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.

Determination of scattering time and of valley occupation in transition-metal dichalcogenides doped by field effect

Abstract: The transition-metal dichalcogenides have attracted a lot of attention as a possible stepping-stone toward atomically thin and flexible field-effect transistors. One key parameter to describe the charge transport is the time between two successive scattering events---the transport scattering time. In a recent report, we have shown that it is possible to use density functional theory to obtain the band structure of two-dimensional semiconductors in the presence of field effect doping. Here, we report a simple method to extract the scattering time from the experimental conductivity and from the knowledge of the band structure. We apply our approach to monolayers and multilayers of ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{MoTe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ in the presence of a gate. In ${\mathrm{WS}}_{2}$, for which accurate measurements of mobility have been published, we find that the scattering time is inversely proportional to the density of states at the Fermi level. Finally, we show that it is possible to identify the critical doping at which different valleys start to be occupied from the doping dependence of the conductivity.

Origin of superconductivity of CaC6 and of other intercalated graphites

Abstract: By intercalation of donor the low conductivity of graphite can be enhanced and a superconducting state can occur at low temperature. The first discovered superconductors were alkali-intercalated compounds, with a critical temperatures (T c ) of the order of 1 K. In 2005 we learned with surprise that Ca intercalated graphite (CaC 6 ) is a superconductor with the sizable T c of 11.5 K. Using density functional theory we demonstrate that superconductivity in CaC 6 is phonon-mediated with an electron-phonon coupling λ equal to 0.83 and a phonon-frequency logarithmic-average equal to 25 meV. Superconductivity is mostly due C vibrations perpendicular and Ca vibrations parallel to the graphite layers. A non zero electron-phonon coupling for these modes can not be associated to the Fermi surface of the graphite pi bands but requires the presence of a second Fermi surface associated to the intercalant atoms. This result suggests a general mechanism for the occurrence of superconductivity in intercalated graphite. In order to stabilize a superconducting state it is necessary to have an intercalant Fermi surface since the simple doping of the π bands in graphite does not lead to a sizeable electron-phonon coupling. This condition occurs if the intercalant band is partially occupied, i.e. when the intercalant is not fully ionized.

SiCCSi antisite pairs in SiC identified as paramagnetic defects with strongly anisotropic orbital quenching

Abstract: The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have been identified using electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor. Based on the theoretical predictions, the positively charged defect has been found experimentally also in the cubic 3C-SiC polytype where it is characterized by spin 1/2 and highly anisotropic g values of g(xx)=2.0030, g(yy)=2.0241, and g(zz)=2.0390 within C-1h symmetry. The exceptional large g values are explained by details of the spin-orbit coupling causing a strongly anisotropic quenching of the orbital angular momentum of the p-like unpaired electron.

Electronic structure of TiSe 2 from a quasi-self-consistent G 0 W 0 approach

Abstract: In a previous work, it was shown that the inclusion of exact exchange is essential for a first-principles description of both the electronic and the vibrational properties of $\mathrm{Ti}{\mathrm{Se}}_{2}$, M. Hellgren et al. [Phys. Rev. Lett. 119, 176401 (2017)]. The $GW$ approximation provides a parameter-free description of screened exchange but is usually employed perturbatively (${G}_{0}{W}_{0}$), making results more or less dependent on the starting point. In this work, we develop a quasi-self-consistent extension of ${G}_{0}{W}_{0}$ based on the random phase approximation (RPA) and the optimized effective potential of hybrid density functional theory. This approach generates an optimal ${G}_{0}{W}_{0}$ starting point and a hybrid exchange parameter consistent with the RPA. While self-consistency plays a minor role for systems such as Ar, BN, and ScN, it is shown to be crucial for $\mathrm{Ti}{\mathrm{S}}_{2}$ and $\mathrm{Ti}{\mathrm{Se}}_{2}$. We find the high-temperature phase of $\mathrm{Ti}{\mathrm{Se}}_{2}$ to be a semimetal with a band structure in good agreement with experiment. Furthermore, the optimized hybrid functional agrees well with our previous estimate and therefore accurately reproduces the low-temperature charge-density-wave phase.

Nitrogen Donor Aggregation in 4H-SiC: g-Tensor Calculations

Abstract: The microscopic origin of the Nx EPR-lines observed in heavily nitrogen doped 4H-SiC and 6H-SiC is discussed with the help of EPR parameters calculated from first principles. Based on the symmetry of the g-tensors we propose a model with distant NC donor pairs on inequivalent lattice sites which are coupled to S=1 centers but with nearly vanishing zero-field splittings, giving rise to an essentially S=1/2 like spectrum. The proposed aggregation in neutral donor pairs can contribute to the saturation of the free concentration observed in heavily nitrogen doped SiC.

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.