Electromagnetic gauge invariance and orbital motion in field-induced spin-density-wave collective modes: Magnetorotons (abstract)

Abstract

The metallic ground state of strongly anisotropic metals with an open Fermi surface is known to be destroyed by a magnetic field. A field-induced spin-density-wave (FISDW) phase appears. As the field varies a cascade of transitions between SDW subphases is observed. The physics of FISDW is characterized by two very different lengths. One, the modulation wavelength, is of the order of the interatomic distance. The other is the magnetic length x0, which is much larger and which is connected to electronic orbital motion in the presence of the magnetic field. The order parameters breaks the continuous translation and spin rotational symmetries. We present a theory of the collective excitation spectrum of the FISDW phases. We find that this spectrum exhibits, beside the Goldstone bosons related to the two continuous broken symmetries, a series of rotonlike modes within the single particle electronic gap at the Fermi level, both in the transverse (to the order parameter) spin-wave spectrum and the phase fluctuation spectrum. The magnetoroton minima are centered at quantized values of the wave vector in terms of x−10. The degeneracy between the modes is lifted by the interaction between fluctuations transverse and parallel to the magnetic field. The rotonlike structure is connected to the interplay between orbitally broken translation invariance and electromagnetic gauge invariance. The spectrum of collective modes proves that FISDW is a new type of electron-hole condensate. The theory applies to Bechgaard’s salts.