| Summary: | 碩士 === 國立交通大學 === 物理研究所 === 101 === According to theoretical calculations, the asymmetric spin-orbit coupling from non-centrosymmetric structure may lead to an order parameter of two gaps or an anisotropic gap. Low temperature specific heat C(T,H) of Re24Ti5, Re21.75Ta7.25 and Re24.9Hf4.1 were investigated, and we fitted their superconducting electronic specific heat by various order parameter symmetries. These compounds are the of non-centrosymmetric superconductors with α-Mn structure. The specific heat can be well described within the context of an isotropic s-wave for all three compounds, and the 2∆⁄(k_B T_c ) is equal to 3.68 ± 0.01 for Re24Ti5, 3.70 ± 0.07 for Re21.75Ta7.25 and 3.69 for Re24.9Hf4.1. The results of ∆C/(γ_n T_c), 2∆⁄(k_B T_c ) and electron-phonon coupling λ_ep indicate a moderate coupling in these compounds. For Re24.9Hf4.1, the upper critical field H_c2 (0)=9.01±0.22 T was estimated from the Werthamer-Helfand-Hohenberg (WHH), consistent with H_c2 (0)=9.84±0.60 T from γ(H).
For two-gap fitting results, the results of Re24Ti5 are (2∆_L)⁄(k_B T_c=) 3.87±.07 and (2∆_S)⁄(k_B T_c=) 3.50±0.06, and those of Re24.9Hf4.1 are (2∆_L)⁄(k_B T_c=) 3.97±0.04 and (2∆_S)⁄(k_B T_c=) 3.41±0.04. As for the fitting of the anisotropic gap, all values of 2∆⁄(k_B T_c ) are the same as those with s-wave, but the individual gap anisotropic α is 0.070 ± 0.025 for Re24Ti5, 0.149 ± 0.033 for Re21.75Ta7.25 and 0.106 ± 0.015 for Re24.9Hf4.1, respectively. Although we found these superconductors are within at the dirty limit, the existence of p-wave order parameter in Re21.75Ta7.25 and Re24.9Hf4.1 cannot be totally excluded considering the case of the two-gap superconductor MgB2 which is also at the dirty limit.
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