1,444 research outputs found

### Electronic structure calculations and molecular dynamics simulations with linear system-size scaling

We present a method for total energy minimizations and molecular dynamics
simulations based either on tight-binding or on Kohn-Sham hamiltonians. The
method leads to an algorithm whose computational cost scales linearly with the
system size. The key features of our approach are
(i) an orbital formulation with single particle wavefunctions constrained to
be localized in given regions of space, and (ii) an energy functional which
does not require either explicit orthogonalization of the electronic orbitals,
or inversion of an overlap matrix.
The foundations and accuracy of the approach and the performances of the
algorithm are discussed, and illustrated with several numerical examples
including Kohn-Sham hamiltonians. In particular we present calculations with
tight-binding hamiltonians for diamond, graphite, a carbon linear chain and
liquid carbon at low pressure. Even for a complex case such as liquid carbon --
a disordered metallic system with differently coordinated atoms -- the
agreement between standard diagonalization schemes and our approach is very
good. Our results establish the accuracy and reliability of the method for a
wide class of systems and show that tight binding molecular dynamics
simulations with a few thousand atoms are feasible on small workstations

### Electronic structure of heavily-doped graphene: the role of foreign atom states

Using density functional theory calculations we investigate the electronic
structure of graphene doped by deposition of foreign atoms. We demonstrate
that, as the charge transfer to the graphene layer increases, the band
structure of the pristine graphene sheet is substantially affected. This is
particularly relevant when Ca atoms are deposed on graphene at CaC$_{6}$
stoichiometry. Similarly to what happens in superconducting graphite
intercalated compounds, a Ca bands occurs at the Fermi level. Its hybridization
with the C states generates a strong non-linearity in one of the $\pi^{*}$
bands below the Fermi level, at energies comparable to the graphene E$_{2g}$
phonon frequency. This strong non-linearity, and not manybody effects as
previously proposed, explains the large and anisotropic values of the apparent
electron-phonon coupling measured in angular resolved photoemission.Comment: 4 pages, 2 figures, see also M. Calandra and F. Mauri,arXiv:0707.146

### Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

Using density functional theory we investigate the occurrence of
superconductivity in AC$_6$ with A=Mg,Ca,Sr,Ba. We predict that at zero
pressure, Ba and Sr should be superconducting with critical temperatures
(T$_c$) 0.2 K and 3.0 K, respectively. We study the pressure dependence of
T$_c$ assuming the same symmetry for the crystal structures at zero and finite
pressures. We find that the SrC$_6$ and BaC$_6$ critical temperatures should be
substantially enhanced by pressure. On the contrary, for CaC$_6$ we find that
in the 0 to 5 GPa region, T$_c$ weakly increases with pressure. The increase is
much smaller than what shown in several recent experiments. Thus we suggest
that in CaC$_6$ a continous phase transformation, such as an increase in
staging, occurs at finite pressure. Finally we argue that, although MgC$_6$ is
unstable, the synthesis of intercalated systems of the kind
Mg$_x$Ca$_{1-x}$C$_y$ could lead to higher critical temperatures.Comment: 9 page

### Charge density wave and superconducting dome in TiSe2 from electron-phonon interaction

At low temperature TiSe2 undergoes a charge density wave instability.
Superconductivity is stabilized either by pressure or by Cu intercalation. We
show that the pressure phase diagram of TiSe2 is well described by
first-principles calculations. At pressures smaller than 4 GPa charge density
wave ordering occurs, in agreement with experiments. At larger pressures the
disappearing of the charge density wave is due to a stiffening of the
short-range force-constants and not to the variation of nesting with pressure.
Finally we show that the behavior of Tc as a function of pressure is entirely
determined by the electron-phonon interaction without need of invoking
excitonic mechanisms. Our work demonstrates that phase-diagrams with competing
orders and a superconducting dome are also obtained in the framework of the
electron-phonon interaction.Comment: 4 pages, 7 picture

### 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.Comment: 4 pages, 3 figure

### First principles theory of the EPR g-tensor in solids: defects in quartz

A theory for the reliable prediction of the EPR g-tensor for paramagnetic
defects in solids is presented. It is based on density functional theory and on
the gauge including projector augmented wave (GIPAW) approach to the
calculation of all-electron magnetic response. The method is validated by
comparison with existing quantum chemical and experimental data for a selection
of diatomic radicals. We thenperform the first prediction of EPR ${\rm g}$-tensors in the solid state and find the results to be in excellent agreement
with experiment for the $E'_1$ and substitutional P defect centers in quartz.Comment: 5 pages, 4 table

### High-$T_c$ superconductivity in weakly electron-doped HfNCl

We investigate the magnetic and superconducting properties in electron-doped
Li$_x$HfNCl. HfNCl is a band insulator that undergoes an insulator to
superconductor transition upon doping at $x\approx0.13$. The persistence of the
insulating state for $x<0.13$ is due to an Anderson transition probably related
to Li disorder. In the metallic and superconducting phase, Li$_x$HfNCl is a
prototype two-dimensional two-valley electron gas with parabolic bands. By
performing a model random phase approximation approach as well as
first-principles range-separated Heyd-Scuseria-Ernzerhof (HSE06) calculations,
we find that the spin susceptibility $\chi_s$ is strongly enhanced in the low
doping regime by the electron-electron interaction. Furthermore, in the low
doping limit, the exchange interaction renormalizes the intervalley
electron-phonon coupling and results in a strong increase of the
superconducting critical temperature for $x<0.15$. On the contrary, for
$x>0.15$, $T_c$ is approximately constant, in agreement with experiments. At
$x=0.055$ we found that $T_c$ can be as large as 40 K, suggesting that the
synthesis of cleaner samples of Li$_x$HfNCl could remove the Anderson
insulating state competing with superconductivity and generate a high-$T_c$
superconductor.Comment: 8 pages, 6 figure

### Zener tunneling in the electrical transport of quasi-metallic carbon nanotubes

We study theoretically the impact of Zener tunneling on the charge-transport
properties of quasi-metallic (Qm) carbon nanotubes (characterized by forbidden
band gaps of few tens of meV). We also analyze the interplay between Zener
tunneling and elastic scattering on defects. To this purpose we use a model
based on the master equation for the density matrix, that takes into account
the inter-band Zener transitions induced by the electric field (a quantum
mechanical effect), the electron-defect scattering and the electron-phonon
scattering. In presence of Zener tunnelling the Qm tubes support an electrical
current even when the Fermi energy lies in the forbidden band gap. In absence
of elastic scattering (in high quality samples), the small size of the band gap
of Qm tubes enables Zener tunnelling for realistic values of the the electric
field (above $\sim$ 1 V/\mu m). The presence of a strong elastic scattering (in
low quality samples) further decreases the values of the field required to
observe Zener tunnelling. Indeed, for elastic-scattering lengths of the order
of 50 nm, Zener tunnelling affects the current/voltage characteristic already
in the linear regime. In other words, in quasi-metallic tubes, Zener tunneling
is made more visible by defects.Comment: 10 pages, 8 figure

### 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.Comment: 4 pages, 1 figur

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