130 research outputs found
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
Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2
We demonstrate that the thermopower (S) can be used to probe the spin
fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based
superconductors. The sensitivity of S to the entropy of charge carriers allows
us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the
spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy
conduction electrons to two-dimensional SFs, similar to heavy-fermion systems.
The low-temperature enhancement of S/T in the Co substitution range 0.02 < x <
0.1 is bordered by two Lifshitz transitions, and it corresponds to the
superconducting region, where a similarity between the electron and
non-reconstructed hole pockets exists. The maximal S/T is observed in proximity
to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05,
close to the highest superconducting T_c. This analysis indicates that low-T
thermopower is influenced by critical spin fluctuations which are important for
the superconducting mechanism
Boron isotope effect in single crystals of ErNiBC superconductor
The influence of local moment magnetism on the boron isotope effect of T
was studied on single crystals of ErNiBC. Values of the partial isotope
effect exponent of =0.100.02 and =0.100.04 were
obtained based on two different criteria applied to extract . No
significant change in the partial isotope effect exponent compared to the ones
obtained for LuNiBC was observed. Based on this result we conclude that
pair-breaking due to the Er local magnetic moment appears to have no detectable
influence on boron isotope effect of T.Comment: 7 pages, 3 figure
Evaluation of a long-time temperature drift in a commercial Quantum Design MPMS SQUID magnetometer using GdO as a standard
The long-time temperature drift in a commercial Quantum Design MPMS SQUID
magnetometer was evaluated using time-dependent magnetization measurements of
GdO. In contrast to earlier claims, the amplitude of the drift was
found not to exceed 1-1.5 K. 30 minutes after system stabilization the
temperature deviation did not exceed 0.2 K and the temperature was fully
stabilized in less than 3 hours
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
Temperature-dependent anisotropy in MgB as inferred from measurements on polycrystals
We present data on temperature-dependent anisotropy of the upper critical
field of MgB obtained from the analysis of measurements on high purity, low
resistivity polycrystals. The anisotropy decreases in a monotonic fashion with
increase of temperature
Distinguishing local moment versus itinerant ferromagnets: Dynamic magnetic susceptibility
Radio-frequency measurements of dynamic magnetic susceptibility of various ferromagnets show striking differences between local-moment ferromagnetism (LFM) and weak itinerant ferromagnetism (IFM) ferromagnetic systems. LFMs show a very sharp peak in susceptibility in the vicinity of the Curie temperatureTC that rapidly decreases in amplitude and shifts to higher temperature with the application of a weak dc bias field. In stark contrast, the generally accepted IFM systems show no peak, but rather a broad maximum well below TC. The temperature of this maximum shifts to lower values and the amplitude is suppressed with an applied dc field
Controlling crystal-electric field levels through symmetry-breaking uniaxial pressure in a cubic super heavy fermion
Financial support by the Max Planck Society is gratefully acknowledged. In addition, we gratefully acknowledge funding through the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through TRR 288–422213477 (project A10) and the SFB 1143 (project-id 247310070; project C09). Research in Dresden benefits from the environment provided by the DFG Cluster of Excellence ct.qmat (EXC 2147, project ID 390858940). Work at the Ames National Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The Ames National Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DEAC02-07CH11358.YbPtBi is one of the heavy-fermion systems with largest Sommerfeld coefficient γ and is thus classified as a ‘super’-heavy fermion material. In this work, we resolve the long-debated question about the hierarchy of relevant energy scales, such as crystal-electric field (CEF) levels, Kondo and magnetic ordering temperature, in YbPtBi. Through measurements of the a.c. elastocaloric effect and generic symmetry arguments, we identify an elastic level splitting that is unambiguously associated with the symmetry-allowed splitting of a quartet CEF level. This quartet, which we identify to be the first excited state at Δ/kB ≈ 1.6 K above the doublet ground state at ambient pressure, is well below the proposed Kondo temperature TK ≈ 10 K. Consequently, this analysis of the energy scheme can provide support models that predict that the heavy electron mass is a result of an enhanced degeneracy of the CEF ground state, i.e., a quasi-sextet in YbPtBi. At the same time, our study shows the potential of the a.c. elastocaloric effect to control and quantify strain-induced changes of the CEF schemes, opening a different route to disentangle the CEF energy scales from other relevant energy scales in correlated quantum materials.Publisher PDFPeer reviewe
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