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arXiv:2604.24208

Dichroic Raman probes for chiral edge modes

Avedis Neehus, Johannes Knolle

Published Apr 27, 2026• 0 citations
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Abstract

The identification and manipulation of charge-neutral fractionalized quasi-particles, in particular chiral edge modes (CEM), is a long-standing quest in physics. Remarkably, the microscopically mediated interaction between light and charge-neutral excitations in Mott-Hubbard insulators can take an identical form to the Raman coupling between light and particles with electric charge. However, since CEMs are Raman-inactive due to conservation of lattice momentum, Raman probes have been deemed unsuitable for their identification. Here, using the Kitaev quantum spin liquid (KSL) as an illustrative example, we demonstrate that the long-range correlated disorder inherent to a closed edge can lead to a Raman circular dichroism (RCD) signal that avoids suppression by linear and angular momentum selection rules, and exhibits a dependence on experimentally tunable length and energy scales that are characteristic of CEM. Having calculated the low-frequency RCD response of generic KSL, we argue that the interaction of the chiral matter fermion with the $\Ztwo$ boundary charge leaves a unique fingerprint of the KSL via the anisotropic Zeeman field dependence.

DOI
10.48550/arxiv.2604.24208

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The identification and manipulation of charge-neutral fractionalized quasi-particles, in particular chiral edge modes (CEM), is a long-standing quest in physics. Remarkably, the microscopically mediated interaction between light and charge-neutral excitations in Mott-Hubbard insulators can take an identical form to the Raman coupling between light and particles with electric charge. However, since CEMs are Raman-inactive due to conservation of lattice momentum, Raman probes have been deemed unsuitable for their identification. Here, using the Kitaev quantum spin liquid (KSL) as an illustrative example, we demonstrate that the long-range correlated disorder inherent to a closed edge can lead to a Raman circular dichroism (RCD) signal that avoids suppression by linear and angular momentum selection rules, and exhibits a dependence on experimentally tunable length and energy scales that are characteristic of CEM. Having calculated the low-frequency RCD response of generic KSL, we argue that the interaction of the chiral matter fermion with the \Ztwo\Ztwo\Ztwo boundary charge leaves a unique fingerprint of the KSL via the anisotropic Zeeman field dependence.
Optical probes have emerged as versatile tools for detecting exotic fractionalized phases in quantum materials. We calculate the low-energy Raman response arising from mobile, interacting Ising anyons in the chiral Kitaev spin liquid perturbed by symmetry allowed interactions---a phase relevant to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ under a magnetic field. At zero temperature, the two-anyon continuum response shows a leading power-law scaling of the intensity near the onset of the signal: $I(\ensuremath{\omega})\ensuremath{\sim}{(\ensuremath{\omega}\ensuremath{-}{E}_{2\ensuremath{\sigma}}^{0})}^{\frac{1}{8}}$ for linear and parallel-circular polarization channels, where ${E}_{2\ensuremath{\sigma}}^{0}$ is the two-particle gap. Strong corrections due to short-range interactions arise at order 1/41/41/4. For cross-circularly polarized channels, the scaling is given by $I(\ensuremath{\omega})\ensuremath{\sim}{(\ensuremath{\omega}\ensuremath{-}{E}_{2\ensuremath{\sigma}}^{0})}^{|l\ifmmode\pm\else\textpm\fi{}1/8|}$, where the value of l=0,1,2l=0,1,2l=0,1,2 is determined by the number of minima in the single anyon dispersion. The exponents are directly related to the topological spin of Ising anyons ${\ensuremath{\theta}}_{\ensuremath{\sigma}}=\ensuremath{\pi}/8$, describing their exchange statistics. Our theory generalizes to spectral probes of anyonic quasiparticles with multiple band minima in other quantum liquids. Interaction between anyons may also induce bound states, resulting in sharp peaks that show strong polarization dependence.
Quantum spin liquids, a highly topologically entangled, dynamically correlated state where quantum fluctuations preclude any long-range ordering down to absolute zero. In the search for a topologically robust qubit, the scientific community has been in continuous hunt for real quantum spin liquid systems. Alexei Kitaev in his exactly solvable model for a spin-1/2 two-dimensional honeycomb lattice, presented a system that hosts a topologically protected state (Majorana zero-modes). Under an applied external field, the Kitaev spin liquids turn into a topologically non-trivial chiral spin-liquid state with non-abelian anionic excitations, which is crucial for quantum computing. Earlier theoretical predictions advocated that Kitaev physics can be realized in spin-orbit-coupled Mott insulators such as honeycomb irradiates and ruthenates. However, the experimental findings continuously challenge the theoretical aspects, indicating the presence of non-Kitaev interactions in real materials, where the dimensionality, disorder (vacancy), chemical composition, generalized spin-S, and external perturbations (pressure, magnetic field, temperature) can actively tune the Kitaev interactions and the ground state excitations. In this review article, a comprehensive discussion is included with an updated literature survey in the context of the potential of Raman spectroscopy as a probe for Kitaev quantum spin liquids.
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Scalar spin chirality, a three-body spin correlation that breaks time-reversal symmetry, is revealed to couple directly to circularly polarized laser. This is shown by the Floquet formalism for the periodically driven repulsive Hubbard model with a strong-coupling expansion. A systematic derivation of the effective low-energy Hamiltonian for a spin degree of freedom reveals that the coupling constant for scalar spin chirality can become significant for a situation in which the driving frequency and the on-site interaction are comparable. This implies that the scalar chirality can be induced by circularly polarized lights, or that it can be used conversely for probing the chirality in Mott insulators as a circular dichroism.
The Kitaev chiral spin liquid has captured widespread interest in recent decades because of its intrinsic non-Abelian excitations, yet the experimental realization is challenging. Here we propose to realize and detect Kitaev chiral spin liquid in a deformed honeycomb array of Rydberg atoms. With a laser-assisted dipole-dipole interaction mechanism which realizes both effective hopping and pairing terms for hard-core bosons, together with van der Waals interactions, we achieve the pure Kitaev model with high precision. The gapped non-Abelian spin liquid phase is then obtained under experimental conditions. Moreover, we propose innovative strategies to probe the chiral Majorana edge modes by light Bragg scattering and by imagining their chiral motion. Our proposed scheme broadens the range of quantum spin models with exotic topological phases that can be realized and detected in atomic systems, and makes an important step toward manipulating non-Abelian anyons. Published by the American Physical Society 2024
Quantum spin liquids realize massive entanglement and fractional quasiparticles from localized spins, proposed as an avenue for quantum science and technology. In particular, topological quantum computations are suggested in the non-abelian phase of Kitaev quantum spin liquid with Majorana fermions, and detection of Majorana fermions is one of the most outstanding problems in modern condensed matter physics. Here, we propose a concrete way to identify the non-abelian Kitaev quantum spin liquid by magnetic field angle dependence. Topologically protected critical lines exist on a plane of magnetic field angles, and their shapes are determined by microscopic spin interactions. A chirality operator plays a key role in demonstrating microscopic dependences of the critical lines. We also show that the chirality operator can be used to evaluate topological properties of the non-abelian Kitaev quantum spin liquid without relying on Majorana fermion descriptions. Experimental criteria for the non-abelian spin liquid state are provided for future experiments.