Seminarios y Eventos
Materiales para las Tecnologías de la Información
Cordinador: Harvey Amorín
07 de mayo de 2019, 12:00 h. Salón de Actos
Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors
Aday J. Molina-Mendoza
Institute of Photonics, Vienna University of Technology, Vienna, Austria
Monolayer transition metal dichalcogenide (TMD) semiconductors provide a unique platform to study light-matter interaction and many-body effects at the atomic scale. The strong Coulomb interaction in these materials leads to the formation of tightly bound electron-hole pairs (excitons). Moreover, excitons residing in the two different valleys at the K points of the Brillouin zone can also interact, giving rise to four- and five-particle states, which have been recently identified in tungsten diselenide (WSe2) as neutral and charged biexcitons, as well as biexcitons in WS2.
In this talk, I will present the electroluminescence from monolayer WSe2 and WS2 by pulsed transient EL, which triggers the formation of exciton complexes and thus their light emission. The high sample quality, enabled by encapsulating the monolayer semiconductor in hexagonal boron nitride (hBN), allows for the observation of electroluminescence from multi-particle exciton complexes, including neutral and negatively charged biexcitons, with narrow emission linewidths down to 2.8 meV. Furthermore, by tailoring the pulse parameters, it is possible to create either an electron- or a hole-rich environment in the 2D semiconductor, consequently favoring the enhanced or diminished emission from the different exciton species. Our technique extends and complements gate-dependent PL spectroscopy and will enable further investigations of many-body phenomena in 2D materials. From an applied point of view, our devices may find application as wavelength tunable light emitters or furnish new opportunities for quantum light sources.
20 de febrero de 2019, 12:00 h. Salón de Actos
Magnetocaloric Effect: From Energy Efficient Refrigeration to Fundamental Studies of Phase Transitions
University of Seville, Seville, Spain
The magnetocaloric effect, that is, the reversible temperature change experienced by a magnetic material upon the application or removal of a magnetic field, has become a topic of increasing research interest due to its potential applications in refrigeration at ambient temperature that is energy efficient and environmentally friendly. From a technological point of view, the improvement of magnetic refrigeration systems can have a notable impact on society: a large fraction of the electricity consumed in residential and commercial markets is used for temperature and climate control. From the point of view of magnetic materials, research on this topic mainly focuses on the discovery of new materials with lower cost and enhanced performance. In addition, the characterization of the magnetocaloric effect can be used for more fundamental studies of the characteristics of phase transitions.
I will cover an overview of the phenomenon and a classification of the most relevant families of alloys and compounds. I will analyze possible limitations for the optimal performance of the materials in magnetic refrigerators, including hysteretic response and cyclability. Regarding phase transitions, I will present a new method to quantitatively determine the order of thermomagnetic phase transitions using the field dependence of the magnetic entropy change. For second-order phase transition materials, I will show that critical exponents can be determined using the magnetocaloric effect even in cases where the usual methods are not applicable. In the case of first-order phase transitions, more details about their hysteretic response can be obtained using T-FORC.
28 de enero de 2019, 10:30 h. Sala de Seminarios, 182
Infrared spectromicroscopy and imaging with six decades of dynamic range
Soleil Synchrotron, France
Infrared spectroscopy has been in the toolbox of scientists from a variety of fields for many decades to obtain information about vibrational properties and low energy electrodynamics. The beginning of the 1980s brought the first commercial far-field infrared microscopes and the possibility to look into the details. Infrared spectromicroscopy had been pushed to its limits starting in the 1990s in synchrotron facilities by exploiting the unmatchable quality of synchrotron radiation, i.e. low angular divergence and extremely broad bandwidth. Synchrotron infrared spectromicroscopy beamlines provide diffraction limited spatial resolution covering the whole IR range and allowing measurements not possible otherwise. Using two-dimensional detectors in far-field instruments allowed measurements of very large area samples with high spatial resolution. The turn of the century brought the advent of near-field IR techniques and thus the breaking of the diffraction limit. Combining high brightness IR sources with atomic force microscopes to detect either photothermal expansion or near-field scattering allowed measurements hundreds of times below the diffraction limit reaching as high as ten-nanometer spatial resolution. Most recently, optically sampled photothermal spectromicroscopy has become available to bridge to resolution gap between the nanometer and micrometer range and provide sub-diffraction limited information relevant to various kinds of samples.
The instrumentation at the SMIS beamline in SOLEIL covers six orders of magnitude spatial dynamic range, therefore providing an unprecedented facility that employs all of the above-mentioned techniques combined with custom instrumentation to support scientific discoveries. In my talk, I will review the capabilities of SMIS through experiments done by SMIS staff and users highlighting a variety of fields and also comment on the benefits of emerging, alternative sources.