稳定和不稳定核巨共振性质的相对论研究

在相对论平均场的基态上自洽的相对论无规位相近似(RRPA)理论框架下,研究稳定核和不稳定核的巨共振性质.研究了稳定核²⁰⁸Pb,¹⁴⁴Sm,¹¹⁶Sn,⁹⁰Zr,⁴⁰Ca,¹⁶O和不稳定核Ca同位素链同位旋标量和同位旋矢量集体巨共振激发,并讨论了Dirac海负能核子态和矢量介子空间分量对核的巨共振性质的影响.研究的结果表明,Dirac海负能核子态和矢量介子空间分量对同位旋标量激发有贡献,特别是对重核,而对轻核它的贡献减弱,对于同位旋矢量激发的贡献可忽略.几组常用的相对论平均场非线性模型参量,不仅能成功的描述有限核的基态性质,也能很好地描述核的巨共振激发.对于N/Z极端情况下,同位旋矢量巨偶极激发模式存在低能集体激发,它是由于费密面附近弱束缚核子的激发和同位旋混杂效应 关键词： 相对论无规位相近似 核巨共振     

Conserving RPA and the response of 48Ca

The connection between the single-particle self-energy and the corresponding conserving particle-hole (ph) interaction, discussed long ago by Kadanoff and Baym, is employed to study the response of ⁴⁸Ca. Second order self-energy contributions are taken into account in the construction of the energy dependent ph interaction. From this perspective it is possible to make contact with other approaches which also aim to incorporate the coupling to 2p2h excitations within the RPA framework. The method is used to study both the discrete low-energy states as well as the giant resonances in both ⁴⁸Ca and ⁴⁸Sc using a realistic G matrix interaction based on meson exchange. The calculated strength distribution compares favorably with experiment but the strength below 15 MeV is still somewhat too large as compared to experiment for all types of excitations. The quenching of magnetic and Gamow-Teller strength due to 2p2h admixture amounts to about 30%.     

Collective potentials and dipole and quadrupole giant resonances

The dynamic collective model is extended into the energy region immediately above the giant dipole resonances, i.e. into an energy region between 20 and 28 MeV. The total Hamiltonian is constructed and the dynamical problem is solved by diagonalizing the Hamiltonian in the basis of a five-dimensional harmonic oscillator. In schematical studies the splitting of giant quadrupole resonances is shown. For some elements the potential energy surfaces (PES) are constructed within the collective model developed by Gneuss et al. and the quadrupole resonances have been calculated in the framework of the dynamic collective model. In the last part the agreement with experimental data is shown.     

Isospin symmetry and giant resonance in nuclei of mass number <Emphasis Type="Italic">A</Emphasis> = 14

An analysis of the cross sections for photon absorption by isobar nuclei ¹⁴C and ¹⁴N in the giant-dipole-resonance region demonstrates a high degree of isospin symmetry for this type of collective excitations in the above nuclei. Giant resonances in A = 14 isobar nuclei are related by a simple rescaling procedure that is based on the fact that, for these nuclei, the isospin remains a good quantum number in the process of dipole excitations of energy up to about 40 MeV. The features of the isospin splitting of the giant resonance in the ¹⁴C nucleus are refined. The shape of the cross section for photoabsorption in the ¹⁴C nucleus—in particular, a large width of the giant resonance in this nucleus—is exhaustively explained.     

Spin and phonon contribution to the superconductivity in La2−xSrxCuO4

Anisotropy of high-energy spin excitations and low-energy conductive electronic excitations in Raman spectra can be interpreted, if quantum spin fluctuation is weak. The low-energy states at (π/2, π/2) in the underdoped phase are composed of insulating states induced by the insulator-metal transition and the conductive states induced by electron-phonon coupling. The superconducting states are created in the latter states and the pairing symmetry is s or d(xy). On the other hand, the electronic states at (π, 0) in the overdoped phase are electron-spin coupled states and the pairing symmetry is d(x²−y²).     

