Thermodynamic Properties of the Spin-1/2 Antiferromagnetic ladder Cu2(C2H12N2)2Cl4 under Magnetic Field

TL;DR

This study investigates the thermodynamic properties of a spin-1/2 antiferromagnetic ladder compound under magnetic fields, revealing deviations from the Heisenberg ladder model due to lattice fluctuations and proposing a new incommensurate gapped state explanation.

Contribution

It demonstrates the limitations of the Heisenberg ladder model under high magnetic fields and introduces the role of lattice fluctuations and distortions in the observed quantum phases.

Findings

Accurate modeling of susceptibility and specific heat at low fields.

Observation of enhanced quantum fluctuations above the critical field.

Identification of lattice coupling effects leading to incommensurate gapped states.

Abstract

Specific heat ($C_V$) measurements in the spin-1/2 Cu$_2$(C$_2$H$_{12}$N$_2$)$_2$Cl$_4$ system under a magnetic field up to $H=8.25 T$ are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature dependences of both the susceptibility and the low field specific heat are accurately reproduced by this model, deviations are observed below the critical field $H_{C1}$ at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations. Finally, we show that such a Heisenberg ladder, for $H>H_{C1}$, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation for the observed low temperature ($T_C\sim 0.5K$ -- $0.8K$) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.