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arXiv 1409.3181cond-mat.supr-con
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超导体的双通道点接触隧穿理论

Two-channel point-contact tunneling theory of superconductors

Mikael Fogelstrom, Matthias J. Graf, V. A. Sidorov, Xin Lu, E. D. Bauer, J. D. Thompson

摘要:

我们引入一个双通道隧穿模型来推广被广泛使用的BTK理论的点接触电导之间的正常金属接触和超导体。电子的隧穿可以通过局域表面态发生或直接发生,导致微分电导$G=dI/dV$中的Fano共振。我们提出了一个分析$G$内的双通道模型时,适用于正常的金属银粒子和原型重费米子超导体CeCoIn $_5$和CeRhIn $_5$在高压下的软点接触。在正常状态下的Fano线形状的测量$G$是很好地描述了一个模型与两个隧道通道和一个大的温度无关的背景电导。在超导状态下,强烈抑制Andreev反射信号被解释的背景电导的存在。我们报告了CeCoIn $_5$中的Andreev信号与标准$d_{x^2-y^2}$波配对一致,假设隧道进入[100]和[110]晶体界面的相等混合。而在1.8和2.0 GPa的CeRhIn $_5$中,信号由具有减小的节区的$d_{x^2-y^2}$波隙描述,即,费米表面上的间隙开口的增加的斜率。一种可能性是,高压Andreev信号的形状受到从1.75 GPa延伸到2.3 GPa的量子临界点线附近的影响,这在我们对重费米子超导体的描述中没有考虑。

英文摘要:

We introduce a two-channel tunneling model to generalize the widely used BTK theory of point-contact conductance between a normal metal contact and superconductor. Tunneling of electrons can occur via localized surface states or directly, resulting in a Fano resonance in the differential conductance $G=dI/dV$. We present an analysis of $G$ within the two-channel model when applied to soft point-contacts between normal metallic silver particles and prototypical heavy-fermion superconductors CeCoIn$_5$ and CeRhIn$_5$ at high pressures. In the normal state the Fano line shape of the measured $G$ is well described by a model with two tunneling channels and a large temperature-independent background conductance. In the superconducting state a strongly suppressed Andreev reflection signal is explained by the presence of the background conductance. We report Andreev signal in CeCoIn$_5$ consistent with standard $d_{x^2-y^2}$-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces. Whereas in CeRhIn$_5$ at 1.8 and 2.0 GPa the signal is described by a $d_{x^2-y^2}$-wave gap with reduced nodal region, i.e., increased slope of the gap opening on the Fermi surface. A possibility is that the shape of the high-pressure Andreev signal is affected by the proximity of a line of quantum critical points that extends from 1.75 to 2.3 GPa, which is not accounted for in our description of the heavy-fermion superconductor.

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