无氧钙钛矿MgCNi3铁磁性附近的超导电性：实验和DFT（密度泛函理论）研究
Superconductivity in the vicinity of ferromagnetism in oxygen free perovskite MgCNi3: An experimental and DFT (Density Functional Theory) study
摘要：
本文报道了MgCNi_3超导体的合成、结构、磁性、比热和密度泛函理论（DFT）研究。采用标准固相反应法合成了MgCNi3多晶样品，其晶体结构为立方钙钛矿结构，空间群为Pm3m，未检测到Ni杂质。在7.25K附近，交流磁化和直流磁化都表现出超导转变（Tc）.在2K时，低临界场（Hc1）和不可逆场（Hirr）分别约为140Oe和11kOe。由现场交流磁化率测量确定的上临界磁场（Hc2）分别为11.6 kOe和91.70 kOe，抗磁性标准分别为50%和90%。在高达140 kOe和低至2 K的外加磁场下进行热容（Cp）测量。由Cp（T）数据对Sommerfeld-Debye模型的低温拟合得到的Sommerfeld常数（γ）和德拜温度（θ D）分别为36.13mJ/mole-K~2和263.13K。Bardeen-Cooper-Schrieffer（BCS）参数（2 Δ/KBTc）约为3.62，表明MgCNi3是一种中等耦合BCS超导体，λ = 0.69。虽然密度泛函理论（DFT）计算表明该化合物是非磁性的，但具有自旋涨落，但在20、50、100、200和300 K下的实验等温磁化MH环表明该化合物具有一定的铁磁性，在20 K下的矫顽场（Hc）约为50 Oe。Ni 3d态在费米能级附近起主导作用，Ni 3d和C2 p态之间存在较强的杂交。MgCNi_3在铁磁性附近似乎是超导的。
英文摘要：
We report synthesis, structural, magnetic, specific heat and Density Functional Theory (DFT) studies on MgCNi3 superconductor. Polycrystalline MgCNi3 samples are synthesized through standard solid state reaction route and found to crystallize in cubic perovskite structure with space group Pm3m, without any detectable trace of Ni impurity. Both AC and DC magnetization exhibited superconducting transition (Tc) at around 7.25 K. The lower critical field (Hc1) and irreversibility field (Hirr) are around 140 Oe and 11 kOe respectively at 2 K. The upper critical field (Hc2) being determined from in-field AC susceptibility measurements is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively. Heat capacity (Cp) measurements are carried out under applied field of up to 140 kOe and down to 2 K. The Sommerfeld constant (γ) and Debye temperature (ΘD) as determined from low temperature fitting of Cp(T) data to Sommerfeld-Debye model are 36.13 mJ/mole-K2 and 263.13 K respectively. The Bardeen-Cooper-Schrieffer (BCS) parameter (2Δ/KBTc) is around 3.62, suggesting MgCNi3 to be an intermediate coupled BCS superconductor with value λ = 0.69. Although the density functional theory (DFT) calculations exhibited the compound to be non-magnetic but with spin fluctuations, the experimental isothermal magnetization MH loops at 20, 50, 100, 200 and 300 K showed some ferromagnetic nature in this compound with coercive field (Hc) of around 50 Oe at 20 K. The Ni3d states play dominant roles near the Fermi levels and there is strong hybridization between Ni3d and C2p states. It seems that MgCNi3 is superconducting in close proximity of ferromagnetism.
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