Search results for "Excitation"
showing 10 items of 1290 documents
One-dimensional quantum-spin—phonon solitons
1983
The quantum dynamics of a compressible harmonic chain of $N$ two-level atoms strongly interacting with the phonons of the lattice is investigated. Two types of mixed excitations are discussed which propagate through the lattice exhibiting solitonic properties. The first type of solitonlike excitation describes the motion of the wall separating two magnetoelastic domains. This transports less energy than the second type of solitonlike excitation which describes the motion of a single spin reversal in the chain. An explicit expression is obtained for the speed of these excitations as a function of an appropriate shape parameter $h$. These results are obtained by approximate self-consistent in…
Behavior of one-magnon frequency in antiferromagneticNicMg1−cOsolid solutions
2005
The one-magnon scattering was studied in antiferromagnetic ${\mathrm{Ni}}_{c}{\mathrm{Mg}}_{1\ensuremath{-}c}\mathrm{O}$ solid solutions. We observed unpredicted behavior of both temperature and composition dependences of one-magnon excitation energies. First, the abrupt change of the one-magnon frequency by $7--8\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ occurs between $c=0.98$ and $c=0.9$ in the limit of $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Second, upon increasing temperature, the one-magnon energy for highly diluted nickel oxide vanishes significantly below the N\'eel temperature.
Generation and Evolution of Spin-, Valley-, and Layer-Polarized Excited Carriers in Inversion-Symmetric WSe2
2016
We report the spin-selective optical excitation of carriers in inversion-symmetric bulk samples of the transition metal dichalcogenide (TMDC) ${\mathrm{WSe}}_{2}$. Employing time- and angle-resolved photoelectron spectroscopy (trARPES) and complementary time-dependent density functional theory (TDDFT), we observe spin-, valley-, and layer-polarized excited state populations upon excitation with circularly polarized pump pulses, followed by ultrafast ($l100\text{ }\text{ }\mathrm{fs}$) scattering of carriers towards the global minimum of the conduction band. TDDFT reveals the character of the conduction band, into which electrons are initially excited, to be two-dimensional and localized wit…
Electronic correlation effects and the Coulomb gap at finite temperature.
2000
We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type Germanium, using tunneling spectroscopy on mechanically controllable break junctions. The tunnel conductance was measured as a function of energy and temperature. At low temperatures, the spectra reveal a minimum at zero bias voltage due to the Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by thermal excitations. This behavior is reflected in the temperature dependence of the variable-range hopping resitivity measured on the same samples: Up to a few degrees Kelvin the Efros-Shkovskii ln$R \propto T^{-1/2}$ law is obeyed, whereas at higher tempe…
Full triples contribution in coupled-cluster and equation-of-motion coupled-cluster methods for atoms and molecules in strong magnetic fields
2020
Coupled-cluster as well as equation-of-motion coupled-cluster methods play an important role whenever high accuracy is warranted. Concerning excitation energies, consideration of triple excitations is typically required to reach an accuracy better than 0.1–0.3 eV. In the context of strong magnetic fields such accuracy is needed for the prediction of spectra of strongly magnetized White Dwarfs. In addition it turns out that in order to correctly model the behavior of energies with respect to the magnetic field strength, triple excitations are required. Due to avoided crossings which are extremely often encountered in the context of strong magnetic fields, double-excitation character can be t…
Insights into the orbital invariance problem in state-specific multireference coupled cluster theory.
2010
In this communication we report the results of our studies on the orbital invariance properties of the state-specific multireference coupled cluster approach suggested by Mukherjee and co-workers (Mk-MRCC). In particular, we have gathered numerical evidence to show that even when the linear excitation manifold is modified in order to span the same space for each reference, the resulting method is not orbital invariant. In order to test this conjecture we have proposed a new truncation scheme (Mk-MRCCSDtq) which, in addition to full single and double excitations, contains partial triple and quadruple excitations. For a reference space generated by all possible combinations of two electrons i…
Excited states with internally contracted multireference coupled-cluster linear response theory.
2014
In this paper, the linear response (LR) theory for the variant of internally contracted multireference coupled cluster (ic-MRCC) theory described by Hanauer and Kohn [J. Chem. Phys. 134, 204211 (2011)] has been formulated and implemented for the computation of the excitation energies relative to a ground state of pronounced multireference character. We find that straightforward application of the linear-response formalism to the time-averaged ic-MRCC Lagrangian leads to unphysical second-order poles. However, the coupling matrix elements that cause this behavior are shown to be negligible whenever the internally contracted approximation as such is justified. Hence, for the numerical impleme…
Explicitly correlated connected triple excitations in coupled-cluster theory.
2009
A way to incorporate explicit electron correlation into connected triple excitations in coupled-cluster theory is proposed. The new ansatz is applied to the coupled-cluster singles and doubles model with noniterative triple excitations [CCSD(T)] and does not introduce any further sets of equations to be solved. A first implementation using automated generation and string-based evaluation of the explicit expressions is reported. The results demonstrate that the ansatz significantly enhances the basis set convergence of the noniterative triple excitation correction and thus improves upon previous approaches to explicitly correlated CCSD(T).
Isobar excitation in proton-proton bremsstrahlung
1978
Abstract The contribution of intermediate nucleon isobars to proton-proton bremsstrahlung is calculated in a relativistic model. Using conventional coupling constants and form factors for the exchange of π - and ρ-mesons we find that for photon energies ω > 100 MeV isobar excitation increases the cross section markedly over the prediction of soft-photon calculations. Our results agree well with recent experiments at incident proton energy T lab =730 MeV.
Electron spectra in forbidden β decays and the quenching of the weak axial-vector coupling constant gA
2017
Evolution of the electron spectra with the effective value of the weak axial-vector coupling constant ${g}_{\mathrm{A}}$ was followed for 26 first-, second-, third-, fourth- and fifth-forbidden ${\ensuremath{\beta}}^{\ensuremath{-}}$ decays of odd-$A$ nuclei by calculating the involved nuclear matrix elements (NMEs) in the framework of the microscopic quasiparticle-phonon model (MQPM). The next-to-leading-order terms were included in the $\ensuremath{\beta}$-decay shape factor of the electron spectra. The spectrum shapes of third- and fourth-forbidden nonunique decays were found to depend strongly on the value of ${g}_{\mathrm{A}}$, while first- and second-forbidden decays were mostly unaff…