76 research outputs found
Trion formation dynamics in monolayer transition metal dichalcogenides
We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ∼50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides
Measurement of the beam-helicity asymmetry in the photoproduction of -pairs off protons and off neutrons
Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz
for the photoproduction of mixed-charge pion pairs in the reactions
off free protons and
and
off quasi-free nucleons bound
in the deuteron for incident photon energies up to 1.4 GeV. Circularly
polarized photons were produced from bremsstrahlung of longitudinally polarized
electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged
pions, recoil protons, recoil neutrons, and decay photons from mesons
were detected in the 4 electromagnetic calorimeter composed of the Crystal
Ball and TAPS detectors. Using a complete kinematic reconstruction of the final
state, excellent agreement was found between the results for free and
quasi-free protons, suggesting that the quasi-free neutron results are also a
close approximation of the free-neutron asymmetries. A comparison of the
results to the predictions of the Two-Pion-MAID reaction model shows that the
reaction mechanisms are still not well understood, in particular at low
incident photon energies in the second nucleon-resonance region.Comment: accepted for publication in Eur. phys. J.
The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region
Photoproduction of -pairs from nucleons has been investigated from
threshold up to incident photon energies of ~1.4~GeV. The quasi-free
reactions , ,
, and were for
the first time measured from nucleons bound in the deuteron. The corresponding
reactions from a free-proton target were also studied to investigate
final-state interaction effects (for neutral pions the free-proton results
could be compared to previous measurements; the reaction was measured for the first time). For the
final state coherent production via the
reaction was also investigated. The experiments were performed at the tagged
photon beam of the Mainz MAMI accelerator using an almost coverage
electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors.
The total cross sections for the four different final states obey the relation
as expected for a
dominant contribution from a
reaction chain,
which is also supported by the shapes of the invariant-mass distributions of
nucleon-meson and - pairs. The experimental results are compared to
the predictions from an isobar reaction model.Comment: accepted for publication in Phys. Lett.
Photoproduction of -pairs off protons and off neutrons
Total cross sections, angular distributions, and invariant-mass distributions
have been measured for the photoproduction of pairs off free
protons and off nucleons bound in the deuteron. The experiments were performed
at the MAMI accelerator facility in Mainz using the Glasgow photon tagging
spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered
electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin
radiator foils. The tagged photon beam covered energies up to 1400~MeV. The
data from the free proton target are in good agreement with previous
measurements and were only used to test the analysis procedures. The results
for differential cross sections (angular distributions and invariant-mass
distributions) for free and quasi-free protons are almost identical in shape,
but differ in absolute magnitude up to 15\%. Thus, moderate final-state
interaction effects are present. The data for quasi-free neutrons are similar
to the proton data in the second resonance region (final state invariant masses
up to 1550~MeV), where both reactions are dominated by the
decay. At higher energies,
angular and invariant-mass distributions are different. A simple analysis of
the shapes of the invariant-mass distributions in the third resonance region is
consistent with strong contributions of an decay
for the proton, while the reaction is dominated by a sequential decay via a
intermediate state for the neutron. The data are compared to
predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel
analysis.Comment: accepted for publication in Eur. Phys. J.
Photoproduction of π0-pairs off protons and off neutrons
Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of π0π0 pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to ≈1550 MeV), where both reactions are dominated by the N(1520)3/2−→Δ(1232)3/2+π decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an N⋆→Nσ decay for the proton, while the reaction is dominated by a sequential decay via a Δπ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis
Measurements of 12C(→γ,pp) photon asymmetries for Eγ= 200–450 MeV
The 12C (→γ ,pp) reaction has been studied in the photon energy range 200-450 MeV at the Mainz microtron MAMI-C, where linearly polarised photons were energy-tagged using the Glasgow-Mainz Tagged Photon Spectrometer and protons were detected in the Crystal Ball detector. The photon asymmetry Σ has been measured over a wider Eγ range than previous measurements. The strongest asymmetries were found at low missing energies where direct emission of nucleon pairs is expected. Cuts on the difference in azimuthal angles of the two ejected protons increased the magnitude of the observed asymmetries. At low missing energies the Σ data exhibit a strong angular dependence, similar to deuteron photodisintegration
Novel experimental setup for megahertz X-ray diffraction in a diamond anvil cell at the High Energy Density (HED) instrument of the European X-ray Free-Electron Laser (EuXFEL)
The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC
Dielectric disorder in two-dimensional materials
Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures
Photoproduction of the meson off the proton near threshold
An experimental study of photoproduction on the proton was conducted
by using the Crystal Ball and TAPS multiphoton spectrometers together with the
photon tagging facility at the Mainz Microtron MAMI. The
differential cross sections are measured from threshold to the incident-photon
energy GeV ( GeV for the center-of-mass energy) with
15-MeV binning in and full production-angle coverage. The quality of
the present data near threshold gives access to a variety of interesting
physics aspects. As an example, an estimation of the scattering
length is provided.Comment: 8 pages, 5 figures. Published version; some typos were fixe
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