arXiv subjects

S. Leoni

Publications and source records attributed to S. Leoni.

At least 37 records · Page 2Linked to original sources

arXiv2018

Testing the capability of low-energy light ions identification of the TRACE silicon detectors

The in-beam tests of two Si pixel type TRACE detectors have been performed at Laboratori Nazionali di Legnaro (Italy). The aim was to investigate the possibility of identifying heavy-ion reactions products with mass A~10 at low kinetic energy, i.e., around 10 MeV. Two separate read-out chains, digital and analog, were used. The Pulse Shape Analysis technique was employed to obtain the identification matrices for the digitally processed part of the data. Separation in both charge and mass was obtained, however, the $α$ particles contaminated significantly the recorded data in the lower energy part. Due to this effect, the identification of the light products ($^{7,6}$Li isotopes) could be possible down only to ~20 MeV

physics.ins-det↗

arXiv2017

Dynamic Displacement Disorder of Cubic BaTiO$_3$

The three dimensional distribution of the X-ray diffuse scattering intensity of BaTiO$_3$ has been recorded in a synchrotron experiment and simultaneously computed using molecular dynamics simulations of a shell-model. Together these have allowed the details of the disorder in paraelectric BaTiO$_3$ to be clarified. The narrow sheets of diffuse scattering, related to the famous anisotropic longitudinal correlations of Ti ions, are shown to be caused entirely by the overdamped anharmonic soft phonon branch. This finding demonstrates that the occurrence of narrow sheets of diffuse scattering agrees with a displacive picture of the cubic phase of this textbook ferroelectric material.

cond-mat.mtrl-sci↗

arXiv2017

Low-energy enhancement and fluctuations of $γ$-ray strength functions in $^{56,57}$Fe: test of the Brink-Axel hypothesis

Nuclear level densities and $γ$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$γ$ coincidences. A low-energy enhancement in the $γ$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $γ$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $γ$-ray strength function, in support of the generalized Brink-Axel hypothesis.

nucl-ex↗

arXiv2016

Microscopic description of insulator-metal transition in high-pressure oxygen

Unusual metallic states involving breakdown of the standard Fermi-liquid picture of long-lived quasiparticles in well-defined band states emerge at low temperatures near correlation-driven Mott transitions. Prominent examples are ill-understood metallic states in $d$- and $f$-band compounds near Mott-like transitions. Finding of superconductivity in solid O$_{2}$ on the border of an insulator-metal transition at high pressures close to 96~GPa is thus truly remarkable. Neither the insulator-metal transition nor superconductivity are understood satisfactorily. Here, we undertake a first step in this direction by focussing on the pressure-driven insulator-metal transition using a combination of first-principles density-functional and many-body calculations. We report a striking result: the finding of an orbital-selective Mott transition in a pure $p$-band elemental system. We apply our theory to understand extant structural and transport data across the transition, and make a specific two-fluid prediction that is open to future test. Based thereupon, we propose a novel scenario where soft multiband modes built from microscopically coexisting itinerant and localized electronic states are natural candidates for the pairing glue in pressurized O$_{2}$.

cond-mat.mtrl-sci↗

arXiv2016

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus 133Sb

The gamma-ray decay of excited states of the one-valence-proton nucleus 133Sb has been studied using cold-neutron induced fission of 235U and 241Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between gamma-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 micros isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, reveals a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus 132Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin.

nucl-ex↗

arXiv2015

Charged particle decay of hot and rotating $^{88}$Mo nuclei in fusion-evaporation reactions

A study of fusion-evaporation and (partly) fusion-fission channels for the $^{88}$Mo compound nucleus, produced at different excitation energies in the reaction $^{48}$Ti + $^{40}$Ca at 300, 450 and 600 MeV beam energies, is presented. Fusion-evaporation and fusion-fission cross sections have been extracted and compared with the existing systematics. Experimental data concerning light charged particles have been compared with the prediction of the statistical model in its implementation in the Gemini++ code, well suited even for high spin systems, in order to tune the main model parameters in a mass region not abundantly covered by exclusive experimental data. Multiplicities for light charged particles emitted in fusion evaporation events are also presented. Some discrepancies with respect to the prediction of the statistical model have been found for forward emitted $α$-particles; they may be due both to pre-equilibrium emission and to reaction channels (such as Deep Inelastic Collisions, QuasiFission/QuasiFusion) different from the compound nucleus formation.

