Quasiparticle-phonon Interaction Research Articles

Distinguishing Quasiparticle-Phonon Interactions by Ultrahigh-Resolution Lifetime Measurements.

We present a determination of quasiparticle-phonon interaction strengths at surfaces through measurements of phonon spectra with ultrahigh energy resolution. The lifetimes of low energy surface phonons on a pristine Ru(0001) surface were determined over a wide range of temperatures and an analysis of the temperature dependence enables us to attribute separate contributions from electron-phonon interactions, phonon-phonon interactions, and defect-phonon interactions. Strong electron-phonon interactions are evident at all temperatures and we show they dominate over phonon-phonon interactions below 400K.

Electromagnetic transitions between low-lying nonrotational states of odd-neutron nuclei in α -decay chains starting from Hs265,267,269

Background: Calculations of the structure of the low-lying states of nuclei with $Z\ensuremath{\ge}100$ play an important role in understanding the properties of nuclei belonging to the new region of the nuclide chart, which is available now for experimental study.Purpose: We calculate quasiparticle-phonon structure and the reduced $\ensuremath{\gamma}$-transition probabilities for the excited states with excitation energies up to 1 MeV for nuclei with $Z\ensuremath{\ge}100$.Methods: The quasiparticle-phonon model, which takes into account the quasiparticle-phonon interaction of different multipolarities, is used as a basis for the calculations.Results: The quasiparticle-phonon structure and the $\ensuremath{\gamma}$-reduced transition probabilities of odd-neutron $^{265,267,269}\mathrm{Hs}$, $^{261,263,265}\mathrm{Sg}$, $^{257,259,261}\mathrm{Rf}$, $^{253,255,257}\mathrm{No}$, and $^{249,251,253}\mathrm{Fm}$ are calculated. The $\ensuremath{\alpha}$-decay chains starting from $^{265,267,269}\mathrm{Hs}$ are analyzed.Conclusion: It is shown that below 500 keV the structure of the nuclear states is mainly exhausted by the single-quasiparticle component. The quasiparticle-phonon interaction starts to play an important role at excitation energies above 500 keV. The nuclei before Fm in the $\ensuremath{\alpha}$-decay chains starting from $^{265,267,269}\mathrm{Hs}$ have at least two $\ensuremath{\alpha}$-decay lines.

Microscopic Model to Take into Account Complex Configurations for Pygmy and Giant Resonances

A microscopic model for taking into account quasiparticle–phonon interaction in magic nuclei is considered within nuclear quantum many-body theory. This model is of interest for constructing a microscopic theory of pygmy and giant multipole resonances—first of all, a description of their fine structure. This article reports on a continuation and development of our earlier study [1]. Basic physics results of that study are confirmed here, and new results are obtained: (i) exact (not approximate, as in [1]) expressions for the first and second variations of the vertex in the phonon field are found and employed; (ii) a new equation involving, in addition to the known effective interaction, the total amplitude for particle–hole interaction is derived for the vertex, which is the main ingredient in the theory of finite Fermi systems; (iii) the required two-phonon configurations are obtained owing to the last result. The new equation for the vertex now contains complex configurations such as 1p1hotimestextrm{phonon} and two-phonon ones, along with numerous ground-state correlations.

Numerical Analysis of the Ground-State Magnetic Moments of ${}^{143,145,147}{\rm{Sm}}$ Isotopes

The explanation of the ground state magnetic properties of odd-mass nuclei is very informative in understanding of the complex structure of the deformed nuclei. The ground-state magnetic moments of most of the odd-A deformed nuclei have been measured by various experimental studies and there are numerous studies in the literature. However, many of the theoretical studies on magnetic moments and spin polarization effects affecting them are far from explaining these measured values. In this paper, the magnetic moments and effective spin g factors of 143,145,147Sm isotopes in the lanthanides region of the periodic table were investigated within the framework of the Quasiparticle-Phonon Nuclear Model (QPNM) for the first time. Spin-spin interaction parameters (χ) were determined by comparing theoretical and experimental values of magnetic moments of the related isotopes and it was determined that these interactions were found to have an isovector character (q = -1). It has been observed that the ground-state structures of the studied isotopes are weakly affected by quasiparticlephonon interactions and the contribution of these interactions ( values) to the ground-state wave functions is quite small (around 0.01%). Theoretical explanation of the renormalization of spin gyromagnetic factor is one of the most important problems of nuclear structure physics. The results obtained in this study for the effective spin gyromagnetic factor also agree with the phenological value .

