Series Expansion Analysis of a Frustrated Four-Spin-Tube

TL;DR

This paper investigates the magnetic properties of a frustrated four-spin tube using series expansion and DMRG, revealing a first-order phase transition, new elementary excitations, and the impact of frustration on the system's energy landscape and excitations.

Contribution

It introduces a detailed analysis of the frustrated four-spin tube, highlighting the destabilization of the spin-gap phase and the emergence of additional elementary excitations due to frustration.

Findings

Frustration destabilizes the spin-gap phase leading to a first-order quantum phase transition.

The FFST supports additional elementary excitations like singlons and extra triplons.

Frustration causes flattening of the energy landscape and enhances excitation masses and binding strengths.

Abstract

We study the magnetism of a frustrated four-leg spin-1/2 ladder with transverse periodic boundary conditions: the frustrated four-spin tube (FFST). Using a combination of series expansion (SE), based on the continuous unitary transformation method and density-matrix renormalization group (DMRG) we analyze the ground-state, the one-, and the two-particle excitations in the regime of strong rung-coupling. We find several marked differences of the FFST with respect to standard two-leg ladders. First we show that frustration destabilizes the spin-gap phase of the FFST which is adiabatically connected to the limit of decoupled rung singlets, leading to a first order quantum phase transition at finite inter-rung coupling. Second, we show that apart from the well-know triplon branch of spin-ladders, the FFST sustains additional elementary excitations, including a singlon, and additional triplons. Finally we find, that in the two-particle sector the FFST exhibits collective (anti)bound states similar to two-leg ladders, however with a different ordering of the spin-quantum numbers. We show that frustration has significant impact on the FFST leading to a flattening of the ground-state energy landscape, a mass-enhancement of the excitations, and to a relative enhancement of the (anti)binding strength. Where possible we use DMRG to benchmark the findings from our SE calculations, showing excellent agreement.