10,894 research outputs found
Revealing the Exciton Fine Structure in PbSe Nanocrystal Quantum Dots
We measure the photoluminescence (PL) lifetime, , of excitons in
colloidal PbSe nanocrystals (NCs) at low temperatures to 270~mK and in high
magnetic fields to 15~T. For all NCs (1.3-2.3~nm radii), increases
sharply below 10~K but saturates by 500~mK. In contrast to the usual picture of
well-separated ``bright" and ``dark" exciton states (found, e.g., in CdSe NCs),
these dynamics fit remarkably well to a system having two exciton states with
comparable - but small - oscillator strengths that are separated by only
300-900 eV. Importantly, magnetic fields reduce below 10~K,
consistent with field-induced mixing between the two states. Magnetic circular
dichroism studies reveal exciton g-factors from 2-5, and magneto-PL shows
10\% circularly polarized emission.Comment: To appear in Physical Review Letter
Stochastic theory of spin-transfer oscillator linewidths
We present a stochastic theory of linewidths for magnetization oscillations
in spin-valve structures driven by spin-polarized currents. Starting from a
nonlinear oscillator model derived from spin-wave theory, we derive Langevin
equations for amplitude and phase fluctuations due to the presence of thermal
noise. We find that the spectral linewidths are inversely proportional to the
spin-wave intensities with a lower bound that is determined purely by
modulations in the oscillation frequencies. Reasonable quantitative agreement
with recent experimental results from spin-valve nanopillars is demonstrated.Comment: Submitted to Physical Review
Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators
We study the magnetization dynamics in a nanocontact magnetic vortex
oscillators as function of temperature. Low temperature experiments reveal that
the dynamics at low and high currents differ qualitatively. At low currents, we
excite a temperature independent standard oscillation mode, consisting in the
gyrotropic motion of a free layer vortex about the nanocontact. Above a
critical current, a sudden jump of the frequency is observed, concomitant with
a substantial increase of the frequency versus current slope factor. Using
micromagnetic simulation and analytical modeling, we associate this new regime
to the creation of a vortex-antivortex pair in the pinned layer of the spin
valve. The vortex-antivortex distance depends on the Oersted field which favors
a separation, and on the exchange bias field, which favors pair merging. The
pair in the pinned layer provides an additional spin torque altering the
dynamics of the free layer vortex, which can be quantitatively accounted for by
an analytical model
Synthesis and bioactivity of a conjugate composed of green tea catechins and hyaluronic acid
(-)-Epigallocatechin-3-gallate (EGCG) is a green tea polyphenol that has several biological activities, including anti-cancer activity and anti-inflammation. Hyaluronic acid (HA) is a naturally-occurring polysaccharide that is widely used as a biomaterial for drug delivery and tissue engineering due to its viscoelastic, biocompatible and biodegradable properties. By conjugating HA with EGCG, the resulting HA-EGCG conjugate is expected to exhibit not only the inherent properties of HA but also the bioactivities of EGCG. Toward this end, we report the synthesis of an amine-functionalized EGCG as an intermediate compound for conjugation to HA. EGCG was reacted with 2,2-diethoxyethylamine (DA) under acidic conditions, forming ethylamine-bridged EGCG dimers. The EGCG dimers were composed of four isomers, which were characterized by HPLC, high-resolution mass spectrometry and NMR spectroscopy. The amine-functionalized EGCG dimers were conjugated to hyaluronic acid (HA) through the formation of amide bonds. HA-EGCG conjugates demonstrated several bioactivities which were not present in unmodified HA, including resistance to hyaluronidase-mediated degradation, inhibition of cell growth and scavenging of radicals. The potential applications of HA-EGCG conjugates are discussed
Generation of Three-Qubit Entangled W-State by Nonlinear Optical State Truncation
We propose an alternative scheme to generate W state via optical state
truncation using quantum scissors. In particular, these states may be generated
through three-mode optical state truncation in a Kerr nonlinear coupler. The
more general three-qubit state may be also produced if the system is driven by
external classical fields.Comment: 7 pages, 2 figur
Simulations of black hole air showers in cosmic ray detectors
We present a comprehensive study of TeV black hole events in Earth's
atmosphere originated by cosmic rays of very high energy. An advanced fortran
Monte Carlo code is developed and used to simulate black hole extensive air
showers from ultrahigh-energy neutrino-nucleon interactions. We investigate the
characteristics of these events, compare the black hole air showers to standard
model air showers, and test different theoretical and phenomenological models
of black hole formation and evolution. The main features of black hole air
showers are found to be independent of the model considered. No significant
differences between models are likely to be observed at fluorescence telescopes
and/or ground arrays. We also discuss the tau ``double bang'' signature in
black hole air showers. We find that the energy deposited in the second bang is
too small to produce a detectable peak. Our results show that the theory of
TeV-scale black holes in ultrahigh-energy cosmic rays leads to robust
predictions, but the fine prints of new physics are hardly to be investigated
through atmospheric black hole events in the near future.Comment: 18 pages, 9 figure
The states of W-class as shared resources for perfect teleportation and superdense coding
As we know, the states of triqubit systems have two important classes:
GHZ-class and W-class.
In this paper, the states of W-class are considered for teleportation and
superdense coding, and are generalized to multi-particle systems. First we
describe two transformations of the shared resources for teleportation and
superdense coding, which allow many new protocols from some known ones for
that. As an application of these transformations, we obtain a sufficient and
necessary condition for a state of W-class being suitable for perfect
teleportation and superdense coding. As another application, we find that state
can be used to
transmit three classical bits by sending two qubits, which was considered to be
impossible by P. Agrawal and A. Pati [Phys. Rev. A to be published]. We
generalize the states of W-class to multi-qubit systems and multi-particle
systems with higher dimension. We propose two protocols for teleportation and
superdense coding by using W-states of multi-qubit systems that generalize the
protocols by using proposed by P. Agrawal and A. Pati. We obtain an
optimal way to partition some W-states of multi-qubit systems into two
subsystems, such that the entanglement between them achieves maximum value.Comment: 10 pages, critical comments and suggestions are welcom
The strong coupling constant from lattice QCD with N_f=2 dynamical quarks
We compute for two flavors of light dynamical quarks
using non-perturbatively improved Wilson fermions. We improve on a
recent calculation by employing Pad\'e-improved two-loop and three-loop
perturbation theory to convert the lattice numbers to the scheme.Comment: Contribution to Lattice 2001 (matrix elements), typo correcte
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