2,288 research outputs found
Effect of proton irradiation on the normal state low-energy excitations of Ba(FeRh)As superconductors
We present a \asnmr Nuclear Magnetic Resonance (NMR) and resistivity study of
the effect of 5.5 MeV proton irradiation on the optimal electron doped (
0.068) and overdoped ( 0.107) Ba(FeRh)As iron based
superconductors. While the proton induced defects only mildly suppress the
critical temperature and increase residual resistivity in both compositions,
sizable broadening of the NMR spectra was observed in all the irradiated
samples at low temperature. The effect is significantly stronger in the
optimally doped sample where the Curie Weiss temperature dependence of the line
width suggests the onset of ferromagnetic correlations coexisting with
superconductivity at the nanoscale. 1/T measurements revealed that the
energy barrier characterizing the low energy spin fluctuations of these
compounds is enhanced upon proton irradiation, suggesting that the defects are
likely slowing down the fluctuations between ( and (,0) nematic
ground states.Comment: 9 pages, 9 figure
Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavy fermion system
We report temperature dependent resistivity, specific heat, magnetic
susceptibility and thermoelectric power measurements made on the heavy fermion
system YbNi2B2C, for both as grown and annealed single crystals. Our results
demonstrate a significant variation in the temperature dependent electrical
resistivity and thermoelectric power between as grown crystals and crystals
that have undergone optimal (150 hour, 950 C) annealing, whereas the
thermodynamic properties: (c_p(T) and chi(T)) remain almost unchanged. We
interpret these results in terms of redistributions of local Kondo temperatures
associated with ligandal disorder for a small (~ 1%) fraction of the Yb sites.Comment: 5 pages, 4 figures, submitted to PR
Strong Enhancement of the Critical Current at the Antiferromagnetic Transition in ErNi2B2C Single Crystals
We report on transport and magnetization measurements of the critical current
density Jc in ErNi2B2C single crystals that show strongly enhanced vortex
pinning at the Neel temperature TN and low applied fields. The height of the
observed Jc peak decreases with increasing magnetic field in clear contrast
with that of the peak effect found at the upper critical field. We also
performed the first angular transport measurements of Jc ever conducted on this
compound. They reveal the correlated nature of this pinning enhancement, which
we attribute to the formation of antiphase boundaries at TN.Comment: 3 figure
Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6
We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized
their anisotropic behaviour through low field magnetic susceptibility,
field-dependent magnetization, resistivity and heat capacity measurements. The
Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit
a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique
effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order
anti-ferromagnetically with Neel temperatures above 450 K and the R ions'
magnetism appears to behave independently. The possible origins of this unusual
behaviour of the ordered Fe moments in this compound are discussed.Comment: 12 pages, 8 figures, accepted in J. Phys.: Cond. Matte
Intrinsic pinning on structural domains in underdoped single crystals of Ba(FeCo)As
Critical current density was studied in single crystals of
Ba(FeCo)As for the values of spanning the entire doping
phase diagram. A noticeable enhancement was found for slightly underdoped
crystals with the peak at . Using a combination of polarized-light
imaging, x-ray diffraction and magnetic measurements we associate this behavior
with the intrinsic pinning on structural domains in the orthorhombic phase.
Domain walls extend throughout the sample thickness in the direction of
vortices and act as extended pinning centers. With the increasing domain
structure becomes more intertwined and fine due to a decrease of the
orthorhombic distortion. This results in the energy landscape with maze-like
spatial modulations favorable for pinning. This finding shows that iron-based
pnictide superconductors, characterized by high values of the transition
temperature, high upper critical fields, and low anisotropy may intrinsically
have relatively high critical current densities.Comment: estimation of Jc correcte
Pressure induced superconductivity in CaFeAs
CaFeAs has been found to be exceptionally sensitive to the
application of hydrostatic pressure and superconductivity has been found to
exist in a narrow pressure region that appears to be at the interface between
two different phase transitions. The pressure - temperature () phase
diagram of CaFeAs reveals that this stoichiometric, highly ordered,
compound can be easily tuned to reveal all the salient features associated with
FeAs-based superconductivity without introducing any disorder. Whereas at
ambient pressure CaFeAs does not superconduct for K and
manifests a first order structural phase transition near K, the
application of kbar hydrostatic pressure fully suppresses the
resistive signature of the structural phase transition and instead
superconductivity is detected for K. For kbar a different
transition is detected, one associated with a clear reduction in resistivity
and for kbar superconductivity is no longer detected. This higher
pressure transition temperature increases rapidly with increasing pressure,
exceeding 300 K by kbar. The low temperature, superconducting dome
is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This
superconducting phase appears to exist when the low pressure transition is
suppressed sufficiently, but before the high pressure transition has reduced
the resistivity, and possibly the associated fluctuations, too dramatically
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