Pseudogap in Cuprates and other Metals or How to Almost Elude Bloch's Theorem

TL;DR

This paper explains the pseudogap phenomenon in cuprates as a consequence of finite correlation lengths of loop-current order, leading to angle-dependent spectral gaps and fluctuating low-frequency excitations, despite no true translational symmetry breaking.

Contribution

It introduces a model where finite correlation lengths of loop-current order cause a pseudogap and spectral features consistent with experimental observations, almost eluding Bloch's theorem.

Findings

Angle-dependent pseudogap in spectral function near the chemical potential.

Fermi surface peaks are shifted proportionally to the square of the loop order parameter.

Presence of low-frequency excitations at all temperatures due to finite correlation length.

Abstract

The loop-current state discovered in under-doped cuprates is characterized by a vector ${\bf \Omega}$ which has four possible orientations which correspond to different domains of order in a perfect sample. Since translational symmetry remains unchanged in the pure limit, no gap occurs at the chemical potential. On the other hand Scanning tunneling microscopy (STM) has revealed that the magnitude of the pseudo-gap in under-doped cuprates varies spatially and is correlated with disorder. For disorder coupling also to the direction of ${\bf \Omega}$, there can only be a finite temperature dependent static correlation length for the loop-current state below the ordering temperature of the pure problem. It is shown that, in this situation, singular forward scattering of fermions for large correlation lengths induces an angle dependent pseudo-gap in the single-particle spectral function near the chemical potential. The peaks in the spectral function at the fermi-vectors are away from the chemical potential proportionally to the square of the average loop order parameter measurable by polarized neutron scattering. This result is tested. Due to the finite correlation length there always exist low frequency excitations at long wavelength at all temperatures in the "ordered" phase. Such fluctuations motionally average over the shifts in frequencies of local probes such as NMR and muon resonance expected for a truly static order.