1,716 research outputs found
Role of non-collective excitations in low-energy heavy-ion reactions
We investigate the effect of single-particle excitations on heavy-ion
reactions at energies near the Coulomb barrier. To this end, we describe
single-particle degrees of freedom with the random matrix theory and solve the
coupled-channels equations for one-dimensional systems. We find that the
single-particle excitations hinder the penetrability at energies above the
barrier, leading to a smeared barrier distribution. This indicates that the
single-particle excitations provide a promising way to explain the difference
in a quasi-elastic barrier distribution recently observed in Ne +
Zr systems.Comment: 8 pages, 7 figure
Non-collective excitations in low-energy heavy-ion reactions: applicability of the random-matrix model
We investigate the applicability of a random-matrix model to the description
of non-collective excitations in heavy-ion reactions around the Coulomb
barrier. To this end, we study fusion in the reaction O + Pb,
taking account of the known non-collective excitations in the Pb
nucleus. We show that the random-matrix model for the corresponding couplings
reproduces reasonably well the exact calculations, obtained using empirical
deformation parameters. This implies that the model may provide a powerful
method for systems in which the non-collective couplings are not so well known.Comment: 6 pages, 4 figure
Subbarrier fusion reactions with dissipative couplings
Using the random matrix model, we discuss the effect of couplings to
non-collective states on the penetrability of a one dimensional potential
barrier. We show that these non-collective excitations hinder the penetrability
and thus smear the barrier distribution at energies above the barrier, while
they do not affect significantly the penetrability at deep subbarrier energies.
The energy dependence of the Q-value distribution obtained with this model is
also discussed.Comment: 4 pages, 2 figures. A talk given at the 10th international conference
on nucleus-nucleus collisions (NN2009), Aug. 16-21, 2009, Beijing, Chin
Role of non-collective excitations in heavy-ion fusion reactions and quasi-elastic scattering around the Coulomb barrier
Despite the supposed simplicity of double-closed shell nuclei, conventional
coupled-channels calculations, that include all of the known collective states
of the target and projectile, give a poor fit to the fusion cross section for
the O + Pb system. The discrepancies are highlighted through the
experimental barrier distribution and logarithmic derivative, that are both
well defined by the precise experimental fusion data available. In order to
broaden our search for possible causes for this anomaly, we revisit this system
and include in our calculations a large number of non-collective states of the
target, whose spin, parity, excitation energy and deformation paramter are
known from high-precision proton inelastic-scattering measurements. Although
the new coupled-channels calculations modify the barrier distribution, the
disagreemnt with experiment remains both for fusion and for quasi-elastic (QE)
scattering. We find that the Q-value distributions for large-angle QE
scattering become rapidly more important as the incident energy increases,
reflecting the trend of the experimental data. The mass-number dependence of
the non-collective excitations is discussed.Comment: 8 pages, 7 figure
Nanometre-scale nuclear-spin device for quantum information processing
We have developed semiconductor point contact devices in which nuclear spins
in a nanoscale region are coherently controlled by all-electrical methods.
Different from the standard nuclear-magnetic resonance technique, the
longitudinal magnetization of nuclear spins is directly detected by measuring
resistance, resulting in ultra-sensitive detection of the microscopic quantity
of nuclear spins. All possible coherent oscillations have been successfully
demonstrated between two levels from four nuclear spin states of I = 3/2
nuclei. Quantum information processing is discussed based on two fictitious
qubits of an I = 3/2 system and methods are described for performing arbitrary
logical gates both on one and two qubits. A scheme for quantum state tomography
based on Mz-detection is also proposed. As the starting point of quantum
manipulations, we have experimentally prepared the effective pure states for
the I = 3/2 nuclear spin system.Comment: 16 pages, 5 figure
Nuclear Spins in a Nanoscale Device for Quantum Information Processing
Coherent oscillations between any two levels from four nuclear spin states of
I=3/2 have been demonstrated in a nanometre-scale NMR semiconductor device,
where nuclear spins are all-electrically controlled. Using this device, we
discuss quantum logic operations on two fictitious qubits of the I=3/2 system,
and propose a quantum state tomography scheme based on the measurement of
longitudinal magnetization, .Comment: 5 pages, 4 figure
A CRISPR Dropout Screen Identifies Genetic Vulnerabilities and Therapeutic Targets in Acute Myeloid Leukemia
Acute myeloid leukemia (AML) is an aggressive cancer with a poor prognosis, for which mainstream treatments have not changed for decades. To identify additional therapeutic targets in AML, we optimize a genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) screening platform and use it to identify genetic vulnerabilities in AML cells. We identify 492 AML-specific cell-essential genes, including several established therapeutic targets such as , , and , and many other genes including clinically actionable candidates. We validate selected genes using genetic and pharmacological inhibition, and chose as a candidate for downstream study. inhibition demonstrated anti-AML activity by inducing myeloid differentiation and apoptosis, and suppressed the growth of primary human AMLs of diverse genotypes while sparing normal hemopoietic stem-progenitor cells. Our results propose that KAT2A inhibition should be investigated as a therapeutic strategy in AML and provide a large number of genetic vulnerabilities of this leukemia that can be pursued in downstream studies.This work was funded by the Kay Kendall Leukaemia Fund (KKLF) and the Wellcome Trust (WT098051). G.S.V. is funded by a Wellcome Trust Senior Fellowship in Clinical Science (WT095663MA) and work in his laboratory is funded by Bloodwise. C.P. is funded by a Kay Kendall Leukaemia Fund Intermediate Fellowship (KKL888)
Shake-up Processes in a Low-Density Two-Dimensional Electron Gas: Spin-Dependent Transitions to Higher Hole Landau Levels
A theory of shake-up processes in photoabsorption of an interacting
low-density two-dimensional electron gas (2DEG) in strong magnetic fields is
presented. In these processes, an incident photon creates an electron-hole pair
and, because of Coulomb interactions, simultaneously excites one particle to
higher Landau levels (LL's). In this work, the spectra of correlated charged
spin-singlet and spin-triplet electron-hole states in the first hole LL and
optical transitions to these states (i.e., shake-ups to the first hole LL) are
studied. Our results indicate, in particular, the presence of optically-active
three-particle quasi-discrete states in the exciton continuum that may give
rise to surprisingly sharp Fano resonances in strong magnetic fields. The
relation between shake-ups in photoabsorption of the 2DEG and in the 2D hole
gas (2DHG), and shake-ups of isolated negative X^- and positive X^+ trions are
discussed.Comment: 8 pages, 8 figures. References updated, one figure added (Fig. 6).
Accepted in Phys. Rev.
Full coherent control of nuclear spins in an optically pumped single quantum dot
Highly polarized nuclear spins within a semiconductor quantum dot (QD) induce
effective magnetic (Overhauser) fields of up to several Tesla acting on the
electron spin or up to a few hundred mT for the hole spin. Recently this has
been recognized as a resource for intrinsic control of QD-based spin quantum
bits. However, only static long-lived Overhauser fields could be used. Here we
demonstrate fast redirection on the microsecond time-scale of Overhauser fields
of the order of 0.5 T experienced by a single electron spin in an optically
pumped GaAs quantum dot. This has been achieved using full coherent control of
an ensemble of 10^3-10^4 optically polarized nuclear spins by sequences of
short radio-frequency (rf) pulses. These results open the way to a new class of
experiments using rf techniques to achieve highly-correlated nuclear spins in
quantum dots, such as adiabatic demagnetization in the rotating frame leading
to sub-micro K nuclear spin temperatures, rapid adiabatic passage, and spin
squeezing
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