Counting rule for Nambu-Goldstone modes in nonrelativistic systems

Abstract

The counting rule for Nambu-Goldstone modes is discussed using Mori's projection operator method in nonrelativistic systems at zero and finite temperatures. We show that the number of Nambu-Goldstone modes is equal to the number of broken charges, Q_a, minus half the rank of the expectation value of [Q_a,Q_b].Comment: 5 pages, no figures; typos corrected; some discussion added and clarifie