Direct Observation of Itinerant Spin Polarons in a Kinetically Frustrated Hubbard System

Core Concepts

Itinerant spin polarons, bound states of a dopant and a spin flip, are directly observed in a triangular-lattice Hubbard system realized with ultracold atoms, revealing enhanced antiferromagnetic correlations around hole dopants and ferromagnetic correlations around charge dopants.

Abstract

The article reports the direct observation of itinerant spin polarons in a triangular-lattice Hubbard system realized with ultracold atoms. Spin polarons are quasiparticles that arise from the interplay between the kinetic energy of doped charge carriers and spin interactions in strongly correlated materials. In kinetically frustrated lattices, itinerant spin polarons are theoretically predicted to exist even without superexchange coupling. However, a microscopic observation of these polarons has been lacking. The authors use their ultracold atom system to directly image the local magnetic correlations around hole and charge dopants. Around hole dopants, they observe enhanced antiferromagnetic correlations, a signature of itinerant spin polarons. In contrast, around charge dopants, they find ferromagnetic correlations, a manifestation of the elusive Nagaoka effect. The authors study the evolution of these correlations with interactions and doping, and use higher-order correlation functions to elucidate the relative contributions of superexchange and kinetic mechanisms. The robustness of itinerant spin polarons at high temperature suggests potential mechanisms for hole pairing and superconductivity in frustrated systems. Furthermore, the work provides microscopic insights into related phenomena in triangular-lattice moiré materials.

Stats

The emergence of quasiparticles in quantum many-body systems underlies the rich phenomenology in many strongly interacting materials. In the context of doped Mott insulators, magnetic polarons are quasiparticles that usually arise from an interplay between the kinetic energy of doped charge carriers and superexchange spin interactions. In kinetically frustrated lattices, itinerant spin polarons—bound states of a dopant and a spin flip—have been theoretically predicted even in the absence of superexchange coupling.

Quotes

"Despite their important role in the theory of kinetic magnetism, a microscopic observation of these polarons is lacking." "In contrast, around a charge dopant, we find ferromagnetic correlations, a manifestation of the elusive Nagaoka effect." "The robustness of itinerant spin polarons at high temperature paves the way for exploring potential mechanisms for hole pairing and superconductivity in frustrated systems."

Key Insights Distilled From

Directly imaging spin polarons in a kinetically frustrated Hubbard system - Nature

by Max L. Prich... at www.nature.com 05-08-2024

https://www.nature.com/articles/s41586-024-07356-6

Deeper Inquiries

How do the properties of itinerant spin polarons, such as their size and binding energy, depend on the specific parameters of the Hubbard model and the degree of kinetic frustration?

The properties of itinerant spin polarons in a Hubbard system are intricately linked to the model parameters and the level of kinetic frustration present in the system. In a kinetically frustrated lattice, where the motion of charge carriers is hindered due to geometric constraints, the size and binding energy of spin polarons can exhibit distinct behavior compared to non-frustrated systems. The size of a spin polaron, which characterizes the spatial extent of the bound state between the dopant and the spin flip, is influenced by the competition between the kinetic energy of the carriers and the strength of the interactions. In highly frustrated systems, the limited mobility of charge carriers can lead to larger spin polarons as the carriers tend to localize around the dopant site due to the frustration of hopping processes. This localization effect can result in an increase in the size of the spin polaron compared to non-frustrated systems. Moreover, the binding energy of itinerant spin polarons is also affected by the Hubbard model parameters and the degree of kinetic frustration. In a Hubbard system with strong on-site repulsion, the binding energy of the spin polaron is determined by the balance between the potential energy associated with the interactions and the kinetic energy of the carriers. In kinetically frustrated lattices, where the hopping of carriers is hindered, the binding energy of spin polarons can be enhanced due to the increased localization of the carriers around the dopant site. This localization effect can lead to a stronger binding between the dopant and the spin flip, resulting in higher binding energies for itinerant spin polarons in frustrated systems compared to non-frustrated ones. Therefore, the properties of itinerant spin polarons, including their size and binding energy, are intricately dependent on the specific parameters of the Hubbard model, such as the strength of interactions and the hopping terms, as well as the degree of kinetic frustration present in the lattice. Understanding these dependencies is crucial for unraveling the unique behavior of spin polarons in kinetically frustrated systems and their implications for the overall magnetic and electronic properties of the material.

