Conservation laws, vertex corrections, and screening in Raman spectroscopy

Article

关键词: FE-BASED SUPERCONDUCTORS;    COLLECTIVE MODES;    LIGHT-SCATTERING;    FLUCTUATIONS;    SYMMETRY;    QUANTUM;    PROBE;   

DOI  :  10.1103/PhysRevB.96.014503

来源: SCIE

【 摘 要 】

We present a microscopic theory for the Raman response of a clean multiband superconductor, with emphasis on the effects of vertex corrections and long-range Coulomb interaction. The measured Raman intensity, R(Omega), is proportional to the imaginary part of the fully renormalized particle-hole correlator with Raman form factors gamma((k) over right arrow). In a BCS superconductor, a bare Raman bubble is nonzero for any ((k) over right arrow) and diverges at Omega = 2 Delta(max), where Delta(max) is the largest gap along the Fermi surface. However, for gamma((k) over right arrow) = constant, the full R(Omega) is expected to vanish due to particle number conservation. It was sometimes stated that this vanishing is due to the singular screening by long-range Coulomb interaction. In our general approach, we show diagrammatically that this vanishing actually holds due to vertex corrections from the same short-range interaction that gives rise to superconductivity. We further argue that long-range Coulomb interaction does not affect the Raman signal for any gamma((k) over right arrow). We argue that vertex corrections eliminate the divergence at 2 Delta(max). We also argue that vertex corrections give rise to sharp peaks in R(Omega) at Omega < 2 Delta(min) (the minimum gap along the Fermi surface), when Omega coincides with the frequency of one of the collective modes in a superconductor, e.g., Leggett and Bardasis-Schrieffer modes in the particle-particle channel, and an excitonic mode in the particle-hole channel.

【 授权许可】

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