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

高温层状硼化物超导体中多阶参数和电子声子相互作用的隧道研究

基本信息

批准号：
15540346
负责人：
EKINO Toshikazu
金额：
$ 2.24万
依托单位：
HIROSHIMA UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2005
项目状态：
已结题

项目摘要

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.

我们通过电子隧道和点接触光谱研究了高温层状硼化物超导体MgB_2。低温下的隧道电导显示出明显的多能隙结构，这是由McMillan提出的二能隙模型描述的。这表明多重差距是由于非平凡的起源造成的。我们将该模型称为“相关两间隙”模型。具体的间隙值是Δ=2-2.5,5-7,10-12 meV，存在到T_c=39K。最大间隙的间隙比2Δ/k_BT_c=5-6表明耦合极强，是非铜超导体中最大的。另一方面，给出的比率为 1.2-1.5 的最小间隙令人惊讶地小于 BCS 的 3.5，这表明感应超导性的不成熟间隙特征。根据温度变化，我们将中间间隙归因于邻近效应。从这些分析中，可以认为最大和最小的间隙来自硼σ和π带产生的2能带超导性。间隙的磁场依赖性使得间隙之间有明显的区别，即较大的间隙在较高的临界场处消失。这与显示相同临界温度的这些间隙的温度依赖性形成对比。最大和最小间隙的外推临界场 18-23 T 和 7-11 T 分别对应于面内和 c 轴上临界场。这些一致性表明多个能隙与层状结构密切相关，并且最大的能隙主要是MgB_2。点接触谱显示峰值位置与中子散射实验的声子谱一致。更重要的是，60-90 meV 左右的高能硼振动模式在 MgB_2 中的电子-声子相互作用中发挥着重要作用。