| 1. |
- 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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| 2. |
- Liu, Bingbing, et al.
(författare)
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Electric resistance of single-walled carbon nanotubes under hydrostatic pressure
- 2001
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Ingår i: Solid State Communications. - : Elsevier B.V.. - 0038-1098 .- 1879-2766. ; 118:1, s. 31-36
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Tidskriftsartikel (refereegranskat)abstract
- The electric resistance of single-walled nanotube mats has been studied systematically under both ambient and high hydrostatic pressures up to 1.5 GPa. Both the temperature dependence of the resistance and the magnetoresistance indicate that electrical transport occurs by variable range hopping, apparently in 2D. We suggest that this unexpected dimensionality arises from a fractal network of tubes and bundles. Under hydrostatic pressure (HP) the resistance still shows 2D variable range hopping and decreases with increasing HP. An irreversible increase in localization length and DOS is induced below 0.5 GPa. The behavior is reversible and due to strong interaction of tubes from 0.5 GPa up to 1.05 GPa. These results indicate that 2D variable range hopping occurs within bundles.
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| 3. |
- Liu, Bingbing, et al.
(författare)
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Synthesis and characterization of single-walled nanotubes produced with Ce as catalyst
- 2000
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Ingår i: Molecular Materials: Molecular Crystals and Liquid Crystals Section C, vol. 13. - : Overseas Publishers Association N.V.. ; , s. 75-80
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Konferensbidrag (refereegranskat)abstract
- We have studied the effect of using rare-earth elements C (typically +4 oxide state) and Eu (typically +2 oxide state) together with nanometer Ni as catalysts to synthesize SWNT in high yield by arc evaporation. A black collar deposit containing mainly SWNT was formed in the presence of Ce/Ni while only nanometer carbon particles were formed using Eu/Ni, as identified by SEM, TEM and Raman spectra. The Raman spectra of our SWNT mats show difference from spectra shown in the literature in a low frequency range, indicating that our mats contain chiral tubes. The temperature dependent resistance of SWNT mats was also measured. A semiconducting behaviour was observed with a negative dR/dT and no resistance minimum from 2 to 350 K. The resistance follows a 2D variable range hopping behaviour.
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| 4. |
- Makarova, Tatiana L, et al.
(författare)
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Conductivity of two-dimensional C60 polymers
- 2000
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Ingår i: Proceedings of the 4th International Workshop on Fullerenes and Atomic Clusters (IWFAC'99), St. Petersburg 1999. - : Overseas Publishers Association N.V.. ; , s. 151-156
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Konferensbidrag (refereegranskat)abstract
- The general trend in the shift of electronic properties as the result of polymerization has been studied with the aid of temperature dependences of conductivity. Samples representing the pure rhombohedral phase exhibit great anisotropy in their electrical properties. The main features of the behaviour of these samples are: (i) in the z-direction the resistivity is of the order of several kOhm cm and decreases with temperature; (ii) in the x-y direction resistivity is less than 1 Ohm cm and shows a minimum at a certain temperature.
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| 5. |
- Maksimov, Igor S., et al.
(författare)
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Low-Temperature Magnetoresistive Effect in Bulk La0.7Ca0.3-xSrxMnO3 Perovskites.
- 2001
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Ingår i: Materials Science Forum volumes 373-376. - Zürich : Trans Tech Publications. - 9780878498802 ; , s. 585-588
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Konferensbidrag (refereegranskat)abstract
- The temperature and field dependencies of the magnetoresistivity (MR) were studied and analysed for two series of polycrystalline samples of La0.7Ca0.3-xSrxMnO3 (0
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| 6. |
- Maksimov, Igor S., et al.
(författare)
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Pressure effects in granular La0.7Ca0.3-xSrxMnO3.
- 2002
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Ingår i: Physica Status Solidi A, Applied Research vol. 189, issue 2. - : Wiley Interscience. ; , s. 281-285
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Konferensbidrag (refereegranskat)abstract
- This paper discusses the effect of hydrostatic pressure up to 0.8 GPa on magnetic properties of manganites La0.7Ca0.3 - xSrxMnO3 (0 x 0.3, x = 0.03) and magnetoresistance data in an applied magnetic field of 5.0 T. Application of pressure enlarges the temperature range of the ferromagnetic phase. Curie temperature, TC, as a function of pressure and the temperature of resistance maximum, Tp, showed an anomaly for x = 0.15. The slope of pressure dependence of TC for x < 0.15 and x > 0.15 is higher than for x = 0.15. Dependence of temperature Tp on x consists of two curves: for x 0.15 and for x 0.15. There is a sharp bend on the Tp curve at x = 0.15. The structural phase transition from orthorhombic phase (x < 0.15) to rhombohedral one (x 0.15) corresponds to that concentration.
