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Computer Simulations of Simple Liquids with Tetrahedral Local Order: the Supercooled Liquid, Solids and Phase Transitions
Stockholm University, Faculty of Science, Numerical Analysis and Computer Science (NADA) (together with KTH). (Numerisk analys)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The understanding of complex condensed matter systems is an area of intense study. In this thesis, some properties of simple liquids with strong preference for tetrahedral local ordering are explored. These liquids are amenable to supercooling, and give complex crystalline structures on eventual crystallisation. All liquids studied are simple, monatomic and are similar to real metallic liquids.

The vibrational density of states of a glass created in simulation is calculated. We show a correspondence between the vibrational properties of the crystal and the glass, indicating that the vibrational spectra of crystals can be used to understand the more complex vibrational spectra of the glass of the same substance.

The dynamics of supercooled liquids is investigated using a previously not implemented comprehensive measure of structural relaxation. This new measure decays more slowly in the deeply supercooled domain than the commonly used measure.

A new atomic model for octagonal quasicrystals is presented. The model is based on findings from a molecular dynamics simulation that resulted in 45˚ twinned β-Mn. A decoration is derived from the β-Mn unit cell and the unit cell of the intermediate structure found at the twinning interface.

Extensive simulations are used to explore the phase diagram of a liquid at low densities. The resulting phase diagram shows a spinodal line and a phase coexistence region between a liquid and a crystalline phase ending in a critical point. This contradicts the old conclusion of the Landau theory -- that continuous transitions between liquids and crystals cannot exist

The same liquid is explored at higher densities. Upon cooling the liquid performs a first order liquid-liquid phase transition. The low temperature liquid is shown to be strong and to have very good glass forming abilities. This result offers new insights into fragile to strong transitions and suggests the possibility of a good metallic glass former.

Place, publisher, year, edition, pages
Stockholm: Numerical Analysis and Computer Science (NADA) (together with KTH), Stockholm University , 2009. , vi, 65 p.
Series
Trita-CSC-A, ISSN 1653-5723 ; 2009:14
Keyword [en]
Simple liquids, glasses, liquid-glass transition, supercooled liquids, tetrahedral liquids, phase transitions, quasicrystals, molecular dynamics
National Category
Condensed Matter Physics Condensed Matter Physics
Research subject
Numerical Analysis
Identifiers
URN: urn:nbn:se:su:diva-27876ISBN: 978-91-7155-901-2 (print)OAI: oai:DiVA.org:su-27876DiVA: diva2:228961
Public defence
2009-09-11, Sydvästra galleriet, Biblioteket, Kungl Tekniska Högskolan, Osquars backe 25, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: In progress. Available from: 2009-08-20 Created: 2009-05-27 Last updated: 2009-08-27Bibliographically approved
List of papers
1. Evidence for compact cooperatively rearranging regions in a supercooled liquid
Open this publication in new window or tab >>Evidence for compact cooperatively rearranging regions in a supercooled liquid
2009 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, Vol. 21, no 24, 245101-1-245101-5 p.Article in journal (Refereed) Published
Abstract [en]

We examine structural relaxation in a supercooled glass-forming liquid simulated by constant-energy constant-volume (NVE) molecular dynamics. Time correlations of the total kinetic energy fluctuations are used as a comprehensive measure of the system’s approach to the ergodic equilibrium. We find that, under cooling, the total structural relaxation becomes delayed as compared with the decay of the component of the intermediate scattering function corresponding to the main peak of the structure factor. This observation can be explained by collective movements of particles preserving many-body structural  orrelations within compact three-dimensional (3D) cooperatively rearranging regions.

Place, publisher, year, edition, pages
IOP Publishing Limited, 2009
Keyword
Supercooled liquid, liquid-glass transition, glass, liquid relaxation
National Category
Condensed Matter Physics Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:su:diva-27897 (URN)10.1088/0953-8984/21/24/245101 (DOI)
Available from: 2009-05-28 Created: 2009-05-28 Last updated: 2009-08-10Bibliographically approved
2. Evidence for a liquid-solid critical point in a simple monatomic system
Open this publication in new window or tab >>Evidence for a liquid-solid critical point in a simple monatomic system
2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 104502Article in journal (Refereed) Published
Abstract [en]

It is commonly believed that the transition line  separating a liquid and a solid cannot be interrupted by a  critical point. This opinion is based on the traditional  symmetry argument that an isotropic liquid cannot be  continuously transformed into a crystal with a discrete  rotational and translational symmetry. We present here a  molecular-dynamics simulation of a simple monatomic system  suggesting the existence of a liquid-solid spinodal terminating  at a critical point. We show that, in the critical region, the  isotropic liquid continuously transforms into a phase with a  mesoscopic order similar to that of the smectic liquid  crystals. We argue that the existence of both the spinodal and  the critical point can be explained by the close structural  proximity between the mesophase and the crystal. This indicates  a possibility of finding a similar thermodynamic behaviour in  gelating colloids, liquid crystals and polymers.

