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Three-dimensional bond orientational order is studied via computer simulations of 864 particles interacting through a Lennard-Jones pair potential. Long-range orientational fluctuations appear upon supercooling about ten percent below the equilibrium melting temperature. The fluctuations suggest a broken icosahedral symmetry with extended correlations in the orientations of local icosahedral packing units. PACS numbers: 61.20.Ja f 61.25.Bi, 61.40.DfCrystalline solids display both translational and rotational broken symmetries. The two symmetries are not independent, since rotating one region of a perfect crystal relative to another also disrupts the translational correlations between the regions. It is possible, however, to imagine a material with no translational order yet with persistent correlations in local coordinate axes defined by the angular positions of a cluster of atoms. An example is the hexatic phase uncovered in recent theoretical investigations of twodimensional melting. 1 The hexatic phase is a fluid characterized by anomalous sixfold correlations in the orientations of "bonds" connecting near-neighbor atoms. Although computer simulations have produced conflicting evidence for an equilibrium hexatic phase, 2 the discovery of a "stacked hexatic" phase in bulk smectic liquid crystals has been reported recently. 3 It seems reasonable to ask if other three-dimensional phases with bond orientational order exist. This question has been investigated recently by Nelson and Toner, 4 who found that crystals disordered by an equilibrium concentration of unbound dislocation loops can display bond orientational order with a cubic symmetry. It was argued that supercooled liquids might drop into such a phase prior to the glass transition.Another possibility is bond orientational order based upon an icosahedral symmetry. It is well known that an icosahedral clustering of twelve atoms about a central sphere is energetically preferable to crystalline packings for Lennard-Jones pair potentials. 5 In this paper we investi-gate bond orientational order in a molecular-dynamics simulation of 864 atoms in a box with periodic boundary conditions interacting via a Lennard-Jones pair potential. The system was supercooled below the freezing temperature at constant density 6 p*=0.97, and equilibrated by integrating the equations of motion with a time step T 0 =4.65xl0" 3 (cr 2 m/e) l/2 . Runs on a system of 600 atoms with the same density gave qualitatively similar results. Upon cooling 0.07± 0.02 reduced units below the melting temperature T m * =0.70, we find an apparent phase transition into a phase with icosahedral bond orientational order. The results are consistent with longrange correlations in the orientations of local icosahedral clusters.To study bond orientational order, we first define a set of "bonds" joining an atom to its neighbors. Neighbors of a given atom are defined as all atoms lying within 1.2 times the minimum in the Lennard-Jones potential. It is convenient to associate a set of order parameters {Q lm (...

A new high-frequency and short-wavelength collective mode specific to binary liquid mixtures with large mass difference is observed in a computer simulation of Lio.8Pbo.2 and discussed within the framework of the Mori-Zwanzig formalism. The mode shows linear dispersion in a wave-number regime 0.1 A "* < ^ < 0.6 A~^ but its propagation velocity is higher than the ordinary sound velocity by more than a factor of 3. Its attenuation is only weakly q dependent in contrast to the damping of ordinary sound. In Lio.8Pbo.2 "fast sound" entails motion of the lighter atoms only.PACS numbers: 61.25.MvWe report the existence of an additional propagating collective mode in binary liquid mixtures, confined to high frequencies and large wave numbers well beyond the hydrodynamic regime. It can be observed in inelastic neutron-scattering experiments or in computer simulation studies of two-component systems with large atomic-mass difference. Some of these systems may respond to a high-frequency short-wavelength perturbation with a density wave, which is supported by the light particles alone, essentially, without the heavy particles participating in the collective motion. Since the dispersion law of this excitation mode is much steeper in the linear region than that of the Brillouin peak of ordinary sound, we call the new mode "fast sound." We have observed fast sound in a computer simulation study ^ of a liquid alloy system of 250 particles in a periodic cell modeling Lio.gPbo.i^ at temperature r=1085 K and total number density w =0.045 58 A ~l Results for the partial dynamic structure factorsSss '(q\(o)

Abstract-Having worked for a long time with videolectures, we devised a new teaching model that makes use of the recorded videos to free the teacher from part of his work and allowing him to concentrate his efforts on a more effective and involving teaching strategy.

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We investigate the structure of 13-particle clusters in binary alloys for various size ratios and different concentrations via molecular-dynamics simulation. Our goal is to predict which systems are likely to form local icosahedral structures when rapidly supercooled from the melt. We calculate the energy spectrum of the minimal energy structures, and characterize all detected minima from both their relative probability and a structural point of view. We identify regions in our parameter space where the icosahedral structure is dominant ͑like in the corresponding monatomic case͒, regions where the icosahedral structure disappears, and others where icosahedral structures are present but not dominant. Finally, we compare our results with simulations reported in the literature and performed on extended binary systems with various size ratios and at different concentrations. ͓S0163-1829͑96͒05018-7͔

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