Non-Kitaev Interactions and Field-Angle Dependence in Topological Magnon Transport of α-RuCl3

Core Concepts

The orientation of an applied magnetic field and the strength of various spin interactions significantly influence the thermal Hall conductivity and spin Nernst coefficient in α-RuCl3, a potential Kitaev quantum spin liquid material.

Abstract

• Bibliographic Information: Mosadeq, H., & Zare, M. (2024). Unveiling Non-Kitaev Interactions and Field-Angle Dependence in Topological Magnon Transport of α-RuCl3. arXiv preprint arXiv:2411.02894v1.

• Research Objective: This study investigates the impact of magnetic field orientation and exchange parameters on the transverse thermal conductivities (thermal Hall conductivity and spin Nernst coefficient) in α-RuCl3, a potential Kitaev quantum spin liquid material.

• Methodology: The researchers employed spin-wave theory (SWT) to analyze the nearest-neighbor Heisenberg-Kitaev-Gamma-Gamma′ (JKΓΓ′) model, incorporating the influence of an external magnetic field. They calculated the thermal Hall conductivity and spin Nernst coefficient for four different parameter sets of α-RuCl3 under various magnetic field orientations.

• Key Findings:

  • The sign and magnitude of the transverse thermal conductivities are highly sensitive to both the applied magnetic field's orientation and the exchange parameters within the JKΓΓ′ model.
  • A positive thermal Hall conductivity was observed when the magnetic field was aligned with the [001] and [11¯2] directions, while a negative value was found for the [111] direction.
  • The thermal Hall conductivity vanished when the field was aligned along the [¯110] direction due to C2 rotational symmetry.
  • Increasing off-diagonal exchange interactions (Γ and Γ′) led to a decrease in both the thermal Hall conductivity and the spin Nernst coefficient, correlating with a reduction in the band gap.
  • The anisotropic Landé g-factor in α-RuCl3 significantly influenced the field-angle dependence of the thermal Hall conductivity.
  • In the partially-polarized ferromagnet (PPF) phase, the sign change of the thermal Hall conductivity was primarily determined by the dominance of the in-plane magnetic field component.

• Main Conclusions: The study highlights the crucial role of magnetic field orientation and exchange interactions in determining the topological properties and thermal transport in α-RuCl3. The findings provide insights into the complex interplay between these factors and their influence on the material's potential for hosting exotic magnetic phases and exhibiting unique thermal transport phenomena.

• Significance: This research contributes to the understanding of topological magnon transport in Kitaev materials and provides valuable insights for the development of spintronic and quantum technologies based on these materials.

• Limitations and Future Research: The study focuses on the linear spin-wave theory, which might not fully capture the complexities of α-RuCl3. Further investigations using more sophisticated theoretical approaches and experimental validation of the predicted effects are necessary. Exploring the influence of other factors like pressure and doping on the topological properties of α-RuCl3 could be promising avenues for future research.

Stats

The thermal Hall conductivity (κxyTH/T) in the a-polarized state is approximately 0.35 × 10−3mW/K2m.
The maximum value of κxy,2DTH/T for the model 4(JKΓΓ′) is approximately 0.2 × (πk2B/6ℏ), which is roughly 40% of the half-quantized value.
The interlayer distance (d) for α-RuCl3 is 5.72 Å.

Quotes

"It is noteworthy that a pronounced anisotropy of magnonic properties with respect to the in-plane direction of an external magnetic field is indicative of a dominant Kitaev interaction."
"Recent thermal Hall conductivity measurements of the intermediate QSL phase have revealed a half-integer quantized plateau for magnetic fields applied along the a direction. Conversely, no such plateau was observed when the field was oriented along the b direction."

Key Insights Distilled From

Unveiling Non-Kitaev Interactions and Field-Angle Dependence in Topological Magnon Transport of $\alpha$-RuCl$_3$

by Hamid Mosade... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.02894.pdf

Deeper Inquiries

How would the inclusion of further-neighbor interactions in the model affect the predicted thermal transport properties?

Answer:
Including further-neighbor interactions in the JKΓΓ′ model for α-RuCl3 could significantly impact the predicted thermal transport properties, particularly the thermal Hall conductivity (κxyTH) and spin Nernst coefficient (κxyN). Here's how:

Modification of Magnon Band Structure: Further-neighbor interactions can alter the magnon band structure, potentially changing the band gap, Berry curvature, and Chern numbers. These changes directly influence the topological nature of the magnon bands and consequently the thermal transport properties. For instance, introducing further-neighbor Heisenberg interactions could lead to band inversions, modifying the Chern number and potentially even changing the sign of κxyTH.

Emergence of New Topological Phases: The inclusion of further-neighbor interactions, especially those with anisotropic and frustrated nature, can give rise to novel topological phases beyond those captured by the nearest-neighbor model. These phases might host exotic excitations with distinct thermal transport signatures. For example, incorporating further-neighbor Kitaev interactions could stabilize a chiral spin liquid phase with a quantized thermal Hall conductivity.

Influence on Magnon-Magnon Interactions: While the provided context focuses on linear spin-wave theory, which neglects magnon-magnon interactions, further-neighbor terms can enhance these interactions. At higher temperatures, these interactions can lead to magnon scattering, impacting the thermal conductivity magnitudes.

