296 research outputs found
Effect of annealing on the specific heat of Ba(Fe1-xCox)2As2
We report on the effect of annealing on the temperature and field
dependencies of the low temperature specific heat of the electron-doped
Ba(FeCo)As for under-(x = 0.045), optimal- (x = 0.08)
and over-doped (x = 0.105 and 0.14) regimes. We observed that annealing
significantly improves some superconducting characteristics in
Ba(FeCo)As. It considerably increases ,
decreases in the superconducting state and suppresses the
Schottky-like contribution at very low temperatures. The improved sample
quality allows for a better identification of the superconducting gap structure
of these materials. We examine the effects of doping and annealing within a
self-consistent framework for an extended s-wave pairing scenario. At optimal
doping our data indicates the sample is fully gapped, while for both under and
overdoped samples significant low-energy excitations possibly consistent with a
nodal structure remain. The difference of sample quality offers a natural
explanation for the variation in low temperature power laws observed by many
techniques.Comment: 9 pages: added references, two figures and supplementary information;
Accepted to Physical Review B (Jan 10, 2010
Photoemission Evidence for a Remnant Fermi Surface and d-Wave-Like Dispersion in Insulating Ca2CuO2Cl2
An angle resolved photoemission study on Ca2CuO2Cl2, a parent compound of
high Tc superconductors is reported. Analysis of the electron occupation
probability, n(k) from the spectra shows a steep drop in spectral intensity
across a contour that is close to the Fermi surface predicted by the band
calculation. This analysis reveals a Fermi surface remnant even though
Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion
with approximately the |cos(kxa)-cos(kya)| form along this remnant Fermi
surface. Together with the data from Dy doped Bi2Sr2CaCu2O(8 + delta) these
results suggest that this d-wave like dispersion of the insulator is the
underlying reason for the pseudo gap in the underdoped regime.Comment: 9 pages, including 7 figures. Published in Science, one figure
correcte
Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P
We report LDA calculated band structure, densities of states and Fermi
surfaces for recently discovered Pt-pnictide superconductors APt3P
(A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is
essentially three dimensional, in contrast to Fe pnictides and chalcogenides.
LDA calculated Sommerfeld coefficient agrees rather well with experimental
data, leaving little space for very strong coupling superconductivity,
suggested by experimental data on specific heat of SrPt3P. Elementary estimates
show, that the values of critical temperature can be explained by rather weak
or moderately strong coupling, while the decrease of superconducting transition
temperature Tc from Sr to La compound can be explained by corresponding
decrease of total density of states at the Fermi level N(E_F). The shape of the
density of states near the Fermi level suggests that in SrPt3P electron doping
(such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g.
partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F)
and possibly Tc.Comment: 5 pages, 5 figure
Novel multiple-band superconductor SrPt2As2
We present LDA calculated electronic structure of recently discovered
superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and
crystal structure are somehow similar to FeAs-based high-temperature
superconductors, the electronic structure of SrPt2As2 is very much different.
Crystal structure is orthorhombic (or tetragonal if idealized) and has layered
nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr
ions. The Fermi level is crossed by Pt-5d states with rather strong admixture
of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional,
with complicated sheets corresponding to multiple bands. We compare SrPt2As2
with 1111 and 122 representatives of FeAs-class of superconductors, as well as
with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of
superconductivity in SrPt2As2 is also presented.Comment: 5 pages, 4 figure
Coexistence of Superconductivity and Charge Density Wave in SrPt2As2
SrPt2As2 is a novel arsenide superconductor, which crystallizes in the
CaBe2Ge2-type structure as a different polymorphic form of the ThCr2Si2-type
structure. SrPt2As2 exhibits a charge-density-wave (CDW) ordering at about 470
K and enters into a superconducting state at Tc = 5.2 K. The coexistence of
superconductivity and CDW refers to Peierls instability with a moderately
strong electron-phonon interaction. Thus SrPt2As2 can be viewed as a
nonmagnetic analog of iron-based superconductors, such as doped BaFe2As2, in
which superconductivity emerges in close proximity to spin-density-wave
ordering.Comment: 4 pages, 5 figure
Kondo behavior, ferromagnetic correlations, and crystal fields in the heavy Fermion compounds Ce3X (X=In, Sn)
We report measurements of inelastic neutron scattering, magnetic
susceptibility, magnetization, and the magnetic field dependence of the
specific heat for the heavy Fermion compounds CeIn and CeSn. The
neutron scattering results show that the excited crystal field levels have
energies = 13.2 meV, = 44.8 meV for CeIn and = 18.5 meV,
= 36.1 meV for CeSn. The Kondo temperature deduced from the
quasielastic linewidth is 17 K for CeIn and 40 K for CeSn. The low
temperature behavior of the specific heat, magnetization, and susceptibility
can not be well-described by J=1/2 Kondo physics alone, but require
calculations that include contributions from the Kondo effect, broadened
crystal fields, and ferromagnetic correlations, all of which are known to be
important in these compounds. We find that in CeIn the ferromagnetic
fluctuation makes a 10-15 % contribution to the ground state doublet entropy
and magnetization. The large specific heat coefficient in this heavy
fermion system thus arises more from the ferromagnetic correlations than from
the Kondo behavior.Comment: 8 pages, 6 figure
Anisotropy on the Fermi Surface of the Two-Dimensional Hubbard Model
We investigate anisotropic charge fluctuations in the two-dimensional Hubbard
model at half filling. By the quantum Monte Carlo method, we calculate a
momentum-resolved charge compressibility , which shows effects of an infinitesimal doping. At the temperature
, shows peak structure at the points along the line. A similar peak
structure is reproduced in the mean-filed calculation for the d-wave pairing
state or the staggered flux state.Comment: 5 pages, 3 figures, figures and presentation are modifie
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