5,412 research outputs found
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
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
Evolution of transport properties of BaFe2-xRuxAs2 in a wide range of isovalent Ru substitution
The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are
investigated by measuring resistivity and Hall coefficient on high-quality
single crystals in a wide range of doping (0 < x < 1.4). Ru substitution
weakens the antiferromagnetic (AFM) order, inducing superconductivity for
relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the
transport properties show non-Fermi-liquid-like behaviors with a
linear-temperature dependence of resistivity and a strong temperature
dependence of Hall coefficient with a sign change. Upon higher doping, however,
both of them recover conventional Fermi-liquid behaviors. Strong doping
dependence of Hall coefficient together with a small magnetoresistance suggest
that the anomalous transport properties can be explained in terms of
anisotropic charge carrier scattering due to interband AFM fluctuations rather
than a conventional multi-band scenario.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Effect of Pressure on Superconducting Ca-intercalated Graphite CaC
The pressure effect on the superconducting transition temperature () of
the newly-discovered Ca-intercalated graphite compound CaC has been
investigated up to 16 kbar. is found to increase under pressure
with a large relative ratio / of +0.4 %/kbar. Using
first-principles calculations, we show that the large and positive effect of
pressure on can be explained in the scope of electron-phonon theory due
to the presence of a soft phonon branch associated to in-plane vibrations of Ca
atoms. Implications of the present findings on the current debate about the
superconducting mechanism in graphite intercalation compounds are discussed.Comment: 6 pages, 5 figs, final PRB versio
Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x
The nature and value of the order parameters (OPs) in the superconducting
Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of
intense debate, also connected to the pairing mechanism which is probably
unconventional. Point-contact Andreev-reflection experiments on
LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the
superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV,
which scales with the local Tc of the contact; a larger unconventional OP that
gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases
on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression
in the conductance around zero bias), that survives in the normal state up to
T* ~ 140 K (close to the Neel temperature of the undoped compound), which we
associate to antiferromagnetic spin fluctuations (AF SF) coexisting with
superconductivity. These findings point toward a complex, unconventional nature
of superconductivity in LaFeAsO_(1-x)F_x.Comment: 19 pages, 12 figures - one figure and some insets added, minor
changes to the tex
Self-similar chain conformations in polymer gels
We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.Comment: 4 pages, RevTex, 3 Figure
Interacting Growth Walk - a model for hyperquenched homopolymer glass?
We show that the compact self avoiding walk configurations, kinetically
generated by the recently introduced Interacting Growth Walk (IGW) model, can
be considered as members of a canonical ensemble if they are assigned random
values of energy. Such a mapping is necessary for studying the thermodynamic
behaviour of this system. We have presented the specific heat data for the IGW,
obtained from extensive simulations on a square lattice; we observe a broad
hump in the specific heat above the -point, contrary to expectation.Comment: 4 figures; Submitted to PR
Fermi-surface topological phase transition and horizontal order-parameter nodes in CaFeAs under pressure
Iron-based compounds (IBS) display a surprising variety of superconducting
properties that seems to arise from the strong sensitivity of these systems to
tiny details of the lattice structure. In this respect, systems that become
superconducting under pressure, like CaFeAs, are of particular
interest. Here we report on the first directional point-contact
Andreev-reflection spectroscopy (PCARS) measurements on CaFeAs crystals
under quasi-hydrostatic pressure, and on the interpretation of the results
using a 3D model for Andreev reflection combined with ab-initio calculations of
the Fermi surface (within the density functional theory) and of the order
parameter symmetry (within a random-phase-approximation approach in a
ten-orbital model). The almost perfect agreement between PCARS results at
different pressures and theoretical predictions highlights the intimate
connection between the changes in the lattice structure, a topological
transition in the hole-like Fermi surface sheet, and the emergence on the same
sheet of an order parameter with a horizontal node line.Comment: 13 pages, 8 color figures. This is an author-created, un-copyedited
version of an article published in Scientific Reports. The published version
is available online, together with Supplementary Information, at
http://www.nature.com/articles/srep2639
Signatures of Electronic Correlations in Optical Properties of LaFeAsOF
Spectroscopic ellipsometry is used to determine the dielectric function of
the superconducting LaFeAsOF ( = 27 K) and undoped LaFeAsO
polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 350 K. The free charge carrier response in both samples is heavily
damped with the effective carrier density as low as 0.0400.005 electrons
per unit cell. The spectral weight transfer in the undoped LaFeAsO associated
with opening of the pseudogap at about 0.65 eV is restricted at energies below
2 eV. The spectra of superconducting LaFeAsOF reveal a
significant transfer of the spectral weight to a broad optical band above 4 eV
with increasing temperature. Our data may imply that the electronic states near
the Fermi surface are strongly renormalized due to electron-phonon and/or
electron-electron interactions.Comment: 4 pages, 4 figures, units in Fig.2 adde
Heat capacity of -GaN: Isotope Effects
Until recently, the heat capacity of GaN had only been measured for
polycrystalline powder samples. Semiempirical as well as
\textit{first-principles} calculations have appeared within the past few years.
We present in this article measurements of the heat capacity of hexagonal
single crystals of GaN in the 20-1400K temperature range. We find that our data
deviate significantly from the literature values for polycrystalline materials.
The dependence of the heat capacity on the isotopic mass has also been
investigated recently for monatomic crystals such as diamond, silicon, and
germanium. Multi-atomic crystals are expected to exhibit a different dependence
of these heat capacities on the masses of each of the isotopes present. These
effects have not been investigated in the past. We also present
\textit{first-principles} calculations of the dependence of the heat capacities
of GaN, as a canonical binary material, on each of the Ga and N masses. We show
that they are indeed different, as expected from the fact that the Ga mass
affects mainly the acoustic, that of N the optic phonons. It is hoped that
these calculations will encourage experimental measurements of the dependence
of the heat capacity on isotopic masses in binary and more complex
semiconductors.Comment: 12 pages, 5 Figures, submitted to PR
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