# M. AcquaroneUniversità di Parma | UNIPR · Department of Physics and Earth Sciences

M. Acquarone

Ph.D in Solid State Physics

## About

68

Publications

2,554

Reads

**How we measure 'reads'**

A 'read' is counted each time someone views a publication summary (such as the title, abstract, and list of authors), clicks on a figure, or views or downloads the full-text. Learn more

292

Citations

Citations since 2017

## Publications

Publications (68)

The hydrogen molecules $H_2$ and $\left( H_2 \right) _2$ are analyzed with
electronic correlations taken into account between the $1s$ electrons exactly.
The optimal single-particle Slater orbitals are evaluated in the correlated
state of $H_2$ by combining their variational determination with the
diagonalization of~the~full Hamiltonian in the~seco...

We evaluate all the electron–phonon couplings derived from the one-body and two-body electronic interactions, in both the adiabatic and extreme non-diabatic limit, for a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape. We find largely different values of the coupling parameters in the two limits, as well as di...

From the complete extended Hubbard-Holstein Hamiltonian we obtain a polaronic effective Hamiltonian by successive application of generalized displacement and squeezing transformations, with wavevector dependent characteristic parameters which make possible a phonon-induced long range interaction between charges, whose sign can change with the dista...

We perform a finite-size cluster study of the Kondo-Heisenberg model, describing a system of itinerant electrons locally coupled via a Kondo-type interaction to a set of magnetic ions. The ionic moments are assumed to have antiferromagnetic order, due to a nearest-neighbor Heisenberg coupling. By consecutive application of displacement and squeezin...

We focus on the definition of the unitary transformation leading to an effective second order Hamiltonian, inside degenerate eigensubspaces of the non-perturbed Hamiltonian. We shall prove, by working out in detail the Su–Schrieffer–Heeger Hamiltonian case, that the presence of degenerate states, including fermions and bosons, which might seemingly...

For a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape we evaluate all the electron-phonon couplings derived from the one-body and two-body electronic interactions, considering both the adiabatic and extreme non-adiabatic limit. Not only the values of the coupling parameters in the two limits, but also the expr...

Recent inelastic neutron scattering experiments in CeIn3 and CePd2Si2 single crystals measured spin wave excitations at low temperatures. These two heavy fermion compounds exhibit antiferromagnetic (AF) long-range order, but a strong competition between the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and Kondo effect is evidenced by their near...

Recent inelastic neutron scattering experiments in CeIn3 and CePd2Si2 single crystals, measured spin wave excitations at low temperatures. These two heavy fermion compounds exhibit antiferromagnetic long-range order, but a strong competition between the Ruderman-Kittel-Kasuya-Yosida(RKKY) interaction and Kondo effect is evidenced by their nearly eq...

Well defined ferromagnetic spin waves were observed when the spin dynamics of Tl2Mn2O7, the first pyrochlore compound found to exhibit colossal magnetoresistance, was measured [ J. W. Lynn et al. Phys. Rev. Lett. 80 4582 (1998)], in stark contrast to the experimental results on the larger family of magnetoresistive manganites with perovskite struct...

We argue that the interband coupling in MgB2 originates from strong anharmonicity of the phonons and the presence of both linear and quadratic electron–phonon interactions due to the bond-stretching modes.

The observed spin dynamics in colossal magnetoresistive pyrochlores is
in stark contrast with that of perovskite manganites. We performed spin
wave calculations for Tl2Mn2O7, using
the microscopic generic model for the compound proposed recently.
Through a canonical transformation we determine the renormalization of
the ferromagnetic spin waves rel...

Well defined spin waves were observed when the spin dynamics of Tl2Mn2O7, the first pyrochlore compound found to exhibit colossal magnetoresistance, was measured [J.W.Lynn et al., Phys.Rev.Lett. 80,4582(1998)], in stark contrast with the experimental results on the larger family of magnetoresistive manganites with perovskite structure. In this work...

Taking as a starting point the results of LDA calculations, which show that in ${\rm MgB_{2}}$ the phonons have a strong quartic anharmonicity and that the bond-stretching electron-phonon interaction (EPI) has both a linear and a large quadratic component, we propose a model Hamiltonian which successfully matches a number of experimental evidences....

We supplement (and critically overview) the existing extensive analysis of antiferromagnetic solution for the Hubbard model
with a detailed discussion of two specific features, namely (i) the evolution of the magnetic (Slater) gap (here renormalized
by the electronic correlations) into the Mott-Hubbard or atomic gap, and (ii) a rather weak renormal...

We investigate the origin of the inter-band coupling in MgB2 by focusing on its unusual phononic features, namely, the strong anharmonicity of the phonons and the presence of both linear and quadratic electron-phonon interactions of the Su-Schrieffer-Heeger (SSH) type. The bare electronic Hamiltonian has two bands with intra- and inter-band hopping...

