8 research outputs found
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
Strong Pinning in High Temperature Superconductors
Detailed measurements of the critical current density jc of YBa2Cu3O7 films
grown by pulsed laser deposition reveal the increase of jc as function of the
filmthickness. Both this thickness dependence and the field dependence of the
critical current are consistently described using a generalization of the
theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024
(1991)]. From the model, we deduce values of the defect density (10^21 m^-3)
and the elementary pinning force, which are in good agreement with the
generally accepted values for Y2O3-inclusions. In the absence of clear evidence
that the critical current is determined by linear defects or modulations of the
film thickness, our model provides an alternative explanation for the rather
universal field dependence of the critical current density found in YBa2Cu3O7
films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002
Comparison of pressure, magnetic field and excess manganese effects on transport properties of film and bulk ceramic La–Ca manganites
The pressure, magnetic field and excess manganese effects on transport and magnetoresistance
effect (MRE) have been studied in both the epitaxial films and bulk ceramics of manganites
(La₀.₇Ca₀.₃)₁₋xMn₁₊xO₃₋y (x = 0–0.2). A comparison of electrical behavior in both kinds of samples
of similar composition at hydrostatic pressures of up to 1.8 GPa and in a magnetic fields of up
to 8 kOe has been performed. The pressure and magnetic field effects are shown to increase with
increasing manganese content. Experimental data show that the pressure and magnetic field effects
on temperatures of both metal–insulator transition (TMD) and MRE peak (TMR) are considerably
stronger in the films than in ceramics. The hydrostatic pressure increases TMD and TMR.
Magnetoresistance effect for both types of samples was shown to be favored by the pressure and
magnetic field in an opposite way. A direct correlation is established between TMD and conductivity
bandwidth as well as between MRE and concentration of charge carriers at applied pressure.
The differences in the values of pressure effect on resistance, MRE and TMD temperature in the
films and ceramics are connected with both granular structure of ceramics and the oxygen
nonstoichiometry in ceramic and film samples of the same content as well as with the film strain
induced by lattice mismatch between the film and the substrate. The origin of pressure–magnetic
field effects is analyzed in the framework of double exchange interaction and small polaron hopping,
and variable range hopping models