Low-lying dipole excitations in nuclei in the mass regions of the new dynamical symmetries

Experimental indications for the nuclear phase transitions described by the new symmetries E(5) and X(5) are discussed for the stable Ba, Nd, and Gd isotopes. Their low-lying dipole excitations, systematically investigated at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator, provide indications for the phase transitional behavior, including the proposed critical point candidates.     

Microscopic nature of the pygmy dipole resonance: the stable Ca isotopes

The electric dipole strength distribution in 44Ca has been measured up to 10 MeV in high resolution photon scattering experiments for the first time. The data obtained have been compared to earlier measurements on (40,48)Ca in order to view the evolution of the electric pygmy dipole resonance (PDR). Calculations that were performed within the framework of the microscopic extended theory of finite Fermi systems, which adds contributions of the quasiparticle-phonon coupling to random phase approximation calculations, give a qualitative agreement with the experimental data for all three isotopes. We have shown that it is necessary to include this coupling to describe the PDR.     

Low-lying excitations in the odd-odd nucleus154Eu

The analysis of experiments on parity mixing in compound nuclear resonances is described, with the nuclear spectroscopic aspects of the parity violation experiments emphasized. Expressions are derived for polarized neutron scattering and polarized neutron capture on unpolarized targets, and the resonance interference is treated explicitly. Examples are presented to illustrate the effects of entrance channel mixing for the case of target spin 1/2 and resonance spin 1. Results are also presented for targets with spins 0 and 3/2.     

Pigmy and giant dipole states in stable and unstable oxygen isotopes

We have studied dipole states of oxygen isotopes in large scale shell model calculations. The calculated photoreaction cross sections in ¹⁶O, ¹⁷O and ¹⁸O give reasonable agreement with experimental observations both in the low energy region below ?ω=15 MeV and in the high energy giant resonance region (15 MeV < ?ω ≤ 30 MeV). We found that the transition strength below dipole giant resonance (?ω ≤ 15 MeV) exhausts about 10% of the classical Thomas-Reiche-Kuhn sum rule and more than 40% of the cluster sum rule in heavier oxygen isotopes than ¹⁷O. The predicted Pigmy strengths in ²⁰O and ²²O are also confirmed by recent Coulomb excitation experiment at GSI.     

Low-lying dipole strength for weakly bound systems: A simple analytic estimate

A simple analytic expression is proposed for the total low-lying dipole strength observed in the break-up of weakly bound systems. The model assumes pure single-particle transitions from the halo weakly bound state to continuum states treated as plane waves (neutrons) or regular Coulomb functions (protons). The analytic expression for the total strength approximately depends on the inverse of the binding energy and the reduced mass of the halo, with correction factors accounting for the charge of the halo and the finite size of the potential.     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of 64-70Ge isotopes has been studied within the framework of the interacting boson model-3 (IBM-3). The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states (T>Tz) have been assigned for the 64Ge (N=Z) nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Low-lying states and isospin excitation in the Ge isotopes

The level structure of ^64-70Ge isotopes has been studied within the framework of the interacting boson model-3（IBM-3） . The symmetry character in the proton and neutron degrees of freedom of the energy levels has been investigated. The isospin excitation states（T 〉 Tz） have been assigned for the ^64Ge（N = Z） nucleus. Some intruder states in these nuclei have been suggested. The calculated energy levels and transition probabilities are in good agreement with recent experimental data. The study indicates that the Ge isotopes are in transition from γ-unstable to vibrational.     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

Low-lying excitations induced by non-magnetic impurities in d-wave superconductors

We have performed numerical calculations for low-lying excitations induced by a single non-magnetic impurity in a d-wave superconductor on the basis of two different frameworks. One is the Bogoliubov–de Gennes theory in real coordinates, while the other is the Born approximation in wave-number coordinates. This paper shows that these results for low-lying excitations are identical by using Fourier transformations.     

Low-lying octupole excitations in 17O

Inelastic electron scattering experiments were performed on a¹⁷O gas target in a range of momentum transfer q = 0.6?1.1fm^?1; B(E3) values for twelve odd-parity levels were obtained from the measured Coulomb form factors. The results are discussed in terms of the weak coupling model.     

Double giant dipole excitations in deformed nuclei

A calculation is presented for the excitation spectra of two superimposed dipole resonances for deformed nuclei by using theSU(3) limit of the interaction boson model for the case of many bosons. The results indicate that experiments to test the model may be feasible.     

Electromagnetic excitations in Cd and Zn

Results of investigation of Cd and Zn surface resistance in magnetic field are given. This field is normal to the metal surface and parallel to hexagonal crystal axis. Oscillations observed in these conditions are discussed and possibly explained by propagation of light damping electromagnetic wave.     

Low-lying quadrupole collective states of the light and medium xenon isotopes

Collective low-lying levels of light and medium xenon isotopes are deduced from the Generalized Bohr Hamiltonian (GBH). The microscopic seven functions entering into the GBH are built from a deformed mean field of the Woods-Saxon type. Theoretical spectra are found to be close to the ones of the experimental data, taking into account that the calculations are completely microscopic, that is to say, without any fitting of parameters.     

Delocalization of excitations in disordered media with dipole transfer

The migration of excitations over a system of impurity centers due to dipole-dipole interactions is studied. A numerical and analytical study of the probability of finding an excitation on the center of its primary localization is carried out for the main models describing the kinetics of spin systems and systems of localized excitons. The results obtained are characterized by a uniformly high accuracy over the entire range of times available for experimental investigations. It is revealed that the preasymptotic effects are in qualitative agreement with theoretical predictions but are quantitatively twice as large as those.     

Low-lying magnetic dipole strength distribution in the γ-soft even-even 130-136Ba

In this study the scissors mode 1⁺ states are systematically investigated within the rotational invariant Quasiparticle Random Phase Approximation (QRPA) for ^130-136Ba isotopes. We consider the 1⁺ vibrations generated by the isovector spin-spin interactions and the isoscalar and isovector quadrupole-type separable forces restoring the broken symmetry by a deformed mean field according to A.A. Kuliev et al. (Int. J. Mod. Phys. E 9, 249 (2000)). It has been shown that the restoration of the broken rotational symmetry of the Hamiltonian essentially decreases the B(M1) value of the low-lying 1⁺ states and increases the collectivization of the scissors mode excitations in the spectroscopic energy region. The agreement between the calculated mean excitation energies as well as the summed B(M1) value of the scissors mode excitations and the available experimental data of ¹³⁴Ba and ¹³⁶Ba is rather good. A destructive interference between the orbit and spin part of the M1 strength has been found for barium isotopes near the shell closer. For all the nuclei under investigation, the low-lying M1 transitions have ΔK = 1 character as it is the case for the well-deformed nuclei.