65,146 research outputs found
Dense attosecond electron sheets from laser wakefields using an up-ramp density transition
Controlled electron injection into a laser-driven wakefield at a well defined space and time is reported based on particle-in-cell simulations. Key novel ingredients are an underdense plasma target with an up-ramp density profile followed by a plateau and a fairly large laser focus diameter that leads to an essentially one-dimensional (1D) regime of laser wakefield, which is different from the bubble (complete blowout) regime occurring for tightly focused drive beams. The up-ramp profile causes 1D wave breaking to occur sharply at the up-ramp-plateau transition. As a result, it generates an ultrathin (few nanometer, corresponding to attosecond duration), strongly overdense relativistic electron sheet that is injected and accelerated in the wakefield. A peaked electron energy spectrum and high charge (∼nC) distinguish the final sheet
Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)
The transport properties of high-performance thin-film transistors (TFT) made
with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The
room-temperature field-effect mobility of the devices varied between 0.004
cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the
material, which modified the structural order. The transport properties of TFTs
were studied as a function of temperature. The field-effect mobility is
thermally activated in all films at T<200 K and the activation energy depends
on the charge density in the channel. The experimental data is compared to
theoretical models for transport, and we argue that a model based on the
existence of a mobility edge and an exponential distribution of traps provides
the best interpretation of the data. The differences in room-temperature
mobility are attributed to different widths of the shallow localized state
distribution at the edge of the valence band due to structural disorder in the
film. The free carrier mobility of the mobile states in the ordered regions of
the film is the same in all structural modifications and is estimated to be
between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure
Control of electron spin decoherence caused by electron-nuclear spin dynamics in a quantum dot
Control of electron spin decoherence in contact with a mesoscopic bath of
many interacting nuclear spins in an InAs quantum dot is studied by solving the
coupled quantum dynamics. The nuclear spin bath, because of its bifurcated
evolution predicated on the electron spin up or down state, measures the
which-state information of the electron spin and hence diminishes its
coherence. The many-body dynamics of nuclear spin bath is solved with a
pair-correlation approximation. In the relevant timescale, nuclear pair-wise
flip-flops, as elementary excitations in the mesoscopic bath, can be mapped
into the precession of non-interacting pseudo-spins. Such mapping provides a
geometrical picture for understanding the decoherence and for devising control
schemes. A close examination of nuclear bath dynamics reveals a wealth of
phenomena and new possibilities of controlling the electron spin decoherence.
For example, when the electron spin is flipped by a -pulse at , its
coherence will partially recover at as a consequence of quantum
disentanglement from the mesoscopic bath. In contrast to the re-focusing of
inhomogeneously broadened phases by conventional spin-echoes, the
disentanglement is realized through shepherding quantum evolution of the bath
state via control of the quantum object. A concatenated construction of pulse
sequences can eliminate the decoherence with arbitrary accuracy, with the
nuclear-nuclear spin interaction strength acting as the controlling small
parameter
Primary Isotope Yields and Characteristic Properties of the Fragmenting Source in Heavy-ion Reactions near the Fermi Energies
For central collisions of Ca Ca at 35 MeV/nucleon, the
density and temperature of a fragmenting source have been evaluated in a
self-consistent manner using the ratio of the symmetry energy coefficient
relative to the temperature, , extracted from the yields of primary
isotopes produced in antisymmetrized molecular dynamics (AMD) simulations. The
values are extracted from all isotope yields using an improved
method based on the Modified Fisher Model (MFM). The values of
obtained, using different interactions with different density dependencies of
the symmetry energy term, are correlated with the values of the symmetry
energies at the density of fragment formation. Using this correlation, the
fragment formation density is found to be . Using
the input symmetry energy value for each interaction temperature values are
extracted as a function of isotope mass . The extracted temperature values
are compared with those evaluated from the fluctuation thermometer with a
radial flow correction.Comment: 10 pages, 8 figure
Spectroscopy of the rotating BTZ black hole via adiabatic invariance
According to Bohr-Sommerfeld quantization rule, an equally spaced horizon
area spectrum of a static, spherically symmetric black hole was obtained under
an adiabatic invariant action. This method can be extended to the rotating
black holes. As an example, we apply this method to the rotating BTZ black hole
and obtain the quantized spectrum of the horizon area. It is shown that the
area spectrum of the rotating BTZ black hole is equally spaced and irrelevant
to the rotating parameter, which is consistent with the Bekenstein conjecture.
Specifically, the derivation do not need the quasinormal frequencies and the
small angular momentum limit.Comment: 6 pages, 0 figures, to appear in Sci China Ser G-Phys Mech Astron.
arXiv admin note: text overlap with arXiv:1106.229
Alfvenic Ion Temperature Gradient Activities in a Weak Magnetic Shear Plasma
We report the first experimental evidence of Alfvenic ion temperature
gradient (AITG) modes in HL-2A Ohmic plasmas. A group of oscillations with
kHz and is detected by various diagnostics in high-density
Ohmic regimes. They appear in the plasmas with peaked density profiles and weak
magnetic shear, which indicates that corresponding instabilities are excited by
pressure gradients. The time trace of the fluctuation spectrogram can be either
a frequency staircase, with different modes excited at different times or
multiple modes may simultaneously coexist. Theoretical analyses by the extended
generalized fishbone-like dispersion relation (GFLDR-E) reveal that mode
frequencies scale with ion diamagnetic drift frequency and , and they
lie in KBM-AITG-BAE frequency ranges. AITG modes are most unstable when the
magnetic shear is small in low pressure gradient regions. Numerical solutions
of the AITG/KBM equation also illuminate why AITG modes can be unstable for
weak shear and low pressure gradients. It is worth emphasizing that these
instabilities may be linked to the internal transport barrier (ITB) and H-mode
pedestal physics for weak magnetic shear.Comment: 9 pages, 7 figure
Tunneling Anisotropic Magnetoresistance in Co/AlOx/Au Tunnel Junctions
We observe spin-valve-like effects in nano-scaled thermally evaporated
Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and
depends on the relative orientation of the magnetization direction of the Co
electrode with respect to the current direction. We attribute this effect to a
two-step magnetization reversal and an anisotropic density of states resulting
from spin-orbit interaction. The results of this study points to future
applications of novel spintronics devices involving only one ferromagnetic
layer.Comment: 11 pages, 5 figures. Accpted for publishing on Nano Letters, 200
A spinal neural circuitry for converting touch to itch sensation
Touch and itch sensations are crucial for evoking defensive and emotional responses, and light tactile touch may induce unpleasant itch sensations (mechanical itch or alloknesis). The neural substrate for touch-to-itch conversion in the spinal cord remains elusive. We report that spinal interneurons expressing Tachykinin 2-Cre (Tac2Cre) receive direct A\u3b2 low threshold mechanoreceptor (LTMR) input and form monosynaptic connections with GRPR neurons. Ablation or inhibition markedly reduces mechanical but not acute chemical itch nor noxious touch information. Chemogenetic inhibition of Tac2Cre neurons also displays pronounced deficit in chronic dry skin itch, a type of chemical itch in mice. Consistently, ablation of gastrin-releasing peptide receptor (GRPR) neurons, which are essential for transmitting chemical itch, also abolishes mechanical itch. Together, these results suggest that innocuous touch and chemical itch information converge on GRPR neurons and thus map an exquisite spinal circuitry hard-wired for converting innocuous touch to irritating itch
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