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Register a new userNon-thermal hot electrons ultrafastly generating hot optical phonons in graphite
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Ultrafast dynamics of graphite is investigated by time-resolved photoemission spectroscopy. We observe spectral features of direct photoexcitations, non-thermal electron distributions, and recovery dynamics occurring with two time scales having distinct pump-power dependences. Additionally, we find an anomalous increase of the spectral intensity around the Fermi level, and we attribute this to spectral broadenings due to coupled optical phonons in the transient. The fingerprints of the coupled optical phonons occur from the temporal region where the electronic temperature is still not definable. This implies that there is a mechanism of ultrafast-and-efficient phonon generations beyond a two-temperature model.
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We have investigated the anisotropic thermal expansion of graphite using ab-initio calculation of lattice dynamics and anharmonicity of the phonons, which reveal that the negative thermal expansion (NTE) in the a-b plane below 600 K and very large positive thermal expansion along the c-axis up to high temperatures arise due to various phonons polarized along the c-axis. While the NTE arises from the anharmonicity of transverse phonons over a broad energy range up to 60 meV, the large positive expansion along the c-axis occurs largely due to the longitudinal optic phonon modes around 16 meV and a large linear compressibility along the c-axis. The hugely anisotropic bonding in graphite is found to be responsible for wide difference in the energy range of the transverse and longitudinal phonon modes polarized along the c-axis, which are responsible for the anomalous thermal expansion behavior. This behaviour is in contrast to other nearly isotropic hexagonal structures like water-ice, which show anomalous thermal expansion in a small temperature range arising from a narrow energy range of phonons.
We have developed a model describing the non-proportional response in scintillators based on non-thermalised carrier and phonon transport. We show that the thermalization of e-h distributions produced in scintillators immediately after photon absorption may take longer than the period over which the non-proportional signal forms. The carrier and LO-phonon distributions during this period remain non-degenerate at quasi-equilibrium temperatures far exceeding room temperature. We solve balance equations describing the energy exchange in a hot bipolar plasma of electrons/holes and phonons. Taking into account dynamic screening we calculate the ambipolar diffusion coefficient at all temperatures. The non-proportional light yields calculated for NaI are shown to be consistent with experimental data. We discuss the implications of a non-equilibrium model, comparing its predictions with a model based on the transport of thermalised carriers. Finally, evidence for non-equilibrium effects is suggested by the shape of non-proportionality curve and wide dispersion in data observed in K-dip spectroscopy near the threshold. A comparison of the predicted curves shows good agreement for deformation potential value in the range 7-8 eV.
Thin film metal-insulator-metal junctions are used in a novel approach to investigate the dissipation of potential energy of multiply charged ions impinging on a polycrystalline metal surface. The ion-metal interaction leads to excited electrons and holes within the top layer. A substantial fraction of these charge carriers is transported inwards and can be measured as an internal current in the thin film tunnel junction. In Ag-AlOX-Al junctions, yields of typically 0.1-1 electrons per impinging ion are detected in the bottom Al layer. The separate effects of potential and kinetic energy on the tunneling yield are investigated by varying the charges state of the Ar projectile ions from 2+ to 9+ for kinetic energies in the range from 1 to 12 keV. The tunneling yield is found to scale linearly with the potential energy of the projectile.
The mechanisms for spin relaxation in semiconductors are reviewed, and the mechanism prevalent in p-doped semiconductors, namely spin relaxation due to the electron-hole exchange interaction, is presented in some depth. It is shown that the solution of Boltzmann-type kinetic equations allows one to obtain quantitative results for spin relaxation in semiconductors that go beyond the original Bir-Aronov-Pikus relaxation-rate approximation. Experimental results using surface sensitive two-photon photoemission techniques show that the spin relaxation-time of electrons in p-doped GaAs at a semiconductor/metal surface is several times longer than the corresponding bulk spin relaxation-times. A theoretical explanation of these results in terms of the reduced density of holes in the band-bending region at the surface is presented.
In a recent letter [Phys. Rev. Lett. 105 (2010) 036804] the unoccupied electronic states of single layers of graphene on ruthenium are investigated. Here we comment on the interpretation, which deviates in four points from [J. Phys.: Condens. Matter 22 (2010) 302001] and outline the corresponding consequences.
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