Tracking Primary Thermalization Events in Graphene with Photoemission at Extreme Time Scales.
Physical review letters 115:8 (2015) 086803
Abstract:
Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is, instead, predicted to be dominant at the earliest time delays. Here, <8 fs extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time-and angle-resolved photoemission spectroscopy. Over a time window of approximately 25 fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme time scales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for petahertz electronics.Trace phase detection and strain characterization from serial X-ray free-electron laser crystallography of a Pr0.5Ca0.5MnO3 powder
Powder Diffraction Cambridge University Press (CUP) 30:S1 (2015) s25-s30
Wavelength-dependent optical enhancement of superconducting interlayer coupling in La1.885Ba0.115CuO4
Physical Review B American Physical Society (APS) 91:17 (2015) 174502
Population inversion in monolayer and bilayer graphene.
Journal of physics. Condensed matter : an Institute of Physics journal 27:16 (2015) 164204
Abstract:
The recent demonstration of saturable absorption and negative optical conductivity in the Terahertz range in graphene has opened up new opportunities for optoelectronic applications based on this and other low dimensional materials. Recently, population inversion across the Dirac point has been observed directly by time- and angle-resolved photoemission spectroscopy (tr-ARPES), revealing a relaxation time of only ∼130 femtoseconds. This severely limits the applicability of single layer graphene to, for example, Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived population inversion in bilayer graphene. The effect is attributed to the small band gap found in this compound. We propose a microscopic model for these observations and speculate that an enhancement of both the pump photon energy and the pump fluence may further increase this lifetime.Proposed parametric cooling of bilayer cuprate superconductors by terahertz excitation.
Physical review letters 114:13 (2015) 137001