Characterization and electrical properties of sol-gel synthesized (Sr, Pb)TiO3 powders

Y-doped (Sr, Pb)TiO₃ powders were prepared by a sol-gel route as well as the calcination of gel precursors. The results of DTA/TG showed that the thermal decomposition of dry precursors mainly occurred below 600°C. Meanwhile, infrared ray (IR) spectrum meter, X-ray diffraction (XRD) meter and transmission electron microscope (TEM) were used to characterize the synthesized powders, respectively. Using the synthesized powders as starting materials, Sr_0.5Pb_0.5TiO₃ semiconducting ceramics were fabricated at 1050°C. Sample's room temperature resistivity is 1.51 × 10² · cm, its resistivity jumps more than 5 orders of magnitude above the Curie temperature (T _c). With increasing the soaking time, the room temperature resistivity and the negative temperature coefficient of resistance (NTCR) effect below T _c increased, showing the electrical properties of (Sr, Pb)TiO₃ thermistors are obviously affected by PbO loss.     

Temperature dependence of resistivity for thermally diffused V3Si multilayer films

Results concerning V₃Si films produced by a simple annealed multilayer technique are reported together with X-ray diffraction patterns, Auger spectroscopy, and Rutherford backscattering analysis. Low-temperature electrical resistivity measurements are discussed. It is found that the V₃Si films exhibit aT ² dependence in the temperature rangeT _c T23 K and aT ^2.6 dependence in the rangeT _c T40 K. The normal-state resistivity in the whole temperature range (T _c T600 K) is analyzed in the framework of Cote-Meisel theory. Consistent values of the saturation resistivity _m and of the Debye temperature are obtained by fitting the experimental data with the Cote-Meisel expression for (T).     

A proximity effect model for superconductivity in titanium-transition metal alloys

We present a unified model for the variation in superconducting transition temperature T _c of quenched titanium-transition metal alloys with intermediate-temperature aging. Experimental results on T _c and the low-temperature resistivity of -quenched Ti-27 at % Nb solid solution, a representative system, measured as a function of annealing are given. It is shown that the enrichment of the matrix in Nb as a result of precipitation of a Ti-rich phase is too small to be of any consequence in influencing T _c. Our resistivity data give, for the first time, evidence for the formation of athermal phase in this system. In the light of the present investigation a clear picture has emerged for the interpretation of T _c in terms of varying extent of superconducting proximity effect, which is shown to be intimately related to the microstructural evolution. The result is shown to be general, applicable to any Ti-transition metal alloy system.     

Effect of annealing on the anomalous electrical resistivity of dilute copper-iron alloys at low temperatures

Electrical resistivity measurements in the temperature range 1.5–35 K on two copper alloys containing 115 and 380 atomic ppm iron are reported, in their unannealed state and also after annealing for 16 and 66 h in fore-vacuum at 530–550°C. Below the temperature of the resistivity minimum the impurity resistivity has the Kondo lnT behavior. However, in the liquid helium region the resistivity drops from its value atT=0, in proportion toT ², conforming to Nagaoka's theory forT<T _K/5. The Kondo temperatureT _K is evaluated from the versusT ² plots using Nagaoka's equation and is found to decrease with increasing concentration. Annealing is found to reduce the effective iron concentration and alsoT _K. The impurity resistivity per atomic percent in our samples can be expressed as a universal function ofT/T _K at the lowest temperatures underT _K/4.     

Upper Critical Fields of Nb/Pd Multilayers

We measured critical temperature and critical magnetic fields of Nb/Pd multilayers where the Nb thickness is held constant (d _Nb =250 Å) while the Pd thickness, d _Pd , is systematically varied from 10 to 200 Å. The critical temperature shows a monotonic decrease as a function of the Pd thickness, which can be tentatively described using the classical de Gennes-Werthamer theory for proximity coupled systems. The critical magnetic field measurements reveal unusual behavior, like a positive curvature of H_c2(T) close to T _c and a dimensional 3D-2D crossover, already present in multilayers with very thin Pd layers. In particular the angular behavior of the upper critical field, in the case of small d _Pd values, confirm the 2D nature of all the samples at T=4.2 K, giving also indication of the columnar nature of the Nb layers. These unusual behaviors might be related to the strong paramagnetic nature of the Pd layers. On the other hand also the proximity theory for S/N systems describes the perpendicular critical field data, but some inconsistencies are found quantitatively.     

Effect of Bi/Pb ratio and annealing temperature onTc and formation of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.80.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

The Transport Properties of Li(1+x)Ti2O4 in High Magnetic Field

The spinel oxide superconductor LiTi₂O₄ shows intricate transport properties in both normal state and superconducting state. The Hall effect measurements exhibit an abrupt change in carrier density near T=170 K where the temperature derivative of resistivity changes sign from negative to positive on cooling. In addition, the sample shows significant positive magnetoresistance below 40 K, e.g., R(H)/R(H=0)3% at T=14 K and H=16 T. A most interesting feature is the magnetic field effect on the resistivity near the superconducting state. In addition to the field-induced broadening of R(T), there exhibits an anomalous upturn in R(T) near the superconducting regime when magnetic field is above 3T. The temperature T ₁ at which R(T) shows upturn linearly correlates with magnetic field H and can be expressed as T ₁/T _C(H=0)=1–0.04H for 3T<H<12T. The superconductivity is completely suppressed at field above 14T and the low temperature resistance behaves semiconductor-like.     

The Transport Properties of Bi-Riched Bi2Sr2CuO6+δ Single Crystal

As-grown superconducting Bi-riched Bi₂Sr₂CuO₆₊ single crystals have been grown by the traveling solvent floating zone technique. The superconducting transition temperature T _c was about 6 K and the room temperature resistivity was about 2×10^–3 Ohm-cm. Transport properties, such as resistivity, magnetoresistance and Hall effect were measured from overdoped to underdoped samples annealed in inert atmosphere at 650°C. The transition temperature can be raised to 12 K after post annealing. The Hall measurement shows that the hole carrier density decrease after annealing. The temperature dependence of Hall angle is T ^1.5, not quadratic as observed for most high-T _c superconducting oxides such as YBa₂Cu₃O₇. The variation of onset T _c with different external magnetic field is very different from high-T _c superconductors. The in-plane conductivity shows the dependence of ln T and can be explained by weak localization theory.     

Kondo effect and impurity interactions in the resistivity of diluteZnMn alloys

The electrical resistivity of diluteZnMn alloys (c=1.7–2400 ppm Mn) has been investigated in the temperature range from 0.05 to 14 K. For the most dilute sample, single-impurity Kondo behavior is found, well described by the Hamann formula withT _K=0.9 K,S=3/2. ForT<50 mK, aT ² law with _R =0.3 K is expected. In the dilute limit the Kondo slope is –(1/c)d()/d(logT)=3.7±0.2 µ-cm/at % dec. In the more highly concentrated alloys, the slope decreases with increasing c and the lnT-like variation of the Kondo resistivity roughly terminates near a temperatureT _W(c),T _W being related to the average Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction strength between the Mn impurities. For 20 ppm c 1000 ppm, the resistivity slightly decreases at low temperatures and a broad resistivity maximum is observed atT _m (c), withT _m c ^0.7. ForT 2 _m, the resistivity dependence is linear inT, and for the most concentrated alloy aT ^3/2 orT ² dependence is measured at the lowest temperatures attainable. The investigation of the transition temperatureT _c (c) to superconductivity ofZnMn results in a critical concentrationc _cr=18 ppm Mn. The concentration dependence ofT _c below 0.3 K suggests the presence of the Kondo effect, although impurity interactions may also influenceT _c in this temperature range.Supported by Deutsche Forschungsgemeinschaft.     

Thermoelectric Power of Deoxygenated Bi1.6Pb0.4Sr2Ca2Cu3O10 + δ Sintered Superconducting Pellets

The thermal variation of the electrical resistivity and thermoelectric power of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_{10 +} pellets subjected to various degrees of deoxygenation is reported. The temperature dependence of the electrical resistivities of deoxygenated samples displays gradual transformation from metallic-like to semiconductor-like features in the normal state. All the samples however, show superconducting transition, but increasing deoxygenation depresses T _C0 from 102 to 45 K. Gross features of the temperature variation of thermoelectric power observed in properly oxygenated (Bi, Pb)-2223 cuprates are retained in all the deoxygenated samples. Our results on electrical resistivity and thermoelectric power in the normal state have been found to be consistent with a two-band model.     

