Twisted-Boundary-Condition Formalism for Thermal Transport and an Application to the One-Dimensional XY Spin Chain

We introduce and formulate the boundary condition twisted by the energy (time translation) in one-dimensional quantum many-body systems. The stiffness against this boundary condition quantifies thermal analogues of the Drude weight and the Meissner stiffness. We apply this formalism to the one-dimensional quantum XY spin chain and estimate the thermal Meissner stiffness.