Boson-fermion Model Research Articles

Metal-insulator transition of spinless fermions coupled to dispersive optical bosons

Including the previously ignored dispersion of phonons we revisit the metal-insulator transition problem in one-dimensional electron-phonon systems on the basis of a modified spinless fermion Holstein model. Using matrix-product-state techniques we determine the global ground-state phase diagram in the thermodynamic limit for the half-filled band case, and show that in particular the curvature of the bare phonon band has a significant effect, not only on the transport properties characterized by the conductance and the Luttinger liquid parameter, but also on the phase space structure of the model as a whole. While a downward curved (convex) dispersion of the phonons only shifts the Tomonaga-Luttinger-liquid to charge-density-wave quantum phase transition towards stronger EP coupling, an upward curved (concave) phonon band leads to a new phase-separated state which, in the case of strong dispersion, can even completely cover the charge-density wave. Such phase separation does not occur in the related Edwards fermion-boson model.

Intertwined quantum phase transitions in the zirconium and niobium isotopes

Nuclei in the A ≈ 100 region exhibit intricate shape-evolution and configuration crossing signatures. Exploring both even–even and their adjacent odd-mass nuclei gives further insight on the emergence of deformation and shape-phase transitions. We employ the algebraic frameworks of the interacting boson model with configuration mixing and the new interacting boson-fermion model with configuration mixing in order to investigate the even–even zirconium with neutron number 52–70 (40Zr) and odd-mass niobium (41Nb) isotopes with 52–62. We compare between the evolution in energy levels, configuration and symmetry content of the wave functions, two neutron separation energies and E2 transition rates, alongside comparison with the experimental data. The comparisons between the two chains of isotopes denote the occurrence of two types of QPTs, a crossing of normal and intruder configurations (named Type II QPT), and a shape-evolution of the intruder configuration (named Type I QPT). The latter QPT begins from spherical shape to axially deformed rotor in the Zr isotopes and from weak to strong coupling scenario in the Nb isotopes. This occurrence, named intertwined quantum phase transitions (IQPTs), in the even–even Zr chain is thus demonstrated to persists when coupling a proton to the boson core for the odd-mass Nb chain, even as deformation increases.

Charge-order melting in the one-dimensional Edwards model

We use infinite matrix-product-state techniques to study the time evolution of the charge-density-wave (CDW) order after a quench or a light pulse in a fundamental fermion-boson model. The motion of fermions in the model is linked to the creation of bosonic excitations, which counteracts the melting of the CDW order. For low-energy quenches corresponding to a change of the boson relaxation rate, we find behavior similar to that in an effective t−V model. When the boson energy is quenched instead or a light pulse is applied to the system, the transient dynamics are more complex, with the CDW order first quickly decreasing to an intermediate value while the density-wave-like order of the bosons rises. In the case of pulse irradiation, the subsequent time evolution of the CDW order depends strongly on the photon frequency. For frequencies slightly below the boson energy, we observe a temporary increase of the CDW order parameter. Our results reveal the complex physics of driven Mott insulators in low-dimensional systems with strong correlations. Published by the American Physical Society 2024

Octupole deformation in 143,145,146Ba

Gamma rays emitted by fission fragments of 252Cf and measured using the Gammasphere array continue to give insight into neutron rich nuclei. Using our high statistics data, we have reexamined high-spin states and linking transitions associated with octupole correlations in 143,145,146Ba. This has been a region of interest, when it was proposed that near 145Ba there was a region of octupole deformation. In an even-A system, it is possible that rotational band structures are explainable by simplex quantum numbers, s=+1 and s=−1. The previous bands including octupole bands in 143,145,146Ba have been extended. The new s=i octupole bands in 145Ba and new s=−1 octupole bands in 146Ba have been identified. In addition, collective model analysis of energy displacement, rotational frequency and B(E1)/B(E2) ratios have been carried out. Evidence of the rotational consequences of octupole deformation in even-even and even-odd isotopes is given. These nuclei have trends as rotation stabilizes the vibrational degrees of freedom. The experimental results are in agreement with theoretical calculations using the interacting boson and boson-fermion models that are based on a universal nuclear energy density functional.

