New Structural and Electronic Degrees of Freedom in Epitaxial Square-Net Materials

Citation

Llanos, Adrian (2025) New Structural and Electronic Degrees of Freedom in Epitaxial Square-Net Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/41fj-5137. https://resolver.caltech.edu/CaltechTHESIS:03232025-064022463

Abstract

Materials belonging to the "square-net" (SN) family of crystal structures share the structural motif of highly conducting, 2D square-planar sheets sandwiched between complex spacer layers. Materials in this class have attracted attention for their diverse array of electronic properties such as topological, magnetic, charge/spin density wave (CDW/SDW) and superconducting ground states. Familiar examples include the cuprate and pnictide superconductors, the rare-earth tellurides and the Dirac semimetals such as ZrSiS. In this thesis we exploit the abilities of molecular beam epitaxy to synthesize and study ultra-thin films of the SN compounds and uncover several unexpected behaviors.

The first compound we explore is DyTe₂, a member of the telluride family of SN materials known for their charge density wave ground states. We begin by describing the methods to fabricate epitaxial films using MBE. The high crystalline quality allows for characterization of subtle superlattice modulations with X-ray diffraction. Combinations of this experimental data with theoretical calculations reveal the origin of this superlattice to be an ordering of Te vacancies driven by Fermi-surface nesting.

We then turn to the related compound LaSb₂. This material is thought to undergo a CDW transition that can be suppressed under pressure and replaced by a superconducting ground state. To our surprise, thin films of LaSb2 adopt a crystal structure distinct from that of the bulk crystals. We characterize this new structure comprehensively and find that concomitant with this new structure is an enhancement of superconducting Tc relative to the bulk.

Finally, we exploit this enhanced T꜀ to observe magnetic field-induced superconductivity in ultra-thin LaSb2 doped with magnetic Ce dopants. This is the result of the unique robustness of the material to application of a parallel magnetic field. The combination of strong spin orbit coupling and reduced dimensionality allows the magnetic field to polarize paramagnetic spins, thereby reducing their deleterious impact on T꜀, before the field itself destroys superconductivity. This allows a superconducting ground state to be induced from an otherwise normal metal ground state at T = 0.

The results of this thesis highlight the unique degrees of freedom that can be accessed via epitaxial growth of single crystalline films of quantum materials.