| 1. |
- Aleksandrovskii, A. N., et al.
(författare)
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Thermal expansion of single-crystal fullerite C60 at helium temperatures
- 2000
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 26:1, s. 75-80
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Tidskriftsartikel (refereegranskat)abstract
- The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.
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| 2. |
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| 3. |
- Wågberg, Thomas, et al.
(författare)
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2D polymerization and doping of fullerenes under pressure
- 2000
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Ingår i: High Pressure Research vol. 18. - : Gordon & Breach. ; 18:1-6, s. 139-143
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Konferensbidrag (refereegranskat)abstract
- Tetragonal polymeric C60 has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C60 has also been intercalated by potassium metal.
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| 4. |
- Lebegue, Sebastien, et al.
(författare)
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Ab initio study of the electronic properties and Fermi surface of the uranium dipnictides
- 2006
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Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 73:4, s. 045119-
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of the uranium dipnictides UX2 (X=As, Sb, and Bi) is investigated by means of ab initio calculations based on density functional theory. The calculated Fermi surfaces are presented and compared to available experimental models obtained from de Haas-van Alphen experiments. In agreement with experiments they are found to have a significant two-dimensional character. Also, the change of the electronic properties through the series is discussed.
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| 5. |
- Soldatov, Alexander, et al.
(författare)
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Topochemical polymerization of C70 controlled by monomer crystal packing
- 2001
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Ingår i: Science. - : American Association for the Advanement of Science. - 0036-8075 .- 1095-9203. ; 293:5530, s. 680-683
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Tidskriftsartikel (refereegranskat)abstract
- Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300°C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
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| 6. |
- Wågberg, Thomas, et al.
(författare)
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Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers
- 2006
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Ingår i: The European Physical Journal B: Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 49:1, s. 59-65
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C60. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.
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| 7. |
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| 8. |
- G.E. Grechnev,, et al.
(författare)
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Electronic structure and magnetic properties of Lithium manganese spinels
- 2003
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Ingår i: J.Magn. Magn. Matt.. ; 258, s. 287-289
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Tidskriftsartikel (refereegranskat)abstract
- Electronic and magnetic structures of the spinel-type lithium–manganese oxides LixMn2O4, x=0,0.5,1, are studied ab initio by employing a full-potential LMTO method. The effect of the orthorhombic distortion on electronic structure and magnetism of LiMn2O4 was investigated, and our calculations do not show a substantial charge ordering at the structural transition from the cubic spinel to the orthorhombic structure.
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| 9. |
- Joyce, J.J., et al.
(författare)
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Photoemission and the electronic structure of PuGaCo$_5$
- 2003
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Ingår i: Physical Review Letters. ; 91, s. 176401-
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of the first Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Exceptional agreement between calculation and experiment defines a path forward for understanding the electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. The results for PuCoGa5 clearly indicate 5f electron behavior on the threshold between localized and itinerant. Comparisons to delta phase Pu metal show a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.
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| 10. |
- Vidya, R., et al.
(författare)
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Tailor-made electronic and magnetic properties in one-dimensional perovskite-like oxides
- 2003
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Ingår i: Physical Review Letters. ; 91, s. 186404-
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Tidskriftsartikel (refereegranskat)abstract
- Full-potential density-functional calculations show that the electronic structure of one-dimensional ferrimagnetic Ca3Co2O6 varies from metal to half metal to insulator as its magnetic ordering changes from the ferrimagnetic through the ferromagnetic to the paramagnetic state. The present Letter is the first to establish the occurrence of half metallicity in one-dimensional oxides. Moreover, the electronic and magnetic properties of this material can be tuned by substitution of Y for Ca, as shown by our detailed study on Ca3–xYxCo2O6 (x = 0, 0.3, 0.75, and 1). The Co ions are in two different valence states [Co4+ (low-spin) and Co2+ (high-spin)], and hence the occurrence of charge ordering in addition to spin ordering is established. For specific Y concentrations we predict a rarely seen combination of ferromagnetic and insulating behavior.
