Intrinsic State Density Research Articles

Anharmonic Vibrational Properties from Intrinsic n-Mode State Densities

A method for calculating fully anharmonic vibrational state counts, state densities, and partition functions for molecules is presented. The method makes use of a new quantity, the intrinsic density of states, which is associated with the states that uniquely arise from a given mode, mode pairing, or higher-order mode coupling. By using only low-order intrinsic densities, the fully coupled anharmonic vibrational result can be constructed, as shown by our application of the method to methane, CH4, and cyclopropene, C3H4. Truncation of the intrinsic expansion at the coupling of pairs of modes yields greatly improved scaling over direct evaluation of the full-dimensional result and recovers a large fraction of the total anharmonicity. We also discuss the relation of the new quantities to the structure of the potential energy surface.

Crossover from vibrational to rotational collectivity in heavy nuclei in the shell-model Monte Carlo approach.

Heavy nuclei exhibit a crossover from vibrational to rotational collectivity as the number of neutrons or protons increases from shell closure towards midshell, but the microscopic description of this crossover has been a major challenge. We apply the shell model Monte Carlo approach to families of even-even samarium and neodymium isotopes and identify a microscopic signature of the crossover from vibrational to rotational collectivity in the low-temperature behavior of ⟨J(2)⟩(T), where J is the total spin and T is the temperature. This signature agrees well with its values extracted from experimental data. We also calculate the state densities of these nuclei and find them to be in very good agreement with experimental data. Finally, we define a collective enhancement factor from the ratio of the total state density to the intrinsic state density as calculated in the finite-temperature Hartree-Fock-Bogoliubov approximation. The decay of this enhancement factor with excitation energy is found to correlate with the pairing and shape phase transitions in these nuclei.

Improved microscopic nuclear level densities within the Hartree-Fock-Bogoliubov plus combinatorial method

New developments have been brought to our energy-, spin-, and parity-dependent nuclear level densities based on the microscopic combinatorial model. As in our previous study, a detailed calculation of the intrinsic state density and of the rotational enhancement factor is included, but this time the vibrational contributions explicitly take the phonon excitations into account. The present model predicts the experimental $s$- and $p$-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives a reliable extrapolation at low energies where experimental data on the cumulative number of levels can be extracted. The predictions are also in good agreement with the experimental data extracted from the analysis of particle-$\ensuremath{\gamma}$ coincidence in the ($^{3}\mathrm{He}$, $\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$, $^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$) reactions. Total as well as partial level densities for more than 8500 nuclei are made available in a table format for practical applications. For the nuclei for which experimental $s$-wave spacings and enough low-lying states exist, renormalization factors are provided to reproduce simultaneously both observables. The same combinatorial method is used to estimate the nuclear level densities at the fission saddle points of actinides and at the shape isomer deformation. Finally, the new nuclear level densities are applied to the calculation of radiative neutron capture cross sections.

Trap distribution and the impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene)

The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of the second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements that exhibited a strong decrease in the mobility with air exposure time.

Influence of traps on charge transport in organic semiconductors

The effect of extrinsic traps on the charge transport in organic semiconductors has been investigated. An analytical model describing hopping transport with traps is formulated on the basis of percolation theory. The results show that the presence of a trap distribution with energy offset and width different from that of the intrinsic density of states does not change the basic phenomenology of hopping transport, as revealed by the temperature dependence of the conductivity at high temperature. However the traps may significantly affect the transport at low temperature. The relation between trap concentration and conductivity is discussed. The model predictions are in good agreement with experimental results.

Global microscopic nuclear level densities within the HFB plus combinatorial method for practical applications

New energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial model are proposed for practical applications. The combinatorial model includes a detailed microscopic calculation of the intrinsic state density and of the rotational enhancement factor, but a phenomenological treatment of the vibrational effect. The calculations make a coherent use of nuclear structure properties determined within the deformed Skyrme–Hartree–Fock–Bogolyubov framework. The present model predicts the experimental s-wave and p-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives reliable extrapolations to low energies where experimental data on the cumulative number of levels can be extracted. Level densities for more than 8500 nuclei are made available in a table format for practical applications.

