Microscopic approach to the antiproton-nucleus optical potential

By using the self-energy of an antiproton evaluated in nuclear matter, the antiproton effective interaction with a bound nucleon is derived and is found to have a drastic density dependence. The p?-nucleus optical potential constructed by folding the effective interaction well reproduces experimental data on the p?-nucleus absorption cross section as well as on the complex level shift of p?-atoms. In addition, it is pointed out that the strong spin-orbit interaction spreads each doublet and considerably affects the lower complex level shift.     

A self-consistent F-center calculation for the optical absorption and emission

A theoretical investigation of the luminescence processes associated with F-centers has been made. It assumes that the marked Stokes's shift arises from the electrostatic interaction between the F-electron and the neighboring ions. A semi-continuum model was employed. First, an examination of most of the previous F -center calculations in NaCI and KC1 was made. These were classified into four categories: (1) the semi-continuum model; (2) the semi-continuum polaron model; (3) the pointion model (a) without polarization and (b) with polarization: and (4) the Hartree-Fock model where the internal structure of the ions which surround the vacancy is taken into account. Some models were omitted. In all calculations, the differences in the eigenvalues associated with the lowest and the first excited state give approximately the measured values of the peak absorption and the peak emission bands. The actual eigenvalues show a large scatter, indicating no reliability in the present calculating techniques. Next, a new self-consistent approach is made. The following results were derived for NaCI and KC1: (1) self-consistent potentials for the absorption and emission; (2) energy level diagrams (relative to the bottom of the conduction band) and wavefunctions for both processes; (3) self-consistent values for the displacements of the first nearest neighbors; and (4) the lifetime of the first excited state (which agrees approximately with the measured result). The loose binding approximation is manifested during emission, whereas tight binding occurs during absorption. These calculations indicate that a self-consistent use of the semi-continuum model can describe the simplest optical properties of the F-center.     

Semi-microscopic optical potential calculation by the nuclear matter approach

In this paper the semi-microscopic nuclear matter approach has been introduced to calculate the microscopic optical potential. The first- and second-order mass operators in asymmetric nuclear matter have been derived with Skyrme effective interactions and the real and imaginary parts of the optical potential for finite nuclei have been obtained by applying a local density approximation. Five Skyrme interactions II–VI have been used and compared with the experimental data to determine how well these Skyrme interaction function for our purposes. Our results obtained in this simple way are to some extent comparable with those obtained with the “nuclear matter” and “nuclear structure” approaches without adjusting the parameters of the Skyrme interactions.     

Increasing absorption optical bistability operating in the front surface layer of high absorption samples

The increasing absorption optical bistability in a bulk ZnSe sample with quite high absorption has been observed. Our experiments show that the bistability operates only in the front surface layer of sample and that the back layer of sample can be regarded as a linear absorber and heat sink.     

Assessing the adequacy of the bare optical potential in near-barrier fusion calculation

We critically examine the differences among the different bare nuclear interactions used in near-barrier heavy-ion fusion analysis and coupled-channels calculations, and discuss the possibility of extracting the barrier parameters of the bare potential from above-barrier data. We show that the choice of the bare potential may be critical for the analysis of the fusion cross sections. Although this may seem trivial, several recent papers use different bare potentials and reach different conclusions, especially when weakly bound systems are considered and possible relatively small fusion cross section enhancements or suppressions are found. We show also that the barrier parameters taken from above-barrier data may be very wrong.     

Microscopic model of the optical absorption of carbon nanotubes functionalized with molecular spiropyran photoswitches

The adsorption of molecules to the surface of carbon nanostructures opens a new field of hybrid systems with distinct and controllable properties. We present a microscopic study of the optical absorption in carbon nanotubes functionalized with molecular spiropyran photoswitches. The switching process induces a change in the dipole moment leading to a significant coupling to the charge carriers in the nanotube. As a result, the absorption spectra of functionalized tubes reveal a considerable redshift of transition energies depending on the switching state of the spiropyran molecule. Our results suggest that carbon nanotubes are excellent substrates for the optical readout of spiropyran-based molecular switches. The gained insights can be applied to other noncovalently functionalized one-dimensional nanostructures in an externally induced dipole field.     

