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J. E. García-Ramos

Publications and source records attributed to J. E. García-Ramos.

18 recordsLinked to original sources
arXiv2025

An intrinsic-state formalism for the Interacting Boson-Fermion Model with configuration mixing

In this letter, we present the intrinsic state formalism of the Interacting Boson-Fermion Model incorporating both 0p-0h and 2p-2h (regular and intruder) configurations. The framework allows to deal with the unpaired fermion in either a single-j or multi-j orbital and is applicable to both axial and triaxial nuclear shapes. The formalism is applied to a schematic transitional region where regular and intruder configurations coexist and cross. We show that the boson-fermion part of the Hamiltonian can either enhance or suppress the emergence of a type II Quantum Phase Transition.

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arXiv2024

The extended Lipkin model: proposal for implementation in a quantum platform and machine learning analysis of its phase diagram

We investigate the Extended Lipkin Model (ELM), whose phase diagram mirrors that of the Interacting Boson Approximation model (IBA). Unlike the standard Lipkin model, the ELM (as the IBA) features both first- and second-order quantum shape phase transitions depending on the model parameters. Our goal is to implement the ELM on a quantum platform, leveraging Machine Learning techniques to identify its quantum phase transitions and critical lines. To achieve this, we offer: i) ground state energy calculations using a variational quantum eigensolver; ii) a detailed formulation for ELM dynamics within quantum computing, facilitating experimental exploration of the IBA phase diagram; and iii) a phase diagram determination using various Machine Learning methods. We successfully replicate the ELM ground-state energy using the Adaptive Derivative-Assembled Pseudo-Trotter ansatz Variational Quantum Eigensolver (ADAPT-VQE) algorithm across the entire phase space. Our framework ensures ELM implementation on quantum platforms with controlled errors. Lastly, our ML predictions yield a meaningful phase diagram for the model. Keywords: Quantum Platforms Nuclear Models ADAPT-VQE Quantum Shape Phase Transitions Interacting Boson Approximation Extended Lipkin Model Machine Learning

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arXiv2023

At the borderline of shape coexistence: Mo and Ru

Background Even-even isotopes of Mo ($Z=42$) and Ru ($Z=44$) are nuclei close to the subshell closure at $Z=40$, where shape coexistence plays a significant role. As a result, their spectroscopic properties are expected to resemble those of Sr ($Z=38$) and Zr ($Z=40$). Exploring the evolution of these properties as they move away from the subshell closure is of great interest. Purpose The purpose of this study is to reproduce the spectroscopic properties of even-even $^{96-110}_{\phantom{961-}42}$Mo and $^{98-114}_{\phantom{961-}44}$Ru isotopes and to determine the influence of shape coexistence. Method We have employed the interacting boson model with configuration mixing as the framework to calculate all the observables for Mo and Ru isotopes. We have considered two types of configurations: 0-particle-0-hole and 2-particle-2-hole excitations. The model parameters have been determined using a least-squares fitting to match the excitation energies and the $B(E2)$ transition rates. Results We have obtained the excitation energies, $B(E2)$ values, two-neutron separation energies, nuclear radii, and isotope shifts for the entire chain of isotopes. Our theoretical results have shown good agreement with experimental data. Furthermore, we have conducted a detailed analysis of the wave functions and obtained the mean-field energy surfaces and the nuclear deformation parameter, $β$, for all considered isotopes. Conclusions Our findings reveal that shape coexistence plays a significant role in Mo isotopes, with the crossing of intruder and regular configurations occurring at neutron number $60$ ($A=102$), which induces a quantum phase transition. In contrast, in Ru isotopes, the intruder states have minimal influence, remaining at higher energies. However, at neutron number $60$, also a quantum phase transition occurs in Ru isotopes.

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arXiv2023

Nuclear Physics in the Era of Quantum Computing and Quantum Machine Learning

In this paper, the application of quantum simulations and quantum machine learning to solve low-energy nuclear physics problems is explored. The use of quantum computing to deal with nuclear physics problems is, in general, in its infancy and, in particular, the use of quantum machine learning in the realm of nuclear physics at low energy is almost nonexistent. We present here three specific examples where the use of quantum computing and quantum machine learning provides, or could provide in the future, a possible computational advantage: i) the determination of the phase/shape in schematic nuclear models, ii) the calculation of the ground state energy of a nuclear shell model-type Hamiltonian and iii) the identification of particles or the determination of trajectories in nuclear physics experiments.

