Carriers In CuO Research Articles

Si doped Cu 0.5Tl 0.5Ba 2Ca 2Cu 3O 10− δ superconductors

The Si doped (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3− y Si y O 10− δ ( y=0, 0.25, 0.5, 0.75, 1.0, 1.25) superconductor samples have been synthesized to investigate the effect of mobile carriers on the critical temperature and magnitude of diamagnetism of this family of cuprates. The Si doped samples have tetragonal structure as observed from the X-ray diffraction spectra. The c-axis length of the unit cell of (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3− y Si y O 10− δ was increased after the increase of Si concentration, whereas the critical temperature and the magnitude of diamagnetism have been decreased. The decrease in T c(0) and magnitude of diamagnetism is possibly due to the deficiency of carriers in CuO 2/SiO 2 planes caused by the +4 state of Si atoms. However, the post-annealing of these samples in oxygen atmosphere has increased the critical temperature and the magnitude of diamagnetism. The FTIR absorption measurements of (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3− y Si y O 10− δ samples have shown a softening of the apical oxygen mode of the type Cu(1)O ACu(2) and TlO ACu(2) with increased concentration of Si in the unit cell; the softening of this mode is directly linked with the increase in the c-axis lattice parameter of the (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3− y Si y O 10− δ superconductor.

Studies of fluctuation induce conductivity of Mg doped Cu 0.5Tl 0.5Ba 2(Ca 2− yMg y)(Cu 0.5Zn 2.5)O 10− δ ( y=0, 0.5, 1.0) superconductors

The electrical resistivity ρ( T) versus temperature data of as-prepared and O 2-annealed Cu 0.5Tl 0.5Ba 2(Ca 2− y Mg y )(Cu 0.5Zn 2.5)O 10− δ ( y=0, 0.5, 1.0) samples is analyzed for fluctuations induced conductivity analysis in the temperature regime well above the critical temperature. The analysis has been carried out employing Aslamazov and Larkin (AL) and Lawrence and Doniach (LD) models. The coherence length, inter-planar coupling, exponent and dimensionality of fluctuations are determined from this analysis. It has been observed in as-prepared and oxygen post-annealed samples that the crossover temperature associated with two distinct exponents and the excess conductivity data fits very well with the two-dimensional (2D) and three-dimensional (3D) AL equations. The cross-over temperature T o is shifted to lower temperatures with enhanced Mg doping. The width of 3D AL region is reduced with increased Mg doping, however, some improvement is observed after post-annealing in the oxygen atmosphere. Moreover, a decrease in the coherence length along the c-axis is observed with the doping of Mg in the final compound. We have elucidated from these analyses that higher thallium contents in the final compound promote a decrease in the density of mobile carriers in CuO 2 planes, which suppresses the coherence length along the c-axis and superconductivity parameters.

The (Cu 0.5Tl 0.5)Ba 2O 4−δ charge reservoir layer as an efficient source of carriers to the CuO 2 planes: Doping of Bi in (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3O 10−δ superconductors

In order to study the higher carrier’s doping efficiency of (Cu 0.5Tl 0.5)Ba 2O 4− δ charge reservoir layer to the conducting CuO 2 planes we have incorporated Bi in (Cu 0.5Tl 0.5)Ba 2Ca 2Cu 3O 10− δ superconductors; the presence of Bi in the charge reservoir layer promotes highest anisotropy to these compounds. For such studies, we have successfully synthesized (Cu 0.25Tl 0.5Bi 0.25)Ba 2Ca 2Cu 3O 10− δ , (Bi 0.5Tl 0.5)Ba 2Ca 2Cu 3O 10− δ , (Cu 0.25Tl 0.25Bi 0.25Li 0.25)Ba 2Ca 2Cu 3O 10− δ and (Cu 0.25Tl 0.25Bi 0.25Li 0.25)Ba 2Ca 2− y Be y Cu 3O 10− δ superconductors at normal pressure and studied their superconducting properties. These studies have shown that incorporation of Bi at the charge reservoir layer promoted a decrease in the density of carriers in the conducting CuO 2 planes which in turn suppressed the magnitude of diamagnetism of final compound. The decreased density of the carriers in the CuO 2 planes, which most likely promotes inferior superconducting properties, was replenished by doping of Li in the charge reservoir layer and optimizing their number by post-annealing in oxygen atmosphere. It is observed that Bi doping in (Cu 0.5Tl 0.5)Ba 2O 4− δ charge reservoir layer promotes an increase in the c-axis length which in turn increase the anisotropy and decrease the Fermi-vector [ k F = (3 π 2 N/ V) 1/3] and Fermi-velocity [ v F = ( π ε c Δ / ℏ ) ] of the carriers. We have decreased the c-axis length of Bi doped compounds by doping Be at the Ca sites in (Cu 0.25Tl 0.25Bi 0.25Li 0.25)Ba 2Ca 2− y Be y Cu 3O 10− δ superconductors. It is observed from these experiments that Beryllium doping of y = 0.5 has produced excellent superconducting properties in terms of enhancement of T c( R = 0) and magnitude of diamagnetism in this compound. A maximum decrease in the unit cell volume is observed with this Be doping concentration, which enhance k F, v F and superconducting order parameter of the carriers and hence promote the enhancement of superconducting properties. These studies have also shown that the role of density of the carriers in CuO 2 planes is pivotal in the mechanism of superconductivity in these compounds and Bi doping significantly affects them due to its presence in the charge reservoir layer.

