26 research outputs found
Doping dependence of the Nernst effect in Eu(Fe1-xCox)2As2 - departure from Dirac fermions physics
We report a systematic study of the transport properties in the series of
Eu(Fe1-xCox)2As2 single crystals with x = 0, 0.15, 0.20 and 0.30.
Spin-density-wave order is observed in the undoped and the least doped samples
(x = 0, 0.15), while for x = 0.15 and 0.20 Eu(Fe1-xCox)2As2 becomes a
superconductor. We found the properties of the parent EuFe2As2 compound well
described by the Dirac fermions model, whereas cobalt doping caused an
evolution of the system toward a regular metallic state. The antiferromagnetic
ordering of the Eu2+ ions at T_N ~ 20 K has only minor influence on the
measured quantities.Comment: 5 pages, 5 figures; ver.3: the sign convention for the Nernst
coefficient is change
Non-adiabatic effects in the phonon dispersion of Mg 1--x Al x B 2
Superconducting MgB shows an E zone center phonon, as measured
by Raman spectroscopy, that is very broad in energy and temperature dependent.
The Raman shift and lifetime show large differences with the values elsewhere
in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its
dispersion can be accounted for by standard harmonic phonon theory, adding only
a moderate electron-phonon coupling. Here we show that the effects rapidly
disappear when electron-phonon coupling is switched off by Al substitution on
the Mg sites. Moreover, using IXS with very high wave-vector resolution in
MgB, we can follow the dispersion connecting the Raman and the IXS signal,
in agreement with a theory using only electron-phonon coupling but without
strong anharmonic terms. The observation is important in order to understand
the effects of electron-phonon coupling on zone center phonons modes in
MgB, but also in all metals characterized by a small Fermi velocity in a
particular direction, typical for layered compounds
Structural and Superconducting Properties of RbOs2O6 Single Crystals
Single crystals of RbOs2O6 have been grown from Rb2O and Os in sealed quartz
ampoules. The crystal structure has been identified at room temperature as
cubic with the lattice constant a = 10.1242(12) A. The anisotropy of the
tetrahedral and octahedral networks is lower and the displacement parameters of
alkali metal atoms are smaller than for KOs2O6, so the "rattling" of the alkali
atoms in RbOs2O6 is less pronounced. Superconducting properties of RbOs2O6 in
the mixed state have been well described within the London approach and the
Ginzburg-Landau parameter kappa(0) = 31 has been derived from the reversible
magnetization. This parameter is field dependent and changes at low
temperatures from kappa = 22 (low fields) to kappa = 31 at H_{c2}. The
thermodynamic critical field H_{c}(0) = 1.3 kOe and the superconducting gap
2delta/k_{B}T_{c} = 3.2 have been estimated. These results together with
slightly different H_{c2}(T) dependence obtained for crystals and
polycrystalline RbOs2O6 proof evidently that this compound is a weak-coupling
BCS-type superconductor close to the dirty limit.Comment: 20 pages, 8 figures, 3 table
High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications
Superconducting technology provides most sensitive field detectors, promising
implementations of qubits and high field magnets for medical imaging and for
most powerful particle accelerators. Thus, with the discovery of new
superconducting materials, such as the iron pnictides, exploring their
potential for applications is one of the foremost tasks. Even if the critical
temperature Tc is high, intrinsic electronic properties might render
applications rather difficult, particularly if extreme electronic anisotropy
prevents effective pinning of vortices and thus severely limits the critical
current density, a problem well known for cuprates. While many questions
concerning microscopic electronic properties of the iron pnictides have been
successfully addressed and estimates point to a very high upper critical field,
their application potential is less clarified. Thus we focus here on the
critical currents, their anisotropy and the onset of electrical dissipation in
high magnetic fields up to 65 T. Our detailed study of the transport properties
of optimally doped SmFeAs(O,F) single crystals reveals a promising combination
of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities
along all crystal directions. This favorable intragrain current transport in
SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a
crucial requirement for possible applications. Essential in these experiments
are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with
sub-\mu\m^2 cross-section, with current along and perpendicular to the
crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed
magnetic fields. The pinning forces have been characterized by scaling the
magnetically measured "peak effect"