215 research outputs found
Unconventional Superconductivity in UPd_2Al_3 from Realistic Selfconsistent Calculations
Realistic selfconsistent calculations of unconventional superconductivity in
a heavy-fermion material are reported. Our calculations for UPdAl start
from accurate energy band dispersions that are computed within the local
spin-density functional theory and provide Fermi surfaces in agreement with
experiment. Using physically motivated, realistic pairing potentials it is
shown that the superconducting gap has two lines of nodes around the z-axis,
thus exhibiting d-wave symmetry in the representation of the
point group. Our results suggest that in a superconductor with gap nodes, the
prevailing gap symmetry is dictated by the constraint that nodes must be as far
as possible from high-density areas.Comment: 4 pages and 3 figures (two color figures
Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
In pump-probe experiments an intensive laser pulse creates non-equilibrium
excited electron distributions in the first few hundred femtoseconds after the
pulse. The influence of non-equilibrium electron distributions caused by a pump
laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is
investigated theoretically here for the first time, considering electron
distributions immediately after the pulse as well as thermalized ones, that are
not in equilibrium with the lattice or spin systems. The XMCD signal is shown
not to be simply proportional to the spin momentum in these situations. The
computed spectra are compared to recent pump-probe XMCD experiments on Ni. We
find that the majority of experimentally observed features considered to be a
proof of ultrafast spin momentum transfer to the lattice can alternatively be
attributed to non-equilibrium electron distributions. Furthermore, we find the
XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid,
even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure
Anomalous Ferromagnetism of Monatomic Co Wire at the Pt(111) Surface Step Edge
A first-principles investigation of the anomalous ferromagnetism of a
quasi-one-dimensional Co chain at the Pt(111) step edge is reported. Our
calculations show that the symmetry breaking at the step leads to an easy
magnetization axis at an odd angle of {\em towards} the Pt
step, in agreement with experiment [P. Gambardella {\em et al.}, {\em Nature}
{\bf 416}, 301 (2002)]. Also, the Co spin and orbital moments become
noncollinear, even in the case of a collinear ferromagnetic spin arrangement. A
significant enhancement of the Co orbital magnetic moment is achieved when
modest electron correlations are treated within LSDA+ calculations.Comment: Presented at MRS Meeting in Boston, Dec. 2003; 4 pages including 3
figure
Small-q Phonon Mediated Unconventional Superconductivity in the Iron Pnictides
We report self-consistent calculations of the gap symmetry for the iron-based
high-temperature superconductors using realistic small-q phonon mediated
pairing potentials and four-band energy dispersions. When both electron and
hole Fermi surface pockets are present, we obtain the nodeless state
that was first encountered in a spin-fluctuations mechanism picture. Nodal gap
structures such as and and even a p-wave
triplet state, are accessible upon doping within our phononic mechanism. Our
results resolve the conflict between phase sensitive experiments reporting a
gap changing sign attributed previously only to a non-phononic mechanism and
isotope effect measurements proving the involvement of phonons in the pairing.Comment: Final version. Corrected typos. Reference adde
Dynamical mean field theory of correlated gap formation in Pu monochalcogenides
The correlated Kondo insulator state of the plutonium monochalcogenides is
investigated using the dynamical mean field theory (DMFT) and the local density
approximation +U (LDA+U). The DMFT-dynamical fluctuations lead to a correlated
insulator state at elevated temperature, in sharp contrast to the static LDA+U
approach that fails to reproduce both the insulating behavior and anomalous
lattice constant. The DMFT conversely predicts the experimentally observed
anomalous increase of the gap with pressure and explains the lattice constant
very well.Comment: 4 pages, 4 figure
Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets
We report a computational theoretical investigation of electron spin-flip
scattering induced by the electron-phonon interaction in the transition-metal
ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon
spin-flip scattering is computed from first-principles, employing a generalized
spin-flip Eliashberg function as well as ab initio computed phonon dispersions.
Aiming at investigating the amount of electron-phonon mediated demagnetization
in femtosecond laser-excited ferromagnets, the formalism is extended to treat
laser-created thermalized as well as nonequilibrium, nonthermal hot electron
distributions. Using the developed formalism we compute the phonon-induced spin
lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated
demagnetization rate is evaluated for laser-created thermalized and
nonequilibrium electron distributions. Nonthermal distributions are found to
lead to a stronger demagnetization rate than hot, thermalized distributions,
yet their demagnetizing effect is not enough to explain the experimentally
observed demagnetization occurring in the subpicosecond regime.Comment: 14 pages, 8 figures, to appear in PR
Ab initio investigation of Elliott-Yafet electron-phonon mechanism in laser-induced ultrafast demagnetization
The spin-flip (SF) Eliashberg function is calculated from first-principles
for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet
electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization.
This is used to compute the SF probability and demagnetization rate for
laser-created thermalized as well as non-equilibrium electron distributions.
Increased SF probabilities are found for thermalized electrons, but the induced
demagnetization rate is extremely small. A larger demagnetization rate is
obtained for {non-equilibrium} electron distributions, but its contribution is
too small to account for femtosecond demagnetization.Comment: 5 pages, 3 figures, to appear in PR
Evolution of multi-gap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB
Starting from first principles, we show the formation and evolution of
superconducting gaps in MgB at its ultrathin limit. Atomically thin MgB
is distinctly different from bulk MgB in that surface states become
comparable in electronic density to the bulk-like - and -bands.
Combining the ab initio electron-phonon coupling with the anisotropic
Eliashberg equations, we show that monolayer MgB develops three distinct
superconducting gaps, on completely separate parts of the Fermi surface due to
the emergent surface contribution. These gaps hybridize nontrivially with every
extra monolayer added to the film, owing to the opening of additional coupling
channels. Furthermore, we reveal that the three-gap superconductivity in
monolayer MgB is robust over the entire temperature range that stretches up
to a considerably high critical temperature of 20 K. The latter can be boosted
to 50 K under biaxial tensile strain of 4\%, which is an enhancement
stronger than in any other graphene-related superconductor known to date.Comment: To appear in Phys. Re
Theoretical de Haas-van Alphen Data and Plasma Frequencies of MgB2 and TaB2
The de Haas-van Alphen-frequencies as well as the effective masses for a
magnetic field parallel to the crystallographic c-axis are calculated within
the local spin density approximation (LSDA) for MgB2 and TaB2. In addition, we
analyze the plasma frequencies computed for each Fermi surface sheet. We find a
large anisotropy of Fermi velocities in MgB2 in difference to the nearly
isotropic behavior in TaB2. We compare calculations performed within the
relativistic non-full potential augmented-spherical-wave (ASW) scheme and the
scalar-relativistic full potential local orbital (FPLO) scheme. A significant
dependence for small cross sections on the bandstructure method is found. The
comparison with the first available experimental de Haas-van Alphen-data by
Yelland et al. (Ref. 19) shows deviations from the electronic structure
calculated within both L(S)DA approaches although the cross section predicted
by FPLO are closer to the experimental data. The elucidation of the relevant
many-body effects beyond the standard LDA is considered as a possible key
problem to understand the superconductivity in MgB2.Comment: Typos corrected, 3references added. Extended and corrected version of
S. Elgazzar et al., Solid State Comm. v. 121, 99 (2002). 7pages, 4figures,
AIP Conference Proc. "Correlated Electron Systems and High-Tc
Superconductors" (ed. F. Mancini) (October 2001, Salerno, Italy
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