Turner, S. R. ; Pailhès, S. ; Bourdarot, F. ; Ollivier, J. ; Raymond, S. ; Keller, T.MLZ* ; Sidis, Y. ; Castellan, J.-P. ; Lory, P.-F. ; Euchner, H. ; Baitinger, M. ; Grin, Y. ; Schober, H. ; de Boissieu, M. ; Giordano, V. M.

2021
APS College Park, MD

Abstract: The low and weakly temperature-varying lattice thermal conductivity,κL(T), in crystals with a complex unitcell such as type-I clathrates is assumed to originate from a reduced momentum and energy space available forpropagative lattice vibrations, which is caused by the occurrence of low-energy optical phonon modes. In thecontext ofab initioself-consistent phonon (SCP) theory, it has been shown that the cubic and quartic anharmonicinteractions result in a temperature-induced energy renormalization of these low-lying optical branches whichcontributes to the anomalous behavior ofκL(T) in structurally ordered type-I clathrates [T. Tadano and S.Tsuneyuki,Phys.Rev.Lett.120, 105901 (2018)]. By means of inelastic neutron scattering, we provide evidencefor this energy renormalization in temperature, which has been resolved for transversely and longitudinallypolarized phonons in the single crystal type-I clathrate Ba7.81Ge40.67Au5.33. By mapping the neutron intensityin the momentum space, we demonstrate the coherent character of the low-lying optical phonons. The overallphonon spectrum and dynamical structure factors are satisfactorily reproduced byab initioharmonic calculationsusing density functional theory with the meta-GGA SCAN functional and a fully ordered structure. However, apolarization-dependent cutoff energy with opposing temperature shifts for longitudinal and transverse acousticdispersions is experimentally observed which is not reproduced by the simulations. Anharmonicity affects theenergies of the low-lying optical phonons in the transverse polarization, which compares quantitatively well withavailable results from SCP theory, whereas differences are observed for the longitudinal polarization.

Keyword(s): Basic research (1st) ; Condensed Matter Physics (2nd)

1. TRISP (TRISP)

1. TRISP: Three axes spin echo spectrometer (SR5b)