45 research outputs found
Ultrafast (but Many-Body) Relaxation in a Low-Density Electron Glass
We present a study of the relaxation dynamics of the photoexcited
conductivity of the impurity states in the low-density electronic glass,
phosphorous-doped silicon Si:P. Using optical pump-terahertz probe spectroscopy
we find strongly temperature and fluence dependent glassy power-law relaxation
occurring over sub-ns time scales. Such behavior is in contrast to the much
longer time scales found in higher electron density glassy systems. We also
find evidence for both multi-particle relaxation mechanisms and/or coupling to
electronic collective modes and a low temperature quantum relaxational regime.Comment: 4 pages, 4 figures, Appeared in Phys. Rev. Let
Critical Quadrupole Fluctuations and Collective Modes in Iron Pnictide Superconductors
The multiband nature of iron pnictides gives rise to a rich
temperature-doping phase diagram of competing orders and a plethora of
collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural
transition is closely followed by a spin density wave transition both being in
close proximity to the superconducting phase. A key question is the nature of
high- superconductivity and its relation to orbital ordering and
magnetism. Here we study the NaFeCoAs superconductor using
polarization-resolved Raman spectroscopy. The Raman susceptibility displays
critical enhancement of non-symmetric charge fluctuations across the entire
phase diagram which are precursors to a -wave Pomeranchuk instability at
temperature \theta(\mbox{x}). The charge fluctuations are interpreted in
terms of quadrupole inter-orbital excitations in which the electron and hole
Fermi surfaces breathe in-phase. Below , the critical fluctuations acquire
coherence and undergo a metamorphosis into a coherent ingap mode of
extraordinary strength.Comment: 21 pages, 15 figures. To appear in Phys. Rev.
Reflectivity and Microwave Absorption in Crystals with Alternating Intrinsic Josephson Junctions
We compute the frequency and magnetic field dependencies of the reflectivity
in layered superconductors with two alternating intrinsic Josephson
junctions with different critical current densities and quasiparticle
conductivities for the electric field polarized along the c-axis. The parameter
describing the electronic compressibility of the layers and the charge
coupling of neighboring junctions was extracted for the
SmLaSrCuO superconductor from two independent
optical measurements, the fit of the loss function at zero magnetic
field and the magnetic field dependence of the peak positions in .
The experiments are consistent with a free electron value for near
the Josephson plasma frequencies.Comment: 4 pages, 4 postscript figures, misprints in table correcte
Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors
We present a comparative study of ultrafast photo-conversion dynamics in
tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical
pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is
activated and temperature-independent respectively, demonstrating dominant
singlet-triplet exciton fission. In Pc films (as well as C-doped films)
this decay channel is suppressed by electron trapping. These results
demonstrate the central role of crystallinity and purity in photogeneration
processes and will constrain the design of future photovoltaic devices.Comment:
Evidence for LineLike Vortex Liquid Phase in TlBaCaCuO Probed by the Josephson Plasma Resonance
We measured the Josephson plasma resonance (JPR) in optimally doped
TlBaCaCuO thin films using terahertz time-domain
spectroscopy in transmission. The temperature and magnetic field dependence of
the JPR frequency shows that the c-axis correlations of pancake vortices remain
intact at the transition from the vortex solid to the liquid phase. In this
respect TlBaCaCuO films, withanisotropy parameter
, are similar to the less anisotropic
YBaCuO rather than to the most
anisotropic BiSrCaCuO single crystals ).Comment: Submitted to Physical Review Letter
Structure of vortex liquid phase in irradiated BSCCO(2212) crystals
The c-axis resistivity in irradiated and in pristine BSCCO(2212) crystals is
measured as a function of the in-plane magnetic field component at fixed
out-of-plane component B_\perp in the vortex liquid phase at T=67 K. From this
data we extract the dependence of the phase difference correlation length
inside layers on B_\perp and estimate the average length of pieces of vortex
lines confined inside columnar defects as a function of the filling factor
f=B_\perp / B_\phi. The maximum length, about 15 interlayer distances, is
reached near f=0.35.Comment: 4 pages, 4 figure
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Ultrafast optical pump terahertz-probe spectroscopy of strongly correlated electron materials.
We have used optical-pump far-infrared probe spectroscopy to probe the low energy electron dynamics of high temperature superconductors and colossal magnetoresistance manganites. For the superconductor YBa2Cu3O7, picosecond conductivity measurements probe the interplay between Cooper-pairs and quasiparticles. In optimally doped films, the recovery time for long-range phase-coherent pairing increases from {approx}1.5 ps at 4K to {approx}3.5 ps near Tc, consistent with the closing of the superconducting gap. For underdoped films, the measured recovery time is temperature independent (3.5 ps) in accordance with the presence of a pseudogap. Ultrafast picosecond measurements of optically induced changes in the absolute conductivity of La0:7M0:3MnO3 thin films (M = Ca, Sr) from 10K to {approx}0.9Tc reveal a two-component relaxation. A fast, {approx}2 ps, conductivity decrease arises from optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. Our results indicate that for T<<Tc, the conductivity is determined by incoherent phonons while spin fluctuations dominate near Tc