Calculations Of Excitation Spectra Research Articles

Fully discrete model of kinetic ion-induced electron emission from metal surfaces

Ion-induced electron emission (IIEE) is an important process whereby ions impinging on a material surface lead to net emission of electrons into the vacuum. While relevant for multiple applications, IIEE is a critical process of electric thruster (ET) operation and testing for space propulsion, and, as such, it must be carefully quantified for safe and reliable ET performance. IIEE is a complex physical phenomenon, which involves a number of ion-material and ion-electron processes, and is a complex function of ion mass, energy, and angle, as well as host material properties, such as mass and electronic structure. In this paper, we develop a discrete model of kinetic IIEE to gain a more accurate picture of the electric thruster chamber and facility material degradation processes. The model is based on three main developments: (i) the use of modern electronic and nuclear stopping databases, (ii) the use of the stopping and range of ions in matter to track all ion and recoil trajectories inside the target material, and (iii) the use of a scattering Monte Carlo approach to track the trajectories of all mobilized electrons from the point of first energy transfer until full thermalization or escape. This represents a substantial advantage in terms of physical accuracy over existing semi-analytical models commonly used to calculate kinetic IIEE. We apply the model to Ar, Kr, and Xe irradiation of W and Fe surfaces and calculate excitation spectra as a function of ion depth, energy, and angle of incidence. We also obtain minimum threshold ion energies for net nonzero yield for each ion species in both Fe and W and calculate full IIEE yields as a function of ion energy and incidence angle. Our results can be used to assess the effect of kinetic electron emission in models of full ET facility testing and operation.

Performance of Common Density Functionals for Excited States of Tetraphenyldibenzoperiflanthene.

Time-dependent density functional theory is the method of choice to efficiently calculate excitation spectra with the functional and basis set choice allowing one to compromise between accuracy and computational cost. In this work, the performance of different functionals as well as the second-order approximate coupled cluster singles and doubles model CC2 is evaluated by comparing the results to experimental results of the example molecule tetraphenyldibenzoperiflanthene (DBP). The choice of the functional has a significant impact on the calculated spectrum of DBP. The performance of a number of different functionals was evaluated, quantified, and, where possible, discussed. The best functional, tuned-CAM-B3LYP, is used to investigate DBP on a surface of hexagonal boron nitride (h-BN). The resulting spectrum shows excellent agreement with experimental results for a monolayer of DBP on h-BN.

Excitation Spectra of Cubic Perovskite Titanates

We calculate excitation spectra of cubic perovskites ATiO3 (A = Ca, Sr, Ba, Pb). The calculations are performed within the time-dependent density functional theory, including local field effects. The theoretical calculations show that the perovskites have a plasmon mode at around 12 eV, which is not observed in experiments.

Excitation spectra in fluids: How to analyze them properly

Although the understanding of excitation spectra in fluids is of great importance, it is still unclear how different methods of spectral analysis agree with each other and which of them is suitable in a wide range of parameters. Here, we show that the problem can be solved using a two-oscillator model to analyze total velocity current spectra, while other considered methods, including analysis of the spectral maxima and single mode analysis, yield rough results and become unsuitable at high temperatures and wavenumbers. To prove this, we perform molecular dynamics (MD) simulations and calculate excitation spectra in Lennard-Jones and inverse-power-law fluids at different temperatures, both in 3D and 2D cases. Then, we analyze relations between thermodynamic and dynamic features of fluids at (Frenkel) crossover from a liquid- to gas-like state and find that they agree with each other in the 3D case and strongly disagree in 2D systems due to enhanced anharmonicity effects. The results provide a significant advance in methods for detail analysis of collective fluid dynamics spanning fields from soft condensed matter to strongly coupled plasmas.

