When continuous symmetries are spontaneously broken, there appear gapless modes called Nambu-Goldstone (NG) modes. Due to their gapless nature, they are important degrees of freedom at low energies. Recently, a generalized kind of symmetries, called higher-form symmetries, have been discussed, whose charged objects are extended. The analogue of the NG theorem exists, and photons can be regarded as NG modes for a spontaneously broken higher-form symmetry. In this talk, I will discuss how to count the number of NG modes associated with spontaneously broken symmetries that can include higher-form ones [1]. I will give a formula for the number of NG modes, which is also applicable for systems without Lorentz invariance.
Reference:
[1] Y. Hidaka, Y. Hirono, R. Yokokura, Phys. Rev. Lett. 126, 071601(2021).
[arXiv:2007.15901]