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arXiv2023

Simultaneous $γ$-ray and electron spectroscopy of $^{182,184,186}$Hg isotopes

Background: The mercury isotopes around $N=104$ are a well-known example of nuclei exhibiting shape coexistence. Mixing of configurations can be studied by measuring the monopole strength $ρ^2(E0)$, however, currently the experimental information is scarce and lacks precision, especially for the $I^π\rightarrow I^π$ ($I \neq 0$) transitions. Purpose: The goals of this study were to increase the precision of the known branching ratios and internal conversion coefficients, to increase the amount of available information regarding excited states in $^{182,184,186}$Hg and to interpret the results in the framework of shape coexistence using different models. Method: The low-energy structures in $^{182,184,186}$Hg were populated in the $β$ decay of $^{182,184,186}$Tl, produced at ISOLDE and purified by laser ionization and mass separation. The $γ$-ray and internal conversion electron events were detected by five germanium clover detectors and a segmented silicon detector, respectively, and correlated in time to build decay schemes. Results: In total, 193, 178 and 156 transitions, including 144, 140 and 108 observed for the first time in a $β$-decay experiment, were assigned to $^{182,184,186}$Hg, respectively. Internal conversion coefficients were determined for 23 transitions, out of which 12 had an $E0$ component. Extracted branching ratios allowed the sign of the interference term in $^{182}$Hg as well as $ρ^2(E0;0^+_2\rightarrow 0^+_1)$ and $B(E2;0^+_2\rightarrow 2^+_1)$ in $^{184}$Hg to be determined. By means of electron-electron coincidences, the $0^+_3$ state was identified in $^{184}$Hg. The experimental results were qualitatively reproduced by five theoretical approaches, the IBM with configuration mixing with two different parametrizations, the General Bohr Hamiltonian, the BMF model and the SCCM model. However, a quantitative description is lacking.

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arXiv2021

Evolution of octupole deformation and collectivity in neutron-rich lanthanides

The onset of octupole deformation and its impact on related spectroscopic properties is studied in even-even neutron-rich lanthanide isotopes Xe, Ba, Ce, and Nd with neutron number $86\leqslant N\leqslant 94$. Microscopic input comes from the Hartree-Fock-Bogoliubov approximation with constrains on the axially symmetric quadrupole and octupole operators using the Gogny-D1M interaction. At the mean-field level, reflection asymmetric ground states are predicted for isotopes with neutron number around $N=88$. Spectroscopic properties are studied by diagonalizing the interacting boson model Hamiltonian, with the parameters obtained via the mapping of the mean-field potential energy surface onto the expectation value of the Hamiltonian in the $s$, $d$, and $f$ boson condensate state. The results obtained for low-energy positive- and negative-parity excitation spectra as well as the electric dipole, quadrupole, and octupole transition probabilities indicate the onset of pronounced octupolarity for $Z\approx 56$ and $N\approx 88$ nuclei.

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arXiv2021

Quadrupole-octupole coupling and the onset of octupole deformation in actinides

The evolution of quadrupole and octupole collectivity and their coupling is investigated in a series of even-even isotopes of the actinide Ra, Th, U, Pu, Cm, and Cf with neutron number in the interval $130\leqslant N\leqslant 150$. The Hartree-Fock-Bogoliubov approximation, based on the parametrization D1M of the Gogny energy density functional, is employed to generate potential energy surfaces depending upon the axially-symmetric quadrupole and octupole shape degrees of freedom. The mean-field energy surface is then mapped onto the expectation value of the $sdf$ interacting-boson-model Hamiltonian in the boson condensate state as to determine the strength parameters of the boson Hamiltonian. Spectroscopic properties related to the octupole degree of freedom are produced by diagonalizing the mapped Hamiltonian. Calculated low-energy negative-parity spectra, $B(E3;3^{-}_{1}\to 0^{+}_{1})$ reduced transition rates, and effective octupole deformation suggest that the transition from nearly spherical to stable octupole-deformed, and to octupole vibrational states occurs systematically in the actinide region.

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arXiv2020

Octupole correlations in light actinides from the interacting boson model based on the Gogny energy density functional

The quadrupole-octupole coupling and the related spectroscopic properties have been studied for the even-even light actinides $^{218-238}$Ra and $^{220-240}$Th. The Hartree-Fock-Bogoliubov approximation, based on the Gogny-D1M energy density functional, has been employed as a microscopic input, i.e., to obtain (axially symmetric) mean-field potential energy surfaces as functions of the quadrupole and octupole deformation parameters. The mean-field potential energy surfaces have been mapped onto the corresponding bosonic potential energy surfaces using the expectation value of the $sdf$ Interacting Boson Model (IBM) Hamiltonian in the boson condensate state. The strength parameters of the $sdf$-IBM Hamiltonian have been determined via this mapping procedure. The diagonalization of the mapped IBM Hamiltonian provides energies for positive- and negative-parity states as well as wave functions which are employed to obtain transitional strengths. The results of the calculations compare well with available data from Coulomb excitation experiments and point towards a pronounced octupole collectivity around $^{224}$Ra and $^{226}$Th.

