Non-Abelian Free Magnetic Phase

• Non-Abelian Free Magnetic Phase is a quantum state characterized by emergent non-Abelian anyons and quantized geometric phases in lattice models and quantum particles.
• In the Kitaev honeycomb model, a weak [111] magnetic field induces a cubic gap scaling, leading to robust topological order with chiral Majorana edge states and Ising fusion rules.
• A parallel realization in supersymmetric quantum systems reveals SU(2) Berry connections and magnetic monopole phases, uniting field-induced and adiabatic non-Abelian mechanisms.

The non-Abelian free magnetic phase describes distinct quantum states in which non-Abelian gauge structure manifests either as emergent anyonic excitations in exactly solvable lattice models under weak magnetic fields, or as robust geometric phases—specifically non-Abelian Berry connections—arising from adiabatic manipulation of degenerate subspaces in quantum systems. The two dominant physical realizations are: (1) the non-Abelian Ising topological phase induced by a small [111] field in the Kitaev honeycomb model, featuring chiral Majorana edge states and non-Abelian anyons; and (2) the SU(2) magnetic monopole geometric phase observed for a quantum particle on a circle with point-like interactions tuned for $\mathscr{N}=2$ supersymmetry. Both frameworks showcase how magnetic effects, either physical or parameter-space, drive quantum systems into non-Abelian topological or geometric phases with quantized holonomy and robust ground state degeneracy.

1. Kitaev Model in a [111] Magnetic Field: Hamiltonian and Emergent Non-Abelian Phase

The canonical lattice realization of a non-Abelian free magnetic phase is found in the Kitaev honeycomb model subject to a magnetic field along the [111] crystalline axis. The full Hamiltonian is

$$H = \sum_{\langle i,j\rangle_\gamma} K_\gamma S_i^\gamma S_j^\gamma - h\sum_i (S_i^x+S_i^y+S_i^z)$$

where $S_i^\gamma = \sigma_i^\gamma/2$ are spin-1/2 operators, $K_\gamma$ are the Kitaev couplings (typically chosen isotropic, $K_x = K_y = K_z \equiv K = \pm 1$), and $h$ is the magnetic field strength. In terms of Pauli operators, the Zeeman term is written as

$$-\frac{h}{\sqrt{3}} \sum_i (\sigma_i^x + \sigma_i^y + \sigma_i^z).$$

At small $h$, the system is adiabatically connected to the integrable Kitaev plus three-spin model, in which the additional

$$K_3 \sum_{\langle\langle i, j, k\rangle\rangle} S_i^x S_j^y S_k^z$$

interaction gaps out the Majorana spectrum while preserving exact solvability. Importantly, the weak magnetic field explicitly breaks time-reversal symmetry and drives the originally gapless spin liquid into a topological phase with non-Abelian anyons (Gohlke et al., 2018, Zhu et al., 2017).

2. Gap Opening and Its Field Dependence

The opening of a bulk excitation gap in the non-Abelian phase is a direct consequence of a weak [111] field breaking time-reversal. At lowest nonvanishing order in perturbation theory, the magnetic field induces an effective three-spin interaction of the form $\sim h^3 S_i^x S_j^y S_k^z$, leading to a gap scaling

$\Delta(h) \propto h^3.$

Matrix product state numerics (iDMRG) extract the correlation length $\xi(h)$ in the topological phase, demonstrating the scaling

$$\xi(h) \sim (32 h^3)^{-1} \quad\Longrightarrow\quad \Delta(h) \sim 1/\xi(h) \propto h^3.$$

This cubic dependence reproduces exactly the integrable limit with three-spin interactions ($\xi(K_3) \sim 1/K_3$) and indicates that even a very weak field suffices to fully gap the bulk Majorana spectrum [(Gohlke et al., 2018), Fig. 3].

3. Topological Order and Non-Abelian Anyon Content

The field-induced phase is characterized by Ising non-Abelian topological order with three types of anyons: $\{\mathbb{1}, \psi, \sigma\}$, obeying fusion rules

$$\sigma \times \sigma = \mathbb{1} + \psi,\quad \sigma \times \psi = \sigma,\quad \psi \times \psi = \mathbb{1}.$$

The nontrivial F- and R-symbols encode Ising statistics, where the braiding phases are

$$R^{\sigma\sigma}_\mathbb{1} = e^{-i\pi/8}, \quad R^{\sigma\sigma}_\psi = e^{3i\pi/8}.$$

Each vortex binds a Majorana zero mode, and the total Chern number of the two Majorana bands is $\pm 1$. The quantum dimension for the vortex is $d_\sigma = \sqrt{2}$, while $d_\mathbb{1} = d_\epsilon = 1$. The topological entanglement entropy, extracted from scaling of the bipartite von Neumann entropy on infinite cylinders, confirms this structure: $\gamma \approx \log 2$, signaling total quantum dimension $\mathcal{D} = 2$ [(Gohlke et al., 2018), Fig. 4; (Zhu et al., 2017)].

