492 research outputs found
Broadband electromagnetic response and ultrafast dynamics of few-layer epitaxial graphene
We study the broadband optical conductivity and ultrafast carrier dynamics of
epitaxial graphene in the few-layer limit. Equilibrium spectra of nominally
buffer, monolayer, and multilayer graphene exhibit significant terahertz and
near-infrared absorption, consistent with a model of intra- and interband
transitions in a dense Dirac electron plasma. Non-equilibrium terahertz
transmission changes after photoexcitation are shown to be dominated by excess
hole carriers, with a 1.2-ps mono-exponential decay that reflects the
minority-carrier recombination time.Comment: 4 pages, 3 figures, final versio
Stimulated emission of Cooper pairs in a high-temperature cuprate superconductor
The concept of stimulated emission of bosons has played an important role in
modern science and technology, and constitutes the working principle for
lasers. In a stimulated emission process, an incoming photon enhances the
probability that an excited atomic state will transition to a lower energy
state and generate a second photon of the same energy. It is expected, but not
experimentally shown, that stimulated emission contributes significantly to the
zero resistance current in a superconductor by enhancing the probability that
scattered Cooper pairs will return to the macroscopically occupied condensate
instead of entering any other state. Here, we use time- and angle-resolved
photoemission spectroscopy to study the initial rise of the non-equilibrium
quasiparticle population in a BiSrCaCuO cuprate
superconductor induced by an ultrashort laser pulse. Our finding reveals
significantly slower buildup of quasiparticles in the superconducting state
than in the normal state. The slower buildup only occurs when the pump pulse is
too weak to deplete the superconducting condensate, and for cuts inside the
Fermi arc region. We propose this is a manifestation of stimulated
recombination of broken Cooper pairs, and signals an important momentum space
dichotomy in the formation of Cooper pairs inside and outside the Fermi arc
region.Comment: 16 pages, 4 figure
Transient terahertz spectroscopy of excitons and unbound carriers in quasi two-dimensional electron-hole gases
We report a comprehensive experimental study and detailed model analysis of
the terahertz dielectric response and density kinetics of excitons and unbound
electron-hole pairs in GaAs quantum wells. A compact expression is given, in
absolute units, for the complex-valued terahertz dielectric function of
intra-excitonic transitions between the 1s and higher-energy exciton and
continuum levels. It closely describes the terahertz spectra of resonantly
generated excitons. Exciton ionization and formation are further explored,
where the terahertz response exhibits both intra-excitonic and Drude features.
Utilizing a two-component dielectric function, we derive the underlying exciton
and unbound pair densities. In the ionized state, excellent agreement is found
with the Saha thermodynamic equilibrium, which provides experimental
verification of the two-component analysis and density scaling. During exciton
formation, in turn, the pair kinetics is quantitatively described by a Saha
equilibrium that follows the carrier cooling dynamics. The terahertz-derived
kinetics is, moreover, consistent with time-resolved luminescence measured for
comparison. Our study establishes a basis for tracking pair densities via
transient terahertz spectroscopy of photoexcited quasi-two-dimensional
electron-hole gases.Comment: 14 pages, 8 figures, final versio
Clean and Dirty Superconductivity in Pure, Al doped, and Neutron Irradiated MgB2: a Far-Infrared Study
The effects of Al substitution and neutron irradiation on the conduction
regime (clean or dirty) of the - and -band of MgB have been
investigated by means of far-infrared spectroscopy. The intensity reflected by
well characterized polycrystalline samples was measured up to 100 cm in
both normal and superconducting state. The analysis of the superconducting to
normal reflectivity ratios shows that only the effect of the opening of the
small gap in the dirty -band can be clearly observed in pure MgB,
consistently with previous results. In Al-doped samples the dirty character of
the -band is increased, while no definitive conclusion on the conduction
regime of the -band can be drawn. On the contrary, results obtained
for the irradiated sample show that the irradiation-induced disorder drives the
-band in the dirty regime, making the large gap in -band
observable for the first time in far-infrared measurements.Comment: 11 pages, 1 figur
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
High- cuprate superconductors are characterized by a strong
momentum-dependent anisotropy between the low energy excitations along the
Brillouin zone diagonal (nodal direction) and those along the Brillouin zone
face (antinodal direction). Most obvious is the d-wave superconducting gap,
with the largest magnitude found in the antinodal direction and no gap in the
nodal direction. Additionally, while antinodal quasiparticle excitations appear
only below , superconductivity is thought to be indifferent to nodal
excitations as they are regarded robust and insensitive to . Here we
reveal an unexpected tie between nodal quasiparticles and superconductivity
using high resolution time- and angle-resolved photoemission on optimally doped
BiSrCaCuO. We observe a suppression of the nodal
quasiparticle spectral weight following pump laser excitation and measure its
recovery dynamics. This suppression is dramatically enhanced in the
superconducting state. These results reduce the nodal-antinodal dichotomy and
challenge the conventional view of nodal excitation neutrality in
superconductivity.Comment: 7 pages, 3 figure. To be published in Nature Physic
A comprehensive study of the vibrationally resolved S 2p −1 Auger electron spectrum of carbonyl sulfide
High-resolution normal Auger-electron spectra of carbonyl sulfide subsequent
to S 2p −1 photoionization at photonenergies of 200, 220, and 240 eV are
reported along with corresponding photoelectron spectra. In addition,
theoretical results are presented that take the core-hole orientation of the
various spin-orbit-split and molecular-field-split S 2p −1 states into
account. Auger transitions to eight metastable dicationic final states are
observed and assigned on the basis of the theoretical results. From Franck-
Condon analysis, assuming Morse potentials along the normal coordinates for
seven of the observed quasi-stable dicationic final states, information on the
potential-energysurfaces is derived and compared with theoretical results from
the literature
Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors
In the transient state of d-wave superconductors, we investigate the temporal
variation of photoinduced changes in the superfluid weight. We derive the
formula that relates the nonlinear response function to the nonequilibrium
distribution function. The latter qunatity is obtained by solving the kinetic
equation with the electron-electron and the electron-phonon interaction
included. By numerical calculations, a nonexponential decay is found at low
temperatures in contrast to the usual exponential decay at high temperatures.
The nonexponential decay originates from the nonmonotonous temporal variation
of the nonequilibrium distribution function at low energies. The main physical
process that causes this behavior is not the recombination of quasiparticles as
previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80,
No.
Metal-insulator Crossover Behavior at the Surface of NiS_2
We have performed a detailed high-resolution electron spectroscopic
investigation of NiS and related Se-substituted compounds
NiSSe, which are known to be gapped insulators in the bulk at all
temperatures. A large spectral weight at the Fermi energy of the room
temperature spectrum, in conjunction with the extreme surface sensitivity of
the experimental probe, however, suggests that the surface layer is metallic at
300 K. Interestingly, the evolution of the spectral function with decreasing
temperature is characterized by a continuous depletion of the single-particle
spectral weight at the Fermi energy and the development of a gap-like structure
below a characteristic temperature, providing evidence for a metal-insulator
crossover behavior at the surfaces of NiS and of related compounds. These
results provide a consistent description of the unusual transport properties
observed in these systems.Comment: 12 pages, 3 figure
- …