arXiv subjects

Alexis Diaz-Torres

Publications and source records attributed to Alexis Diaz-Torres.

At least 19 recordsLinked to original sources

arXiv2025

Enhancing the creation of elements in laser-assisted heavy-ion fusion reactions

Low-energy fusion of heavy ions is a fascinating coupling-assisted quantum tunnelling problem, whose understanding is crucial for advancing the synthesis of new elements and isotopes. Quantum dynamical coupled-channels calculations of laser-assisted $^{16}$O + $^{238}$U fusion are presented for both a central collision and the total fusion cross-sections, suggesting that laser-nucleus interaction can enhance the average $^{16}$O + $^{238}$U fusion probability by $6-60 \%$ at subbarrier energies using quasi-static laser fields of intensity $10^{27}-10^{29}$ Wcm$^{-2}$ and photon's energy of $1$ eV. Femtosecond laser pulses are shown to reduce this enhancement by many orders of magnitude.

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arXiv2025

Quantum Dynamical Microscopic Approach to Stellar Carbon Burning

The process of carbon burning is vital to understanding late stage stellar evolution of massive stars and the conditions of certain supernovae. Carbon burning is a complex problem, involving quantum tunnelling and nuclear molecular states. Quantum dynamical calculations of carbon burning are presented, combining the time-dependent wave-packet method and the density-constrained time-dependent Hartree-Fock (DC-TDHF) approach. By limiting the contribution of triaxial molecular configurations to fusion, we demonstrate that the DC-TDHF interaction potential successfully explains the appearance of some resonant structures in the sub-barrier fusion cross-section. This result shows the critical role of nucleon-nucleon interactions in the 12C + 12C fusion resonances observed at astrophysical energies.

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arXiv2023

Thermal and atomic effects on coupled-channels heavy-ion fusion

Stellar nuclear fusion reactions take place in a hot, dense plasma within stars. To account for the effect of these environments, the theory of open quantum systems is used to conduct pioneering studies of thermal and atomic effects on fusion probability at a broad range of temperatures and densities. Since low-lying excited states are more likely to be populated at stellar temperatures and increase nuclear plasma interaction rates, a 188Os nucleus was used as a target that interacts with an inert 16O projectile. Key results showed thermal effects yield an average increase in fusion probability of 15.5% and 36.9% for our test nuclei at temperatures of 0.1 and 0.5 MeV respectively, compared to calculations at zero temperature. Thermal effects could be tested in a laboratory using targets prepared in excited states as envisaged in facilities exploiting laser-nucleus interactions.

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arXiv2022

Coherence dynamics in low-energy nuclear fusion

Low-energy nuclear fusion reactions have been described using a dynamical coupled-channels density matrix method, based on the theory of open quantum systems. For the first time, this has been combined with an energy projection method, permitting the calculation of energy resolved fusion probabilities. The results are benchmarked against calculations using stationary Schrödinger dynamics and show excellent agreement. Calculations of entropy, energy dissipation and coherence were conducted, demonstrating the capability of this method. It is evident that the presence of quantum decoherence does not affect fusion probability. This framework provides a basis for quantum thermodynamic studies using thermal environments.

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arXiv2021

Tracing the dynamical interplay of low-energy reaction processes of exotic nuclei using a two-center molecular continuum

The competition among reaction processes of a weakly-bound projectile at intermediate times of a slow collision has been unraveled. This has been done using a two-center molecular continuum within a semiclassical, time-dependent coupled-channel reaction model. Dynamical probabilities of elastic scattering, transfer and breakup agree with those derived from the direct integration of the time-dependent Schrödinger equation, demonstrating the usefulness of a two-center molecular continuum for gaining insights into the reaction dynamics of exotic nuclei.

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arXiv2019

Identifying resonances with wave-packet dynamics

A new method for the study of resonant behavior - using wave-packet dynamics - is presented, based on the powerful window operator technique. The method is illustrated and quantified by application to the astrophysically-important example of low-energy $^{12}$C + $^{12}$C collisions. For this selected, potential model test case, the technique is shown to provide both resonance energies and widths in agreement with alternative methods, such as complex-energy scattering-matrix pole searches and scattering phase-shift analyses. The method has a more general capability to study resonance phenomena across disciplines, that involve particles temporarily trapped by potential pockets.

