Onset of Energy Dissipation in Ballistic Atomic Wires

Abstract

Electronic transport at finite voltages in free-standing gold atomic chains of up to seven atoms in length is studied at low temperatures using a scanning tunneling microscope. The conductance vs voltage curves show that transport in these single-mode ballistic atomic wires is nondissipative up to a finite voltage threshold of the order of several mV. The onset of dissipation and resistance within the wire corresponds to the excitation of the atomic vibrations by the electrons traversing the wire and is very sensitive to strain. DOI: 10.1103/PhysRevLett.88.216803 PACS numbers: 73.63.Nm, 68.37.Ef, 73.40.Jn The trend toward miniaturization in electronics will soon lead to devices of nanometer scale in which quantum effects become relevant. The ultimate quantum conductor is a perfect one-dimensional wire, such as an atomic chain [1,2] or semiconducting heterostructure [3]. In these wires the electrons are ballistic since there are no defects to inhibit resistance-free currents [3]. The limiting factor in the current-carrying capacity of a wire is dissipation, which results in heating. Two mechanisms contribute to the resistance of a metallic wire: elastic scattering with defects and impurities and inelastic scattering with the lattice vibrations [4]. In the absence of scattering, electrons can propagate freely and transport is said to be ballistic. This situation is possible in the nanoscale where the mean-free path of electrons can be much longer than the length of the device.The two-terminal zero-bias resistance of a single-mode ballistic wire is the resistance quantum h͞2e 2 . This resistance is entirely associated with the connections of the wire to the electrodes [5], being the intrinsic resistance of the wire zero, as recently demonstrated in quantum wires fabricated from GaAs͞AlGaAs heterostructures [3], and in agreement with Landauer framework [6,7]. Within this framework, the applied voltage serves to unbalance the chemical potentials for propagating electrons in each direction and drops entirely at the contacts and not within the wire. The Joule dissipation associated with this resistance is assumed to take place far away from the contact (at an inelastic relaxation length), where electrons and holes relax to the Fermi level of the electrodes. This picture is correct for bias voltages close to zero, which implies vanishingly small currents (note that the resistance-free currents in the experiment of Ref.[3] were smaller than 1 nA).In this Letter we study transport at finite voltages and the mechanism of dissipation in ballistic wires. Our experiments are performed in freely suspended gold atomic wires of up to seven atoms in length, fabricated using a low-temperature scanning tunneling microscope (STM) [1,2]. Very recently the forces and conductance have been measured simultaneously [8] during the process of chain formation giving insight into the formation mechanisms. The mechanical and electronic properties of these metallic nanostructures are of great interest not only from the point of view of the...

Phonons softening in tip-stretched monatomic nanowires

“…Incorporation of each new atom in the chain was observed after a elongation of nearly 1.5Å and involved a sudden downward jump in bond lengths. This crude model shows that the wire of given length can only be stretched elastically over a limited distance which compares well with the 1Å in the experiments [5]. The calculated change of nanowire interatomic distance for a total elongation tip-tip distance of 0.75Å and a chain of 7 atoms was 0.03Å, instead of nearly 0.1Å as expected the perfect chain.…”

“…2 the frequencies deduced from the experiment of Ref. [5] (long dashed line). In order to decide the position of these points we made two assumptions.…”

“…At sufficiently low voltage, the conductance of these wires is ballistic. However, a recent experiment [5] showed that above a voltage threshold of about 15 meV a monatomic gold wire, seven atoms long, displays a drop of about 1 % in the differential conductance below the ballistic value of one. The symmetric drop of conductance with respect to the voltage sign and the value of the voltage threshold of around 15 meV suggested the effect to be due to inelastic scattering of electrons by longitudinal wire vibrations.…”

Modeling inelastic phonon scattering in atomic- and molecular-wire junctions

“…Inelastic effects have in the recent years been studied in a variety of nanoscale systems, e.g., single molecules on surfaces probed with the scanning tunneling microscope ͑STM͒, 2 molecules in break junctions, 3 and metallic atomic wires. 4 Theoretical descriptions of inelastic transport through small devices connected to metallic contacts include the many-body theory in the Coulomb blockade regime, 5 singleparticle first-order perturbation approaches, 6,7 i.e., Fermi's golden rule ͑FGR͒, as well as calculations to infinite order based on the self-consistent Born approximation ͑SCBA͒ combined with nonequilibrium Green's functions ͑NEGF͒. [8][9][10] Our paper is based on the SCBA, which in contrast to FGR takes the many-particle nature of the problem into account.…”

Single-Molecule Analysis Methods Using Nanogap Electrodes and Their Application to DNA Sequencing Technologies

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