I'm having some troubles understanding the unit used in solid state physics. In a paper I read [tex] \Lambda a \sim 1 [/tex] where [itex] \Lambda [/itex] is a momentum cutoff and [itex] a [/itex] is the lattice spacing of a crystal. Questions: 1) What kind of units are customarily used in solid state physics scientific articles? Can I be confident that they are atomic units? 2) The aforementioned equation is not correct as far as the units of measure are concerned. If this was solid state physics, I would say that one could set the speed of light and the Planck constant to 1 and make the equation correct. However in a low energy treatment in solid state physics, I don't see the point in using the speed of light or the Planck constant, and making the physics of the system dependent upon these quantities. Thanks in advance for your answers.