纳米碳掺杂MgB2超导体的临界参数增强
Enhanced Critical parameters of nano-Carbon doped MgB2 Superconductor
摘要：
通过高场磁化强度和磁输运测量确定了MgB_（2-x）Cx系统的临界超导参数。合成的样品为纯相，并使用Rietveld细化进行晶格参数评估。R-T（H）测量在140 kOe的磁场下进行。上临界场值Hc 2基于90%正常电阻率的标准从该数据获得，即Hc 2 =H，其中Rho=90%Rho;其中RhoN是正常电阻率，即，在我们的情况下，电阻率约为40 K。Werthamer-Helfand-Hohenberg（WHH）对Hc（0）的预测低估了临界场值，甚至低于进行测量的场。在此基础上，应用Ginzburg-Landau理论（GL方程）对R-T（H）数据进行处理，不仅计算了Hc 2（0）值，而且确定了低温高场区Hc 2随温度的变化关系。在MgB_2-xCx系列中，纯MgB_2的Hc（0）=157.2 kOe，x=0.15时的Hc（0）= 297.5 kOe。在不同温度下测量了120 kOe的磁化强度，并计算了不可逆场、Hirr和临界电流密度Jc（H）等参数。与纯MgB 2样品相比，纳米碳掺杂导致临界参数如Hc 2、Hirr和Jc（H）的显著增强。
英文摘要：
The high field magnetization and magneto transport measurements are carried out to determine the critical superconducting parameters of MgB2-xCx system. The synthesized samples are pure phase and the lattice parameters evaluation is carried out using the Rietveld refinement. The R-T(H) measurements are done up to a field of 140 kOe. The upper critical field values, Hc2 are obtained from this data based upon the criterion of 90% of normal resistivity i.e. Hc2=H at which Rho=90%Rho; where RhoN is the normal resistivity i.e., resistivity at about 40 K in our case. The Werthamer-Helfand-Hohenberg (WHH) prediction of Hc(0) underestimates the critical field value even below than the field up to which measurement is carried out. After this the model, the Ginzburg Landau theory (GL equation) is applied to the R-T(H) data which not only calculates the Hc2(0) value but also determines the dependence of Hc2 on temperature in the low temperature high field region. The estimated Hc(0)=157.2 kOe for pure MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx series. Magnetization measurements are done up to 120 kOe at different temperatures and the other parameters like irreversibility field, Hirr and critical current density Jc(H) are also calculated. The nano carbon doping results in substantial enhancement of critical parameters like Hc2, Hirr and Jc(H) in comparison to the pure MgB2 sample.
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