Matera D, Bonura M, Černý R, McKeown Walker S, Buta F, LeBoeuf D, Chaud X, Giannini E, Senatore C
Department of Quantum Matter Physics (DQMP), University of Geneva, Geneva, Switzerland.
Laboratory of Advanced Technology (LTA), Geneva, Switzerland.
Sci Rep. 2020 Oct 19;10(1):17656. doi: 10.1038/s41598-020-74300-9.
The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB for magnet applications. This work presents the results of an extensive experimental campaign on MgB bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB takes place: this enhances the upper critical field while reducing the critical temperature only by few Kelvins. On the other hand, the critical temperature in bulk samples decreases monotonically when structural disorder increases, and this imposes a limit to the maximum achievable upper critical field.
上临界场设定了超导性的热力学极限。在MgB多晶块体超导体中测量的上临界场值与薄膜的上临界场值之间存在很大差距，在薄膜中，在4.2K时已实现高达约50T的值。填补这一差距将释放MgB在磁体应用方面的潜力。这项工作展示了在MgB块体样品上进行的广泛实验活动的结果，该活动由实验设计指导。我们对合成主要参数对上临界场的依赖性进行了建模，并通过一种非常规的快速合成路线制备了碳掺杂块体样品，创造了新的记录（在4.2K时约为35T）。该值似乎是块体样品上临界场的一个上限。薄膜中的结构无序似乎选择性地作用于MgB发生超导的两个能带之一：这提高了上临界场，而仅使临界温度降低了几开尔文。另一方面，当结构无序增加时，块体样品的临界温度单调下降，这对上临界场的最大可实现值施加了限制。
