Solving models with generalized free fermions II: Path-product expansion and conserved charges

Free-fermion solvability in quantum spin systems is increasingly understood to be governed by a graph Clifford algebra defined from the frustration graph of the Hamiltonian. When the frustration graph belongs to certain classes, such as the even-hole-free and claw-free (ECF) class, the Hamiltonian is solvable by hidden free fermions: it admits a free-fermion solution although it does not reduce to a Majorana bilinear under the Jordan-Wigner transformation. However, unlike in the Jordan-Wigner case, where each mode is a linear combination of single Majorana fermions, the explicit operator structure of the hidden free-fermion modes -- and that of the local conserved charges -- has remained obscure. In this work, we derive a path-product expansion that expresses each free-fermion mode as a linear combination of path products along induced paths in the extended frustration graph. The expansion is obtained from the generating function of the Krylov basis and yields the modes directly, without using the transfer matrix or the nonlocal conserved charges as input. As an application, the mode decomposition computes infinite-temperature dynamical correlation functions for arbitrary ECF frustration graphs. We further obtain explicit expressions for local conserved charges as linear combinations of path products along induced paths; these charges apply beyond the free-fermion (ECF) class to more general claw-free frustration graphs. We also identify a unified family of generalized conserved charges that contains both the previously known nonlocal conserved charges and these local conserved charges as special cases. For Fendley's original FFD chain with homogeneous couplings and periodic boundary conditions, in a suitable basis, the structure of these local conserved charges exhibits the same Catalan-tree pattern as in the spin-$1/2$ XXX chain.