Signatures of electronic nematicity in (111) LaAlO3/SrTiO3 interfaces

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]

Strong spin-orbit coupling and magnetism in (111) (La0.3Sr0.7)(Al0.65Ta0.35Bal1, Huang2, Han3 et al. 2018Phys. Rev. BSelf Cite4290View full textAdd to dashboardCite

“…21,22 How this complex band structure affects various properties is just beginning to be explored. 23,24 Recent transport experiments on (111) LaAlO 3 /STO (LAO/STO) have shown surprising anisotropies related to the crystalline axes, dependent on an applied V g , 9,[25][26][27][28][29] as well as a non-monotonic dependence of phase coherence time τ φ and spin-orbit scattering time τ so on V g . 30 Here we report studies of the MR of LSAT deposited epitaxially on (111) STO.…”

Anisotropic superconductivity and frozen electronic states at the (111) LaAlO3/SrTiO3 interface

“…In (111) LAO/STO interface structures, the normal state transport characteristics (longitudinal resistance, Hall coefficient, and quantum capacitance) are strongly anisotropic, [17][18][19][20] being different along the [112] and [110] directions, the anisotropy increasing with decreasing oxygen vacancy concentration and more negative V g . 18 While in-plane anisotropy in transport characteristics has also been observed in (001) LAO/STO interface structures, and has been variously ascribed to microstructural effects 26 or ferroelectric twin domains walls, [27][28][29] the in-plane anisotropy in the (111) devices is characteristically different in magnitude and qualitative behavior, and thus has a different origin.…”

“…The electronic nematicity in the normal state manifests itself as an anisotropy with respect to crystal surface direction that is observed in almost all the electronic properties of the (111) interface, including the longitudinal resistance, Hall effect and quantum capacitance. [17][18][19][20] At lower temperatures, the nematicity is predicted to manifest itself as an anisotropy in the superconducting properties of the (111) LAO/STO interface. 15 We show below that the superconducting properties of the (111) LAO/STO interface are indeed anisotropic, being different when measured along different surface crystal directions.…”

Anisotropic magnetoresistance in multiband systems: Two-dimensional electron gases and polar metals at oxide interfaces

“…Our work suggests that the AMR and its symmetries may be used to indirectly detect such symmetry breaking. Furthermore, the nematic resistivity observed 37,51,58 in insulating ultra-low density (111) 2DEGs may be viewed as arising from an "Anderson-localized polar metal" which exhibits anisotropic variable-range hopping. It would be interesting to further explore this regime.…”

“…We turn next to the question of how directional symmetry breaking at the (111) interface, which leads to a 2D polar metal phase, might impact the AMR in the 2DEG. Our work is partly motivated by experimental reports of nematicity in transport measurements in such 2DEGs [35][36][37]41,58 . Such symmetry breaking might have its origin in the bulk 3D structure; e.g., bulk SrTiO 3 has a structural cubic-to-tetragonal transition upon lowering temperature 60 T 100K.…”