1,228 research outputs found
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
Small Fermi energy, zero point fluctuations and nonadiabaticity in MgB
Small Fermi energy effects are induced in MgB by the low hole doping in
the bands which are characterized by a Fermi energy eV. We show that, due to the particularly strong deformation
potential relative to the phonon mode, lattice fluctuations are
reflected in strong fluctuations in the electronic band structure. Quantum
fluctuations associated to the zero-point lattice motion are responsible for an
uncertainty of the Fermi energy of the order of the Fermi energy itself,
leading to the breakdown of the adiabatic principle underlying the
Born-Oppenheimer approximation in MgB even if , where are the characteristic phonon
frequencies. This amounts to a new nonadiabatic regime, which could be relevant
to other unconventional superconductors.Comment: to appear on Physical Review
Is LaOFFeAs an electron-phonon superconductor ?
In this paper we calculate the electron-phonon coupling of the
newly-discovered superconductor LaOFFeAs from first-principles,
using Density Functional Perturbation Theory. For pure LaOFeAs, the calculated
electron-phonon coupling constant and logarithmic-averaged
frequency , give a maximum of 0.8 K, using the
standard Migdal-Eliashberg theory.
For the doped compounds, we predict even smaller coupling constants, due
to the strong suppression of the electronic Density of States at the Fermi
level. To reproduce the experimental , a 5-6 times larger coupling
constant would be needed.
Our results indicate that electron-phonon coupling is not sufficient to
explain superconductivity in the newly-discovered LaOFFeAs
superconductor, probably due to the importance of strong correlation effects
Specific Heat of the Ca-Intercalated Graphite Superconductor CaC
The superconducting state of Ca-intercalated graphite CaC6 has been
investigated by specific heat measurements. The characteristic anomaly at the
superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of
the superconductivity. The temperature and magnetic field dependence of the
electronic specific heat are consistent with a fully-gapped superconducting
order parameter. The estimated electron-phonon coupling constant is lambda =
0.60 - 0.74 suggesting that the relatively high Tc of CaC6 can be explained
within the weak-coupling BCS approach.Comment: 4 pages, 4 figs, submitted to Phys. Rev. Let
Linear response separation of a solid into atomic constituents: Li, Al, and their evolution under pressure
We present the first realization of the generalized pseudoatom concept
introduced by Ball, and adopt the name enatom to minimize confusion. This
enatom, which consists of a unique decomposition of the total charge density
(or potential) of any solid into a sum of overlapping atomiclike contributions
that move rigidly with the nuclei to first order, is calculated using
(numerical) linear response methods, and is analyzed for both fcc Li and Al at
pressures of 0, 35, and 50 GPa. These two simple fcc metals (Li is fcc and a
good superconductor in the 20-40 GPa range) show different physical behaviors
under pressure, which reflects the increasing covalency in Li and the lack of
it in Al. The nonrigid (deformation) parts of the enatom charge and potential
have opposite signs in Li and Al; they become larger under pressure only in Li.
These results establish a method of construction of the enatom, whose potential
can be used to obtain a real-space understanding of the vibrational properties
and electron-phonon interaction in solids.Comment: 13 pages, 9 figures, 1 table, V2: fixed problem with Fig. 7, V3:
minor correction
The role of the dopant in the superconductivity of diamond
We present an {\it ab initio} study of the recently discovered
superconductivity of boron doped diamond within the framework of a
phonon-mediated pairing mechanism. The role of the dopant, in substitutional
position, is unconventional in that half of the coupling parameter
originates in strongly localized defect-related vibrational modes, yielding a
very peaked Eliashberg function. The electron-phonon
coupling potential is found to be extremely large and T is limited by the
low value of the density of states at the Fermi level
Effects of magnetism and doping on the electron-phonon coupling in BaFeAs
We calculate the effect of local magnetic moments on the electron-phonon
coupling in BaFeAs using the density functional perturbation
theory. We show that the magnetism enhances the total electron-phonon coupling
by , up to , still not enough to explain the
high critical temperature, but strong enough to have a non-negligible effect on
superconductivity, for instance, by frustrating the coupling with spin
fluctuations and inducing order parameter nodes. The enhancement comes mostly
from a renormalization of the electron-phonon matrix elements. We also
investigate, in the rigid band approximation, the effect of doping, and find
that versus doping does not mirror the behavior of the density of
states; while the latter decreases upon electron doping, the former does not,
and even increases slightly.Comment: 4 pages, 3 figure
Superconductivity in Heavy Alkaline-Earths Intercalated Graphites
We report the discovery of superconductivity below 1.65(6) K in
Sr-intercalated graphite SrC6, by susceptibility and specific heat (Cp)
measurements. In comparison with CaC6, we found that the anisotropy of the
upper critical fields for SrC6 is much reduced. The Cp anomaly at Tc is smaller
than the BCS prediction indicating an anisotropic superconducting gap for SrC6
similar to CaC6. The significantly lower Tc of SrC6 as compared to CaC6 can be
understood in terms of "negative" pressure effects, which decreases the
electron-phonon coupling for both in-plane intercalant and the out-of-plane C
phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Electrons and phonons in the ternary alloy CaAlSi} as a function of composition
We report a detailed first-principles study of the structural, electronic and
vibrational properties of the superconducting C phase of the ternary
alloy CaAlSi, both in the experimental range ,
for which the alloy has been synthesised, and in the theoretical limits of high
aluminium and high silicon concentration. Our results indicate that, in the
experimental range, the dependence of the electronic bands on composition is
well described by a rigid-band model, which breaks down outside this range.
Such a breakdown, in the (theoretical) limit of high aluminium concentration,
is connected to the appearance of vibrational instabilities, and results in
important differences between CaAl and MgB. Unlike MgB, the
interlayer band and the out-of-plane phonons play a major role on the stability
and superconductivity of CaAlSi and related C intermetallic compounds
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