Effect of Fe and Co doping on electrical and thermal properties of La0.5Ce0.5Mn1−x(Fe,Co)xO3 manganites

The effect of Fe and Co doping on structural, electrical and thermal properties of half doped La_0.5Ce_0.5Mn_1−x(Fe, Co)_xO₃ is investigated. The structure of these crystallizes in to orthorhombically distorted perovskite structure. The electrical resistivity of La_0.5Ce_0.5MnO₃ exhibits metal-semiconductor transition (T_MS at ∼225 K). However, La_0.5Ce_0.5Mn_1−xTM_xO₃ (TM = Fe, Co; 0.0 ≤ x ≤ 0.1) manganites show semiconducting behavior. The thermopower measurements infer hole as charge carriers and electron–magnon as well spin wave fluctuation mechanism are effective at low temperature domain and SPC model fits the observed data at high temperature. The magnetic susceptibility measurement confirms a transition from paramagnetic to ferromagnetic phase. The observed peaks in the specific heat measurements, shifts to lower temperatures and becomes progressively broader with doping of transition metals on Mn-site. The thermal conductivity is measured in the temperature range of 10–350 K with a magnitude in between 10 and 80 mW/cm K.     

Thermoelectric properties of GdBaCo2−x Fe x O5+δ ceramics

The influence of Fe doping on the lattice structure and thermoelectric properties of GdBaCo_2?x Fe _x O_5+δ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ceramics was studied from room temperature to 525 K. The results show that presence of Fe will increase both electrical resistivity and Seebeck coefficients of the samples. Due to the lattice misfit and carrier concentration reduction caused by Fe, the thermal conductivity of the sample decreases. The activation energy of conductivity is larger than that of thermopower calculated in the semiconductive region, which means that the conduction mechanism may be determined as a small polaron hopping model. The optimum Fe doping amount is x = 0.6, which results in a ZT value 0.02 at 373 K.     

Electrical transport properties of Nd0.67−x Eu x Sr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd_0.67−x Eu _x Sr_0.33MnO₃ (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.     

Effect of Vanadium-Titanium Co-doping on the BPSCCO Superconductor

We have produced the (BiPb)₂V _x Sr₂Ca₃Cu_4?y Ti _y O_12+?? compounds for x=0.05 and y=0, 0.05, 0.10 and 0.20 by glass-ceramic method. The effects of vanadium adding and Ti doping on the structure have been investigated by electrical resistance, scanning electron micrographs (SEM), XRD patterns and magnetic hysteresis loop measurements. It has been found that the high-T _c superconducting phase, (2223), is formed in the samples annealed at 845?°C for 185, with concentration x=y=0.05. However, with increasing Ti doping the (2223) phase transforms into the (2212) phase. We have observed no superconducting properties for x=0.05 and y=0.20 compound. It has completely transformed to semiconductor. In addition, the critical current densities (J _c), calculated from the hysteresis loop measurements by using Bean??s critical state model are obtained for the samples in the same doping range. Our data have indicated that J _c decreases with increasing temperature and Ti concentration.     

High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4

Sn doping in an n-type transparent conducting oxide MgIn₂O₄ is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg_1+xIn_2−2xSn_xO₄ for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10⁻⁴ K⁻¹ for the parent compound at 600 K.     

Two Pseudogap Behavior in La2−x Sr x CuO4: Thermoelectric Power at High Temperature

Thermoelectric power (TEP) of high-T _C superconductors has been investigated in a wide range of temperature (T _C < T < 700 K) for La_2?x Sr _x CuO₄. TEP of La_2?x Sr _x CuO₄ shows different temperature dependences in three temperature regions. In the low-temperature region, a positive broad TEP peak is observed near T _p, which shifts to lower temperature upon doping. As temperature increases, TEP decreases linearly at intermediate temperature. In the high-temperature region, TEP deviates from the linear temperature dependence at a certain temperature, T _h showing a saturation behavior. As the doping concentration increases, the characteristic temperatures, T _C, T _p, and T _h, show systematic changes. In comparison with pseudogap temperature estimated from other experiments, the large pseudogap behavior in TEP at high temperature has been discussed and distinguished from the small pseudogap observed at lower temperature. A possibility of bound pairs formation in the normal state opening the pseudogap at high temperature is discussed briefly. The coexistence of bound pairs and the normal independent carriers for T _C < T < T_h could be the origin of the intrinsic inhomogeneity.     