Effects of the isoscalar and isovector interaction on the ground-state magnetic moments of odd-mass 137–145Ce nuclei

A systematic study of the magnetic properties of deformed odd-neutron 137–145Ce isotopes using the microscopic quasiparticle phonon nuclear model (QPNM) has been presented. The QPNM includes residual spin–spin interaction in both isoscalar and isovector channels. The analysis shows that in the isoscalar channel contributions to the magnetic moment coming from the neutron and proton systems practically cancel out each other. On the other hand, in the isovector channel, the coherent contribution coming from the quasiparticle–phonon interactions leads to a spin polarization (core polarization), which is important for determination of the quenched spin gyromagnetic factors (gs). The quenched spin gyromagnetic factors so called [Formula: see text] have been found to range from [Formula: see text] to [Formula: see text] in the odd-mass 137–145Ce isotopic chain, which is similar to its phenomenological value ([Formula: see text] between [Formula: see text] and [Formula: see text]). By taking into consideration the core polarization effects, the available experimental data are satisfactorily reproduced with an accuracy of 0.01μN–0.1μN.

Results of Microscopic Self-Consistent Theory of Quasiparticle—Phonon Interaction in Nuclei

A self-consistent approach in the problem of taking into account quasiparticle-phonon interaction provides a high predictive power and is free from adjustable parameters (this is of crucial importance for astrophysics). Moreover, it is consistent and makes it possible to take into account new effects. A brief survey of the results obtained within this approach on the basis of Skyrme or Fayans functionals by employing the smallness parameter g2, where g is the phonon-production amplitudes, with allowance for tadpole effects is presented. The contribution of quasiparticle-phonon interaction to ground-state electromagnetic moments of odd nuclei; second-order anharmonic effects in g2, including quadrupole moments of the first 2+ and 3− states and EL transitions between one-phonon states; third-order anharmonic effects; pygmy dipole and giant resonances; and the contribution of quasiparticle-phonon interaction to radiative properties of nuclear reactions are considered. For magic and semimagic nuclei, additional effects and structures arising in nuclear features because of quasiparticle-phonon interaction are discussed along with new—that is, three- and four-quasiparticle ground-state—correlations. Earlier unknown values of the above features, including the features of the pygmy dipole resonance in neutron-rich nickel isotopes, are predicted. It is shown that, in all of the aforementioned problems, the contributions of quasiparticle-phonon interaction is sizable, is of fundamental importance, and is necessary for explaining experimental data.

Fine Structure and Collectivity of the Levels of the Pygmy Dipole Resonance in 208Pb in a Self-Consistent Model

A novel fully self-consistent microscopic approach based on the energy density functional method is employed to calculate the fine structure of the pygmy dipole resonance in 208Pb, i.e., the energies and reduced probabilities of E1 transitions for the states with energies below 10 MeV. The approach includes the random-phase approximation, quasiparticle–phonon interaction and the single-particle continuum. The theoretical results are compared to the available high-resolution data and found to agree with measured integral characteristics of the pygmy dipole resonance at energies above 5.7 MeV. Residual spin–spin forces are quantified, and their contribution is found to be significant at both low and high energies. A recently proposed criterion is employed to analyze the collectivity of the 1–states in 208Pb.

