Weak phonon coupling to nematic quantum critical mode in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>BaFe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>As</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">P</mml:mi> <mml:mi>x</mml:mi> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math>

Despite intense focus on quantum critical behavior, microscopic information on the evolution of spin and nematic correlations remains scarce. Using high-resolution inelastic scattering, the authors investigate here in-plane transverse acoustic phonons in BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ near its nematic quantum critical point. The authors observe a weak coupling between the critical nematic fluctuations and the lattice near the critical regime. Together with theory, this result suggests the nature of nematicity evolves from spin-driven with strong lattice coupling in the underdoped regime to orbital-driven with weak coupling near the critical point.