Electronic and Magnetic Properties of MBE Grown Topological Semimetals
Open Access
- Author:
- Pillsbury, Timothy
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 11, 2020
- Committee Members:
- Nitin Samarth, Dissertation Advisor/Co-Advisor
Nitin Samarth, Committee Chair/Co-Chair
Cuizu Chang, Committee Member
Chaoxing Liu, Committee Member
Venkatraman Gopalan, Outside Member
Nitin Samarth, Program Head/Chair - Keywords:
- topological materials
magnetism
molecular beam epitaxy
ARPES
angle-resolved photoemission spectroscopy - Abstract:
- Exploring and expanding the menagerie of topological materials has been at the forefront of condensed matter physics for over a decade. With the discovery of each new class of materials, starting with graphene, followed by topological insulators, and continuing most recently into topological semimetals, new paradigms have been discovered that provide a platform for exploring fundamental physics as well as novel practical applications. Crucial to all these innovations has been the topologically protected edge and surface states, which in addition to their robust nature, also have strong spin or valley correlation with momentum, offering direct coupling of macroscopic electronic properties to the quantum state of electrons. The interaction between magnetism and topology has lead to novel quantum phenomena, such as the quantum anomalous Hall effect, as well as enhanced device capabilities, such as the switching of magnetic polarization via the topological surface states. In topological semimetals, it is predicted that magnetism creates a transition from a Dirac to a Weyl semimetal phase by breaking time-reversal symmetry. Experimentally realizing such a transition will provide a fundamental platform for examining the emergence of Weyl fermions, as well as a practical platform for engineering tailored Weyl semimetals, including the ideal case of two Weyl nodes which has yet to be discovered in any intrinsic Weyl semimetal. The material system explored most extensively throughout this dissertation is the transition metal dichalcogenide Dirac semimetal ZrTe2. Thus far, a few thin film studies have demonstrated the existence of a Dirac semimetal phase in this material directly through angle resolved photoemission spectroscopy (ARPES) analysis of the Dirac cone as well as through transport measurements of the chiral anomaly. Additionally, a very recent experiment demonstrates the existence of a superconducting phase in ZrTe2 that could lead to further interest if it is shown to coexist with the Dirac semimetal phase. To facilitate further research into this promising material platform, the first portion of this dissertation focuses on the the development of high quality ZrTe2 thin films on insulating substrates. This is a necessary step for characterizing the electronic states of the material as well as adapting it for spintronics. Through transmission electron microscopy, scanning tunneling microscopy, and x-ray diffraction, the lattice structure and quality are assessed. ARPES and transport measurements demonstrate hole-like carriers. While this matches theoretical predictions for this material, it is at odds with most of the current literature, which demonstrates n-type carriers due to the high defect density. To observe the Dirac cone, tellurium vacancies are intentionally introduced for ARPES measurements, although this method was not effective for transport measurements. Ultimately, this prevents the observation of many of the desirable transport phenomena expected of a Dirac semimetal. Finally, we report the presence of weak anti-localization in ZrTe2 thin films at low temperatures. After establishing the MBE growth of ZrTe2 thin films, chromium dopants are introduced into these films to form (CrxZr1-x)Te2. Structural characterization is consistent with a transition between the Dirac semimetal ZrTe2 and the recently discovered metallic 2D ferromagnet CrTe2. This makes it phenomenologically distinct from a recently reported Cr-intercalated ZrTe2, which did not demonstrate similar changes in lattice structure and demonstrates completely different ARPES and transport phenomena. The (CrxZr1-x)Te2 thin films presented in this work are demonstrated to be ferromagnetic, with a TC of ∼150 K, as revealed via the anomalous Hall effect as well as magnetometry measurements. This system could be a critical component for realizing the transition from a Dirac to a Weyl semimetal, although currently it is limited by the location of the Fermi level. Finally, the next steps to utilizing the Dirac semimetal ZrTe2 in real devices are presented. Additionally, other methods of inducing magnetism based on the foundation laid by the study of (CrxZr1-x)Te2 are explored, such as magnetic proximity effect and alternative dopants. Ultimately, this research presents an approach towards tailoring magnetism in topological semimetals that has important implications for examining the transition between Dirac and Weyl semimetals through time-reversal symmetry breaking as well as for designing functional materials for spintronics applications.