Date of Graduation

12-2016

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

Huaxiang Fu

Fourth Committee Member

Ryan Tian

Abstract

As the representative family of complex oxides, transition metal oxides, where the lattice,

charge, orbital and spin degrees of freedom are tightly coupled, have been at the forefront

of condensed matter physics for decades. With the advancement of state-of-the-art heteroepitaxial deposition techniques, it has been recognized that combining these oxides on the atomic scale, the interfacial region offers great opportunities to discover emergent phenomena and tune materials' functionality. However, there still lacks general guiding principles for experimentalists, following which one can design and fabricate artificial systems on demand. The main theme of this dissertation is to devise and propose some basic rules for heterostructure engineering.

Towards this goal, I first report the growth of high quality YTiO3/CaTiO3 superlattices

by pulsed laser deposition. Electrical transport measurements reveal that a novel, non-

SrTiO3 based two-dimensional electron gas system has formed at the interface. What is

more, these studies add solid evidences that interface engineering via charge modulation is

an effective approach to realizing exotic many-body phenomena.

Secondly, a new concept, denoted as \geometrical lattice engineering" is proposed with pioneering experimental efforts. Aiming at designing magnetically frustrated systems, (111)-

oriented CoCr2O4 thin films and CoCr2O4/Al2O3 heterostructures have been fabricated

for the first time. Comprehensive structural and electronic characterizations reveal that

no disorder in the cation distribution or multivalency issue is present. As a result, unique

quasi-two dimensional geometrically frustrated lattices composed of alternating kagome

and triangular lattices, are naturally established via this topological approach.

These CoCr2O4/Al2O3 heterostructures have been found to exhibit remarkably different

behaviors from the bulk compounds. Towards the two dimensional limit, the ground state

of the ultrathin superlattices transforms from the bulk-like nonlinear ferrimagnetic phase,

into an emergent collinear ferrimagnetic phase, and finally into an exotic magnetically

disordered phase with an extensively large frustration parameter, which is a hallmark of

quantum spin liquid. These findings corroborate geometrical lattice engineering has excellent

potential in achieving novel electronic, magnetic, and topological phases.

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