4. Dynamical Correlations and Experimental Signatures

Dynamical spin structure factor calculations (MPO-based real-time evolution) in the topological phase reveal characteristic reshaping of spectral features:

• At $h=0$, a sharp two-flux gap $\Delta_2 \approx 0.03$ appears as a delta-peak, superposed on a broad Majorana continuum extending up to $\omega \sim 1.5$.
• For increasing $h$ ($0.1 < h < 0.2$), the low-energy delta-peak broadens and splits, tracking flux mobility, while the broad continuum persists.
• In the fully integrable three-spin model, this delta-peak shifts to higher energies ($\omega \approx 0.2$) and the spectral edges remain sharp.
• When both field and three-spin terms are present, the gap collapses rapidly and dispersive subpeaks reflect mobile vortices.

Such dynamical features, especially the well-defined multi-peak structure and the persistence of the low-energy gap, serve as direct diagnostics of the non-Abelian phase [(Gohlke et al., 2018), Fig. 5].

5. Magnetic Field Robustness and Phase Diagrams: Ferromagnetic vs. Antiferromagnetic Kitaev Couplings

The stability of the non-Abelian phase is acutely sensitive to the sign of the Kitaev coupling:

• For ferromagnetic ($K < 0$) interactions, the phase is fragile: the topological order collapses at a small $h_{c,\text{FM}} \approx 0.014$.
• For antiferromagnetic ($K > 0$) interactions, robustness is enhanced, with topological order persisting up to $h_{c1,\text{AF}} \approx 0.22$, more than an order of magnitude larger.
• In the AF regime, an intermediate, likely gapless, phase emerges for $0.22 < h < 0.36$.
• At even larger fields, both FM and AF models transition into a trivial, fully polarized state. The FM transition is close to the linear spin-wave prediction, while the AF transition substantially undershoots the classical $h_{c,\text{clas}} = 1/\sqrt{3}$.

This distinction arises because FM and AF Kitaev models, though gauge equivalent at zero field, differ strongly in their uniform field responses. Frustration and spin–orbit anisotropy in the AF case suppress field-induced magnetization, protecting the bulk flux gap and stabilizing the non-Abelian phase to much larger $h$. Once time-reversal is broken, the Majorana sector acquires Chern number $C = \pm 1$, ensuring chiral order (Zhu et al., 2017).

6. Non-Abelian Geometric Phases: Free Particle with Point-like Interactions

A parallel realization of the non-Abelian free magnetic phase emerges in the setting of a quantum particle on a circle with point-like interactions at antipodal points and $\mathscr{N}=2$ supersymmetry. The model's parameter space is $S^1_\alpha \times S^2_Z$, with $Z \in U(2)/U(1)^2 \cong S^2$ serving as an effective SU(2) orientation parameter. The spectrum—except potentially for a single ground state—is exactly two-fold degenerate for all excited levels due to supersymmetry.

Adiabatic variation of $Z$ along loops in $S^2$ produces a Wilczek–Zee connection in the degenerate subspace,

$$A_{ab}(\mathbf{n}) = i\int dx\; \Psi_{a, n}^*(x; \mathbf{n})\, d\Psi_{b, n}(x; \mathbf{n})$$

which is precisely the Wu–Yang SU(2) magnetic monopole connection for most states. The non-Abelian magnetic charge, calculated via projection on the unbroken U(1) direction and integration over $S^2$, is quantized to $Q=1$, and the corresponding second Chern character is nontrivial. Physically, the system thus realizes a free, background SU(2) magnetic phase: there are no dynamical gauge fields, but the degenerate submanifold accumulates a non-Abelian geometric phase upon adiabatic traversal of the parameter space (Ohya, 2014, Ohya, 2014).

7. Unified Perspective and Physical Implications

The non-Abelian free magnetic phase unites two threads in quantum many-body physics and quantum geometry: (1) the realization of Ising topological order and Majorana modes in lattice spin liquids with field-induced time-reversal breaking, and (2) the emergence of non-Abelian Berry connection structure—specifically, the SU(2) Wu–Yang monopole—when degeneracies protected by symmetries (such as supersymmetry) are manipulated adiabatically in parameter space.

A plausible implication is that the underlying mechanism—adiabatic evolution in the presence of nontrivial symmetry-enforced degeneracy and magnetic perturbations—provides a general route to engineer non-Abelian gauge structures and holonomy in both quantum materials and low-dimensional quantum systems. These phases are characterized by robust degeneracy, quantized holonomy (from either physical or synthetic gauge fields), and topologically nontrivial excitations or geometric phases. The connection between topological entanglement measures, dynamical structure factors, and geometric holonomies furnishes a comprehensive diagnostic toolkit for identifying such phases in theoretical and experimental platforms (Gohlke et al., 2018, Zhu et al., 2017, Ohya, 2014, Ohya, 2014).