Negative flat band magnetism in a spin-orbit coupled correlated kagome magnet (1901.04822v2)

Abstract: It has long been speculated that electronic flat band systems can be a fertile ground for hosting novel emergent phenomena including unconventional magnetism and superconductivity. Although flat bands are known to exist in a few systems such as heavy fermion materials and twisted bilayer graphene, their microscopic roles and underlying mechanisms in generating emergent behavior remain elusive. Here we use scanning tunneling microscopy to elucidate the atomically resolved electronic states and their magnetic response in the kagome magnet Co3Sn2S2. We observe a pronounced peak at the Fermi level, which is identified to arise from the kinetically frustrated kagome flat band. Increasing magnetic field up to +-8T, this state exhibits an anomalous magnetization-polarized Zeeman shift, dominated by an orbital moment in opposite to the field direction. Such negative magnetism can be understood as spin-orbit coupling induced quantum phase effects tied to non-trivial flat band systems. We image the flat band peak, resolve the associated negative magnetism, and provide its connection to the Berry curvature field, showing that Co3Sn2S2 is a rare example of kagome magnet where the low energy physics can be dominated by the spin-orbit coupled flat band. Our methodology of probing band-resolved ordering phenomena such as spin-orbit magnetism can also be applied in future experiments to elucidate other exotic phenomena including flat band superconductivity and anomalous quantum transport.

• The paper reveals that Co₃Sn₂S₂ exhibits a pronounced flat band at the Fermi level with robust negative orbital magnetism.
• Using STM under varying magnetic fields up to ±8T, the study demonstrates that spin-orbit coupling induces a Zeeman shift dominated by an oppositely aligned orbital moment.
• Band-structure calculations attribute the negative magnetism to Berry curvature effects, highlighting its impact on quantum transport and flatband superconductivity.

Analyzing Negative Flat Band Magnetism in a Spin-Orbit-Coupled Correlated Kagome Magnet

The paper investigates robust electronic states within the kagome magnet Co₃Sn₂S₂ using scanning tunneling microscopy (STM) to explore the manifestation of flat band magnetism coupled with spin-orbit interactions. Notable findings from this paper focus on the identification of a pronounced peak at the Fermi level, revealed to be associated with a kagome flat band. This flat band is distinct for its kinetic frustration and plays a pivotal role in the observed quantum behavior.

Key Observations and Methodologies

In studying Co₃Sn₂S₂ under varying magnetic fields up to ±8T, the authors capture a unique magnetization-polarized Zeeman shift, suggesting the dominance of an orbital moment oppositely aligned to the external magnetic field. This "negative magnetism" phenomenon is attributed to quantum phase effects induced by spin-orbit coupling (SOC)—a critical finding as it extends understanding into SOC's influence within flatband systems. Through meticulous STM imaging, the flatband peak was visualized, corroborating its magnetic properties and deducing its link to the Berry curvature field.

The comprehensive band-structure calculations substantiate the hypothesis that the flatband's negative orbital magnetism originates significantly from the Berry curvature effect within Co₃Sn₂S₂. This interplay infers a critical relationship between flatband physics and the associated anomalous magnetic phenomena.

Implications and Future Directions

The paper posits significant implications for the theoretical and practical advancement of quantum materials, particularly those exhibiting strong correlation effects intertwined with topology. The observed behavior of the spin-orbit coupled flatband enriches the understanding of emergent phenomena, like anomalous quantum transport and flatband superconductivity—a promising route for quantum materials' future applications.

The paper provides a novel methodological framework for unraveling band-resolved ordering phenomena such as SOC-induced magnetism. This sets a foundation for future explorations into the subtleties of exotic phenomena in quantum materials and is an exemplar for experimental probing of SOC and Berry phase effects at the microscopic scale. Further research may expand upon quantifying the electron-electron interactions within such correlated systems using comprehensive quantum many-body theories.

In conclusion, this paper explores the microscopic phenomena linked to negative flat band magnetism, underscoring the significance of SOC in correlated electron systems. It portrays Co₃Sn₂S₂ as a fertile ground for uncovering unconventional ferromagnetic properties due to spin-orbit interactions, propelling further inquiries into the design and understanding of materials with non-trivial topologies and interactions.