductivity
in conventional s-wave superconductors. In most superconductors, the degree of
disorder is fixed during sample preparation. Here we report measurements of the
superconducting properties of the two-dimensional gas that forms at the
interface between LaAlO (LAO) and SrTiO (STO) in the (111) crystal
orientation, a system that permits \emph{in situ} tuning of carrier density and
disorder by means of a back gate voltage . Like the (001) oriented LAO/STO
interface, superconductivity at the (111) LAO/STO interface can be tuned by
. In contrast to the (001) interface, superconductivity in these (111)
samples is anisotropic, being different along different interface crystal
directions, consistent with the strong anisotropy already observed other
transport properties at the (111) LAO/STO interface. In addition, we find that
the (111) interface samples "remember" the backgate voltage at which they
are cooled at temperatures near the superconducting transition temperature
, even if is subsequently changed at lower temperatures. The low
energy scale and other characteristics of this memory effect ( K)
distinguish it from charge-trapping effects previously observed in (001)
interface samples.Comment: 6 pages, 5 Figure
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