Interlayer magnetic coupling of the antiferromagnetic delafossite metal, PdCrO2

Abstract

Kondo physics explores the intricate interaction between conduction elec- trons and localized magnetic moments, revealing diverse phases and strong correlations within Kondo lattices. In these systems, localized magnetic mo- ments compete between magnetic ordering and Kondo screening, resulting in phenomena such as heavy fermions and superconductivity at quantum critical points. The rich variety of phases and behaviors exhibited by Kondo systems underscores their complex nature and offers abundant opportunities for further exploration and understanding.

Interlayer Kondo lattice systems, which involve coupling between layers containing localized spins and itinerant electrons, have emerged as promising platforms for research. Studies using artificial heterostructures have observed Kondo-like peaks and enhanced quasiparticle masses despite weaker interlayer couplings, demonstrating the persistence of Kondo physics. Moreover, natural bulk heterostructure systems like 4Hb-TaS2 and PdCrO2, influenced by local spin layers within the conduction layers, display phenomena such as chiral su- perconducting phases or non-Fermi liquid behaviors. These experimental and theoretical investigations highlight interlayer Kondo lattice systems not only as ideal settings for exploring traditional Kondo physics but also as innovative arenas for investigating intriguing physical phenomena.

Delafossite materials are of significant interest due to their unique structural and electronic properties. Especially, PdCrO2 features alternating layers of con- ducting Pd and magnetic insulating CrO2, creating a natural heterostructure that is ideal for studying interlayer Kondo lattice. In this material, the inter- layer magnetic coupling between Pd electrons and Cr localized spins is crucial and influences the electronic properties, providing a rich ground for exploring the interplay between conduction electrons and localized spins. In this thesis, various electronic transport properties of PdCrO2, such as magnetic quantum oscillations and the unconventional anomalous Hall effect, are explored in detail, with a comparative analysis against PdCoO2, a non-magnetic metal delafossite. Additionally, sophisticated bulk device fabrication at the microscale using FIB techniques enables more precise and intricate experiments.

In the first part, we introduce the Fermi surface reconstruction from the staggered chiral spin texture in PdCrO2. Utilizing quantum oscillation experi- ments and DFT+U calculations, we confirm that in addition to the known in- plane spin periodicity, there is an out-of-plane periodicity that breaks additional Pd band degeneracy. Through DFT+DMFT calculations of the Mott-localized spin and the on-site energy of Cr, we study the hybridization between Pd and Cr eg orbitals, revealing the unique interlayer magnetic coupling. Our research suggests that PdCrO2, with its natural heterostructure, serves as an excellent model system for studying the interplay between electrons and localized spins.

In the second part, we explore the spin-cluster skew scattering in in-plane transport from a highly frustrated antiferromagnet, PdCrO2, and its influence on the unconventional anomalous Hall effect. Above the spin ordering temper- ature (TN), fluctuating spin textures induce a substantial AHE, proportional to the thermal average of fluctuating spin chirality. Our experimental results demonstrate that in PdCrO2, a highly mobile single Fermi surface contributes to a giant anomalous Hall conductivity, which persists up to ∼ 150 K. This study provides an alternative route to realizing high-temperature giant anoma- lous Hall responses by exploiting magnetic frustration in the ultraclean regime.

In the third part, we extend our exploration to out-of-plane transport by fabricating precise devices capable of measuring the c-axis Hall effect. The re- sults showcase the characteristics of an open Fermi surface and reveal that, similar to in-plane transport, PdCrO2 exhibits a large, unconventional Hall re- sponse above its ordering temperature. This behavior persists even at extremely high temperatures, highlighting the influence of short-range spin correlations and suggesting further investigation into the transverse transport properties of open Fermi surface.

In the last part, we investigate the Hall effect in open three-dimensional Fermi surfaces using a semiclassical approach. Our analysis reveals that Hall conductivity in an open Fermi surface is determined by contributions from both closed and open orbits, with Fermi surface geometry and anisotropic mean-free- path vectors playing significant roles. Experimental results on PdCoO2 demon- strate notable anisotropy in magneto-transport properties, aligning well with theoretical predictions. Additionally, we observed an unexpected kink in the Hall response at high magnetic fields, indicating distinct mechanisms in longi- tudinal and transverse conductivities. This study enhances our understanding of transport properties in open Fermi surface, laying the groundwork for ad- vancements in materials such as goniopolar materials.

Our study reveals that PdCrO2 serves as an excellent model system for studying the interaction between nearly free electrons and localized spins. This natural heterostructure allows for detailed exploration of the interplay between conduction electrons and localized magnetic moments. Further research on this system is expected to yield deeper insights into the fundamental principles of strongly correlated electron systems.