Sunday, August 7, 2016

Pseudogap and strange metal

Pseudogap and strange metal
Defining a pseudogap as a depression in the single-particle spectral weight at the Fermi level, or in other observable quantities such as the spin susceptibility, the appearance of a pseudogap in the normal state is a rather common occurrence in correlated-electron systems. There are several broad classes of mechanism that can produce a pseudogap. In one scenario, backed by experiments that reveal broken symmetries, a phase transition occurs at the pseudogap. That phase transition is proposed to be in the Ashkin-Teller universality class so that it is essentially undetectable by thermodynamic measurements. 

Long wavelength fluctuations involving composite order parameters (charge plus superconducting for example) have been proposed as another mechanism. There seemed to be agreement amongst participants that in electron-doped cuprates, antiferromagnetic fluctuations in the two-dimensional renormalized classical regime can explain the main experimental results on the pseudogap.  

This is not the case in hole-doped cuprates. A purely d=2 doped Mott insulator mechanism vs antiferromagnetic quantum critical point are still on the line, an issue related with the 1+x vs x number of carriers discussed under the cuprate section. New related experimental results in half-filled quasi-two dimensional Mott insulating organics were first announced at this meeting, namely: A pseudogap appears in the proximity of the Mot transition to the superconducting state if the insulating phase is antiferromagnetic, but not if it is a spin liquid.

Other fluctuation scenarios were discussed, including Ising nematic fluctuations and fluctuations with various power-law spectra that can be treated within the Eliashberg formalism.    

Strange metal behavior also motivated discussions. At half-filling, DMFT seems to explain the scaling of the resistivity observed in layered organics, opening a possibility for explanation of that behavior in cuprates. It seems to be clear also that in the overdoped regime for the cuprates, a phenomenlogical anisotropic marginal Fermi liquid explains much of the behavior. Standard Fermi liquid behavior occurs just at the end of the superconducting dome, a phenomenon begging for a microscopic explanation.  

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