Gapless line for the anisotropic Heisenberg spin-1/2 chain in a magnetic field and the quantum axial next-nearest-neighbor Ising Chain

TL;DR

This paper identifies a critical line in the anisotropic Heisenberg spin-1/2 chain under a magnetic field, revealing a gapless phase with no continuous symmetry and its relation to the ANNNI chain transition.

Contribution

It demonstrates the existence of a non-trivial critical line in the (a,h) parameter space using bosonization and RG analysis, linking it to the quantum spin-flop transition.

Findings

Existence of a gapless critical line in the (a,h) plane.

The critical line corresponds to a spin-flop transition.

Properties of the model at higher magnetic fields and the disorder line.

Abstract

We study the anisotropic Heisenberg (XYZ) spin-1/2 chain placed in a magnetic field pointing along the x-axis. We use bosonization and a renormalization group analysis to show that the model has a non-trivial fixed point at a certain value of the XY anisotropy a and the magnetic field h. Hence, there is a line of critical points in the (a,h) plane on which the system is gapless, even though the Hamiltonian has no continuous symmetry. The quantum critical line corresponds to a spin-flop transition; it separates two gapped phases in one of which the Z_2 symmetry of the Hamiltonian is broken. Our study has a bearing on one of the transitions of the axial next-nearest neighbor Ising (ANNNI) chain in a transverse magnetic field. We also discuss the properties of the model when the magnetic field is increased further, in particular, the disorder line on which the ground state is a direct product of single spin states.