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| 11. |
- Dick Thelander, Kimberly
(författare)
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Epitaxial Growth and Design of Nanowires and Complex Nanostructures
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the epitaxial growth of III-V semiconductor nanowires using Au seed particles, and the design of more complex three-dimensional branched structures from these wires. Growth was performed by metallorganic vapour phase epitaxy, in which precursor molecules for the semiconductor material components are introduced in a low-pressure vapour. Nanowires grow epitaxially (with controlled crystal orientation) on a semiconductor substrate; diameter control is achieved via the Au particles, while length is controlled by growth parameters. Particle-assisted nanowire growth is used extensively today to achieve well-controlled structures. The current understanding of this growth mechanism was developed over forty years ago, and is known as the Vapour-Liquid-Solid (VLS) mechanism. This model indicates that a liquid alloy is formed between the seed particle and the growth material(s), and growth proceeds by precipitation from a supersaturated particle. The enhanced growth rate compared to the bulk growth from the vapour is typically attributed to preferential decomposition of precursor materials at or near the particle surface. Recently, however, several inconsistencies have been observed between this model, which was developed for Au-assisted Si whiskers (micro-scale wires), and particle-assisted growth of other materials. In particular, there have been many reports of nanowire growth at temperatures too low for a liquid alloy to form. As well, nanowire growth has been reported for systems where no precursor molecules are used, and thus growth enhancement cannot be explained by preferential decomposition. Other reports have given evidence that such preferential decomposition does not necessarily occur when precursors are used. The first part of this thesis presents the current understanding of particle-assisted growth, both generally and for the specific materials and growth systems used here. Semiconductor nanowires present the possibility for numerous applications, and many simple device components have been demonstrated. The development of practical applications of such prototypes may rely on the ability to assemble nanowires into more complex structures. The second part of this thesis presents techniques for the production of three-dimensional branched nanowire structures, including methods to achieve controlled structure and morphology. The assembly of branched structures into large-scale interconnected networks is also presented.
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| 12. |
- Bergkvist Sylvan, Sara, 1977-
(författare)
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Numerical studies of spin chains and cold atoms in optical lattices
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An important, but also difficult, research field in condensed matter physics is that of strongly correlated systems. This thesis considers two topics in this field. The first topic is disorder and frustration in spin models. The introduction of disorder into quantum spin chains creates a complex problem. The ground state of the random-bond spin-1 Heisenberg chain is studied by means of stochastic series expansion quantum Monte Carlo simulation, applying the concept of directed loops. It is found that this system undergoes a phase transition to the random-singlet phase if the bond disorder is strong enough. Further a frustrated spin system is investigated. The frustration is introduced by having spins positioned on a triangular lattice. Performing a quantum Monte Carlo simulation for such a frustrated lattice leads to the occurrence of the infamous sign problem. This problem is investigated and it is shown that it is possible to use a meron cluster approach to reduce its effect for some specific models. The second topic concerns atomic condensates in optical lattices. A system of trapped bosonic atoms in such a lattice is described by a Bose-Hubbard model with an external confining potential. Using quantum Monte Carlo simulations it is demonstrated that the local density approximation that relates the observables of the unconfined and the confined models yields quantitatively correct results in most of the interesting parameter range of the model. Further, the same model with the addition that the atoms carry spin-1 is analyzed using density matrix renormalization group calculations. The anticipated phase diagram, with Mott insulating regions of dimerized spin-1 chains for odd particle density, and on-site singlets for even density is confirmed. Also an ultracold gas of bosonic atoms in an anisotropic two dimensional optical lattice is studied. It is found that if the system is finite in one direction it exhibits a quantum phase transition. The Monte Carlo simulations performed show that the transition is of Kosterlitz-Thouless type.
