Highly conductive molecular junctions were formed by direct binding of benzene molecules between two Pt electrodes. Measurements of conductance, isotopic shift in inelastic spectroscopy, and shot noise compared with calculations provide indications for a stable molecular junction where the benzene molecule is preserved intact and bonded to the Pt leads via carbon atoms. The junction has a conductance comparable to that for metallic atomic junctions (around 0:1-1G 0 ), where the conductance and the number of transmission channels are controlled by the molecule's orientation at different interelectrode distances.

We report measurements of shot noise in the current through a single D2 molecule. The molecular junctions were formed by means of the mechanically controllable break junction technique. The configuration of the D2 molecule bridging the gap between two Pt tips is verified by use of point contact spectroscopy. Maintaining the same junction shot noise measurements were performed and the observed quantum suppression shows that conductance is carried dominantly by a single, almost fully transparent conductance channel. This observation allows us to decide between conflicting model calculations for this system, and this may serve as a benchmark for further computations on molecular junctions.

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.

Using the mechanically controlled break junction technique at low temperatures and under cryogenic vacuum conditions we have studied atomic contacts of several magnetic ͑Fe, Co, and Ni͒ and nonmagnetic ͑Pt͒ metals, which recently were claimed to show fractional conductance quantization. In the case of pure metals we see no quantization of the conductance nor half quantization, even when high magnetic fields are applied. On the other hand, features in the conductance similar to ͑fractional͒ quantization are observed when the contact is exposed to gas molecules. Furthermore, the absence of fractional quantization when the contact is bridged by H 2 indicates the current is never fully polarized for the metals studied here. Our results are in agreement with recent model calculations. DOI: 10.1103/PhysRevB.69.081401 PACS number͑s͒: 73.63.Rt, 75.75.ϩa 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 formulawhere the summation is extended to all the available channels for the electrons traversing the contact, T i is a number between 0 and 1 for the transmission of the ith channel, and G 0 ϭ2e 2 /h the quantum of conductance ͑assuming degeneracy of spin͒ in terms of the electron charge e and Planck's constant h. In the case in which the degeneracy of spin would be removed the channels would have to be redefined for each spin and each of these would carry up to 1 2 G 0 . The number of channels available in a one-atom contact is determined by the valence of the metal, 2 and the transmission of each channel is influenced by other parameters such as the number of neighbors or the bond distance. 3,4 For special cases (s-type metals such as Au or Na͒, electronic transport through a single atom will be due to a single channel with a transmission close to unity, but this will not be true for other metals where all kinds of combinations of channels with different transmissions will add up to produce the total conductance of the atom. This is the case for transition metals with partial occupation of the d orbitals and therefore they are not expected to have a one-atom conductance of 1 2 G 0 or 1 G 0 for magnetic or nonmagnetic metals, r...

Single-molecule junctions are found to show anomalous spikes in dI/dV spectra. The position in energy of the spikes are related to local vibration mode energies. A model of vibrationally induced two-level systems reproduces the data very well. This mechanism is expected to be quite general for single-molecule junctions. It acts as an intrinsic amplification mechanism for local vibration mode features and may be exploited as a new spectroscopic tool.PACS numbers: 81.07. Nb, 73.63.Rt, 85.65.+h, 63.22.+m A single atom or molecule with an almost transparent single conductance channel leading to a conductance near the conductance quantum 2e 2 /h (= 1 G 0 ) can be contacted to leads. Conduction electrons can pass through such junction ballistically for low bias voltages since the mean free path of the electrons is much larger than the size of the contact. However the contact is not entirely ballistic in the sense that once the excess energy of the conduction electrons becomes equal or larger than the energy of a local mode of the contact, the electrons can scatter inelastically by exciting a local mode. This results in the case of a perfectly transmitting single channel contact to a small decrease in the conductance, since the forward travelling electrons are backscattered due to the energy loss in the inelastic scattering process. Differential conductance (dI/dV) measurements have identified vibration modes of single molecules in an atomic contact [1]. This technique, also called Point Contact Spectroscopy (PCS) is analogous to inelastic electron tunnelling spectroscopy (IETS) for single molecules [2,3], with the difference that the conductance in the latter case increases due to the opening of an additional conductance channel.In this letter we present the observation of anomalous spikes, rather than steps, in dI/dV measurements on various single molecule contacts. We present a model that involves two-level systems, which describes our data very well. It may be used as a new spectroscopic tool for identifying molecular vibration modes in single molecule junctions.We create atomic contacts using a mechanically controlled break junction (MCBJ) setup in cryogenic vacuum at 4.2 K (see ref.[4] for a detailed description). Break junctions for the metals Au, Ag, Pt and Ni have been investigated with the molecules H 2 , D 2 , O 2 , C 2 H 2 , CO, H 2 O and benzene. In most of these cases regular vibration mode spectra displaying a step down in conductance have been observed, but for all systems anomalous spectral features as displayed in Fig. 1 were also found.In order to admit these molecules to the metal atomic contacts at 4.2 K, the insert is equipped with a capillary that has a heating wire running all along its interior to prevent premature condensation of the gasses. The amount of gas admitted is of order 10 µmol. Previous measurements have clearly demonstrated that it is possible to capture a single molecule in the atomic junction and measure its vibration modes [1,5].Figure 1(a) shows a dI/dV spectrum of a single ...