Chiral bosonic phases on the Haldane honeycomb lattice

Recent experiments in ultracold atoms and photonic analogs have reported the implementation of artificial gauge fields in lattice systems, facilitating the realization of topological phases. Motivated by such advances, we investigate the Haldane honeycomb lattice tight-binding model, for bosons with local interactions at the average filling of one boson per site. We analyze the ground state phase diagram and uncover three distinct phases: a uniform superfluid (SF), a chiral superfluid (CSF) and a plaquette Mott insulator with local current loops (PMI). Nearest-neighbor and next-nearest neighbor currents distinguish CSF from SF, and the phase transition between them is first order. We apply bosonic dynamical mean field theory and exact diagonalization to obtain the phase diagram, complementing numerics with calculations of excitation spectra in strong and weak coupling perturbation theory. The characteristic density fluctuations, current correlation functions, and excitation spectra are measurable in ultracold atom experiments.

Microscopic analysis of quadrupole-octupole shape evolution

We analyze the quadrupole-octupole collective states based on the microscopic energy density functional framework. By mapping the deformation constrained self-consistent axially symmetric mean-field energy surfaces onto the equivalent Hamiltonian of the sd f interacting boson model (IBM), that is, onto the energy expectation value in the boson coherent state, the Hamiltonian parameters are determined. The resulting IBM Hamiltonian is used to calculate excitation spectra and transition rates for the positive- and negative-parity collective states in large sets of nuclei characteristic for octupole deformation and collectivity. Consistently with the empirical trend, the microscopic calculation based on the systematics of β2 − β3 energy maps, the resulting low-lying negative-parity bands and transition rates show evidence of a shape transition between stable octupole deformation and octupole vibrations characteristic for β3-soft potentials.

Microscopic analysis of shape evolution and triaxiality in germanium isotopes

Background: The motivation for this study is the experimental evidence for rigid triaxial deformation at low energy in ^76Ge that was recently observed. Purpose: Quadrupole shapes and low-energy spectra of the isotopes ^72–82Ge are analyzed using a theoretical framework based on nuclear density functional theory. Method: The relativistic functional DD-PC1, supplemented by a finite-range pairing force, is used to perform constrained triaxial mean-field calculations of energy surfaces as functions of quadrupole deformation parameters. The corresponding collective Hamiltonian, based on DD-PC1, is employed in the calculation of excitation spectra and transition rates. Results: Model calculations reproduce the empirical trend of collective observables and predict the evolution of shapes from weakly triaxial in ^74Ge to γ soft in ^78,80Ge. For ^76Ge, in particular, the theoretical excitation spectrum is in good agreement with available data, the experimental ratio E(2^+_2)/E(2^+_1) is reproduced, as well as the pattern and amplitude of the staggering in energy between odd- and even-spin states in the γ band. Conclusions: The mean-field potential of ^76Ge appears to be γ soft. Collective correlations drive the nucleus toward triaxiality but do not stabilize a rigid triaxial shape. Both the experimental and theoretical staggering of levels in the γ band display a pattern consistent with triaxial shapes but the amplitudes are negligible and do not present evidence for rigid triaxiality.

Microscopic description of octupole shape-phase transitions in light actinide and rare-earth nuclei

A systematic analysis of low-lying quadrupole and octupole collective states is presented, based on the microscopic energy density functional framework. By mapping the deformation constrained self-consistent axially symmetric mean-field energy surfaces onto the equivalent Hamiltonian of the $sdf$ interacting boson model (IBM), that is, onto the energy expectation value in the boson condensate state, the Hamiltonian parameters are determined. The study is based on the global relativistic energy density functional DD-PC1. The resulting IBM Hamiltonian is used to calculate excitation spectra and transition rates for the positive- and negative-parity collective states in four isotopic chains characteristic for two regions of octupole deformation and collectivity: Th, Ra, Sm and Ba. Consistent with the empirical trend, the microscopic calculation based on the systematics of $\beta_{2}$-$\beta_{3}$ energy maps, the resulting low-lying negative-parity bands and transition rates show evidence of a shape transition between stable octupole deformation and octupole vibrations characteristic for $\beta_{3}$-soft potentials.