Shot noise in diffusive ferromagnetic metals

We show that shot noise in a diffusive ferromagnetic wire connected by tunnel contacts to two ferromagnetic electrodes can probe the intrinsic density of states and the extrinsic impurity scattering spin-polarization contributions in the polarization of the wire conductivity. The effect is more pronounced when the electrodes are perfectly polarized in opposite directions. While in this case the shot noise has a weak dependence on the impurity scattering polarization, it is strongly affected by the polarization of the density of states. For a finite spin-flip scattering rate the shot noise increases well above the normal state value and can reach the full Poissonian value when the density of states tends to be perfectly polarized. For the parallel configuration we find that the shot noise depends on the relative sign of the intrinsic and the extrinsic polarizations.

Measured intrinsic defect density throughout the entire band gap at the Si/SiO2 interface

Conductance-frequency measurements down to temperatures of 100 K have been performed on both p-type and n-type <100> silicon oxidized in dry oxygen at 900 degrees C. The metal electrode capacitors used were not given a post-metallization anneal in forming gas. This has allowed measurements of the intrinsic density of states, capture cross section, and surface potential fluctuations to within 0.06 eV of the band edges. Two peaks in the defect density at energies of 0.3 eV and 0.85 eV above the valence band are clearly visible above an asymmetric background, which rises rapidly towards the conduction band edge. The capture cross section is near constant at approximately 1016 cm2 across the gap and independent of temperature. The surface potential fluctuations reveal a peak value of approximately 70 meV centred at 0.4 eV above the valence band superimposed on a constant background of approximately 40 meV. The authors attribute the peaks in the density of states to the amphoteric trivalent silicon Pb centres. The probable causes of the asymmetric background are either the tail of a defect peak centred around the conduction band edge, or states descending from the conduction band induced by stress within the oxide.

Charge transfer in metal/polymer contacts

The authors have investigated the contact charging of polymers by metals, paying particular attention to the variation of charge transfer from place to place on the sample surface, and from one sample to another. For most polymers they find that charge transfer is unaffected by attempts at purification and is much the same for samples prepared in quite different ways, and they conclude that charge transfer is an intrinsic property of polymers, not normally dominated by the presence of accidental impurities etc. There is one exception however: charge transfer to PTFE appears to be determined by physical damage. For some polymers the charge transfer tends to increase as the work function of the contacting metal increases but this tendency is not universal; charge transfer to some polymers is insensitive to the metal work function. (They show that this is true for multiple contacts and for sliding as well as for single non-sliding contacts.) The apparent charge density shows remarkably little variation from one polymer to another, and this implies that the density of the electron states responsible for contact electrification is much the same for a wide range of polymers. However, the density of states required to account for the observed charge densities is much smaller than the density of intrinsic states, such as the orbitals of 'pendant' chemical groups, and too small to allow significant back-tunnelling during separation. The observations cannot be satisfactorily reconciled with current models of charge transfer which assume electron exchange between the metal and localised states in the polymer unless intrinsic states of sufficiently low density can be plausibly identified.

Shell-correction approach to nuclear state densities and the competition between fission and neutron emission of 210Po

Starting from the Hartree-Fock approximation to the grand canonical partition function we formulate a consistent renormalization of the ground-state energy and the intrinsic state density as a function of deformation. The competition between fission and neutron emission, Γ f Γ n (E) , of 210Po is studied within the framework of the statistical theory as an example. Calculations using renormalized state densities are compared with usual shell-model calculations and experimental data. We find that the usual calculations reflect the incorrect uniform deformamation dependence of the shell-model spectral function. As important changes due to renormalization we find: there is a rapid change of the shape of the transition state at ≈ 45 MeV excitation energy, Γ f Γ n (E) remains smaller than unity for all excitation energies and the deformation of the transition state increases after the “shape transition” at 45 MeV monotonically towards the liquid drop saddle-point deformation.