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Theoretical Studies of Two-Dimensional Magnetism and Chemical Bonding
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is divided into two parts. In the first part we study thermodynamics of the two-dimensional Heisenberg ferromagnet with dipolar interaction. This interaction breaks the conditions of the Mermin-Wagner theorem, resulting in a finite transition temperature. Our calculations are done within the framework of the self-consistent spin-wave theory (SSWT), which is modified in order to include the dipolar interaction. Both quantum and classical versions of the Heisenberg model are considered.

The second part of the thesis investigates the chemical bonding in solids from the first principles calculations. A new chemical bonding indicator called balanced crystal orbital overlap population (BCOOP) is developed. BCOOP is less basis set dependent than the earlier indicators and it can be used with full-potential density-functional theory (DFT) codes. We apply BCOOP formalism to the chemical bonding in the high-T_c superconductor MgB2 and the theoretically predicted MAX phase Nb3SiC2. We also study how the chemical bonding results in a repulsive hydrogen–hydrogen interaction in metal hydrides. The role of this interaction in the structural phase transition in Ti3SnHx is investigated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2005. , p. vii + 87
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 21
Keywords [en]
Physics, spin Hamiltonians, quantized spin models, Heisenberg model, spin waves, self-consistent spin-wave theory, dipolar interaction, density functional theory, chemical bonding, overlap population, MAX phases, metal hydrides
Keywords [sv]
Fysik
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-4815ISBN: 91-554-6164-6 (print)OAI: oai:DiVA.org:uu-4815DiVA, id: diva2:165865
Public defence
2005-04-01, Siegbahnsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 10:15
Opponent
Supervisors
Available from: 2005-03-10 Created: 2005-03-10Bibliographically approved
List of papers
1. Thermodynamics of a two-dimensional Heisenberg ferromagnet with dipolar interaction
Open this publication in new window or tab >>Thermodynamics of a two-dimensional Heisenberg ferromagnet with dipolar interaction
2005 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 71, no 2, p. 024427-Article in journal (Refereed) Published
Abstract [en]

Thermodynamics of quantum and classical two-dimensional (2D) Heisenberg models with long-range dipole-dipole interaction has been investigated using various forms of self-consistent spin-wave theory (SSWT). It has been found that SSWT gives a much lower transition temperature T-c than the free-magnon (spin-wave) theory. For the classical spin, the T-c from SSWT lies within 9% of the Monte Carlo value, making SSWT the best approximation among those considered. It is proven that the random phase approximation vertex corrections to SSWT are rather small. The results depend strongly on the value of the spin, emphasizing the importance of using the quantum and not the classical 2D Heisenberg model even for large spins such as S=7/2.

Keywords
ULTRATHIN MAGNETIC-FILMS, GREENS-FUNCTION THEORY, SPIN-WAVE THEORY, THIN-FILMS, REORIENTATION TRANSITION, LOW-TEMPERATURE, 2 DIMENSIONS, MONTE-CARLO, RANGE ORDER, ANISOTROPY
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-92687 (URN)10.1103/PhysRevB.71.024427 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
2. Geometry of the Valence Transition Induced Surface Reconstruction of Sm(0001)
Open this publication in new window or tab >>Geometry of the Valence Transition Induced Surface Reconstruction of Sm(0001)
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2002 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 88, no 13, p. 136102-Article in journal (Refereed) Published
Abstract [en]

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initiocalculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-92688 (URN)10.1103/PhysRevLett.88.136102 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
3. Balanced crystal orbital overlap population: a tool for analysing chemical bonds in solids
Open this publication in new window or tab >>Balanced crystal orbital overlap population: a tool for analysing chemical bonds in solids
2003 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 15, no 45, p. 7751-7761Article in journal (Refereed) Published
Abstract [en]