Excitation of deformation vibrations by radiationless muonic transitions

An investigation is carried out for selected doubly even nuclei in the rare-earth and actinide regions for possible resonances between pairs of muonic levels of the appropriate quantum numbers and certain nuclear collective states. Such resonances can lead to the excitation of the particular nuclear state involved, and could be used as an alternate means of nuclear excitation. Electric monopole excitations were found which can result in the excitation of the nuclear 0⁺ level of the β-band. The muonic states involved, however, are such that the probability of nuclear excitation may be small. An electric quadrupole resonance in ¹⁸²W involving the excitation of the 2⁺ state of the β-band is found to be a very likely candidate. Dipole and octupole resonances were also investigated; however no possible resonances were found. Also unsuccessful was the search for quadrupole excitations of the 2⁺ state in the γ-band. The matrix elements involved in the EO and E2 resonances are calculated, the nuclear contribution being computed using the nuclear collective model of Davydov and Chaban. The probability for nuclear excitation in the E2 resonance in ¹⁸²W is computed.     

对关联对核基态和集体激发态性质的影响

基于相对论平均场和BCS理论,研究了共振连续对奇特核对关联性质的影响. 利用S矩阵方法，通过设定合理的散射态边界条件来得到单粒子共振态的能量和宽度. 通过引入连续态能级密度的方法来处理共振态宽度对核对关联的贡献. 计算结果显示合理地处理共振态对对关联性质的贡献在研究滴线附近核性质时很重要. 它可以影响中子的对隙、费米能级、对关联能以及总结合能. 其次,基于RMF+BCS基态,采用线性响应理论给出了描述开壳核集体激发态性质的准粒子相对论无规位相近似理论. 并且将该方法应用于开壳核120Sn的各种同位旋标量集体激发态性质的研究中. 研究表明：对关联对核的集体激发性质的影响主要表现在低能集体激发态上,考虑对关联后的相对论无规位相近似理论能够很好地再现低能集体激发的实验结果.     

Anisotropic exchange and magnetic excitations in KCoF3

A superexchange mechanism is used for the determination of the form of the exchange Hamiltonian in KCoF₃ introducing only two independent parameters. The Co²⁺ ions are orbitally degenerate and this leads to the anisotropic exchange in KCoF₃. The collective excitations are introduced being based on the space of selfconsistently determined states of Co²⁺ ions in the low temperature approximation. The effective exchange field is introduced in this many-level system so that the one-ion eigenvalue problem can be solved. The agreement with two lowest branches of magnetic excitation spectra is satisfactory. It is emphasized that the appropriate general form of the exchange Hamiltonian which includes the orbital operators should be used for the interpretation of experimental data instead of the simple isotropic form. In the case of KCoF₃, however, it appears that the dispersion relation for magnetic excitations is rather insensitive on the form of the model Hamiltonian.     

Baryon resonances in the mean field approach and a simple explanation of the Θ<Superscript>+</Superscript> pentaquark

We suggest to classify baryon resonances as single-quark states in a mean field, and/or as its collective excitations. Identifying the Roper resonance N(1440, 1/2⁺), the nucleon resonance N(1535, 1/2⁻), and the singlet hyperon Λ(1405, 1/2⁻) as single-quark excitations, we find that there must be an exotic S = +1 baryon resonance Θ⁺ (the “pentaquark”) with a mass about 1440 + 1535 − 1405 = 1570 MeV and spin-parity 1/2⁺. We argue that Θ⁺ is an analog of the Gamov-Teller excitation long known in nuclear physics.     

Damping of single-particle states and giant resonances in208Pb

The damping width of single-particle states and of giant resonances is estimated in ²⁰⁸Pb, based on the excitation of surface modes and T = 1 pairs. The damping is dominated by the collectivity of the surface modes, with the pairing modes playing a weaker role. For both the single-particle states and giant resonances, the predicted damping is somewhat small. This could be due to the neglect of T = 0 particle-particle correlations or to the neglect of the volume modes. The damping for the giant resonances is reduced by the coherence between particle and hole.