Cooper Pair Splitting by Means of Graphene Quantum Dots

Tan, Zhongchao; Cox, Daniel J.; Nieminen, Tero; Lähteenmäki, Pasi; Golubev, Dmitry S.; Lesovik, G. B.; Hakonen, Pertti

doi:10.1103/physrevlett.114.096602

Cited by 117 publications

(125 citation statements)

References 30 publications

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“…Since Cooper pairs are spin-singlet states, such devices could serve as a source of nonlocal spin entangled electron pairs. Similar geometries are also relevant in the search for Majorana bound states [4] in local S-N junction experiments and in threeterminal devices, where an increase in CPS efficiency might serve as a signature of the elusive exotic states [5].In a series of recent experiments on semiconducting nanowires (NWs) [6][7][8][9], carbon nanotubes [10-12], and graphene [13], CPS was demonstrated by positive conductance correlations between the currents from S into N1 and N2. In these experiments, external magnetic fields were solely used to suppress the superconductivity for control experiments, but not as a parameter to tune CPS.…”

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confidence: 99%

Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter

et al. 2015

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Cooper pair splitting (CPS) is a process in which the electrons of the naturally occurring spin-singlet pairs in a superconductor are spatially separated using two quantum dots. Here, we investigate the evolution of the conductance correlations in an InAs CPS device in the presence of an external magnetic field. In our experiments the gate dependence of the signal that depends on both quantum dots continuously evolves from a slightly asymmetric Lorentzian to a strongly asymmetric Fano-type resonance with increasing field. These experiments can be understood in a simple three-site model, which shows that the nonlocal CPS leads to symmetric line shapes, while the local transport processes can exhibit an asymmetric shape due to quantum interference. These findings demonstrate that the electrons from a Cooper pair splitter can propagate coherently after their emission from the superconductor and how a magnetic field can be used to optimize the performance of a CPS device. In addition, the model calculations suggest that the estimate of the CPS efficiency in the experiments is a lower bound for the actual efficiency. In the Cooper pair splitting (CPS) process the electrons of the Cooper pairs in a superconductor are separated spatially using two quantum dots (QDs) coupled in parallel to a central superconductor contact (S) in a three-terminal geometry [1-3], see Fig. 1(a). The Coulomb repulsion on the QDs and the quasiparticle energy gap of the superconductor enforce the electrons to separate into different normal metal electrodes (N1 and N2). Since Cooper pairs are spin-singlet states, such devices could serve as a source of nonlocal spin entangled electron pairs. Similar geometries are also relevant in the search for Majorana bound states [4] in local S-N junction experiments and in threeterminal devices, where an increase in CPS efficiency might serve as a signature of the elusive exotic states [5].In a series of recent experiments on semiconducting nanowires (NWs) [6][7][8][9], carbon nanotubes [10-12], and graphene [13], CPS was demonstrated by positive conductance correlations between the currents from S into N1 and N2. In these experiments, external magnetic fields were solely used to suppress the superconductivity for control experiments, but not as a parameter to tune CPS. In addition, most experiments were interpreted in terms of an incoherent picture with independent transport mechanisms, only coupled by the QD dynamics [9,11].Here, we report experiments in a NW-based Cooper pair splitter with a Nb superconducting electrode. The large critical magnetic field of Nb allows us to explore CPS up to ∼1 T. We find that the conductance correlations can not only manifest as symmetric peaks and dips in the gate dependence, but also as strongly asymmetric shapes reminiscent of Fano resonances. We interpret the experimental results in a minimal model that incorporates the superconducting proximity effect, the tunnel coupling between the QDs, and quantum interference [see Fig. 1(b)]. Interference results in asymmetric...

