173 research outputs found
Electric field gradients in s-, p- and d-metal diborides and the effect of pressure on the band structure and T in MgB
Results of FLMTO-GGA (full-potential linear muffin-tin orbital -- generalized
gradient approximation) calculations of the band structure and boron electric
field gradients (EFG) for the new medium-T superconductor (MTSC), MgB,
and related diborides MB, M=Be, Al, Sc, Ti, V, Cr, Mo and Ta are reported.
The boron EFG variations are found to be related to specific features of their
band structure and particularly to the M-B hybridization. The strong charge
anisotropy at the B site in MgB is completely defined by the valence
electrons - a property which sets MgB apart from other diborides. The boron
EFG in MgB is weakly dependent of applied pressure: the B p electron
anisotropy increases with pressure, but it is partly compensated by the
increase of core charge assymetry. The concentration of holes in bonding
bands is found to decrease slightly from 0.067 to 0.062 holes/B under
a pressure of 10 GPa. Despite a small decrease of N(E), the Hopfield
parameter increases with pressure and we believe that the main reason for the
reduction under pressure of the superconducting transition temperature, T,
is the strong pressure dependence of phonon frequencies, which is sufficient to
compensate the electronic effects.Comment: 12 pages, 3 figure
Ab initio probing of the electronic band structure and Fermi surface of fluorine-doped WO3 as a novel low-TC superconductor
First-principles calculations were performed to investigate the electronic
structure and the Fermi surface of the newly discovered low-temperature
superconductor: fluorine-doped WO3. We find that F doping provides the
transition of the insulating tungsten trioxide into a metallic-like phase
WO3-xFx, where the near-Fermi states are formed mainly from W 5d with admixture
of O 2p orbitals. The cooperative effect of fluorine additives in WO3 consists
in change of electronic concentration as well as the lattice constant. At
probing their influence on the near-Fermi states separately, the dominant role
of the electronic factor for the transition of tungsten oxyfluoride into
superconducting state was established. The volume of the Fermi surface
gradually increases with the increase of the doping. In the sequence WO3
\rightarrow WO2.5F0.5 the effective atomic charges of W and O ions decrease,
but much less, than it is predicted within the idealized ionic model - owing to
presence of the covalent interactions W-O and W-F.Comment: 8 pages, 4 figure
Nature of bonding and electronic structure in MgB2, a boron intercalation superconductor
Chemical bonding and electronic structure of MgB2, a boron-based newly
discovered superconductor, is studied using self-consistent band structure
techniques. Analysis of the transformation of the band structure for the
hypothetical series of graphite - primitive graphite - primitive graphite-like
boron - intercalated boron, shows that the band structure of MgB2 is
graphite-like, with pi-bands falling deeper than in ordinary graphite. These
bands possess a typically delocalized and metallic, as opposed to covalent,
character. The in-plane sigma-bands retain their 2D covalent character, but
exhibit a metallic hole-type conductivity. The coexistence of 2D covalent
in-plane and 3D metallic-type interlayer conducting bands is a peculiar feature
of MgB2. We analyze the 2D and 3D features of the band structure of MgB2 and
related compounds, and their contributions to conductivity.Comment: 4 pages in revtex, 3 figures in 4 separate EPS file
Superconductivity of metallic boron in MgB_2
Boron in MgB_2 forms layers of honeycomb lattices with magnesium as a space
filler. Band structure calculations indicate that Mg is substantially ionized,
and the bands at the Fermi level derive mainly from B orbitals. Strong bonding
with an ionic component and considerable metallic density of states yield a
sizeable electron-phonon coupling. Using the rigid atomic sphere approximation
and an analogy to Al, we estimate the coupling constant lambda to be of order
1. Together with high phonon frequencies, which we estimate via zone-center
frozen phonon calculations to be between 300 and 700 cm^-1, this produces a
high critical temperature, consistent with recent experiments reporting Tc=39 K
(J. Akimitsu et al., to be published). Thus MgB_2 can be viewed as an analog of
the long sought, but still hypothetical, superconducting metallic hydrogen.Comment: several typos corrected, most importantly, units in the tables fixed
and a missing zero in the expression for the resistivity restore
Bonding Nature in MgB2
The accurate charge density of MgB2 was observed at room temperature(R.T.)
