关键词: 格林函数方法, 长程自旋三重态关联, 零能峰, Andreev束缚态

Abstract: Based on the Green's function method, we study theoretically the pair correlations and the local density of states (DOS) in a ferromagnet/ferromagnet/s-wave superconductor (F₁F₂S) heterostructure in the clean limit. The Green's functions which satisfy the Gor'kov equations are calculated numerically without taking the quasiclassical approximation. We find that, when the angle α between the exchange fields in F₁ and F₂ is finite (α≈0.5π) and further F₁ is a half-metal, only the equal-spin triplet correlation can be induced within the F₁ layer. In this circumstance, the Andreev reflection is governed by the anomalous equal-spin-band Andreev reflection which can lead to a zero-energy Andreev bound state in the F₂ layer. In the presence of the zero-energy Andreev bound state, the induced DOS in the F₁ layer exhibits a rather sharp zero-energy peak (ZEP). We derive the condition for the occurrence of the zero-energy Andreev bound state analytically and verify that it can predict the numerical results excellently. In particular, we find that the ZEP structure of DOS can penetrate deep into the F₁ layer reflecting the long-range nature of the equal-spin triplet correlation. Experimental detection of the long-range ZEP in our F₁F₂S system (by using STM, for instance) may provide unambiguous evidence for the presence of the long-range triplet pair correlation induced by the superconducting proximity effect.

Key words: Green's function method, long-range spin-triplet correlation, zero-energy peak, Andreev bound state