The continuing miniaturization of microelectronics raises the prospect of nanometre-scale devices with mechanical and electrical properties that are qualitatively different from those at larger dimensions. The investigation of these properties, and particularly the increasing influence of quantum effects on electron transport, has therefore attracted much interest. Quantum properties of the conductance can be observed when 'breaking' a metallic contact: as two metal electrodes in contact with each other are slowly retracted, the contact area undergoes structural rearrangements until it consists in its final stages of only a few bridging atoms [1][2][3]. Just before the abrubt transition to tunneling occurs, the electrical conductance through a monovalent metal contact is always close to a value of 2e 2 /h(≈ (12.9kΩ) −1 ), where e is the charge on an electron and h is Plack's constant [4][5][6]. This value corresponds to one quantum unit of conductance, thus indicating that the 'neck' of the contact consists of a single atom [7]. In contrast to previous observations of only single-atom necks, here we describe the breaking of atomic-scale gold contacts, which leads to the formation of gold chains one atom thick and at least four atoms long. Once we start to pull out a chain, the conductance never exceeds 2e2 /h, confirming that it acts as a one-dimensional quantized nanowire. Given their high stability and the ability to support ballistic electron transport, these structures seem well suited for the investigation of atomic-scale electronics.Previous studies on metallic contacts of atomic dimensions have shown remarkable properties of such structures, including conductance quantisation and superior mechanical strength compared to the bulk. Experimental techniques, most common being scanning tunnelling microscopy (STM) and mechanically controllable break-junctions (MCB), are all based on piezoelectric transducers which allow fine positioning of two metal electrodes with respect to each other. STM, in which the tip is driven into contact with a metal surface and the conductance is measured during subsequent retraction, has been widely used for this purpose [4,5,8,9]. In the alternative method of MCB one starts with a macroscopic notched wire [10], or a nanofabricated metal bridge [11] mounted on a flexible substrate. The wire (or bridge) is broken at low temperatures in vacuum, and contact is re-established between the fracture surfaces by piezoelectric control of substrate bending. In this work we have used both MCB and a very stable STM at liquid helium temperatures to produce and study chains of single gold atoms. In each case, high purity (99.99 + %) gold was used. Conductance was measured at a 10 mV DC voltage bias with 1% accuracy.An example of a conductance curve obtained while stretching a gold nanocontact is presented in Fig. 1. The curve reflects the evolution of some particular atomic configuration, during which the conductance decreases in a series of sharp vertically descending steps, with a gradual slop...

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.

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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 ...