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arXiv · 2608.21316

Ultralow-Field Triplon Condensation in a Spin-Ladder Magnet

Ankit Labh·Ross H. Colman·Jakub Šebesta·Noah Oefele·Elsa Lhotel·Adam Berlie·Paul Steffens·Oksana Zaharko·Pascal Manuel·Iurii Kibalin·Philipp Gegenwart·Dominik Legut·Johanna K. Jochum·Alexander A. Tsirlin·Petr Čermák
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Abstract

We realise the first ultralow-field Bose-Einstein condensation of triplons in a spin-ladder magnet, uncovering a quantum critical point at only $\mu_0 H_{c1}=0.17$ T in Henmilite ($\mathrm{Ca_2Cu(OH)_4[B(OH)_4]_2}$). Unlike dimer magnets, a ladder retains extended one-dimensional correlations in its gapped parent state, making this limit strongly fluctuation dominated. Thermodynamic, magnetoelastic, $\mu$SR, and neutron-diffraction measurements overturn the previous assignment of zero-field antiferromagnetic order, establishing a quantum-disordered coupled-ladder parent state with persistent low-energy dynamics. The weak low-temperature anomaly instead marks a gap-controlled crossover from the correlated ladder regime into the activated quantum-disordered state. These measurements further reveal an exceptionally asymmetric ordered dome extending to $\mu_0 H_{c2}\simeq 8.2$ T. Quantum Monte Carlo simulations for the relevant spin Hamiltonian place Henmilite just on the gapped side of the zero-field ladder-ordering instability, naturally accounting for the strong separation between the exchange and residual-gap scales and the tiny critical field. Our findings extend ultralow-field triplon condensation beyond the dimer paradigm and establish Henmilite as a platform for controlled tuning across quantum criticality in a fluctuation-dominated spin ladder.

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Ankit Labh, Ross H. Colman, Jakub Šebesta, Noah Oefele, Elsa Lhotel, Adam Berlie, Paul Steffens, Oksana Zaharko, Pascal Manuel, Iurii Kibalin, Philipp Gegenwart, Dominik Legut, Johanna K. Jochum, Alexander A. Tsirlin, Petr Čermák. 2026-08-21. Ultralow-Field Triplon Condensation in a Spin-Ladder Magnet. https://arxiv.org/abs/2608.21316

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