Gauge Theory for a Doped Antiferromagnet in a Rotating Reference-Frame

TL;DR

This paper develops a U(1) gauge theory framework for doped antiferromagnets in a rotating reference frame, revealing a quantum phase transition and long-range interactions that lead to bound states and charge-spin separation.

Contribution

It introduces a U(1) gauge theory approach for doped antiferromagnets using a rotating reference frame, connecting spin-gap opening with charge-spin separation and pairing.

Findings

Identifies a quantum phase transition from Ne9el to spin-liquid phase.

Calculates the U(1) gauge field propagator proportional to the spin-excitation gap.

Predicts formation of bound states such as spin-waves and fermion pairs in the spin-liquid phase.

Abstract

We study a doped antiferromagnet (AF) using a rotating reference-frame. Whereas in the laboratory reference-frame with a globally fixed spin-quantization axis (SQA) the long-wavelength, low-energy physics is given by the O(3) non-linear $\sigma$-model with current-current interactions between the fermionic degrees of freedom and the order-parameter field for the spin-background, an alternative description in form of an U(1) gauge theory can be derived by choosing the SQA defined by the local direction of the order-parameter field via a SU(2) rotation of the fermionic spinor. Within a large-$N$ expansion of this U(1) gauge theory we obtain the phase diagram for the doped AF and identify the relevant terms due to doping that lead to a quantum phase transition at $T=0$ from the antiferromagnetically ordered N\'eel phase to the quantum-disordered (QD) spin-liquid phase. Furthermore, we calculate the propagator of the corresponding U(1) gauge field, which mediates a long-range transverse interaction between the bosonic and fermionic fields. It is found that the strength of the propagator is proportional to the gap of the spin-excitations. Therefore, we expect as a consequence of this long-range interaction the formation of bound states when the spin-gap opens, i.e.\ in the QD spin-liquid phase. The possible bound states are spin-waves with a (spin-) gap in the excitation spectrum, spinless fermions and pairs of fermions. Thus, an alternative picture for charge-spin separation emerges, with composite charge-separated excitations. Moreover, the present treatment shows an intimate connection between the opening of the spin-gap and charge-spin separation as well as pairing.