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| 7. |
- Ulyanov, A.N., et al.
(författare)
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Crystal symmetry and pressure effects on the properties of mixed-valence manganites
- 2003
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Ingår i: Journal of Magnetism and Magnetic Materials vol. 258-259, Special Issue SI. - Amsterdam : Elsevier B.V.. ; , s. 312-314
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Konferensbidrag (refereegranskat)abstract
- A lattice structure effect on the magnetic and magnetotransport properties of La0.7Ca(0.3-x)SrxMnO3 lanthanum manganites have been studied. The Curie temperature pressure coefficient increased almost linearly under an applied pressure as dTc/dP=14K/GPa and as dTc/dP=7.5K/GPa in the orthorhombic and rhombohedral structures,respectively. This is caused by the different pressure effect on the Mn–O bond distances and on the Mn–O–Mn bond angles in those phases. The anomalous low value of dTc/dP(=3.5K/GPa) for La0.7Ca0.15Sr0.15MnO3 composition and a jump of the magnetoresistance maximum value at the concentrational Pbnm2R%3c phase transition were observed and discussed.
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| 8. |
- Ulyanov, A.N., et al.
(författare)
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La0.7Ca0.3-xSrxMnO3 manganites: Effect of structure on the magnetic and transport properties.
- 2002
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Ingår i: Journal of the Physical Society of Japan. - : The Physical Society of Japan. - 0031-9015 .- 1347-4073. ; 71:3, s. 927-929
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Tidskriftsartikel (refereegranskat)abstract
- The effect of lattice structure on the magnetic and transport properties of La0.7Ca0.3-xSrxMnO3 (x= 0; 0.03; 0.06; ...; 0.3) lanthanum manganites have been studied. Curie temperature pressure coefficient, dTc/dP, showed the dependence on x value. Temperature Tc increased almost linearly under applied pressure as dTc/dP≈14 K/GPa and as dTc/dP≈7.5 K/GPa in the orthorhombic (Pbnm) and rhombohedral (R3c) phases, respectively. The value of dTc/dP(≈3.5 K/GPa) showed a minimum and a temperature of the resistance maximum, Tp(x), showed a change of slope at x=0.15, corresponding to the concentration structural phase transition. Differences between the values of dTc/dP and slopes of Tp versus x in Pbnm and R3c phases are explained by the different effect of external pressure on the Mn–O bond length and Mn–O–Mn bond angle, and by the different internal pressure effect in those phases, respectively
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| 9. |
- Ulyanov, A. N., et al.
(författare)
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Structure and pressure effects on the properties of La0.7Ca0.3-xSrxMnO3 manganites.
- 2002
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Ingår i: Journal of Applied Physics vol. 91. - : American Institute of Physics. ; , s. 7739-7741
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Konferensbidrag (refereegranskat)abstract
- We report a study of the structure and pressure effects on magnetic and transport properties of a lanthanum manganite, La0.7Ca0.3–xSrxMnO3 (0<=x<=0.3, Deltax = 0.03). The pressure coefficient of the Curie temperature (Tc), dTc/dP, is shown to depend on x. The temperature Tc increases approximately linearly under applied pressure as dTc/dP[approximate]14 K/GPa in the orthorhombic (Pbnm) phase and as dTc/dP[approximate]7.5 K/GPa in the rhombohedral (R3-bar c). The value of dTc/dP shows a minimum ([approximate]3.5 K/GPa) and the temperature of the resistance maximum, Tp(x), shows a change of slope at x = 0.15, corresponding to a concentration structural phase transition. Differences between the values of dTc/dP and the slopes of Tp vs x in Pbnm and R3-bar c phases are explained by the different effect of external pressure on the Mn–O bond length and the Mn–O–Mn bond angle, and by the different internal pressure effect (depending on the value of x) in those phases, respectively.
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| 10. |
- 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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