| 21. |
- Sundqvist, Bertil, et al.
(författare)
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Constant-pressure versus constant-volume resistivities of some high transition temperature superconductors
- 1994
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Ingår i: Physica C, volume 235-240. - : Elsevier Science B.V.. ; , s. 1407-1408
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Konferensbidrag (refereegranskat)abstract
- The resistivity varrho is always measured at constant p, while theory assumes constant V. For high temperature superconductors there are large differences between constant-p and constant-V resistivities. We have previously shown constant-V results for YBa2Cu3O7−δ and show here similar data for La2−xSrxCuO4. Constant-V data for varrho agree with the behaviour expected for a normal metal, with low-T linearity in T and saturation at high T. We speculate that similar effects will be observed in doped fullerenes (M3C60, M = Li, K, Rb, …).
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| 22. |
- Sundqvist, Bertil
(författare)
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Point defects and thermal conductivity of C60
- 1993
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 48:19, s. 14712-14713
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Tidskriftsartikel (refereegranskat)abstract
- Recent literature data for the thermal conductivity κ of single-crystal C60 are analyzed using a standard model. It is shown that in the intermediate range 100–250 K, below the rotational transition, κ can be described extremely well by a model taking into account phonon and point defect scattering only. The magnitude derived for the phonon-phonon scattering term agrees very well with an estimate from a simple model.
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| 23. |
- Sundqvist, Bertil, et al.
(författare)
-
Resistivity of high-Tc superconductors: Linear in T at constant p, non-linear at constant V
- 1990
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Ingår i: Solid State Communications. - : Elsevier Science Ltd. - 0038-1098 .- 1879-2766. ; 76:8, s. 1019-1022
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Tidskriftsartikel (refereegranskat)abstract
- The observed constant-pressure resistivity varrhop of most high-Tc superconductors is linear in temperature T over large ranges. We show that this is not true for the constant-volume resistivity varrhov; for 1–2–3 YBCO the correction from varrhop to varrhov is -32% at 500 K relative to the low-T value. Since theory predicts varrhov rather than varrhop, the observed linearity of varrho probably cannot be used as a tool to choose between theories for high-Tc superconductivity.
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| 24. |
- Sundqvist, Bertil
(författare)
-
Resistivity saturation in fcc La under high pressure
- 1992
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 69:18, s. 2693-2696
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Tidskriftsartikel (refereegranskat)abstract
- Data are presented for the electrical resistivity ρ of fcc La as a function of T and p in the range 70 to 700 K and 0 to 1.25 GPa. The data are analyzed using the phenomenological ‘‘parallel resistivity’’ formula for resistivity saturation, which describes very well the T dependences of both ρ and its pressure coefficient. The pressure coefficients of the electron-phonon interaction parameter and Tc are calculated from the p dependence of the electron-phonon component, and the p dependence of the parallel saturation resistivity is obtained from band-structure calculations using a simple Bloch-Boltzmann model. In both cases excellent agreement with experiment is found.
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| 25. |
- Sundqvist, Bertil, et al.
(författare)
-
Structure, disorder, and phase diagram of C60 up to 1 GPa and below 300 K
- 1994
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Ingår i: High Pressure in Material Science and Geoscience. - Praha, Czech Republic : Prometheus. - 8085849933 ; , s. 109-112
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Konferensbidrag (refereegranskat)abstract
- We have measured the compressibility and the thermal conductivity of C60 in the range below 300 K and up to 1 GPa (10 kbar). From the data obtained we have deduced the p-T phase diagram of C60. Literature data show that the ratio of pentagon-to-hexagon oriented molecules in the orientationally ordered phase decreases with increasing pressure, and from our data we conclude that C60 forms a new, completely "hexagon oriented" phase above 0.6 GPa at 150 K, or 1.2 GPa at 300 K. The glass transition temperature (90 K at atmospheric pressure) is found to increase by 54 K/GPa.
