Superconductivity and Magnetism in Two-Dimensional MBE Grown Chalcogenide Films

Open Access
- Author:
- Stanley, Max
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 02, 2024
- Committee Members:
- Nitin Samarth, Chair & Dissertation Advisor
Vincent Crespi, Major Field Member
Cuizu Chang, Major Field Member
Mauricio Terrones, Program Head/Chair
Stephanie Law, Outside Unit & Field Member - Keywords:
- Molecular beam epitaxy
Two-dimensional materials
Superconductivity
Magnetism
Chalcogenides - Abstract:
- In the last two decades, following the discovery of graphene, the field of two-dimensional (2D) materials has surged to the forefront of condensed matter physics. Numerous van der Waals crystals have now been brought to the ultrathin limit, yielding a diverse array of physical phenomena and material properties. Semiconducting, topological, superconducting, and most recently ferromagnetic 2D crystals now serve as platforms for fundamental research and as potential boons of technological innovation. Two of these emergent phenomena, superconductivity and magnetism, will take precedence in this dissertation, due their strong ties with dimensionality and the fascinating physics in which they are rooted. In particular, we will focus on the enhanced, interfacial superconductivity of FeSe grown on SrTiO3 substrates, and the ferromagnetism in ultrathin films of the transition metal dichalcogenide CrTe2. A driving force of research related to superconductivity and our understanding of it is the identification and creation of new superconducting material systems, ideally with ever higher critical temperatures. An alternative approach to understanding superconductivity, however, comes from studying its destruction. To this end, we will first investigate a fundamental parameter of superconductivity in FeSe/SrTiO3 - the upper critical field, Hc2. By growing FeSe films with varying amounts of disorder, we are able to map out Hc2(T) using magnetoresistance measurements in pulsed, high magnetic fields up to 65 T. These measurements provide insight to the superconducting pair breaking mechanisms and anisotropies of FeSe, and how they relate to disorder when analyzed in the context of Werthamer-Helfand-Hohenberg (WHH) theory. We observe strong spin-orbit driven scattering enhancements of Hc2, as well as psuedoisotropic and multiband superconductivity, and the possible emergence of a low-temperature, high field superconducting phase. We will then discuss a systematic study on the influence of Co impurity doping on the superconductivity of FeSe/SrTiO3. We find a consistent suppression of superconductivity with increased cobalt concentration, likely resulting from overdoping evidenced by a Lifshitz transition seen in ARPES band mappings. An analysis of the residual resistance of our Fe1−xCoxSe films provides evidence of an s++-wave pairing symmetry in FeSe. Finally, we will turn our attention to the 2D magnet CrTe2, and discuss the growth, characterization, and ferromagnetism of our ultrathin films. We interface CrTe2 with the Dirac semimetal ZrTe2 to achieve high quality growth and realize long-range ferromagnetism persisting down to the CrTe2 monolayer. With this heterostructure, we put forward a proof of concept, all van der Waals device which utilizes the spin Hall conductivity of ZrTe2 to electrically switch the magnetization orientation in the adjacent CrTe2 layer.