Wobbling Motion in Nuclei

Wobbling motion as an exotic collective mode in nuclei without axial symmetry, was intensively discussed during the last few years. The observation of the newly proposed transverse wobbling, first reported in 135Pr and soon after in nuclei from other mass regions, was considered as a significant discovery in low-spin nuclear structure. However, both the reported experimental results and the proposed theoretical models were actively questioned in work devoted to the study of the low-spin wobbling mode in the same nuclei. We recently re-measured the electromagnetic character of the ΔI=1 transitions connecting the one- to zero-phonon and the two- to one-phonon wobbling bands in 135Pr, showing their predominant M1 magnetic character, which is in contradiction with the wobbling interpretation. These new experimental results, which were reproduced by either the quasiparticle-plus-triaxial-rotor model and interacting boson-fermion model calculations, are against the previously proposed wobbling nature of the low-spin bands in 135Pr. On the other hand, we obtained conclusive experimental evidence for the theoretically proposed transverse wobbling bands at medium spin in 136Nd. The comparison of the experimental data with calculations using the triaxial projected shell model as well as a new particle-rotor model with frozen orthogonal geometry of the active nucleons, supports the description in terms of transverse wobbling of medium-spin bands in triaxial even-even nuclei.

Microscopic calculation of the β− decays of 151Sm, 171Tm, and 210Pb with implications to detection of the cosmic neutrino background

The electron spectral shapes corresponding to the low-Qβ−-decay transitions Sm151(5/2g.s.−)→Eu151(5/2g.s.+), Sm151(5/2g.s.−)→Eu151(7/21+), Tm171(1/2g.s.+)→Yb171(1/2g.s.−), Tm171(1/2g.s.+)→Yb171(3/21−), Pb210(0g.s.+)→Bi210(1g.s.−), and Pb210(0g.s.+)→Bi210(01−) have been computed using beta-decay theory with several refinements for these first-forbidden nonunique (ff-nu) β− transitions. These ff-nu β− transitions have non-trivial electron spectral shapes with transition nuclear matrix elements (NMEs) computed by using the microscopic Interacting Boson-Fermion Model (IBFM-2) for the decays of 151Sm and 171Tm, and the nuclear shell model (NSM) for the decay of 210Pb. Within the respective Q windows, the computed ff-nu electron spectral shapes deviate maximally at sub-percent level from the universal allowed shape, except for the transition Pb210(0g.s.+)→Bi210(1g.s.−), where the maximal deviation is some 2.7%. This confirms that the so-called ξ approximation is fairly good for most of these low-Qβ− transitions and thus the allowed shape is a rather good first approximation. Our computed spectral shapes could be of interest for experiments aiming to measure the cosmic neutrino background (CνB), like the PTOLEMY experiment. We have also derived CνB cross sections for the ground-state transitions of the considered nuclei at the β endpoint. Our findings indicate that more work on the atomic mismatch correction is needed in the future in order to extract reliable and precise CνB cross sections for any nuclear target.

Negative Doublet Bands with Different Shape in the 107Ag Nucleus

The existence of chirality in the negative high spin states in the 107Ag nucleus has been confirmed by the Interacting Boson-Fermion Model (IBFM). IBFM excited states and electromagnetic properties are in good agreement with the available experimental data. According to IBFM analysis, the negative parity doublet bands (3) and ( 4) in the 107Ag nucleus were interpreted as triaxial and axial shap respectively.

Number parity effects in the normal state of SrTiO3

We study the recently discovered even-odd effects in the normal state of single-electron devices manufactured at strontium titanium oxide/lanthanum aluminum oxide interfaces (STO/LAO). Within the framework of the number parity-projected formalism and a phenomenological fermion-boson model, we find that, in sharp contrast to conventional superconductors, the crossover temperature ${T}^{*}$ for the onset of number parity effect is considerably larger than the superconducting transition temperature ${T}_{c}$ due to the existence of a pairing gap above ${T}_{c}$. Furthermore, the finite lifetime of the preformed pairs reduces by several orders of magnitude the effective number of states ${N}_{\mathrm{eff}}$ available for the unpaired quasiparticle in the odd-parity state of the Coulomb blockaded STO/LAO island. Our findings are in qualitative agreement with the experimental results reported by Levy and coworkers for STO/LAO-based single electron devices.

