Simulating Liquid GeTe
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0918-H03-01
Simulating Liquid GeTe James R. Chelikowsky Institute for Computational Engineering and Sciences, University of Texas at Austin, 1 University Station C0200, Austin, Texas, 78735 ABSTRACT One of the most difficult problems in condensed matter physics is to describe the microscopic liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss specific microscopic structures, only ensemble averages can be specified. Such averages can be performed via molecular dynamics simulations. A problematic issue in performing such simulations is computing accurate interatomic forces. Although classical many-body potentials can be use for simulations of covalent materials, one must effectively map quantum phenomena such as hybridization onto such potentials. This mapping is complex and lacking a well-defined prescription. This step can be avoided by employing quantum forces from the pseudopotentialdensity functional method. Using molecular dynamics with quantum forces, we examine the local atomic order as well as some dynamic and electronic properties of the semiconducting liquid GeTe. Near the melting temperature, the Peierls distortion responsible for the lower temperature crystal phase of GeTe is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder leads to an eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. Using an ensemble average, we predict the dc conductivity of the melt to be consistent with recent measurements. INTRODUCTION The GexTe1-x semiconductor family exhibits many interesting properties, e.g., the ability to generate easily the amorphous phase, and the possibility of thermally-induced local crystallization [1]. Previous work has focused on the Te rich compounds, which are excellent glass-forming materials. Despite the importance of the glassy phase, the only studies examining the precursory liquid phase have been performed on the eutectic Ge85Te15 and GeTe2 compounds [2]. The local atomic structure of these liquids was not elucidated in this work. The structure of the liquid, and especially its relation with the semiconducting behavior of the melt, remain unclear. In contrast to most tetrahedrally coordinated semiconductors (e.g., IV, III-V, and some II-VI semiconductors), melting in GeTe is not associated with a semiconductor (SC) to metal (M) transition [3]. For compounds experiencing SC to M transitions upon melting, the sudden increase in d.c.conductivity is accompanied by a density increase, indicating a structural modification of the local atomic order in the liquid. For these semiconductors, neutron diffraction has demonstrated that the coordination number, Z, increases from the value of 4 in the crystal to a value 6 in the melt [4]. Although this behavior is widely observed for IV and III-V semiconductors, some II-VI materials (HgSe, CdTe and
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