299 research outputs found
Microscopic origin of shape coexistence in the N=90, Z=64 region
A microscopic explanation of the nature of shape coexistence in the N=90,
Z=64 region is suggested, based on calculations of single particle energies
through standard covariant density functional theory. It is suggested that
shape coexistence in the N=90 region is caused by the protons, which create
neutron particle-hole (p-h) excitations across the N=112 3-dimensional
isotropic harmonic oscillator (3D-HO) magic number, signaling the start of the
occupation of the 1i13/2 intruder orbital, which triggers stronger
proton-neutron interaction, causing the onset of the deformation and resulting
in the shape/phase transition from spherical to deformed nuclei described by
the X(5) critical point symmetry. A similar effect is seen in the N=60, Z=40
region, in which p-h excitations across the N=70 3D-HO magic number occur,
signaling the start of the occupation of the 1h11/2 intruder orbital.Comment: 6 pages, 7 figure
Experimental study of weak antilocalization effect in a high mobility InGaAs/InP quantum well
The magnetoresistance associated with quantum interference corrections in a
high mobility, gated InGaAs/InP quantum well structure is studied as a function
of temperature, gate voltage, and angle of the tilted magnetic field.
Particular attention is paid to the experimental extraction of phase-breaking
and spin-orbit scattering times when weak anti- localization effects are
prominent. Compared with metals and low mobility semiconductors the
characteristic magnetic field in high mobility
samples is very small and the experimental dependencies of the interference
effects extend to fields several hundreds of times larger. Fitting experimental
results under these conditions therefore requires theories valid for arbitrary
magnetic field. It was found, however, that such a theory was unable to fit the
experimental data without introducing an extra, empirical, scale factor of
about 2. Measurements in tilted magnetic fields and as a function of
temperature established that both the weak localization and the weak
anti-localization effects have the same, orbital origin. Fits to the data
confirmed that the width of the low field feature, whether a weak localization
or a weak anti-localization peak, is determined by the phase-breaking time and
also established that the universal (negative) magnetoresistance observed in
the high field limit is associated with a temperature independent spin-orbit
scattering time.Comment: 13 pages including 10 figure
Islands of shape coexistence in proxy-SU(3) symmetry and in covariant density functional theory
Shape coexistence in even-even nuclei is observed when the ground state band
of a nucleus is accompanied by another K=0 band at similar energy but with
radically different structure. We attempt to predict regions of shape
coexistence throughout the nuclear chart using the parameter-free proxy-SU(3)
symmetry and standard covariant density functional theory. Within the
proxy-SU(3) symmetry the interplay of shell model magic numbers, formed by the
spin-orbit interaction, and the 3-dimensional isotropic harmonic oscillator
magic numbers, leads to the prediction of specific horizontal and vertical
stripes on the nuclear chart in which shape coexistence should be possible.
Within covariant density functional theory, specific islands on the nuclear
chart are found, in which particle-hole excitations leading to shape
coexistence are observed. The role played by particle-hole excitations across
magic numbers as well as the collapse of magic numbers as deformation sets in
is clarified.Comment: 12 pages, 2 figures, to appear in the Proceedings of the 39th
International Workshop on Nuclear Theory (Rila 2022), ed. M. Gaidarov and N.
Minkov (Heron Press, Sofia, 2022
Shape coexistence in even-even nuclei: A theoretical overview
The last decade has seen a rapid growth of our understanding of the
microscopic origins of shape coexistence, assisted by the new data provided by
the modern radioactive ion beam facilities built worldwide. Islands of the
nuclear chart in which shape coexistence can occur have been identified, and
the different microscopic particle-hole excitation mechanisms leading to
neutron-induced or proton-induced shape coexistence have been clarified. The
relation of shape coexistence to the islands of inversion, appearing in light
nuclei, to the new spin-aligned phase appearing in N=Z nuclei, as well as to
shape/phase transitions occurring in medium mass and heavy nuclei, has been
understood. In the present review, these developments are considered within the
shell model and mean field approaches, as well as by symmetry methods. In
addition, based on systematics of data, as well as on symmetry considerations,
quantitative rules are developed, predicting regions in which shape coexistence
can appear, as a possible guide for further experimental efforts, which can
help in improving our understanding of the details of the nucleon-nucleon
interaction, as well as of its modifications occurring far from stability.Comment: 80 pages, 14 figures, 837 reference
Islands of shape coexistence from single-particle spectra in covariant density functional theory
Using covariant density functional theory with the DDME2 functional and
labeling single-particle energy orbitals by Nilsson quantum numbers, a search
for particle-hole (p-h) excitations connected to the appearance of shape
coexistence is performed for Z=38 to 84. Islands of shape coexistence are found
near the magic numbers Z=82 and Z=50, restricted in regions around the relevant
neutron midshells N=104 and N=66 respectively, in accordance to the well
accepted p-h interpretation of shape coexistence in these regions, which we
call neutron-induced shape coexistence, since the neutrons act as elevators
creating holes in the proton orbitals. Similar but smaller islands of shape
coexistence are found near N=90 and N=60, restricted in regions around the
relevant proton midshells Z=66 and Z=39 respectively, related to p-h
excitations across the 3-dimensional isotropic harmonic oscillator (3D-HO)
magic numbers N=112 and N=70, which correspond to the beginning of the
participation of the opposite parity orbitals 1i13/2 and 1h11/2 respectively to
the onset of deformation. We call this case proton-induced shape coexistence,
since the protons act as elevators creating holes in the neutron orbitals, thus
offering a possible microscopic mechanism for the appearance of shape
coexistence in these regions. In the region around N=40, Z=40, an island is
located on which both neutron p-h excitations and proton p-h excitations are
present.Comment: 21 pages, 17 figure
Signatures for shape coexistence and shape/phase transitions in even-even nuclei
Systematics of B(E2) transition rates connecting the first excited 0+ state
to the first excited 2+ state of the ground state band in even-even nuclei
indicates that shape coexistence of the ground state band and the first excited
K=0 band should be expected in nuclei lying within the stripes of nucleon
numbers 7-8, 17-20, 34-40, 59-70, 96-112 predicted by the dual shell mechanism
of the proxy-SU(3) model, avoiding their junctions, within which high
deformation is expected. Systematics of the excitation energies of the first
excited 0+ states in even-even nuclei show that shape coexistence due to
proton-induced neutron particle-hole excitations is related to a first-order
shape/phase transition from spherical to deformed shapes, while shape
coexistence due to neutron-induced proton particle-hole excitations is observed
along major proton shell closures.Comment: 13 pages, 4 figures, 4 table
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