Energy gap, penetration depth, and surface resistance of MgB(2) thin films determined by microwave resonator measurements
Energy gap, penetration depth, and surface resistance of MgB(2) thin films determined by microwave resonator measurements
复制标题
通过微波谐振器测量确定 MgB(2) 薄膜的能隙、穿透深度和表面电阻
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
Sung
中科院分区：
文献类型：
--
作者：
B.B.Jin;N.Klein;W.N.Kang;Hyeong;E. Choi;Sung
我们采用介电谐振器技术测量了两种c轴取向MgB(2)薄膜的微波表面阻抗Z(s)=R(s)+iomegamu(0) λ的温度依赖性。假设两个样品的能隙减小后δ (0)/kT(c)分别为1.13和1.03，用标准的BCS积分表达式可以很好地拟合出蓝宝石介电谐振器在17.9 GHz下测得的磁场穿透深度λ与温度的关系。对于零温度下的穿透深度，根据拟合确定了102 nm和107 nm的值。我们的结果清楚地显示了序参量的s波特征。采用各向异性s波BCS模型对侵彻深度数据进行了拟合。在这个模型中，我们必须假设一个长序参数，在c轴方向上有一个大的间隙值，在ab平面上有一个小的间隙值。这与最近的各向异性s波模型与上临界场数据的拟合形成对比，其中必须使用扁序参数，并提出了关于MgB超导状态性质的有趣问题(2)。采用金红石介质谐振器，高精度地获得了R(s)的温度依赖关系。在约T(c)/2以下，R(s)(T)-R(s)（5 K）表现出指数温度依赖关系，与穿透深度数据确定的减小的能隙一致。在4.2 K时的R(s)值在7.2 GHz时低至19 μ ω，与高温超导氧化铜薄膜相当。
We have measured the temperature dependence of the microwave surface impedance Z(s)=R(s)+iomegamu(0)lambda of two c-axis oriented MgB(2) films employing dielectric resonator techniques. The temperature dependence of the magnetic-field penetration depth lambda determined by a sapphire dielectric resonator at 17.9 GHz can be well fitted from 5 K close to T(c) by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kT(c) to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 102 and 107 nm were determined from the fit. Our results clearly indicate the s-wave character of the order parameter. A similar fit of the penetration depth data was obtained with an anisotropic s-wave BCS model. Within this model we had to assume a prolate order parameter, having a large gap value in the c-axis direction and a small gap within the ab plane. This is in contrast to recent fits of the anisotropic s-wave model to upper critical-field data, where an oblate order parameter had to be used, and raises interesting questions about the nature of the superconducting state in MgB(2). A rutile dielectric resonator was employed to obtain the temperature dependence of R(s) with high accuracy. Below about T(c)/2, R(s)(T)-R(s)(5 K) exhibits an exponential temperature dependence with a reduced energy gap consistent with that determined from the penetration depth data. The R(s) value at 4.2 K was found to be as low as 19 muOmega at 7.2 GHz, which is comparable with a high-temperature superconducting copper oxide thin film.