Public defence in Engineering Physics, M.Sc. Mohammad Amini

Title of the thesis: Manipulating Quantum States in 2D Ferroics

Doctoral student: Amini Mohammad
Opponent: Ikerbasque Research ProfessorJosé Pascual, Basque Foundation for Science, Spain
Custos: Professor Peter Liljeroth, Aalto University School of Science, Department of Applied Physics

In the past 20 years, experimental condensed matter physics has experienced a drastic shift. The discovery of graphene has revolutionized the field and brought huge attention to studying 2D materials. Since then, many van der Waals (vdW) 2D materials with different properties, such as ferroelectricity, ferromagnetism, and multiferroicity, have been discovered. 

In this thesis, I will focus on creating and studying these 2D systems. I will use molecular beam epitaxy (MBE) technique to design and fabricate van der Waals (vdW) heterostructures and scanning tunneling microscope (STM) to study their properties at cryogenic temperatures. I will showcase how we can engineer different materials with novel properties, which are at the core of condensed matter physics.

The first topic concerns artificial multiferroics, which we achieve by growing a magnetic layer of iron phthalocyanine (FePc) on a monolayer ferroelectric tin telluride (SnTe). I will show how we can control the orbital ordering of the molecule using the substrate's electric polarization. Secondly, I will focus on inelastic magnon excitations in monolayer ferromagnet chromium(III) bromide (CrBr3). We will show how their energy can be tuned by the moiré pattern between the CrBr3 and the highly ordered pyrolytic graphite (HOPG) substrate. These elementary excitations, which are the property of ferromagnets, are probed for the first time at the atomic scale using STM.

Finally, I will showcase our atomic scale study of the first known 2D multiferroic NiI2. I will show the existence of multiferroic properties in this system for the first time using STM. Our experiments allow atomic scale understanding of the mechanism of the multiferroicity in this system. By manipulating the multiferroic domains using external magnetic and electric fields, I will confirm the magnetoelectric coupling in this system.

This thesis will provide an overview of 2D systems with novel ferroic properties and paves the way for further advancements in condensed matter physics and quantum materials research.

Key words: Scanning Tunneling Microscopy, Multiferroicity, Ferromagnetism, Ferroelectricity, Quantum Matter, Two-dimensional materials

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

Contact information:

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