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| 26. |
- Sundqvist, Bertil
(författare)
-
The electrical resistivity of La under pressure between 70 and 300 K
- 1991
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Ingår i: High Pressure Research, volume 7 and volume 8, issues 1-3. - Reading : Gordon and Breach. ; , s. 250-252
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Konferensbidrag (refereegranskat)abstract
- The electrical resistivity of fcc La has been measured as a function of temperature and pressure between 70 and 300 K. The pressure coefficient of resistance dlnR/dp is found to decrease from +0. 022 GPa-1 at 75 K to -0. 014 GPa-1 at 300 K.
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| 27. |
- Sundqvist, Bertil, et al.
(författare)
-
Thermal conductivity of bulk YBa2Cu4O8 and the phonon transport model
- 1994
-
Ingår i: Physica C, volume 235-240. - : Elsevier Science B.V.. ; , s. 1377-1378
-
Konferensbidrag (refereegranskat)abstract
- We have measured the thermal conductivity varkappa of bulk YBa2Cu4O8 (1-2-4) between 30 and 310 K and at pressures p up to 1 GPa. At p = 0, varkappa = 10 Wm−1K−1 near 100 K, twice that of sintered YBa2Cu3O−δ (1-2-3). varkappa also decreases with increasing T more rapidly than in 1-2-3, to 7.4 Wm−1K−1 at 300 K. Under pressure, varkappa increases slowly with p above 150 K, but dvarkappa/dp decreases at lower T. Our results are in excellent agreement with a semi-classical model for phonon thermal conductivity, provided we assume significant charge transfer with pressure. The calculated phonon mean free path is much larger than the lattice spacing at all T.
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| 28. |
- Sundqvist, Bertil
(författare)
-
Thermal diffusivity and thermal conductivity of Chromel, Alumel, and Constantan in the range 100-450 K
- 1992
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 72:2, s. 539-545
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Tidskriftsartikel (refereegranskat)abstract
- Data are presented for the thermal diffusivity and the thermal conductivity of commercially available Chromel, Alumel, and Constantan alloys, and the specific heat capacity of Alumel, over the temperature range 100 to 450 K. Over this range, the thermal conductivity of all materials increases by more than 50%; room-temperature values are 17, 29, and 23 W m−1 K−1, respectively. The thermal conductivity data are discussed in terms of simple theory.
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| 29. |
- Sundqvist, Bertil
(författare)
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Thermal diffusivity measurements by Ångström's method in a fluid environment
- 1991
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Ingår i: International journal of thermophysics. - 0195-928X .- 1572-9567. ; 12:1, s. 191-206
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Tidskriftsartikel (refereegranskat)abstract
- Ångström's method has been used for measuring the thermal diffusivity, a, and the radial heat loss coefficients of a thin constantan wire, both in a vacuum and with the wire immersed in air and in four different liquids, and using temperature wave periods of 1 to 1000 s. The presence of a surrounding fluid medium causes errors of up to 90% in the measured values of a. It is shown that both the experimental errors and the radial heat loss coefficients can be accurately calculated using simple models, both at high and at low frequencies, and that a previously developed two-frequency model can be used to obtain accurate data for a even under these conditions, provided the frequency is high enough. We also present a new variety of Ångström's model, valid only at very low frequencies and with purely convective heat loss.
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| 30. |
- Ullakko, Kari, et al.
(författare)
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Isothermal Martensite Transformation of Fe-Ni-C Alloys as a Function of Hydrostatic Pressure
- 1990
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Ingår i: Materials Science Forum volumes 56-58. - Zürich : Trans Tech Publiations. ; , s. 197-200
-
Konferensbidrag (refereegranskat)abstract
- Isothermal martensite transformation as a function of hydrostatic pressure was studied in Fe-Ni-C alloys. It was observed that the transformation was much the same as it happens as a function of time. Any noticeable differences in the morphology between isothermal and athermal martensite were not seen.
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