Effect of Ho-doping on structural,electrical and magnetic properties of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics prepared by Plasma-Activated Sintering

Different components of La_0.7?x Ho _x Sr_0.3MnO₃ (LHSMO, x = 0, 0.1, 0.2, 0.3) ceramics were fabricated by Plasma-Activated Sintering (PAS), so as to study the correlation between the contents of Ho³⁺ and the structural, electrical, magnetic properties. XRD and SEM confirmed that LHSMO ceramics prepared by PAS exhibited high-purity phase and dense microstructure. The measurement of electrical resistivity showed that the resistivity of LHSMO ceramics increased, and the metal–insulator transition temperature decreased with the increasing Ho-doping content. The resistivity data were then fitted using various empirical equations, and the conduction mechanism of LHSMO ceramics was found to be in accord with the electron–magnon scattering process in the low-temperature region and the small polaron hopping model in the high-temperature region. Lastly, we calculated the values of magnetoresistance of the LHSMO ceramics, which increased with increasing Ho-doping content, from 3.5% for x = 0 to 14.6% for x = 0.3. Therefore, the doping of Ho³⁺ into La_0.7Sr_0.3MnO₃ can effectively enhance the low-field magnetoresistance effect.     

Physical, Mechanical and Magnetic Properties of the Yb-Substituted Bi2Sr2Ca1Cu2O y Textured Superconductor

In this study, samples with nominal composition of Bi₂Sr₂Ca₁Cu_2?x Yb _x O _y , where x=0.0, 0.05, 0.1 and 0.25, have been textured using the Laser Floating Zone technique. The effects of different Yb doping on the structure have been investigated by electrical resistivity, scanning electron microscopy, XRD, mechanical and dc-magnetization techniques. It has been found that with increasing Yb³⁺ doping for Cu, the physical and superconducting properties of structure gradually decrease. J _C values, calculated using the Bean??s model, significantly change, although T _C is approximately the same for all samples.     

Critical Behavior in the Low-Energy Spin Fluctuations in High-T c Superconductors

We study the spin fluctuations of pure and Zn-substituted La_{2 ? x} Sr _x CuO₄ using the muon spin relaxation technique. Superconductivity is found to coexist with low-frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by μSR decreases with increasing x and vanishes above a critical doping x _c ≈ 0.19. This value of x _c coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states.     

Elastic and Thermal Properties of Double Doped Orthovanadate: Eu1−x (La0.254Y0.746) x VO3

We present the effect of A-site double doping (La_0.254Y_0.746) on elastic and thermal properties of EuVO₃ in a wide doping range (0≤x≤1), using a modified rigid ion model (MRIM). Various lattice distortions and their relation to bulk modulus has been investigated systematically. The effect of the lattice distortions on thermodynamic properties of Eu_1?x (La_0.25Y_0.74) _x VO₃ has been explored by an atomistic approach. The computed results emphasize that the Debye temperature decreases while the specific heat increases with increase in doping concentration (x). The computed temperature dependent (1 K≤T≤300 K) specific heat trends are in accordance with the corresponding experimental data at various compositions. Future scope of MRIM has also been discussed.     

