45,820 research outputs found
Exact isovector pairing in a shell-model framework: Role of proton-neutron correlations in isobaric analog states
We utilize a nuclear shell model Hamiltonian with only two adjustable
parameters to generate, for the first time, exact solutions for pairing
correlations for light to medium-mass nuclei, including the challenging
proton-neutron pairs, while also identifying the primary physics involved. In
addition to single-particle energy and Coulomb potential terms, the shell model
Hamiltonian consists of an isovector pairing interaction and an average
proton-neutron isoscalar interaction, where the term describes the
average interaction between non-paired protons and neutrons. This Hamiltonian
is exactly solvable, where, utilizing 3 to 7 single-particle energy levels, we
reproduce experimental data for 0 state energies for isotopes with mass
through exceptionally well including isotopes from He to Ge.
Additionally, we isolate effects due to like-particle and proton-neutron
pairing, provide estimates for the total and proton-neutron pairing gaps, and
reproduce (neutron) = (proton) irregularity. These results provide a
further understanding for the key role of proton-neutron pairing correlations
in nuclei, which is especially important for waiting-point nuclei on the
rp-path of nucleosynthesis.Comment: 10 pages, 4 figure
The effect of electromechanical coupling on the strain in AlGaN/GaN heterojunction field effect transistors
The strain in AlGaN/GaN heterojunction field-effect transistors (HFETs) is
examined theoretically in the context of the fully-coupled equation of state
for piezoelectric materials. Using a simple analytical model, it is shown that,
in the absence of a two-dimensional electron gas (2DEG), the out-of-plane
strain obtained without electromechanical coupling is in error by about 30% for
an Al fraction of 0.3. This result has consequences for the calculation of
quantities that depend directly on the strain tensor. These quantities include
the eigenstates and electrostatic potential in AlGaN/GaN heterostructures. It
is shown that for an HFET, the electromechanical coupling is screened by the
2DEG. Results for the electromechanical model, including the 2DEG, indicate
that the standard (decoupled) strain model is a reasonable approximation for
HFET calculataions. The analytical results are supported by a self-consistent
Schr\"odinger-Poisson calculation that includes the fully-coupled equation of
state together with the charge-balance equation.Comment: 6 figures, revte
A posteriori teleportation
The article by Bouwmeester et al. on experimental quantum teleportation
constitutes an important advance in the burgeoning field of quantum
information. The experiment was motivated by the proposal of Bennett et al. in
which an unknown quantum state is `teleported' by Alice to Bob. As illustrated
in Fig. 1, in the implementation of this procedure, by Bouwmeester et al., an
input quantum state is `disembodied' into quantum and classical components, as
in the original protocol. However, in contrast to the original scheme,
Bouwmeester et al.'s procedure necessarily destroys the state at Bob's
receiving terminal, so a `teleported' state can never emerge as a freely
propagating state for subsequent examination or exploitation. In fact,
teleportation is achieved only as a postdiction.Comment: 1 page LaTeX including 1 figure. Scientific Correspondence about:
"Experimental quantum teleportation" Nature 390, 575 (1997
Demonstration of Non-Deterministic Quantum Logic Operations using Linear Optical Elements
Knill, Laflamme, and Milburn recently showed that non-deterministic quantum
logic operations could be performed using linear optical elements, additional
photons (ancilla), and post-selection based on the output of single-photon
detectors [Nature 409, 46 (2001)]. Here we report the experimental
demonstration of two logic devices of this kind, a destructive controlled-NOT
(CNOT) gate and a quantum parity check. These two devices can be combined with
a pair of entangled photons to implement a conventional (non-destructive) CNOT
that succeeds with a probability of 1/4.Comment: 4 pages, 5 figures; Minor change
Simple protocol for secure decoy-state quantum key distribution with a loosely controlled source
The method of decoy-state quantum key distribution (QKD) requests different
intensities of light pulses. Existing theory has assumed exact control of
intensities. Here we propose a simple protocol which is secure and efficient
even there are errors in intensity control. In our protocol, decoy pulses and
signal pulses are generated from the same father pulses with a two-value
attenuation. Given the upper bound of fluctuation of the father pulses, our
protocol is secure provided that the two-value attenuation is done exactly. We
propose to use unbalanced beam-splitters for a stable attenuation. Given that
the intensity error is bounded by , with the same key rate, our method
can achieve a secure distance only 1 km shorter than that of an ideal protocol
with exactly controlled source
Direct visualization of iron sheath shielding effect in MgB_2 superconducting wires
Local magneto-optical imaging and global magnetization measurement techniques
were used in order to visualize shielding effects in the superconducting core
of MgB_2 wires sheathed by ferromagnetic iron (Fe). The magnetic shielding can
provide a Meissner-like state in the superconducting core in applied magnetic
fields up to ~1T. The maximum shielding fields are shown to correlate with the
saturation fields of magnetization in Fe-sheaths. The shielding has been found
to facilitate the appearance of an overcritical state, which is capable of
achieving a critical current density (J_c) in the core which is larger than J_c
in the same wire without the sheath by a factor of ~2. Other effects caused by
the magnetic interaction between the sheath and the superconducting core are
discussed.Comment: 4 pages, 3 figure
- …