Entanglement Perturbation Theory for Infinite Quasi-1D Quantum Systems

TL;DR

This paper introduces Entanglement Perturbation Theory (EPT) for analyzing infinite quasi-one-dimensional quantum systems, providing precise phase diagrams and insights into phase transitions in a spin chain model.

Contribution

The paper develops a novel EPT method for infinite quasi-1D systems and applies it to accurately identify phase transitions in a complex spin chain model.

Findings

Accurate phase diagram for the spin chain model.

Detection of phase transitions at specific anisotropy values.

Questioning the existence of a second Neel phase.

Abstract

We develop Entanglement Perturbation Theory (EPT) for infinite Quasi-1D quantum systems. The spin 1/2 Heisenberg chain with ferromagnetic nearest neighbor (NN) and antiferromagnetic next nearest neighbor (NNN) interactions with an easy-plane anisotropy is studied as a prototypical system. The obtained accurate phase diagram is compared with a recent prediction [Phys.Rev.B,81,094430(2010)] that dimer and Neel orders appear alternately as the XXZ anisotropy Delta approaches the isotropic limit Delta=1. The first and second transitions (across dimer, Neel, and dimer phases) are detected with improved accuracy at Delta\approx 0.722 and 0.930. The third transition (from dimer to Neel phases), previously predicted to be at Delta\approx 0.98, is not detected at this Delta in our method, raising the possibility that the second Neel phase is absent.