113 research outputs found
Isospin dependence of nucleon Correlations in ground state nuclei
The dispersive optical model (DOM) as presently implemented can investigate
the isospin (nucleon asymmetry) dependence of the Hartree-Fock-like potential
relevant for nucleons near the Fermi energy. Data constraints indicate that a
Lane-type potential adequately describes its asymmetry dependence. Correlations
beyond the mean-field can also be described in this framework, but this
requires an extension that treats the non-locality of the Hartree-Fock-like
potential properly. The DOM has therefore been extended to properly describe
ground-state properties of nuclei as a function of nucleon asymmetry in
addition to standard ingredients like elastic nucleon scattering data and level
structure. Predictions of nucleon correlations at larger nucleon asymmetries
can then be made after data at smaller asymmetries constrain the potentials
that represent the nucleon self-energy. A simple extrapolation for Sn isotopes
generates predictions for increasing correlations of minority protons with
increasing neutron number. Such predictions can be investigated by performing
experiments with exotic beams. The predicted neutron properties for the double
closed-shell 132Sn nucleus exhibit similar correlations as those in 208Pb.
Future relevance of these studies for understanding the properties of all
nucleons, including those with high momentum, and the role of three-body forces
in nuclei are briefly discussed. Such an implementation will require a proper
treatment of the non-locality of the imaginary part of the potentials and a
description of high-momentum nucleons as experimentally constrained by the
(e,e'p) reactions performed at Jefferson Lab.Comment: 7 pages and 7 figure
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
Transfer reactions and the dispersive optical-model
The dispersive optical-model is applied to transfer reactions. A systematic
study of reactions on closed-shell nuclei using the finite-range
adiabatic reaction model is performed at several beam energies and results are
compared to data as well as to predictions using a standard global
optical-potential. Overall, we find that the dispersive optical-model is able
to describe the angular distributions as well as or better than the global
parameterization. In addition, it also constrains the overlap function.
Spectroscopic factors extracted using the dispersive optical-model are
generally lower than those using standard parameters, exhibit a reduced
dependence on beam energy, and are more in line with results obtained from
measurements.Comment: Phys. Rev. C 84, 044611 (2011
Microscopic self-energy calculations and dispersive optical-model potentials
Nucleon self-energies for 40Ca, 48Ca, 60Ca isotopes are generated with the
microscopic Faddeev-random-phase approximation (FRPA). These self-energies are
compared with potentials from the dispersive optical model (DOM) that were
obtained from fitting elastic-scattering and bound-state data for 40Ca and
48Ca. The \textit{ab initio} FRPA is capable of explaining many features of the
empirical DOM potentials including their nucleon asymmetry dependence. The
comparison furthermore provides several suggestions to improve the functional
form of the DOM potentials, including among others the exploration of parity
and angular momentum dependence. The non-locality of the FRPA imaginary
self-energy, illustrated by a substantial orbital angular momentum dependence,
suggests that future DOM fits should consider this feature explicitly. The
roles of the nucleon-nucleon tensor force and charge-exchange component in
generating the asymmetry dependence of the FPRA self-energies are explored. The
global features of the FRPA self-energies are not strongly dependent on the
choice of realistic nucleon-nucleon interaction.Comment: Submitted to Phys. Rev.
Microscopic self-energy of Ca from the charge-dependent Bonn potential
The effects of short-range correlations on the nucleon self-energy in
Ca are investigated using the charge-dependent Bonn (CDBonn)
interaction. Comparisons are made with recent results for the self-energy of
Ca derived from the dispersive optical-model (DOM). Particular emphasis
is placed on the non-locality of the imaginary part of the microscopic
self-energy which suggests that future DOM analyses should include this
feature. In particular, data below the Fermi energy appear sensitive to the
implied orbital angular momentum dependence of the self-energy. Quasiparticle
properties obtained for the CDBonn interaction are substantially more
mean-field-like than the corresponding DOM results with spectroscopic factors
larger by about 0.2 e.g. Reaction cross sections obtained from the microscopic
self-energy for scattering energies up to 100 MeV indicate that an adequate
description of volume absorption is obtained while a considerable fraction of
surface absorption is missing. The analysis of the non-locality of the
imaginary part of the microscopic self-energy suggests that a simple gaussian
provides an adequate description, albeit with rather large values for ,
the non-locality parameter.Comment: 18 pages, 19 figures, 4 table
Nonlocal extension of the dispersive-optical-model to describe data below the Fermi energy
Present applications of the dispersive-optical-model analysis are restricted
by the use of a local but energy-dependent version of the generalized
Hartree-Fock potential. This restriction is lifted by the introduction of a
corresponding nonlocal potential without explicit energy dependence. Such a
strategy allows for a complete determination of the nucleon propagator below
the Fermi energy with access to the expectation value of one-body operators
(like the charge density), the one-body density matrix with associated natural
orbits, and complete spectral functions for removal strength. The present
formulation of the dispersive optical model (DOM) therefore allows the use of
elastic electron-scattering data in determining its parameters. Application to
Ca demonstrates that a fit to the charge radius leads to too much
charge near the origin using the conventional assumptions of the functional
form of the DOM. A corresponding incomplete description of high-momentum
components is identified, suggesting that the DOM formulation must be extended
in the future to accommodate such correlations properly. Unlike the local
version, the present nonlocal DOM limits the location of the deeply-bound hole
states to energies that are consistent with (\textit{e,e}\textit{p})
and (\textit{p,2p}) data.Comment: 14 pages, 10 figures, submitted to Physical Review
Experimental Observation of ABCB Stacked Tetralayer Graphene
In tetralayer graphene, three inequivalent layer stackings should exist; however, only rhombohedral (ABCA) and Bernal (ABAB) stacking have so far been observed. The three stacking sequences differ in their electronic structure, with the elusive third stacking (ABCB) being unique as it is predicted to exhibit an intrinsic bandgap as well as locally flat bands around the K points. Here, we use scattering-type scanning near-field optical microscopy and confocal Raman microscopy to identify and characterize domains of ABCB stacked tetralayer graphene. We differentiate between the three stacking sequences by addressing characteristic interband contributions in the optical conductivity between 0.28 and 0.56 eV with amplitude and phase-resolved near-field nanospectroscopy. By normalizing adjacent flakes to each other, we achieve good agreement between theory and experiment, allowing for the unambiguous assignment of ABCB domains in tetralayer graphene. These results establish near-field spectroscopy at the interband transitions as a semiquantitative tool, enabling the recognition of ABCB domains in tetralayer graphene flakes and, therefore, providing a basis to study correlation physics of this exciting phase
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