Date of Graduation

5-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Chakhalian, Jacques

Committee Member

Bellaiche, Laurent

Second Committee Member

Singh, Surendra P.

Third Committee Member

Thibado, Paul M.

Fourth Committee Member

Tian, Z. Ryan

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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