nucl-ex↗

arXiv2015

Mott-Kondo Insulator Behavior in the Iron Oxychalcogenides

We perform a combined experimental-theoretical study of the Fe-oxychalcogenides (FeO$\emph{Ch}$) series La$_{2}$O$_{2}$Fe$_{2}$O\emph{M}$_{2}$ (\emph{M}=S, Se), which is the latest among the Fe-based materials with the potential \ to show unconventional high-T$_{c}$ superconductivity (HTSC). A combination of incoherent Hubbard features in X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectra, as well as resitivity data, reveal that the parent FeO$\emph{Ch}$ are correlation-driven insulators. To uncover microscopics underlying these findings, we perform local density approximation-plus-dynamical mean field theory (LDA+DMFT) calculations that unravel a Mott-Kondo insulating state. Based upon good agreement between theory and a range of data, we propose that FeO$\emph{Ch}$ may constitute a new, ideal testing ground to explore HTSC arising from a strange metal proximate to a novel selective-Mott quantum criticality.

cond-mat.str-el↗

arXiv2014

Formation of light exotic nuclei in low-energy multinucleon transfer reactions

Low-energy multinucleon transfer reactions are shown to be very effective tool for the production and spectroscopic study of light exotic nuclei. The corresponding cross sections are found to be significantly larger as compared with high energy fragmentation reactions. Several optimal reactions for the production of extremely neutron rich isotopes of elements with Z=6-14 are proposed.

nucl-th↗

arXiv2014

Evidence for dipole nature of the low-energy $γ$ enhancement in $^{56}$Fe

The $γ$-ray strength function of $^{56}$Fe has been measured from proton-$γ$ coincidences for excitation energies up to $\approx 11$ MeV. The low-energy enhancement in the $γ$-ray strength function, which was first discovered in the ($^3$He,$αγ$)$^{56}$Fe reaction, is confirmed with the ($p,p^\primeγ$)$^{56}$Fe experiment reported here. Angular distributions of the $γ$ rays give for the first time evidence that the enhancement is dominated by dipole transitions.

nucl-ex↗

arXiv2013

Measurement of light charged particles in the decay channels of medium-mass excited compound nuclei

The 48Ti on 40Ca reactions have been studied at 300 and 600 MeV focusing on the fusion-evaporation (FE) and fusion-fission (FF) exit channels. Energy spectra and multiplicities of the emitted light charged particles have been compared to Monte Carlo simulations based on the statistical model. Indeed, in this mass region (A about 100) models predict that shape transitions can occur at high spin values and relatively scarce data exist in the literature about coincidence measurements between evaporation residues and light charged particles. Signals of shape transitions can be found in the variations of the lineshape of high energy gamma rays emitted from the de-excitation of GDR states gated on different region of angular momenta. For this purpose it is important to keep under control the FE and FF processes, to regulate the statistical model parameters and to control the onset of possible preequilibrium emissions from 300 to 600 MeV bombarding energy.

nucl-ex↗

arXiv2012

Response of AGATA Segmented HPGe Detectors to Gamma Rays up to 15.1 MeV

The response of AGATA segmented HPGe detectors to gamma rays in the energy range 2-15 MeV was measured. The 15.1 MeV gamma rays were produced using the reaction d(11B,ng)12C at Ebeam = 19.1 MeV, while gamma-rays between 2 to 9 MeV were produced using an Am-Be-Fe radioactive source. The energy resolution and linearity were studied and the energy-to-pulse-height conversion resulted to be linear within 0.05%. Experimental interaction multiplicity distributions are discussed and compared with the results of Geant4 simulations. It is shown that the application of gamma-ray tracking allows a suppression of background radiation following neutron capture by Ge nuclei. Finally the Doppler correction for the 15.1 MeV gamma line, performed using the position information extracted with Pulse-shape Analysis, is discussed.

nucl-ex↗

arXiv2011

Unconventional Mott Transition in KxFe2-ySe2

Whether the newly discovered KxFe2-ySe2 systems are doped Mott or band insulators is key to how superconductivity emerges at lower temperature. With extant theoretical studies supporting conflicting scenarios, a more realistic approach is urgently called for. Here, we use LDA+DMFT to study this issue in KxFe2-ySe2. We find that the undoped KFe1.6Se2 system is a new kind of Mott-Kondo insulator (MKI). Electron doping this MKI drives a Mott transition to an orbital-selective non-Fermi liquid metal. Good agreement with spectral and transport responses supports our view, implying that superconductivity arises from a doped Mott insulator, as in the high-Tc cuprates.

cond-mat.supr-con↗

arXiv2009

$α$-FeSe as an orbital-selective incoherent metal: An LDA+DMFT study

$α$-FeSe, a prototype iron-chalcogenide superconductor, shows clear signatures of a strange incoherent normal state. Motivated thereby, we use LDA+DMFT to show how strong multi-band correlations generate a low-energy pseudogap in the normal state, giving an incoherent metal in very good semi-quantitative agreement with observations. We interpret our results in terms of $α$-FeSe being close to Mottness. A wide range of anomalous responses in the "normal" state are consistently explained, lending strong support for this view. Implications for superconductivity arising from such an anomalous state are touched upon.

