arXiv subjects

M. Mariani

Publications and source records attributed to M. Mariani.

10 recordsLinked to original sources

arXiv2021

Hybrid stars with sequential phase transitions: the emergence of the g$_2$ mode

Neutron stars are the densest objects in the Universe, with $M \sim 1.4 M_{\odot}$ and $R \sim 12$ km, and the equation of state associated to their internal composition is still unknown. The extreme conditions to which matter is subjected inside neutron stars could lead to a phase transition in their inner cores, giving rise to a hybrid compact object. The observation of $2M_{\odot}$ binary pulsars (PSR~J1614-2230, PSR~J0343$+$0432 and PSR~J0740$+$6620) strongly constraints theoretical models of the equation of state. Moreover, the detection of gravitational waves emitted during the binary neutron star merger, GW170817, and its electromagnetic counterpart, GRB170817A, impose additional constraints on the tidal deformability. In this work, we investigate hybrid stars with sequential phase transitions hadron-quark-quark in their cores. We assume that both phase transitions are sharp and analyse the rapid and slow phase conversion scenarios. For the outer core, we use modern hadronic equations of state. For the inner core we employ the constant speed of sound parametrization for quark matter. We analyze more than 3000 hybrid equations of state, taking into account the recent observational constraints from neutron stars. The effects of hadron-quark-quark phase transitions on the normal oscillation modes $f$ and $g$, are studied under the Cowling relativistic approximation. Our results show that, in the slow conversion regime, a second quark-quark phase transition gives rise to a new $g_2$~mode. We discuss the observational implications of our results associated to the gravitational waves detection and the possibility of detecting hints of sequential phase transitions and the associated $g_2$~mode.

astro-ph.HE↗

arXiv2019

NMR and $μ^{+}$SR detection of unconventional spin dynamics in Er(trensal) and Dy(trensal) molecular magnets

Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and relaxation and of Muon Spin Relaxation ($μ^{+}$SR) have been performed as a function of temperature and external magnetic field on two isostructural lanthanide complexes, Er(trensal) and Dy(trensal) featuring crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon $λ$ longitudinal relaxation rates, LRR, exhibit a peak for temperatures T lower than 30K, associated to the slowing down of the spin dynamics, and the width of the NMR absorption spectra starts to increase significantly at T ca. 50K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks have a field and temperature dependence different from those previously reported for all Molecular Nanomagnets. They do not follow the Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature and field and thus cannot be explained in terms of a single dominating correlation time $τ$c determined by the spin slowing down at low temperature. Further, for T lower than 50K the spectral width does not follow the temperature behavior of the magnetic susceptibility chi. We suggest, using simple qualitative considerations, that the observed behavior is due to a combination of two different relaxation processes characterized by the correlation times $τ$LT and $τ$HT, dominating for T lower than 30K and T higher than 50K, respectively. Finally, the observed flattening of LRR for T lower than 5K is suggested to have a quantum origin.

cond-mat.mtrl-sci↗

arXiv2018

Dirichlet's and Thomson's principles for non-selfadjoint elliptic operators with application to non-reversible metastable diffusion processes

We present two variational formulae for the capacity in the context of non-selfadjoint elliptic operators. The minimizers of these variational problems are expressed as solutions of boundary-value elliptic equations. We use these principles to provide a sharp estimate for the transition times between two different wells for non-reversible diffusion processes. This estimate permits to describe the metastable behavior of the system.

math.PR↗

arXiv2017

Estrellas híbridas: una aproximación semi-analítica a T finita

Starting from the semi-analytic construction of a equation of state (EoS) which takes into account nuclear and quark matter at finite temperature, we study the possibility that proto-neutron stars, be proto-hybrid stars whose cores are composed of quark matter. We obtain the mass-radius relationship and discuss the latest constraints on masses and radii of neutron stars, considering the pulsars PSR J1614-2230 and PSR J0348 + 0432.

nucl-th↗

arXiv2017

Simplified Thermal Evolution of Proto-hybrid Stars

We study the possibility of a hadron-quark phase transition in the interior of neutron stars, taking into account different schematic evolutionary stages at finite temperature. Furthermore, we analyze the astrophysical properties of hot and cold hybrid stars, considering the constraint on maximum mass given by the pulsars J1614-2230 and J1614-2230. We obtain cold hybrid stars with maximum masses $\geq 2$ M$_{\odot}$. Our study also suggest that during the proto-hybrid star evolution a late phase transition between hadronic matter and quark matter could occur, in contrast with previous studies of proto-neutron stars.

nucl-th↗

arXiv2017

Constant entropy hybrid stars: a first approximation of cooling evolution

We aim to study the possibility of a hadron-quark phase transition in the interior of neutron stars, taking into account different schematic evolutionary stages at finite temperature. We also discuss the strange quark matter stability in the quark matter phase. Furthermore, we aim to analyze the astrophysical properties of hot and cold hybrid stars, considering the constraint on maximum mass given by the pulsars J1614-2230 and J0348+0432. We have developed a computational code to construct semi-analytical hybrid equations of state at fixed entropy per baryon and to obtain different families of hybrid stars. An analytical approximation of the Field Correlator Method is developed for the quark matter equation of state. For the hadronic equation of state we use a table based on the relativistic mean field theory, without hyperons. We solved the relativistic structure equations of hydrostatic equilibrium and mass conservation for hybrid star configurations. For the different equations of state obtained, we calculated the stability window for the strange quark matter, lepton abundances, temperature profiles and contours profiles for the maximum mass star depending on the parameters of the Field Correlator Method. We also computed the mass-radius and gravitational mass-baryonic mass relationships for different hybrid star families. We have analyzed different stages of hot hybrid stars as a first approximation of the cooling evolution of neutron stars with quark matter cores. We obtain cold hybrid stars with maximum masses $\geq 2 M_\odot$ for different combinations of the Field Correlator Method parameters. In addition, our study based on the gravitational mass - baryonic mass plane shows a late phase transition between hadronic and quark matter during the proto-hybrid star evolution, in contrast with previous studies of proto-neutron stars.

