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High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
Published on: October 31, 2019
First principles study of the optical contrast in phase change materials
S Caravati1, M Bernasconi, M Parrinello
1Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, 6900 Lugano, Switzerland.
Summary
Phase change materials like Germanium Antimony Telluride (Ge2Sb2Te5) exhibit significant optical contrast between crystalline and amorphous states. This difference, crucial for data storage, stems from altered optical matrix elements due to structural disorder in amorphous phases.
Area of Science:
- Materials Science
- Condensed Matter Physics
- Computational Materials Science
Background:
- Phase change materials (PCMs) like Ge2Sb2Te5 (GST), GeTe, and Sb2Te3 are critical for optical data storage due to their distinct crystalline and amorphous states.
- Understanding the origin of the optical contrast between these phases is essential for optimizing device performance.
Purpose of the Study:
- To investigate the optical properties of crystalline and amorphous Ge2Sb2Te5 (GST), GeTe, and Sb2Te3 using first-principles calculations.
- To elucidate the fundamental reasons behind the optical contrast exploited in optical data storage applications.
Main Methods:
- First-principles calculations were employed to study the electronic and optical properties.
- Realistic amorphous phase models were generated using ab initio molecular dynamics simulations with quenching from the melt.
- Analysis focused on changes in optical matrix elements across the phase transition.
Main Results:
- The calculations accurately reproduced the experimentally observed strong optical contrast between crystalline and amorphous phases for all studied materials.
- The optical contrast was definitively linked to changes in optical matrix elements during the phase transition.
- A reduction in optical matrix elements in the amorphous phases was identified as the primary cause of the contrast.
Conclusions:
- Angular disorder in p-bonding within the amorphous network is the dominant factor responsible for the reduced optical matrix elements and the observed optical contrast.
- These findings align with and support previous theoretical proposals regarding the structural origins of optical contrast in PCMs.
- The study provides a fundamental understanding of the optical properties of these key phase change materials.
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