arXiv subjects

R. Pucci

Publications and source records attributed to R. Pucci.

At least 37 records · Page 2Linked to original sources

arXiv2009

Ab initio prediction of Boron compounds arising from Borozene: Structural and electronic properties

Structure and electronic properties of two unusual boron clusters obtained by fusion of borozene rings has been studied by means of first principles calculations, based on the generalized-gradient approximation of the density functional theory, and the semiempirical tight-binding method was used for the transport calculations. The role of disorder has also been considered with single vacancies and substitutional atoms. Results show that the pure boron clusters are topologically planar and characterized by (3c-2e) bonds, which can explain, together with the aromaticity (estimated by means of NICS), the remarkable cohesive energy values obtained. Such feature makes these systems competitive with the most stable boron clusters to date. On the contrary, the introduction of impurities compromises stability and planarity in both cases. The energy gap values indicate that these clusters possess a semiconducting character, while when the larger system is considered, zero-values of the density of states are found exclusively within the HOMO-LUMO gap. Electron transport calculations within the Landauer formalism confirm these indications, showing semiconductor-like low bias differential conductance for these stuctures. Differences and similarities with Carbon clusters are highlighted in the discussion.

cond-mat.mtrl-sci↗

arXiv2009

Proposed lower bound for the shear viscosity to entropy density ratio in some dense liquids

Starting from relativistic quantum field theories, Kovtun et al. (2005) have quite recently proposed a lower bound eta/s >= hbar /(4 pi kB), where eta is the shear viscosity and s the volume density of entropy for dense liquids. If their proposal can eventually be proved, then this would provide key theoretical underpinning to earlier semiempirical proposals on the relation between a transport coefficient eta and a thermodynamic quantity s. Here, we examine largely experimental data on some dense liquids, the insulators nitrogen, water, and ammonia, plus the alkali metals, where the shear viscosity eta(T) for the four heaviest alkalis is known to scale onto an `almost universal' curve, following the work of Tankeshwar and March a decade ago. So far, all known results for both insulating and metallic dense liquids correctly exceed the lower bound prediction of Kovtun et al.

cond-mat.stat-mech↗

arXiv2008

Modeling vacancies and hydrogen impurities in graphene: A molecular point of view

We have followed a "molecular" approach to study impurity effects in graphene. This is thought as the limiting case of an infinitely large cluster of benzene rings. Therefore, we study several carbon clusters, with increasing size, from phenalene, including three benzene rings, up to coronene 61, with 61 benzene rings. The impurities considered were a chemisorbed H atom, a vacancy, and a substitutional proton. We performed HF and UHF calculations using the STO-3G basis set. With increasing cluster size in the absence of impurities, we find a decreasing energy gap, here defined as the HOMO-LUMO difference. In the case of H chemisorption or a vacancy, the gap does not decrease appreciably, whereas it is substantially reduced in the case of a substitutional proton. The presence of an impurity invariably induces an increase of the density of states near the HOMO level. We find a zero mode only in the case of a substitutional proton. In agreement with experiments, we find that both the chemisorbed H, the substitutional proton, and the C atom near a vacancy acquire a magnetic moment. The relevance of graphene clusters for the design of novel electronic devices is also discussed.

cond-mat.str-el↗

arXiv2008

Statistical correlations of an anyon liquid at low temperatures

Using a proposed generalization of the pair distribution function for a gas of non-interacting particles obeying fractional exclusion statistics in arbitrary dimensionality, we derive the statistical correlations in the asymptotic limit of vanishing or low temperature. While Friedel-like oscillations are present in nearly all non-bosonic cases at T=0, they are characterized by exponential damping at low temperature. We discuss the dependence of these features on dimensionality and on the value of the statistical parameter alpha.

cond-mat.stat-mech↗

arXiv2008

Molecules in clusters: the case of planar LiBeBCNOF built from a triangular form LiOB and a linear four-center species FBeCN

Krueger some years ago proposed a cluster LiBeBCNOF, now called periodane. His ground-state isomer proposal has recently been refined by Bera et al. using DFT. Here, we take the approach of molecules in such a cluster as starting point. We first study therefore the triangular molecule LiOB by coupled cluster theory (CCSD) and thereby specify accurately its equilibrium geometry in free space. The second fragment we consider is FBeCN, but treated now by restricted Hartree-Fock (RHF) theory. This four-center species is found to be linear, and the bond lengths are obtained from both RHF and CCSD calculations. Finally, we bring these two entities together and find that while LiOB remains largely intact, FBeCN becomes bent by the interaction with LiOB. Hartree-Fock and CCSD theories then predict precisely the same lowest isomer found by Bera et al. solely on the basis of DFT.

cond-mat.other↗

arXiv2007

Statistical correlations in an ideal gas of particles obeying fractional exclusion statistics

