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1D chain of atoms. Each unit cell contains one atom.
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In a recent letter [Appl. Phys. Lett.92, 011901 (2008)], Tang reported a simulation of structuraldynamics in metal films induced by ultrafast laser heating using the two-temperature model [P. B. Allen, Phys. Rev. Lett.59, 1460 (1987) and R. W. Schoenlein et al., Phys. Rev. Lett.58, 1680 (1987)] and one-dimensional anharmonic chain model [E. Fermi J. Pasta S. Ulam, No. LA-1940 (1955)]. In this comment, we would like to point out several issues in the physical concepts and formulations in the simulation which we strongly disagree with the author. Consequently, we believe that the main conclusion of Tang's paper that the interpretation of ultrafast diffraction data requires both nonlocal collective atomic motion and the conventional linear thermal expansion lacks physical justification and is questionable.
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