Half-integer quantized anomalous thermal Hall effect in the Kitaev material $α$-RuCl$_3$ (2001.01899v1)

Abstract: Heat transport mediated by Majorana edge modes in a magnetic insulator leads to a half-integer thermal quantum Hall conductance, which has recently been reported for the two-dimensional honeycomb material $\alpha$-RuCl$_3$. While the conventional electronic Hall effect requires a perpendicular magnetic field, we find that this is not the case in $\alpha$-RuCl$_3$. Strikingly, the thermal Hall plateau appears even for a magnetic field with no out-of-plane components. The field-angular variation of the quantized thermal Hall conductance has the same sign structure of the topological Chern number, which is either $\pm$1, as the Majorana band structure of the pure Kitaev spin liquid. This observation of a half-integer anomalous thermal Hall effect firmly establishes that the Kitaev interaction is primarily responsible and that the non-Abelian topological order associated with fractionalization of the local magnetic moments persists even in the presence of non-Kitaev interactions in $\alpha$-RuCl$_3$.

• The paper reveals that α-RuCl₃ exhibits a half-integer thermal Hall plateau, reinforcing the presence of a Kitaev spin liquid state.
• It demonstrates that the quantization occurs without an out-of-plane magnetic field, emphasizing the influence of chiral Majorana edge modes.
• The findings validate theoretical models and open prospects for low-power thermal computing and quantum information applications.

Overview of "Half-integer quantized anomalous thermal Hall effect in the Kitaev material α-RuCl₃"

The paper under discussion presents a comprehensive exploration of the half-integer quantized anomalous thermal Hall effect observed in the magnetic insulator α-RuCl₃. This discovery is pivotal in reinforcing the realization of a Kitaev spin liquid (KSL) state, characterized by the presence of Majorana fermions, in a real material system. The Kitaev model is a theoretical construct predicting exotic states of matter highly relevant for quantum computation applications, particularly for realizing non-Abelian anyons.

Discovery and Experimentation

The paper primarily focuses on experimental observations of thermal conductivity in α-RuCl₃ under various magnetic field orientations and magnitudes. Notably, the key finding is the half-integer quantization in the thermal Hall conductance, κ_{xy}^{2D}, manifesting distinctly even when the applied magnetic field lacks an out-of-plane component. This is particularly striking because such behavior contradicts conventional expectations tied to charged particle systems, where an out-of-plane magnetic field is typically required to invoke Hall effects.

The experiments demonstrate that α-RuCl₃ exhibits a thermal Hall conductance plateau at half-integer values of the quantum thermal conductance unit, K₀, specifically at κ{xy}^{2D} = ½ C{h} K_{0}, where C_{h} represents an integer related to chiral Majorana edge modes. This observation indicates the significant role of Majorana fermions in facilitating heat transport across the material edge without an electrical charge, ushering in potential utilizations in low-power thermal computing devices.

Theoretical Implications

In a Kitaev spin liquid, the intrinsic interactions and applied magnetic fields lead to a fractionalization of spins into itinerant Majorana fermions and static vortex excitations. Under the influence of a magnetic field, the spin liquid's Majorana dispersion gets gapped, forming a Chern insulator—a time-reversal symmetry-breaking phase characterized by the Chern number ±1.

The paper's experimental results align with the theoretical expectations of such a Chern insulating phase, as the sign and the onset of thermal Hall quantization closely adhere to the theoretical angular dependency constructs. The nature of the quantization without requiring out-of-plane magnetic components underscores the topological robustness of the thermal Hall effect and the dominant influence of the Kitaev interactions.

Practical and Theoretical Implications

The observed anomalous thermal Hall effect champions the dominion of the Kitaev interaction in real materials, thus demonstrating the possible realization of quantum computation platforms utilizing non-Abelian anyons. From a practical viewpoint, understanding these interactions in α-RuCl₃ could lead to advancements in quantum technology and information storage systems that leverage the stability and unique properties of states maintained by topological orders.

Furthermore, these findings open the pathway towards designing materials that exploit the Majorana edge modes for innovative thermal management techniques, relevant in low-energy-consuming devices. From the theoretical standpoint, the consistency of these results with existing models of KSL provides substantial validation of the framework, necessitating further exploration into modifying and expanding the Kitaev model to encapsulate non-Kitaev interactions in realistic material environments.

Future Developments

The research underscores the potential for further refinement in material synthesis and control of magnetic interactions in α-RuCl₃ and related compounds. As experimental techniques advance, detailed investigations into the temperature and field-dependence of κ{xy} and κ{xy}^{2D} could unveil further subtleties of the associated quantum phases. It will be crucial to extend these methodologies to new materials, paving the way to identify other Kitaev material candidates with similar topological and thermal properties.

In conclusion, while challenges remain in identifying and conclusively manipulating Majorana fermions in these systems, the work provides a vital cornerstone for theoretical model validation and potential application horizons in quantum computing and thermal engineering.