Title: Pinning of longitudinal phonons in holographic spontaneous helices,
Abstract: We consider the spontaneous breaking of translational symmetry and identify
the associated Goldstone mode -- a longitudinal phonon -- in a holographic
model with Bianchi VII helical symmetry. For the first time in holography, we
observe the pinning of this mode after introducing a source for explicit
breaking compatible with the helical symmetry of our setup. We study the
dispersion relation of the resulting pseudo-Goldstone mode, uncovering how its
speed and mass gap depend on the amplitude of the source and temperature. In
addition, we extract the optical conductivity as a function of frequency, which
reveals a metal-insulator transition as a consequence of the pinning.