arXiv subjects

K. Heyde

Publications and source records attributed to K. Heyde.

At least 37 records · Page 2Linked to original sources

arXiv2007

Microscopic calculation of symmetry projected nuclear level densities

We present a quantum Monte Carlo method with exact projection on parity and angular momentum that is free of sign-problems for seniority-conserving nuclear interactions. This technique allows a microscopic calculation of angular momentum and parity projected nuclear level densities. We present results for the $^{55}$Fe, $^{56}$Fe and $^{57}$Fe isotopes. Signatures of the pairing phase transition are observed in the angular momentum distribution of the nuclear level density.

nucl-th↗

arXiv2007

Quadrupole collective variables in the natural Cartan-Weyl basis

The matrix elements of the quadrupole collective variables, emerging from collective nuclear models, are calculated in the natural Cartan-Weyl basis of O(5) which is a subgroup of a covering $SU(1,1)\times O(5)$ structure. Making use of an intermediate set method, explicit expressions of the matrix elements are obtained in a pure algebraic way, fixing the $γ$-rotational structure of collective quadrupole models.

nucl-th↗

arXiv2006

Solution of the Bohr hamiltonian for soft triaxial nuclei

The Bohr-Mottelson model is solved for a generic soft triaxial nucleus, separating the Bohr hamiltonian exactly and using a number of different model-potentials: a displaced harmonic oscillator in $γ$, which is solved with an approximated algebraic technique, and Coulomb/Kratzer, harmonic/Davidson and infinite square well potentials in $β$, which are solved exactly. In each case we derive analytic expressions for the eigenenergies which are then used to calculate energy spectra. Here we study the chain of osmium isotopes and we compare our results with experimental information and previous calculations.

nucl-th↗

arXiv2005

A theoretical description of energy spectra and two-neutron separation energies for neutron-rich zirconium isotopes

Very recently the atomic masses of neutron-rich Zr isotopes, from $^{96}$Zr to $^{104}$Zr, have been measured with high precision. Using a schematic Interacting Boson Model (IBM) Hamiltonian, the evolution from spherical to deformed shapes along the chain of Zr isotopes, describing at the same time the excitation energies as well as the two-neutron separation energies, can be rather well reproduced. The interplay between phase transitions and configuration mixing of intruder excitations in this mass region is succinctly addressed.

nucl-th↗

arXiv2005

Spin-dependent neutrino-induced nucleon knockout

We study neutrino-induced nucleon knockout off atomic nuclei and examine the polarization properties of the ejectile. A detailed study of the spin dependence of the outgoing nucleon is presented. The numerical results are derived within a non-relativistic plane-wave impulse-approximation approach. Our calculations reveal large polarization asymmetries, and clear dissimilarities between neutrino- and antineutrino-induced reactions. They reflect the fact that neutrino-induced nucleon knockout is dominated by the transverse axial current and gains its major contributions from forward nucleon emission and backward lepton scattering.

nucl-th↗

arXiv2004

Optimal Monte Carlo Updating

Based on Peskun's theorem it is shown that optimal transition matrices in Markov chain Monte Carlo should have zero diagonal elements except for the diagonal element corresponding to the largest weight. We will compare the statistical efficiency of this sampler to existing algorithms, such as heat-bath updating and the Metropolis algorithm. We provide numerical results for the Potts model as an application in classical physics. As an application in quantum physics we consider the spin 3/2 XY model and the Bose-Hubbard model which have been simulated by the directed loop algorithm in the stochastic series expansion framework.

cond-mat.other↗

arXiv2004

Soft triaxial rotor in the vicinity of $γ=π/6$ and its extensions

The collective Bohr hamiltonian is solved for the soft triaxial rotor around $γ_0=π/6$ with a displaced harmonic oscillator potential in $γ$ and a Kratzer-like potential in $β$. The properties of the spectrum are outlined and a generalization for the more general triaxial case with $0<γ<π/6$ is proposed.

nucl-th↗

arXiv2004

Configuration mixing in $^{188}$Pb : band structure and electromagnetic properties

In the present paper, we carry out a detailed analysis of the presence and mixing of various families of collective bands in $^{188}$Pb. Making use of the interacting boson model, we construct a particular intermediate basis that can be associated with the unperturbed bands used in more phenomenological studies. We use the E2 decay to construct a set of collective bands and discuss in detail the B(E2)-values. We also perform an analysis of these theoretical results (Q, B(E2)) to deduce an intrinsic quadrupole moment and the associated quadrupole deformation parameter, using an axially deformed rotor model.

nucl-th↗

arXiv2004

Identifying neutrinos and antineutrinos in neutral-current scattering reactions

We study neutrino-induced nucleon knockout from nuclei. Expressions for the induced polarization are derived within the framework of the independent-nucleon model and the non-relativistic plane-wave approximation. Large dissimilarities in the nucleon polarization asymmetries are observed between neutrino- and antineutrino-induced processes. These asymmetries represent a potential way to distinguish between neutrinos and antineutrinos in neutral-current neutrino-scattering on nuclei. We discuss astrophysical applications of these polarization asymmetries. Our findings are illustrated for neutrino scattering on $^{16}$O and $^{208}$Pb.

