22 research outputs found
Triple-gap superconductivity of MgB2 - (La,Sr)MnO3 composite. Which of the gaps is proximity induced?
Interplay of superconductivity and magnetism in a composite prepared of the
ferromagnetic half-metallic La_0.67Sr_0.33MnO (LSMO) nanoparticles and the
conventional s-wave superconductor MgB_2 has been studied. A few principal
effects have been found in bulk samples. With an onset of the MgB_2
superconductivity, a spectacular drop of the sample resistance has been
detected and superconductivity has been observed at temperature up to 20K.
Point-contact (PC) spectroscopy has been used to measure directly the
superconducting energy coupling. For small voltage, an excess current and
doubling of the PC's normal state conductance have been found. Conductance
peaks corresponding to three energy gaps are clearly observed. Two of these
gaps we identified as enhanced \Delta_{\pi} and \Delta_{\sigma} gaps
originating from the MgB_2; the third gap \Delta_{tr} is more than three times
larger than the largest MgB_2 gap. The experimental results provide unambiguous
evidences for a new type of proximity effect which follows the phase coherency
scenario of proximity induced superconductivity. Specifically, at low
temperature, the p-wave spin-triplet condensate with pairing energy \Delta_{tr}
is essentially sustained in LSMO but is incapable to display long-range
supercurrent response because of a phase-disordering state. The proximity
coupling to MgB_2 restores the long-range phase coherency of the triplet
superconducting state, which, in turn, enhances superconducting state of the
MgB_2.Comment: 10 pages, 6 figure
Possibility of local pair existence in optimally doped SmFeAsO(1-x) in pseudogap regime
We report the analysis of pseudogap Delta* derived from resistivity
experiments in FeAs-based superconductor SmFeAsO(0.85), having a critical
temperature T_c = 55 K. Rather specific dependence Delta*(T) with two
representative temperatures followed by a minimum at about 120 K was observed.
Below T_s = 147 K, corresponding to the structural transition in SmFeAsO,
Delta*(T) decreases linearly down to the temperature T_AFM = 133 K. This last
peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of
Fe spins. It is believed that the found behavior can be explained in terms of
Machida, Nokura, and Matsubara (MNM) theory developed for the AFM
superconductors.Comment: 5 pages, 2 figure
ab-plane tunneling and Andreev spectroscopy of superconducting gap and pseudogap in (Bi,Pb)2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8
We have measured the temperature dependence of gap features revealed by
Andreev reflection Delta_s and by tunneling Delta in the ab-plane of optimal
and slightly overdoped microcrystals of (BiPb)2Sr2Ca2Cu3O10 (Bi2223) with
critical temperature Tc=110-115 K, and Bi2Sr2CaCu2O8 (Bi2212) with Tc=80-84 K.
The tunneling conductance of Bi2223-Insulator-Bi2223 junction shows peaks at
the 2Delta gap voltage, as well as dips and broad humps at other voltages. In
Bi2223, similarly to the well known Bi2212 spectra, the energies corresponding
to 2Delta, to the dip, and to the hump structure are in the ratio of 2:3:4.
This confirms that the dip and hump features are generic to the high
temperature superconductors, irrespective of the number of CuO2 layers or the
BiO superstructure. On the other hand, in both compounds Delta(T) and
Delta_s(T) dependences are completely different, and we conclude that the two
entities have different nature.Comment: LaTeX 2e, 17 pages, 7 figures in .eps forma
Paraconductivity of K-doped SrFe2As2 superconductor
Paraconductivity of the optimally K-doped SrFe2As2 superconductor is
investigated within existing fluctuation mechanisms. The in-plane excess
conductivity has been measured in high quality single crystals, with a sharp
superconducting transition at Tc=35.5K and a transition width less than 0.3K.
The data have been also acquired in external magnetic field up to 14T. We show
that the fluctuation conductivity data in zero field and for temperatures close
to Tc, can be explained within a three-dimensional Lawrence-Doniach theory,
with a negligible Maki-Thompson contribution. In the presence of the magnetic
field, it is shown that paraconductivity obeys the three-dimensional
Ullah-Dorsey scaling law, above 2T and for H||c. The estimated upper critical
field and the coherence length nicely agree with the available experimental
data.Comment: 12 pages, 5 figure
Two band gap field-dependent thermal conductivity of
The thermal conductivity of the new superconductor was
studied as a function of the temperature and a magnetic field. No anomaly in
the thermal conductivity is observed around the superconducting
transition in absence or presence of magnetic fields up to 14 Tesla; upon that
field the superconductivity of persisted. The thermal conductivity in
zero-field shows a -linear increase up to 50K. The thermal conductivity is
found to increase with increasing field at high fields. We interpret the
findings as if there are two subsystems of quasiparticles with different
field-dependent characters in a two ( and )-band superconductor reacting
differently with the vortex structure. The unusual enhancement of at low temperature but higher than a () critical field
is interpreted as a result of the overlap of the low energy states outside the
vortex cores in the -band.Comment: 6 pages,3 figure
Subgap magnetotransport in
Andreev reflection spectroscopy has been used to probe mutual
influence of superconductivity and ferromagnetism in contacts of a
low-temperature
superconductor (S), Pb, and a half-metallic ferromagnet, La0.65Ca0.35
MnO3 (LCMO). In the contacts, which we distinguish as
proximity-affected ones, a few unusual effects have been detected:
i) a spectacular drop in the resistance of the contacts at the onset
of the Pb superconductivity; ii) excess current and doubling of the
normal-state conductance; iii) subharmonic gap structure; and iv)
the ratio of the single-particle gap to the Pb superconducting
transition temperature is anomalously large. Microscopically, our
results can most reasonably be explained assuming a conversion from
spin singlet pairs to spin triplets at the S/LCMO interface and a
long-range proximity effect
Tunneling Conductivity of Manganites with Negative Magnetoresistance
Tunneling investigations of ferromagnetic LaSrMnO ceramic samples showing negative magnetoresistance effect were carried out in two types of junctions: symmetric "break junctions" and asymmetric ceramics-insulator-metal junctions. The results are consistent with the presence of small ferromagnetic clusters in the insulating barriers of both types of junctions and suggest a two-phase state realized by impurity mechanism of phase separation