Engineering Weyl and Dirac Fermions in Transition Metal Perovskites

Abstract

The emergence of topological materials has fundamentally altered the landscape of condensed matter physics, introducing phases of matter defined by the topological properties of their electronic wavefunctions. A particularly compelling subclass is topological semimetals, which host quasiparticle excitations that are solid-state analogues of relativistic Dirac and Weyl fermions. These materials exhibit a host of remarkable properties, including ultra-high carrier mobilities, large non-saturating magnetoresistance, and topologically protected surface states, positioning them as key candidates for next-generation spintronic and quantum computing technologies.

Complex oxides, with their rich interplay of spin-orbit coupling, electron correlations, and structural diversity, present a highly tunable platform for realizing these exotic topological states. However, the synthesis of these materials in high-quality thin-film form is a significant experimental hurdle. The manifestation of their subtle topological characteristics is critically contingent upon achieving atomic-level precision in stoichiometry and structural perfection, as even minor defects can mask or annihilate the desired quantum states.

This doctoral thesis addresses this challenge by focusing on two exemplary transition metal perovskites: the itinerant ferromagnet SrRuO3, a candidate magnetic Weyl semimetal, and the strong spin-orbit-coupled paramagnet SrIrO3, a candidate Dirac semimetal. The core motivation of this research is to develop and utilize controlled synthesis methodologies to engineer and probe the emergent transport phenomena associated with Weyl and Dirac fermions in these oxide thin films and their heterostructures.

The thesis has been organized as follows:

Chapter 1: Introduction This foundational chapter provides a comprehensive overview of the theoretical and experimental landscape of topological semimetals. It elucidates the defining characteristics of Dirac, Weyl, and nodal-line semimetals, including their unique band structures and symmetry-protection mechanisms. The roles of linear dispersion and Berry curvature in producing hallmark transport signatures—such as high carrier mobility, large magnetoresistance, and the chiral anomaly—are discussed in detail. The chapter also reviews the key experimental techniques, particularly quantum oscillation measurements, that are essential for the characterization of these quantum states, thereby setting the stage for the experimental work presented in the subsequent chapters.

Chapter 2: Experimental methods The experimental methodologies employed throughout this research are detailed in this chapter. It outlines the use of Pulsed Laser Deposition (PLD) for the epitaxial growth of perovskite thin films. A suite of characterization techniques is described, including X-ray Diffraction (XRD) for structural analysis and SQUID magnetometry for probing magnetic properties. The chapter further details the microfabrication workflow for creating Hall bar devices using optical lithography and ion beam etching, followed by low-temperature magnetotransport measurements using a Physical Property Measurement System to probe the electronic properties of the engineered films.

Chapter 3: Predictive Modeling and Quantum Magnetotransport in Stoichiometry-Dependent Epitaxial SrRuO3 Thin Films This chapter reports a systematic investigation into the synthesis of epitaxial SrRuO3 (SRO) films on SrTiO3 (111) substrates, with a central focus on the critical role of stoichiometry and the emergence of novel quantum phenomena. To overcome the challenges of Ru volatility in PLD, a Gradient Boosting regression model was successfully employed to navigate the complex growth parameter space, enabling the synthesis of near-stoichiometric films with a high residual resistivity ratio (RRR) of 31.4 and a Curie temperature of 158 K. A key discovery was the observation of strong coupling between the magnetic and electric properties and clear Shubnikov–de Haas oscillations exclusively in these high-quality films. Analysis of these quantum oscillations revealed a non-trivial Berry phase of approximately 0.96π, which provides compelling evidence of a topologically non-trivial band structure and underscores the necessity of material perfection for observing intrinsic quantum transport phenomena.

Chapter 4: Intrinsic Magnetic Weyl Semimetallicity in Ultra-Thin (111) SrRuO3 Films Building upon the optimized synthesis, this chapter presents the realization of intrinsic magnetic Weyl semimetallicity in ultra-thin SrRuO3 (111) films (4 nm and 18 nm) without external electric-field gating. The evidence for Weyl fermions is multifaceted and robust, including a significantly high, non-saturating positive magnetoresistance (up to 95 percent at 2 K and 14 T) and high carrier mobilities (up to 10,000 cm² V⁻¹ s⁻¹) extracted from Shubnikov–de Haas oscillations. The analysis revealed ultralow effective carrier masses (for example, 0.08 times the free electron mass) and mean free paths significantly exceeding the film thickness, indicating a two-dimensional transport regime likely dominated by topological surface states. These experimental results were corroborated by Density Functional Theory calculations, confirming the presence of Weyl nodes near the Fermi level.

Chapter 5: Magnetotransport Signatures of Dirac Semimetallicity in SrIrO3 and in SrRuO3/SrIrO3 Heterostructures The research was then extended to the 5d-electron perovskite SrIrO3 (SIO). High-quality epitaxial SIO films were synthesized, which exhibited the characteristic transport signatures of a Dirac semimetal, including high carrier mobility (approximately 4000 cm² V⁻¹ s⁻¹) and a pronounced linear magnetoresistance at high fields. To probe the interplay of magnetism and topology, SRO/SIO bilayer heterostructures were engineered, and remarkably, interfacing SIO with ferromagnetic SRO led to a significant enhancement of the transport signatures. This was evidenced by a five-fold increase in magnetoresistance (to approximately 50 percent) and a substantial boost in the high-mobility channel to approximately 6700 cm² V⁻¹ s⁻¹. These results demonstrate that interfacial engineering is a powerful strategy to stabilize and improve the properties of topological quasiparticles.

Chapter 6: Conclusion and Outlook This chapter summarizes the principal findings of the thesis and proposes future research directions. This work successfully established a methodology, combining controlled PLD synthesis and heterostructure engineering, to realize and probe topological fermionic states in transition metal oxides. The key achievements of this thesis are: (i) establishing the critical role of stoichiometry in SrRuO3 for observing quantum phenomena, leading to the measurement of a non-trivial Berry phase; (ii) demonstrating intrinsic magnetic Weyl semimetallicity in ultra-thin SrRuO3 films, characterized by high magnetoresistance and mobility; (iii) identifying robust Dirac semimetal signatures in epitaxial SrIrO3 films; and (iv) showing that these topological transport properties are significantly enhanced through interfacial engineering in SrRuO3/SrIrO3 bilayers. These findings pave the way for future investigations into novel oxide heterostructures and superlattices. The demonstrated high-performance transport characteristics also underscore the potential of these engineered films for developing next-generation spintronic and quantum devices, warranting focused efforts on theoretical modeling and device fabrication.