557 research outputs found
Room temperature Peierls distortion in small radius nanotubes
By means of {\it ab initio} simulations, we investigate the phonon band
structure and electron-phonon coupling in small 4-\AA diameter nanotubes. We
show that both the C(5,0) and C(3,3) tubes undergo above room temperature a
Peierls transition mediated by an acoustical long-wavelength and an optical
phonons respectively. In the armchair geometry, we verify that the
electron-phonon coupling parameter originates mainly from phonons at
and is strongly enhanced when the diameter decreases. These results
question the origin of superconductivity in small diameter nanotubes.Comment: submitted 21oct2004 accepted 6jan2005 (Phys.Rev.Lett.
Magnetic moment of the 2083 keV level of [140]Ce
For the magnetic moment of the 2083 keV level of [140]Ce, there are four published data, all obtained by applying an external magnetic field of less than 5 T to a liquid sample containing [140]La using the time-differential perturbed angular correlation (TDPAC) technique. Although these four values are consistent within two times their uncertainties (2σ), the range of values in 2σ extends from μ=+3.0to +5.2 (in units of nuclear magneton, μ[N]). This time, the TDPAC technique was successfully applied to the 2083 keV level of 140Ce implanted in an Fe foil. The magnetic moment of this level was determined to be μ=+4.00(20)μ[N], employing the known hyperfine field at 141Ce in Fe, −41(2) T, which agrees very well with one of the values, μ=+4.06(15)μ[N]. The present value is compared with two shell-model calculations
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 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
Lattice distortion and energy level structures in doped C_{60} and C_{70} studied with the extended Su-Schrieffer-Heeger model: Polaron excitations and optical absorption
We extend the Su-Schrieffer-Heeger model of polyacetylene to C_{60} and
C_{70} molecules, and solve numerically. The calculations of the undoped
systems agree well with the known results. When the system (C_{60} or C_{70})
is doped with one or two electrons (or holes), the additional charges
accumulate almost along an equatorial line of the molecule. The dimerization
becomes weaker almost along the same line. Two energy levels intrude largely in
the gap. The intrusion is larger in C_{70} than in C_{60}. Therefore,
``polarons'' are predicted in doped buckminster- fullerenes. We calculate
optical absorption coefficient for C_{60} in order to look at how ``polarons''
will be observed. It is predicted that there appears a new peak at the lower
energy than the intergap transition peaks. It is also found that C_{60} and
C_{70} are related mutually with respect to electronical structures as well as
lattice geometries. (to be published in Phys. Rev. B 45, June 15 issue)Comment: 21 page
Nonadiabatic Pauli susceptibility in fullerene compounds
Pauli paramagnetic susceptibility is unaffected by the electron-phonon
interaction in the Migdal-Eliashberg context. Fullerene compounds however do
not fulfill the adiabatic assumption of Migdal's theorem and nonadiabatic
effects are expected to be relevant in these materials. In this paper we
investigate the Pauli spin susceptibility in nonadiabatic regime by following a
conserving approach based on Ward's identity. We find that a sizable
renormalization of due to electron-phonon coupling appears when
nonadiabatic effects are taken into account. The intrinsic dependence of
on the electron-phonon interaction gives rise to a finite and negative isotope
effect which could be experimentally detected in fullerides. In addition, we
find an enhancement of the spin susceptibility with temperature increasing, in
agreement with the temperature dependence of observed in fullerene
compounds. The role of electronic correlation is also discussed.Comment: Revtex, 10 pages, 8 figures include
First-Principles Electronic Structure of Solid Picene
To explore the electronic structure of the first aromatic superconductor,
potassium-doped solid picene which has been recently discovered by Mitsuhashi
et al with the transition temperatures K, we have obtained a
first-principles electronic structure of solid picene as a first step toward
the elucidation of the mechanism of the superconductivity. The undoped crystal
is found to have four conduction bands, which are characterized in terms of the
maximally localized Wannier orbitals. We have revealed how the band structure
reflects the stacked arrangement of molecular orbitals for both undoped and
doped (Kpicene) cases, where the bands are not rigid. The Fermi surface for
Kpicene is a curious composite of a warped two-dimensional surface and a
three-dimensional one.Comment: 5 pages, 4 figure
Structural study on hole-doped superconductors Pr1-xSrxFeAsO
The structural details in Pr1-xSrxFeAsO (1111) superconducting system are
analyzed using data obtained from synchrotron X-ray diffraction and the
structural parameters are carefully studied as the system is moving from
non-superconducting to hole-doped superconducting with the Sr concentration.
Superconductivity emerges when the Sr doping amount reaches 0.221. The linear
increase of the lattice constants proves that Sr is successfully introduced
into the system and its concentration can accurately be determined by the
electron density analyses. The evolution of structural parameters with Sr
concentration in Pr1-xSrxFeAsO and their comparison to other similar structural
parameters of the related Fe-based superconductors suggest that the interlayer
space between the conducting As-Fe-As layer and the insulating Pr-O-Pr layer is
important for improving Tc in the hole-doped (1111) superconductors, which
seems to be different from electron-doped systems.Comment: 17 pages, 7 figures, 1 tabl
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