106 research outputs found
Photoexcited electron dynamics in Kondo insulators and heavy fermions
We have studied the photoexcited carrier relaxation dynamics in the Kondo
insulator SmB6 and the heavy fermion metal YbAgCu4 as a function of temperature
and excitation level. The dynamic response is found to be both strongly
temperature dependent and nonlinear. The data are analyzed with a
Rothwarf-Taylor bottleneck model, where the dynamics are governed by the
presence of a narrow gap in the density of states near the Fermi level. The
remarkable agreement with the model suggests that carrier relaxation in a broad
class of heavy electron systems (both metals and insulators) is governed by the
presence of a (weakly temperature dependent) hybridization gap.Comment: accepted for publication in Physical Review Letter
Josephson Plasma Resonance in TlBaCaCuO in a Magnetic Field Measured using THz Spectroscopy
We report the first measurements of the -axis Josephson Plasma Resonance
(JPR) in TlBaCaCuO as a function of temperature with and
without a -axis magnetic field using terahertz time-domain spectroscopy in
transmission. The JPR is sensitive to the alignment of pancake vortices along
the -axis, and is observed to decrease when applying a magnetic field as
expected.Comment: 3 pages, 3 ps-figures (can also be obtained from
[email protected]), to be published in Physica
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
Extraordinarily Efficient Conduction in a Redox-Active Ionic Liquid
Iodine added to iodide-based ionic liquids leads to extraordinarily efficient
charge transport, vastly exceeding that expected for such viscous systems.
Using terahertz time-domain spectroscopy, in conjunction with dc conductivity,
diffusivity and viscosity measurements we unravel the conductivity pathways in
1-methyl-3-propylimidazolium iodide melts. This study presents evidence of the
Grotthuss mechanism as a significant contributor to the conductivity, and
provides new insights into ion pairing processes as well as the formation of
polyiodides. The terahertz and transport results are reunited in a model
providing a quantitative description of the conduction by physical diffusion
and the Grotthuss bond-exchange process. These novel results are important for
the fundamental understanding of conduction in molten salts and for
applications where ionic liquids are used as charge-transporting media such as
in batteries and dye-sensitized solar cells.Comment: Article: 17 pages, 3 figures. Supplementary information: 14 pages, 6
figures. Table of contents figure: 1 page. Accepted for publication in
ChemPhysChe
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
Electronic compressibility and charge imbalance relaxation in cuprate superconductors
In the material SmLaSrCuO with alternating intrinsic
Josephson junctions we explain theoretically the relative amplitude of the two
plasma peaks in transmission by taking into account the spatial dispersion of
the Josephson Plasma Resonance in direction due to charge coupling. From
this and the magnetic field dependence of the plasma peaks in the vortex solid
and liquid states it is shown that the electronic compressibility of the
CuO layers is consistent with a free electron value. Also the London
penetration depth near can be
determined. The voltage response in the -curve of a
BiSrCaCuO mesa due to microwave irradiation or current
injection in a second mesa is related to the nonequilibrium charge imbalance of
quasiparticles and Cooper pairs and from our experimental data the relaxation
time is obtained.Comment: 2 pages, 2 figures, phc-proc4-auth.cls, to be published in Physica C
as a proceeding of M2S-HTSC Rio 200
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
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:
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.
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