Composite materials fabricated by annealing of nonsuperconducting ceramics La{sub 2}CuO{sub 4} and La{sub 1.56}Sr{sub 0.44}CuO{sub 4} at 910 °C during various time are investigated. Areas of ...superconducting La{sub 1.85}Sr{sub 0.15}CuO{sub 4} phase arises at boundaries of contacting nonsuperconducting granules. The volume fraction of the superconducting phase increases with increasing annealing time. A model describing the magnetic and transport properties of the samples at low magnetic fields is constructed. The magnetotransport characteristics of obtained samples at low magnetic fields (∼ 100 Oe) are defined by weak links network formed by superconducting areas. At high fields, behavior of the system is defined by a magnetization of the disconnected superconducting islands. The average size of the superconducting areas has been estimated from an extended critical state model.
Electrical conductivity σ(
T
) of the paper consisting of multiwalled carbon nanotubes (MWCNTs) is studied in the temperature range 4.2-295 K, and its magnetoresistivity ρ(
B
) at various ...temperatures in magnetic fields up to 9 T is analyzed. The temperature dependence of the paper electrical conductivity σ(
T
) exhibits two-dimensional quantum corrections to the conductivity below 10 K. The dependences of negative magnetoresistivity ρ(
B
) measured at various temperatures are used to estimate the wavefunction phase breakdown length
L
ϕ
of conduction electrons and to obtain the temperature dependence
L
ϕ
= const
T
−
p
/2
, where
p
≈ 1/3. Similar dependences of electrical conductivity σ(
T
), magnetoresistivity ρ(
B
), and phase breakdown length
L
ϕ
(
T
) are detected for the initial MWCNTs used to prepare the paper.
The influence of thermomagnetic prehistory on the behavior of a resistive transition
R
(
T
) in external magnetic fields of polycrystalline YBa
2
Cu
3
O
7
and Bi
1.8
Pb
0.3
Sr
1.9
Ca
2
Cu
3
O
x
...high-temperature supercon-ductors and the Bi
1.8
Pb
0.3
Sr
1.9
Ca
2
Cu
3
O
x
+ Ag texture has been investigated. It has been found that, for YBa
2
Cu
3
O
7
, the thermomagnetic prehistory exerts a substantial influence on the dissipation in the subsystem of grain boundaries in magnetic fields up to ∼10
3
Oe, and this effect becomes insignificant in fields higher than ∼10
4
Oe. This behavior has been explained by the influence of magnetic moments of high-temperature superconductor grains on the effective magnetic field in the intergranular medium. For bismuth high-temperature superconductors, no influence of thermomagnetic prehistory on the resistive transition has been observed; however, this effect manifests itself in current-voltage characteristics at high transport current densities. There is also a radical difference in the behavior of isotherms of the magnetoresistance
R
(
H
) for the yttrium and bismuth systems. For YBa
2
Cu
3
O
7
, there is a clear separation between the dissipation regimes in the intergranular medium and in grains, which manifests itself even at low transport current densities as a change of sign in the curvature of the dependence
R
(
H
). For a texture based on the bismuth high-temperature superconductor, this feature has been observed only at high current densities (comparable to the critical current density at
H
= 0). This difference in the behavior of magnetoresistive properties of the classical high-temperature superconductor systems under investigation has been explained by relatively low irreversibility fields of the bismuth high-temperature superconductors. In these materials, simultaneous processes of dissipation can occur in an external magnetic field both in the subsystem of grain boundaries between crystallites and in the crystallites themselves.
The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TC system Ba0.6K0.4BiO3 (TC≈30 K) is confirmed on Ba0.6K0.4BiO3 single ...crystal. At T* (T*<TC, T*≈17 K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gapes on the Fermi surface: semi- and superconducting, that are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. Nonlinear I-V curves, depended on temperature and magnetic field, that are typical to a Josephson system, are observed on Ba0.6K0.4BiO3 single crystal at temperatures below T*. Besides, a step like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier then critical current. Perhaps the last phenomenon is the consequence of “coherent” state of several successive Josephson junctions, appeared in the exfoliation state.
The behavior of the hysteresis of the magnetoresistance
R
(
H
) of granular high-temperature superconductors has been investigated under the conditions where the resistive response of the subsystem ...of grain boundaries close to saturation. The hysteretic dependences
R
(
H
) have been measured for Y
1 −
x
Pr
x
Ba
2
Cu
3
O
7
samples at
x
= 0.11 and 0.04 with the transition temperatures
T
C
≈ 85.5 and 91.0 K, respectively. The evolution of the field width of the hysteresis
R
(
H
) has been examined by varying the measuring current. The limit of the applicability has been established for the concept of the effective field in the intergranular medium, which was previously proposed for the description of the hysteretic behavior of the magnetoresistance
R
(
H
) and thermal magnetic prehistory of the granular high-temperature superconductors. In the studied samples, the approximation of the effective field in the intergranular medium is applicable until the magnetoresistance of the subsystem of grain boundaries exceeds (90 ± 5)% of the maximum value.
The current–voltage (I–V) characteristics of break junctions of polycrystalline La1.85Sr0.15CuO4, Y0.75Lu0.25Ba2Cu3O7−δ, Bi1.8Pb0.3Sr1.9Ca2Cu3Ox and composite YBa2Cu3O7−δ+Ag are investigated. The ...experimental I–V curves exhibit the specific peculiarities of superconductor/normal-metal/superconductor junctions. The relation between an I–V characteristic of network of weak links and I–V dependencies of typical weak links is suggested to describe the experimental data. The I–V curves of typical weak links are calculated by the Kümmel–Gunsenheimer–Nicolsky model considering the multiple Andreev reflections.
The current-voltage characteristics of a porous superconductor Bi
2
Sr
2
Ca
2
Cu
3
O
x
(Bi2223) have been measured at temperatures in the range from 10 to 90 K in magnetic fields of 0–80 kOe. The ...experimental dependences have been analyzed within the model allowing for pinning by clusters of a normal phase with fractal boundaries, as well as the model taking into account phase transformations of vortex matter. It has been found that the electrical resistance of the superconductor material significantly increases at temperatures of 60–70 K over the entire range of magnetic fields under consideration without changing in the sign of the curvature of the dependence
R
(
I
). It has been assumed that this behavior is associated with the specific feature of the pinning in a highly porous high-temperature superconductor, which lies in the fractality of the distribution of pinning centers in a wide range of self-similarity scales. The studied material at the aforementioned temperatures is characterized by the melting of the vortex structure.
Magnetoresistance curves
R(
H) and time relaxation of the residual resistance of bulk YBCO
+
CuO composites have been studied. These composites represent the network of Josephson junctions, where CuO ...acts as a barrier between YBCO grains. It has been shown that hysteresis of the
R(
H) dependences is related to the flux trapping within the YBCO crystallites. This flux induces magnetic field in the intergrain media. Time relaxation of the residual resistance is caused by relaxation of the magnetization of YBCO grains. The time dependence of the residual resistance obeys a logarithmic law, which describes the Anderson-like relaxation processes in the YBCO grains.
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