39,227 research outputs found
Elliptic flow of thermal photons and dileptons
In this talk we describe the recently discovered rich phenomenology of
elliptic flow of electromagnetic probes of the hot matter created in
relativistic heavy-ion collisions. Using a hydrodynamic model for the
space-time dynamics of the collision fireball created in Au+Au collisions at
RHIC, we compute the transverse momentum spectra and elliptic flow of thermal
photons and dileptons. These observables are shown to provide differential
windows into various stages of the fireball expansion.Comment: 8 pages, including 9 figures. Invited talk at the Hard Probes 2006
Conference (Asilomar, June 9-16, 2006), to appear in the Proceedings
(Elsevier
HBT: A (mostly) experimental overview
I will present a review of the field of Hanbury Brown-Twiss interferometry in
relativistic heavy-ion collisions. The "HBT puzzle" is explored in detail,
emphasizing recent theoretical attempts to understand the persisting puzzle. I
also present recent experimental results on azimuthally sensitive HBT, HBT of
direct photons, and some surprises in the comparison of HBT results from p+p
and Au+Au collisions at RHIC.Comment: 8 pages, 3 figures. Proceedings of the Quark Matter 2004 conference
(Oalkland, CA, USA, January 2004
Photon HBT interferometry for non-central heavy-ion collisions
Currently, the only known way to obtain experimental information about the
space-time structure of a heavy-ion collision is through 2-particle momentum
correlations. Azimuthally sensitive HBT interferometry (Hanbury Brown-Twiss
intensity interferometry) can complement elliptic flow measurements by
constraining the spatial deformation of the source and its time evolution.
Performing these measurements on photons allows us to access the fireball
evolution at earlier times than with hadrons. Using ideal hydrodynamics to
model the space-time evolution of the collision fireball, we explore
theoretically various aspects of 2-photon intensity interferometry with
transverse momenta up to 2 GeV, in particular the azimuthal angle dependence of
the HBT radii in non-central collisions. We highlight the dual nature of
thermal photon emission, in both central and non-central collisions, resulting
from the superposition of QGP and hadron resonance gas photon production. This
signature is present in both the thermal photon source function and the HBT
radii extracted from Gaussian fits of the 2-photon correlation function.Comment: 18 pages, 12 figure
Family Ties: Multigenerational Family Foundation Board Engagement
Based on interviews with CEOs and family board chairs at seven large, multigenerational family foundations, this publication spotlights the governance practices and structures that these family foundations have created to maintain family involvement; select, orient, and engage family members across generations; and keep the board and foundation focused on impact
Exciton Trapping Is Responsible for the Long Apparent Lifetime in Acid-Treated MoS2
Here, we show that deep trapped "dark" exciton states are responsible for the
surprisingly long lifetime of band-edge photoluminescence in acid-treated
single-layer MoS2. Temperature-dependent transient photoluminescence
spectroscopy reveals an exponential tail of long-lived states extending
hundreds of meV into the band gap. These sub-band states, which are
characterized by a 4 microsecond radiative lifetime, quickly capture and store
photogenerated excitons before subsequent thermalization up to the band edge
where fast radiative recombination occurs. By intentionally saturating these
trap states, we are able to measure the "true" 150 ps radiative lifetime of the
band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature.
These experiments reveal the dominant role of dark exciton states in
acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at
room temperature in trap states below the band edge. We hypothesize that these
states are associated with native structural defects, which are not introduced
by the superacid treatment; rather, the superacid treatment dramatically
reduces non-radiative recombination through these states, extending the exciton
lifetime and increasing the likelihood of eventual radiative recombination
Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation
Hybrid quantum dot (QD) / transition metal dichalcogenide (TMD)
heterostructures are attractive components of next generation optoelectronic
devices, which take advantage of the spectral tunability of QDs and the charge
and exciton transport properties of TMDs. Here, we demonstrate tunable
electronic coupling between CdSe QDs and monolayer WS using variable length
alkanethiol ligands on the QD surface. Using femtosecond time-resolved second
harmonic generation (SHG) microscopy, we show that electron transfer from
photoexcited CdSe QDs to single-layer WS occurs on ultrafast (50 fs - 1 ps)
timescales. Moreover, in the samples exhibiting the fastest charge transfer
rates ( 50 fs) we observed oscillations in the time-domain signal
corresponding to an acoustic phonon mode of the donor QD, which coherently
modulates the SHG response of the underlying WS layer. These results reveal
surprisingly strong electronic coupling at the QD/TMD interface and demonstrate
the usefulness of time-resolved SHG for exploring ultrafast
electronic-vibrational dynamics in TMD heterostructures
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