Chemical potential influence on the condensation energy from a Boson–Fermion model of superconductivity

Influence of the temperature dependent chemical potential on the condensation energy from a ternary Boson–Fermion model of superconductivity is reported, it consist of unbound electrons/holes which are fermions plus two-electron and two-hole Cooper pairs which are bosons. When solving simultaneously the set of equations of the mixture (two gap-like equations, one for electron pairs and another one for hole pairs, plus the particle number conservation equation) within the weak-coupling (BCS regime), the resulting superconducting chemical potential shows a shift from its normal state counterpart, which is related to both the magnitude of the temperature-dependent superconducting gap and to the Fermi energy of the superconductor. As predicted by van der Marel in the early 1990s we also find that the superconducting chemical potential has a prominent kink at critical temperature Tc, which in turn coincides with the normal state chemical potential. Also there is a discontinuity in its first derivative which directly affects the magnitude in the specific heat jump. We show that the difference between the superconducting and normal state chemical potentials is of the same order of magnitude as the corresponding difference between the thermodynamic potentials for the mixture, and must therefore be accounted for in the condensation energy calculations instead of ignoring it as is done often. The condensation energy obtained here shows very good agreement with experimental data for elemental superconductors.

Investigation of Structure and Nuclear Shape Phase Transition in Odd Nuclei in a multi-j model

In this paper‎, ‎we study the nature of the dynamics in second-order Quantum Phase Transition (QPT) between vibrational ( ) and -unstable ( ) nuclear shapes‎. ‎Using a transitional Hamiltonian according to an affine SU(1,1) algebra in combination with a coherent state formalism, Shape Phase Transitions (SPT) in odd-nuclei in the framework of the Interacting Boson Fermion Model (IBFM) are investigated‎. ‎Classical analysis reveals a change in the system along with the transition in a critical point‎. ‎The role of a fermion with angular momentum j at the critical point on quantum phase transitions in bosonic systems is investigated via a semi-classical approach‎. ‎The effect of the coupling of the odd particle to an even-even boson core is discussed along with the shape transition and‎, ‎in particular‎, ‎at the critical point‎. Our study confirms the importance of the odd nuclei as necessary signatures to characterize the occurrence of the phase transition and determine the critical point's precise position‎.

Analysis of the one-neutron transfer reaction in O18+Se76 collisions at 275 MeV

Purpose: We want to analyze transitions to low-lying excited states of the residual and ejectile nuclei in the 76Se(18O, 17O) 77Se one-neutron stripping reaction at 275-MeV incident energy and determine the role of single-particle and core excitation in the description of the measured cross sections. In addition, we explore the sensitivity of the calculated cross section to different nuclear structure models. Methods: The excitation energy spectrum and the differential cross-section angular distributions are measured using the MAGNEX large acceptance magnetic spectrometer for the detection of the ejectiles and the missing mass technique for the reconstruction of the reaction kinematics. The data are compared with calculations based on distorted-wave Born approximation, coupled-channels Born approximation, and coupled reaction channels adopting spectroscopic amplitudes for the projectile and target overlaps derived by large-scale shell-model calculations and interacting boson-fermion model. Results: Peaks in the energy spectra corresponding to groups of unresolved transitions to 77Se and 17O are identified. The experimental cross sections are extracted and compared to theoretical calculations. A remarkable agreement is found, without using any scaling factors, demonstrating that the adopted models for nuclear structure and reaction take into account the relevant aspects of the studied processes. The main transitions which contribute to the cross section of each peak are identified.

Questioning the wobbling interpretation of low-spin bands in γ -soft nuclei within the interacting boson-fermion model

An alternative interpretation of the recently reported low-lying excited bands in $\gamma$-soft odd-mass nuclei as wobbling bands is presented in terms of the interacting boson-fermion model. The model Hamiltonian is determined based on the mean-field calculations with the nuclear energy density functionals. The predicted mixing ratios of the $\Delta{I}=1$ electric quadrupole to magnetic dipole transition rates between yrast bands and those yrare bands previously interpreted as wobbling bands in $^{135}$Pr, $^{133}$La, $^{127}$Xe, and $^{105}$Pd are consistently smaller in magnitude than the experimental values on which the wobbling interpretation is based. These calculated mixing ratios indicate predominant magnetic character in agreement with the new experimental data. The earlier wobbling assignments are severely questioned.

