Spin Fluctuations in Magnetically Coupled Bi-layer Cuprates

TL;DR

This paper proposes a mechanism for the pseudo spin gap anomaly in bi-layer cuprates driven by inter-layer magnetic correlations and dynamical screening effects, without requiring fermion pairing or spin-charge separation.

Contribution

It introduces a novel explanation for PSGA based on inter-layer spin fluctuations and dynamical screening, independent of fermion pairing theories.

Findings

Inter-layer antiferromagnetic coupling is enhanced by intra-layer spin fluctuations.

Dynamical screening suppresses low-energy spin excitations near the phase boundary.

PSGA arises from inter-layer spin fluctuation dynamics, not fermion pairing.

Abstract

We propose a possible mechanism of pseudo spin gap anomaly(PSGA) in magnetically coupled bi-layer cuprates without any fermion pairing instability. In our proposal PSGA does not necessarily require the spin-charge separation or the breakdown of the Fermi liquid description of a normal state of the cuprate superconductor.The low energy magnetic excitations are mainly governed by the {\it itinerant nature of the intra-layer system} and {\it the inter-layer antiferromagnetic coupling}. No matter how weak the bare inter-layer coupling is, it can be dramatically enhanced due to the intra-layerspin fluctuations. As the temperature decreases near the antiferromagnetic phase boundary the strongly enhanced inter-layer correlation induces the inter-layer particle-hole exchange scattering processes that tend to enhance the inter-layer spin singlet formation and kill the triplet formation. We propose that the coupling of spin fluctuations on the adjacend layers via the strong repulsive interaction between parallel spins travelling on each layer give rise to the dynamical screening effects. As a result the low energy part of the spin excitation spectrum is strongly suppressed as the temperature decreases near the antiferromagnetic phase boundary. We ascribe PSGA to this dynamical screening effects.