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arXiv2020

The subtle connection between shape coexistence and quantum phase transition. The Zr case

Background: Zr region is characterized by very rapid changes in the ground state structure of the nuclei. In particular, the onset of deformation when passing from $^{98}$Zr to $^{100}$Zr is one of the fastest ever observed in the nuclear chart. It has been probed both experimental and theoretically that certain low-lying excited states of Zr isotopes own different shapes than the ground state. Purpose: We intend to disentangle the interplay between the sudden changes in the ground state shape, i.e., the existence of a quantum phase transition, and the presence in the spectra of coexisting states with very different deformation, i.e., the presence of shape coexistence. Method: We rely on a previous calculation using the Interacting Boson Model with Configuration Mixing (IBM-CM) which reproduces in detail the spectroscopic properties of $^{96-110}$Zr. This IBM-CM calculation allows to compute mean-field energy surfaces, wave functions and any other observable related with the presence of shape coexistence or with a quantum phase transition. Results: We obtain energy surfaces and the equilibrium value of the deformation parameter $β$, the U(5) decomposition of the wave functions and the density of states. Conclusions: We confirm that Zr is a clear example of quantum phase transition that originates from the crossing of two configurations with a very different degree of deformation. Moreover, we observe how the intruder configuration exhibits its own evolution which resembles a quantum phase transition too.

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arXiv2020

Two-neutron transfer reactions as a tool to study the interplay between shape coexistence and quantum phase transitions

The goal of this study is to find an observable that could distinguish between both phenomena, shape coexistence and quantum phase transitions. The selected observable to be analyzed is the two-neutron transfer intensity between the 0+ states in the parent and daughter nuclei. The framework in which the study is done is the Interacting Boson Model (IBM), including its version with configuration mixing (IBM-CM). In order to generate the wave functions of the isotope chains of interest, needed for calculating transfer intensities, previous systematic studies with IBM and IBM-CM are taken without changing the parameters. Results for two-neutron transfer intensities are presented for Zr, Hg and Pt isotopic chains using IBM-CM and, moreover, the same is done for Zr, Pt and Sm isotopic chains using IBM with just a single configuration, i.e., without using configuration mixing. In the case of Zr, the two-neutron transfer intensities between the ground states provide a clear observable indicating that normal and intruder configurations coexist in the low-lying spectrum and that they cross at A=98->100, and this could allow to disentangle whether or not shape coexistence is inducing a given QPT. In the case of Pt, where shape coexistence is present and the regular and the intruder configurations cross for the ground state, there is almost no influence in the value of the two-neutron transfer, neither in the case of Hg where the ground state always has regular nature. For the Sm isotope chain that is one of the quantum phase transition paradigms, the value of the two-neutron transfer is strongly affected.

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arXiv2019

Number conserving particle-hole RPA for superfluid nuclei

We present a number conserving particle-hole RPA theory for collective excitations in the transition from normal to superfluid nuclei. The method derives from an RPA theory developed long ago in quantum chemistry using antisymmetric geminal powers, or equivalently number projected HFB states, as reference states. We show within a minimal model of pairing plus monopole interactions that the number conserving particle-hole RPA excitations evolve smoothly across the superfluid phase transition close to the exact results, contrary to particle-hole RPA in the normal phase and quasiparticle RPA in the superfluid phase that require a change of basis at the broken symmetry point. The new formalism can be applied in a straightforward manner to study particle-hole excitations on top of a number projected HFB state.

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arXiv2019

Scaling Properties of the Lipkin Model at the Critical Point

The influence of Franco Iachello in Physics during the last 50 years and, in particular, in the use of algebraic methods applied to very different physical problems has been broad, extense and have permeated most branches of Physics, from Nuclear and Molecular to Particle and Condensed Matter physics. Apart of many other contributions, at the beginning of the 2000's he introduced the concept of critical point symmetry and triggered the study of the many faces of quantum phase transitions in nuclei and other mesoscopic systems. In this contribution, we present the analysis of the scaling properties of the Lipkin model spectrum at the phase transition point and we focus on the differences between first and second order quantum phase transitions. Moreover, we explain how the obtained results can be also of application for the interacting boson model.

