El Dimoni de Maxwell

El dimoni de Maxwell és el resultat d'un experiment mental que va proposar el físic escocès James Clerk Maxwell (1831-1879), que si es complís amenaçaria la validesa de la segona llei de la termodinàmica. Segons aquest experiment, seria possible la transmissió de calor d'un cos a un altre de més calent sense cap altre canvi. S'hi ex- posen diverses solucions, que van des de la interacció entre la mesura i el sistema mesurat, fins a la teoria de la informació. Aquest article, originalment escrit en anglès, va rebre una menció especial en el Second Year Essay Prize (2008- 09) del Departament de Física de l'Imperial College. La traducció al català és del propi autor

Temperature Effects in the Band Structure of Topological Insulators.

We study the effects of temperature on the band structure of the Bi_{2}Se_{3} family of topological insulators using first-principles methods. Increasing temperature drives these materials towards the normal state, with similar contributions from thermal expansion and from electron-phonon coupling. The band gap changes with temperature reach 0.3 eV at 600 K, of similar size to the changes caused by electron correlation. Our results suggest that temperature-induced topological phase transitions should be observable near critical points of other external parameters

Strong electron-phonon coupling and carrier self-trapping in Sb2_2S3_3

Antimony sulphide (Sb$_2$S$_3$) is an Earth-abundant and non-toxic material that is under investigation for solar energy conversion applications. However, it still suffers from poor power conversion efficiency and a large open circuit voltage loss that have usually been attributed to point or interfacial defects and trap states. More recently, a self-trapped exciton has been suggested as the microscopic origin for the performance loss. By using first-principles methods, we demonstrate that Sb$_2$S$_3$ exhibits strong electron-phonon coupling, which results in a large renormalization of 200 meV of the absorption edge when temperature increases from 10K to 300K, and in a quasi-1D electron polaron that is delocalized in the ribbon direction of the crystal structure, but localized in the inter-ribbon directions. The calculated polaron formation energy of 67 meV agrees well with experimental measurements, suggesting that self-trapped excitons are likely to form with the mediation of an electron polaron. Our results demonstrate the importance of systematically investigating electron-phonon coupling and polaron formation in the antimony chalcogenide family of semiconductors for optoelectronic applications.Comment: 4 figure