# Breakdown of intermediate one-half magnetization plateau of spin-1/2 Ising-Heisenberg and Heisenberg branched chains at triple and Kosterlitz-Thouless critical points

## TL;DR

This paper investigates the magnetic and entanglement properties of spin-1/2 Ising-Heisenberg and Heisenberg branched chains, revealing how intermediate magnetization plateaus behave at critical points, including Kosterlitz-Thouless transitions.

## Contribution

It provides a detailed analysis of ground states, phase diagrams, and magnetization processes for both models, highlighting the disappearance of the one-half plateau at critical points.

## Key findings

- Intermediate magnetization plateaus depend on coupling constants.
- The one-half plateau vanishes at a Kosterlitz-Thouless critical point.
- Different ground states are characterized by distinct entanglement and magnetic properties.

## Abstract

The spin-1/2 Ising-Heisenberg branched chain composed of regularly alternating Ising spins and Heisenberg dimers involving an additional side branching is rigorously solved in a magnetic field by the transfer-matrix approach. The ground-state phase diagram, the magnetization process and the concurrence measuring a degree of bipartite entanglement within the Heisenberg dimers are examined in detail. Three different ground states were found depending on a mutual interplay between the magnetic field and two different coupling constants: the modulated quantum antiferromagnetic phase, the quantum ferrimagnetic phase and the classical ferromagnetic phase. Two former quantum ground states are manifested in zero-temperature magnetization curves as intermediate plateaus at zero and one-half of the saturation magnetization, whereas the one-half plateau disappears at a triple point induced by a strong enough ferromagnetic Ising coupling. The ground-state phase diagram and zero-temperature magnetization curves of the analogous spin-1/2 Heisenberg branched chain were investigated using DMRG calculations. The latter fully quantum Heisenberg model involves, besides two gapful phases manifested as zero and one-half magnetization plateaus, gapless quantum spin-liquid phase. The intermediate one-half plateau of the spin-1/2 Heisenberg branched chain vanishes at Kosterlitz-Thouless quantum critical point between gapful and gapless quantum ground states unlike the triple point of the spin-1/2 Ising-Heisenberg branched chain.