Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3
journal contributionposted on 04.11.2019 by Arian Arab, Xiaoran Liu, Okan Köksal, Weibing Yang, Ravini U. Chandrasena, Srimanta Middey, Mikhail Kareev, Siddharth Kumar, Marius-Adrian Husanu, Zhenzhong Yang, Lin Gu, Vladimir N. Strocov, Tien-Lin Lee, Jan Minár, Rossitza Pentcheva, Jak Chakhalian, Alexander X. Gray
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Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic  direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic–structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.