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SUMMARY:Investigating the optically silent phonons in hexagonal boron nitr
 ide (hBN) - Aleksandar Radić\, Cavendish Laboratory
DTSTART:20260331T130000Z
DTEND:20260331T140000Z
UID:TALK246190@talks.cam.ac.uk
CONTACT:Boyao Liu
DESCRIPTION:Understanding lattice vibrations in two-dimensional (2D) mater
 ials is essential for controlling thermal transport\, mechanical response\
 , and energy dissipation in nanoscale devices. However\, several low-energ
 y phonon modes in hexagonal boron nitride (hBN) remain inaccessible using 
 conventional probes. Here we use helium-3 spin-echo spectroscopy to direct
 ly resolve the low-energy phonons at the surface of hBN. We measure the di
 spersions of the flexural and hybrid Rayleigh wave modes and extract the m
 echanical properties of a quasi-freestanding hBN monolayer. We simultaneou
 sly observe multiple surface-confined interlayer shear modes whose frequen
 cies agree closely with theoretical predictions\, and determine their intr
 insic linewidths\, revealing an order-of-magnitude reduction in phonon lif
 etimes with few-layer confinement. Finally\, we show that the amplitudes o
 f the observed phonon modes can be used to perform non-perturbative thermo
 metry based on their equilibrium (Bose-Einstein) occupations. These result
 s demonstrate the ability of helium atom scattering to access optically si
 lent vibrational modes and provide a direct route to measuring intrinsic p
 honon dynamics in two-dimensional materials.
LOCATION:B2.002\, Ray Dolby Centre\, Cavendish Laboratory
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