Calculation of the optical absorption between surface states of silicon

A detailed calculation of the optical absorption between surface states on silicon is reported and the results compared with the observed absorption. Actually the observations relate to the (111) surface whereas the calculations involved the better understood (110) surface, but arguments are advanced to suggest that similar results should be obtained for both and a simulated (111) calculation presented. A model, which accurately reproduces the results of complete theoretical calculations, is used to obtain the surface state energies throughout the two-dimensional Brillouin zone, and to obtain the momentum matrix elements at each k point by utilising the effective mass sum rule. The formation of a band gap between the surface state bands, arising from surface rearrangement, is incorporated into the model in a physically sensible manner. There is very good agreement in both shape and absolute magnitude between the calculated and observed absorption clearly indicating that the measured absorption does arise from surface states. The (1, 1, 0) calculations show large polarisation effects which should be observable on reconstructed (1, 1, 0) silicon surfaces or on the III–V compounds.     

Microscopic calculation of the constitutive relations

Homogenization theory is used to calculate the macroscopic dielectric constant from the quantum microscopic dielectric function in a periodic medium. The method can be used to calculate any macroscopic constitutive relation, but it is illustrated here for the case of electrodynamics of matter. The so-called cell problem of homogenization theory is solved and an explicit expression is given for the macroscopic dielectric constant in a form akin to the Clausius-Mossotti or Lorentz-Lorenz relation. The validity of this expression is checked by showing that the standard formula is recovered for cubic materials and that the average of the microscopic energy density is the macroscopic one. Finally, the general expression is applied to Bloch eigenstates.     

Microscopic optical potential for 208Pb in the nuclear structure approach

The optical potential for nucleon-²⁰⁸Pb scattering below 30 MeV is calculated microscopically as the sum of a real Hartree-Fock term and a complex correction term arising from the coupling to excited states of the target. The Skyrme effective interaction is used to generate the Hartree-Fock field, the RPA excited states and the coupling. A complex local equivalent potential is defined and used to calculate scattering and absorption cross sections. The real part of the optical potential is reasonably well described in this approach while the imaginary part is too weak. Inclusion of rearrangement processes could improve the agreement with experiment.     

Self-consistent calculation of the optical potential and ground-state properties of nuclear matter

On the basis of the Green-function formalism, we performed a self-consistent calculation of the self-energy ∑(k, ω) of a particle interacting with the infinite nuclear medium. The function ∑(k, ω) was mapped out in the energy-momentum plane, and the single-particle energy ω(k), momentum distribution ?(k) and the “on-shell” part of the self-energy, ∑(k, ω(k)), were defined, from which all physical properties followed. In particular we investigated the ground-state properties of nuclear matter in two Λ-approximations of the T-matrix. In one, the intermediate two-particle propagator, Λ₀₀, represented free-particle propagation; in the other, called Λ₁₁, intermediate states included both interacting particles and holes. Pauli principle effects were included in both approximations. The second approximation was expected to be conserving because it included a large part of the rearrangement effects which, we found, contributed ～6 MeV per particle to the average energy and ～28 MeV to the singleparticle energy at zero momentum. The Hugenholtz-van Hove theorem was nearly satisfied, with only 1 MeV separating the chemical potential from the average energy. We also studied, in the Λ₀₀-approximation, the optical potential for the scattering of a particle by a large nucleus; it was directly related to the “on-shell” part of the self-energy. It was found that, below 100 MeV, the real part varied as (?90 + 0.584E) [MeV], and the imaginary part as (2.4 + 0.009 E) [MeV].     