We perform a finite-size cluster study of the Kondo-Heisenberg model, describing a system of itinerant electrons locally coupled via a Kondo-type interaction to a set of magnetic ions. The ionic moments are assumed to have antiferromagnetic order, due to a nearest-neighbor Heisenberg coupling. By consecutive application of displacement and squeezin...

Discusses the equilibrium configurations and the phase transitions of a semi-classical model system consisting of two equal magnetic moments coupled by isotropic, anisotropic and antisymmetric exchange, with uniaxial anisotropy terms up to fourth order, in the presence of a transverse magnetic field. No restriction is imposed on the values of the i...

The electronic contribution to the cohesive energy is calculated in narrow band systems where the splitting of bands due to intra-atomic correlation is larger than the band width by solving the effective Hamiltonian for the lower Hubbard subband in the non-degenerate band approximation on the basis of different decouplings of the quasielectron Gree...

We study the complete extended Hubbard-Holstein Hamiltonian on a four-site chain with equally spaced sites, with spacing-dependent electronic interaction parameters evaluated in terms of Wannier functions built from Gaussian atomic orbitals. By successive application of generalized displacement and squeezing transformations, an effective polaronic...

Starting from the Hamiltonian for a dimer which includes all the electronic and electron-phonon terms consistent with a non-degenerate orbital, by a sequence of displacement and squeezing transformation we obtain an effective polaronic Hamiltonian. The renormalized electronic interactions differ from the results of semiclassical or perturbative tre...

For a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape we evaluate all the electron-phonon couplings derived from the one-body and two-body electronic interactions, considering both the adiabatic and extreme non-adiabatic limit. Not only the values of the coupling parameters in the two limits, but also the expr...

We derive an effective polaronic Hamiltonian by applying the displacement and the squeezing transformations to an extended version of the Hubbard–Holstein model, which includes all the electronic one- and two-body interactions for electrons coupled to dispersionless optical phonons. This effective Hamiltonian is then exactly solved on a four-site c...

The effective fermionic Hamiltonian derived from the extended Hubbard–Holstein model by a sequence of generalized displacement and squeezing transformations, contains new, long-range, intersite charge interactions. By exact diagonalization on a four-site ring with equally spaced sites, followed by variational optimization of the energy, we obtain t...

We evaluate all the electron–phonon coupling terms derived from one and two-body electronic interactions, in both the adiabatic and the extreme nonadiabatic limit, for a dimer with a nondegenerate orbital built from atomic wave functions of Gaussian shape. Different forms of the Hamiltonian contributions result in the two limits.

We evaluate all the electron-phonon couplings derived from the one-body and two-body electronic interactions, in both the adiabatic and extreme non-adiabatic limit, for a dimer with a non-degenerate orbital built from atomic wave functions of Gaussian shape. We find largely different values of the coupling parameters in the two limits, as well as d...

A polaronic Hamiltonian is obtained for a dimer of length a, including an Einstein phonon of frequency , a Holstein coupling g
0, and all the electronic one- and two-body terms consistent with a single orbital per site (the latter evaluated in terms of Wannier functions built from Gaussian atomic orbitals). In the quarter- and half-filled orbital c...

By a Gutzwiller-type treatment of the t–J model in an antiferromagnetic background, we explain the 1/δ variation of the spin polaron delocalization energy observed in doped 3d-oxides of different effective dimensionality.

Using the displacement and the squeezing transformations, we obtain the polaronic Hamiltonian for a chain of four sites including all the electronic one- and two-body terms for a single orbital, the free phonon term for a single frequency Ω, and an electron–phonon coupling g0 of the Holstein type. The ground state was studied for up to four electro...

From the complete extended Hubbard- Holstein Hamiltonian we obtain a polaronic effective Hamiltonian by successive application of generalized displacement and squeezing transformations, with wavevector dependent characteristic parameters which make possible a phonon- induced long range interaction between charges, whose sign can change with the dis...

Effective Hamiltonians for phonon and spin polarons are obtainedby applying a sequence of displacement and squeezing transformations to electron--phonon, or electron--magnon, interacting Hamiltonians. The basic techniques of calculation are shown in details, with explicit applications to the case of two- and four-sites systems.

We consider a model Hamiltonian for a dimer including all the electronic one- and two-body terms consistent with a single orbital per site, a free Einstein phonon term, and an electron-phonon coupling of the Holstein type. The bare electronic interaction parameters were evaluated in terms of Wannier functions built from Gaussian atomic orbitals. An...

A polaronic Hamiltonian is obtained for a dimer of lengtha, including an Einstein phonon of frequency Ω, a Holstein coupling g0, and all the electronic one- and two-body terms consistent with a single orbital per site (the tatter evaluated in terms
of Wannier functions built from Gaussian atomic orbitals). Its ground state for givena,g
0, and Ω is...

