Dynamical susceptibility of a near-critical non-conserved order parameter and B2g Raman response in Fe-based superconductors

Abstract

We analyze the dynamical response of a two-dimensional system of itinerant fermions coupled to a scalar boson $\varphi$, which undergoes a continuous transition towards nematic order with $d-$wave form-factor. We consider two cases: (a) when $\varphi$ is a soft collective mode of fermions near a Pomeranchuk instability, and (b) when it is an independent critical degree of freedom, such as a composite spin order parameter near an Ising-nematic transition. In both cases, the order-parameter is not a conserved quantity and the $d-$wave fermionic polarization $\Pi (q,\Omega )$ remains finite even at $q=0$. The polarization $\Pi (0,\Omega )$ has similar behavior in the two cases, but the relations between $\Pi (0,\Omega )$ and the bosonic susceptibility $\chi (0,\Omega )$ are different, leading to different forms of ${\chi }^{\prime \prime }(0,\Omega )$, as measured by Raman scattering. We compare our results with polarization-resolved Raman data for the Fe-based superconductors FeSe${}_{1-x}$S${}_x$, NaFe${}_{1-x}$Co${}_x$As and BaFe${}_2$As${}_2$. We argue that the data for FeSe${}_{1-x}$S${}_x$ are well described within Pomeranchuk scenario, while the data for NaFe${}_{1-x}$Co${}_x$As and BaFe${}_2$As${}_2$ are better described within the "independent" scenario involving a composite spin order.

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Cited by in corpus (11)

• Scattering mechanisms and electrical transport near an Ising nematic quantum critical point
• Superconductivity near a nematic quantum critical point -- the interplay between hot and lukewarm regions
• Superconductivity-induced nematicity
• Field-tuned ferroquadrupolar quantum phase transition in the insulator TmVO${}_4$
• Collective modes near a Pomeranchuk instability
• Raman response in the nematic phase of FeSe
• Low frequency Raman response near Ising-nematic quantum critical point: a memory matrix approach
• Fluctuations and pairing in Fe-based superconductors: Light scattering experiments
• Nematic state of the FeSe superconductor
• Quadrupolar charge dynamics in the nonmagnetic FeSe${}_{1-x}$S${}_x$ superconductors
• Collective excitations and stability of a non-Fermi liquid state near a quantum-critical point of a metal