Thermal Properties of Superconductor MgB2-XCX (X=0、0.02、0.06、0.08)
| Autor: | Hsun-Kai Chang, 張洵愷 |
|---|---|
| Rok vydání: | 2006 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 94 The transition temperature of MgB2 superconductor is 39 K, which is currently the highest among the metallic superconductors. The purpose of this study is to investigate thermal properties of MgB2-xCx (x = 0, 0.02, 0.06, 0.08) superconductors. The thermal conductivity, Seebeck coefficient and electrical resistivity of the MgB2-xCx (x = 0, 0.02, 0.06, 0.08) samples were measured by using Thermal Transport Option of Physical Properties Measurement System (PPMS) under a wide temperature range between 8 K and 300 K. The effective Lorenz number in the Wiedemann- Franz law was used to estimate the contributions of phonon and electron to the thermal conductivity. A more quantitative analysis of thermal conductivity suggests that the phonon contribution is relatively small in comparison with the field-induced electronic contribution. The contribution of electrons to the thermal conductivity in the superconducting state was further analyzed by the two-band theory. Our result showed that electrons in the π-band play a significant role in the heat transport. The substitution of carbon for boron leads to a considerable reduction of the electronic heat transport in bothπ-band and σ-band. The polarity of the free carriers in this work was mainly determined by the Seebeck coefficient. In the carbon-doped MgB2 compounds the Seebeck coefficient data demonstrates that the Fermi energy and the density of states decrease with the increasing x. Also, the reductions result in a lower transition temperature. But the upper critical fields are surprisingly enhanced by the carbon doping. We believe that the enhancement is mostly attributed to the flux pinning of the defects which act as pinning centers. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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