Documentation says: "The state data is a structure array with the fields state.u, state.ux, state.uy, state.uz (for 3-D problems), and state.time (for transient problems). The state.u field contains the solution vector. The state.ux, state.uy, state.uz fields are estimates of the solution’s partial derivatives at the corresponding points of the location structure. The state.time field contains time at evaluation points."
When I try to write function handle for internal heat source and print out 'state' variable, it shows that the 'state' is empty. That must be the reason why I can't get 'state.u'.