What are the implications of the observed ferromagnetic correlations around charge dopants for the understanding of the Nagaoka effect and its potential role in correlated electron systems?

The observation of ferromagnetic correlations around charge dopants in the context of the Nagaoka effect provides valuable insights into the behavior of correlated electron systems and the role of kinetic frustration in driving magnetic phenomena. The Nagaoka effect, named after Yosuke Nagaoka, is a fundamental concept in the field of strongly correlated electron systems, particularly in the context of doped Mott insulators. It describes the emergence of ferromagnetic order in a system with a single hole or electron doped into an antiferromagnetically ordered background, leading to the destabilization of the antiferromagnetic state and the formation of ferromagnetic correlations around the dopant. The presence of ferromagnetic correlations around charge dopants in the Hubbard system studied indicates the manifestation of the Nagaoka effect in a kinetically frustrated lattice. In the absence of superexchange interactions, the kinetic frustration of the lattice plays a crucial role in promoting ferromagnetic correlations around charge dopants, highlighting the significance of the kinetic mechanisms in driving magnetic ordering in frustrated systems. The observation of the Nagaoka effect in this context provides a deeper understanding of the interplay between kinetic energy, interactions, and frustration in correlated electron systems, shedding light on the complex magnetic behavior that can emerge in these materials. Furthermore, the implications of the observed ferromagnetic correlations extend to the potential role of the Nagaoka effect in the context of hole pairing and superconductivity in frustrated systems. By elucidating the magnetic properties around charge dopants and their connection to the Nagaoka effect, this work opens up new avenues for exploring the mechanisms underlying unconventional superconductivity and other emergent phenomena in correlated electron systems. Understanding the interplay between ferromagnetic correlations, kinetic frustration, and the Nagaoka effect is essential for unraveling the complex physics of these systems and harnessing their unique properties for technological applications.

Given the connection to moiré materials, how can the insights from this work be leveraged to explore novel quantum phenomena in other frustrated lattice systems, such as those involving spin-orbit coupling or topological band structures?

The insights gained from the study of itinerant spin polarons in a Hubbard system with a triangular lattice can be leveraged to explore novel quantum phenomena in other frustrated lattice systems, particularly those involving spin-orbit coupling or topological band structures, such as moiré materials. The unique properties of itinerant spin polarons, their dependence on kinetic frustration, and their impact on magnetic correlations provide a valuable framework for investigating the behavior of correlated electron systems in diverse lattice geometries and interaction regimes. In the context of moiré materials, which are characterized by the presence of a moiré pattern resulting from the stacking of two-dimensional layers with a slight twist angle, the insights from this work offer a foundation for studying the interplay between frustration, interactions, and quantum effects in these systems. By extending the understanding of itinerant spin polarons to moiré materials, researchers can explore how the geometric constraints imposed by the moiré pattern influence the formation and dynamics of spin polarons, as well as their implications for magnetic and electronic properties. Moreover, the connection to moiré materials opens up opportunities to investigate the role of spin-orbit coupling and topological band structures in shaping the behavior of itinerant spin polarons and related phenomena. By incorporating spin-orbit interactions or engineering topological features in frustrated lattice systems, researchers can explore how these additional factors modulate the properties of spin polarons, magnetic correlations, and emergent quantum states. The insights from this work provide a roadmap for probing the rich physics of correlated electron systems in a variety of lattice configurations, paving the way for the discovery of novel quantum phenomena and potential applications in quantum technologies.