Evidence for strong correlations at finite temperatures in the dimerized magnet Na$_2$Cu$_2$TeO$_6$

TL;DR

This study demonstrates that in the dimerized magnet Na$_2$Cu$_2$TeO$_6$, quasiparticles maintain strong correlations at finite temperatures, counteracting thermal decoherence, as evidenced by inelastic neutron scattering revealing persistent singlet-triplet excitations.

Contribution

The paper provides experimental evidence that strong correlations persist at finite temperatures in a dimerized quantum magnet, highlighting the universality of such states.

Findings

Persistent singlet-triplet excitations at elevated temperatures

Temperature-dependent gap and bandwidth changes

Asymmetric energy broadening indicating strong correlations

Abstract

Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound Na$_2$Cu$_2$TeO$_6$ that the excited quasiparticles can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, we observe clear dispersive singlet-triplet excitations arising from the dimers formed along the crystalline $b$-axis. The excitation gap is of $\sim$18 meV and the bandwidth is about half of the gap. The band top energy has a weak modulation along the [100] direction, indicative of a small interchain coupling. The gap increases while the bandwidth decreases with increasing temperature, leading to a strong reduction in the available phase space for the triplons. As a result, the Lorentzian-type energy broadening becomes highly asymmetric as the temperature is raised. These results are associated with a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. We consider these results to be not only evidence for strong correlations at finite temperatures in Na$_2$Cu$_2$TeO$_6$, but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems.