We report experimental evidence for atomic chain formation during stretching of atomic-sized contacts for gold and silver, that is strongly enhanced due to oxygen incorporation. While gold has been known for its tendency to form atomic chains, for silver this is only observed in the presence of oxygen. With oxygen the silver chains are as long as those for gold, but the conductance drops with chain length to about 0.1 conductance quantum. A relation is suggested with previous work on surface reconstructions for silver (110) surfaces after chemisorption of oxygen.

Abstract. Current innovations in optical imaging, measurement techniques, and data analysis algorithms express the need for reliable testing and comparison methods. We present the design and characterization of silicone elastomer-based optical phantoms. Absorption is included by adding a green dye and scattering by adding TiO 2 or SiO 2 particles. Optical coherence tomography measurements demonstrate a linear dependence of the attenuation coefficient with scatterer concentration in the absence of absorbers. Optical transmission spectroscopy of the nonscattering absorbing phantoms shows a linear concentration dependent absorption coefficient. Both types of samples are stable over a period of 6 months. Confocal microscopy of the samples demonstrates a homogeneous distribution of the scatterers, albeit with some clustering. Based on layers with thicknesses as small as 50 m, we make multifaceted structures resembling flow channels, ͑wavy͒ skin-like structures, and a layered and curved phantom resembling the human retina. Finally, we demonstrate the ability to incorporate gold nanoparticles within the phantoms. In conclusion, our phantoms are easy to make, are based on affordable materials, exhibit well-defined and controllable thickness, refractive index, absorption, and scattering coefficients, are homogeneous, and allow the incorporation of novel types of nanoparticle contrast agents. We believe our phantoms fulfill many of the requirements for an "ideal" tissue phantom, and will be particularly suited for novel optical coherence tomography applications.

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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 ...