40 research outputs found
Intercalant-Driven Superconductivity in YbC and CaC
Recently deiscovered superconductivity in YbC and CaC at temperatures
substantially higher than previously known for intercalated graphites, raised
several new questions: (1) Is the mechanism considerably different from the
previously known intercalated graphites? (2) If superconductivity is
conventional, what are the relevant phonons? (3) Given extreme similarity
between YbC and CaCa, why their critical temperatures are so different?
We address these questions on the basis of first-principles calculations and
conclude that coupling with intercalant phonons is likely to be the main force
for superconductivity in YbC and CaC, but not in alkaline-intercalated
compounds, and explain the difference in by the ``isotope effect'' due to
the difference in Yb and Ca atomic masses.Comment: 4 pages, embedded postscript figire
X-ray magnetic circular dichroism in (Ge,Mn) compounds: experiments and modeling
X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra
at the L edges of Mn in (Ge,Mn) compounds have been measured and are
compared to the results of first principles calculation. Early \textit{ab
initio} studies show that the Density Functional Theory (DFT) can very well
describe the valence band electronic properties but fails to reproduce a
characteristic change of sign in the L XMCD spectrum of Mn in
GeMn, which is observed in experiments. In this work we demonstrate
that this disagreement is partially related to an underestimation of the
exchange splitting of Mn 2 core states within the local density
approximation. It is shown that the change in sign experimentally observed is
reproduced if the exchange splitting is accurately calculated within the
Hartree-Fock approximation, while the final states can be still described by
the DFT. This approach is further used to calculate the XMCD in different
(Ge,Mn) compounds. It demonstrates that the agreement between experimental and
theoretical spectra can be improved by combining state of the art calculations
for the core and valence states respectively.Comment: 8 page
Structural, electronic, and magneto-optical properties of YVO
Optical and magneto-optical properties of YVO single crystal were studied
in FIR, visible, and UV regions. Two structural phase transitions at 75 K and
200 K were observed and established to be of the first and second order,
respectively. The lattice has an orthorhombic symmetry both above 200 K
as well as below 75 K, and is found to be dimerized monoclinic in
between. We identify YVO as a Mott-Hubbard insulator with the optical gap
of 1.6 eV. The electronic excitations in the visible spectrum are determined by
three -bands at 1.8, 2.4, and 3.3 eV, followed by the charge-transfer
transitions at about 4 eV. The observed structure is in good agreement with
LSDA+ band structure calculations. By using ligand field considerations, we
assigned these bands to the transitions to the , , and states. The strong temperature dependence of these
bands is in agreement with the formation of orbital order. Despite the small
net magnetic moment of 0.01 per vanadium, the Kerr effect of the order
of was observed for all three -bands in the magnetically
ordered phase . A surprisingly strong enhancement of
the Kerr effect was found below 75 K, reaching a maximum of . The
effect is ascribed to the non-vanishing net orbital magnetic moment.Comment: Submitted to Phys. Rev.
Transport, optical and electronic properties of the half metal CrO2
The electronic structure of CrO_2 is critically discussed in terms of the
relation of existing experimental data and well converged LSDA and GGA
calculations of the electronic structure and transport properties of this half
metal magnet, with a particular emphasis on optical properties. We find only
moderate manifestations of many body effects. Renormalization of the density of
states is not large and is in the typical for transition metals range. We find
substantial deviations from Drude behavior in the far-infrared optical
conductivity. These appear because of the unusually low energy of interband
optical transitions. The calculated mass renormalization is found to be rather
sensitive to the exchange-correlation functional used and varies from 10%
(LSDA) to 90% (GGA), using the latest specific heat data. We also find that
dressing of the electrons by spin fluctuations, because of their high energy,
renormalizes the interband optical transition at as high as 4 eV by about 20%.
Although we find no clear indications of strong correlations of the Hubbard
type, strong electron-magnon scattering related to the half metallic band
structure is present and this leads to a nontrivial temperature dependence of
the resistivity and some renormalization of the electron spectra.Comment: 9 Revtex 2 column pages, including 8 postscript figures. Two more
figures are included in the submission that are not embedded in the paper,
representing DOS and bandstructure of the paramagnetic CrO
Effects of Crystal Structure and the On-Site Coulomb Interactions on the Electronic and Magnetic Structure of Pyrochlores MoO (A= Y, Gd, and Nd)
Being motivated by recent experimental studies, we investigate magnetic
structures of the Mo pyrochlores MoO (= Y, Nd, and Gd) and
their impact on the electronic properties. The latter are closely related with
the behavior of twelve Mo() bands, located near the Fermi level and
well separated from the rest of the spectrum. We use a mean-field Hartree-Fock
approach, which combines fine details of the electronic structure for these
bands, extracted from the conventional calculations in the local-density
approximation, the spin-orbit interaction, and the on-site Coulomb interactions
amongst the Mo(4d) electrons, treated in the most general rotationally
invariant form. The Coulomb repulsion U plays a very important role in the
problem, and the semi-empirical value U1.5-2.5 eV accounts simultaneously
for the metal-insulator (M-I) transition, the ferromagnetic (FM) - spin-glass
(SG) transition, and for the observed enhancement of the anomalous Hall effect
(AHE). The M-I transition is mainly controlled by . The magnetic structure
at the metallic side is nearly collinear FM, due to the double exchange
mechanism. The transition into the insulating state is accompanied by the large
canting of spin and orbital magnetic moments. The sign of exchange interactions
in the insulating state is controlled by the Mo-Mo distances. Smaller distances
favor the antiferromagnetic coupling, which preludes the SG behavior in the
frustrated pyrochlore lattice. Large AHE is expected in the nearly collinear FM
state, near the point of M-I transition, and is related with the unquenched
orbital magnetization at the Mo sites. We also predict large magneto-optical
effect in the same FM compounds.Comment: 26 pages, 17 figures (low resolution is used for Figs. 6, 8, and 9,
please contact directly if you need the originals), 1 tabl
Capillary discharge tabletop soft X-ray lasers reach new wavelengths and applications
Includes bibliographical references (pages 1080-1081).For many years, researchers have envisioned the development of compact high repetition rate tabletop soft X-ray lasers that could be routinely used in application in numerous disciplines. With demonstrated average powers of several mW and mJ-level pulse energy at 46.9 nm, capillary discharge-pumped lasers are the first compact lasers to reach this goal. In this paper we summarize the development status of high repetition rate tabletop soft X-ray lasers based on capillary discharge excitation, and give examples of their successful use in several applications. Results of the use of a capillary discharge pumped 46.9 nm laser in dense plasma interferometry, soft X-ray reflectometry for the determination of optical constants, and laser ablation are described. The observation of lasing at 53 nm line in Ne-like Cl with output pulse energy up to 10 μJ is also reported