The theoretical understanding of the spin-orbit coupling (SOC) effects at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces is still in its infancy. We perform first-principles density-functional-theory calculations and derive from these a simple tight-binding Hamiltonian, through a Wannier function projection and group theoretical analysis. We find striking differences to the standard Rashba theory for spin-orbit coupling in semiconductor heterostructures due to multi-orbital effects: by far the biggest SOC effect is at the crossing point of the xy and yz (or zx) orbitals; and around the Γ point a Rashba spin splitting with a cubic dependence on the wave vector k is possible.

Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with non-collinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, we demonstrate how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result we possess control of the lateral magnetic and electronic anisotropies by atomic-scale design of the oxygen octahedral rotation.

Corresponding authors: W.J. (email: wji@ruc.edu.cn) and Z.Z. (email: zhong@nimte.ac.cn) † These authors contributed equally to this work.Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer antiferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with 3 as previously believed. Moreover, we demonstrated that the intra-and interlayer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong-and weakinteraction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the S2 freedom of stacking order in two dimensional layered materials.Introduction.-Magnetism in two dimensions has received growing attention since the two ferromagnetic monolayers, namely CrI3 [1] and Cr2Ge2Te6 [2], were successfully fabricated in 2017. The ferromagnetism in these two layers was believed to be stabilized by magnetic anisotropy as enhanced by spin-orbit coupling or external magnetic fields.Their Curie temperatures were up to ~50 K. Very recently, a room-temperature Tc were achieved in monolayer VSe2 [3] and MnSex [4], two members of the transition-metal dichalcogenides family. This shed considerable light on the search for high Tc ferromagnetic (FM) magnets. However, the tunability of magnetism has been emerging as a new challenge. The coupling strengths of two-dimensional (2D) materials are significantly different between intra-and inter-layer interactions. Such difference may offer diverse magnetic coupling mechanisms at strong and weak interacting limits. The interlayer magnetic coupling is of peculiar interest, as the effective coupling is relatively weak and confined within few atomic layers, which is much easier to model and more feasible to tune than strong and periodic couplings in three-dimension.Recent experiments demonstrated that the anti-ferromagnetic (AFM) interlayer order in bilayer CrI3 can be manipulated to a FM order by electric gating or reasonably large magnetic fields [5][6][7][8][9][10][11][12]. As a consequence, a magnetic tunnel junction with giant tunneling magnetoresistance values was achieved in bilayer CrI3 d...

The recently discovered nickelate superconductors appear, at first glance, to be even more complicated multi-orbital systems than cuprates. To identify the simplest model describing the nickelates, we analyse the multi-orbital system and find that it is instead the nickelates which can be described by a one-band Hubbard model, albeit with an additional electron reservoir and only around the superconducting regime. Our calculations of the critical temperature T C are in good agreement with experiment, and show that optimal doping is slightly below 20% Sr-doping. Even more promising than 3d nickelates are 4d palladates.

Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG--the SrTiO 3 (110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising "semiheavy" band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system. oxide surface | electronic structure | quantum confinement | perovskite | ARPES T he 2D electron gas (2DEG) observed in oxide heterostructures such as LaAlO 3 /SrTiO 3 (1, 2) offers a possible alternative to conventional semiconductors, not only for electronics at the nanoscale (3) but also because of the possibility of spin-polarized (4) and superconducting (5, 6) currents. An even simpler setup is to create a 2DEG directly at SrTiO 3 . Recently this was achieved by irradiating a (001) surface (7, 8) with synchrotron radiation, albeit the origin of the resulting 2DEG is still under debate (7-9). This system has two major drawbacks: (i) surface oxygen vacancies are very reactive and (ii) the (001) surface has no unique surface termination, as TiO 2 and SrO terraces may develop, and the surface structure strongly depends on sample treatment and history (10).Here, we show that a 2DEG can also be induced at SrTiO 3 (110), which is stabilized and covered by a reconstructed overlayer. This overlayer automatically forms to compensate the intrinsic polarity of the system. A SrTiO 3 crystal can be viewed as a stack of alternating (SrTiO) 4+ and (O 2 ) 4− planes along the [110] orientation, resulting in a dipole moment that diverges with increasing crystal thickness (11). As is often true for polar surfaces, this is prevented by one of several compensation mechanisms (11). Specifically, the SrTiO 3 (110) surface spontaneously forms a (4 × 1) reconstruction upon various different sample treatments, including annealing in a tube furnace with flowing high-purity oxygen (12) and standard ultrahigh vacuum preparation procedures (13,14). The reconstruction consists of a 2D, tetrahedrally coordinated titania overlayer (Fig. 1A), which, with a nominal stoichiometry of (Ti 1.5 O 4 ) 2− , quenches the overall dipole moment (12, 15). Because the Ti atoms in the tetrahedral titania surface layer of the reconstruction are saturated by strong, directional bonds, the (4 × 1) surface is chemically quite inert (16). Results and DiscussionExposing the SrTiO 3 ...

The synthesis of transition metal heterostructures is currently one of the most vivid fields in the design of novel functional materials. In this paper we propose a simple scheme to predict band alignment and charge transfer in complex oxide interfaces. For semiconductor heterostructures band alignment rules like the well known Anderson or Schottky-Mott rule are based on comparison of the work function or electron affinity of the bulk components. This scheme breaks down for oxides due to the invalidity of a single workfunction approximation as recently shown (Phys. Rev. B 93, 235116; Adv. Funct. Mater. 26, 5471). Here we propose a new scheme which is built on a continuity condition of valence states originating in the compounds' shared network of oxygen. It allows for the prediction of sign and relative amplitude of the intrinsic charge transfer, taking as input only information about the bulk properties of the components. We support our claims by numerical density functional theory simulations as well as (where available) experimental evidence. Specific applications include i) controlled doping of SrTiO3 layers with the use of 4d and 5d transition metal oxides and ii) the control of magnetic ordering in manganites through tuned charge transfer. 79.60.Jv

Using first-principles density functional theory calculations, we find a strong position and thickness dependence of the formation energy of oxygen vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an analytical capacitor model. Oxygen vacancies are preferentially formed at p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons introduced by the oxygen vacancies reduce their energy by moving to the n-type interface. This asymmetric behavior makes an important contribution to the conducting (insulating) nature of n-type (p-type) interfaces while providing a natural explanation for the failure to detect evidence for the polar catastrophe in the form of core level shifts.

Manipulating physical properties using the spin degree of freedom constitutes a major part of modern condensed matter physics and is a key aspect for spintronics devices. Using the newly discovered two-dimensional van der Waals ferromagnetic CrI as a prototype material, we theoretically demonstrated a giant magneto band-structure (GMB) effect whereby a change of magnetization direction significantly modifies the electronic band structure. Our density functional theory calculations and model analysis reveal that rotating the magnetic moment of CrI from out-of-plane to in-plane causes a direct-to-indirect bandgap transition, inducing a magnetic field controlled photoluminescence. Moreover, our results show a significant change of Fermi surface with different magnetization directions, giving rise to giant anisotropic magnetoresistance. Additionally, the spin reorientation is found to modify the topological states. Given that a variety of properties are determined by band structures, our predicted GMB effect in CrI opens a new paradigm for spintronics applications.