Date of Graduation

5-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor

Jak Chakhalian

Committee Member

Laurent Bellaiche

Second Committee Member

Surendra P. Singh

Third Committee Member

Paul Thibado

Fourth Committee Member

Ryan Tian

Keywords

Pure sciences; Applied sciences; Complex oxides; Correlated electrons; Electronic structure; Heterostructures; Interfaces; Metal-insulator trnasistion

Abstract

Strongly correlated electron systems, particularly transition metal oxides, have been a focus of condensed matter physics for more than two decades since the discovery of high-temperature superconducting cuprates. Diverse competing phases emerge, spanning from exotic magnetism to unconventional superconductivity, in proximity to the localized-itinerant transition of Mott insulators. While studies were concentrated on bulk crystals, the recent rapid advance in synthesis has enabled fabrication of high-quality oxide heterostructures, offering a new route to create novel artificial quantum materials.

This dissertation details the investigation on ultrathin films and heterostructures of 3d7(t2g6eg1) systems with spin (S=1/2) and orbital degeneracies. Perovskite RNiO3 (R = rare earth) was chosen as a representative model, since its Ni 3+ valence corresponds to the low-spin 3d7 configuration. The heteroepitaxial growth of RNiO3 ultrathin films and heterostructures was studied by laser molecular beam epitaxy. To achieve a layer-by-layer growth mode crucial for stabilizing the proper stoichiometry and creating sharp interfaces, a fast pumping plus interruption growth method was developed.

In addition to conventional transport measurement, resonant x-ray absorption spectroscopy was used to characterize the resulting electronic structures. The results demonstrate that the effect of polarity mismatch on the initial growth may lead to a chemical pathway for compensating the dipolar field. By utilizing the x-ray linear dichroic effect, an asymmetric heteroepitaxial strain-induced d orbital response in LaNiO3 was revealed. Moreover, the interfacial lattice constraint was found to modulate the Ni-O covalency in RNiO3 by simultaneously tuning the Madelung energy and the p-d hybridization, leading to a self-doped mechanism that controls the collective phase behavior in NdNiO3.

The electronic reconstructions in correlated quantum wells were also investigated in superlattices of LaNiO3/LaAlO3. In proximity to the confinement limit, a Mott-type metal-insulator transition was observed with tendency towards charge ordering as a competing ground state. The interfacial Ni-O-Al bond was found to highly suppress the apical ligand hole density and result in confinement-induced orbital polarization. The key role of the interfacial boundary in selecting the many-body electronic ground state was directly demonstrated in quantum wells of NdNiO3.

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