Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6.

Byung-Gu Jeon,Kee Hoon Kim,C. B. Park,S. C. Riggs,E. G. Moon,B. Koteswararao,G. J. Shu,F. C. Chou,S. B. Chung

doi:10.1038/srep36970

Abstract

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu2PO6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu2PO6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of Hc ≅ 23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at Hc, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at Hc.

Highlights

The bc plane[12,13,14]
The application of magnetic field leads to reduction in the spin gap (Fig. 1b) and induces cascades of magnetic phase transitions above critical magnetic fields μ0Hc = 20–24 T, the values of which depend on the direction of the applied field[21,22]
The spin excitation branches, i(k) (i = 0, +1, −1), become split even at H = 0 because of giant anisotropic spin interactions originating from strong spin-orbit coupling, which can be described by antisymmetric Dzyaloshinskii-Moriya (D-M) and symmetric anisotropic spin exchange terms

Summary

Introduction

The bc plane[12,13,14]. Various thermodynamic, light scattering and nuclear magnetic resonance measurements have indicated anisotropic spin-gap with the excitation energy (Δ/kB) ranging from 32 to 45 K, without any long-range spin order down to 0.1 K12–20. It has been suggested that the triplons can even become fractionalized and reconstructed into a soliton lattice at higher magnetic fields above Hc17 All those unusual spin excitation spectra and postulated high field phases suggest that study of thermal transport could be useful to understand the evolution of spin and phonon excitations of BiCu2PO6 in the high field region where conventional neutron scattering cannot be performed. We found that the dip located between the double peaks in κ(T) moves systematically toward lower temperatures as H is increased to Hc, which result in the strongest field-induced suppression of κ known among the magnetic materials (by a factor of ~30) Such large T- and H-dependent variations in κ could be explained in terms of the resonant scattering of the phonons induced by magnetic transitions involving two-triplon bound states[8,18]. Κb(T) measured along the ladder leg direction revealed remnant magnetic contribution that scaled almost linearly in temperature at Hc., supporting that the magnetic heat transport nearly becomes gapless at the magnetic QCP

Methods

Results

Conclusion