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arXiv2019

An extended Agassi model: algebraic structure, phase diagram, and large size limit

The Agassi model is a schematic two-level model that involves pairing and monopole-monopole interactions. It is, therefore, an extension of the well known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic formulation of an extension of the Agassi model as well as its bosonic realization through the Schwinger representation. Moreover, a mean-field approximation for the model is presented and its phase diagram discussed. Finally, a $1/j$ analysis, with $j$ proportional to the degeneracy of each level, is worked out to obtain the thermodynamic limit of the ground state energy and some order parameters from the exact Hamiltonian diagonalization for finite$-j$.

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arXiv2018

Phase diagram of an extended Agassi model

Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick model that incorporates the pairing interaction. It is a schematic model that describes the interplay between particle-hole and pair correlations. It was proposed in the 1960's by D. Agassi as a model to simulate the properties of the quadrupole plus pairing model. Purpose: The aim of this work is to extend a previous study by Davis and Heiss generalizing the Agassi model and analyze in detail the phase diagram of the model as well as the different regions with coexistence of several phases. Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov (HFB) approximation, introducing two variational parameters that play the role of order parameters. We also compare the HFB calculations with the exact ones. Results: We obtain the phase diagram of the model and classify the order of the different quantum phase transitions appearing in the diagram. The phase diagram presents broad regions where several phases, up to three, coexist. Moreover, there is also a line and a point where four and five phases are degenerated, respectively. Conclusions: The phase diagram of the extended Agassi model presents a rich variety of phases. Phase coexistence is present in extended areas of the parameter space. The model could be an important tool for benchmarking novel many-body approximations.

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arXiv2018

On the nature of the shape coexistence and the quantum phase transition phenomena: lead region and Zr isotopes

The goal of this contribution is to analyze the connection between shape coexistence and quantum phase transition, two seemingly unrelated phenomena that share common aspects, namely, the rapid change in the ground state structure along an isotope chain or the presence of several minima at the mean-field level. To illustrate the similarities and differences between both phenomena, we will focus in the Pb region, in particular in Pt and Hg isotopes, as well as in Zr isotopes.

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arXiv2016

Phase diagram of the two-fluid Lipkin model: a butterfly catastrophe

Background: In the last few decades quantum phase transitions have been of great interest in Nuclear Physics. In this context, two-fluid algebraic models are ideal systems to study how the concept of quantum phase transition evolves when moving into more complex systems, but the number of publications along this line has been scarce up to now. Purpose: We intend to determine the phase diagram of a two-fluid Lipkin model, that resembles the nuclear proton-neutron interacting boson model Hamiltonian, using both numerical results and analytic tools, i.e., catastrophe theory, and to compare the mean-field results with exact diagonalizations for large systems. Method: The mean-field energy surface of a consistent-Q-like two-fluid Lipkin Hamiltonian is studied and compared with exact results coming from a direct diagonalization. The mean-field results are analyzed using the framework of catastrophe theory. Results: The phase diagram of the model is obtained and the order of the different phase-transition lines and surfaces is determined using a catastrophe theory analysis. Conclusions: There are two first order surfaces in the phase diagram, one separating the spherical and the deformed shapes, while the other separates two different deformed phases. A second order line, where the later surfaces merge, is found. This line finishes in a transition point with a divergence in the second order derivative of the energy that corresponds to a tricritical point in the language of the Ginzburg-Landau theory for phase transitions.

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arXiv2010

Relationship between X(5)-models and the interacting boson model

The connections between the X(5)-models (the original X(5) using an infinite square well, X(5)-$β^8$, X(5)-$β^6$, X(5)-$β^4$, and X(5)-$β^2$), based on particular solutions of the geometrical Bohr Hamiltonian with harmonic potential in the $γ$ degree of freedom, and the interacting boson model (IBM) are explored. This work is the natural extension of the work presented in [1] for the E(5)-models. For that purpose, a quite general one- and two-body IBM Hamiltonian is used and a numerical fit to the different X(5)-models energies is performed, later on the obtained wave functions are used to calculate B(E2) transition rates. It is shown that within the IBM one can reproduce well the results for energies and B(E2) transition rates obtained with all these X(5)-models, although the agreement is not so impressive as for the E(5)-models. From the fitted IBM parameters the corresponding energy surface can be extracted and it is obtained that, surprisingly, only the X(5) case corresponds in the moderate large N limit to an energy surface very close to the one expected for a critical point, while the rest of models seat a little farther.

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