Quantitative and Qualitative Changes: The specific impact of further-neighbor interactions depends on their strength and type. Small perturbations might only quantitatively modify the existing thermal transport features. However, stronger further-neighbor interactions could lead to qualitative changes, including the emergence of new topological phases with distinct thermal Hall and spin Nernst responses.
In conclusion, incorporating further-neighbor interactions in the model is crucial for a complete understanding of thermal transport in α-RuCl3. It allows for exploring a wider range of topological phases and capturing the influence of magnon-magnon interactions, leading to more accurate predictions of the material's thermal transport behavior.

Could the observed anisotropy in thermal transport be exploited for developing novel spintronic devices?

Answer:
The observed anisotropy in the thermal transport properties of α-RuCl3, particularly the dependence of κxyTH and κxyN on the magnetic field orientation, presents intriguing possibilities for novel spintronic devices. Here are some potential avenues for exploitation:

Field-Controlled Thermal Transistors: The strong dependence of κxyTH on the in-plane magnetic field direction could be utilized to create thermal transistors. By controlling the magnetic field orientation, one could switch the thermal current flow on or off, paving the way for heat management in nanoscale devices.

Directional Heat Flow Devices: The anisotropic thermal Hall effect enables the design of devices that conduct heat preferentially in specific directions. This could be valuable for thermal management applications, such as heat dissipation in integrated circuits or thermoelectric energy conversion.

Spin Current Generation and Detection: The anisotropic spin Nernst effect suggests the possibility of generating and detecting spin currents using temperature gradients. By applying a temperature gradient along a specific crystallographic direction, one could generate a spin current in a perpendicular direction, enabling the development of spin-based logic and memory devices.

Topological Magnonic Logic Gates: The presence of topologically protected edge states in certain magnetic field configurations suggests the possibility of constructing magnonic logic gates. These gates would exploit the dissipationless nature of edge magnon transport, potentially leading to low-power spintronic devices.

Sensing Applications: The sensitivity of thermal transport properties to magnetic fields and potentially other external stimuli makes α-RuCl3 a promising candidate for sensing applications. For instance, it could be used to detect small magnetic field variations or temperature gradients with high precision.
However, realizing these applications requires overcoming several challenges. These include:

Material Synthesis and Control: Synthesizing high-quality α-RuCl3 with controlled properties, such as low defect density and precise control over exchange interactions, is crucial.

Operating Temperatures: The observed effects are prominent at low temperatures, limiting the practical applicability of devices based on these phenomena. Exploring ways to enhance the effects at higher temperatures is essential.

Device Integration: Integrating α-RuCl3 with existing spintronic materials and architectures is necessary for practical device fabrication.
Despite these challenges, the unique anisotropic thermal transport properties of α-RuCl3 offer exciting possibilities for developing novel spintronic devices with potential applications in heat management, spin current control, and sensing.

How does the understanding of topological magnon transport in α-RuCl3 contribute to the broader search for materials hosting Majorana fermions?

Answer:
While the study of topological magnon transport in α-RuCl3 doesn't directly confirm the existence of Majorana fermions, it significantly contributes to the broader search for these elusive particles in several ways:

Understanding Kitaev Physics: α-RuCl3 is a prime candidate for realizing the Kitaev spin liquid, a theoretical model predicted to host Majorana fermions as emergent excitations. Investigating topological magnon transport in this material provides valuable insights into the underlying Kitaev physics, which is crucial for identifying other potential Majorana platforms.

Identifying Relevant Energy Scales: The observation of a sign change in κxyTH in the partially polarized ferromagnetic (PPF) phase, reminiscent of Majorana fermion behavior, highlights the importance of specific magnetic field orientations and energy scales for potentially uncovering Majorana signatures. This knowledge guides the search for materials and experimental conditions conducive to Majorana physics.

Developing Experimental Probes: The study of topological magnons in α-RuCl3 advances the development of sensitive experimental probes for detecting exotic excitations. Techniques used to measure thermal Hall conductivity and spin Nernst effect can be adapted and refined to search for the unique signatures of Majorana fermions in other candidate materials.

Exploring Connections Between Different Topological Phases: The observation of different topological phases in α-RuCl3 under varying magnetic fields suggests a potential connection between topological magnon phases and those hosting Majorana fermions. Understanding these connections could provide a pathway to engineer materials that exhibit both phenomena, potentially facilitating the manipulation and detection of Majorana fermions.

Expanding the Search Scope: The challenges encountered in definitively proving Majorana fermions in α-RuCl3, such as the need for extremely low temperatures and the influence of non-Kitaev interactions, provide valuable lessons for the broader search. These insights help refine experimental techniques and focus on materials with potentially more favorable properties for hosting and detecting Majorana fermions.
In conclusion, while not a definitive proof, the study of topological magnon transport in α-RuCl3 significantly advances the search for Majorana fermions. It deepens our understanding of Kitaev physics, identifies relevant energy scales and experimental probes, and highlights potential connections between different topological phases, ultimately guiding the search for these elusive particles in other quantum materials.