I am currently reading S. Sachdevs Book on Quantum Phase Transitions focusing on the Bose-Hubbard Model (Chapter 9) and especially the Dilute-Boson Field Theory (Chapter 16).
When describing the fluid phase of the one dimensional model Sachdev says that this phase has quasi-long range order in the superfluid order parameter, intermediate boson occupation number and a non-zero superfluid stiffness.
I could not find any definition of a superfluid stiffness in the entire book and also doing some research on the internet I was not able to find a clean definition of superfluid stiffness in this context (Most likely because of my incapability :D to find something).
Therefore my question:
- Could somebody provide a definition of a superfluid stiffness in the context of the Bose-Hubbard Model?
- Any further explaination of this quantity in the "quasi long-range ordered" phase of the XY-chain would also be very kind?
Thank you all in advance.