862 research outputs found
Transition Rates between Mixed Symmetry States: First Measurement in 94Mo
The nucleus 94Mo was investigated using a powerful combination of
gamma-singles photon scattering experiments and gamma-gamma-coincidence studies
following the beta-decay of 94mTc. The data survey short-lived J^pi=1+,2+
states and include branching ratios, E2/M1 mixing ratios, lifetimes, and
transition strengths. The mixed-symmetry (MS) 1+ scissors mode and the 2+ MS
state are identified from M1 strengths. A gamma transition between MS states
was observed and its rate was measured. Nine M1 and E2 strengths involving MS
states agree with the O(6) limit of the interacting boson model-2 using the
proton boson E2 charge as the only free parameter.Comment: 9 pages, 3 PostScript figures included, ReVTeX, accepted for
publication in Physical Review Letters, tentatively scheduled for August 9,
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Signature of Martensite transformation on conductivity noise in thin films of NiTi shape memory alloys
Slow time-dependent fluctuations, or noise, in the electrical resistance of
dc magnetron sputtered thin films of Nickel Titanium shape memory alloys have
been measured. Even in equilibrium, the noise was several orders of magnitude
larger than that of simple diffusive metallic films, and was found to be
non-monotonic around the martensitic transformation regime. The results are
discussed in terms of dynamics of structural defects, which also lay foundation
to a new noise-based characterization scheme of martensite transformation.Comment: 4 pages, 3 figure
Effects of guanidine on synaptic transmission in the spinal cord of the frog
The effects of guanidine on motoneurons of the isolated frog spinal cord were studied by adding the drug to the solution bathing the cord during intracellular recording. Guanidine (5·10–4 M) did not alter the membrane potential of motoneurons.
The main effect was a marked increase of the amplitudes and frequencies of small spontaneously occurring inhibitory postsynaptic potentials. The hyperpolarizing component of postsynaptic potentials evoked by stimulation of dorsal roots was also enhanced by guanidine. Higher concentrations of guanidine (5·10–3 M) resulted in a very large and irreversible increase of the small spontaneously occurring inhibitory potentials, which now appeared in a regular, rhythmic pattern.
The effects of guanidine could easily be blocked by increasing the magnesium ions (15 mM) in the bath solution.
These results indicate that guanidine facilitates the release of an inhibitory transmitter in afferent terminals of the frog spinal cord either by a direct action on these terminals or indirectly by an action on nerve endings impinging on inhibitory interneurons
Lifetime Measurements in 120Xe
Lifetimes for the lowest three transitions in the nucleus Xe have
been measured using the Recoil Distance Technique. Our data indicate that the
lifetime for the transition is more than a factor of
two lower than the previously adopted value and is in keeping with more recent
measurements performed on this nucleus. The theoretical implications of this
discrepancy and the possible reason for the erroneous earlier results are
discussed. All measured lifetimes in Xe, as well as the systematics of
the lifetimes of the 2 states in Xe isotopes, are compared with
predictions of various models. The available data are best described by the
Fermion Dynamic Symmetry Model (FDSM).Comment: 9 pages, RevTeX, 3 figures with Postscript file available on request
at [email protected], [email protected]. Submitted to Phys.
Rev.
Nonadiabatic Dynamics of Ultracold Fermions in Optical Superlattices
We study the time-dependent dynamical properties of two-component ultracold
fermions in a one-dimensional optical superlattice by applying the adaptive
time-dependent density matrix renormalization group to a repulsive Hubbard
model with an alternating superlattice potential. We clarify how the time
evolution of local quantities occurs when the superlattice potential is
suddenly changed to a normal one. For a Mott-type insulating state at quarter
filling, the time evolution exhibits a profile similar to that expected for
bosonic atoms, where correlation effects are less important. On the other hand,
for a band-type insulating state at half filling, the strong repulsive
interaction induces an unusual pairing of fermions, resulting in some striking
properties in time evolution, such as a paired fermion co-tunneling process and
the suppression of local spin moments. We further address the effect of a
confining potential, which causes spatial confinement of the paired fermions.Comment: 4 pages, 5 figure
F-spin as a Partial Symmetry
We use the empirical evidence that F-spin multiplets exist in nuclei for only
selected states as an indication that F-spin can be regarded as a partial
symmetry. We show that there is a class of non-F-scalar IBM-2 Hamiltonians with
partial F-spin symmetry, which reproduce the known systematics of collective
bands in nuclei. These Hamiltonians predict that the scissors states have good
F-spin and form F-spin multiplets, which is supported by the existing data.Comment: 14 pages, 1 figur
Magnetic Dipole Sum Rules for Odd-Mass Nuclei
Sum rules for the total- and scissors-mode M1 strength in odd-A nuclei are
derived within the single-j interacting boson-fermion model. We discuss the
physical content and geometric interpretation of these sum rules and apply them
to ^{167}Er and ^{161}Dy. We find consistency with the former measurements but
not with the latter.Comment: 13 pages, Revtex, 1 figure, Phys. Rev. Lett. in pres
Extended M1 sum rule for excited symmetric and mixed-symmetry states in nuclei
A generalized M1 sum rule for orbital magnetic dipole strength from excited
symmetric states to mixed-symmetry states is considered within the
proton-neutron interacting boson model of even-even nuclei. Analytic
expressions for the dominant terms in the B(M1) transition rates from the first
and second states are derived in the U(5) and SO(6) dynamic symmetry
limits of the model, and the applicability of a sum rule approach is examined
at and in-between these limits. Lastly, the sum rule is applied to the new data
on mixed-symmetry states of 94Mo and a quadrupole d-boson ratio
is obtained in a largely
parameter-independent wayComment: 19 pages, 3 figures, Revte
Nuclear forces from chiral EFT: The unfinished business
In spite of the great progress we have seen in recent years in the derivation
of nuclear forces from chiral effective field theory (EFT), some important
issues are still unresolved. In this contribution, we discuss the open problems
which have particular relevance for microscopic nuclear structure, namely, the
proper renormalization of chiral nuclear potentials and sub-leading many-body
forces.Comment: 16 pages, 3 figures; contribution to J. Phys. G, Special Issue, Focus
Section: Open Problems in Nuclear Structur
Chiral three-nucleon forces and bound excited states in neutron-rich oxygen isotopes
We study the spectra of neutron-rich oxygen isotopes based on chiral two- and
three-nucleon interactions. First, we benchmark our many-body approach by
comparing ground-state energies to coupled-cluster results for the same
two-nucleon interaction, with overall good agreement. We then calculate bound
excited states in 21,22,23O, focusing on the role of three-nucleon forces, in
the standard sd shell and an extended sdf7/2p3/2 valence space. Chiral
three-nucleon forces provide important one- and two-body contributions between
valence neutrons. We find that both these contributions and an extended valence
space are necessary to reproduce key signatures of novel shell evolution, such
as the N = 14 magic number and the low-lying states in 21O and 23O, which are
too compressed with two-nucleon interactions only. For the extended space
calculations, this presents first work based on nuclear forces without
adjustments. Future work is needed and open questions are discussed.Comment: 6 pages, 4 figures, published versio
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