We investigate SrIrO 3 /LaNiO 3 superlattices in which we observe a full electron transfer at the interface from Ir to Ni, triggering a massive structural and electronic reconstruction. Through experimental characterization and first-principles calculations, we determine that a large crystal field splitting from the distorted interfacial IrO 6 octahedra surprisingly dominates over the spinorbit coupling and together with the Hund's coupling results in the high-spin (S = 1) configurations on both the Ir and Ni sites. This demonstrates the power of interfacial charge transfer in coupling lattice, charge, orbital, and spin degrees of freedom, opening fresh avenues of investigation of quantum states in oxide superlattices.superlattice | interfacial charge transfer | iridates | nickelates

We highlight recent advances in the theory, materials fabrication, and experimental characterization of strongly correlated and topological states in [111] oriented transition metal oxide thin films and heterostructures, which are notoriously difficult to realize compared to their [001] oriented counterparts. We focus on two classes of complex oxides, with the chemical formulas ABO3 and A2B2O7, where the B sites are occupied by an open-shell transition metal ion with a local moment and the A sites are typically a rare earth element. The [111] oriented quasi-two-dimensional lattices derived from these parent compound lattices can exhibit peculiar geometries and symmetries, namely, a buckled honeycomb lattice, as well as kagome and triangular lattices. These lattice motifs form the basis for emergent strongly correlated and topological states expressed in exotic magnetism, various forms of orbital ordering, topological insulators, topological semimetals, quantum anomalous Hall insulators, and quantum spin liquids. For transition metal ions with high atomic number, spin–orbit coupling plays a significant role and may give rise to additional topological features in the electronic band structure and in the spectrum of magnetic excitations. We conclude this perspective by articulating open challenges and opportunities in this actively developing field.

We report on the emergent magnetic state of (111)-oriented CoCr 2 O 4 ultrathin films sandwiched by Al 2 O 3 in the quantum confined geometry. At the two-dimensional crossover, polarized neutron reflectometry reveals an anomalous enhancement of the total magnetization compared to the bulk value. Synchrotron x-ray magnetic circular dichroism (XMCD) demonstrates the appearance of long-range ferromagnetic ordering of spins on both Co and Cr sublattices. Brillouin function analyses further corroborates 1 arXiv:1905.12024v1 [cond-mat.str-el] 28 May 2019 that the observed phenomena are due to the strongly altered magnetic frustration, manifested by the onset of a Yafet-Kittel type ordering as the new ground state in the ultrathin limit, which is unattainable in the bulk. Keywords spinels, ultrathin films, emergent properties, magnetismThe quest to design, discover and manipulate new quantum states of matter has fostered tremendous research activity among condensed matter physicists. Recent progress in the fabrication of epitaxial thin films has empowered this effort with additional means and led to a plethora of interesting artificial multilayers and heterostructures grown with atomic level of precision. 1-4 Nowadays, to realize exotic physics linked to many-body phenomena the interest has shifted to tailoring the magnetic states in quasi two-dimensional (2D) limit. 5,6 On one hand, according to the Mermin-Wagner theorem, in an isotropic Heisenberg spin system of dimensionality D ≤2, enhanced thermal fluctuations prohibit the onset of a long-range (ferroor antiferro-) magnetic ordering at any finite temperature. 7 On the other hand, lowering the dimensionality brings about several new factors that can radically alter a quantum system including changes in band topology, ionic coordinations and covalency, crystal fields, exchange pathways, magnetic anisotropy, quantum confinement, and universality class. 2,[8][9][10][11] As a result, in the crossover to low dimensions the magnetic ground state of a material can be distinctly different from its three-dimensional (3D) counterpart thus opening an opportunity for emergent or hidden materials phases.In this context, it is interesting to ask whether we can "dial-in" dimensionality of a system from 3D to 2D in a controllable way, and what can happen to the quantum state when low dimensionality entwines with frustration? Here we recap that frustrated magnets are systems where the localized spins are entangled in an incompatible way due to either multiple competing exchange interactions, or the underlaying lattice geometry or both. [12][13][14][15][16] Generally, frustration tends to suppress spin ordering and promotes a complex magnetic

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