Coherent transmission of nodal Dirac fermions through a graphene-based superconducting double barrier junction
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Coherent transmission of nodal Dirac fermions through a graphene-based superconducting double barrier junction Chunxu Bai · Ke-Wei Wei · Gui Yang · Yanling Yang
Received: 30 June 2012 / Accepted: 9 September 2012 / Published online: 29 September 2012 © Springer-Verlag Berlin Heidelberg 2012
Abstract Transport characteristics of relativistic electrons through graphene-based d-wave superconducting double barrier junction and ferromagnet/d-wave superconductor/ normal metal double junction have been investigated based on the Dirac–Bogoliubov–de Gennes equation. We have first presented the results of superconducting double barrier junction. In the subgap regime, both the crossed Andreev and nonlocal tunneling conductance all oscillate with the bias voltage due to the formation of Andreev bound states in the normal metal region. Moreover, the critical voltage beyond which the crossed Andreev conductance becomes to zero decreases with increasing value of superconducting pair potential α. In the presence of the ferromagnetism, the MR through graphene-based ferromagnet/ d-wave superconductor/normal metal double junction has been investigated. It is shown that the MR increases from exchange splitting h0 = 0 to h0 = EF (Fermi energy), and then it goes down. At h0 = EF , MR reaches its maximum 100. In contrast to the case of a single superconducting barrier, Andreev bound states also manifest itself in the zero bias MR, which result in a series of peaks except the maximum one at h0 = EF . Besides, the resonance peak of the MR can appear at certain bias voltage and structure parameter. Those phenomena mean that the coherent transmission can be tuned by superconducting pair potential, structure parameter, and external bias voltage, which benefits the spin-polarized electron device based on the graphene materials.
C. Bai () · K.-W. Wei · G. Yang · Y. Yang School of Physics, Anyang Normal University, Anyang 455000, China e-mail: [email protected]
1 Introduction Graphene, a flat monolayer of carbon atoms densely packed into a two-dimensional honeycomb lattice, possesses unusual and unique band structure (so-called Dirac cone structure) has been known for 60 years [1]. For years, however, graphene was considered as a pure academic molecule that does not exist as a free standing material due to its unstable nature. A breakthrough in graphene research was achieved by Novoselov et al. who were the first people to successfully isolate the elusive free-standing graphene membrane experimentally in 2004 [2]. Following the pioneering contributions, a huge = interest in studying the physical properties of graphene has been raised [3–8]. Superconductivity, one of the major and the long list of graphene remarkable properties, has attracted considerable attention due to the unique possibility to bridge the gap between relativity and superconductivity in experimental [3]. Although graphene itself is not superconducting, superconductivity in a graphene layer can be induced by the following two routes: First, it can be realized by the presenc
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