2:15 PM - EP08.10.03
A New Structural Phase Transition and C-Axis Transport in HfS2
Jie Peng1,Peter Chung1,Sina Najmaei2,Madan Dubey2
University of Maryland, College Park1,U.S. Army Research Laboratory2
HfS2 has attracted interests recently due to its distinctive properties such as high room-temperature mobility , finite bandgap , and high on-off current ratio . The majority of the focus appears to be on in-plane properties. However, many exciting physical behaviors including the thermoelectric effect , field effect tunneling , and superconductivity  are tightly related to the c-axis (out-of-plane) electronic and thermal transport properties. However, the understanding of c-axis transport is still formative.
In recent X-ray and Raman spectroscopy measurements , an abnormal behavior was observed where a frequency increase – referred to as phonon “stiffening” - in the out-of-plane phonon mode A1g accompanied an increase in temperature from 80K to 300K. This is in contrast to the normal behavior where increasing temperature leads to lattice expansion, thus resulting in decreasing phonon frequency or phonon “softening”. Beyond 300K, however, the trend in the Raman shift was found to reverse and phonon softening was observed. A correlated behavior also occurred in the thermal expansion coefficients (TECs) where a transition at 300K occurred between a region of low TECs at lower temperatures and high TECs at higher temperatures. To explain these observations, we performed first principles simulations using the Quasi-Harmonic Approximation (QHA) as well as first principles Car-Parrinello molecular dynamics (CPMD) simulations from 80K to 500K. The simulations revealed a fundamental change in the stacking pattern between adjacent Hf-S-Hf layers from AAA to ABC at 300K. The increasing temperature gave rise to a unique anharmonic mechanism. This was confirmed by comparing the calculated A1g mode frequencies from temperature dependent phonon density of states with experimental data. Thus, we attribute the surprising phonon stiffening to a lattice phase transition that takes place at 300K where the transition is marked primarily by a change in stacking sequence. We also report for the first time a first principles fully-anharmonic temperature-dependent estimation of c-axis transport properties such as the carrier density, electron and phonon mean free path, mobility, and thermal conductivity.
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