C.E.M. de Oliveira

Thermal stability of Pt Schottky contacts to 4H–SiC

Depth profiles by x-ray photoelectron spectroscopy have been used in conjunction with current–voltage measurements to study the thermal stability of a 50-nm-thick Pt contact to n-4H–SiC substrate. A reaction between the Pt and the SiC substrate is observed at temperatures of 600 °C and above. Annealing below that temperature improves the ideality and the uniformity of the Schottky characteristics, while annealing above this temperature degrades the electrical performance and uniformity. Thermodynamic stability is not reached even after annealing for 1 h at 900 °C. A local improvement of the characteristics at 800 °C is correlated with the formation of a second graphite film in the Pt–SiC reaction.

Super-crystals in composite ferroelectrics

As atoms and molecules condense to form solids, a crystalline state can emerge with its highly ordered geometry and subnanometric lattice constant. In some physical systems, such as ferroelectric perovskites, a perfect crystalline structure forms even when the condensing substances are non-stoichiometric. The resulting solids have compositional disorder and complex macroscopic properties, such as giant susceptibilities and non-ergodicity. Here, we observe the spontaneous formation of a cubic structure in composite ferroelectric potassium–lithium–tantalate–niobate with micrometric lattice constant, 104 times larger than that of the underlying perovskite lattice. The 3D effect is observed in specifically designed samples in which the substitutional mixture varies periodically along one specific crystal axis. Laser propagation indicates a coherent polarization super-crystal that produces an optical X-ray diffractometry, an ordered mesoscopic state of matter with important implications for critical phenomena and applications in miniaturized 3D optical technologies.

Control of laser plasma accelerated electrons for light sources

With gigaelectron-volts per centimetre energy gains and femtosecond electron beams, laser wakefield acceleration (LWFA) is a promising candidate for applications, such as ultrafast electron diffraction, multistaged colliders and radiation sources (betatron, compton, undulator, free electron laser). However, for some of these applications, the beam performance, for example, energy spread, divergence and shot-to-shot fluctuations, need a drastic improvement. Here, we show that, using a dedicated transport line, we can mitigate these initial weaknesses. We demonstrate that we can manipulate the beam longitudinal and transverse phase-space of the presently available LWFA beams. Indeed, we separately correct orbit mis-steerings and minimise dispersion thanks to specially designed variable strength quadrupoles, and select the useful energy range passing through a slit in a magnetic chicane. Therefore, this matched electron beam leads to the successful observation of undulator synchrotron radiation after an 8 m transport path. These results pave the way to applications demanding in terms of beam quality.Electron beam quality in accelerators is crucial for light source application. Here the authors demonstrate beam conditioning of laser plasma electrons thanks to a specific transport line enabling the control of divergence, energy, steering and dispersion and the application to observe undulator radiation.

Measuring Curie temperature on potassium lithium tantalate niobate crystals : an innovative approach

A new approach to measure Curie temperature (TC) of materials with phase transition in the range from ?5 ?C to +70 ?C is presented. In this work, measurements on potassium lithium tantalate niobate (KLTN) crystals are used to demonstrate the method. In this new approach the measurement is done directly over KLTN plates after cutting, without any additional processing. Contacts of conductive rubber are used as measurement points on the plates. Compared to traditional methods of TC measurement, the presented method is faster, less expensive, non-destructive and easily enables the TC topography on KLTN plates. The measurement set-up presented in this paper is fully automated and can measure TC of 64 points on plates with area up to 40 mm ? 40 mm with a resolution of 0.1 ?C.