Liquid-Liquid Phase Transition in Undercooled Silicon: Structural, Electronic and Dynamical Aspects
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Liquid-Liquid Phase Transition in Undercooled Silicon: Structural, Electronic and Dynamical Aspects Noel Jakse1 and Alain Pasturel1,2 1 Sciences et Ingénierie des Matériaux et Procédés, INP Grenoble, UJF-CNRS 1130, rue de la Piscine, BP 75, 38402 Saint-Martin d’Hères Cedex, France 2 Laboratoire de Physique et Modélisation des Milieux Condensés, Maison des Magistères, BP 166 CNRS, 38042 Grenoble Cedex 09, France ABSTRACT We have proposed a new mixed approach by combining efficiently classical and firstprinciples molecular dynamics to study undercooling of liquid silicon, regardless of a specific empirical interaction model. Our results show an enhancement of the local tetrahedral ordering in the deep undercooled region associated to the appearance of propagating shear waves and give a strong support to the existence of a transition between a high density liquid to a low density liquid near 1050 K. An analysis of the structural, dynamics and electronic properties is proposed to elucidate these features. INTRODUCTION Despite the occurrence of advanced materials in the recent years, silicon still keeps the leadership in semiconductor technology and attracts growing interest in advanced materials as well as for photovoltaic applications in the context of energetic and environmental issues. As most of technological applications begin with crystalline silicon elaborated from the melt, the properties of normal and undercooled liquid silicon are therefore of utmost importance for the manufacturing process and material design. Unusual behavior of the density in the undercooled liquid states [1] as deep as 200 K below the melting point was observed, leading researchers to consider plausible the existence of a liquid-liquid transition at even lower temperatures. Unfortunately these are out of reach to state-of-the-art experimental facilities [2]. Therefore, the absence of direct evidence from experiments has prompted to look for it using numerical simulations [3-5]. However, simulations based on empirical potentials have to be taken with caution since the values of the potential parameters might not be transferable with the change of the bonding environment upon undercooling. We have therefore proposed a new mixed approach by combining efficiently classical and first-principles molecular dynamics to attain a deep undercooling and to describe the system from first principles. Our results show a transition between a high density liquid (HDL) to a low density liquid (LDL) near 1050 K regardless of a specific empirical interaction model [4], giving a strong support to the existence of liquid-liquid transition (LLT). The aim of the present paper is to analyze the structural, electronic, and dynamical aspects of the undercooling effects of silicon including those associated to the LLT. SIMULATION METHOD The liquid and undercooled states have been investigated using ab initio molecular dynamics simulations (AIMD) with N = 512 silicon atoms in the canonical ensemble (NVT)
within the density functional theory (DFT) with
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