Showing papers by "Ricardo P. S. M. Lobo published in 2002"
TL;DR: It is shown that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.
Abstract: We have investigated the polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots by pump-probe midinfrared spectroscopy. A long T1 polaron decay time is measured at both low temperature and room temperature, with values around 70 and 37 ps, respectively. The decay time decreases for energies closer to the optical phonon energy. The relaxation is explained by the strong coupling for the electron-phonon interaction and by the finite lifetime of the optical phonons. We show that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.
115 citations
TL;DR: In this article, the ab-plane reflectance of Bi-2212 thin films was measured in the $30 -25000 -€ 25000 range for one under-doped sample and one overdoped sample.
Abstract: The ab-plane reflectance of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi-2212) thin films was measured in the $30--25000{\mathrm{cm}}^{\ensuremath{-}1}$ range for one underdoped ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}70\mathrm{K}$), and one overdoped sample ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}63\mathrm{K}$) down to 10 K. We find similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above ${T}_{c}$, the effective spectral weight, obtained from the integrated conductivity, does not decrease when $T$ decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening in the $\mathbf{k}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(0,\ensuremath{\pi})$ direction and of the in-plane infrared conductivity being mostly sensitive to the $\mathbf{k}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{\pi},\ensuremath{\pi})$ direction.
64 citations
TL;DR: In this article, the abplane reflectivity of thin films was measured in the $30\char21{}30000 {\mathrm{cm}}^{\ensuremath{-}1}$ range for samples with $x=0$.
Abstract: The ab-plane reflectivity of $({\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}){\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ thin films was measured in the $30\char21{}30000 {\mathrm{cm}}^{\ensuremath{-}1}$ range for samples with $x=0$ ${(T}_{c}=90 \mathrm{K}),$ $x=0.4$ ${(T}_{c}=35 \mathrm{K}),$ and $x=0.5$ ${(T}_{c}=19 \mathrm{K})$ as a function of temperature in the normal state. The effective charge density obtained from the integrated spectral weight decreases with increasing x. The variation is consistent with the higher dc resistivity for $x=0.4,$ but is one order of magnitude smaller than what would be expected for $x=0.5.$ In the latter sample, the conductivity is dominated at all temperatures by a large localization peak. Its magnitude increases as the temperature decreases. We relate this peak to the dc resistivity enhancement. A simple localization-by-disorder model accounts for the optical conductivity of the $x=0.5$ sample.
12 citations
07 Nov 2002
TL;DR: In this paper, the abplane infrared and visible response of Bi2Sr2CaCu2O8þd (Bi-2212) thin films has been measured between 300 and 10 K for different doping levels, and the spectral weight change, below the critical temperature and beyond the intraband transitions, may be assigned to a change of electronic kinetic energy, a figure which addresses the issue of a kinetic energy driven mechanism.
Abstract: The ab-plane infrared and visible (3 meV–3 eV) response of Bi2Sr2CaCu2O8þd (Bi-2212) thin films has been measured between 300 and 10 K for different doping levels. In the superconducting state, dramatic differences appear between the underdoped and optimally/overdoped regimes regarding the electrodynamics of the formation of the superfluid condensate. In the optimally doped and overdoped regimes, the superfluid grows up by removing states from energies below 60 meV. This energy is of the order of a few times the superconducting gap. In this respect, optimally doped and overdoped Bi-2212 exhibit a conventional behavior. In the underdoped regime, states extending up to 2 eV, far beyond any conventional scale for a BCS-like mechanism, contribute to the superfluid. The spectral weight change, below the critical temperature and beyond the intraband transitions, may be assigned to a change of electronic kinetic energy � 1 meV, a figure which addresses the issue of a kinetic energy driven mechanism.