468 research outputs found
Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds
We show that low-energy spectral features in the cuprates can be separated
into different components by the measurement of the recombination dynamics of
different low-energy excitations in real-time using femtosecond laser
spectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap
localized state relaxation processes exhibit qualitatively different time- and
temperature-dependences. The relaxation measurements also show the existence of
two distinct coexisting energy gaps near optimum doping and in the overdoped
region, one more or less temperature independent (which exists above and below
Tc) and one which closes at Tc in a mean-field like fashion. Thus systematic
studies of QP recombination as a function of doping and temperature suggest
that the ground state of the cuprates is a mixed Boson-Fermion system with
localised states present over the entire region of the phase diagram.Comment: 4 pages, acepted for publication in Physica C, invited paper given at
M2S, Feb. 20 - 25, 2000, Houston, US
Photoinduced absorption from localized intra-gap states
A model is developed for photoinduced absorption from localized states
observed in femtosecond pump-probe experiments in high-Tc superconductors and
other materials. The dynamics of localized carriers are described in terms of
phenomenological approach similar to that originaly proposed by Rothwarf and
Taylor. Expanding the relaxation rate in powers of the order parameter we have
shown that density of localized carriers is sensitive to Tc. From the analysis
of the experimental data on YBa2Cu3O(7-x) and K0.3MoO3 we conclude that
significant intra-gap density of localized states exists in these materials.
Temperature dependence of the density of photoexcited localized carriers in
underdoped YBa2Cu3O(7-x) and in K0.3MoO3 is consistent with the observation of
the pseudogap above Tc.Comment: 4 pages, 2 figures, acepted for publication in Physica C, invited
poster presented at M2S, Feb. 20 - 25, 2000, Houston, US
Ultrafast quasiparticle relaxation dynamics in normal metals and heavy fermion materials
We present a detailed theoretical study of the ultrafast quasiparticle
relaxation dynamics observed in normal metals and heavy fermion materials with
femtosecond time-resolved optical pump-probe spectroscopy. For normal metals, a
nonthermal electron distribution gives rise to a temperature (T) independent
electron-phonon relaxation time at low temperatures, in contrast to the
T^{-3}-divergent behavior predicted by the two-temperature model. For heavy
fermion compounds, we find that the blocking of electron-phonon scattering for
heavy electrons within the density-of-states peak near the Fermi energy is
crucial to explain the rapid increase of the electron-phonon relaxation time
below the Kondo temperature. We propose the hypothesis that the slower Fermi
velocity compared to the sound velocity provides a natural blocking mechanism
due to energy and momentum conservation laws.Comment: 10 pages, 11 figure
Low-energy electronic structure in Y1-xCaxBa2Cu3O7-y comparison of t ime-resolved optical spectroscopy, NMR, neutron and tunneling data
Time-resolved optical measurements give information on the quasiparticle
relaxation dynamics in YBCO, from which the evolution of the gap with doping
and temperature can be systematically deduced. In this paper these optical
charge-channel `pseudogap' data are compared with the `pseudogap' obtained from
the NMR Knight shift Ks, spin polarized neutron scattering (SPNS) and single
particle tunneling measurements. A simple energy level diagram is proposed to
explain the different `gap' magnitudes observed by different spectroscopies in
Y1-xCaxBa2Cu3O7-y, whereby the spin gap Delta_s in NMR and SPNS corresponds to
a triplet local pair state, while Delta_p in the charge excitation spectrum
corresponds to the pair dissociation energy. At optimum doping and in the
overdoped state, an additional T-dependent gap becomes evident, which closes at
T_c, suggesting a cross-over to a more conventional BCS-like superconductivity
scenario.Comment: 9 pages, 4 figures. Presented in HTS99, Miami, January 9
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