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Pini D, Rovelli T, Mambretti F, Galli DE. Structural transition, orientational order, and anomalous specific heat in a two-dimensional dimer crystal of core-softened particles. Phys Rev E 2024; 109:034128. [PMID: 38632752 DOI: 10.1103/physreve.109.034128] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2023] [Accepted: 02/21/2024] [Indexed: 04/19/2024]
Abstract
Systems featuring hard-core-soft-shell repulsive pair potentials can form ordered phases, where particles organize themselves in aggregates with nontrivial geometries. The dimer crystal formed by one such potential, namely, the hard-core plus generalized exponential model of order 4, has been recently investigated, revealing a low-temperature structural phase transition, with the onset of nematic ordering of the dimers. In the present work, we aim to characterize this phase transition via a mean-field theory, by which a detailed analysis of the low-temperature properties of the system is carried out under quadrupole approximation. We determine the transition temperature and identify its order parameter, highlighting the link between the structural transition and the nematic ordering of the system. The first-order character of the transition is established and supported by the Landau expansion of the free energy in powers of the order parameter. The theory is subsequently generalized to take into account lattice vibrations and dimer length fluctuations. Finally, we provide an explanation for the anomalous behavior displayed by the specific heat in the vanishing-temperature limit, which is also supported by Monte Carlo simulations.
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Affiliation(s)
- D Pini
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli", via Celoria 16, 20133 Milano, Italy
| | - T Rovelli
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli", via Celoria 16, 20133 Milano, Italy
| | - F Mambretti
- Istituto Italiano di Tecnologia, via Melen 83, 16152 Genova, Italy
- Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, via Marzolo 8, 35131 Padova, Italy
| | - D E Galli
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli", via Celoria 16, 20133 Milano, Italy
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Mambretti F, Martinelli M, Civillini F, Bertoletti M, Riva S, Manini N, Galli DE, Pini D. Low-temperature ordering of the dimer phase of a two-dimensional model of core-softened particles. Phys Rev E 2021; 104:044602. [PMID: 34781531 DOI: 10.1103/physreve.104.044602] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2021] [Accepted: 09/17/2021] [Indexed: 11/07/2022]
Abstract
Purely pairwise interactions of the core-softened type, i.e., featuring a soft repulsion followed by a hard-core interaction at shorter distance, give rise to nontrivial equilibrium structures entirely different from the standard close packing of spheres. In particular, in a suitable low-temperature region of their phase diagram, such interactions are well known to favor a transition from a fluid to a cluster crystal. The residual mutual interaction between individual clusters can lead to the formation of patterns of their reciprocal orientations. In this work, we investigate two examples of such models in two dimensions, at the density most appropriate to the dimer phase, whereby clusters consist of just two particles, studying them with optimization techniques and Monte Carlo simulations. We focus on the dimer crystal, and unveil a second phase transition at extremely low temperature. This transition leads from a triangular dimer lattice with randomly disordered dimer orientations at high temperature to a reduced-symmetry ground state with nematic orientational order and a slightly distorted structure characterized by a centered-rectangular lattice at low temperature.
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Affiliation(s)
- F Mambretti
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy.,Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, via Marzolo 8, 35131 Padua, Italy
| | - M Martinelli
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - F Civillini
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - M Bertoletti
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - S Riva
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - N Manini
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - D E Galli
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
| | - D Pini
- Università degli Studi di Milano, Dipartimento di Fisica "Aldo Pontremoli," via Celoria 16, 20133 Milan, Italy
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