Superconductivity of Ba1 − x K x BiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 ? x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

Low-temperature thermoelectric and magnetic characteristics of Ca2.9Bi0.1Co4−xFexO9+δ (0 ≤ x ≤ 0.10)

The effect of Fe ion doping on the low-temperature thermoelectric and magnetic properties of Ca_2.9Bi_0.1Co_4?xFe_xO_9+δ (x = 0.00, 0.025, 0.05 and 0.10) have been investigated. The samples were prepared by conventional solid-state synthesis. The X-ray diffraction patterns revealed that all the samples are single phase. The electrical resistivity results indicated that all the samples obey the variable range hopping in the low temperature regime. The T^* (transition temperature from Fermi liquid metal to incoherent metal) was increased and the slope of A value (Fermi-liquid transport coefficient) was decreased with increasing Fe content. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity, thermopower and total thermal conductivity were decreased with increasing Fe content. Among the doped samples, Ca_2.95Bi_0.10Co_3.90Fe_0.10O_9+δ had the highest dimensionless figure of merit of 0.056 at 300 K. Magnetic measurements indicated that all the samples exhibit a low-spin state of cobalt ion. The effective magnetic moments were decreased with increasing Fe content.     

Magnetic and Transport Properties of Single Crystalline K 0.8Fe 2 − x Ir x Se2

A series of K_0.8Fe_2?xIr _x Se₂ single-crystal compounds with nominal compositions x = 0, 0.01, 0.03, 0.05, 0.07, and 0.1 were successfully synthesized using self-flux method. It is found that the Ir doping in the present system will induce a suppression of the c-axis lattice constants. However, the zero-resistance temperatures, \(\mathrm {T}_{\mathrm {c}}^{\text {zero}}\) , are nearly maintained around 32 K for the samples with x ≤ 0.05, and it drops to 29 K for the one with x = 0.07. The superconductivity vanishes when the doping level reaches to 0.1. The hump in resistivity which corresponds to the transition from the insulating to metallic phase shifts towards low temperatures. However, the magnitude of the normal-state resistivity above 100 K for Ir-doped samples with x ≤ 0.05 is smaller than the one of the undoped sample, indicating the doping at Fe vacancy site. An onset transition temperature of 44 K has been observed in the ρ-T curve for K_0.8Fe_1.95Ir_0.05Se_2.     

Wide gap p-type degenerate semiconductor: Mg-doped LaCuOSe

Hole transport and optical properties were investigated on undoped and Mg-doped LaCuOS_1−xSe_x (x=0-1) epitaxial films. Both electrical conductivity and Hall mobility were found to increase monotonously with increasing Se content in the films. The increase in Hall mobility is considered to be associated with the increase in valence band dispersion. Mg ion doping increased hole concentrations in the undoped films by an order of magnitude to ∼2×10²⁰ cm⁻³, while Mg doping reduced mobility to merely half that of undoped films. The results suggest that hole scattering due to Mg impurity ions is suppressed by natural modulation doping originating from the layered structure of LaCuOS_1−xSe_x. Hole concentrations showed no temperature dependence, indicating degenerate conduction. The largest value for conductivity, 140 S cm⁻¹, was obtained with Mg-doped LaCuOSe epitaxial film. Accompanying characteristics included moderately high optical transparency in the visible region and blue photoluminescence.     

Thermoelectric power factor of Ti<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NiSn alloys

Ti_{1 − x} V _x NiSn (x = 0–0.10) substitutional solid solutions have been prepared by doping the intermetallic semiconductor n-TiNiSn (half-Heusler phase) with vanadium, a donor impurity, and their resistivity and thermopower have been measured at temperatures from 80 to 380 K. The results demonstrate that, when doping of TiNiSn causes no type inversion, the thermoelectric power factor of the solid solution markedly exceeds that of the undoped ternary compound.     

Effect of Cr-Doping on the Magnetic State of?Er0.55Sr0.45Mn1?x Cr x O3

Magnetic measurements performed on Er_0.55Sr_0.45Mn_1?x Cr _x O₃ revealed an antiferromagnetic ground state at low temperatures (??4.2 K). Curie-type behavior has been used to investigate the effects of Cr-ions doping on the magnetic state. We found that Cr-doping weaken the antiferromagnetic exchange interaction, and the magnetic state gradually transforms to ferromagnetic state near x=0.1 doping level. However, Arrott?CKouvel plots did not reveal the formation of spontaneous moment with increasing Cr-concentration.     

