345 research outputs found
Phase diagram of the screened two-dimensional electron crystal
No full textThe melting line of the two-dimensional crystal formed by surface electrons above liquid helium films adsorbed on PMMA has been mapped out by means of a microwave cavity technique at temperatures above 1 K. The influence of the dielectric substrate on such a transition has been measured at different temperatures and at various helium film thicknesses. The results are found to be in good agreement with existing theories. In general, screening reduces the stability range of the Wigner crystal in the phase diagram of the surface electrons. (C) 1998 Elsevier Science B.V. All rights reserved
Microwave study of screened two-dimensional electron crystals on helium films
No full textWe report investigations at microwave frequencies on surface-state electrons (SSE) which were supported by helium films adsorbed on different polymer substrates at temperatures above 1 K. The real and imaginary parts of the dielectric response epsilon(k,omega) of the SSE were measured at fixed k and omega inside a 10 GHz microwave cavity. The solidification of the two-dimensional electron crystal was detected by a kink occurring simultaneously in both components of epsilon(k,omega) as the electron density was slowly increased at constant temperature. The influence of the dielectric substrate on such a freezing transition has been measured at different temperatures and at various helium-film thicknesses. The results are found to be in good agreement with existing theories. This work represents the first systematic study of a screened two-dimensional electron crystal on helium
Evidence for quantum melting in the two-dimensional electron system on a thin helium film
No full textThe real and imaginary parts of the dielectric response of surface state electrons (SSE) on helium films adsorbed on oxidized Si platelets have been measured with a microwave cavity at 10 GHz. Preliminary measurements taken at T=1.2 K show an abrupt increase of the SSE mobility at electron densities near 10(11) cm(-2), which is suggestive of quantum melting of the Wigner solid. Reproducibility of this effect on different Si wafers is discussed
Cyclotron resonance for two-dimensional electrons on thin helium films
No full textWe present a systematic investigation of the microwave absorption for two-dimensional electron layers on thin helium films and in the presence of a cyclotron resonance (CR) magnetic field. To explain the measured data, a recently proposed two-fraction structure of the electron system is used and here described in detail. Hereby the problem of substrate roughness, usually always present for electrons on thin helium films, is taken into account and it turns out to be an important parameter. Within this model the general structure of the microwave absorption becomes understandable and the fraction of localized and free electrons can be precisely determined. The details of the observed asymmetry and shift of the CR line shape are discussed
Microwave study of surface electrons on helium films in a magnetic field
No full textWe report preliminary results of a cyclotron resonance study of surface electrons (SE) on saturated helium films covering a PMMA substrate at T > 1 K. The real and imaginary parts of the dielectric response epsilon(k, omega) of the SE are measured at fixed k and omega in B-fields up to 10 T in a 12 GHz cavity. The cyclotron resonance of the SE is determined at different helium film thicknesses d(He) and at various electron densities. At small d(He) we find significant anomalies in the cyclotron resonance lineshape and position. As d(He) increases the lineshape becomes progressively more symmetric and its peak moves towards the cyclotron field value expected for a free electron. To fit these data we have modified the classical Drude expression, introducing two different relaxation times for the low and high B-field regions. The phenomenological formulas fit the data quite well. A systematic theoretical analysis of these results is in progress
Exploiting optical near fields for phase change memories
No full textWe apply a recently developed technique based on optical near fields to achieve reversible phase switching in Ge2 Sb2 Te 5 films. By placing dielectric microspheres at the film surface and exposing them to pulsed laser light, a complex intensity distribution due to the optical near field can be created at the film surface. We demonstrate writing and erasing operations of patterns through phase switching. Spheres can be removed after an operation by optical near fields without ablation. Data erasure is achieved with and without near fields. The erasure method used can be determined from the result and erased information can be retrieved although being inverted. Three distinct material states are identified within patterns, showing clear contrast and sharp borders between them, thus opening the possibility of three-level data storage. Our results suggest that optical near fields are a promising candidate for developing strategies in data storage, encryption, and multiplexing. © 2011 American Institute of Physics.Peer Reviewe
Quantum fluids on alkali metal surfaces
No full textThe behaviour of quantum fluid films on alkali metal surfaces is investigated. Using the quartz microbalance technique, the wetting phase diagram can be explored by adsorption isotherm measurements. Optically excited surface plasmons are used to obtain real-time images of the dynamical behaviour of He-4 droplets on, cesiated surfaces
Superhard, conductive coatings for atomic force microscopy cantilevers
No full textBoron carbide thin films were grown by mass selected ion beam deposition using low energy 11B+ and 12C+ ions at room temperature. The amorphous films exhibit any desired stoichiometry controlled by the ion charge ratio B+/C+. Films with a stoichiometry of B4C showed the optimal combination of a high mechanical strength and a low electrical resistivity for the coating of atomic force microscopy (AFM) silicon cantilevers. The properties of such AFM tips were evaluated and simultaneous topography and Kelvin mode AFM measurements with high lateral resolution were performed on the systems (i) Au nanoparticles on a p-WS2 surface and (ii) conducting/ superconducting YBa2Cu3O72x.publishe
Two-fraction electron system on a thin helium film
No full textA systematic theoretical investigation of microwave absorption of 2-dimensional electron systems above a thin helium film in the presence of a cyclotron resonance magnetic field is presented. To explain the measured data, a two-fraction structure of the electron system is introduced. One component corresponds to the free electron motion, the second one takes into account electron localization near the potential minimum caused by the roughness of the substrate. Within this model the general dependence of microwave absorption becomes understandable. The details of the observed cyclotron resonance line-shift are discussed
TIME RESOLVED DYNAMICS OF ELECTRONIC EXCITATIONS IN
No full text S. Minemoto, J. M\""{u}ller, G. Gantef\""{o}r, H. J. M\""{u}nzer, J. Boneberg, and P. Leiderer, Phys. Rev. Lett. accepted for publicationAuthor Institution: Universit\""at KonstanzExperiments have been performed an the ultrafast electron dynamics of mass selected cluster anions. The setup consists of a pulsed arc cluster ion source (PACIS), a time-of-flight mass spectrometer, a magnetic-bottle-type time of flight electron spectrometer, and a femtosecond laser. Time resolved pump/probe photoelectron spectra are recorded. We present first data on . Photoelectron spectra were obtained for a set of increasing delays between pump pulse (excitation of the anion) and probe pulse (detachment of the ad-electron). Interpretation of the series of spectra yields an assignment of the involved electron states and, moreover, the lifetime of the excited state. The method is suitable to study all kinds of electronic excitation and relaxation processes in mass selected nanoparticles. [FIGURE
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