Keyword
Critical point, Spinodal, phase transition
National Category
Condensed Matter Physics Condensed Matter Physics Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:su:diva-29017 (URN)10.1063/1.3213616 (DOI)
Available from: 2009-08-04 Created: 2009-08-04 Last updated: 2017-12-13Bibliographically approved
3. A polyamorphous fragile to strong transition under equilibrium cooling in a simple monatomic liquid
Open this publication in new window or tab >>A polyamorphous fragile to strong transition under equilibrium cooling in a simple monatomic liquid
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Glass transition remains one of the deepest and most interesting  problems of condensed-matter physics. One fundamental aspect of the problem is how to avoid  crystalline nucleation when cooling a liquid towards the glass  transition at an arbitrarily slow rate. The prototype ``ideal glass  formers'' are silica and other inorganic network-forming systems. No  monatomic glass has been obtained so far by cooling from melt.  Whether a monatomic system can reproduce the behavior of  silica-like glass formers is a question of great interest, both  conceptual and for technological applications. We present a  molecular-dynamics simulation of a simple monatomic system based on  a metallic-like pair potential. We demonstrate that, while remaining  stable with respect to crystallization, the system performs under  cooling a first order phase transition from a fragile to an  extremely strong liquid state. The low-temperature  liquid phase can be cooled to a state of very high viscosity and low  diffusivity while remaining in equilibrium. This result may provide  a significant insight into the formation mechanisms of metallic  glasses.

National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-29018 (URN)
Available from: 2009-08-04 Created: 2009-08-04 Last updated: 2013-07-09Bibliographically approved
4. Structural model for octagonal quasicrystals derived from octagonal symmetry elements arising in beta-Mn crystallization of a simple monatomic liquid
Open this publication in new window or tab >>Structural model for octagonal quasicrystals derived from octagonal symmetry elements arising in beta-Mn crystallization of a simple monatomic liquid
Show others...
2009 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 79, no 14, 144201-1-144201-10 p.Article in journal (Refereed) Published
Abstract [en]

While performing molecular-dynamics simulations of a simple monatomic liquid, we observed the crystallization of a material displaying octagonal symmetry in its simulated diffraction pattern. Inspection of the atomic arrangements in the crystallization product reveals large grains of the beta-Mn structure aligned along a common fourfold axis, with 45° rotations between neighboring grains. These 45° rotations can be traced to the intercession of a second crystalline structure fused epitaxially to the beta-Mn domain surfaces, whose primitive cell has lattice parameters a=b=c=a_{beta-Mn}, alpha =beta =90°, and gamma =45°. This secondary phase adopts a structure which appears to have no known counterpart in the experimental literature, but can be simply derived from the Cr3Si and Al3Zr4 structure types. We used these observations as the basis for an atomistic structural model for octagonal quasicrystals, in which the beta-Mn and the secondary phase structure unit cells serve as square and rhombic tiles (in projection), respectively. Its diffraction pattern down the octagonal axis resembles those experimentally measured. The model is unique in being consistent with high-resolution electron microscopy images showing square and rhombic units with edge-lengths equal to that of the beta-Mn unit cell. Energy minimization of this configuration, using the same pair potential as above, results in an alternative octagonal quasiperiodic structure with the same tiling but a different atomic decoration and diffraction pattern.

Place, publisher, year, edition, pages
Colllege Park, Maryland, USA: American Physical Society, 2009
Keyword
Crystallisation, electron microscopy, liquid metals, liquid structure, liquid theory, manganese, molecular dynamics method
National Category
Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:su:diva-27901 (URN)10.1103/PhysRevB.79.144201 (DOI)000265943200050 ()
Available from: 2009-05-28 Created: 2009-05-28 Last updated: 2017-12-13Bibliographically approved
5. Nature of vibrational eigenmodes in topologically disorderedsolids
Open this publication in new window or tab >>Nature of vibrational eigenmodes in topologically disorderedsolids
Show others...
2002 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 65, 104302- p.Article in journal (Refereed) Published
Abstract [en]

We use a local projectional analysis method to investigate theeffect of topological disorder on the vibrational dynamics in a modelglass simulated by molecular dynamics. It is shown that the vibrational eigenmodes in the glass aregenerically related to the corresponding eigenmodes of its crystallinecounterpart via disorder-induced level-repelling and hybridizationeffects. It is demonstrated that the effect of topological disorder in theglass on the dynamical matrix can be simulated by introducingpositional disorder in a crystalline counterpart.

Place, publisher, year, edition, pages
APS, 2002
Keyword
Disordered solids, Phonons, Vibrational states
National Category
Condensed Matter Physics Atom and Molecular Physics and Optics
Research subject
Materials Science
Identifiers
urn:nbn:se:su:diva-29016 (URN)10.1103/PhysRevB.65.104302 (DOI)
Available from: 2009-08-04 Created: 2009-08-04 Last updated: 2017-12-13Bibliographically approved

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