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arXiv2018

Characterizing the astrophysical S-factor for $^{12}$C+$^{12}$C with wave-packet dynamics

A quantitative study of the astrophysically important sub-barrier fusion of $^{12}$C+$^{12}$C is presented. Low-energy collisions are described in the body-fixed reference frame using wave-packet dynamics within a nuclear molecular picture. A collective Hamiltonian drives the time propagation of the wave-packet through the collective potential-energy landscape. The fusion imaginary potential for specific dinuclear configurations is crucial for understanding the appearance of resonances in the fusion cross section. The theoretical sub-barrier fusion cross sections explain some observed resonant structures in the astrophysical S-factor. These cross sections monotonically decline towards stellar energies. The structures in the data that are not explained are possibly due to cluster effects in the nuclear molecule, which are to be included in the present approach.

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arXiv2018

Effects of unconventional breakup modes on incomplete fusion of weakly bound nuclei

The incomplete fusion dynamics of $^6$Li + $^{209}$Bi collisions at energies above the Coulomb barrier is investigated. The classical dynamical model implemented in the {\sc platypus} code is used to understand and quantify the impact of both $^6$Li resonance states and transfer-triggered breakup modes (involving short-lived projectile-like nuclei such as $^8$Be and $^5$Li) on the formation of incomplete fusion products. Model calculations explain the experimental incomplete-fusion excitation function fairly well, indicating that (i) delayed direct breakup of $^6$Li reduces the incomplete fusion cross-sections, and (ii) the neutron-stripping channel practically determines those cross-sections.

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arXiv2015

Systematic study of complete fusion suppression in reactions involving weakly bound nuclei at energies above the Coulomb barrier

Complete fusion excitation functions of reactions involving breakup are studied by using the empirical coupled-channel (ECC) model with breakup effects considered. An exponential function with two parameters is adopted to describe the prompt-breakup probability in the ECC model. These two parameters are fixed by fitting the measured prompt-breakup probability or the complete fusion cross sections. The suppression of complete fusion at energies above the Coulomb barrier is studied by comparing the data with the predictions from the ECC model without the breakup channel considered. The results show that the suppression of complete fusion are roughly independent of the target for the reactions involving the same projectile.

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arXiv2011

PLATYPUS: a code for fusion and breakup in reactions induced by weakly-bound nuclei within a classical trajectory model with stochastic breakup

A self-contained Fortran-90 program based on a classical trajectory model with stochastic breakup is presented, which should be a powerful tool for quantifying complete and incomplete fusion, and breakup in reactions induced by weakly-bound two-body projectiles near the Coulomb barrier. The code calculates complete and incomplete fusion cross sections and their angular momentum distribution, as well as breakup observables (angle, kinetic energy and relative energy distributions).

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arXiv2010

Coupled-channels density-matrix approach to low-energy nuclear collision dynamics: A technique for quantifying quantum decoherence effects on reaction observables

The coupled-channels density-matrix technique for nuclear reaction dynamics, which is based on the Liouville-von Neumann equation with Lindblad dissipative terms, is developed with the inclusion of full angular momentum couplings. It allows a quantitative study of the role and importance of quantum decoherence in nuclear scattering. Formulae of asymptotic observables that can reveal effects of quantum decoherence are given. A method for extracting energy-resolved scattering information from the time-dependent density matrix is introduced. As an example, model calculations are carried out for the low-energy collision of the $^{16}$O projectile on the $^{154}$Sm target.

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arXiv2010

Coupled-channels density-matrix approach to low-energy nuclear reaction dynamics

Atomic nuclei are complex, quantum many-body systems whose structure manifests itself through intrinsic quantum states associated with different excitation modes or degrees of freedom. Collective modes (vibration and/or rotation) dominate at low energy (near the ground-state). The associated states are usually employed, within a truncated model space, as a basis in (coherent) coupled channels approaches to low-energy reaction dynamics. However, excluded states can be essential, and their effects on the open (nuclear) system dynamics are usually treated through complex potentials. Is this a complete description of open system dynamics? Does it include effects of quantum decoherence? Can decoherence be manifested in reaction observables? In this contribution, I discuss these issues and the main ideas of a coupled-channels density-matrix approach that makes it possible to quantify the role and importance of quantum decoherence in low-energy nuclear reaction dynamics. Topical applications, which refer to understanding the astrophysically important collision $^{12}$C + $^{12}$C and achieving a unified quantum dynamical description of relevant reaction processes of weakly-bound nuclei, are highlighted.

