arXiv subjects

Matteo Calandra

Publications and source records attributed to Matteo Calandra.

133 records · Page 8Linked to original sources

arXiv2005

Superconductivity in C6Ca

Using density functional theory we demonstrate that superconductivity in C6Ca is due to a phonon-mediated mechanism with electron-phonon coupling 0.83 and phonon-frequency logarithmic-average 24.7 meV. The calculated isotope exponents are 0.24 for Ca and 0.26 for C. Superconductivity is mostly due C vibrations perpendicular and Ca vibrations parallel to the graphite layers. Since the electron-phonon couplings of these modes are activated by the presence of an intercalant Fermi surface, the occurrence of superconductivity in graphite intercalated compounds requires a non complete ionization of the intercalant.

cond-mat.supr-con↗

arXiv2004

Density functional theory description of hole-trapping in SiO$_2$: a successful self-interaction-corrected approach

We present a self-interaction-corrected (SIC) density-functional-theory (DFT) approach for the description of systems with an unpaired electron or hole such as spin 1/2 defect-centers in solids or radicals. Our functional is easy-to-implement and its minimization does not require additional computational effort with respect to ordinary DFT functionals. In particular it does not present multi-minima, as the conventional SIC functionals. We successfully validate the method studying the hole self-trapping in quartz associated to the Al substitutional impurity. We show that our approach corrects for the well known failures of standard DFT functionals in this system.

cond-mat.mtrl-sci↗

arXiv2004

Electron-phonon coupling and phonon self-energy in MgB$_2$: do we really understand MgB$_2$ Raman spectra ?

We consider a model Hamiltonian fitted on the ab-initio band structure to describe the electron-phonon coupling between the electronic $σ-$bands and the phonon E$_{2g}$ mode in MgB$_2$. The model allows for analytical calculations and numerical treatments using very large k-point grids. We calculate the phonon self-energy of the E$_{2g}$ mode along two high symmetry directions in the Brillouin zone. We demonstrate that the contribution of the $σ$ bands to the Raman linewidth of the E$_{2g}$ mode via the electron-phonon coupling is zero. As a consequence the large resonance seen in Raman experiments cannot be interpreted as originated from the $E_{2g}$ mode at $Γ$. We examine in details the effects of Fermi surface singularities in the phonon spectrum and linewidth and we determine the magnitude of finite temperature effects in the the phonon self-energy. From our findings we suggest several possible effects which might be responsible for the MgB$_2$ Raman spectra.

cond-mat.supr-con↗

arXiv2004

Superconductivity from doping boron icosahedra

We propose a new route to achieve the superconducting state in Boron-rich solids, the hole doping of B$_{12}$ icosahedra. For this purpose we consider a prototype metallic phase of B$_{13}$C$_2$. We show that in this compound the Boron icosahedral units are mainly responsible for the large phonon frequencies logarithmic average, 65.8 meV, and the moderate electron-phonon coupling 0.8. We suggest that this high critical temperature could be a general feature of hole doped boron icosahedral solids.

cond-mat.supr-con↗

arXiv2003

Anharmonic phonon frequency shift in MgB2

We compute the anharmonic shift of the phonon frequencies in MgB2, using density functional theory. We explicitly take into account the scattering between different phonon modes at different q-points in the Brillouin zone. The shift of the E2g mode at the Gamma point is +5 % of the harmonic frequency. This result comes from the cancellation between the contributions of the four- and three-phonon scattering, respectively +10 % and -5 %. A similar shift is predicted at the A point, in agreement with inelastic X-ray scattering phonon-dispersion measurements. A smaller shift is observed at the M point.

cond-mat.supr-con↗

arXiv2003

Phonon dispersion and lifetimes in MgB2

We measure phonon dispersion and linewidth in a single crystal of MgB_2 along the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We use Density Functional Theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E_2g phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.

cond-mat.supr-con↗

arXiv1999

From antiferromagnetism to d-wave superconductivity in the 2D t-J model

We have found that the two dimensional t-J model, for the physical parameter range J/t = 0.4 reproduces the main experimental qualitative features of High-Tc copper oxide superconductors: d-wave superconducting correlations are strongly enhanced upon small doping and clear evidence of off diagonal long range order is found at the optimal doping δ~ 0.15. On the other hand antiferromagnetic long range order, clearly present at zero hole doping, is suppressed at small hole density with clear absence of antiferromagnetism at δ>~ 0.1.

cond-mat.str-el↗