Neutron tomography of magnetic Majorana fermions in a proximate quantum spin liquid (1609.00103v1)

Abstract: Quantum matter provides an effective vacuum out of which arise emergent particles not corresponding to any experimentally detected elementary particle. Topological quantum materials in particular have become a focus of intense research in part because of the remarkable possibility to realize Majorana fermions, with their potential for new, decoherence-free quantum computing architectures. In this paper we undertake a study on high-quality single crystal of $\alpha-RuCl_3$ which has been identified as a material realizing a proximate Kitaev state, a topological quantum state with magnetic Majorana fermions. Four-dimensional tomographic reconstruction of dynamical correlations measured using neutrons is uniquely powerful for probing such magnetic states. We discover unusual signals, including an unprecedented column of scattering over a large energy interval around the Brillouin zone center which is remarkably stable with temperature. This is straightforwardly accounted for in terms of the Majorana excitations present in Kitaev's topological quantum spin liquid. Other, more delicate, features in the scattering can be transparently associated with perturbations to an ideal model. This opens a window on emergent magnetic Majorana fermions in correlated materials.

• The paper demonstrates that inelastic neutron scattering reveals a robust magnetic continuum in α-RuCl₃, supporting the presence of emergent Majorana fermions in a QSL state.
• It employs four-dimensional tomographic reconstruction of momentum–energy correlations to capture temperature-dependent magnetic fluctuation signatures beyond conventional spin-wave theory.
• Results indicate enhanced Kitaev couplings and challenge traditional models, paving the way for research into decoherence-free quantum computing architectures.

Neutron Tomography of Magnetic Majorana Fermions in a Proximate Quantum Spin Liquid

The paper presented in the paper investigates the magnetic behavior of the quantum spin liquid (QSL) state using neutron tomography techniques on the α-RuCl₃ material. The authors' principal objective is to provide experimental insights into the nature of emergent magnetic Majorana fermions, attributed to Kitaev quantum spin liquids (KQSLs), constituted by S=1/2 spins on a honeycomb lattice.

Key Findings and Observations

The authors utilized four-dimensional tomographic reconstruction of dynamical correlations across momentum-energy space using inelastic neutron scattering to probe these quantum states. Their central finding is the unusual two-dimensional continuum spectrum of magnetic fluctuations centered at the Brillouin zone (BZ) center, which was examined across temperatures spanning below and beyond the Néel temperature $T_N = 7 K$. This paper provides experimental evidence corroborating the theoretical specification of a KQSL state manifesting in α-RuCl₃.

Key experimental results include:

• A robust magnetic scattering continuum at the BZ center (Γ point) that persists beyond 120 K, significantly larger than the estimated temperature associated with the Kitaev coupling.
• Scattering maintained a broad energy dependence of magnetic fluctuation signatures cosmopolitan in the proposed Majorana fermions in the system, thus aligning with the emergent Majorana excitations proposed by Kitaev's theoretical model.
• The temperature and momentum-dependent measurements show patterns not compatible with standard spin-wave theory (SWT) but instead align with Majorana fermionic activity.
• Results from isotropic Kitaev model calculations, adjusted with a phenomenological fitting parameter R, show strong alignment with experimental data, implying enhanced Kitaev couplings beyond nearest neighbors.

Implications and Future Work

The experimental elucidation of a QSL in α-RuCl₃ suggests its strategic importance in advancing our understanding of topological phases of matter and their fractionalized excitations, such as Majorana fermions. These findings have considerable implications for future research into decoherence-free quantum computation architectures due to the non-locality of Majorana fermions.

From a theoretical perspective, the results provide an opportunity for future analytical models to better align the observed experimental expectations of quantum spin liquid phases with theory. Enhancements in computational approaches to accurately capture extended Kitaev terms and any emerging interactions within realistic materials can better inform the scheme for predicting and tuning majorana excitations.

On the experimental front, expanding the scope to test phenomena under different physical perturbations, such as doping, external field application, or mechanical strain, could yield insights into the behavior of emergent states near zero temperature edges, validating or challenging current QSL models.

In conclusion, this paper provides a comprehensive and detailed picture of the Kitaev quantum spin liquid state interactions in α-RuCl₃, thus opening new directions for research into QSLs and the practical applications of exotic quantum states such as Majorana fermions.