Euclidian crystals in many-body systems: breakdown of Goldstone's theorem

TL;DR

This paper demonstrates that Goldstone's theorem can be violated in a novel Euclidian crystal state of interacting fermions, leading to gapped Goldstone modes and unique physical properties with potential implications for high-temperature superconductors.

Contribution

It introduces the Euclidian crystal state where Goldstone modes are gapped, challenging the traditional understanding of spontaneous symmetry breaking.

Findings

Goldstone modes can have finite gaps in Euclidian crystal states.

QOP's Green's function exhibits infinite periodic poles along the real frequency axis.

Potential connection of Euclidian crystal state to phenomena in cuprate superconductors.

Abstract

In accord with Nambu-Goldstone theorem spontaneous breaking of a global continuous symmetry of the system by a mean-field order parameter causes appearance of massless Goldstone bosons, called also gapless Goldstone modes. It is demonstrated below that the theorem can be violated, i.e. the Goldstone modes possess finite gap, in the case of a novel "Euclidian crystal" (quantum ordered) state of interacting fermions, that was proposed recently {mukhin 2011}. This result may lead to the important consequences when considered in combination with another peculiar properties of the quantum order parameter (QOP) that where recently found: QOP's Green's function is finite and periodic along the Matsubara's axis, but Wick-rotated to the axis of real frequencies, possesses infinite periodic "chain" of \it{second order poles} along this axis {mukhin 2011}. This property of QOP leads to a remarkable physical consequence: \it{QOP does not scatter anything in Minkowski world} ! Hence, the present theory, we believe, may naturally indicate that two effects: "hidden order" revealed in ARPES spectra of fermi-excitations and "neutron resonance" feature at finite frequency in the magnetic (bosonic) excitations spectrum in lightly hole-doped copper oxides, could be emerging fingerprints of the Euclidian crystal state. A peculiar manifestation of the QOP self-consistent solution in the form of non-classical phonon states is described for an "electromagnetic Euclidian crystal" predicted recently {mukhin,fistul 2013} in the array of Josephson junctions coupled via electromagnetic field in the superconducting resonator. It is demonstrated that it may constitute a stable state of the system, that, for a particular quantum disordering model, exists inside the stability domain of the classical photonic condensate delimited by the critical value of the quantum disordering strength.