近来超薄MgB2薄膜被认为在研制新型超导电子器件方面具有比较重要的应用前景,对其超导特性的研究同时对理解低维度情形下的多带超导电性也有重要意义.在(0001)Al2O3衬底上,我们利用混合物理化学气相沉积方法生长了一系列厚度在50 nm至10 nm的MgB2超导薄膜,测量了薄膜的电阻率—温度曲线及其在不同磁场下的超导转变,考察了薄膜的基本特性参数如超导转变温度Tc、剩余电阻比RRR及超导上临界磁场Hc2等随薄膜厚度d的变化.测量发现H2 (T)随温度变化呈现出线性依赖关系,反映出MgB2薄膜的两带超导特性.随着d的降低,发现薄膜的Tc及RRR值逐渐下降,0K时的H2(0)逐渐升高.进一步地,我们发现薄膜的Tc与Hc2(0)都随RRR值呈现出比较单调的变化:Tc随RRR值降低而降低,Hc2(0)随RRR值降低而升高,表明RRR值是影响薄膜超导特性的比较重要的变量.依照两带超导理论对此现象的分析表明,与薄膜的RRR值降低相伴随的可能是MgB2薄膜内σ带电子和π带电子的带间散射率的增强.
It has been recently recognized that MgB2 ultrathin films have good prospects in developing novel superconducting electronic devices.Investigating their superconducting properties has also important implications in exploring the phenomenon of multiband superconductivity in low-dimensions.On (0001) Al2 O3 substrates we have grown a series of MgB2 thin films with thicknesses d varying between 50 and 10 nm by employing the hybrid physical-chemical vapor deposition method and studied the thickness dependence of characteristic parameters of the films such as the superconducting transition temperature Tc,the residual resistivity ratio RRR,and the upper critical field He2 by measuring the temperature dependence of resistivity in zero and different applied magnetic fields.It has been observed that the Hc2 (T) exhibits a linear temperature dependence for all samples,clearly indicating the two-band superconductivity in MgB2 thin films.As d reduces,overall,the Tc and RRR of the films are shown to drop and the Hc2 (0) at 0 K rises.It has been further found that the Tc and Hc2 (0) of the films show in approximate a monotonic dependence on the RRR,with Tc decreasing and Hc2 (0) increasing as the RRR decreases,suggesting the RRR as an important factor in influencing the superconducting properties of the fabricated MgB2 ultrathin films.By analyzing the above phenomenon within the framework of two-band superconductivity,it is proposed that the interband scattering between electrons from σ band and zr band in MgB2 may be intensified as the RRR of the films decreases.
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