The interacting quasiparticle–phonon picture and odd–even nuclei. Overview and perspectives

The role of the nucleon correlations in the ground states of even–even nuclei on the properties of low-lying states in odd–even spherical and transitional nuclei is studied. We reason about this subject using the language of the quasiparticle–phonon model which we extend to take account of the existence of quasiparticle⊗phonon configurations in the wave functions of the ground states of the even–even cores. Of paramount importance to the structure of the low-lying states happens to be the quasiparticle–phonon interaction in the ground states which we evaluated using both the standard and the extended random phase approximations. Numerical calculations for nuclei in the barium and cadmium regions are performed using pairing and quadrupole–quadrupole interaction modes which have the dominant impact on the lowest-lying states’ structure. It is found that states with same angular momentum and parity become closer in energy as compared to the predictions of models disregarding the backward amplitudes, which turns out to be in accord with the experimental data. In addition we found that the interaction between the last quasiparticle and the ground-state phonon admixtures produces configurations which contribute significantly to the magnetic dipolemoment of odd-A nuclei. It also reveals a potential for reproducing their experimental values which proves impossible if this interaction is neglected.

Radiative strength function and the pygmy dipole resonance in 208Pb and 70Ni

The pygmy-resonance parameters and the E1 strength function are derived for 208Pb using a fully self-consistent microscopic formalism recently developed for magic nuclei, which takes into account quasiparticle phonon interactions (or coupling to phonons) in addition to the random phase approximation. For the radiative strength function of 208Pb at energies above 5 MeV, the experimental data of the Oslo group are adequately described by our predictions, whereby the important role of coupling to phonons is confirmed. By comparing the measurements based on the (3He, 3He′γ) and (γ, γ′) reactions, we discuss the physical properties of the radiative strength function measured for 208Pb. For the neutron-rich 70Ni nucleus, predictions for the radiative strength function and the pygmy resonance are obtained using a partially self-consistent approach, which invokes the Skyrme forces in deriving the mean field, effective nucleon–nucleon interaction, and phonon characteristics.

Structure of the low-lying states of the odd-neutron nuclei with Z ≈ 100

Theoretical investigations of the structure of the low-lying states of the nuclei with Z ≈ 100 play an important role in understanding the properties of nuclei belonging to the new region of the nuclide chart which are now available for the experimental study. We perform the calculations of the excitation energies and the wave functions of the low-lying states of the nuclei with Z ≈ 100. The quasiparticle-phonon model, which takes into account the interaction of quasiparticles with phonons of different multipolarities, is used as a basis for the calculations. The excitation energies and the quasiparticle-phonon structure of the low-lying states with excitation energies up to 1200 keV of the odd-neutron nuclei 245–251Cm, 249–255Cf, 249–259Fm, 253–259No and 257–261Rf are calculated. It is shown that the excitation of the phonons and the quasiparticle-phonon interaction play an important role in the description of the properties of the excited states of the nuclei with Z ≈ 100 with excitation energy exceeding 600 keV.

Zero sound and first sound in thin arbitrarily polarized Fermi-liquid films

We study the propagation and attenuation of zero sound and first sound in thin, arbitrarily polarized Fermi-liquid films. Following Khalatnikov and Abrikosov, we solve Landau's linearized kinetic equation in the relaxation-time approximation for a complex speed of sound. Analytic solutions are obtained in the hydrodynamic and ballistic limits for arbitrary polarization. By solving the collision integral in two dimensions, we find the well-known result that quasiparticle-quasiparticle collisions contribute to the collision frequencies $1/{\ensuremath{\tau}}_{\ensuremath{\sigma}}$, with a low-temperature term proportional to ${T}^{2}\mathrm{ln}({T}_{F\ensuremath{\sigma}}/T)$, where $\ensuremath{\sigma}=\ensuremath{\uparrow},\phantom{\rule{0.16em}{0ex}}\ensuremath{\downarrow}$ is the spin state. If the films are adsorbed to a dynamic substrate, we find additional possible contributions to the collision frequency that come from quasiparticle-phonon interactions. We show, however, that for ${}^{3}$He thin films, the mismatch between possible maximum values of the Fermi velocity and the substrate speed of sound freezes this contribution out at usual experimental temperatures. Thus, we can conclude that zero sound propagates at absolute zero in this type of adsorbed Fermi-liquid film. By utilizing previous results for the Landau parameters of an arbitrarily polarized ${}^{3}$He film, we compute numerical solutions for the sound speeds and attenuation in the hydrodynamic and ballistic regimes, thereby studying the transition from first sound to zero sound as a function of temperature.