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| 13. |
- Colarieti Tosti, Massimiliano
(författare)
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Theory of Crystal Fields and Magnetism of f-electron Systems
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A parameter free approach for the calculation of the crystal field splitting of the lowest Russel-Saunders J-multiplet in f-electron systems has been developed and applied to selected compounds. The developed theory is applicable to general symmetries and is based on symmetry constrained density functional theory calculations in the local density or in the generalised gradient approximation.The magnetocrystalline anisotropy of Gd has been analysed. It has been shown that the peculiar orientation of the easy axis of magnetisation is consistent with an S-ground state. Further, the temperature dependence of the easy axis of magnetisation has been investigated and it has been shown that the temperature driven reduction of the effective magnetisation is the principal mechanism responsible for it.A new method has been developed that allows for theoretical studies of the electronic structure and total energy of elements and compounds in an intermediate valence regime. The method combines model and first-principles band structure calculations, therefore being accurate and computationally efficient. It has been applied to Yb metal under pressure obtaining a remarkable agreement with experimental observations for the equation of state and the x-ray absorption spectroscopy.
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| 14. |
- Seung Ki, Baek, 1979-, et al.
(författare)
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Phase transition of q-state clock models on heptagonal lattices
- 2009
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Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 80:1, s. 011133-
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Tidskriftsartikel (refereegranskat)abstract
- We study the q-state clock models on heptagonal lattices assigned on a negatively curved surface. We show that the system exhibits three classes of equilibrium phases; in between ordered and disordered phases, an intermediate phase characterized by a diverging susceptibility with no magnetic order is observed at every q >= 2. The persistence of the third phase for all q is in contrast with the disappearance of the counterpart phase in a planar system for small q, which indicates the significance of nonvanishing surface-volume ratio that is peculiar in the heptagonal lattice. Analytic arguments based on Ginzburg-Landau theory and generalized Cayley trees make clear that the two-stage transition in the present system is attributed to an energy gap of spin-wave excitations and strong boundary-spin contributions. We further demonstrate that boundary effects break the mean-field character in the bulk region, which establishes the consistency with results of clock models on boundary-free hyperbolic lattices.
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| 15. |
- Waltersson, Erik, 1979-, et al.
(författare)
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Controlled operations in a strongly correlated two-electron quantum ring
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:11, s. 115318-
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Tidskriftsartikel (refereegranskat)abstract
- We have analyzed the electronic spectrum and wave-function characteristics of a strongly correlated two-electron quantum ring with model parameters close to those observed in experiments. The analysis is based on an exact diagonalization of the Hamiltonian in a large B-spline basis. We propose a qubit pair for storing quantum information, where one component is stored in the total electron spin and one multivalued “quMbit” is represented by the total angular momentum. In this scheme the controlled-NOT quantum gate is demonstrated with near 100% fidelity for a realistic far-infrared electromagnetic pulse.
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| 16. |
- Abou-Hamad, Edy, et al.
(författare)
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Molecular dynamics and phase transition in one-dimensional crystal of C60 encapsulated inside single wall carbon nanotubes
- 2009
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Ingår i: ACS Nano. - Washington, DC 20036 USA : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 3:12, s. 3878-3883
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Tidskriftsartikel (refereegranskat)abstract
- One-dimensional crystals of 25% 13C-enriched C60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the 13C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C60α, are blocked at room temperature and 68%, labeled C60β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin−lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C60β dynamics. Above the transition, the C60β exhibits continuous rotational diffusion; below the transition, C60β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C60, suggesting a quiet smooth orientational dependence of the interaction between C60β molecules and the inner surface of the nanotubes.
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| 17. |
- Aleksandrovskii, A. N., et al.
(författare)
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Effect of argon on the thermal expansion of fullerite C60 at helium temperatures
- 2001
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : American Institute of Physics. - 1063-777X .- 1090-6517. ; 27:3, s. 245-246
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Tidskriftsartikel (refereegranskat)abstract
- The linear thermal expansion of compacted Ar-doped fullerite C60(ArxC60) is investigated at 2–12 K using a dilatometric method. The thermal expansion of ArxC60 is also studied after partial desaturation of argon from fullerite. It is revealed that argon doping resulted in a considerable change of the temperature dependence of the thermal expansion of fullerite. An explanation of the observed effects is proposed.
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| 18. |
- Aleksandrovskii, A.N., et al.
