361 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
Unusual signatures of the ferromagnetic transition in the heavy Fermion compound UMnAl
Magnetic susceptibility results for single crystals of the new cubic
compounds UTAl (T=Mn, V, and Mo) are reported. Magnetization,
specific heat, resistivity, and neutron diffraction results for a single
crystal and neutron diffraction and inelastic spectra for a powder sample are
reported for UMnAl. For T = V and Mo, temperature independent Pauli
paramagnetism is observed. For UMnAl, a ferromagnetic transition is
observed in the magnetic susceptibility at = 20 K. The specific heat
anomaly at is very weak while no anomaly in the resistivity is seen at
. We discuss two possible origins for this behavior of UMnAl:
moderately small moment itinerant ferromagnetism, or induced local moment
ferromagnetism.Comment: 5 pages, 5 figures, to be published in Phys. rev.
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
Quantum critical behavior in the heavy Fermion single crystal Ce(NiPd)Ge
We have performed magnetic susceptibility, specific heat, resistivity, and
inelastic neutron scattering measurements on a single crystal of the heavy
Fermion compound Ce(NiPd)Ge, which is believed to
be close to a quantum critical point (QCP) at T = 0. At lowest
temperature(1.8-3.5 K), the magnetic susceptibility behaves as with m/mole
(0.0025 emu/mole). For 1 K, the specific heat can be fit to the formula
with of order 700 mJ/mole-K.
The resistivity behaves as for temperatures below 2
K. This low temperature behavior for and is in accord
with the SCR theory of Moriya and Takimoto\cite{Moriya}. The inelastic neutron
scattering spectra show a broad peak near 1.5 meV that appears to be
independent of ; we interpret this as Kondo scattering with 17 K. In
addition, the scattering is enhanced near =(1/2, 1/2, 0) with maximum
scattering at = 0.45 meV; we interpret this as scattering from
antiferromagnetic fluctuations near the antiferromagnetic QCP.Comment: to be published in J. Phys: Conference Serie
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