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F. Iachello

Publications and source records attributed to F. Iachello.

At least 37 records · Page 2Linked to original sources
arXiv2015

Origin of Low-Lying Enhanced E1 Strength in Rare-Earth Nuclei

The experimental $E1$ strength distribution below 4 MeV in rare-earth nuclei suggests a local breaking of isospin symmetry. In addition to the octupole states, additional $1^-$ states with enhanced E1 strength have been observed in rare-earth nuclei by means of ($γ,γ'$) experiments. By reproducing the experimental results, the spdf interacting boson model calculations provide further evidence for the formation of an $α$ cluster in medium-mass nuclei and might provide a new understanding of the origin of low-lying E1 strength.

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arXiv2014

Evidence for tetrahedral symmetry in 16O

We derive the rotation-vibration spectrum of a 4-alpha configuration with tetrahedral symmetry, T_d, and show evidence for the occurrence of this symmetry in the low-lying spectrum of 16O. All vibrational states with A, E and F symmetry appear to have been observed, as well as the rotational bands with L(P)=0(+), 3(-), 4(+), 6(+) on the A states, and part of the rotational bands built on the E, F states. We derive analytic expressions for the form factors and B(EL) values of the ground state rotational band and show that the measured values support the tetrahedral symmetry of this band.

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arXiv2013

Lifshitz and Excited State Quantum Phase Transitions in Microwave Dirac Billiards

We present experimental results for the density of states (DOS) of a superconducting microwave Dirac billiard which serves as an idealized model for the electronic properties of graphene. The DOS exhibits two sharp peaks which evolve into van Hove singularities with increasing system size. They divide the band structure into regions governed by the \emph{relativistic} Dirac equation and by the \emph{non-relativistic} Schrödinger equation, respectively. We demonstrate that in the thermodynamic limit a topological transition appears as a neck-disrupting Lifshitz transition in the number susceptibility and as an excited state transition in the electronic excitations. Furthermore, we recover the finite-size scaling typical for excited state quantum phase transitions involving logarithmic divergences and identify a quasi-order parameter.

cond-mat.mes-hall↗
arXiv2013

Phase space factors for $β^+β^+$ decay and competing modes of double-$β$ decay

A complete and improved calculation of phase space factors (PSF) for $2νβ^+β^+$ and $0νβ^+β^+$ decay, as well as for the competing modes $2νECβ^+$, $0νECβ^+$, and $2νECEC$, is presented. The calculation makes use of exact Dirac wave functions with finite nuclear size and electron screening and includes life-times, single and summed positron spectra, and angular positron correlations.

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arXiv2013

Two neutrino double-$β$ decay in the interacting boson-fermion model

A calculation of the spectroscopic properties, energy levels and electromagnetic transitions and moments, of the ten nuclei $^{128,130}$Te, $^{128,130}$I, $^{128,130}$Xe, $^{129,131}$I, $^{127,129}$Te within the framework of the interacting boson model (IBM-2) and its extensions (IBFM-2 and IBFFM-2) is presented. The wave functions so obtained are used to calculate single-$β$ and $2νββ$ matrix elements for $^{128,130}$Te $\to$ $^{128,130}$I $\to$ $^{128,130}$Xe ecay. Use of the effective value of the axial vector coupling constant $g_{A,\text{eff},β}$ extracted from single-$β$ produces results for $2νββ$ in agreement with experiment.

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arXiv2013

Nuclear matrix elements for double-β decay

Background: Direct determination of the neutrino mass through double-$β$ decay is at the present time one of the most important areas of experimental and theoretical research in nuclear and particle physics. Purpose: We calculate nuclear matrix elements for the extraction of the average neutrino mass in neutrinoless double-$β$ decay. Methods: The microscopic interacting boson model (IBM-2) is used. Results: Nuclear matrix elements in the closure approximation are calculated for $^{48}$Ca, $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{110}$Pd, $^{116}$Cd, $^{124}$Sn, $^{128}$Te, $^{130}$Te, $^{148}$Nd, $^{150}$Nd, $^{154}$Sm, $^{160}$Gd, and $^{198}$Pt decay. Conclusions: Realistic predictions for the expected half-lives in neutrinoless double-$β$ decay with light and heavy neutrino exchange in terms of neutrino masses are made and limits are set from current experiments.

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arXiv2012

Phase space factors for double-$β$ decay

A complete and improved calculation of phase space factors (PSF) for $2νββ$ and $0νββ$ decay is presented. The calculation makes use of exact Dirac wave functions with finite nuclear size and electron screening and includes life-times, single and summed electron spectra, and angular electron correlations.