Polariton Mott insulator with trapped ions or circuit QED

We consider variants of the Jaynes-Cummings-Hubbard model of lattice polaritons, taking into account next-nearest-neighbor, diagonal, and long-range photon hopping in one and two dimensions. These models are relevant for potential experimental realizations of polariton Mott insulators based on trapped ions or microwave stripline resonators. We obtain the Mott-superfluid phase boundary and calculate excitation spectra in the Mott phase using numerical and analytical methods. Including the additional hopping terms leads to a larger Mott phase in the case of trapped ions, and to a smaller Mott phase in the case of stripline resonators, compared to the original model with nearest-neighbor hopping only. The critical hopping for the transition changes by up to about 50 percent in one dimension and by up to about 20 percent in two dimensions. In contrast, the excitation spectra remain largely unaffected.

The finite-temperature binary collision method for treating spin-dimer systems: Taking TlCuCl 3 as an example

We showed that the bond-operator Hamiltonian for spin-dimer systems above the critical field is equivalent to a single-field (effective) Hamiltonian with finite coupling strength. This equivalence is revealed by going beyond the mean field level of the singlet operator and through an orthogonal transformation. The associated field operator, named as the effective magnon in this single-field Hamiltonian, is the linear combination of the hard-core triplet magnons. The magnetic properties and condensation of spin dimers are ascribed to the behaviors of effective magnons. By recognizing the effective coupling of the Hamiltonian for hard-core effective magnons as the sum of the T -dependent ladder diagrams with hard-core vertex, we study the finite-temperature behaviors of spin-dimer systems in both condensed and noncondensed phases. We calculate excitation spectra, energy gaps, and the parallel and transverse (staggered) magnetizations at finite temperatures. Our results, which are improvements on the ordinary Hartree–Fock–Popov approximation, are consistent with experiments and numerical results.

Microscopic analysis of shape mixing in low-lying states of proton-rich nuclei in the Se-Kr region

Using the five-dimensional quadrupole collective Hamiltonian, we study the oblate-prolate shape coexistence/mixing phenomena in the low-lying states of proton-rich nuclei in the A=70-90 region from a viewpoint of oblate-prolate symmetry and its breaking. To derive the collective Hamiltonian microscopically, we have developed a new method, on the basis of the adiabatic self-consistent collective coordinate method, for determining the collective potential, the vibrational and rotational inertial masses in it. By solving the collective Schrödinger equation, we calculate excitation spectra, spectroscopic quadrupole moments and electric quadrupole transition probabilities among the low-lying states. The result of the calculation clearly indicates the dominant role of the large-amplitude vibration in the triaxial shape degree of freedom. It also exhibits an interesting effect of rotation, which may be called 'rotational hindrance to the oblate-prolate shape mixing', that is, the growth of localization of the collective wave function in the (β, γ) deformation space assisted by the rotational motion.

Itinerant-localized dual character of a strongly correlated superfluid Bose gas in an optical lattice

We investigate a strongly-correlated Bose gas in an optical lattice. Extending the standard-basis operator method developed by Haley and Erdos to a boson Hubbard model, we calculate excitation spectra in the superfluid phase, as well as in the Mott insulating phase, at T=0. In the Mott phase, the excitation spectrum has a finite energy gap, reflecting the localized character of atoms. In the superfluid phase, the excitation spectrum is shown to have an itinerant-localized dual structure, where the gapless Bogoliubov mode (which describes the itinerant character of superfluid atoms) and a band with a finite energy gap coexist. We also show that the rf-tunneling current measurement would give a useful information about the duality of a strongly-correlated superfluid Bose gas near the superfluid-insulator transition.

Absorption spectra of pulse-train-excited sodium two-level atoms

The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.

The electronic spectra of conjugated free radicals

Ensemble averaging schemes are applied within the PPP model to calculate excitation spectra of free conjugated hydrocarbon radicals and further, more general forms of the pairing and particle-hole properties for conjugated hydrocarbons are shown to be valid.