Emergent Topological Phenomena in Thin Films of Pyrochlore Iridates

Abstract

Two quintessential ingredients governing the topological invariant of a system are the dimensionality and the symmetry of the system. Due to the recent development of thin film and artificial superstructure growth technique, it is possible to control the dimensionality of the system, smoothly between the two-dimensions (2D) and three-dimensions (3D). In this work we unveil the dimensional crossover of emergent topological phenomena in correlated topological materials. In particular, by focusing on the thin film of pyrochlore iridate antiferromagnets grown along the [111] direction, we demonstrate that it can show giant anomalous Hall conductance, which is proportional to the thickness of the film, even though there is no Hall effect in 3D bulk material. In addition, we uncover the emergence of a new topological phase, whose nontrivial topological properties are hidden in the bulk insulator but manifest only in thin films. This shows that the thin film of topological materials is a new platform to search for unexplored novel topological phenomena. Dimensionality and symmetry of a system govern the topological nature of a band insulator. [1][2][3][4] For instance, in two dimensional (2D) electronic systems with broken time reversal symmetry (TRS), band insulators can be classified by the quantized Hall conductance, which generally has the form of σ xy = e 2 h N where e is the elementary charge and h is the Planck constant. 5 Since N can be any integer number, there are infinite number of ways distinguishing band insulators. However, when 2D and three dimensional (3D) electronic systems obey TRS, there are only two different ways to discern band insulators based on the Z 2 topological index. [6][7][8][9][10][11] On the other hand, if 3D electronic systems break TRS, there is no way to differentiate band insulators since one insulator can always be deformed to the other insulator adiabatically. Considering such an intimate relationship between the topological property of band insulators and the dimensionality (and also the symmetry), one natural question is how the dimensional crossover from 2D to 3D limit occurs in a thin film structure.In fact, it is recently proposed that in systems with TRS, if we make a thin film by stacking 2D topological insulators, the topological nature of the film dramatically changes depending on the parity of the number of stacked 2D layers. Such an oscillating topological property of the film eventually gives rise to a topologically nontrivial insulator in the 3D bulk limit. 12 The main purpose of this work is to establish such a fundamental relationship between the dimensionality and the topological nature of a system in correlated topological materials.In particular, we focus on the evolution of topological properties of a transition metal oxide (TMO) with strong spin-orbit coupling in its thin film form. The motivation to study TMO with large spin-orbit coupling is two-fold. At first, in this system, due to the strong spinorbit coupling and the resultant enhanced electron cor...

Geometrical lattice engineering of complex oxide heterostructures: a designer approach to emergent quantum states

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Spin ice Thin Film: Surface Ordering, Emergent Square ice, and Strain Effects

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Are the surface Fermi arcs in Dirac semimetals topologically protected?

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