Quasiparticle-phonon interaction in the theory of finite Fermi systems

Within the Green’s function method and on the basis of the method developed by V.A. Khodel for analyzing anharmonic effects, effects of quasiparticle-phonon interaction in the second order in the amplitude of phonon production are studied in two problems as a natural development of A.B. Migdal’s theory of finite Fermi systems. Transitions between excited states and static moments of magic and nonmagic nuclei in excited states, each of which is described in the random-phase approximation, are considered. The results for this problem are found to differ considerably from those in the quasiparticle random-phase approximation. The inclusion of all second-order anharmonic effects in the extended theory of finite Fermi systems that extends the standard theory of finite Fermi systems to the case of taking into account quasiparticle-phonon interaction in order to describe excited states, but which does not take into account all such effects, is also considered. They are taken into account at a level that makes it possible to calculate static moments of odd nuclei—more precisely, the respective equation for the vertex function, which, in the theory of finite Fermi systems, is a basic ingredient that describes the interaction of a nucleus with an external field, is derived. Some numerical results obtained within the recently implemented self-consistent version of the extended theory of finite Fermi systems are also presented for 15 stable and unstable tin isotopes. These results give sufficient grounds to conclude that phenomenological systematics are inapplicable to giant dipole resonances in neutron-rich isotopes. The cross sections for radiative neutron capture that are calculated by usingmicroscopic strength functions for the neutron-rich isotopes 132Sn and 150Sn differ strongly from the cross sections calculated on the basis of a phenomenological description of giant dipole resonances. These results are of paramount importance for astrophysics and for the theory of nuclear data for reactors since they highlight the inapplicability of phenomenological systematics to giant dipole resonances and the need for self-consistent calculations for neutron-rich nuclei.

Self-consistent calculations within the quasiparticle time blocking approximation with exact account taken of the single-particle continuum

A self-consistent version of the model based on the method of Green’s functions which takes into account conventional phonons of the random phase approximation, complex configurations like 2 quasiparticles⊗phonon ones, and exact single-particle continuum is used to describe many discrete natural-parity states and giant resonances in the 123Sn and 208Pb nuclei. The quasiparticle-phonon interaction is shown to be important not only for low-lying and high-lying collective states but also for low-lying noncollective states.

Single-proton-transfer studies of 153Eu levels

Levels in 153Eu have been studied with the (t, α) reaction at 15 MeV. Earlier ( 3He,d) and ( α,t) results have also been supplemented by new measurements with improved resolution at selected angles. Spectroscopic strengths were extracted, using distorted wave Born analysis methodology, and interpreted in terms of the Nilsson model with Coriolis mixing. Several new level and band assignments are proposed. For the 5/2 + levels at 694 and 706 keV an interpretation consistent with all experimental data requires admixtures of the 1/2 +[420] and 5/2 +[402] orbitals, as well as other types of components. Specifically, in addition to the 5/2 −[532]+ Q 30 components previously proposed to explain gamma-decay modes of these levels, (t,p) results indicate that complex configurations involving pairing excitations are important for the 706 keV and other 5/2 + levels. The 5/2 +[402] strength is fragmented over several levels. A successful description of these complex structures will require consideration of several effects, including quasiparticle–phonon interactions, Coriolis mixing, and a detailed treatment of pairing correlations.

