arXiv subjects

Avisek Das

Publications and source records attributed to Avisek Das.

5 recordsLinked to original sources

arXiv2024

Phase behavior of hard sheared cube family

A sheared cube is made out of a cube by giving a shear to the body in one direction keeping one of the faces fixed. We investigate here the thermodynamic phase behavior of a family of such regular hard sheared cubes, each of the members of the family having a distinct angle made by the faces with the perpendicular on the fixed face. Hard particle Monte Carlo (HPMC) has been performed with these anisotropic building blocks resulting entropy-driven self assembly. Thereby computational evidence of discrete plastic crystal phase has been found in crystal. The discrete plastic crystal phase is known to form through the spontaneous self-assembly of certain polyhedra. Throughout the entire solid regime particle orientations exhibit strong specific correlations before melting into a liquid, without any evidence of freely rotating plastic crystal at lower density solid. It has been thoroughly observed that geometrical attributes of the shapes don't determine any of the properties that designate this orientational disorder phase reported here. We also find that particle's rotational symmetric axes and one of the rotational symmetric axes of the unit cell of the crystal have a strong relationship in their alignment in space. These results, achieved with shapes having crystallographic point group symmetry, are investigated as being consistent with the phenomenology of discrete plastic crystal phase established in earlier works with hard particles having non-crystallographic point group symmetry.

cond-mat.soft↗

arXiv2024

Predictive orientational phase behavior in convex polyhedral entropic crystals

Hard convex polyhedra, idealized models for anisotropic colloids and nanoparticles, are known to form variety of orientational phases despite the regular arrangement of particles in the crystalline assemblies. Based on the orientational behavior of the constituents particles, such phases could be categorized into freely rotating plastic crystals (PC), discrete plastic crystals (DPC) and orientationally ordered crystals (OC). In this article, we report an extensive Monte Carlo computer simulation study of sixty hard convex polyhedral shape indicating a direct predictive relationship between the nature of orientational phases in the crystalline assemblies and single-particle shape attributes. The influence of three attributes namely; (i) Isoperimetric Quotient (IQ) i.e., the extent of asphericity; (ii) isotropy of the moment of inertia tensor in the principal frame and (iii) number of symmetry operations in the point group of the particle and self-assembled crystal structure, were observed to control the orientational phase behavior of the entire solid region in many-body system. The translational order in the crystal appeared to play significant role only in the DPC phase, where as, other two phases were completely governed by the combination of two attributes. In this study, the role of shape attributes were characterized by sequential appearance of one or two of the aforementioned rotational phases across the phase diagram in a pressure dependent manner which could be regarded as an important stepping stone towards fully predictive self-assembly behavior of hard particle systems.

cond-mat.soft↗

arXiv2024

Role of symmetry in the orientationally disordered crystals of hard convex polyhedra

The crystalline solids with lack of orientational ordering of anisotropic particles serve the purpose of studying the disordered systems with many fundamental applications in contemporary research. Despite the orientational disorder, multiple unique orientations with fixed angular differences exist in the crystal structures giving rise of "discrete plastic crystal" phase where the particles jump discretely within the unique orientations. We report the computational evidence of the role of symmetries between polyhedral particles and respective crystalline structures in controlling the existence of such phase at comparatively higher range of packing fractions beyond the freely rotating plastic crystals. The point groups of the particle and crystal structure were found to be directly connected in terms of the parallel alignment between the highest order rotational symmetry axes of the particle point group and any rotational axes of crystallographic point group, as a characteristic feature of this phase giving rise of discrete orientations. Based on our previous research [Kundu \textit{et al.}, arXiv:2311.06799, 2023] and new findings reported here, this symmetry relationship appeared to occur at the unit cells of the crystal structures which acted as the source of correlation, where as, all previously reported conserved orientational attributes i.e., number of unique orientations with fixed angular differences, equal population densities within the unique orientations, could be thought as the signatures of correlation present in the entire system. This relationship appeared to control all the aspects of phase which might be useful to draw fundamental insights about the disordered phases with orientational correlation as well as designing the disorder in the crystals.

cond-mat.soft↗

arXiv2024

Algorithmic detection of crystal structures from computer simulation data

Detection of crystal structures from particle positions of crystalline assemblies formed in computer simulations is an unsolved problem. The standard protocol, formulated in the reciprocal space, for structure determination from experimental diffraction data is not suitable for analysis of computer simulation data, after converting them to the Fourier space. There is a long history of attempts to tackle this problem by analyzing the system in the real space by using ideas of local neighbors and broken symmetries of the crystalline state. In this paper, we propose a heuristic solution to this problem by detecting all possible unit cells directly from particle coordinates obtained in a typical computer simulation. The method is based on well known facts about crystal structures, some of which are underutilized in the context of the current problem. These include, the symmetry of the coordination polyhedron and its empirical relationship with directions of lattice vectors for a simple Bravais lattice, and the fact that any complex crystal can be systematically decomposed into multiple Bravais lattices. By using these ideas, along with standard computational techniques like search, clustering and convex hull construction, we were able to handle complex basis and construct all crystallographically viable unit cells from the coordinates. The method is capable of handling statistical noise by employing certain cutoffs and deals with multicomponent systems in a transparent manner. We validated it on real Monte Carlo simulation data and variety of test systems, including crystals with tens of particles in the basis. Our heuristic algorithm, which requires minimal human intervention and computational resources, provides a solution to the long standing problem and would be beneficial to the wider communities of condensed matter physics and computational materials science.

cond-mat.mtrl-sci↗

arXiv2023

Understanding orientational disorder in crystalline assemblies of hard convex polyhedra

Spontaneous self-assembly of hard convex polyhedra are known to form orientationally disordered crystalline phases, where particle orientations do not follow the same pattern as the positional arrangement of the crystal. A distinct type of orientational phase with discrete rotational mobility has been reported in hard particle systems. In this paper, we present a new analysis method for characterizing orientational phase of a crystal, which is based on algorithmic detection of unique orientations. Using this method we collected complete statistics of discrete orientations along the Monte Carlo simulation trajectories and observed that particles were equally partitioned among them, with specific values of pairwise orientational differences. These features remained constant across the pressure range and did not depend on rotational mobility. The discrete mobility was characteristic of a distinct equilibrium thermodynamic phase, qualitatively different from the freely rotating plastic phase with continuous orientations. The high pressure behavior with frozen particle orientations was part of that the same description and not a non-equilibrium arrested state. We introduced a precise notion of orientational order and demonstrated that the system was maximally disordered at the level of unit cell, even though individual particles could only take few discrete orientations. We report the existence of this phase in five polyhedral shapes and in systematically curated shape families constructed around two of them. The symmetry mismatch between the particle and the crystallographic point groups was found to be a predictive indicator for the occurrence of this phase.

cond-mat.soft↗