The conducting gas that forms at the interface between LaAlO3 and SrTiO3 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 lowtemperature 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.

No abstract

Symmetry breaking is a fundamental concept in condensed matter physics whose presence often heralds new phases of matter. For instance, the breaking of time reversal symmetry is traditionally linked to magnetic phases in a material, while the breaking of gauge symmetry can lead to superfluidity/superconductivity. Nematic phases are phases in which rotational symmetry is broken while maintaining translational symmetry, and are traditionally associated with liquid crystals. Electronic nematic states where the orthogonal in-plane crystal directions have different electronic properties have garnered a great deal of attention after their discovery in Sr 3 Ru 2 O 7 , 1 multiple iron based superconductors, 2,3 and in the superconducting state of CuBiSe. 4,5 Here we demonstrate the existence of an electronic nematic phase in the two-dimensional carrier gas that forms at the (111) LaAlO 3 (LAO)/SrTiO 3 (STO) interface that onsets at low temperatures, and is tunable by an electric field.Over more than a decade, the two-dimensional conducting gas that forms at the LAO/STO interface has been intensively studied because of the variety of properties that can be controlled through the application of an in-situ electric field, including conductivity, superconductivity, 6-8 ferromagnetism, 7-11 and the spinorbit interaction. 12,13 Until recently, most of these studies focused on the (001) orientation of the LAO/STO heterostructures, while the (110) and (111) orientations have remained relatively unexplored. The (111) orientation of the LAO/STO interface is especially interesting due to the hexagonal symmetry of the titanium atoms at the interface, shown schematically in Fig. 1(a). This configuration has been likened to a strongly correlated analogue of graphene, and has been predicted to exhibit topological properties, unconventional superconductivity, as well as nematic phases. [14][15][16] Electric transport measurements have shown that the (111) LAO/STO interface exhibits many of the properties already seen in (001) LAO/STO, including a coexistence of superconductivity R⊡ (kΩ) 0 5 10 15 20 Vg (V) 40 50 60 70 80 90 100 [112] [110] Ti 1 Ti 2 Ti3 O [110] [112] [ 1 1 1 ] [110] (a) (b) I VL VH VL VH [112] [110] I T=4.4K [112] FIG. 1. a) Schematic representation of the first three monolayers at the LAO/STO interface with the [112] and [110] labeled. The red atoms represent the oxygen and blue atom titanium. The titanium atoms are further labeled Ti 1/2/3 to indicate their distance away from the interface, with Ti 1 being at the interface. (b) Averaged R vs Vg for the [112]/ [110] in red/black measured simultaneously at 4.4 K. The inset shows a schematic of the device configuration on each measured LAO/STO sample.and magnetism. 17-20 However, the feature that distinguishes the (111) interface from the (001) interface is the strong anisotropy with respect to surface crystal direc-arXiv:1708.04809v2 [cond-mat.str-el]

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 LaAlO3 (LAO) and SrTiO3 (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.

We report measurements of the superconducting properties of the two-dimensional gas that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) in the (111) crystal orientation, a system that permits in-situ tuning of carrier density and disorder by means of a back gate voltage, Vg. Like the (001) oriented LAO/STO interface, superconductivity at the (111) LAO/STO interface can be tuned by Vg. The 2D superconductivity in these (111) LAO/STO samples shows an in-plane anisotropy, being different along different interface crystal directions, and 'remembers' the disorder landscape at which they are cooled through the superconducting transition. The low energy scale and other characteristics of this memory effect distinguish it from charge-trapping effects previously observed in (001) interface samples.