Francesco Mauri

Bio: Francesco Mauri is an academic researcher from Sapienza University of Rome. The author has contributed to research in topics: Graphene & Phonon. The author has an hindex of 85, co-authored 352 publications receiving 69332 citations. Previous affiliations of Francesco Mauri include University of Texas at Arlington & University of California, Berkeley.

Kohn anomalies and nonadiabaticity in doped carbon nanotubes

Abstract: The high-frequency Raman-active phonon modes of metallic single-walled carbon nanotubes (SWCNT) are thought to be characterized by Kohn anomalies (KAs) resulting from the combination of SWCNT intrinsic one-dimensional nature and a significant electron-phonon coupling (EPC). KAs are expected to be modified by the doping-induced tuning of the Fermi energy level ${ϵ}_{F}$, obtained through the intercalation of SWCNTs with alkali atoms or by the application of a gate potential. We present a density-functional theory (DFT) study of the phonon properties of a (9,9) metallic SWCNT as a function of electronic doping. For such study, we use, as in standard DFT calculations of vibrational properties, the Born-Oppenheimer (BO) approximation. We also develop an analytical model capable of reproducing and interpreting our DFT results. Both DFT calculations and this model predict, for increasing doping levels, a series of EPC-induced KAs in the vibrational mode parallel to the tube axis at the $\mathbf{\ensuremath{\Gamma}}$ point of the Brillouin zone, usually indicated in Raman spectroscopy as the ${G}^{\ensuremath{-}}$ peak. Such KAs would arise each time a new conduction band is populated. However, we show that they are an artifact of the BO approximation. The inclusion of nonadiabatic effects dramatically affects the results, predicting KAs at $\mathbf{\ensuremath{\Gamma}}$ only when ${ϵ}_{F}$ is close to a band crossing ${E}_{X}$. For each band crossing, a double KA occurs for ${ϵ}_{F}={E}_{X}\ifmmode\pm\else\textpm\fi{}\ensuremath{\hbar}\ensuremath{\omega}∕2$, where $\ensuremath{\hbar}\ensuremath{\omega}$ is the phonon energy. In particular, for a $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ metallic nanotube, we predict a KA to occur in the so-called ${G}^{\ensuremath{-}}$ peak at a doping level of about ${N}_{\mathrm{el}}∕C=\ifmmode\pm\else\textpm\fi{}0.0015$ atom $({ϵ}_{F}\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and, possibly, close to the saturation doping level $({N}_{\mathrm{el}}∕C\ensuremath{\sim}0.125)$, where an interlayer band crosses the ${\ensuremath{\pi}}^{*}$ nanotube bands. Furthermore, we predict that the Raman linewidth of the ${G}^{\ensuremath{-}}$ peak significantly decreases for $\ensuremath{\mid}{ϵ}_{F}\ensuremath{\mid}\ensuremath{\geqslant}\ensuremath{\hbar}\ensuremath{\omega}∕2$. Thus, our results provide a tool to determine experimentally the doping level from the value of the KA-induced frequency shift and from the linewidth of the ${G}^{\ensuremath{-}}$ peak. Finally, we predict KAs to occur in phonons with finite momentum $\mathbf{q}$ not only in proximity of a band crossing but also each time a new band is populated. Such KAs should be observable in the double-resonant Raman peaks, such as the defect-activated $D$ peak, and the second-order peaks $2D$ and $2G$.

Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe.

Abstract: Since 2014 the layered semiconductor SnSe in the high-temperature $Cmcm$ phase is known to be the most efficient intrinsic thermoelectric material Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge nonperturbative anharmonic effects We show that the transition from the $Cmcm$ phase to the low-symmetry $Pnma$ is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature Our calculations show that the spectral function of the in-plane vibrational modes are strongly anomalous with shoulders and double-peak structures We calculate the lattice thermal conductivity obtaining good agreement with experiments only when nonperturbative anharmonic scattering is included Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the nonperturbative anharmonicity

Current-voltage characteristics of graphene devices: Interplay between Zener-Klein tunneling and defects

Abstract: We report a theoretical/experimental study of current-voltage characteristics $(I\text{\ensuremath{-}}V)$ of graphene devices near the Dirac point. The $I\text{\ensuremath{-}}V$ can be described by a power law ($I\ensuremath{\propto}{V}^{\ensuremath{\alpha}}$ with $1l\ensuremath{\alpha}\ensuremath{\le}1.5$). The exponent is higher when the mobility is lower. This superlinear $I\text{\ensuremath{-}}V$ is interpreted in terms of the interplay between Zener-Klein transport, that is tunneling between different energy bands, and defect scattering. Surprisingly, the Zener-Klein tunneling is made visible by the presence of defects.

First-principles calculation of the infrared spectrum of hematite

Abstract: The theoretical infrared spectrum of lizardite [Mg3Si2O5(OH)4] was computed using first-principles quantum mechanical calculations. Density functional perturbation theory allowed us to derive the low-frequency dielectric tensor of lizardite as a function of the light frequency. The infrared spectrum was then calculated using a model that takes into account the platy shape of particles. A very good agreement was obtained between theory and experiment. This agreement allows us to make an unambiguous assignment of the major absorption bands observed in the IR spectrum of lizardite, including the stretching bands of OH groups.

Calcium Phosphates and Hydroxyapatite: Solid State NMR Experiments and First Principles Calculations

Abstract: Various calcium phosphates and hydroxyapatite (HAp) have been fully characterized by one- and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments and first principles calculations of NMR parameters, such as chemical shift anisotropy (CSA) and electric field gradient tensors for all nuclei. Such compounds act as useful biocompatible materials. The projector augmented wave (PAW) and gauge including PAW methods allowed the complete assignment of spectra, including 1H magic-angle spinning (MAS) spectra for which ultimate resolution is not attained experimentally. 1H CSA tensors and orientation of the principal axes systems have been also discussed. 17O parameters have been calculated for a large variety of oxo-bridges and terminal oxygen atoms, including P–O–Si fragments characteristic for silicophosphate phases. The (δiso, CQ) sets of values allowed the clear distinction between the various oxygen atoms in a calculated 17O 3-quantum MAS experiment. Such an approach should be of great help for the description of interfaces in complex materials, in terms of structure and chemical composition.

The rise of graphene

Abstract: Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

The electronic properties of graphene

Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.