| 1. |
- Campanini, Donato, 1988-
(författare)
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Thermodynamic characterization of superconducting and magnetic materials using nanocalorimetry
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Measurement of specific heat is a powerful technique for the investigation of novel materials. Superconducting and magnetic systems, in particular, can be thoroughly characterized by studying their electronic contribution to the specific heat. To investigate their behavior in magnetic fields, single crystals need to be used, since the magnetic properties are dependent on the crystalline orientation. Crystal quality is often enhanced when sizes are reduced down to below the 100 μm scale, which is lower than the limit of conventional calorimeters. Nanocalorimetry allows to detect the weak electronic signature in the specific heat for such small samples with a preserved combination of high resolution and good accuracy. This is achieved by miniaturizing the device using microsystems technology and by a proper optimization of the measurement conditions.In this thesis, a nanocalorimeter designed for the study of samples with masses from sub-μg to 100 μg in the temperature range 1-350 K is used for studying three different systems, yielding insights into their physical properties.In the magnetocaloric compound Fe2P a deep thermodynamic understanding of the first-order magnetic phase transition at the Curie temperature TC ≈ 217 K is lacking. The nanocalorimeter is used to map the magnetic phase diagram for fields applied parallel and perpendicular to the easy axis of magnetization. Two different phase diagrams are obtained depending on the applied field orientation. The first-order magnetic phase transition is characterized by specific and latent heat, providing a textbook example of thermodynamic properties around such a transition. The results are complemented with a combined nanocalorimetry - x-ray diffraction study and by magnetization measurements.The iron-based high-temperature superconductor BaFe2(As1-xPx)2 shows several anomalous physical properties which have been associated to the presence of a quantum critical point. High-resolution specific heat measurements are an important piece of the puzzle in understanding the behavior of this material. The specific heat is measured as a function of phosphorus doping x in the superoptimally substituted range and several superconducting parameters are extracted. An evolution from a single-gap to a two-gap is seen with doping, as well as a decrease of the London penetration depth close to optimum doping, without signs of divergence.The superconducting properties are as well investigated in the metastable β phase of gallium. β-Ga is obtained in-situ from the stable α-Ga by increasing the temperature about 10 K above the melting point. This novel method to produce β-Ga allows more reproducible and reliable measurements in comparison to traditional methods. A thorough thermodynamic characterization of the metastable phase is obtained, giving insights into the conditions for a strongly enhanced superconductivity in β-Ga in comparison to α-Ga. β-Ga is found to be a strong-coupling superconductor, with a 2.55 higher density of states at the Fermi energy in comparison to α-Ga.These measurements demonstrate how several problems in condensed matter physics can be addressed through nanocalorimetry, which allows mapping various phase diagrams and obtaining fundamental thermodynamic properties on high-quality samples in magnetic fields.
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| 2. |
- Nagel, Peter, et al.
(författare)
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C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization
- 1999
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:24, s. 16920-16927
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Tidskriftsartikel (refereegranskat)abstract
- We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.
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| 3. |
- Nagel, Peter, et al.
(författare)
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High-resolution thermal expansion of high-pressure polymerized and 'normal' C60 from 10-500 K: Kinetics of depolymerization
- 1998
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Ingår i: Molecular Nanostructures. - Singapore : World Scientific. - 9810232616 ; , s. 365-368
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Konferensbidrag (refereegranskat)abstract
- High-resolution capacitance dilatometry was used to study the thermal expansion of both 'normal' and polymer phases of C60. The expansivity, alpha(T), of 'normal' C60 exhibits an unusual temperature dependence above the orientational disordering transition at 260 K; alpha(T) decreases by about 30% from 260 to 500 K and appears to reach a minimum near 500 K. Polymerized C60 has a much smaller expansivity than normal C60 due to the stronger covalent bonding between molecules. The polymerized state converts back to 'normal' C60 at temperatures between 450 and 500 K. We show that this is an activated process with a well-defined activation energy of 1.92 eV and a volume increase of about 2%.
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| 4. |
- Nagel, Peter, et al.