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confidence: 99%

Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter

et al. 2015

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“…Our analysis is motivated by two recent experiments on graphene based Cooper pair splitter device 26,27 . We use the fact that unlike normal BCS type superconductor, the hexagonal lattice structure of graphene exhibits two types of Bogoliuobov quasipartiles with different energy.…”

Section: Discussionmentioning

confidence: 99%

“…The latter type of pairing can arise due to both proximity induced superconductivity and electron-phonon interaction. However, for experimental situation, the proximity induced superconductivity is the only realistic possibility 26,27 . When both types of pairing are considered, CPS visibility exhibits a minimum.…”

Section: Discussionmentioning

confidence: 99%

“…Later, J. Cayssol 25 has investigated quantum transport properties in a normal-superconductor-normal (NSN) hybrid junction made of graphene monolayer, and predicts that graphene could be a better candidate to realize spin-entangled electrons via the Crossed Andreev reflection (CAR) process. Very recently, graphene based CPS has been designed experimentally in order to enhance CPS current 26,27 compared to normal superconductor. In the experiment, superconducting correlation has been induced in graphene via the proximity effect.…”

Section: Introductionmentioning

confidence: 99%

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Amplification of Cooper pair splitting current in a graphene-based Cooper pair beam splitter geometry

Islam

Saha

2017

Phys. Rev. B

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Motivated by the recent experiments [Scientific reports, 6, 23051 (2016), Phys. Rev. Lett. 114, 096602 (2015)], we theoretically investigate Cooper pair splitting current in a graphene based Cooper pair beam splitter geometry. By considering the graphene based superconductor as an entangler device, instead of normal (2D) BCS superconductor, we show that the Cooper pair splitting current mediated by Crossed Andreev process is amplified compared to its normal superconductor counterpart. This amplification is attributed to the strong suppression of local normal Andreev reflection process (arising from the Cooper pair splitting) from the graphene based superconductor to lead via the same quantum dot, in comparison to the usual 2D superconductor. Due to the vanishing density of states at the Dirac point of undoped graphene, a doped graphene based superconductor is considered here and it is observed that Cooper pair splitting current is very insensitive to the doping level in comparison to the usual 2D superconductor. The transport process of non-local spin entangled electrons also depends on the type of pairing i.e., whether the electron-hole pairing is on-site, inter-sublattice or the combination of both. The inter-sublattice pairing of graphene causes the maximum non-local Cooper pair splitting current, whereas presence of both pairing reduces the Cooper pair splitting current.

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“…One of very promising applications of such hybrid systems is the possibility to realize a Cooper pair splitter (CPS) [19,20,21,22,23,24,25,26,27,28,29,30]. In Cooper pair splitting devices the electrons forming a Cooper pair tunnel from superconductor into two separate normal leads, while the tunability of quantum dots embedded in the arms of a CPS enables controlling of the splitting process [20,21,22].…”

Section: Introductionmentioning

confidence: 99%

Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads

Wrześniewski

Trocha

Weymann

2017

J. Phys.: Condens. Matter

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Abstract. We investigate the current cross-correlations in a double quantum dot based Cooper pair splitter coupled to one superconducting and two ferromagnetic electrodes. The analysis is performed by assuming a weak coupling between the double dot and ferromagnetic leads, while the coupling to the superconductor is arbitrary. Employing the perturbative real-time diagrammatic technique, we study the Andreev transport properties of the device, focusing on the Andreev current cross-correlations, for various parameters of the model, both in the linear and nonlinear response regimes. Depending on parameters and transport regime, we find both positive and negative current cross-correlations. Enhancement of the former type of cross-correlations indicates transport regimes, in which the device works with high Cooper pair splitting efficiency, contrary to the latter type of correlations, which imply negative influence on the splitting. The processes and mechanisms leading to both types of current cross-correlations are thoroughly examined and discussed, giving a detailed insight into the Andreev transport properties of the considered device.PACS numbers: 73.21.La,74.45.+c,

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Cooper Pair Splitting by Means of Graphene Quantum Dots

Cited by 117 publications

References 30 publications

Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter

Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter

Amplification of Cooper pair splitting current in a graphene-based Cooper pair beam splitter geometry

Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads

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