V. Yu. Tarenkov

The nature of transport critical current hysteresis in HTSC: magnetic fields and high pressures

Abstract It was found that pressure has a strong influence on the critical current hysteresis loop of ceramics at H ∼ 10 kOe. The phenomenon is attributed to the critical current hysteresis of separate Josephson contacts and is due to the Abrikosov vortex density gradient within granules. The gradient defines both the sign and the value of the pinning current, whereas the sign of Meissner reversible surface current component is determined by the external field H direction. As a result the critical current of Josephson contacts defined by the total surface value depends on the magnetic prehistory of a sample.

Pressure-induced variation of the magnetic structure of the surface of La0.6Sr0.4MnO3 granules

The transport properties of textured films and tunneling junctions of La0.6Sr0.4MnO3, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La0.6Sr0.4MnO3-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.

Fluctuation conductivity and pseudogap in SmFeAsO1−x

The temperature dependences of the fluctuation conductivity and pseudogap in SmFeAsO1−x are determined from resistivity measurements. It is shown that the temperature dependence of the pseudogap contains a number of features that substantially distinguish it from that of HTSC-cuprates.

Magnetic-field and high-pressure dependences of Tc and critical current in the polycrystalline HgBaCaCuO system

Abstract The high-temperature phase Hg 0.8 Pb 0.2 Ba 2 Ca 2 Cu 3 O 8+δ (HBCCO) with the onset transition temperature T c ≈ 133 ± 0.5 K has been investigated under hydrostatic pressure up to 12 kbar. We found a positive pressure effect on T c with a rate 0.28 ± 0.04 K/kbar and a positive pressure effect on the transport critical current J c , dln( J c )/d P = 0.2 ± 0.02 kbar −1 . The observation of a strong hysteresis effect in the J c versus magnetic field H dependences shows the possibility of surface barriers to vortex motion in the high- T c HBCCO system.

Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)

Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreev-reflection (AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained on “classic” s-wave superconductor—ferromagnetic manganites PCs, as well as related theoretical models applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected” contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as well as possibility of unconventional superconducting proximity effect, have been discussed.

An excess current in Bi2223–Ag contacts above the critical temperature

In contacts normal metal–superconductor Bi1.6Pb0.4Sr1.8Ca2.2Cu3O10 (Bi2223), the energy gap Δ(T) of the superconductor and the excess current Iexc(T) were measured by means of the Andreev spectroscopy. It is shown that the parameter Δ(T) turns to zero at the superconducting transition temperature of the cuprate, Tc ≈ 110 K, and the dependence Δ(T) follows the BCS theory; at the same time, the excess current Iexc(T) persists up to the temperature Tpair ≈ 180 K, significantly higher than Tc. The result obtained indicates a possible existence in Bi2223 of a wide temperature range Tc < T < Tpair where a condensate of Cooper pairs starts to form.

Contact phenomena in Ag–YBCO composites

Abstract Current transport properties of YBaCuO–Ag composites with different silver contents have been studied. The pressure response of the resistance of a single Ag/YBCO junction has been determined. The experiments reveal the absence of a dielectric layer in the junction. It has been found that the proximity effect does not contribute significantly to the conductivity of the Ag–YBCO composite for the different ways of introduction of the metallic inclusions into the YBCO. For the strip-like samples of Ag–YBCO composite with the Ag content of 15% by volume the resistivity ρ was found to be ρ =4.7×10 −9 Ω cm at the density of measuring current of J =3×10 4 A/cm 2 and magnetic field B =1 T.

Temperature dependence of the energy gap in Bi2223 metal oxide superconductor

AbstractSpectroscopic studies of the silver-optimum-doped Bi2223 contacts show that the temperature dependence of the parameter Δ follows the BCS curve. However, the tunnel measurements performed for the same series of specimens did not reveal any temperature dependence of the energy gap Δ. The feature observed in the tunnel density of states was retained at temperatures T>Tc, manifesting the presence of the temperature-independent pseudogap Ep. The difference between the data obtained with tunnel spectroscopy and Andreev reflection spectroscopy is explained by the fact that the latter measures the true superconducting energy gap Δs(T), whereas the peaks of the tunneling conductivity are related to the total energy gap Δ of cuprates, which includes both the parameter Δs and the pseudogap % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFH0dXde9LqFHe9Lq% pepeea0xd9q8as0-LqLs-Jirpepeea0-as0Fb9pgea0lrP0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamyramaaBa% aaleaacaWGWbaabeaaieaakiaa-PdacqqHuoarcqGHijYUdaGcaaqa% aiabfs5aenaaDaaaleaacaWFZbaabaGaa8NmaaaakiabgUcaRiaadw% eadaqhaaWcbaGaamiCaaqaaiaaikdaaaaabeaaaaa!45EA!

Andreev spectroscopy of point contacts between a low-temperature superconductor and manganite

E_p :\Delta \approx \sqrt {\Delta _s^2 + E_p^2 }