nucl-th↗

arXiv2004

Shell-model description of monopole shift in neutron-rich Cu

Variations in the nuclear mean-field, in neutron-rich nuclei, are investigated within the framework of the nuclear shell model. The change is identified to originate mainly from the monopole part of the effective two-body proton-neutron interaction. Applications for the low-lying states in odd-$A$ Cu nuclei are presented. We compare the results using both schematic and realistic forces. We also compare the monopole shifts with the results obtained from large-scale shell-model calculations, using the same realistic interaction, in order to study two-body correlations beyond the proton mean-field variations.

nucl-th↗

arXiv2003

Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited

A Bose-Hubbard model, describing bosons in a harmonic trap with a superimposed optical lattice, is studied using a fast and accurate variational technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a Numerical Renormalization Group (NRG) procedure in order to improve on both. Results are presented for one, two and three dimensions, with particular attention to the experimentally accessible momentum distribution and possible satellite peaks in this distribution. In one dimension, a comparison is made with exact results obtained using Stochastich Series Expansion.

cond-mat.stat-mech↗

arXiv2003

Intruder bands and configuration mixing in the lead isotopes

A three-configuration mixing calculation is performed in the context of the interacting boson model with the aim to describe recently observed collective bands built on low-lying $0^+$ states in neutron-deficient lead isotopes. The configurations that are included correspond to the regular, spherical states as well as two-particle two-hole and four-particle four-hole excitations across the Z=82 shell gap.

nucl-th↗

arXiv2002

Allowed Gamow-Teller Excitations from the Ground State of 14N

Motivated by the proposed experiment $^{14}N(d,{^2He})^{14}C$, we study the final states which can be reached via the allowed Gamow-Teller mechanism. Much emphasis has been given in the past to the fact that the transition matrix element from the $J^π=1^+ T=0$ ground state of $^{14}N$ to the $J^π=0^+ T=1$ ground state of $^{14}C$ is very close to zero, despite the fact that all the quantum numbers are right for an allowed transition. We discuss this problem, but, in particular, focus on the excitations to final states with angular momenta $1^+$ and $2^+$. We note that the summed strength to the $J^π=2^+ T=1$ states, calculated with a wide variety of interactions, is significantly larger than that to the $J^π=1^+ T=1$ final states.

nucl-th↗

arXiv2002

Proton-neutron quadrupole interactions: an effective contribution to the pairing field

We point out that the proton-neutron energy contribution, for low multipoles (in particular for the quadrupole component), effectively renormalizes the strength of the pairing interaction acting amongst identical nucleons filling up a single-j or a set of degenerate many-j shells. We carry out the calculation in lowest-order perturbation theory. We perform a study of this correction in various mass regions. These results may have implications for the use of pairing theory in medium-heavy nuclei and for the study of pairing energy corrections to the liquid drop model when studying nuclear masses.

nucl-th↗

arXiv2001

Nuclear binding energies: Global collective structure and local shell-model correlations

Nuclear binding energies and two-neutron separation energies are analyzed starting from the liquid-drop model and the nuclear shell model in order to describe the global trends of the above observables. We subsequently concentrate on the Interacting Boson Model (IBM) and discuss a new method in order to provide a consistent description of both, ground-state and excited-state properties. We address the artefacts that appear when crossing mid-shell using the IBM formulation and perform detailed numerical calculations for nuclei situated in the 50-82 shell. We also concentrate on local deviations from the above global trends in binding energy and two-neutron separation energies that appear in the neutron-deficient Pb region. We address possible effects on the binding energy, caused by mixing of low-lying $0^{+}$ intruder states into the ground state, using configuration mixing in the IBM framework. We also study ground-state properties using a deformed mean-field approach. Detailed comparisons with recent experimental data in the Pb region are amply discussed.

nucl-th↗

arXiv2000

Two-neutron separation energies, binding energies and phase transitions in the interacting boson model

In the framework of the interacting boson model the three transitional regions (rotational-vibrational, rotational-$γ$-unstable and, vibrational-$γ$-unstable transitions) are reanalyzed. A new kind of plot is presented for studying phase transitions in finite systems such as atomic nuclei. The importance of analyzing binding energies and not only energy spectra and electromagnetic transitions, describing transitional regions is emphasized. We finally discuss a number of realistic examples.

nucl-th↗

arXiv1998

Thermodynamical properties of a mean-field plus pairing model and applications for the Fe nuclei

A mean-field plus pairing model for atomic nuclei in the Fe region was studied using a finite-temperature quantum Monte-Carlo method. We present results for thermodynamical quantities such as the internal energy and the specific heat. These results give indications of a phase transition related to the pairing amongst nucleons, around temperatures of 0.7 MeV. The influence of the residual interaction and of the size of the model space on the nuclear level densities is discussed too.

nucl-th↗

arXiv1998

A quantum Monte-Carlo method for fermions, free of discretization errors

In this work we present a novel quantum Monte-Carlo method for fermions, based on an exact decomposition of the Boltzmann operator $exp(-βH)$. It can be seen as a synthesis of several related methods. It has the advantage that it is free of discretization errors, and applicable to general interactions, both for ground-state and finite-temperature calculations. The decomposition is based on low-rank matrices, which allows faster calculations. As an illustration, the method is applied to an analytically solvable model (pairing in a degenerate shell) and to the Hubbard model.

cond-mat.str-el↗