412 research outputs found
Diffusion of Nonequilibrium Quasiparticles in a Cuprate Superconductor
We report a transport study of nonequilibrium quasiparticles in a high-Tc
cuprate superconductor using the transient grating technique. Low-intensity
laser excitation (at photon energy 1.5 eV) was used to introduce a spatially
periodic density of quasiparticles into a high-quality untwinned single crystal
of YBa2Cu3O6.5. Probing the evolution of the initial density through space and
time yielded the quasiparticle diffusion coefficient, and both inelastic and
elastic scattering rates. The technique reported here is potentially applicable
to precision measurement of quasiparticle dynamics, not only in cuprate
superconductors, but in other electronic systems as well.Comment: 5 pages, 4 figure
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
Semiconducting-to-metallic photoconductivity crossover and temperature-dependent Drude weight in graphene
We investigated the transient photoconductivity of graphene at various
gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We
demonstrated that graphene exhibits semiconducting positive photoconductivity
near zero carrier density, which crosses over to metallic negative
photoconductivity at high carrier density. Our observations are accounted for
by considering the interplay between photo-induced changes of both the Drude
weight and the carrier scattering rate. Notably, we observed multiple sign
changes in the temporal photoconductivity dynamics at low carrier density. This
behavior reflects the non-monotonic temperature dependence of the Drude weight,
a unique property of massless Dirac fermions
Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system
We report time-resolved measurements of the photoinduced change in
reflectivity, Delta R, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O(8+delta) (BSCCO) system of
cuprate superconductors as a function of hole concentration. We find that the
kinetics of quasiparticle decay and the sign of Delta R both change abruptly
where the superconducting transition temperature Tc is maximal. These
coincident changes suggest that a sharp transition in quasiparticle dynamics
takes place precisely at optimal doping in the BSCCO system.Comment: 10 pages, 4 figure
Ultrafast dynamics in the presence of antiferromagnetic correlations in electron-doped cuprate LaCeCuO
We used femtosecond optical pump-probe spectroscopy to study the photoinduced
change in reflectivity of thin films of the electron-doped cuprate
LaCeCuO (LCCO) with dopings of x0.08 (underdoped) and
x0.11 (optimally doped). Above T, we observe fluence-dependent
relaxation rates which onset at a similar temperature that transport
measurements first see signatures of antiferromagnetic correlations. Upon
suppressing superconductivity with a magnetic field, it is found that the
fluence and temperature dependence of relaxation rates is consistent with
bimolecular recombination of electrons and holes across a gap (2)
originating from antiferromagnetic correlations which comprise the pseudogap in
electron-doped cuprates. This can be used to learn about coupling between
electrons and high-energy () excitations in these
compounds and set limits on the timescales on which antiferromagnetic
correlations are static
Nonlinear optical probe of tunable surface electrons on a topological insulator
We use ultrafast laser pulses to experimentally demonstrate that the
second-order optical response of bulk single crystals of the topological
insulator BiSe is sensitive to its surface electrons. By performing
surface doping dependence measurements as a function of photon polarization and
sample orientation we show that second harmonic generation can simultaneously
probe both the surface crystalline structure and the surface charge of
BiSe. Furthermore, we find that second harmonic generation using
circularly polarized photons reveals the time-reversal symmetry properties of
the system and is surprisingly robust against surface charging, which makes it
a promising tool for spectroscopic studies of topological surfaces and buried
interfaces
Real time observation of cuprates structural dynamics by Ultrafast Electron Crystallography
The phonon-mediated attractive interaction between carriers leads to the
Cooper pair formation in conventional superconductors. Despite decades of
research, the glue holding Cooper pairs in high-temperature superconducting
cuprates is still controversial, and the same is true as for the relative
involvement of structural and electronic degrees of freedom. Ultrafast electron
crystallography (UEC) offers, through observation of spatio-temporally resolved
diffraction, the means for determining structural dynamics and the possible
role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse
excites the charge carriers, which relax through electron-electron and
electron-phonon coupling, and the consequential structural distortion is
followed diffracting fs electron pulses. In this review, the recent findings
obtained on cuprates are summarized. In particular, we discuss the strength and
symmetry of the directional electron-phonon coupling in Bi2Sr2CaCu2O8+\delta
(BSCCO), as well as the c-axis structural instability induced by near-infrared
pulses in La2CuO4 (LCO). The theoretical implications of these results are
discussed with focus on the possibility of charge stripes being significant in
accounting for the polarization anisotropy of BSCCO, and cohesion energy
(Madelung) calculations being descriptive of the c-axis instability in LCO
Observation of spin Coulomb drag in a two-dimensional electron gas
An electron propagating through a solid carries spin angular momentum in
addition to its mass and charge. Of late there has been considerable interest
in developing electronic devices based on the transport of spin, which offer
potential advantages in dissipation, size, and speed over charge-based devices.
However, these advantages bring with them additional complexity. Because each
electron carries a single, fixed value (-e) of charge, the electrical current
carried by a gas of electrons is simply proportional to its total momentum. A
fundamental consequence is that the charge current is not affected by
interactions that conserve total momentum, notably collisions among the
electrons themselves. In contrast, the electron's spin along a given spatial
direction can take on two values, "up" and "down", so that the spin current and
momentum need not be proportional. Although the transport of spin polarization
is not protected by momentum conservation, it has been widely assumed that,
like the charge current, spin current is unaffected by electron-electron (e-e)
interactions. Here we demonstrate experimentally not only that this assumption
is invalid, but that over a broad range of temperature and electron density,
the flow of spin polarization in a two-dimensional gas of electrons is
controlled by the rate of e-e collisions
Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr_2IrO_4 from time-resolved photocarrier dynamics
We perform a time-resolved optical study of Sr_2IrO_4 to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J_(eff) = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T_N the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J_(eff) = 1/2 band undergoes the sharpest upturn at TN, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr_2IrO_4 to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T_N in transport and thermodynamic measurements
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