Entanglement entropy as a signature of a quantum phase transition in nuclei in the framework of the interacting boson model and interacting boson-fermion model

Background: One of the fundamental problems of quantum information science is quantum entanglement. Recently, quantum phase transition in nuclear systems is studied in connection with quantum entanglement.Purpose: In this paper, the use of entanglement entropy as a suitable signal for the study of quantum phase transition in the even-even and odd-$A$ nuclei is investigated. The effect of the coupling of a single fermion to a boson core on entanglement entropy is studied.Method: By use of the affine $SU(1,1)$ Lie algebra and through the Schmidt decomposition in the framework of the interacting boson model (IBM) and interacting boson-fermion model (IBFM), entanglement entropy in the even-even and odd-$A$ are obtained. The entanglement entropy is used for tracking and studying the shape phase transition in these nuclei.Results: The entanglement entropy in the IBM and the IBFM is calculated. The entanglement entropy values of the low-lying states of $^{122--134}\mathrm{Xe}, ^{102--110}\mathrm{Pd}$, and $^{123--133}\mathrm{Xe}$ were calculated and analyzed. It is found that entanglement entropy is a suitable order parameter to detect shape phase transition in nuclear systems.Conclusions: The obtained results indicate that the entropy of entanglement is sensitive to the shape-phase transition between spherical and $\ensuremath{\gamma}$-unstable regions in nuclei. The results show that entanglement entropy is a powerful tool for identifying shape phase transitions in nuclei. It is found that the coupling of the single fermion with angular momentum $j$ to the even-even system does not change the geometry imposed by the boson core performing the transition and only the entanglement entropy values have been shifted by the addition of the odd particle with respect to the even case.

Quantum phase transitions and band mixing in 135Ba

Quantum phase transitions and band-mixing phenomena are considered within the framework of a two-state boson–fermion model for the odd-mass 135Ba nucleus. The purpose of the study is to further explore the nature of the interplay between these modalities within the framework of an even–even boson core described by an IBM Hamiltonian coupled to a j = 3/2 fermion as a function of the strength of the coupling constant that links the boson and fermion pictures. The results show that excitation energies and the known E2 strengths of 135Ba are sensitive to the strength of the coupling constant, with a best fit to known experimental results yielding results that lie close to those of the recently proposed E(5/4) model.

Toward simulating quantum field theories with controlled phonon-ion dynamics: A hybrid analog-digital approach

Quantum field theories are the cornerstones of modern physics, providing relativistic and quantum mechanical descriptions of physical systems at the most fundamental level. Simulating real-time dynamics within these theories remains elusive in classical computing. This provides a unique opportunity for quantum simulators, which hold the promise of revolutionizing our simulation capabilities. Trapped-ion systems are successful quantum-simulator platforms for quantum many-body physics and can operate in digital, or gate-based, and analog modes. Inspired by the progress in proposing and realizing quantum simulations of a number of relativistic quantum field theories using trapped-ion systems, and by the hybrid analog-digital proposals for simulating interacting boson-fermion models, we propose hybrid analog-digital quantum simulations of selected quantum field theories, taking recent developments to the next level. On one hand, the semi-digital nature of this proposal offers more flexibility in engineering generic model interactions compared with a fully-analog approach. On the other hand, encoding the bosonic fields onto the phonon degrees of freedom of the trapped-ion system allows a more efficient usage of simulator resources, and a more natural implementation of intrinsic quantum operations in such platforms. This opens up ways for simulating complex dynamics of, e.g., Abelian and non-Abelian gauge theories, by combining the benefits of digital and analog schemes.

Positive parity low-spin states of even–odd 129−133Ba isotopes

The positive parity low-spin states of even-odd 129−133Ba isotopes were studied in this study using the Interacting Boson–Fermion Model (IBFM-1). The single fermion is predicted to be in one of three single-particle orbits: 2d5/2, 2d3/2, and 3s1/2. The Interacting Boson Model (IBM-1) was used to investigate the energy levels, electric quadrupole transition probabilities, and potential energy surface of even-even Barium isotopes (a core for even-odd nuclei).The measured positive parity low-state energy spectra and predicted energy levels, as well as the B(E2) transition probabilities, are reasonably consistent with the experimental data and previous research for Ba isotopes. The potential energy surface contour plot reveals that all interesting nuclei are deformed and have γ-unstable-like properties.