cond-mat.supr-con↗

arXiv2009

Orbital-spin order and the origin of structural distortion in MgTi$_2$O$_4$

We analyze electronic, magnetic, and structural properties of the spinel compound MgTi$_2$O$_4$ using the local density approximation+U method. We show how MgTi$_2$O$_4$ undergoes to a canted orbital-spin ordered state, where charge, spin and orbital degrees of freedom are frozen in a geometrically frustrated network by electron interactions. In our picture orbital order stabilize the magnetic ground state and controls the degree of structural distortions. The latter is dynamically derived from the cubic structure in the correlated LDA+U potential. Our ground-state theory provides a consistent picture for the dimerized phase of MgTi$_2$O$_4$, and might be applicable to frustrated materials in general.

cond-mat.other↗

arXiv2009

Self-Doping Induced Orbital-Selective Mott Transition in Hg2Ru2O7

Pyrochlore oxides are fascinating systems where strong, multi-orbital correlations in concert with geometrical frustration give rise to unanticipated physical properties. The detailed mechanism of the insulator-metal transitions (IMT) underpinning these phenomena is, however, ill-understood in general. Motivated thereby, we study the IMT in the pyrochlore ${\rm Hg_{2}Ru_{2}O_{7}}$ using LDA+DMFT. In contrast to the well-known examples of Mott transitions in TMO, we show that, in the negative charge-transfer situation characteristic of \hg, self-doping plays a crucial role in the emergence of an orbital-selective IMT. We argue that this mechanism has broader relevance to other correlated pyrochlore oxides.

cond-mat.str-el↗

arXiv2008

Mottness underpins the anomalous optical response of Iron Pnictides

The recent discovery of high-temperature superconductivity (HTSC) in doped Iron pnictides is the latest example of unanticipated behavior exhibited by $d$- and $f$-band materials. The symmetry of the SC gap, along with the mechanism of its emergence from the ``normal'' state is a central issue in this context. Here, motivated by a host of experimental signatures suggesting strong correlations in the Fe-pnictides, we undertake a detailed study of their normal state. Focussing on symmetry-unbroken phases, we use the correlated band structure method, LDA+DMFT, to study the one-particle responses of both ${\rm LaO_{1-x}FeAsF_{x}}$ and ${\rm SmO_{1-x}FeAsF_{x}}$ in detail. Basing ourselves on excellent quantitative agreement between LDA+DMFT and key experiments probing the one-particle responses, we extend our study, undertaking the first detailed study of their normal state electrodynamic response. In particular, we propose that near-total normal state incoherence, resulting from strong, {\it local} correlations in the Fe $d$-shell in Fe-pnictides, underpins the incoherent normal state transport found in these materials, and discuss the specific electronic mechanisms leading to such behavior. We also discuss the implications of our work for the multi-band nature of the SC by studying the pairing ``glue'' function, which we find to be an overdamped, electronic continuum. Similarities and differences between cuprates and Fe-pnictides are also touched upon. Our study supports the view that SC in Fe-pnictides arises from a bad metallic, incoherent ``normal'' state that is proximate to a Mott insulator.

cond-mat.supr-con↗

arXiv2008

Probing the order-to-chaos region in superdeformed 151Tb and 196Pb nuclei with continuum gamma-transitions

The gamma-decay associated with the warm rotation of the superdeformed (SD) nuclei 151Tb and 196Pb has been measured with the EUROBALL IV array. Several independent quantities provide a stringent test of the population and decay dynamics in the SD well. A Monte Carlo simulation of the gamma-decay based on microscopic calculations gives remarkable agreement with the data only assuming a large enhancement of the B(E1) strength at low excitation energy, which may be related to the evidence for octupole vibrations in both mass regions.

nucl-ex↗

arXiv2008

Normal State Correlated Electronic Structure of Iron Pnictides

We describe the correlated electronic structure of a prototype Fe-pnictide superconductor, $SmO_{1-x}F_{x}FeAs$, using LDA+DMFT. Strong, multi-orbital electronic correlations generate a low-energy pseudogap in the undistorted phase, giving a bad, incoherent metal in qualitative agreement with observations. Very good semi-quantitative agreement with the experimental spectral functions is seen, and interpreted, within a correlated, multi-orbital picture. Our results show that Fe-pnictides should be understood as low-carrier density, incoherent metals, in resemblance to the underdoped cuprate superconductors.

cond-mat.str-el↗