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John O | March 2018

Scientists provide insights into the mechanical properties of crystalline solids

By Josh Perry, Editor


Scientists from the Max Planck Institute for Structure and Dynamics of Matter (MPSD) in Hamburg, Germany have developed a new method for precisely measuring the interatomic forces that hold crystalline solids together, which provides insights into the mechanical properties of matter and the instability near phase changes.


Strong-field mid-infrared excitation allows to drive lattice vibrations of a crystal into the highly anharmonic regime. (J.M. Harms, MPSD)


The researchers, according to a report from the institute, show that a tetrahertz-frequency laser pulse can create large deformations in the crystal and that measuring the unusual atomic trajectories under electromagnetic transients allowed researchers to determine how rigid atomic bonds were at large distances from the equilibrium arrangements.


“Ultrashort laser flashes at mid-infrared frequencies were used to move atoms far away from their equilibrium arrangement,” the report explained. “By measuring how the same atoms were made to ring after the impulse had been turned off, the MPSD research group could reconstruct the nature of the forces that hold the crystal together.”


It continued, “The corresponding atomic displacements, enormous on the scale of the interatomic distances, are nevertheless only of the order of a few picometers, that is a millionth of a billionth of a meter. The vibrations were traced with a second, even shorter laser pulse. Although the atoms were found to oscillate with speeds beyond 1000 m/s, their motion could be traced in ultra-slow motion. This time-resolved measurement was key to reconstructing the forces acting on the atoms.”


Understanding the strong forces that hold crystalline structures together will give scientists a greater understanding of their mechanical and thermal properties.


The research was recently published in Nature. The abstract read:


“Nonlinear optical techniques at visible frequencies have long been applied to condensed matter spectroscopy. However, because many important excitations of solids are found at low energies, much can be gained from the extension of nonlinear optics to mid-infrared and terahertz frequencies. For example, the nonlinear excitation of lattice vibrations has enabled the dynamic control of material functions.


“So far it has only been possible to exploit second-order phonon nonlinearities at terahertz field strengths near one million volts per centimetre. Here we achieve an order-of-magnitude increase in field strength and explore higher-order phonon nonlinearities. We excite up to five harmonics of the A1 (transverse optical) phonon mode in the ferroelectric material lithium niobate.


“By using ultrashort mid-infrared laser pulses to drive the atoms far from their equilibrium positions, and measuring the large-amplitude atomic trajectories, we can sample the interatomic potential of lithium niobate, providing a benchmark for ab initio calculations for the material.


“Tomography of the energy surface by high-order nonlinear phononics could benefit many aspects of materials research, including the study of classical and quantum phase transitions.”

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