A new tool for analysing theoretically the chemical bonding in solids is proposed. A balanced crystal orbital overlap population (BCOOP) is an energy resolved quantity which is positive for bonding states and negative for antibonding states, hence enabling a distinction between bonding and antibonding contributions to the chemical bond. Unlike the conventional crystal orbital overlap population (COOP), BCOOP handles correctly the situation of crystal orbitals being nearly linear dependent, which is often the case in the solid state. Also, BCOOP is much less basis set dependent than COOP. A BCOOP implementation within the full-potential linear muffin tin orbital method is presented and illustrated for Si, TiC and Ru. Thus, BCOOP is compared to the COOP and crystal orbital Hamilton population (COHP) for systems with chemical bonds ranging from metallic to covalent character.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-92689 (URN)10.1088/0953-8984/15/45/014 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
4. Layered compound Nb3SiC2 predicted from first-principles theory
Open this publication in new window or tab >>Layered compound Nb3SiC2 predicted from first-principles theory
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2004 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 85, no 15, p. 3071-3073Article in journal (Refereed) Published
Abstract [en]

A previously unobserved ternary carbide, Nb3SiC2, belonging to the family of the so-called Mn+1AXn or MAX phases is predicted from first-principles calculations. It has a theoretical bulk modulus of 296 Gpa, wich is much higher than that of Ti3SiC2. The new phase is metastable with a formation energy of +0.02 eV/atom. We suggest that the phase may possibly be synthesized using thin film technology. The chemical binding of Nb3SiC2 is investigated using the balanced crystal orbital overlap population indicator and it is found to be dominated by the formation of Nb4D-C 2p covalent bonds.

National Category
Physical Sciences Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-92690 (URN)10.1063/1.1791734 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
5. Elastic properties of Mg(1-x)AlxB2 from first principles theory
Open this publication in new window or tab >>Elastic properties of Mg(1-x)AlxB2 from first principles theory
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2004 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 16, no 29, p. 5241-5250Article in journal (Refereed) Published
Abstract [en]

Elastic properties of Mg(1−x)AlxB2 have been studied from first principles. The elastic constants (c11, c12, c13, c33 and c55) have been calculated, in the regime of x = 0 to 0.25. From these calculations the ratio between the bulk modulus and shear modulus (B/G) as well as the ratio between the two directional bulk moduli (Ba/Bc) have been evaluated. Our calculations show that the ratio Ba/Bc decreases monotonically as the aluminium content is increased, whereas the ratio B/G is well below the empirical ductility limit, 1.75, for all concentrations. In addition, we analyse the electronic structure and the nature of the chemical bonding, using the balanced crystal orbital overlap population (BCOOP) (Grechnev et al 2003 J. Phys.: Condens. Matter 15 7751) and the charge densities. Our analysis suggests that, while aluminium doping decreases the elastic anisotropy of MgB2 in the a and c directions, it will not change the brittle behaviour of the material considerably. 

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-92691 (URN)10.1088/0953-8984/16/29/015 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
6. H-H interaction and structural phase transition in Ti3SnHx
Open this publication in new window or tab >>H-H interaction and structural phase transition in Ti3SnHx
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2002 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 23, p. 235104-Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-92692 (URN)10.1103/PhysRevB.66.235104 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved
7. Phase relations in the Ti3Sn–D system
Open this publication in new window or tab >>Phase relations in the Ti3Sn–D system
2004 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 364, no 1-2, p. 127-131Article in journal (Refereed) Published
Abstract [en]

Ti3Sn forms an orthorhombic metal hydride phase at 25-kPa deuterium pressure and 650 °C. The unit cell parameters were determined to be a=6.179(1) Å, b=9.877(2) Å and c=4.7898(6) Å and the space group to beC2221. The crystal structure was determined from neutron powder diffraction data with the Rietveld method. Three phases are formed in the Ti3Sn–D system upon hydrogenation, and appear in the following order at increasing deuterium pressures: an orthorhombic structure (Ti3SnD0.80), a hexagonal phase and a cubic metal hydride phase (Ti3SnD). The cubic phase, Ti3SnD, crystallises in the CaTiO3-type structure, space group Pm3m, with the unit cell parameter a=4.1776(2) Å. The stability of the three Ti3SnDx phases is in agreement with calculated total energies, based on first principles theory.

Keywords
Transition metal compounds; Metal hydrides; Crystal structure; Neutron diffraction
National Category
Inorganic Chemistry Physical Sciences
Identifiers
urn:nbn:se:uu:diva-92693 (URN)10.1016/S0925-8388(03)00494-8 (DOI)
Available from: 2005-03-10 Created: 2005-03-10 Last updated: 2017-12-14Bibliographically approved

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