For fundamental integrable quantum chains with deformed symmetries we outline a general procedure for defining a continuous family of quasilocal operators whose time derivative is supported near the two boundary sites only. The program is implemented for a spin $1/2$ $XXZ$ chain, resulting in improved rigorous estimates for the high temperature spin Drude weight.

• Received 19 June 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.057203