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Magnetic and phononic dynamics in the two-ladder quantum magnet (C5H9NH3)2CuBr4

J. Philippe1,2, F. Elson3, T. Arh1, S. Sanz4, M. Metzelaars4,5, D. W. Tam3, O. K. Forslund2,6, O. Shliakhtun1,2, C. Jiang3 et al.

J. Lass1, M. D. Le7, J. Ollivier8, P. Bouillot9, T. Giamarchi9, M. Bartkowiak1, D. G. Mazzone1, P. Kögerler5, M. Månsson3, A. M. Läuchli10,11, Y. Sassa3,12, M. Janoschek1,2, B. Normand10, and G. Simutis1,12,*

  • *Contact author: gediminas.simutis@psi.ch
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Phys. Rev. B 113, 134438 – Published 22 April, 2026

DOI: https://doi.org/10.1103/89j6-f5zx

Abstract

In quantum magnetic materials it is common to observe both static and dynamic lattice effects on the magnetic excitation spectrum. Less common is to find that the magnetic correlations have a significant impact on the phonon spectrum. Here we study the metal-organic material (C5H9NH3)2CuBr4(Cu-CPA), in which an explanation of the low-lying excitations depends crucially on a full understanding of both the spin and lattice subsystems. We report high-resolution neutron spectroscopy enabled by large, deuterated single crystals that reveal how both sectors are affected by the recently discovered structural phase transition. By measuring over several Brillouin zones, we disentangle the vibrational contribution to the spectrum in order to obtain an accurate estimate of the quasi-one-dimensional magnetic signal. The low-energy magnetic excitations are dominated by two gaps, Δb=0.41meV and Δa=0.55meV, which contribute with equal intensity ratios, confirming that Cu-CPA realizes a two-ladder spin Hamiltonian, and we deduce the magnetic interaction parameters of both ladders. The phonon spectrum contains highly localized modes at an anomalously low energy around 2 meV. For half of these modes, this characteristic frequency drops by approximately 5% as magnetic correlations become established with decreasing temperature, and we connect this apparent “elastomagnetic” behavior with the location and structure of the cyclopentylammonium rings.

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Phys. Rev. B 113, 134438– Published 22 April, 2026
  • Received 24 October 2025
  • Revised 2 March 2026
  • Accepted 10 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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