# Critical spin-$\frac{1}{2}$ tetramer compound CuInVO$_5$: Exploring the vicinity of two multimerized singlet states

## TL;DR

This study uses the density-matrix renormalization group to analyze a spin-1/2 Heisenberg chain model for CuInVO$_5$, revealing two valence-bond-solid phases and a quantum critical point near the material's experimental parameters.

## Contribution

It identifies two distinct multimerized VBS phases in CuInVO$_5$ and demonstrates the material's proximity to a quantum critical point between these phases.

## Key findings

- Two VBS phases characterized by tetramer and dimer singlets.
- Finite spin gaps vanish at the phase boundary, indicating a second order transition.
- Experimental magnetization curve explained by parameters near the quantum critical point.

## Abstract

Using the density-matrix renormalization group technique, we study a one-dimensional spin-$\frac{1}{2}$ Heisenberg chain consisting of coupled tetramers as an effective spin model for copper vanadate CuInVO$_5$. We obtain the ground-state phase diagram as a function of intra-tetramer and inter-tetramer exchange interactions, exhibiting two multimerized valence-bond-solid (VBS) phases : one is characterized by the formation of tetramer-singlet units; the other by the formation of dimer-singlet pairs. We show that the finite spin gaps in both the VBS phases smoothly vanish at the phase boundary: a second order phase transition defining a quantum critical point (QCP). The phase boundary is also captured by the fact that the central charge is unity at the phase boundary and zero otherwise in the thermodynamic limit. We further demonstrate that the experimental magnetization curve (which starts increasing with zero or tiny field) can be reasonably explained only by assuming the exchange parameters of CuInVO$_5$ to be very close to the phase boundary. Thus, we argue that CuInVO$_5$ may be a first example material which at ambient pressure stands near a QCP between two VBS phases. By varying the balance of exchange interactions with pressure, a transition from N\'eel to either of the VBS phases could be observed.