Increase In Normal State Resistivity Research Articles

Levitation force of Graphene added bulk MgB2 superconductor

The effect of Graphene addition on levitation/suspension force of bulk MgB2 superconductor has been studied in both zero field cooling (ZFC) and field cooling (FC) conditions at different temperatures. For this, the pristine, 0.5, 1 and 3 wt% of Graphene added bulk MgB2 samples are prepared by solid state reaction method under argon atmosphere. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) of prepared samples have been performed for structural characterization and surface morphology respectively. The structural characterization shows well developed phase of MgB2 while hexagonal shaped grains are evident in SEM photographs. Electrical resistivity measured between 20 K and 300 K confirms a decrease in superconducting transition temperature (TC) and increase in normal state resistivity of pristine MgB2 due to Graphene addition. The levitation force measurement under zero field cooling condition reveals that low level (0.5 wt%) of Graphene addition enhances the levitation force throughout the investigated temperature range. However the levitation force measurements performed at different initial field height under field cooling condition confirm the presence of both attractive and repulsive force. Further, the maximum levitation force (FMLF) and maximum attractive force (FMAF) estimated from the field cooling data of pure and Graphene added MgB2 is found to be a strong function of initial field cooling height (HFC). The maximum repulsive/levitation force (FMLF) increases exponentially with increase in initial field cooling height, while an exponential decrease in attractive force (FMAF) of MgB2 has been observed with increase in initial field cooling height.

Enhanced superconductivity in TiO epitaxial thin films

Titanium oxides have many fascinating optical and electrical properties, such as the superconductivity at 2 K in cubic titanium monoxide (TiO) polycrystalline bulk. However, the lack of TiO single crystals or epitaxial films has prevented systematic investigations on its superconductivity. Here, we report the basic superconductivity characterizations of cubic TiO films epitaxially grown on (0001)-oriented α-Al2O3 substrates. The magnetic and electronic transport measurements confirmed that TiO is a type-II superconductor and the recorded high Tc is about 7.4 K. The lower critical field (Hc1) at 1.9 K, the extrapolated upper critical field Hc2(0), and coherence length are about 18 Oe, 13.7 T, and 4.9 nm, respectively. With increasing pressure, the value of Tc shifts to lower temperature while the normal state resistivity increases. Our results on the superconducting TiO films confirm the strategy to achieve higher Tc in the epitaxial films, which may be helpful for finding more superconducting materials in various related systems.

Effects of Ni and Co doping on the physical properties of tetragonal FeSe 0.5Te 0.5 superconductor

Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature ( T c ) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the T c increases and reaches a maximum for FeSe 0.5Te 0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe 0.5Te 0.5 in the nominal composition range Fe 1− x TM x Se 0.5Te 0.5 (TM = Co ( x = 0.05, 0.1, 0.15, 0.2) and Ni ( x = 0.05, 0.1)). Both Co and Ni doping suppress T c and drives the system to metal–insulator transition. The in-plane (‘ a’) and out-of-plane (‘ c’) lattice constants decrease with increasing dopant concentration.

Charge carrier localization and metal to insulator transition in cerium substituted (Bi,Pb)-2212 superconductor

Abstract We report on the details of carrier localization and metal to insulator transition in (Bi,Pb)-2212 by substituting Ce at its Sr site (0.2 ≤  x  ≤ 1.0). Structural and compositional analysis confirm good homogeneity of Ce distribution in the (Bi,Pb)-2212 matrix. The resistivity measurements show that x  = 0.2 sample is a superconductor while those with x  > 0.2 are insulators. Resistivity in the insulating regime (0.4 ≤  x  ≤ 1.0) is analyzed using a generalized hopping approach. The increase in normal state resistivity and disorder of the samples leads to a metal to insulator transition at 0.2  x  ≤ 0.4 and the insulating phases exhibit Mott's variable range hopping phenomenon. Around the transition, a dimensional change over from two to three dimensions is observed in the variable range hopping regime. Results show that substitution reduces the carrier concentration of (Bi,Pb)-2212, leads to suppression of superconductivity and metal to insulator transition in the system by localizing the electronic states at the Fermi level.

The charge transfer induced by Cr doping in MgB 2

Mg 1− x Cr x B 2 polycrystal bulk samples with 0 ⩽ x ⩽ 5% have been synthesized by a solid-state reaction and studied by X-ray diffraction, SEM and Raman spectrum. It is found that the c-axis of the lattice decreases as the Cr content increases, while the a-axis remains unchanged. Moreover, crystal grain size increases apparently with Cr doping concentration increase. The normal-state resistivity increases and the superconducting transition temperature ( T c) decreases from 38.2 K ( x = 0) to 35.1 K ( x = 0.03) with the increase of Cr content. It is suggested that the charge transfer between the Mg-layer and the B-layer causes the decrease of the charge carrier concentration and induces the changes of T c and normal-state resistivity. On the other hand, by the Raman scattering study, it is found that the linewidth of Raman spectrum increases with the increase of Cr content, which is resulted by the competition between the electron–phonon interaction and substitution-induced disorder. The Raman peak has no evident shift due to the countervailing between the effects of the electron–phonon coupling and the grain size.

The interplay of superconductivity and localization in Nd 2− xCe xCuO 4+ δ single crystal films

The influence of nonstoichiometric disorder on the in-plane resistivity and SC-transition has been investigated for Nd 2− x Ce x CuO 4+ δ single crystal films ( x=0.15 and 0.18). It is shown that with increasing of δ the in-plane normal state resistivity increases (the mean free path diminishes) and SC-transition temperature decreases with essential broadening of the transition region. The observed evolution from homogeneous metallic (and superconducting) Nd 2− x Ce x CuO 4+ δ system to inhomogeneous dielectric one is described as Anderson-type disorder-induced transition in a two-dimensional electron system.