We consider the Hamiltonian for a dimer including all the electronic one- and two-body terms for a single orbital, the free
phonon term for a single frequency Ω, and an electron-phonon coupling g of the Holstein type. The bare electronic interaction
parameters were evaluated in terms of Wannier functions, built from atomic Gaussian orbitals. An eff...

We consider the complete extended Hubbard Hamiltonian on a dimer, including all one- and two-body terms. The energy eigenvalues and eigenvectors and various correlation functions have been obtained in explicit form. Introducing model Wannier orbitals we have evaluated the interaction parameters as functions of the dimer length and orbital shape. Mi...

We discuss specific features of quasiparticles in a strong applied magnetic field and near the Mott-Hubbard localization: the strong spin dependence of the de Haas-van Alphen oscillations, the maximum in the field dependence of the linear specific-heat coefficient, and metamagnetic behavior. These properties are obtained within the approach involvi...

The effect of Zn substitution on TN of Nd2Cu1−xZnxO4 for x ≤ 0.05 was measured by neutron diffraction, yielding limx → 0|TN−1 dTN/dx| = 3.9. We find some evidence that Zn has a solubility limit of ∼2%. We also checked that TN is insensitive to the sample annealing atmosphere.

The renormalization of the on-site charge excitation energy U∗ in the presence of on-site Hubbard correlation U and inter-site Coulomb repulsion V is evaluated, at zero temperature, in the large U limit, for arbitrary electron concentration ne = 1 ± δ. We find U∗ = U − V(1 − δ), at variance with earlier studies.

The impossibility to explain doping- dependent delocalization
transitions in strongly correlated system as due to the renormalization
of the charge excitation energy, suggest that they might be due to the
collapse of the spin-polarons.

We give a concise review of the theoretical and experimental activity on diamagnetic-substitution effects in superconducting
copper perovskites developed by Italian researchers since 1991.

We provide a microscopic modelling of the strong variation of the effective exchange interaction J
eff between Cu2+ spins as a function of hole doping δ in La2-δSrδCuO4, as derived from complementary Raman scattering and paramagnetic susceptibility data.

A pulse echo technique was employed to measure the ultrasonic attenuation and velocity in the 120–340 K temperature range on Nd2Cu1−xZnxO4 with x = 0, 0.02 and 0.05. A marked hysteresis in the velocity vs. temperature trend was observed. The Zn doping results in a reduction in the hysteresis loop area. Strong attenuation maxima were observed at 210...

To investigate why the sensitivity of the Néel temperature TN of the antiferromagnetic (AF) layered copper perovskites (typically La2CuO4) to diamagnetic impurities such as Zn is reportedly much larger than in the AF members of the K2NiF4 family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in t...

We discuss a model combining diamagnetic impurity and electron correlation effects to explain, in La2Cu1−cZncO4 and other materials related to HTCSC, the vanishing of TN for small Zn concentration.

For a single band of correlated fermions at T = 0 in a magnetic field h we prove that: (a) if n < 1 and U > 2W, the effective mass for spin-up (-down) particles decreases (increases) with h; (b) if n = 1 and U < 2W, the effective mass is spin-independent, the magnetoresistance is positive, and h induces a first-order transition to the localized, ma...

A re-examination of the available experimental data concerning the metallic phase of the three- dimensional system (V1-xMx)2O3 (M = Ti, Cr) suggests that they are consistent with the realization of the “electronic phase separated” state, which has recently been indicated in theoretical grounds as possible ground state of the t −J model, in connecti...

An interpretation of the Cu-site substitution effects on the critical temperature of La2CuO4-derived superconductors is presented. We argue that the available data on the dependence ofT
c on the amount of Cu-site substitutions indicate that the integrity of the Cu spin lattice is not fundamental to the superconductivity.
A consistent way of systema...

EXAFS measurements on La2Cu0.9Ga0.1O4 and La2Cu0.9Cd0.1O4 were performed at the Cu, Ga and Cd K-edges: Ga atoms, effective in suppressing Tc in La2CuO4 derived superconductors, induce structural distortions and seem not be be properly substituted at Cu sites, whereas Cd atoms, having a weak effect on Tc, substitute for Cu with small changes in the...

An attempt to find a quantitative relationship between magnetic transition temperature and conduction states properties in metallic materials is presented. The exchange interaction among magnetic moments is assumed to be of the Anderson's s-f type and the conduction states have s-d orbital character. The results are analyzed vs. different amount of...

We study a two-band Hamiltonian for hybridized and correlated bands of sc and bcc symmetry, the latter case approximating a two-dimensional band. Magnetic phase diagrams are obtained, showing in the bcc case a region of rapid phase change with filling which is absent in the sc case.