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arXiv2011

Effect of a fermion on quantum phase transitions in bosonic systems

The effect of a fermion with angular momentum j on quantum phase transitions of a (s,d) bosonic system is investigated. It is shown that the presence of a fermion strongly modifies the critical value at which the transition occurs, and its nature, even for small and moderate values of the coupling constant. The analogy with a bosonic system in an external field is mentioned. Experimental evidence for precursors of quantum phase transitions in bosonic systems plus a fermion (odd-even nuclei) is presented.

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arXiv2011

Quantum Phase Transitions in Odd-Mass Nuclei

Quantum shape-phase transitions in odd-even nuclei are investigated in the framework of the interacting boson-fermion model. Classical and quantum analysis show that the presence of the odd fermion strongly influences the location and nature of the phase transition, especially near the critical point. Experimental evidence for the occurrence of spherical to axially-deformed transitions in odd-proton nuclei Pm, Eu and Tb (Z=61, 63, 65) is presented.

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arXiv2011

Quantum phase transitions in Bose-Fermi systems

Quantum phase transitions in a system of N bosons with angular momentum L=0,2 (s,d) and a single fermion with angular momentum j are investigated both classically and quantum mechanically. It is shown that the presence of the odd fermion strongly influences the location and nature of the phase transition, especially the critical value of the control parameter at which the phase transition occurs. Experimental evidence for the U(5)-SU(3) (spherical to axially-deformed) transition in odd-even nuclei is presented.

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arXiv2011

Dual algebraic structures for the two-level pairing model

Duality relations are explicitly established relating the Hamiltonians and basis classification schemes associated with the number-conserving unitary and number-nonconserving quasispin algebras for the two-level system with pairing interactions. These relations are obtained in a unified formulation for both bosonic and fermionic systems, with arbitrary and, in general, unequal degeneracies for the two levels. Illustrative calculations are carried out comparing the bosonic and fermionic quantum phase transitions.

quant-ph↗
arXiv2007

Excited state quantum phase transitions in many-body systems

Phenomena analogous to ground state quantum phase transitions have recently been noted to occur among states throughout the excitation spectra of certain many-body models. These excited state phase transitions are manifested as simultaneous singularities in the eigenvalue spectrum (including the gap or level density), order parameters, and wave function properties. In this article, the characteristics of excited state quantum phase transitions are investigated. The finite-size scaling behavior is determined at the mean field level. It is found that excited state quantum phase transitions are universal to two-level bosonic and fermionic models with pairing interactions.

quant-ph↗
arXiv2007

Analytic descriptions for transitional nuclei near the critical point

Exact solutions of the Bohr Hamiltonian with a five-dimensional square well potential, in isolation or coupled to a fermion by the five-dimensional spin-orbit interaction, are considered as examples of a new class of dynamical symmetry or Bose-Fermi dynamical symmetry. The solutions provide baselines for experimental studies of even-even [E(5)] and odd-mass [E(5|4)] nuclei near the critical point of the spherical to deformed gamma-unstable phase transition.

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arXiv2005

Phase structure of a two-fluid bosonic system

The phase diagram of a two-fluid bosonic system is investigated. The proton-neutron interacting boson model (IBM-2) possesses a rich phase structure involving three control parameters and multiple order parameters. The surfaces of quantum phase transition between spherical, axially-symmetric deformed, and SU*(3) triaxial phases are determined, and the evolution of classical equilibrium properties across these transitions is investigated. Spectroscopic observables are considered in relation to the phase diagram.

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arXiv2004

Phase structure of the two-fluid proton-neutron system

The phase structure of a two-fluid bosonic system is investigated. The proton-neutron interacting boson model (IBM-2) posesses a rich phase structure involving three control parameters and multiple order parameters. The surfaces of quantum phase transition between spherical, axially-symmetric deformed, and SU*(3) triaxial phases are determined.

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arXiv2004

Structure of the nucleon from electromagnetic space-like and time-like form factors

Recent experimental data on space-like and time-like form factors of the nucleon are reviewed in light of a model of the nucleon with an intrinsic (quark-like) structure and a meson cloud. The analysis points to the astonishing result that the proton electric space-like form factor vanishes at Q^2~8 (GeV/c)^2 and becomes negative beyond that point. The intrinsic structure is estimated to have a r.m.s. radius of ~0.34 fm, much smaller than the proton r.m.s. radius ~0.82 fm. The calculations are in perfect agreement with space-like proton data, but deviate drastically from space-like neutron data at Q^2>1 (GeV/c)^2. Analysis of the time-like data appears to indicate excellent agreement with both proton and neutron data in the entire range of measured q^2=-Q^2 values.

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