U(1)xSU(2) Chern--Simons gauge theory of underdoped cuprate superconductors

TL;DR

This paper develops a U(1)xSU(2) Chern-Simons gauge theory framework for underdoped cuprate superconductors, revealing how gauge fields influence holon and spinon dynamics, leading to non-Fermi liquid behavior and insights into the spin gap.

Contribution

It introduces a novel gauge-theoretic approach to model the normal state of underdoped cuprates, connecting gauge fluctuations with electron properties and magnetic order.

Findings

Holons become Dirac fermions due to lux phase.

Spinons acquire a gap that vanishes at zero doping.

The minimal electron gap scales with the square root of doping.

Abstract

The Chern-Simons bosonization with U(1)xSU(2) gauge field is applied to 2-D t-J model in the limit t >> J, to study the normal state properties of underdoped cuprate superconductors. We prove the existence of an upper bound on the partition function for holons in a spinon background, and we find the optimal spinon configuration saturating the upper bound on average--a coexisting flux phase and s+id-like RVB state. After neglecting the feedback of holon fluctuations on the U(1) field B and spinon fluctuations on the SU(2) field V, the holon field is a fermion and the spinon field is a hard--core boson. We show that the B field produces a \pi flux phase for holons, converting them into Dirac--like fermions, while the V field, taking into account the feedback of holons produces a gap for spinons vanishing in zero doping limit. The nonlinear sigma-model with a mass term describes the crossover from short-ranged antiferromagnetic (AF) state in doped samples to long range AF order in reference compounds. Moreover, we derive a low--energy effective action in terms of spinons, holons and a self-generated U(1) gauge field. The gauge fluctuations are not confining due to coupling to holons, but yield an attractive interaction between spinons and holons leading to a bound state with electron quantum numbers. The renormalisation effects due to gauge fluctuations give rise to non--Fermi liquid behaviour for the composite electron.This formalism provides a new interpretation of the spin gap in underdoped superconductors (due to short-ranged AF order) and predicts the minimal gap for the physical electron is proportional to the square root of the doping concentration.