astro-ph.HE↗

arXiv2011

Magnetic properties and spin dynamics of 3d-4f molecular complexes

We present the magnetic properties of three recently synthesized binuclear molecular complexes [NiNd], [NiGd] and [ZnGd] investigated by dc magnetization and proton nuclear magnetic resonance (NMR) measurements. The high-temperature magnetic properties are related to the independent paramagnetic behavior of the two magnetic metal ions within the binuclear entities both in [NiNd] and [NiGd]. On lowering the temperature, the formation of a magnetic dimer, with a low-spin ground state due to antiferromagnetic interaction (J/kB = -25 K) between Ni2+ and Nd3+, is found in the case of [NiNd], while in [NiGd] a ferromagnetic interaction (J/kB = 3.31 K) between the magnetic ions leads to a high-spin (S = 9/2) ground state. The temperature dependence of the proton nuclear spin lattice relaxation rate 1/T1 in [NiNd] is driven by the fluctuation of the hyperfine field at the nuclear site due to relaxation of the magnetization. At high temperature the independent Ni2+ and Nd3+ spins fluctuate fast while at low temperature we observe a slowing down of the fluctuation of the total magnetization of the dimer because of the insurgence of antiferromagnetic spin correlations. The relaxation mechanism in [NiNd] at low temperature is interpreted by a single, temperature dependent, correlation frequency wc\simT^3.5, which reflects the life time broadening of the exchange coupled spins via spin-phonon interaction. The proton NMR signal in [NiGd] could be detected just at room temperature, due to the shortening of relaxation times when T is decreased. The magnetic properties of [ZnGd] are the ones expected from a weakly interacting assembly of isolated moments except for anomalies in the susceptibility and NMR results below 15 K which currently cannot be explained.

cond-mat.mes-hall↗

arXiv2011

Finite-size effects on the dynamic susceptibility of CoPhOMe single-chain molecular magnets in presence of a static magnetic field

The static and dynamic properties of the single-chain molecular magnet [Co(hfac)$_2$NITPhOMe] are investigated in the framework of the Ising model with Glauber dynamics, in order to take into account both the effect of an applied magnetic field and a finite size of the chains. For static fields of moderate intensity and short chain lengths, the approximation of a mono-exponential decay of the magnetization fluctuations is found to be valid at low temperatures; for strong fields and long chains, a multi-exponential decay should rather be assumed. The effect of an oscillating magnetic field, with intensity much smaller than that of the static one, is included in the theory in order to obtain the dynamic susceptibility $χ(ω)$. We find that, for an open chain with $N$ spins, $χ(ω)$ can be written as a weighted sum of $N$ frequency contributions, with a sum rule relating the frequency weights to the static susceptibility of the chain. Very good agreement is found between the theoretical dynamic susceptibility and the ac susceptibility measured in moderate static fields ($H_{\rm dc}\le 2$ kOe), where the approximation of a single dominating frequency turns out to be valid. For static fields in this range, new data for the relaxation time, $τ$ versus $H_{\rm dc}$, of the magnetization of CoPhOMe at low temperature are also well reproduced by theory, provided that finite-size effects are included.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co

The magnetic properties and the spin dynamics of two molecular magnets have been investigated by magnetization and d.c. susceptibility measurements, Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance (NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields, H=0.5 and 1.5 Tesla. The two molecular magnets consist of CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show the presence of non equivalent groups of protons with a measurable contact hyperfine interaction consistent with a small admixture of s-wave function with the d-function of the magnetic ion. The NMR relaxation results are explained in terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an almost temperature independent correlation time due to the weak magnetic exchange interaction. We conclude that the two molecular magnets studied here behave as single molecule paramagnets with a very weak intramolecular interaction, almost of the order of the dipolar intermolecular interaction. Thus they represent a new class of molecular magnets which differ from the single molecule magnets investigated up to now, where the intramolecular interaction is much larger than the intermolecular one.

cond-mat.mtrl-sci↗

arXiv2003

Mesoscopic phase separation in Na$_x$CoO$_2$ ($0.65\leq x\leq 0.75$)

NMR, EPR and magnetization measurements in Na$_x$CoO$_2$ for $0.65\leq x\leq 0.75$ are presented. While the EPR signal arises from Co$^{4+}$ magnetic moments ordering at $T_c\simeq 26$ K, $^{59}$Co NMR signal originates from cobalt nuclei in metallic regions with no long range magnetic order and characterized by a generalized susceptibility typical of strongly correlated metallic systems. This phase separation in metallic and magnetic insulating regions is argued to occur below $T^*(x)$ ($220 - 270$ K). Above $T^*$ an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for $T<T^*$ were localized on Co$^{4+}$ $d_{z^2}$ orbitals. It is pointed out that the in-plane antiferromagnetic coupling $J\ll T^*$ cannot be the driving force for the phase separation.

cond-mat.str-el↗