(författare)
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Thermal expansion of fullerite C60 alloyed with argon and neon
- 2001
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - Woodbury : American Institute of Physics. - 1063-777X .- 1090-6517. ; 27:12, s. 1033-1036
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Tidskriftsartikel (refereegranskat)abstract
- The linear thermal expansion of compacted fullerite C60 alloyed with argon (ArxC60) and neon (NexC60) are investigated by a dilatometric method. The experimental temperature is 2–12 K. In the same temperature interval the thermal expansion of ArxC60 and NexC60 are examined after partial desaturation of the gases from fullerite. It is found that Ar and Ne alloying affects the temperature dependence of the thermal expansion coefficient of C60 quite appreciably. The libration and translation contributions to the thermal expansion of pure C60 are separated. The experimental results on the thermal expansion are used to obtain the Debye temperature of pure C60. The effects observed are tentatively interpreted.
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| 19. |
- Andersson, Ove, et al.
(författare)
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Pressure-induced collapse of ice clathrate and hexagonal ice mixtures formed by freezing
- 2009
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics Publishing LLC. - 0021-9606 .- 1089-7690. ; 131, s. 114503-114513
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Tidskriftsartikel (refereegranskat)abstract
- We report thermal conductivity κ measurements of the pressure-induced collapse of two mixtures of ice and tetrahydrofuran (THF) clathrate hydrate formed by freezing aqueous solutions, THF·23 H2O and THF·20 H2O, one containing twice as much excess water than the other. On pressurizing, κ of the solid mixture first decreases at the onset pressure of 0.8 GPa, as occurs for collapse of pure ice, reaches a local minimum at a pressure of 1.0 GPa, and then increases as occurs for the collapse of the pure clathrate THF·17 H2O. This shows that in the apparently homogeneous mixture, the ice and the clathrate collapse as if the two were in a mechanically mixed state. The manner in which the clathrate aggregate can arrange in the solid indicates that ice occupies the interstitial space in the tightly packed aggregates and H2O molecules belonging to the lattice of one form hydrogen bond with that of the other, a feature that is preserved in their collapsed states. On decompression, the original clathrate is partially recovered in the THF·20 H2O mixture, but the collapsed ice does not transform to the low density amorph. We surmise that on irreversible transformation to the original clathrate, the aggregates expand. Any pressure thus exerted on the small domains of the collapsed ice with a hydrogen bonded interface with the clathrate aggregates could prevent it from transforming to the low density amorph. Measurements of κ are useful in investigating structural collapse of crystals when dilatometry is unable to do so, as κ seems to be more sensitive to pressure-induced changes than the volume.
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| 20. |
- Andersson, Ove, et al.
(författare)
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Tricritical Lifshitz point in the temperature-pressure-composition diagram for (PbySn1-y)2 P2(SexS1-x)6 ferroelectrics
- 2009
-
Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 80, s. 174107-
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Tidskriftsartikel (refereegranskat)abstract
- The heat capacity of Sn2P2S6 ferroelectric crystals has been measured under quasihydrostatic pressures up to 0.7 GPa. The analysis of the heat-capacity and literature data for the birefringence shows that the tricritical point of Sn2P2S6 is in the 0.20–0.25 GPa range. Moreover, in the approximation of a linear change in the free-energy expansion coefficients, with respect to concentration and pressure, thermodynamic trajectories have been constructed for (PbySn1−y)2 P2(SexS1−x)6 solid solutions. We have thereby identified the region of the T-p-y-x diagram for (PbySn1−y)2 P2(SexS1−x)6 showing the tricritical Lifshitz point.
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| 21. |
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| 22. |
- Buga, Sergei G., et al.