We investigate the indirect exchange interaction between localized moments via conduction bands. The origin of the exchange is assumed to be the Anderson s-f mixing mechanism appropriate to anomalous rare earth and light actinide systems. The conduction states originate from s-orbitals in a CsCl structure. Hubbard correlation in Hartree-Fock approx...

We study the effect of the conduction-electron density of states on magnetic transition temperatures in metallic systems with localized magnetic moments. We assume the indirect-exchange interaction to originate from s-f mixing of the Anderson type and we perform calculations of the coupling parameters, up to fourth-nearest neighbors, in the formali...

Two models, one strongly, and one moderately correlated, for the electronic structure of the high Tc superconductors are discussed. We study the superconducting phase (the critical temperature, and its dependence on pressure, the gap and the Josephson current) of the first model, and the magnetic phases of the second one, in bi- or tridimensional l...

The normal state of correlated electrons in a single band near the metal-insulator transition is described by a variational approach, for arbitrary values of the band filling n, and of the ratio between Hubbard correlation U and bandwidth W. A careful reexamination of the BCS equations allows to apply them correctly to the present model for general...

We study the influence of interorbital hybridization gammak on the indirect exchange interaction J(k) in a model of two bcc bands of s-type. We report two cases where the stable magnetic structure changes due to gammak. However, gammak generally leaves unchanged the structure, while reducing greatly its stability vs. temperature.

We discuss both continuous and discontinuous transitions form para- to ferromagnetism within a model of electrons in double degenerate and hybridized band. We transform out rigorously the hybridization and obtain a two-band model with the component bands of substantially different width. This band structure is approximated by a band and a level pla...

A theory of electron correlation effects at nonzero temperatures is discussed which successfully describes the set of metal-insulator transitions of the (V1-xCrx)2O3 alloy system in the paramagnetic state.

We study the transition from the para- to ferro-magnetic phase, accompanied by a transition to the mixed-valent state within a two-narrow-band model. Continuous and discontinuous transitions are possible. Application of the theory to Co(S1-xSex)2 and CoTi1-xAlx is discussed. Laboratoire PMTM, CNRS, Univ. Paris XIII, 93430 Villetaneuse, France.

We show that a model Hamiltonian representing a degenerate band composed of d equally hybridized and equivalent narrow subbands can be diagonalized for arbitrary d.A simple transformation removing the hybridization entirely is constructed. It gives both the eigenvalues and the eigen-functions in explicit form. The conditions under which the electro...

We analyze magnetic and mixed-valent states of electrons composing a band and an atomic level which arise from the removal of strong hybridization between the two components of a degenerate band. We obtain both continuous and discontinuous onsets of ferromagnetic ordering. A brief discussion of results for transition-metal disulphides is given. On...

Conditions for the onset of ferromagnetism in a discontinuous way are given for a model system composed of a band and an atomic level located in the middle of the band. Such model reflects the system of two strongly hybridized bands of equal width. The presence of a linear term in the total magnetization for the ground state energy is demonstrated....

We study the electronic states of a system with a degenerate band of eg type. Removing the off-diagonal hopping terms in the Hamiltonian results in a renormalized band structure exhibiting similarities with mixed valence systems. Inclusion of Coulomb correlation and exchange produces a variety of magnetic configurations. Laboratorio PMTM, Universit...

We study in the molecular-field approximation the equilibrium configuration and the phase transitions of a magnetic model system consisting of two sets of moments of equal magnitude, coupled by isotropic, anisotropic, and antisymmetric exchange, in the presence of uniaxial anisotropy, under the effect of a transverse magnetic field. The anisotropy...

Starting from an analytical expression for the density of states of an s-type band with arbitry Coulomb correlation obtained from a solution of the Hubbard Hamiltonian, an expression for the electronic contribution to the static elastic constant in a narrow doubly degenerate band is derived both in the absence and in the presence of a magnetic fiel...

Fully general solutions to the equilibrium equations of a system of two equal magnetic moments coupled by isotropic, anisotropic and antisymmetric exchange, together with second- and fourth-order uniaxial anisotropy, in vanishing external field, are given. They depend critically upon the fourth-order anisotropy and the antisymmetric exchange, and c...

We solve rigorously the equilibrium equations for a system of two classical magnetic moments in a transverse field, coupled by Dzialoshinskii antisymmetric exchange, as well as isotropic and anisotropic exchange, in the presence of uniaxial second order anisotropy, whose easy axis is parallel to the Dzialoshinskii D vector. The system allows for pl...

## Projects

Projects (3)

We study the electronic and magnetic states in a S=1/2 magnetic dimer with first- and second-order interactions between spins and vibrations , for varying dimer length. A generic Heisenberg coupling term of the XYZ type is assumed. The exchang components and the spin-vibration couplings are evaluated explicitly by using model Wannier functions.
The effect of vibrations is treated non-perturbatively by a sequence of unitary transformations.