S. Thiagarajan | Searcharxiv

arXiv2026

The Effects of Inter-Valley Coupling of Dirac Fermions near Four Dimensions

We analyze the criticality of an Ising Gross-Neveu-Yukawa (GNY) theory of $N_ψ$ Dirac fermion valleys with intra- and inter-valley interactions, using a renormalization-group analysis in $4-ε$ space-time dimensions to one-loop order. The conventional GNY fixed point of decoupled valleys is unstable against inter-valley fluctuations which are naturally present if the symmetry breaking is driven by short-ranged interactions. At the new fixed point, the critical exponents differ from the conventional GNY universality. Most importantly, Lorentz invariance is broken due to interference effects resulting from the relative rotation between valley coordinate frames. In the limit of large $N_ψ$ the critical fixed point with finite inter-valley coupling remains stable but Lorentz invariance is asymptotically restored.

cond-mat.str-el↗

arXiv2025

Nature of the Topological Transition of the Kitaev Model in [111] Magnetic Field

We investigate the nature of the topological phase transition of the antiferromagnetic Kitaev model on the honeycomb lattice in the presence of a magnetic field along the [111] direction. The field opens a topological gap in the Majorana fermion spectrum and leads to a sequence of topological phase transitions before the field polarised state is reached. At mean field level the gap first closes at the three $M$ points in the Brillouin zone, where the Majorana fermions form Dirac cones, resulting in a change of Chern number by three. An odd number of Dirac fermions in the infrared is unusual and requires Berry curvature compensation in the UV, which occurs via topological, ring-like hybridisation gaps with higher-energy bands. We perform a renormalisation-group analysis of the topological phase transition at the three $M$ points within the Yukawa theory, allowing for intra- and inter-valley fluctuations of the spin-liquid bond operators. We find that the latter lead to a breaking of Lorentz invariance and hence a different universality compared to the standard Ising Gross-Neveu-Yukawa class.

cond-mat.str-el↗