Magnetic properties and metal-non metal transition in the YBa 2Cu 3−xLi xO z system

Lithium substitutes copper of the YBCO-structure inducing specific magnetic effects which depend on the concentration range and on the oxygenation degree. With the increase of Li-content and oxygen deficiency, Tc decreases and the normal state resistivity increases. The susceptibility of the air-annealed and moderately concentrated samples obeys the Curie or modified Curie law showing the formation of magnetic moments around the lithium ions with 0,45μ B/at.Li. The results are compared with the local magnetic properties previously investigated by 7Li NMR experiments.

Thermopower of ion-irradiated YBa2Cu3O7??-films

Experimental data of the temperature dependence of the thermoelectric power S(T) for differently doped HTSC-samples recently found renewed interest in the context of the van Hove scenario as well as the spin gap behavior. Furthermore, a universal relation between S(290K) and the hole concentration p has been reported. On the other hand, ion irradiation of HTSC- films with ions of medium energy (typically 350 ke V) is known to produce defects leading to a strong increase in normal-state resistivity and a depression of the superconducting transition temperature Tc. The idea of the present work is to combine both techniques to test whether the ion-induced defects are predominantly oxygen vacancies within the CuO-chains, reducing p as monitored by S(290K). For this purpose, c-axis oriented YBa2Cu3O7-films on (100)-SrTiO3 substrates were irradiated with 350 ke V He+-ions. After each irradiation step an XRD analysis and an S(T)-measurement for Tc < T < 295 K is performed. It is shown that for small fluences the ioninduced S(T)-changes can be attributed to oxygen displacements destroying the CuO-chains, while for higher fluences additional defects within the CuO2-planes have to be taken into account. The corresponding S(T) behavior is comparable to what has been reported for Zn-substituted underdoped HTSC-samples.

Structural and superconducting properties of Bi 2Sr 2Ca 1− xCe xCu 2O y

The structural and superconducting properties of Bi 2Sr 2Ca 1− x Ce x Cu 2O y (0≤x≤0.4) system are reported. Slow increase in normal state resistivity and decrease in superconducting transition temperature, T c is observed with increasing x for x ≤ 0.1. The T c depression rate is unusually high in comparison to the other rare earth dopants at Ca site for 0.1 < x <0.2. Metal-Insulator transition is observed at around x=0.2. The conduction mechanism in the semiconducting samples is found to be Variable Range Hopping. However, it is not possible to distinguish whether the hopping is in 2D or in 3D. From the decrease of c-lattice parameter with increasing x, the existence of Ce predominantly in +4 valence state is inferred. The hole filling mechanism is likely to be responsible for the T c depression and Metal-Insulator transition.

STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF Bi2Sr2Ca1−xPrxCu2Oy

Solid solutions of Bi 2 Sr 2 Ca 1−x Pr x Cu 2 O y (0 ≤ x ≤ 0.6) have been synthesized. The crystal structure and superconducting properties of the samples were examined by XRD and dc electrical resistivity measurements. Linear increase in normal state resistivity is observed with increasing Pr-content (x) for 0 ≤ x ≤ 0.55. Decrease in T c is observed with increasing x and metal–insulator transition is observed around x = 0.55. Although increase in normal state resistivity leads to decrease in T c value, we do not find any definite correlation between these parameters. The conduction mechanism in the semi-conducting sample is found to be Variable Range Hopping. From the decrease of c lattice parameter the possibility of existence of Pr ions in +4 state besides +3 state is inferred.

Irradiation effects in Ba 2YCu 3O 7 superconductor

Superconducting property changes induced by electron, neutron and ion irradiations in Ba 2YCu 3O 7 pellets and thin films were reviewed in consideration of the critical current density enhancement with radiation-produced defects. The transition temperature decreases almost proportionally to the fluence concurrently with a linear increase in normal state resistivity at an early stage of the irradiations. The ratio of the degradation rate for the transition temperature to that for the resistivity is in the range from 0.4 to 0.8 K/μΩ·m in the pellets irradiated with MeV-electrons and reactor-neutrons and 0.1 to 2 K/μΩ·m for high quality thin film irradiated with 600 keV Ar-ions. The critical magnetization current density in sintered pellets is increased by irradiation with electrons, neutrons and high-energy Xe ions to relatively low fluences where the transition temperature is decreased by about 1 K or less. The enhancement is significant in higher magnetic fields for the electron and neutron irradiated samples. Furthermore, the critical transport current density is increased in magnetic fields more than 1 T in an oriented thin film irradiated with 25 MeV 16O ions to a fluence that decreases the transition temperature by about 3 K.

Transition to zero dimensionality in granular aluminum superconducting films

Granular aluminum films prepared by evaporation in the presence of oxygen show two different behaviors when the oxygen content is increased. First, the average grain size (as measured by the dark-field electron microscopy technique) is reduced and the critical temperature is increased. This increase is neither due to a variation of the lattice parameter which, following our measurements, stays constant and equal to the value of pure Al, nor to a purely electronic effect linked to the small grain size, because in this first regime the grains are essentially in good electrical contact. In a second stage, the average grain size changes little but the mean free path becomes much smaller than the grain size, indicating a build up of insulating barriers between the grains. The critical temperature decreases and the transition becomes broader as the normal-state resistivity increases. We attribute this behavior to the appearance of large critical fluctuations, due to the fact that the decoupled grains tend to have a zero dimensionality.