Selection rules for second order infrared and raman processes. I. Caesium chloride structure and interpretation of the second order Raman spectra of CsBr and Csi

Ganesan, S.; Burstein, E.; Karo, A. M.; Hardy, J. R.

doi:10.1051/jphys:019650026011063900

Cited by 14 publications

(2 citation statements)

References 10 publications

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“…The caesium chloride structure also has a two-atom basis and thus three optical-phonon branches. However, the zone-center optical phonons here transform according to T 1u+ [40], i.e., like a spatial vector, which is plausible since the distortions generate an electric dipole moment of each unit cell. We denote the displacement vector by p = (p x , p y , p z ), which serves as the distortional order parameter.…”

Section: Distortion Modesmentioning

confidence: 99%

Distortional weak-coupling instability of Bogoliubov Fermi surfaces

2021

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Centrosymmetric multiband superconductors which break time-reversal symmetry generically have two-dimensional nodes, i.e., Fermi surfaces of Bogoliubov quasiparticles. We show that the coupling of the electrons to the lattice always leads to a weak-coupling instability of such a state towards spontaneous breaking of inversion symmetry at low temperatures. This instability is driven by a Cooper logarithm in the internal energy but the order parameter is not superconducting but distortional. We present a comprehensive symmetry analysis and introduce a measure that allows to compare the strengths of competing distortional instabilities. Moreover, we discuss the instability using an effective single-band model. This framework reveals a duality mapping of the effective model which maps the distortional order parameter onto a superconducting one, providing a natural explanation for the Cooper logarithm and the weak-coupling nature of the instability. Finally, we consider the possibility of a pair-density wave state when inversion symmetry is broken. We find that it can indeed exist but does not affect the instability itself.

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Section: Distortion Modesmentioning

confidence: 99%

Distortional weak-coupling instability of Bogoliubov Fermi surfaces

2021

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“…2. Experimental data are available for momentum-resolved phonon band structures of CsI 57 and NaI. 55,56 These are compared with theoretical predictions in Ref.…”

Section: Resultsmentioning

confidence: 99%