Leggett Modes and the Anderson-Higgs Mechanism in Superconductors without Inversion Symmetry

We develop a microscopic and gauge-invariant theory for collective modes resulting from the phase of the superconducting order parameter in noncentrosymmetric superconductors. Considering various crystal symmetries, we derive the corresponding gauge mode.Gdq_ and find, in particular, new Leggett modes omega(L)(q) with characteristic properties that are unique to noncentrosymmetric superconductors. We calculate their mass and dispersion that reflect the underlying spin-orbit coupling and thus the balance between triplet and singlet superconductivity occurring simultaneously. Finally, we demonstrate the role of the Anderson-Higgs mechanism: while the long-range Coulomb interaction shifts omega(G)(q) to the condensate plasma mode omega(P)(q), it leaves the mass Lambda(0) of the new Leggett mode unaffected and only slightly modifies its dispersion.