Atomic Vibrations Play Key Role in Material Phase Change

Atomic Vibrations Play Key Role in Material Phase Change

Image: FemtoMAX beamline at MAX IV

A research group working with MAX IV’s FemtoMAX beamline has succeeded to slow the phase change from the solid to liquid state in the semiconductor, indium antimonide (InSb), by reducing the inherent vibrations between atoms. An important precursory step in the process was non-thermal melting of the sample, which broke its atomic bonds. This revealed that unbound atoms move with the velocity they had at the instant the bonds were broken. Further it showed that initial velocity is governed by atomic vibrations, which in turn are temperature dependent. The findings are steps toward functional manipulation of material structure during phase transitions.

Imagine a world where we control the structure of materials by subjecting them to short-pulse laser radiation. This is the implication of research that allows us to alter the timing when phase change occurs.

experimental setup, FemtoMAX
Schematic of the experimental setup. With the difference in incident angle between the laser and the X-rays, sample positions T1 and T2 are exposed at different time delays, which converts the spatial extent of the sample (and detector image) into a time axis. Credit: Creative Commons Attribution 4.0 International.

Melting a material with or without heat produces a similar result, at a similar speed. What is going on at the atomic level is quite different, however. Thermal heating excites electrons to a higher energy state. Electron-phonon coupling then equilibrates the electron and lattice temperature which makes the lattice vibrate so violently that atomic bonds break. Non-thermal heating also excites electrons but breaks the bonds instantly—within femtoseconds—and releases atoms from their original structural configuration. Scientists seek to distinguish what happens after bonds sever due to these excited electrons.

The significant factor under investigation is the initial, steady-state vibrational energy that atoms have before atomic bonds break in phase transition.

“We wanted to demonstrate you cannot just do with quantum mechanics calculations to figure out what potentials look like and how [atomic] acceleration takes place after non-thermal melting,” said Jörgen Larsson, Lund University physics professor and principal investigator of the study. “You also need to take into account fundamental aspects even if you cool down [a sample] and break bonds. You will still have initial velocities you cannot get away from.”

Chilly start, sluggish atoms

Scientists from Lund University in Sweden carried out trials on an InSb sample at five different initial temperatures ranging from 35 to 500 Kelvin. At FemtoMAX beamline, the sample was exposed to simultaneous ultrafast X-rays and femtosecond laser pulses in order to measure the melting, or disordering time, from solid to liquid state. Short laser pulses impacting the semiconductor material broke the atomic bonds and caused released atoms to move with the original speed and direction they had in a bonded state. To visualize the structural dynamics and X-ray scattering densities of the sample, researchers utilized MD calculations with Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software.

Researchers observed that the intrinsic atomic vibrations slowed with each decrease in initial temperature. The working premise is that cold atoms vibrate slowly, and hot atoms vibrate quickly. As compared to classical thermal melting, this laser-induced bond breaking occurs simultaneously in the whole exposed volume, enabling the study of the disordering process.

Simulation by molecular dynamics program package (LAMMPS) shows the dynamics of laser-induced melting of InSb when governed by thermal equilibrium dynamics. After laser excitation, atoms move at a constant velocity. (a)–(d) show only 18 atoms for clarity. The atomic positions are shown for (a) time=0  fs and T=500  K, (b) time=0  fs and T=70  K, (c) time=350  fs and T=500  K, and (d) time=350  fs and T=70  K. The time-resolved x-ray scattering intensity for 500 (e) and for 70 K (f) are also shown. Credit: Creative Commons Attribution 4.0 International.

X-ray diffraction measurements of the crystalline sample revealed the limit of control of phase change is absolute zero (-273.15 K), at which point it is no longer possible to reduce atomic vibrations. Weak vibrations will continue in a state of zero-point energy due to quantum effects.

Better control of the phase change in materials paves the way for improved and new innovative technologies such as reading and writing of novel storage media, optical recording, and various types of phase-change memory devices.

For the user community

The study provides evidence of the model of inertial motion proposed in 2005, which states that the random movement of atoms after bond breaking is dependent on their initial velocity before bond breaking. It also verifies the need for new predictive models of atomic motion which take the initial velocities of atoms into account during phase change-laser interactions.

Beyond proof of concept, the user community may take away an additional benefit, according to Larsson. “In this experiment, we really test the stability of the laser with respect to the timing of the linear accelerator. This means checking for unwanted measurement jitters in order to correct for them in your calculations. If users want time zero, we want to be able to give them that.”

FemtoMAX beamline delivers ultrafast X-rays for the study of atomic structure and dynamics of materials. Its short, intense beam serves questions on light-induced programming of materials, novel media storage devices, and sustainable energy research.



Role of Thermal Equilibrium Dynamics in Atomic Motion during Nonthermal Laser-Induced Melting
Xiaocui Wang, J. C. Ekström, Å. U. J. Bengtsson, A. Jarnac, A. Jurgilaitis, Van-Thai Pham, D. Kroon, H. Enquist, and J. Larsson. Phys. Rev. Lett. 124, 105701 – Published 12 March 2020.

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