Microscopic calculation of pre-equilibrium emission

Previous TDHF calculations have shown that a pre-equilibrium emission of nucleons should be seen for asymmetric collisions of nuclei. Recent calculations indicate that the added effect of two-body collisions is to enhance this pre-equilibrium emission, it being seen also for symmetric collisions. More detailed calculations of this phenomenon are presenied here. The two-body collisions are treated by the time-relaxation method. This method is reviewed, and a revised formula for the relaxation time is introduced.Calculations are made in a slab geometry. For real nuclei, as much as 6% of all nucleons are estimated to be emitted at a beam energy of E_c.m. = 20 MeV/A. The energy distribution of the emitted nucleons relates to a temperature of 13 MeV at this energy. At 5 MeV/A, the emission is reduced to about 1 %.     

Microscopic modelling of ultrafast optical switching in coupled arrays of vertical cavity surface emitting lasers for optical interconnects

In view of novel applications of ultra-high-speed vertical cavity surface emitting lasers (VCSEL) for optical interconnects and computing a microscopic theoretical model is presented which allows a general and well-founded modelling of these devices beyond the validity of conventional rate-equation approaches at ultra-fast time-scales. The model takes into account, in particular, the coupling of macroscopic device-related constraints (like the shape of the cavity) together with the ultra-fast intrinsic processes. It thereby provides the basis for the investigation and design of ultra-fast and coupled VCSELs. Simulations demonstrate the overall relevance of internal effects like the combination of microscopic spatial and spectral dynamics of the carrier distribution functions and the nonlinear polarization of the active semiconductor medium for VCSELs on short time-scales.     

Microscopic calculation of the fermi liquid interaction constants for electrons in the surface inversion layer of a semiconductor

The leading Fourier coefficients of both the spin dependent and spin independent Landau interaction function of a quasi-two-dimensional electron gas are evaluated. Three different schemes for calculating the interaction function are studied for both a pure two-dimensional Coulomb interaction and an effective electron-electron interaction appropriate to the actual metal-insulator-semiconductor structure.     

Comments on the method of moments applied to the calculation of optical absorption line shapes

In order to check reliability of the method of moments proposed by Sati et al., the line shape for an A_1g → T_1μ transition (plus T_2g-mode) is investigated in the high temperature limit. This method is applicable even when the original function has a singular point. It is essential to adopt an appropriate generating function which has the same asymptotic behavior as the original function.     

Explicit treatment of internucleon P- and D-states in the calculation of the nucleon optical potential

Usual calculations of the nucleon optical potential using the Brueckner t-matrix treat the internucleon P- and D-states in a nuclear matter approximation. We use the method of Kisslinger to obtain the potential when P- and D-states are treated more precisely. We show phenomenologically that the effect of an explicit treatment of P-states is small, consistent with calculations indicating that the total P-state contribution to the t-matrix is small. This result provides additional verification that the Brueckner method is reasonably satisfactory. The potential obtained from an explicit treatment of the D-states is such that, although large, it can be reduced approximately to the form usually used. We show that the error in this approximation is not large though non-negligible when comparing calculated cross sections with experimental data. We also show that the P- and D-states corrections do not substantially affect phenomenological fits to experimental data.     

Microscopic calculation of the collective motion in76Kr

A microscopic calculation of Bohr's collective Hamiltonian is used to describe the collective motion in the⁷⁶Kr isotope. A single-particle basis calculated in a deformed Woods-Saxon potential leads to the potential energy surface obtained by the Strutinsky renormalization procedure, and to the inertial functions determined in the cranking model approximation. The collective Schrödinger equation is solved numerically. The low-energy, even parity states in⁷⁶Kr are analyzed in the frame of this model. The theoretical results involve the potential energy and the inertial parameters as functions of intrinsic quadrupole deformations, the collective levels and wave functions including their transitions and electromagnetic moments. A good agreement between experiment and theory is obtained without adjusting specifically for this nucleus any parameter in the model. Some results regarding statical and dynamical characteristics of even-even^{74, 78, 80}Kr isotopes are also presented.