We report on theoretical results about contact-dependent effects and tunneling currents through DNA molecules. A tetranucleotide PolyGACT chain, connected in between metallic contacts, is studied as a generic case, and compared to other periodic sequences such as PolyAT or PolyGC. Remarkable resonance conditions are analytically derived, indicating that a strong coupling does not always result in a larger conductance. This result is properly illustrated by considering intrinsic features of

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... dependent tunneling currents in the coherent regime. In recent years, many experimental measurements have directly probed the electrical current as a function of the applied potential across DNA molecules. 1-5 These experiments are performed in a variety of conditions where important factors, including the substrate surface, contacts to the electrodes, counterions, and DNA structure are not kept constant. 3 This state of affairs considerably makes difficult a proper comparison among different experimental reports, which range from completely insulating to semiconducting, and even superconducting, behaviors. 2 In turn, such scatter makes it difficult to set the basis for a meaningful theoretical approach to the intrinsic DNA electrical transport properties. To this end, the role of contacts deserves particular attention. In many measurements, contact with metal electrodes was achieved by laying down the molecules directly on the electrodes. In this case, it is rather difficult to prove that the DNA molecule is in direct contact with the electrodes. Even so, the weak physical adhesion between DNA and metal may produce an insulating contact. Recent transport experiments have shown that deliberate chemical bonding between DNA and metal electrodes is a prerequisite for achieving reproducible conductivity results. [3] [4] [5] Generally, any current measured through a DNA molecule results from the carrier injection onto the stack of bases, combined with the intrinsic conduction along the DNA sequence. At low voltage, the main contribution to the resistance comes from the metal-DNA junction potential mismatch ͑barrier͒, whereas for high enough voltage, new conduction channels are provided by the molecular states. The I͑V͒ characteristics are thus somehow inferred from the energy difference between the metallic work function and the lowest ionization energy levels of the DNA ͑in case of hole transport͒. 6 Besides, charge transfer in DNA has been proven to be mainly conveyed by intrastrandcoupling, 7 through sequential incoherent hopping or coherent tunneling ͑superexchange͒. 7 The latter mechanism might be expected to dominate the conduction in the very low-temperature regime. Despite great experimental efforts, 8,9 few theoretical works have so far precisely addressed the nature of measured currents and its relation with device characteristics. In this work, we present a theoretical study on coherent charge tunneling in DNA molecules connected in between metallic contacts. An effective tight-binding Hamiltonian is constructed from ab initio parameters, and an analytical expression of the transmission coefficient is derived. The role played by the DNA-metal interface in determining the overall transport and I͑V͒ characteristics is also addressed. In this way, we determine the limits for large turn-on currents ͑in the nA regime͒, that result from the resonance ͑alignment͒ between the DNA molecular levels and the bias-modulated Fermi levels of contacts. As a suitable representative example, the properties of a periodic polyGACT tetranucleotide chain, connected to metallic leads at both ends, will be considered. Many details on the geometry and chemical bonding nature of the DNA-lead interface are poorly known currently. In our model the coupling between the metal orbitals and the DNA energy levels at the interface will be described in terms of an effective parameter. Thus, the lead-DNA global system will be described by means of the tight-binding Hamiltonian H as where c j ͑c j † ͒ is the creation ͑annihilation͒ operator for a charge at jth site in the chain. The first term describes the intrastrand hole propagation through the DNA chain, where A = 8.24 eV, T = 9.14 eV, C = 8.87 eV, G = 7.75 eV are the nucleotides on-site energies, t is the hopping term between adjacent nucleotides, and N is the number of nucleotides in the chain. 6,10 The second term describes the DNA-metal coupling, where measures the coupling strength, whereas the third term gives the energetics of the metallic leads, with m = 5.36 eV ͑related to the platinum metallic work function 6 ͒, while t m is the hopping term. Firstprinciples calculations have reported values ranging from t = 0.01 to t = 0.4 eV. 11 Following previous works, properly accounting for experimental I-V curves, we will take t = 0.4. 12 By considering nearest-neighbors interactions the first term of H can be cast in terms of the unimodular matrices