Type II superconductors containing magnetic impurities

A theoretical study is made on the equilibrium as well as on the dynamical properties of type II superconductors containing magnetic impurities. The Kondo effect associated with the impurity spins is taken into account within the pole approximation, which has been used previously by Müller-Hartmann and Zittarz in their calculation of the superconducting transition temperature. In this approximation we have still the additive law of the different pair-breaking mechanisms for the transition temperature; the sum of the pair-breaking parameters due to magnetic impurities and due to magnetic fields is a universal function of the temperature. Since the pair-breaking parameter arising from magnetic impurities has the maximum atTT _K , the upper critical fieldH _c2 (T) reflects the Kondo effect in the system. The ₂(T) parameter, which describes the magnetization as well as the flux-flow resistivity of the vortex state, is obtained. WhenT _K /T _c0 >1, ₂(T) increases as the temperature decreases, but whenT _K /T _c0 <1, ₂(T) first increases as the temperature decreases, then passing a maximum and decreasing at low temperatures, which is in sharp contrast to that obtained previously where the impurity scattering was treated with the Born approximation (i.e., the Kondo effect was completely neglected.) A brief discussion of the density of states in the gapless region is also given, which reflects the Kondo effect in an interesting way.     

Superconducting Properties in Amorphous MgxB1−x Films

We report the observation of superconductivity in a thick amorphous (a-) Mg_xB_1–x film with x0.3, which was prepared by coevaporation of Mg and B. The resistivity in the normal state is by approximately two order of magnitude higher than that for clean MgB₂ crystals. The transition temperature T _c at which the resistivity vanishes is found to be 6 K, that is even lower than T _c=39 K for clean MgB₂ crystals. The upper critical field extrapolated to zero temperature is about 5 T. Based on the data of resistivity measured down to 0.4 K for various fields B, we construct the possible B–T phase diagram over the broad temperature/field region.     

The Effect of Zn-Substitution on the Anomalous Resistance Behavior of La1−x Ca x Mn1−y Zn y O3 Compounds

We have studied the effect of Zn substitution on the temperature dependence of the resistivity and the Curie temperature T _p of La_1–x Ca _x Mn_1–y Zn _y O₃ system (with x=0, y=0, and x=0.33, 0.00y0.15) during their pass to the paramagnetic conducting phase. All the specimens show a clear metal-insulator transition passing through the Curie temperature T _p . With increasing Zn concentration, T _p decreases for La/Ca substitution and also for Zn/Mn substitution with fixed 33 at.% of Ca. T _p is found to be slightly decreases for low concentrations of Zn (up to y=0.075) followed by a sharp decrease up to y=0.15. A possible explanation is given. Moreover, the activation energy E _a is calculated in the three different temperature regions characterized in the (-T) curve. Decreasing the temperature below T _p can increase the parallel alignment of the Mn spins and, thus decreases the resistivity of the samples. E _a increases with Zn content (up to y=0.075) followed by a sharp decrease. A strong correlation in the behavior of the normal state and residual resistivity as well as T _p and E _a with Zn content is reported.     

Transport properties of GdB6 and DyB6

The electrical resistance and absolute thermoelectric power (TEP) have been measured for GdB₆ and DyB₆ in the temperature range 2–30 K. The compounds GdB₆ and DyB₆ order antiferromagnetically atT _N 15.2 and 20.3 K, respectively. Above the Néel temperature the resistivity has a small contribution that is linear inT due to electron-phonon scattering, whereas in the same temperature range the spin-disorder TEP (S _spd) has been evaluated and found to be linear inT. A divergence in the temperature derivative of resistance and TEP has been found atT _N that is consistent with the present theories. There is evidence of a low-temperature phase appearing at 7 K in the resistivity and the TEP data of GdB₆. A minimum in the TEP is found in these compounds below 6 K, which is associated mainly with phonon drag and possibly a magnon contribution. A broad peak in the TEP of DyB₆ around 16 K is thought to be due to crystalline electric field effects.     

Localized magnetism in YMn2 below the néel temperature

We have measured magnetic susceptibility, thermal expansion, electrical resistivity, and magnetoresistance of the intermetallic compound YMn₂. YMn₂ orders antiferromagnetically belowT _N 110 K. There is a large thermal hysteresis such that the ordering temperature isT _N =86 K on cooling andT _N =116 K on warming. This is a first-order phase transition from the paramagnetic to antiferromagnetic phase. Our electrical resistivity and magnetoresistance measurements in the ordered state show a localized magnetic moment for Mn atoms in YMn₂ belowT _N . The Mn moments collapse atT _N which is reflected in the sharp decrease in the thermal expansivity atT _N . AboveT _N the Mn moment gradually recovers with increasing temperature which is reflected as an increase in with increasing temperature in the paramagnetic state. This behavior atT>T _N could be described by weak itinerant electron magnetism.     