(författare)
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Dissociation energy of 3D-polymeric C60 : Calorimetric study and structural analysis
- 2001
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Ingår i: Electronic Properties of Novel Materials - Science and Technology of Molecular Nanostructures. - Melville, NY : American Institute of Physics. - 0735400334 ; , s. 49-53
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Konferensbidrag (refereegranskat)abstract
- Annealing of 2D- and 3D-polymeric C[sub60] fullerene obtained under pressures of 9.5 and 12.5 GPa and temperatures of 670 and 770 K has been investigated by DSC in the range 240–640 K. An endothermal heat effect was observed with a peak maximum just below 540 K, a temperature characteristic for breakdown of (2+2) intermolecular links in dimers, 1D and 2D polymers. Exothermal effects, starting from 380 K, were observed for the first time in polymeric fullerenes. These effects are attributed to relaxation processes and to breakdown of other types of intermolecular bonds such as common four-sided rings and (3+3) interlinks.
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| 23. |
- Buga, Sergei G., et al.
(författare)
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DSC study of annealing and phase transformations of C60 and C70 polymerized under pressures in the range 9.5 - 13 GPa.
- 2002
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier B.V.. - 0022-3697 .- 1879-2553. ; 63:2, s. 331-343
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Tidskriftsartikel (refereegranskat)abstract
- C60 and C70 fullerenes polymerized under pressures between 9.5 and 13 GPa and temperatures between 670 and 1850 K were investigated by differential scanning calorimetry (DSC) in the range 240–640 K. Endothermal heat effects were observed with a peak maximum just below 540 K, a temperature characteristic for breakdown of (2+2) intermolecular links in dimers, 1D and 2D polymers. Exothermal effects, starting from 380 K, were observed for the first time in polymeric fullerenes. These effects are attributed to relaxation processes and to breakdown of other types of intermolecular bonds such as common four-sided rings and (3+3) interlinks.
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| 24. |
- Buga, Sergei G., et al.
(författare)
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Pressure effect on electrical properties and photoluminescence spectra of solid C60 and C70 fullerenes
- 2001
-
Ingår i: Frontiers of High Pressure Research II: Application of High Pressure to Low-Dimensional Novel Electronic Materials. - Dordrecht : Springer/Kluwer. - 9781402001604 ; , s. 483-491
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Konferensbidrag (refereegranskat)abstract
- Electrical resistivity of crystalline and disordered fullerite samples obtained by static high-pressure-high-temperature treatment of C-60 and C-70 at P = 12.5 GPa and T = 820-1500 K was investigated in the temperature range of 2.4-300 K, Room-temperature activation energy of charge carriers was found to be in the range 40-200 meV. T-3/2 and T-4 dependencies of conductivity versus temperature were revealed both in crystalline and disordered structures. Photoluminescence spectra of C-60 samples treated at P = 13 GPa. T = 770-1470 K show 50 nm short-wave length shift of characteristic 750 nm PL band.
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| 25. |
- Buga, Sergei G., et al.
(författare)
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Semimetallic and semiconductor properties of some superhard and ultrahard fullerites in the range 300-2 K
- 2000
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Ingår i: Proceedings of the 5th IUMRS International Conference on Advanced Materials, Beijing 1999. - : Elsevier B.V.. ; , s. 1009-1015
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Konferensbidrag (refereegranskat)abstract
- Electrical resistivity and magnetoresistance were measured on samples with disordered structures synthesized from pure C60 and C70 at pressures in the range 8–12.5 GPa and temperatures of 900–1500 K. Different types of behaviour were observed: semimetallic, VRH and semiconducting, depending on the degree of disorder and the particular short-range order of the samples. A negative magnetoresistance was observed at T<10 K on samples with a semimetallic type of conductivity synthesized at 8 GPa pressure. The temperature dependence of resistivity in the sample with a disordered crystalline structure based on 3D-polymerized C60 molecules fits Mott's law for hopping conductivity. T3/2, T2 and T4 dependencies of conductivity are observed for samples with densities of 2.8 and 3.05 g/cm3 synthesized at a pressure of 12.5 GPa. The effect of hydrostatic pressure on the resistivity of cross-linked layered carbon structures obtained from C60 at P=8 GPa, T=1600 K was investigated up to 0.6 GPa at room temperature. An approximately linear decrease of resistivity was observed with a very small value of the derivative d ln ρ/dp=0.06 /GPa, which correlates with a very low compressibility of the material.
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