Synthesis,Characterization and ESR Studies of Zn1−x Co x O Nanoparticles

Zn_1?x Co _x O polycrystalline nanoparticles with different (x=0.03, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3) compositions were synthesized using the sol-gel technique. The effects of doping ratio and annealing temperature on structure and magnetic properties were investigated systematically. The phase, crystal structure, and microstructure of the Zn_1?x Co _x O nanoparticles were characterized using X-Ray diffraction and scanning electron microscope. Electron spin resonance spectra of Zn_1?x Co _x O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz.     

Low temperature synthesis and properties of MgxZn1?x O:Al UV transparent conducting films by ultrasonic spray pyrolysis

Mg _x Zn_1?x O:Al UV transparent conducting films were synthesized by ultrasonic spray pyrolysis at low temperature. Effects of Mg doping content and substrate temperature on microstructure, electrical and optical properties of Mg_xZn_1?x O:Al films were investigated. With increasing Mg content x from 0.1 to 0.4, the preferred orientation of (002) in Mg_xZn_1?x O:Al films became weak and the grains became to lenticular-like. The electric resistivities of Mg _x Zn_1?x O:Al films with x?<?0.2 are lower than those of ZnO:Al films, but it obviously increases with the increase of Mg doping contents when x?>?0.2. Highly preferred orientation of (002) in Mg_xZn_1?x O:Al films has been observed when the substrate temperature is over 450?°C. Though the Mg doping content and substrate temperature have no significant influence on the optical transmission of Mg _x Zn_1?x O:Al films, the band gap increases and the absorption edge systematically shifts towards the shorter wavelength with the increasing Mg doping contents, which indicate that the transparent spectrum range of Mg _x Zn_1?x O:Al thin films extend to the ultraviolet band.     

The Effect of Co Substitution on the Crystal Structure and Electrical Resistivity of (La0.85Ag0.15)MnO3 Compounds

The effect of Co substitution on the crystal structure and electrical transport properties of La_0.85Ag_0.15Mn_1−y Co _y O₃ compounds (0≥y≤0.50) has been studied. Structural transition from rhombohedral to orthorhombic symmetry has been observed with Co doping. The lattice parameters are found to increase with doping up to y=0.20, then it decreases. It is explained on the basis of transition from high spin state to low spin state of Co ions with increase in doping beyond y=0.20. Ferromagnetic (FM) metallic behavior with colossal magneto-resistivity has been observed up to y=0.10. However, for y≥0.15 compounds, the temperature dependence of resistivity ρ(T) follows semiconducting behavior. The electrical resistivity in the metallic region could be explained based on electron–electron and electron–magnon scattering mechanisms. The data in the semiconducting region could be explained based on the variable range hopping model for y=0.2 and adiabatic small polaron hopping model for y≥0.3.      

Ultrasonic, Transport, and Magnetic Properties of Nd0.5Sr0.5Mn1−x Cr x O3 Perovskites

The resistivity, magnetization and longitudinal ultrasonic velocity (V _l) have been measured in single-phase polycrystalline Nd_0.5Sr_0.5Mn_1?x Cr _x O₃ (x=0, 0.02, 0.05, 0.1) to clarify the Cr doping effect on the metal-insulator and charge ordering transitions. For x=0 sample, the V _l shows two large stiffening just below the metal-insulator transition temperature T _MI and the charge ordering temperature T _CO, respectively. It is suggested that the former ultrasonic anomaly may originate from the spin-lattice effect and the dynamic Jahn-Teller effect, and the latter is due to the electron-lattice effect arising from the static Jahn-Teller effect of Mn³⁺. With the Cr doping, the T _MI shifts to lower temperature, the maximum of magnetization becomes smaller and the resistivity remains metallic type. Furthermore, only one stiffening of ultrasonic velocity is observed below T _MI in these doping samples. The analysis suggests that the Cr doping enhances the double exchange between Mn³⁺ and Mn⁴⁺, and thoroughly destroys the charge ordering state.