# Spin-Peierls transition of the dimer phase of the $J_1-J_2$ model: Energy cusp and CuGeO$_3$ thermodynamics

## TL;DR

This paper models the spin-Peierls transition in a dimerized spin-1/2 chain, revealing how ground state degeneracy causes an energy cusp that affects thermodynamic properties, with implications for CuGeO$_3$.

## Contribution

It introduces a model capturing the energy cusp effect due to ground state degeneracy in the spin-Peierls transition, explaining experimental thermodynamics of CuGeO$_3$.

## Key findings

- Energy cusp alters dimerization temperature dependence.
- Model reproduces magnetic susceptibility and specific heat anomalies.
- Parameters match experimental data for CuGeO$_3$.

## Abstract

The spin-Peierls transition is modeled in the dimer phase of the spin-$1/2$ chain with exchanges $J_1$, $J_2 = \alpha J_1$ between first and second neighbors. The degenerate ground state generates an energy cusp that qualitatively changes the dimerization $\delta(T)$ compared to Peierls systems with nondegenerate ground states. The parameters $J_1 = 160$ K, $\alpha = 0.35$ plus a lattice stiffness account for the magnetic susceptibility of CuGeO$_3$, its specific heat anomaly, and the $T$ dependence of the lowest gap.