Carlos Untiedt

Measurement of the conductance of a hydrogen molecule

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.

Common origin for surface reconstruction and the formation of chains of metal atoms

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.

Observation of a Parity Oscillation in the Conductance of Atomic Wires

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.

Absence of magnetically induced fractional quantization in atomic contacts

When a metallic wire is stretched its conductance becomes smaller as a result of the decrease of its cross section. This process continues until the breaking of the wire, and just before this event takes place, the contact is formed by just one atom. In this way atomic-sized contacts between two metallic electrodes can be formed and studied. The instruments that have made these studies possible are the mechanically controllable break junctions and the scanning tunneling microscope. In both techniques the relative displacement of two electrodes is controlled with a resolution of a few picometers by the use of a piezoelectric element which allows us to monitor the formation and breaking of the contact between the two electrodes. Properties of such atomic-sized contacts have been extensively studied during the past decade 1 for many different metals both magnetic and nonmagnetic. The conductance of these contacts can be described by the Landauer formula

The Kondo effect in ferromagnetic atomic contacts

Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature. The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano–Kondo resonance in the conductance characteristics as observed in other artificial nanostructures. The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory. These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems. Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

Calibration of the length of a chain of single gold atoms

In the last few years, there has been a significant advance in the understanding of the electronic properties of atomicsized contacts. This has been possible thanks to the use of two techniques: scanning tunnelling microscopy ~STM! ~Refs. 1 and 2! and the mechanically controllable break junction ~MCBJ!. 3 In both cases the distance between two electrodes is controlled by means of a piezoelectric transducer which allows for relative displacements of the electrodes down to a resolution in the range of picometers. In these experiments the current that traverses the contact between two electrodes, at a given bias voltage, is measured as a function of the relative displacement of these electrodes. As the contact is broken, the current changes smoothly during elastic elongation stages, decreasing suddenly in plastic deformations stages. 4,5 In the last stage before breaking the contact, just a few atoms determine the electronic transport and the conductance is given by the Landauer formula G5 2e 2

Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt bridge

This work was supported by the Dutch “Stichting FOM,” the Danish Center for Scientific Computing through Grant No. HDW-1101-05, the Spanish MCyT under Contract No. MAT-003-08109-C02-01 and the Ramon y Cajal program, and the ESF through the EUROCORES SONS programme.

Topologically protected quantum transport in locally exfoliated bismuth at room temperature

We report electrical conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 and 300 K. As a function of the elongation of the nanocontact, we measure robust, tens of nanometers long plateaus of conductance G0 = 2e2/h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer, a predicted quantum spin Hall (QSH) insulator, in the retracing process following a tip-surface contact. The formation of the bilayer is further supported by the additional observation of conductance steps below G0 before breakup at both temperatures. Our finding provides the first experimental evidence of the possibility of mechanical exfoliation of Bi bilayers, the existence of the QSH phase in a two-dimensional crystal, and, most importantly, the observation of the QSH phase at room temperature.

The high-bias stability of monatomic chains

For the metals Au, Pt and Ir it is possible to form freely suspended monatomic chains between bulk electrodes. The atomic chains sustain very large current densities, but finally fail at high bias. We investigate the breaking mechanism, that involves current-induced heating of the atomic wires and electromigration forces. We find good agreement of the observations for Au based on models due to Todorov and co-workers. The high-bias breaking of atomic chains for Pt can also be described by the models, although here the parameters have not been obtained independently. In the limit of long chains the breaking voltage decreases inversely proportional to the length.

Formation of a metallic contact: jump to contact revisited

The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determine the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through a combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers, or double-bond contacts.