Public defence in Engineering Physics, M.Eng. Xin Huang

Title of the thesis: Different Is More---Selected Works in Heterostructures of Transition Metal Dichalcogenides

Doctoral student: Xin Huang
Opponent: Professor Teemu Ojanen, University of Tampere
Custos: Professor Adam Foster, Aalto University School of Science, Department of Applied Physics

Introducing two advances in lateral and vertical heterostructures and their new physics.

A heterostructure is a composite structure made of different components. When assembled together, the whole heterostructure exhibits new properties distinct from those of their individual constituents. Heterostructures provide a powerful platform for creating artificial designer quantum matter.

This research focuses on two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDCs). Using molecular beam epitaxy (MBE), we synthesized lateral heterostructures of VSe₂ and NbSe₂, as well as vertical heterostructures of 1T/1H-NbSe₂. We further studied their electronic properties using scanning tunneling microscopy (STM) and spectroscopy (STS).

In the work on VSe₂—NbSe₂ lateral heterostructures, our protocols successfully created atomically sharp 1D interfaces between these two single layers. We systematically studied their structural details and revealed an intriguing phenomenon: the emergence of Kondo resonance around the interfaces. This phenomenon arises from the interactions between localized magnetic moments in VSe₂ and conduction electrons from NbSe₂.

In the work on a 1T/1H-NbSe₂ vertical heterostructure, single-layer 1T-phase NbSe₂  was grown on top of 1H-phase NbSe₂. Charge transfer from the bottom layer to the top layer resulted in the formation of a doped Mott insulator—a starting point for studying high-temperature superconductors. We further analyzed the spatial distribution of the doped/undoped sites and uncovered possible charge correlations between them.

These results pave the way for producing and understanding many-body physics in strongly correlated systems. Together, these selected works highlight the potential of combining different structures to create more unknown phenomena and explore more novel physics.

Keywords: heterostructures, transition metal dichalcogenides, scanning tunneling microscopy

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact information: https://attopunk.xyz 

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52