(författare)
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Intermolecular bond stability of C60 dimers and 2D pressure-polymerized C60
- 1998
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Ingår i: AIP Conference Proceedings Vol. 442. - Woodbury, NY : American Institute of Physics (AIP). - 1563968088 ; , s. 194-197
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Konferensbidrag (refereegranskat)abstract
- The thermal stability of C60 dimers and 2D pressure-polymerized C60 is studied using high-resolution capacitance dilatometry. The transformation of both the dimer and the polymer phases back to 'normal' C60 is excellently described by a simple thermally activated process with activation energies of 1.75 +/- 0.1 eV (dimer) and 1.9 +/- 0.2 eV (polymer). These results are compared to previous data for 1D-polymerized C60 and photo-polymerized C60. The thermal expansivity of the 2D-polymer phase is as much as a factor of ten smaller than that of pure C60 and approaches values for diamond.
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| 5. |
- Persson, Per-Axel, et al.
(författare)
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Physical properties of pressure polymerized C60
- 1996
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Ingår i: High Pressure Science and Technology. - Singapore : World Scientific. - 9810225474 ; , s. 716-718
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Konferensbidrag (refereegranskat)abstract
- The properties of C60 have been studied after treatment at high temperature and high pressure (1.1 GPa and 565 K for 2 h). The treated material is insoluble in organic solvents. We present results obtained in NMR and Raman studies and measured data for the specific heat and the thermal expansion. Our results show clearly that there are no covalent bonds and no molecular rotation, but suggest that the molecules are slightly deformed and held together by weak pi-type bonds.
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| 6. |
- Soldatov, Alexander, et al.
(författare)
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Polymeric fullerenes: from C60 to C70
- 1999
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Ingår i: AIP Conference Proceedings vol. 486. - Melville, NY : American Institute of Physics (AIP). - 1563969009 ; , s. 12-15
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Konferensbidrag (refereegranskat)abstract
- For the first time polymerization of both powder and single crystals of C70 fullerene was established after their subjection to high pressure (1.1 - 2 GPa) at elevated temperature (500 - 580 K). High-resolution capacitance dilatometry, FTIR/Raman spectroscopy and thermal conductivity were employed to characterise the polymeric phase of C70. The results demonstrate drastic changes in the physical properties of C70 on polymerization. We report on a reverse transformation to the monomeric state on heating the polymer to 500 K at ambient pressure. The activation energy of depolymerization was determined to be 1.8(1) eV. We discuss our results in terms of existing structural models for polymerization of C70 and compare the physical properties of C70 and C60 polymers.
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| 7. |
- 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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| 8. |
- Sundqvist, Bertil, et al.
(författare)
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Physical properties of pressure polymerized C60
- 1996
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Ingår i: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, volume 3. - Pennington, NJ : The Electrochemical Society. - 1566771625 ; , s. 1014-1028
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Konferensbidrag (refereegranskat)abstract
- We present in this paper an overview of the physical properties of the high pressure polymerized C60 phase commonly known as "soft fcc". This phase has been studied by several methods over wide ranges in temperature T and/or pressure, p. We present here experimental information about the specific heat capacity, the thermal expansion coefficient, the lattice structure, and the thermal conductivity, and we also show results obtained by NMR and Raman spectroscopy. All data presented agree with the accepted model that the individual molecules in this phase are covalently bound to form linear molecular chains. In particular, the NMR data show clearly the presence of covalent bonds, and the Raman data exhibit several new lines at very low energies connected with chain vibrations. Thermal conductivity data obtained during polymerization show both the time dependence of the process and that polymerization occurs at lower p and T than observed previously for this phase.
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| 9. |
- Sundqvist, Bertil, et al.
(författare)
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Physical properties of two-dimensionally polymerized C60
- 1998
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Ingår i: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 6. - Pennington, NJ : The Electrochemical Society. - 1566772028 ; , s. 705-716
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Konferensbidrag (refereegranskat)abstract
- Preliminary data are reported for the thermal expansion, compressibility, and specific heat of two-dimensionally polymerized C60 obtained by treating fullerene samples at temperatures above 800 K under a pressure of 2 GPa. The samples have also been characterized by Raman spectroscopy and inelastic neutron scattering. The measured data for the thermophysical properties are strongly modified by the creation of strong covalent intermolecular bonds and we show that the results are compatible with the predictions of simple theories.
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