High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement.Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 2 Since the discovery of the iron based superconductors 1 , great efforts have been devoted to the preparation of thin films of the various phases 2 -7 and references therein. Between other reasons, the interest in films deposition is motivated by the strong Tc dependence on external and chemical pressure in iron based pnictides and calcogenides 8 -12 , which has suggested the idea of exploring whether a similar effect can be induced by strain in thin films. Indeed, such expectation turned out to be true: in Ba(Fe1-xCox)2As2 thin films 7 deposited on various substrates, Tc has been observed to increase with the ratio c/a. Also in FeSe0.5Te0.5 thin films an increase of Tc has been obtained by two groups 5,6 in particular a maximum Tc of 17K has been measured. Such increase has been attributed to the observed c axis decrease with respect to the bulk value.Here we present preparation as well as structural, morphological and physical characterization of epitaxial FeSe0.5Te0.5 thin films with different thickness, deposited on different substrates. A maximum Tc =21K was obtained, which is a remarkable 30% increase with respect to the bulk value.The films were grown by Pulsed Laser Ablation Deposition (PLD) in ultra high vacuum system 13 starting from a FeSe0.5Te0.5 (Fe(Se,Te)) target prepared by direct synthesis from high purity materials (Fe 99.9+%, Se 99.9% and Te 99.999%) 5 .With the aim of introducing biaxial strain we deposited films on single crystal substrates with different cell parameters, namely magnesium oxide (MgO a=4.217 Å) , strontium titanate (STO a=3.905 Å ), lantanum aluminate (LAO a=3.789 Å), and yttria stabilized zirconia (ZrO:Y, a=3.637 Å); for all the substrates we used the (001) orientation. The deposition conditions were optimized as reported in a previous paper 5 ; namely, we used a deposition temperature of 550°C at a pressure of 5·10 -Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain Ver. 04/12/2009 14:29:00 3 9 mBar. The laser repetition rate was 3 Hz (248 nm wavelength) and the laser fluency was 2 J/cm 2 (2 mm 2 spot size). The target-substrate distance was maintained at 5 cm.In order to study the residual strain behaviour, films of different thickness from 1.2 nm to 600 nm were deposited; the thickness was calibrated by X-ray reflectometry.XRD analysis allowed to identify the PbO-like tetragon...

The adsorption of the amino acid, (S)-glutamic acid, was investigated on Ag{110} as a function of coverage and adsorption temperature using the techniques of scanning tunneling microscopy, low energy electron diffraction, and reflection absorption infrared spectroscopy. In the monolayer, (S)-glutamic acid was found to adsorb predominantly in the anionic glutamate form. Several discrete ordered adlayer structures were observed depending on preparation conditions. In addition, (S)-glutamic acid was found to induce both one- and two-dimensional faceting of the Ag{110} surface. In some cases, evidence was found that the 2-D faceting involved the creation of a chiral facet distribution. A comparison is made of the Ag/(S)-glutamic acid system with analogous studies of amino acids on Cu.

Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate di-electric, we have successfully tuned carrier density in FeSe0.5Te0.5 thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe0.5Te0.5. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li + ions intercalation into the FeSe0.5Te0.5. It indicates that a systematic change of electronic properties in FeSe0.5Te0.5 arises from the electrostatic doping induced by the accumulation of Li + ions at the interface between FeSe0.5Te0.5 and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which Tc is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions. PACS numbers: 74.25.F-, 74.70.Xa, 74.78.-w Tuning carrier concentration is one of the most powerful approaches in the condensed matter physics for the explorations of novel quantum phases and exotic electronic properties as well as their underlying physical mechanics [1-8]. To overcome the inherent doping limit in the material synthetic methods, field effect transistor (FET) configurations have been applied to tune material properties using gating by electric field [9]. Two types of FET, metal-insulator-semiconductor (MIS) FET and electric double layer (EDL) FET, are widely used to control the charge carrier density on the surface of materials [10, 11]. In order to change the carrier density in the bulk, the so-called ionic field-effect transistor (iFET) with gel-like electrolyte as the gate medium has been used to drive Li + ions into layered materials. This type of FET configuration can effectively modulate 1T-TaS 2 electronic properties by the tunable Li + ion intercalation [12]. However, the heavily-doped electronic states in all these FET configurations are confined at the interfaces or overlaid with electrolyte, which prevents them from being characterized by many physical measurements. On the other hand, conventional MIS-FET devices cannot provide sufficient carriers to induce novel phases, such as superconductivity, by electrostatic doping, and the liquid or gel-like electrolyte is not compatible with modern solid electronics and may react with samples when gating voltage is applied [11, 13, 14]. Recently, we have fabri-[ †] These authors contributed equally to this work.