Green’s function method in the problem of complex configurations in fermi systems with pairing

The Green’s function method is used to derive general equations for describing effects of pairing in Fermi systems where there are two types of interaction, two-particle and quasiparticle-phonon interaction. These equations generalize Bardeen-Cooper-Schrieffertheory to the case of complex configurations involving “strong” phonons. In the approximation of weak coupling to phonons, realistic equations that make it possible to describe excited states of nonmagic even-even nuclei with allowance for a single-particle continuum and complex configurations of the two quasiparticles ⊗ phonon type are formulated for the first time. These equations are solved for an isovector E 1 resonance in the stable isotope 120 Sn and in the unstable isotopes 104,132Sn. It is shown that complex configurations must be taken into account in order to describe E1 excitations—in particular, in a broad energy region around the nucleon binding energy.

Quasiparticle-phonon interaction in non-magic nuclei

A general microscopic approach to describe properties of excited states in non-magic nuclei is formulated. It is based on the consistent use of the Green function method in Fermi systems with Cooper pairing. The main attention is paid to even-even nuclei, but for odd nuclei with pairing some important relations are obtained too. The quasiparticle-phonon interaction which is introduced acts also in the particle-particle channel and gives a quasiparticle-phonon contribution to pairing. When applied to the theory of giant multipole resonances, the approach includes all known sources of resonance width, i.e. QRPA configurations (which correspond to Landau damping in magic nuclei), the single-particle con- tinuum (escape width) and more complex configurations (spreading width). The use of the Green function method makes it possible to include consistently the ground-state correlations induced by the more complex configurations. In the approximation of the collective phonon creation amplitude squared, which is considered in detail here, these are the ground-state correlations caused by two-quasiparticle-phonon configurations; effects of these correlations have been found earlier to be noticeable for magic nuclei. Such a unified approach will give a reasonable description of the giant resonances' integral characteristics including their widths and of some more delicate properties like fine structure and decay characteristics. Physical arguments and earlier results of a similar approach for magic nuclei allow to use the known parameters of the Landau-Migdal non-separable interaction for all non-magic nuclei (except the light ones). This means that the theory developed is suitable for realistic predictions of the properties of unknown nuclei including unstable ones. The inclusion of the single-particle continuum allows to consider also nuclei with separation energy near zero.

Contribution of quasiparticle-phonon, recoil and coriolis interactions in describing low-lying states of 133Nd isotope

A quasiparticle-phonon coupling based on one quadrupole phonon is developed. The coupling is constructed by using a deformed average field of Nilsson, a monopole pairing interaction and a quadrupole-quadrupole force. Microscopic structure of the quadrupole phonon is given from the Tamm-Dancoff Approximation. The two effects of recoil and Coriolis forces are included with the assumption of a symmetric rotational motion. Since theoretical treatment is performed for an odd- A nuclei the configuration of intrinsic states should contain both one-quasiparticle and quasiparticle-phonon components. This method is applied to describe the level scheme of 133Nd for which an agreement with experimental data is obtained. The results are also found similar to previous calculations obtained by the Particle plus Triaxial Rotor Model.

Contribution of quasiparticle-phonon, recoil and coriolis interactions in describing low-lying states of

Contribution of quasiparticle-phonon, recoil and coriolis interactions in describing low-lying states of

The role of ground state correlations in the single-particle strength of odd nuclei with pairing

A method based on the consistent use of the Green function formalism has been developed to calculate the distribution of the single-particle strength in odd nuclei with pairing. The method takes into account the quasiparticle-phonon interaction, ground state correlations and a “refinement” of phenomenological single-particle energies and pairing gap values from the quasiparticle-phonon interaction under consideration. The calculations for 121 Sn and 119 Sn that were performed in the quasiparticle⊗phonon approximation, have shown a reasonable agreement with experiment. The ground state correlations play a noticeable role and mostly improve the agreement with experiment or shift the results to the right direction.

Four stages in the study of quasiparticle-phonon interactions

Four stages are described in the theoretical study of the transport effects of strong interactions between phonons and narrow-band quasiparticles such as electrons and excitons in molecular solids. Recent work is reviewed and future directions are commented upon.