11 research outputs found
Two-gap superconductivity with line nodes in CsCaFeAsF
We report the results of a muon-spin rotation (SR) experiment to
determine the superconducting ground state of the iron-based superconductor
CsCaFeAsF with K. This compound is
related to the fully-gapped superconductor CaCsFeAs, but here the
Ca-containing spacer layer is replaced with one containing CaF. The
temperature evolution of the penetration depth strongly suggests the presence
of line nodes and is best modelled by a system consisting of both an - and a
-wave gap. We also find a potentially magnetic phase which appears below
K but does not appear to compete with the superconductivity. This
compound contains the largest alkali atom in this family of superconductors and
our results yield a value for the in-plane penetration depth of
nm.Comment: 6 pages, 2 figure
Magnetic Monopole Noise
Magnetic monopoles are hypothetical elementary particles exhibiting quantized
magnetic charge and quantized magnetic flux . A classic proposal for detecting such magnetic charges is to measure the
quantized jump in magnetic flux threading the loop of a superconducting
quantum interference device (SQUID) when a monopole passes through it.
Naturally, with the theoretical discovery that a plasma of emergent magnetic
charges should exist in several lanthanide-pyrochlore magnetic insulators,
including DyTiO, this SQUID technique was proposed for their direct
detection. Experimentally, this has proven extremely challenging because of the
high number density, and the generation-recombination (GR) fluctuations, of the
monopole plasma. Recently, however, theoretical advances have allowed the
spectral density of magnetic-flux noise due to GR
fluctuations of magnetic charge pairs to be determined. These
theories present a sequence of strikingly clear predictions for the
magnetic-flux noise signature of emergent magnetic monopoles. Here we report
development of a high-sensitivity, SQUID based flux-noise spectrometer, and
consequent measurements of the frequency and temperature dependence of
for DyTiO samples. Virtually all the elements
of predicted for a magnetic monopole plasma, including the
existence of intense magnetization noise and its characteristic frequency and
temperature dependence, are detected directly. Moreover, comparisons of
simulated and measured correlation functions of the magnetic-flux
noise imply that the motion of magnetic charges is strongly
correlated because traversal of the same trajectory by two magnetic charges of
same sign is forbidden
Robustness of superconducting properties to transition metal substitution and impurity phases in Fe1-xVxSe
We have performed transverse- and zero-field muon spin rotation/relaxation
experiments, as well as magnetometry measurements, on samples of Fe1-xVxSe and
their Li+NH3 intercalates Li0.6(NH2)0.2(NH3)0.8 Fe1-x Vx Se. We examine the low
vanadium substitution regime: x = 0.005, 0.01, and 0.02. The intercalation
reaction significantly increases the critical temperature (Tc) and the
superfluid stiffness for all x. The nonintercalated samples all exhibit Tc =
8.5 K while the intercalated samples all show an enhanced Tc > 40 K. Vanadium
substitution has a negligible effect on Tc, but seems to suppress the
superfluid stiffness for the nonintercalated samples and weakly enhance it for
the intercalated materials. The optimal substitution level for the intercalated
samples is found to be x = 0.01, with Tc = 41 K and {\lambda}_{ab}(0) = 0.18
{\mu}m. The nonintercalated samples can be modeled with either a single d-wave
superconducting gap or with an anisotropic gap function based on recent
quasiparticle imaging experiments, whereas the intercalates display multigap
nodal behavior which can be fitted using s + d- or d + d-wave models.
Magnetism, likely from iron impurities, appears after the intercalation
reaction and coexists and competes with the superconductivity. However, it
appears that the superconductivity is remarkably robust to the impurity phase,
providing an avenue to stably improve the superconducting properties of
transition metal substituted FeSe.Comment: 7 pages, 4 figure