After a brief discussion of the concepts of fractional exchange and fractional exclusion statistics, we report partly analytical and partly numerical results on thermodynamic properties of assemblies of particles obeying fractional exclusion statistics. The effect of dimensionality is one focal point, the ratio mu/kB T of chemical potential to thermal energy being obtained numerically as a function of a scaled particle density. Pair correlation functions are also presented as a function of the statistical parameter, with `Friedel' oscillations developing close to the fermion limit, for sufficiently large density.

cond-mat.str-el↗

arXiv2006

Methods to determine the Hausdorff dimension of vortex loops in the three-dimensional XY model

The geometric properties of critical fluctuations in the 3D XY model are analyzed. The 3D XY model is a lattice model describing superfluids. We present a direct evaluation of the Hausdorff dimension D_H of the vortex loops which are the critical fluctuations of the 3D XY model. We also present analytical arguments for why \vartheta in the scaling relation η_ϕ + D_H = 2 + \vartheta between D_H and the anomalous scaling dimension of the corresponding field theory, must be zero.

cond-mat.supr-con↗

arXiv2006

Majorana: from atomic and molecular, to nuclear physics

In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some aspects of his fundamental scientific production in atomic and molecular physics, including a not well known short communication. There, Majorana critically discusses Fermi's solution of the celebrated Thomas-Fermi equation for electron screening in atoms and positive ions. We argue that some of Majorana's seminal contributions in molecular physics already prelude to the idea of exchange interactions (or Heisenberg-Majorana forces) in his later workson theoretical nuclear physics. In all his papers, he tended to emphasize the symmetries at the basis of a physical problem, as well as the limitations, rather than the advantages, of the approximations of the method employed.

physics.hist-ph↗

arXiv2006

The Gross-Pitaevskii equations and beyond for inhomogeneous condensed bosons

A simple derivation of the static Gross-Pitaevskii (GP) equation is given from an energy variational principle. The result is then generalized heuristically to the time-dependent GP form. With this as background, a number of different experimental areas explored very recently are reviewed, in each case contact being established between the measurements and the predictions of the GP equations. The various limitations of these equations as used on dilute inhomogeneous condensed Boson atomic gases are then summarized, reference also being made to the fact that there is no many-body wave function underlying the GP formulation. This then leads into a discussion of a recently proposed integral equation, derived by taking the Bogoliubov-de Gennes equation as starting point. Some limitations of the static GP differential equation are thereby removed, though it is a matter of further study to determine whether a correlated wave function exists as underpinning for the integral equation formulation.

cond-mat.soft↗

arXiv2005

Scaling of the superconducting transition temperature in underdoped high-Tc cuprates with a pseudogap energy: Does this support the anyon model of their superfluidity?

In earlier work, we have been concerned with the scaling properties of some classes of superconductors, specifically with heavy Fermion materials and with five bcc transition metals of BCS character. Both of these classes of superconductors were three-dimensional but here we are concerned solely with quasi-two-dimensional high-Tc cuprates in the underdoped region of their phase diagram. A characteristic feature of this part of the phase diagram is the existence of a pseudogap (pg). We therefore build our approach around the assumption that kB Tc / E_pg is the basic dimensionless ratio on which to focus, where the energy E_pg introduced above is a measure of the pseudogap. Since anyon fractional statistics apply to two-dimensional assemblies, we expect the fractional statistics parameter allowing `interpolation' between Fermi-Dirac and Bose-Einstein statistical distribution functions as limiting cases to play a significant role in determining kB Tc / E_pg and experimental data are analyzed with this in mind.

cond-mat.supr-con↗

arXiv2005

Equilibrium molecular energies used to obtain molecular dissociation energies and heats of formation within the bond-order correlation approach

Ab initio calculations including electron correlation are still extremely costly except for the smallest atoms and molecules. Therefore, our purpose in the present study is to employ a bond-order correlation approach to obtain, via equilibrium molecular energies, molecular dissociation energies and heats of formation for some 20 molecules containing C, H, and O atoms, with a maximum number of electrons around 40. Finally, basis set choice is shown to be important in the proposed procedure to include electron correlation effects in determining thermodynamic properties. With the optimum choice of basis set, the average percentage error for some 20 molecules is approximately 20% for heats of formation. For molecular dissociation energies the average error is much smaller: ~0.4.

physics.chem-ph↗

arXiv2005

Multiband superconductors close to a 3D-2D electronic topological transition

Within the two-band model of superconductivity, we study the dependence of the critical temperature Tc and of the isotope exponent alpha in the proximity to an electronic topological transition (ETT). The ETT is associated with a 3D-2D crossover of the Fermi surface of one of the two bands: the sigma subband of the diborides. Our results agree with the observed dependence of Tc on Mg content in A_{1-x}Mg_xB_2 (A=Al or Sc), where an enhancement of Tc can be interpreted as due to the proximity to a "shape resonance". Moreover we have calculated a possible variation of the isotope effect on the superconducting critical temperature by tuning the chemical potential.