Magnetic moment of the 11/2− isomeric state in Mo99 and neutron spin g factor quenching in A≈100 nuclei

The gyromagnetic factor of the low-lying ${E}_{x}=684.10(19)\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ isomeric state of the nucleus $^{99}\mathrm{Mo}$ was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of ${J}^{\ensuremath{\pi}}=11/{2}^{\ensuremath{-}}$, with a half-life of ${T}_{1/2}=742(13)\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$. The state of interest was populated and spin-aligned via a single-neutron transfer on a highly enriched $^{98}\mathrm{Mo}$ target. A magnetic moment ${\ensuremath{\mu}}_{\mathrm{expt}.}=\ensuremath{-}0.627(20){\ensuremath{\mu}}_{\mathrm{N}}$ was obtained. This result is far from the Schmidt value expected for a pure single-particle $\ensuremath{\nu}{h}_{11/2}$ state. A comparison of experimental spectroscopic properties of this nucleus is made with results of multishell Interacting boson-fermion Model (IBFM-1) calculations. In this approach, the ${J}^{\ensuremath{\pi}}=11/{2}^{\ensuremath{-}}$ isomeric state in $^{99}\mathrm{Mo}$ has a pure $\ensuremath{\nu}{h}_{11/2}$ configuration. Its magnetic moment, as well as that of other two excited states could be reasonably well reproduced by reducing the free neutron spin $g$ factor with a quenching factor of 0.45. This low value is not appropriate only for this case, similar values for the quenching factor being also required in order to describe magnetic moments in other nuclei from the same mass region.

Effects of an odd particle on shape phase transitions in odd-even systems * *Supported by National Natural Science Foundation of China (11875158)

A scheme to solve the Hamiltonian in the interacting boson-fermion model in terms of the SU(3) coupling basis is introduced, through which the effects of an odd particle on shape phase transitions (SPTs) in odd-A nuclei are examined by comparing the critical behaviors of some selected quantities in odd-even and even-even systems. The results indicate that the spherical to prolate (U(5)-SU(3)) SPT and spherical to -soft (U(5)-O(6)) SPT may clearly occur in the odd-even system with the SPT signatures revealed by various quantities including the excitation energies, energy ratio, ratio, quadrupole moments, and one-particle-transfer spectroscopic intensities. In particular, the results indicate that the spherical to prolate SPT in the odd-even system can even be strengthened by the effects of the odd particle with the large fluctuations of the quadrupole deformations appearing near the critical point.

Description of the negative- and positive-parity states in odd-A Radon and Francium isotopes within the framework of the interacting Boson–Fermion model

The negative- and positive-parity states and electromagnetic transitions for [Formula: see text]Rn and [Formula: see text]Fr nuclei have been studied by using the Interacting Boson–Fermion Approximation (IBFM-1). Based on our calculations, the spin assignments for some states with uncertain spin have been done. Energy spectra, electromagnetic transition probabilities and branching ratios in these isotopes are calculated and compared with available experimental data. The wave functions are analyzed to understand the nuclear structure. In addition, some predictions of levels that have no experimental data are presented such as spin and electromagnetic transitions. Good results were obtained with a decrease in the collectivity properties, especially for the most neutron-deficient isotopes.

First microscopic evaluation of spin-dependent WIMP-nucleus scattering off 183W

We perform the first consistent calculation of elastic-scattering and inelastic-scattering structure functions for spin-dependent WIMP-nucleus scattering off 183W in a microscopic nuclear-theory framework. The nuclear structure calculations are performed in the microscopic interacting boson-fermion model (IBFM-2). Our results show that while 183W is very insensitive to spin-dependent elastic scattering, the structure function for inelastic scattering is quite sizable at small momentum transfers. Moreover, to our knowledge 183W provides the first studied case where inelastic scattering can compete with elastic scattering as the primary detection signal.