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arXiv2010

Absence of decoherence in the complex potential approach to nuclear scattering

Time-dependent density-matrix propagation is used to demonstrate, in a schematic model of an open quantum system, that the complex potential approach and the Lindblad dissipative dynamics are \emph{not} equivalent. While the former preserves coherence, it is destroyed in the Lindblad dissipative dynamics. Quantum decoherence is the key aspect that makes the difference between the two approaches, indicating that the complex potential model is inadequate for a consistent description of open quantum-system dynamics. It is suggested that quantum decoherence should always be explicitly included when modelling low-energy nuclear collision dynamics within a truncated model space of reaction channels.

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arXiv2010

Modelling incomplete fusion dynamics of weakly-bound nuclei at near-barrier energies

The classical dynamical model for reactions induced by weakly-bound nuclei at near-barrier energies is developed further. It allows a quantitative study of the role and importance of incomplete fusion dynamics in asymptotic observables, such as the population of high-spin states in reaction products as well as the angular distribution of direct alpha-production. Model calculations indicate that incomplete fusion is an effective mechanism for populating high-spin states, and its contribution to the direct alpha production yield diminishes with decreasing energy towards the Coulomb barrier. It also becomes notably separated in angles from the contribution of no-capture breakup events. This should facilitate the experimental disentanglement of these competing reaction processes.

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arXiv2009

Reaction dynamics of weakly-bound nuclei at near-barrier energies: impact of incomplete fusion on the angular distribution of direct alpha-production

The classical trajectory model with stochastic breakup for nuclear collision dynamics of weakly-bound nuclei is further developed. It allows a quantitative study of the importance of incomplete fusion dynamics in the angular distribution of direct alpha-production. Model calculations indicate that the incomplete fusion contribution diminishes with decreasing energy towards the Coulomb barrier, notably separating in angles from the contribution of no-capture breakup events. This should facilitate the experimental disentanglement of these competing reaction processes.

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arXiv2008

Dissipative quantum dynamics in low-energy collisions of complex nuclei

Model calculations that include the effects of irreversible, environmental couplings on top of a coupled-channels dynamical description of the collision of two complex nuclei are presented. The Liouville-von Neumann equation for the time-evolution of the density matrix of a dissipative system is solved numerically providing a consistent transition from coherent to decoherent (and dissipative) dynamics during the collision. Quantum decoherence and dissipation are clearly manifested in the model calculations. Energy dissipation, due to the irreversible decay of giant-dipole vibrational states of the colliding nuclei, is shown to result in a hindrance of quantum tunneling and fusion.

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arXiv2008

Coupled-Channels Approach for Dissipative Quantum Dynamics in Near-Barrier Collisions

A novel quantum dynamical model based on the dissipative quantum dynamics of open quantum systems is presented. It allows the treatment of both deep-inelastic processes and quantum tunneling (fusion) within a fully quantum mechanical coupled-channels approach. Model calculations show the transition from pure state (coherent) to mixed state (decoherent and dissipative) dynamics during a near-barrier nuclear collision. Energy dissipation, due to irreversible decay of giant-dipole excitations of the interacting nuclei, results in hindrance of quantum tunneling.

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arXiv2008

Solving the two-center nuclear shell-model problem with arbitrarily-orientated deformed potentials

A general new technique to solve the two-center problem with arbitrarily-orientated deformed realistic potentials is demonstrated, which is based on the powerful potential separable expansion method. As an example, molecular single-particle spectra for $^{12}$C + $^{12}$C $\to$ $^{24}$Mg are calculated using deformed Woods-Saxon potentials. These clearly show that non-axial symmetric configurations play a crucial role in molecular resonances observed in reaction processes for this system at low energy.

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