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Circuit Theory of Crossed Andreev Reflection Physical Review B 74, 214510 ͑2006͒

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Circuit Theory of Crossed Andreev Reflection Physical Review B 74, 214510 ͑2006͒ PHYSICAL REVIEW B 74, 214510 ͑2006͒ Circuit theory of crossed Andreev reflection Jan Petter Morten,1,* Arne Brataas,1,2 and Wolfgang Belzig3 1Department of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway 2Centre for Advanced Study, Drammensveien 78, Oslo, N-0271 Norway 3Department of Physics, University of Konstanz, D-78457 Konstanz, Germany ͑Received 21 June 2006; revised manuscript received 11 September 2006; published 18 December 2006͒ We consider transport in a three-terminal device attached to one superconducting and two normal-metal terminals, using the circuit theory of mesoscopic superconductivity. We compute the nonlocal conductance of the current out of the first normal-metal terminal in response to a bias voltage between the second normal-metal terminal and the superconducting terminal. The nonlocal conductance is given by competing contributions from crossed Andreev reflection and electron cotunneling, and we determine the contribution from each process. The nonlocal conductance vanishes when there is no resistance between the superconducting terminal and the device, in agreement with previous theoretical work. Electron cotunneling dominates when there is a finite resistance between the device and the superconducting reservoir. Dephasing is taken into account, and the characteristic time scale is the particle dwell time. This gives rise to an effective Thouless energy. Both the conductance due to crossed Andreev reflection and electron cotunneling depend strongly on the Thouless energy. We suggest experimental determination of the conductance due to crossed Andreev reflection and electron cotunneling in measurement of both energy and charge flow into one normal-metal terminal in response to a bias voltage between the other normal-metal terminal and the superconductor. DOI: 10.1103/PhysRevB.74.214510 PACS number͑s͒: 74.45.ϩc, 74.25.Fy, 73.23.Ϫb I. INTRODUCTION magnetizations, since Cooper pairs in singlet superconduct- ors consist of two electrons with opposite spins. Electron Crossed Andreev reflection1,2 transforms an incident elec- cotunneling is favored in a parallel configuration. tron from one conductor, attached to a superconductor, into a The predicted dependence of the nonlocal conductance on hole in a geometrically separated second attached conductor. the magnetization configuration was observed experimen- 3 In an alternative, equivalent picture, two quasiparticles from tally in a hybrid superconducting-ferromagnetic device. two separate conductors are transferred into a supercon- Subsequently, a bias dependent nonlocal conductance was observed in a more complicated geometry with only normal- ductor as a Cooper pair. Electrons can also be transferred 4 between the conductors by electron cotunneling, where an metal contacts to the superconductor. For bias voltages cor- responding to energies below the Thouless energy associated incident electron tunnels via a virtual state in the supercon- with the distance between the two normal terminals, a non- ductor. The nonlocal conductance, defined in a three-terminal local signal with sign corresponding to electron cotunneling device as the current response in one normal-metal terminal was seen. Thus, in contrast to the results of Refs. 5 and 14, to a voltage bias between the other normal metal and the experiments showed a finite nonlocal conductance at low superconductor, is determined by contributions from both bias. Additionally, the sign of the nonlocal signal in Ref. 4 crossed Andreev reflection and electron cotunneling. Crossed changes when the bias voltage exceeds the Thouless energy, Andreev reflection and electron cotunneling give opposite and this was interpreted as a consequence of crossed An- contributions to the nonlocal conductance. In this way, dreev reflection dominating the nonlocal signal. These ex- crossed Andreev reflection competes with electron cotunnel- perimental findings are currently not understood. ing. The realization of a system where crossed Andreev re- In previous theoretical works, it is assumed that supercon- flection can be observed, has been the aim of both ducting properties, e.g., the magnitude of the gap, are not experimental3,4 and theoretical5–9 work lately. This interest is modified by the presence of the contacts. This assumption is due to the fact that crossed Andreev reflection is an inher- valid as long as the coupling between the normal or ferro- ently mesoscopic phenomenon, with the prospect of creating magnetic conductor and the superconductor is weak or has a entangled electrons.10–12 small cross section compared to the superconducting coher- The nonlocal conductance of a device where two normal