Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal-molecule-metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.

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...

During the fracture of nanocontacts gold spontaneously forms freely suspended chains of atoms, which is not observed for the isoelectronic noble metals Ag and Cu. Au also differs from Ag and Cu in forming reconstructions at its low-index surfaces. Using mechanically controllable break junctions we show that all the 5d metals that show similar reconstructions (Ir, Pt, and Au) also form chains of atoms, while both properties are absent in the 4d neighbor elements (Rh, Pd, and Ag), indicating a common origin for these two phenomena. A competition between s and d bonding is proposed as an explanation. DOI: 10.1103/PhysRevLett.87.266102 PACS numbers: 68.35.Bs, 73.22. -f, 73.40.Jn It has recently been discovered that nanowires of gold spontaneously evolve into chains of single atoms [1,2], which are surprisingly stable. They form metallic wires with a nearly ideal quantum value of the conductance G Ӎ 2e 2 ͞h, and are able to sustain enormous current densities. Various numerical calculations on these chains have been presented, both in regular and distorted configurations [3], but the question as to why these chains form specifically for Au, and, e.g., not for Cu or Ag, was not addressed [4]. An understanding of the mechanism of the formation of these chains may help to improve our ability to control the fabrication process. This may lead to the formation of chains for different materials with interesting properties (magnetism, superconductivity) or of longer chains. Apart from a possible technical interest longer chains will also inspire new fundamental research as such atomic chains are the closest approximation to ideal one-dimensional metallic systems. These 1D systems are expected to undergo a Peierls distortion and ultimately Tomanaga-Luttinger Liquid effects [5] could appear.In search for properties distinguishing Au from the other noble metals, that can be linked to the chain formation, we are particularly interested in surface effects, where the bonding between the atoms is modified by the reduced dimensions. Among the special features of Au that have been extensively studied are the reconstructions of the lowindex surfaces. The (110) surface shows a missing-row reconstruction, where every second row of atoms on the surface is removed; the (001) surface has a quasihexagonal reconstruction, where the top layer of the sample contracts to form a hexagonal layer on top of the square structure of the bulk.Since these surface reconstructions distinguish Au from Ag and Cu, it is worth looking at the mechanism that has been put forward to explain them [6]. In fact, the end-of-series 5d elements Ir, Pt, and Au have similar surface reconstructions, which are absent in the related 4d elements Rh, Pd, and Ag, suggesting that the explanation for the reconstructions cannot lie in any particular detail of d band electronic structure. There appears to be a growing consensus that a stronger bonding of low-coordination atoms of the 5d metals with respect to the 4d metals is a result of sd competition caused by relativistic ...

Using a scanning tunnel microscope or mechanically controllable break junctions atomic contacts for Au, Pt, and Ir are pulled to form chains of atoms. We have recorded traces of conductance during the pulling process and averaged these for a large number of contacts. An oscillatory evolution of conductance is observed during the formation of the monoatomic chain suggesting a dependence on the numbers of atoms forming the chain being even or odd. This behavior is not only observed for the monovalent metal Au, as was predicted, but is also found for the other chain-forming metals, suggesting it to be a universal feature of atomic wires.

A conducting bridge of a single hydrogen molecule between Pt electrodes is formed in a break junction experiment. It has a conductance near the quantum unit, G0 = 2e 2 /h, carried by a single channel. Using point contact spectroscopy three vibration modes are observed and their variation upon isotope substitution is obtained. The stretching dependence for each of the modes allows uniquely classifying them as longitudinal or transversal modes. The interpretation of the experiment in terms of a Pt-H2-Pt bridge is verified by Density Functional Theory calculations for the stability, vibrational modes, and conductance of the structure.