Transport via coupled states in aC60peapod quantum dot

Abstract

We have measured systematic repetitions of avoided crossings in low temperature three-terminal transport through a carbon nanotube with encapsulated C60 molecules. We show that this is a general effect of the hybridization of a host quantum dot with an impurity. The well-defined nanotube allows identification of the properties of the impurity, which we suggest to be a chain of C60 molecules inside the nanotube. This electronic coupling between the two subsystems opens the interesting and potentially useful possibility of contacting the encapsulated molecules via the tube.PACS numbers: 73.61.Wp, 73.63.Fg, 71.10.Pm Peapod systems represent a next step in complexity of carbon-based electronics, departing from the wellcharacterized single-walled nanotube system. Since the advent 1 of these single-walled carbon nanotubes (CNTs) filled with C 60 molecules (or other fullerenes), there has been an ongoing experimental effort to clarify the modification of the electronic properties of the CNT. In particular, the hybridization of the C 60 molecules with the CNT electronic states is of importance for addressing the molecular scale "peas" via the CNT, for instance using spin-exchange processes. More generally, the interaction of quantum dot systems of different nature and the associated transport signatures are of broad interest. Band structure calculations 2-7 have suggested that the hybridization between the CNT and the encapsulated C 60 molecules could lead to an extra band crossing the Fermi-level in a metallic peapod, depending on tubechirality, but the experimental evidence for such mixing between the two subsystems remains ambiguous.Since the first transmission electron microscopy images of the encapsulated molecules 1 , scanning tunneling microscopy (STM) has been used to probe the electronic states of a single peapod, showing that they were indeed different from those of an empty CNT 8 . These STMdata of Hornbaker et al.8 were rationalized in terms of a semi-empirical model invoking a coupling between the CNT π-orbitals and the t 1u states of a C 60 of the order of 1.25 eV, indicative of substantial hybridization of the two subsystems. DFT-results of Lu et al.4 predicted one order of magnitude smaller hybridization. Subsequent photoemission studies 9 even showed no evidence for hybridization between C 60 molecules and tube.Also low-temperature transport measurements of peapods prepared as three-terminal quantum dots have been performed 10-13 , but the results remain inconclusive. Refs.10,11 find no evidence for electronic structures deviating from that of empty CNT quantum dots, whereas Refs.12,13 showed irregular diamond-structures which were suggested to derive from the encapsulated C 60 system. Since, at present, simultaneous imaging and transport measurements are not possible, these experiments may have probed peapods with rather different electronic structure. For a consistent picture to emerge, more experiments on high-quality peapod samples are clearly necessary. Thus the question still remains whether t...

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