and 15K by the MEM(Maximum Entropy Method)/Rietveld analysis using synchrotron
radiation powder data. The obtained charge density clearly revealed the
covalent bonding feature of boron forming the 2D honeycomb network in the basal
plane, on the other hand, Mg is found to be in divalent state. A subtle but
clear charge concentration was found on boron 2D sheets at 15K, which should be
relating to superconductivity.Comment: 4 pages, 3 figure
Pair-Hopping Mechanism for Layered Superconductors
We propose a possible charge fluctuation effect expected in layered
superconducting materials. In the multireference density functional theory,
relevant fluctuation channels for the Josephson coupling between
superconducting layers include the interlayer pair hopping derived from the
Coulomb repulsion. When interlayer single-electron tunneling processes are
irrelevant in the Kohn-Sham electronic band structure calculation, the two-body
effective interactions stabilize a superconducting phase. This state is also
regarded as a valence-bond solid in a bulk electronic state. The hidden order
parameters coexist with the superconducting order parameter when the charging
effect of a layer is comparable to the pair hopping. Relevant materials
structures favorable for the pair-hopping mechanism are discussed.Comment: 24 pages, 2 figures, to be published in J. Phys. Soc. Jpn. (2009
BN domains included into carbon nanotubes: role of interface
We present a density functional theory study on the shape and arrangement of
small BN domains embedded into single-walled carbon nanotubes. We show a strong
tendency for the BN hexagons formation at the simultaneous inclusion of B and N
atoms within the walls of carbon nanotubes. The work emphasizes the importance
of a correct description of the BN-C frontier. We suggest that BN-C interface
will be formed preferentially with the participation of N-C bonds. Thus, we
propose a new way of stabilizing the small BN inclusions through the formation
of nitrogen terminated borders. The comparison between the obtained results and
the available experimental data on formation of BN plackets within the single
walled carbon nanotubes is presented. The mirror situation of inclusion of
carbon plackets within single walled BN nanotubes is considered within the
proposed formalism. Finally, we show that the inclusion of small BN plackets
inside the CNTs strongly affects the electronic character of the initial
systems, opening a band gap. The nitrogen excess in the BN plackets introduces
donor states in the band gap and it might thus result in a promising way for
n-doping single walled carbon nanotubes
Superconducting properties and c-axis superstructure of Mg1-xAlxB2
The superconducting and structural properties of a series of Mg1-xAlxB2
samples have been investigated. X-ray diffraction results confirmed the
existence of a structural transition associated with the significant change in
inter-boron layer distance as reported previously by Slusky et al.
Moreover,transmission-electron-microscopy observations revealed the existence
of a superstructure with doubled lattice constant along the c-axis direction.
We propose that this superstructure is essentially related to the structural
transition. The modifications of superconducting transition temperature Tc, the
normal state resistivity, and the upper critical field Bc2 by Al doping are
discussed in terms of Al-substitution induced changes in the electronic
structure at the Fermi energy.Comment: 15 pages, 7 figure
Structural, Magnetic and Electronic Properties of the Iron-Chalcogenide AFeSe (A=K, Cs, Rb, Tl and etc.) Superconductors
The latest discovery of a new iron-chalcogenide superconductor
AFeSe(A=K, Cs, Rb, Tl and etc.) has attracted much attention
due to a number of its unique characteristics, such as the possible insulating
state of the parent compound, the existence of Fe-vacancy and its ordering, a
new form of magnetic structure and its interplay with superconductivity, and
the peculiar electronic structures that are distinct from other Fe-based
superconductors. In this paper, we present a brief review on the structural,
magnetic and electronic properties of this new superconductor, with an emphasis
on the electronic structure and superconducting gap. Issues and future
perspectives are discussed at the end of the paper.Comment: 45 pages, 19 figure
Band structure of SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs superconductors
By means of first-principle FLAPW-GGA calculations, we have investigated the
electronic properties of the newly discovered layered quaternary systems
SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs
superconductors. The electronic bands, density of states, Fermi surfaces,
atomic charges, together with Sommerfeld coefficients and molar Pauli
paramagnetic susceptibility have been evaluated and discussed in comparison
with oxyarsenide LaFeAsO - a parent phase for a new class of high-temperature
(Tc about 26-56K) oxygen-containing FeAs superconductors. Similarity of our
data for SrFeAsF and CaFeAsF with the band structure of oxygen-containing FeAs
superconducting materials may be considered as theoretical background
specifying the possibility of superconductivity in these oxygen-free systems.Comment: 12 pages, 3 figure
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