Diluted Oxide Interfaces with Tunable Ground States

Abstract

as 2D superconductivity (SC), [4][5][6][7][8] magnetism, [9][10][11] and possible coexistence of these two, [12][13][14] providing an intriguing platform for multifunctional electronic devices.The carriers in the 2DEL of STObased heterostructures occupy the Ti 3d t 2g orbitals, where the in-plane d xy bands often have a lower energy than the out-ofplane oriented d xz /d yz due to the confinement of these orbitals at the interface. [1,15] Among the systems investigated so far, the epitaxial polar/nonpolar LAO/STO interface has drawn intensive attention and shows a typical carrier density, n s , of ≈4 × 10 13 cm −2 . [4] This high carrier density results in the population of orbitals with both d xy and d xz /d yz symmetry. The occupation of carriers in different subbands has previously been controlled reversibly using electrostatic gating, leading to a versatile tuning of the physical properties of the interface. [16] For example, by back gating the interface through the STO substrate, a Lifshitz transition was revealed at a critical carrier density of n c = 1.7 × 10 13 cm −2 , [6] above which the Fermi level starts to cross the bottom of the d xz /d yz bands. In proximity to n c , superconductivity exhibits a dome-shape as a function of gating voltage The metallic interface between two oxide insulators, such as LaAlO 3 /SrTiO 3 (LAO/STO), provides new opportunities for electronics and spintronics. However, due to the presence of multiple orbital populations, tailoring the interfacial properties such as the ground state and metal-insulator transitions remains challenging. Here, an unforeseen tunability of the phase diagram of LAO/STO is reported by alloying LAO with a ferromagnetic LaMnO 3 insulator without forming lattice disorder and at the same time without changing the polarity of the system. By increasing the Mn-doping level, x, of LaAl 1−x Mn x O 3 / STO (0 ≤ x ≤ 1), the interface undergoes a Lifshitz transition at x = 0.225 across a critical carrier density of n c = 2.8 × 10 13 cm −2 , where a peak T SC ≈255 mK of superconducting transition temperature is observed. Moreover, the LaAl 1−x Mn x O 3 turns ferromagnetic at x ≥ 0.25. Remarkably, at x = 0.3, where the metallic interface is populated by only d xy electrons and just before it becomes insulating, a same device with both signatures of superconductivity and clear anomalous Hall effect (7.6 × 10 12 cm −2 < n s ≤ 1.1 × 10 13 cm −2 ) is achieved reproducibly. This provides a unique and effective way to tailor oxide interfaces for designing on-demand electronic and spintronic devices. Oxide InterfacesThe surface of SrTiO 3 (STO) [1] or its interface to another oxide insulator such as the polar perovskite LaAlO 3 (LAO) [2] or the spinel γ-Al 2 O 3 [3] can host a 2D electron liquid (2DEL). In particular, the 2DEL at LAO/STO interface has been found to exhibit a wide spectrum of emergent phenomena such

Stimulating Oxide Heterostructures: A Review on Controlling SrTiO3‐Based Heterointerfaces with External Stimuli

“…For 2DEGs at STO‐based oxide interfaces, a bell‐like MR has been widely observed when the carrier density is higher than the critical carrier density mentioned in Section above which both d xy and d xz /d yz bands start to become occupied (the Lifshitz transition). In this case, the MR is parabolic at low field, but at a critical field it deviates from the parabola and turns into a bell‐like shape . The corresponding Hall resistance shows a nonlinearity at the same critical field as described in the following subsection on the Hall effect.…”

“…The anomalous Hall effect arises from a coupling between the itinerant electrons and magnetic moments, as observed in several oxide heterostructures . As shown in Figure a, a signature of the anomalous Hall effect is a downward curvature that bends the curve clockwise, in contrast to the aforementioned counter‐clockwise bending with the nonlinear Hall effect stemming from conduction in two n ‐type subbands.…”

“…Subtracting the contribution of the ordinary Hall effect determined from the low‐field slope of R xy versus B makes the anomalous Hall effect clearer, as displayed in Figure b. In contrast to the case in Figure , the anomalous Hall effect typically only gives a small contribution to the total Hall effect due to the weak magnetization and/or a weak coupling between itinerant moments and the magnetization . In these cases, the anomalous Hall effect can be hard to deduce directly from the raw data, whereas d R xy /d B provides a better way of identifying this contribution .…”

“…In contrast to the case in Figure , the anomalous Hall effect typically only gives a small contribution to the total Hall effect due to the weak magnetization and/or a weak coupling between itinerant moments and the magnetization . In these cases, the anomalous Hall effect can be hard to deduce directly from the raw data, whereas d R xy /d B provides a better way of identifying this contribution . In order to make the effects of the magnetic state more pronounced, one of the common strategies used is to introduce ferromagnetism in LAO/STO using the magnetic proximity effect from magnetic dopants as discussed in Section .…”

“…However, these spin‐polarized electrons have been detected often in the high carrier density range where both the d xy and d xz /d yz electrons are populated . Based on the ferromagnetism of LMO thin films on the STO, Gan et al later detected the anomalous Hall effect in oxide interfaces of LaAl 0.7 Mn 0.3 O 3 /STO where only the d xy band are populated …”