ence length. None of the mentioned theoretical works de- conductors are tunnel coupled to a bulk superconductor was scribe a dependence in the conductances on the Thouless calculated in second order perturbation theory for quantum energy. tunneling13 in Ref. 5. The conductance originating from The circuit theory of mesoscopic transport18,19 is a suit- crossed Andreev reflection was predicted to exactly cancel able framework to understand transport properties of mesos- the conductance due to electron cotunneling. Subsequently, copic small normal-metal–superconducting systems. Circuit disorder14 and higher order quantum interference effects15,16 theory is a discretization of the quasiclassical theory of have been incorporated into this approach, and the noise and superconductivity,20 and can treat nonequilibrium effects and cross correlations have been considered.17 Ferromagnetic dephasing. A circuit is modeled as a network of cavities, contacts were also considered in Ref. 5. Crossed Andreev connectors, and terminals—similar to the way we understand reflection is favored in an antiparallel configuration of the classical electric circuits. Terminals are voltage probes in lo- 1098-0121/2006/74͑21͒/214510͑11͒ 214510-1 ©2006 The American Physical Society MORTEN, BRATAAS, AND BELZIG PHYSICAL REVIEW B 74, 214510 ͑2006͒ cal thermodynamic equilibrium, whereas cavities can be driven out of equilibrium. Cavities and terminals may be normal metals or superconductors. The connectors can rep- resent physical interfaces between cavities and terminals or model diffusion. Connectors representing interfaces are de- scribed by their sets of transmission probabilities. “Kirch- hoff’s rules” determine the matrix Green’s functions ͑poten- tials͒ of the cavities and the matrix currents through the connectors. The matrix currents describe not only the flow of charge and energy, but, e.g., also the flow of quasiparticle correlation. Circuit theory has been applied successfully to explain various phenomena in superconductor and normal- metal–ferromagnet hybrid structures, like the proximity effect,18 multiple Andreev reflections,21 spin transport,22,23 and unconventional superconductivity.24 Circuit theory has FIG. 1. Our circuit theory model. A normal-metal chaotic cavity also proved to be a successful approach to calculate the full ͑c͒ is connected to one superconducting ͑S͒ and two normal-metal 25–28 ͑ ͒ counting statistics of hybrid structures. A circuit theory terminals N1 and N2 . The three connectors are described by their of magnetoelectronics has been developed for hybrid sys- sets of transmission probabilities. A coupling to ground represents tems of ferromagnets and normal metals; see Ref. 29 and the “leakage of coherence” ͑see text͒. references therein. We apply circuit theory to calculate the nonlocal conduc- and that the Green’s functions are isotropic due to chaotic tance of a three-terminal device with contacts to one super- scattering. A physical realization of the chaotic cavity could conducting and two normal-metal terminals. We substantially be a small piece of diffusive metal, embedded in a circuit by, generalize previous theoretical approaches by computing e.g., tunneling contacts to the terminals. The assumptions on analytically the nonlocal conductance with general connec- the chaotic cavity are quite general and can also be satisfied tors ranging from, e.g., ballistic point contacts via diffusive for, e.g., a quantum dot with ballistic point contacts, if the contacts to tunnel junctions. We take the proximity effect conductance of the contacts is much less than the Sharvin into account, in which superconducting correlations affect estimation of the cavity conductance.19 The circuit theory the spectral properties of a normal metal. We also take representation of our model is shown in Fig. 1. The normal dephasing into account, where the dwell time gives rise to an terminals N1 and N2, and the superconducting terminal S are effective Thouless energy. We consider low bias transport so connected to the chaotic cavity c through general connectors that the relevant energies are much smaller than the super- ͕ ͑i͖͒ represented by their sets of transmission probabilities Tn conducting gap. The model we consider has a simple and where i=1,2,S, and the index n numbers the conductance generic geometrical structure. We do not consider Josephson channels. These connectors can represent anything from bal- effects. We recover several aspects seen in experiments.4 listic point contacts to tunnel junctions.22 For a ballistic con- Crossed Andreev reflection and electron cotunneling do not nector all transmission eigenvalues are equal to 1 for the cancel each other. However, in the limiting case of strong propagating channels and 0 otherwise. For a tunnel junction, coupling between the device and the superconducting termi- all transmission
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