Thermally Assisted Flux Creep of a Driven Vortex Lattice in Untwinned YBa2Cu3O7−δ Single Crystals

We have studied the driven vortex lattice in untwinned, clean YBa ₂ Cu ₃ O _7– single crystals, showing the first order (melting) transition T _m . At high enough driving currents (j 10 ³ A/cm ² , j j _c ) and temperatures T < T _m , a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T _m , associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid.     

A structural model of Cr/(Ba,Pb)TiO3 positive temperature coefficient composite

The over-current protector is one of the main applications of the positive temperature coefficient (PTC) thermistor. Low room-temperature resistance and a PTC effect are required for the use of the over-current protection. As a result, lowering the room-temperature resistivity of PTC materials becomes very important. From a Japanese patent, the method of adding metal to BaTiO₃-based PTC ceramics to form composites has shown good results. But in recent publications, few papers were related to this area. Furthermore, in the limited literature, the resistance–temperature curve of the material expressed a negative temperature coefficient (NTC) effect when metal was present. In the present work, chromium (Cr), was added to (Ba,Pb)TiO₃ ceramics to form PTC composite with higher Curie temperature (T _C = 180°C). Under a given composition and method, the prepared composite had low room-temperature resistivity (p = 1.33cm) and PTC effect (P_max/P_max = 10). From the experimental results obtained, a structural model of the composite is proposed. The co-function of metal and ceramics, and sintering atmosphere factor on the PTC effect are discussed in this model. By employing this model, the resistance–temperature properties of the composites can be explained satisfactorily.     

Theory of electrical resistivity of metal-matrix composites at cryogenic and higher temperatures

This paper presents a theory describing the electrical resistivity (conductivity) in the axial direction of unidirectional fibre-reinforced metal-matrix composite (MMC) materials at cryogenic and higher temperatures. The theory is derived from the solution of Boltzmann's equation. A triple-integral expression is obtained to describe the change in the electrical conductivity in the matrix metal due to electron scattering from the fibre surfaces. It is shown that at cryogenic temperatures, the electrical resistivity can rise by a factor of 2200 over a decrease in temperature of about 6 K below about 10 K. This effect is due entirely to electron scattering from fibre surfaces. The model developed shows that the composite resistivity agrees well with known experimental data at temperatures above 80 K. At very low temperatures, _c (T) _c (T _R) (1–CT ^–4)^–1, whereT is the absolute temperature. Shortcomings and implications of the theory are discussed.     

Electrical properties of exchange-enhanced systems: Fe in (Pd95Rh5)

The electrical resistivity of the giant moment system Pd₉₅Rh₅ containing between 0.8 and 1.85 at % Fe has been measured from 1.4 to 300 K. The incremental resistivity in alloys containing more than 1.25 at % Fe is found to exhibit aT ² limiting low-temperature form; however for the 0.8 and 1.1 at % Fe alloys such aT ² form is not clearly discernible, with (T) exhibiting a temperature dependence intermediate betweenT ² andT ^3/2. The former result is predicted from conduction electron-magnon scattering for which wave vector conservation holds, from which it is inferred that the criterion for wave vector conservation in this type of alloy isnot determined by mean free path effects. Estimates of the acoustic spin-wave stiffnessD are derived from the measuredT ² coefficients. These resistivity data also enable estimates of the exchange coupling parameterJ _s-local to be made. The magnetic ordering temperatureT _c is considerably less discernible than in single-component hosts but approximate values have been derived for the various alloys, from which estimates of the exchange coupling parameterJ _d-local have been made. In the disordered phase the measured incremental resistivity is found to contain a term which decreases approximately linearly with increasing temperature, at a rate of –(1.1±0.45)10^–3 µ cm/K at % Fe. Using existing pressure data on both Pd- and PdRh-based alloys, it is shown that both the sign and magnitude of this term can be accounted for in terms of the volume dependence of the potential and exchange terms, in conjunction with a large coefficient of thermal expansion.This work has been supported in part by the Defence Research Board under Grant No. 9510-100.