Superconducting epitaxial FeSe0.5Te0.5 thin films are prepared on SrTiO3(001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy are studied with different experimental techniques: x-rays diffraction, reflection high energy electron diffraction, scanning tunneling microscopy and atomic force microscopy. The substrate temperature during the deposition is found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate, without evidence of any other orientation. Moreover, such films exhibit a metallic behavior over the whole measured temperature range and the critical temperature is above 17 K, which is higher than the target value.

Accessing, controlling and understanding nanoscale friction and dissipation is a crucial issue in nanotechnology, where moving elements are central [1][2][3][4]. Recently, ultra-sensitive noncontact pendulum Atomic Force Microscope (AFM) succeeded in detecting the electronic friction drop caused by the onset of superconductivity in Nb [5], raising hopes that a wider variety of mechanisms of mechanical dissipation arising from electron organization into different collective phenomena will become accessible through this unconventional surface probe. Among them, the driven phase dynamics of charge-density-waves (CDWs) represents an outstanding challenge as a source of dissipation. Here we report a striking multiplet of AFM dissipation peaks arising at nanometer distances above the surface of NbSe2 -a layered compound exhibiting an incommensurate CDW. Each peak appears at a well defined tip-surface interaction force of the order of a nN, and persists until T = 70 K where CDW short-range order is known to disappear. A theoretical model is presented showing that the peaks are connected to tip-induced local 2π CDW phase slips. Under the attractive potential of the approaching tip, the local CDW surface phase landscape deforms continuously until a series of 2π jumps occur between different values of the local phase. As the tip oscillates to and fro, each slip gives rise to a hysteresis cycle, appearing at a selected distance, the dissipation corresponding to "pumping" in and out a local slip in the surface CDW phase of NbSe2.Bodies in relative motion separated by large vacuum gaps of one or more nanometers experience a tiny frictional force, whose nature is now beginning to be accessible [6]. Measuring these minute frictional forces, and relating them to the underlying physics and collective phenomena of electrons, ions, spins and their phase transitions represents a challenging and valuable prospective form of local spectroscopy of the solid surface under the tip. Although extremely delicate, this noncontact form of friction is now measurable by a highly sensitive AFM cantilever oscillating like a pendulum over the surface [5,7,8]. The pendulum configuration takes advantage of very small spring constants, typically in the order of mN/m. The corresponding minimal detectable noncontact friction is extremely small, about Γ pend 0 ∼ 10 −12 kg/s, compared with Γ TF 0 ∼ 10 −7 kg/s of more conventional tuning fork tips (see Methods for details), which work at smaller distances, just outside the repulsive regime. Here we use the pendulum to demonstrate a novel example of noncontact friction, occurring when the vibrating tip pumps integer 2π slips onto the local surface phase of a CDW.Layered dichalchogenides have long been known for their phase transitions leading to picometer sized superstructure lattice distortions and corresponding new electronic periodicities in their low temperature ground state [9]. Among them, NbSe 2 (with 2H stacking) stands out as a material exhibiting bulk CDW below the long range order onset tempera...