cond-mat.supr-con↗

arXiv2005

Electronic structure of clusters (LiBC)_n: n=1, 2, 4

A crystalline form of LiBC is known which has been predicted to be superconducting, with a Tc comparable to that of MgB_2. In both compounds, superconductivity is enhanced by the presence of two electronic bands, one of which is close to a dimensional crossover. Here, we take a quantum chemical approach, and investigate the structural and electronic properties of small clusters (LiBC)_n (n=1-4). With increasing cluster size, we find that several properties of crystalline LiBC evolve naturally from the corresponding properties of the clusters. In particular, one may recognize the origin of the solid bilayered structure, typical of magnesium diboride, and the character of the electronic sigma-band, arising from the overlap of the atomic orbitals in the in-plane BC rings. Two aspects especially emphasized are (a) the HOMO-LUMO gap as function of n and (b) the role of different spin multiplicity.

physics.atm-clus↗

arXiv2005

Pressure dependence of superconducting and magnetic critical temperatures in the ruthenocuprates

We study the dependence on filling and pressure of the superconducting and ferromagnetic critical temperatures of the ruthenocuprates, within the two-band model. At zero pressure, we find separate regions of coexistence of superconductivity and ferromagnetism as a function of filling, with contiguous regions merging together as pressure increases. As a function of pressure, a stronger enhancement of the magnetic phase results in a reduced pressure effect on the superconducting critical temperature. Comparison with recent experiments on the determination of the critical temperatures as a function of the pressure is also discussed.

cond-mat.str-el↗

arXiv2005

Tc for heavy Fermion superconductors linked with other physical properties at zero and applied pressure

The superconducting transition temperature Tc has earlier been correlated with coherence length and effective mass for a series of heavy Fermion (HF) materials at atmospheric pressure. Here, a further physical property, the dc electrical conductivity sigma(Tc), is one focal point, another being the pressure dependence of both Tc and sigma(Tc) for several HF materials. The relaxation time tau(Tc) is also studied in relation to an Uncertainty Principle limit, involving only the thermal energy kB Tc and Planck's constant.

cond-mat.supr-con↗

arXiv2004

Bond-order correlation energies for small Si-containing molecules compared with ab initio results from low-order Moller-Plesset perturbation theory

The present study of small molecules containing silicon has been motivated by (a) the considerable interest being shown currently in the kinetics and reactivity of such molecules, and (b) the biotechnological potential of silicon-derivate surfaces as substrates in the adsorption of, for instance, amino acids and proteins. Therefore, we have studied by (i) a semi-empirical approach and (ii) an ab initio procedure employing low-order Moller-Plesset perturbation theory, the molecular correlation energies of some neutral closed and open shell silicon-containing molecules in the series SiXnYm. Procedure (i) is shown to have particular merit for the correlation of the ionic members studied in the above series, while the ab initio procedures employed come into their own for neutral species.

physics.chem-ph↗

arXiv2004

Comment on "Variation of the superconducting transition temperature of hole-doped copper oxides"

We point out the incorrect derivation of the gap equation in X.-J. Chen and H. Q. Lin [Phys. Rev. B 69 (2004) 104518; cond-mat/0306680] within the interlayer tunneling (ILT) model for multilayered cuprates. There, the local structure in k-space of the ILT effective interaction has not been taken into due account when the ILT model is generalized to the case of n layers per unit cell. This is a specific characteristic of the ILT model that, apart from giving rise to a highly nontrivial k-dependence of the gap function, is known to enhance the critical temperature Tc in a natural way. As a consequence, we argue that Chen and Lin's results cannot be employed, in their present form, for a quantitative interpretation of the high-pressure dependence of Tc in Bi-2212, as is done by X.-J. Chen et al. [cond-mat/0408587]. Moreover, when the generalization of X.-J. Chen et al. [cond-mat/0408587] is applied to the case n=2, it fails to reproduce the original ILT gap equation. However, a more careful analysis of the ILT model for multilayered cuprates, taking into account the nonuniform hole distribution among inequivalent layers, has been earlier suggested to describe the observed pressure dependence of Tc in homologous series of high-Tc cuprates.

cond-mat.supr-con↗

arXiv2004

Effect of nonuniform hole-content distribution within the interlayer pair-tunneling mechanism of layered HTSC

The interlayer pair-tunneling (ILT) mechanism for high-$T_c$ superconductivity is able to predict the dependence of the (optimal) critical temperature Tc on the number of layers n within an homologous series of layered cuprate oxides. We generalize the mean-field procedure employed to evaluate Tc within an extended in-plane Hubbard model in presence of ILT, developed for a bilayer complex (n = 2), to the case of n = 3, 4 inequivalent superconducting layers. As a function of doping, we show how a nonuniform hole-content distribution among different layers affects Tc. In particular, depending on doping, the onset of superconductivity may be ruled by inner or outer layers. The latter result may be related to recent experimental data of Tc as a function of pressure in Tl- and Bi-based layered superconductors.

cond-mat.supr-con↗