Gradient: Its interpretation is quite qood in Griffiths. Assume u have a scalar field. For example, we take the case of a solid body where the temperature has differnt values at different point in the solid. We would like to find the temperature difference between two close by points along some...
Example
Anderw has described it correctly. Here is an exmple from a lab experience. Those who work in lowtemperature physics employ Liq.He and N2 and make use of this principle to reach temp. below the (normal) boiling point of these liquids. For example. Liq. He4 boils at 4.2K and by pumping...
Condensematter-Optical properties books
Here are two important books on the "Optical properties of solids:
1. F. Wooten - "Optical Properties of Solids"
Comment : Excellent treatment, the standard reference quoted in many journal articles. Year of Public: 1976
2. G. Gruner & M...
Quantum Manybody Physics
Take a look at the book of P.Coleman, Rutgers on "Quantum Manybody Theory", which discusses strogly correlated systems as well
http://www.physics.rutgers.edu/~coleman/mbody/pdf/bk.pdf