Topological and kinetic origins of fractional thermal conductance at the topological insulator--superconductor interface

Abstract

A recent experiment by Roy et al. (Nat. Commun. 17, 2853 (2026)) demonstrated a robust half-integer thermal conductance plateau, κ_0 T/2, at a bipolar (ν,ν')=(2,-1) junction in bilayer graphene, produced not by non-Abelian topology but by full equilibration of co-propagating electron and hole edge modes. We prove a theorem that fixes when the two origins can be told apart: under full thermal equilibration, the two-terminal thermal conductance of a network of chiral edge segments joined at ideal floating contacts is a rational function of the net chiral central charges of the segments alone, quantities pinned by the gravitational anomaly and invariant under arbitrary local boundary kinetics. The corollary is an impossibility statement: whenever two realizations present the same anomaly data to the same network, no thermal-conductance measurement can distinguish them. That the equilibrated (2,-1) value equals the central charge of a chiral Majorana mode is an arithmetic fact about one filling combination, but wherever such a coincidence occurs it is beyond the reach of thermometry, and the separation must come from the charge sector, which is not anomaly-pinned at a superconducting boundary. We develop the topological insulator-superconductor interface as the application: a vortex carries fractional charge e/4 from the θ= πmagnetoelectric coupling, the boundary hosts a chiral Majorana mode of central charge 1/2, and a laterally adjacent integer quantum Hall (IQH) region supplies the kinetic realization. The Lorenz ratio (anomalous for the isolated Majorana boundary, L_0 / (1 + 4|ν_{IQH}|) in the composite device) and the excess shot noise (growing with B from e/4 vortex tunneling versus locked to IQH plateaus) together resolve the mechanism, provided the quantum Hall edge does not abut the superconductor.