Tunneling study of multiple order parameter and electron-phonon interaction in the high temperature layered boride superconductor

本文用电子隧穿和点接触谱研究了高温层状硼化物超导体MgB_2，低温隧穿电导呈现明显的多能隙结构，可用McMillan提出的双能隙模型描述。这表明多个间隙是由于非平凡的起源。我们称之为“相关双间隙”模型。在T_c = 39K时，比能隙分别为Δ = 2 - 2.5，5 - 7，10 - 12meV。最大能隙的差距比为2 Δ/kBT_c = 5 - 6，表明极强的耦合，这是非铜超导体中最大的耦合。另一方面，给出比率1.2 - 1.5的最小能隙令人惊讶地小于BCS的能隙3.5，表明来自诱导超导性的未成熟能隙特征。从温度的变化，我们归因于中间的间隙可能是由于邻近效应。从这些分析中可以认为，最大和最小的能隙来自于由硼的σ和π带产生的2带超导，能隙的磁场依赖性使能隙之间有明显的区别，即在较高的临界场下，较大的能隙消失。这与显示相同临界温度的这些间隙的温度依赖性相反。最大和最小间隙的18 - 23 T和7 - 11 T的外推临界场分别对应于面内和c轴上临界场。这些一致性表明，多重能隙与层状结构密切相关，最大能隙是MgB_2的主要能隙。点接触谱表明，峰位与中子散射实验得到的声子谱一致。更重要的是，在60 - 90 meV附近的高能硼振动模式被认为在MgB_2中的电子-声子相互作用中起着重要的作用。

We have investigated the high temperature layered boride superconductor MgB_2 by electron tunneling and point contact spectroscopy.The tunneling conductance at low temperatures shows clear multiple-gap structures, which are described by the two-gap model proposed by McMillan. This indicates that the multiple gaps are due to non-trivial origin. We call this model as "the correlated two gap" model. The specific gap values are Δ=2-2.5,5-7,10-12 meV that exist up to T_c=39K. The gap ratio 2Δ/k_BT_c=5-6 for the largest gap indicates extremely strong coupling, which is the largest one among non-copper superconductors. On the other hand, the smallest gap giving the ratio 1.2-1.5 is surprisingly smaller than the BCS one 3.5, indicating an immature gap feature from induced superconductivity. From the temperature variations, we attribute that the middle gap can be due to the proximity effect. From these analyses, it is possible to consider that the largest and smallest gaps are coming from the 2 band superconductivity arising from boron σ and π bands.The magnetic-field dependence of the gaps makes clear distinction between the gaps, namely, the larger gap vanishes at the higher critical field. This is in contrast to the temperature dependence of these gaps showing the same critical temperature. The extrapolated critical fields of 18-23 T and 7-11 T for the largest and smallest gaps correspond to in-plane and c-axis upper critical fields, respectively. These agreements indicate that the multiple gaps are intimately related to the layered structure, and the largest gap is predominant to MgB_2.Point- contact spectra show that the peak positions are in agreement with the phonon spectra from the neutron scattering experiments. More significantly, the high-energy boron vibration modes around 60-90 meV are suggested to play an important role in the electron-phonon interaction in MgB_2.