The role of Nb substitution in Cu 1− xTl xBa 2Ca 2− yNb yCu 3O 10− δ ( y = 0, 0.25, 0.5, 0.75, 1.0) superconductor

The effects of higher electro-negativity Nb substitution at the Ca site in Cu 1− x Tl x Ba 2Ca 2− y Nb y Cu 3O 10− δ [Cu 1− x Tl x Nb y -1223] ( y = 0.25, 0.5, 0.75, 1.0) superconductor have been investigated. It is observed from these studies that lower doping concentration of Nb ( y = 0.25) improves the inter-plane coupling, however, coupling of CuO 2 planes is reduced with higher Nb doping. The higher concentration of Nb increases the density of oxygen in the charge reservoir layer and a material with over-doped carriers in CuO 2 planes is formed. The post-annealing in a N 2 atmosphere has been found to improve the superconducting properties of the final compound, however, O 2 annealing promotes the formation of material with inferior superconducting properties. The magnitude of diamagnetism is increased with post-annealing in a nitrogen atmosphere; as N 2 annealing helps in the formation of material with optimum carrier concentration in CuO 2 planes which is possibly done by the change in charge state of thallium from Tl 3+ to Tl 1+.

Transport properties and Raman scattering studies of superconducting ferromagnet Ru1-xAlxSr2GdCu2O8

Transport properties and Raman spectra of Ru 1-x Al x Si 2 GdCu 2 O 8 (Al-doped Ru-1212) with x=0, 0.05, 0.1 and 0.15 have been investigated. The superconducting transition temperature of the Al-doped Ru-1212 increases with increasing Al content up to 10 %, and then decreases with increasing Al content. The observed result seems to indicate that excess holes induced by Al doping could transfer from RuO 2 plane to CuO 2 plane through apical oxygen for 0 0.1, the holes might be trapped or localized in the RuO 2 plane due to a local disorder induced by Al doping. The temperature dependence of the critical field can be fitted as H c2 (T) = 34(1-T/Tc) 4.8 , suggesting that the vortex dynamics of Ru-1212 might be intrinsically different from that of the other high-T c cuprates. The largest magnetoresistance (MR) of the Al-doped Ru-1212 occurs around magnetic ordering temperature, indicating that the exchange interaction between the carriers in CuO 2 planes and the spins in RuO 2 planes dominates the magneto-transport properties of the Ru-1212 system. The peak position of the 150 cm -1 involving Cn vibration along the c-axis hardly changes as x increases. It reveals that Al doesn't occupy Cu site. A new Raman active mode around 550 cm -1 observed in the Al-doped compounds might be attributed to doping-induced A 1g -like apical oxygen mode.

The electronic structure of CuO2 plane

A theoretical model is developed to describe the electronic structure and the motion of the carriers in CuO 2 plane. The ground state of the CuO 2 plane without extra carriers is described by a many-particles wave function which consists of the background Cu 2+( O 2−) 2 and a series of virtual electron-hole ( Cu 1+ O 1−) pairs. Extra holes mainly occupy O sites but required to orthogonalize to the valence band wave function and undergoes exchange scattering on the virtual e-h pairs. By this model a consistent description of the interactions between the carriers and the spin fluctuations of Cu ions, the electronic phase separation and the evolution from the correlated metals to normal metals, etc. can be obtained.

Chemical pressure effects on the optimization of T C in T′ electron doped superconducting cuprates

We report a systematic variation of the superconducting properties with the Ln 3+ size and x in (Ln) 2−xCe xCuO 4−δT′ compounds. Progressive higher T C's with increasing interatomic Cu-O distances are found at the rate dT C0/d(d Cu−O)∼;240 KÅ −1 in x-optimized samples, having the same density of carriers in CuO 2 layers. Charge transfer mechanisms have been isolated from all the rest modifying T C under pressure. The first can positively contribute to T C in overdoped samples, while the second are estimated to yield dT C0/dP∼;-0.11 K/Kbar.

The Kondo anomaly in the low temperature specific heat of Pr doped YBCO

Low-temperature specific heat measurements on Y 1−xPr xBa 2Cu 3 O 7 as a function of temperature T for X=0.2, 0.45, 0.6 with 90mK<T< 30K are reported. The specific heat of this system consists of a nuclear Schottky anomaly, a Pr magnetic Kondo anomaly and the background contribution. The large linear term of specific heat has been found to be connected with Kondo scattering of the mobile hole carriers by Pr magnetic moments. The spin-1 character of the Kondo specific heat anomaly indicates that the Pr valence state is close to +3 in the Y 1−xPr xBa 2CuO 7 system. The Tc depression and metal-insulator transition are believed to be attributed to the localization due to the interaction of Pr 4f electrons with hole carriers in CuO 2 planes.

Range of thet-Jmodel parameters forCuO2planes: Experimental data constraints

The t-J model effective hopping integral is determined from the three-band Hubbard model for the charge carriers in CuO$_{2}$ plane. For this purpose the values of the superexchange constant $J$ and the charge-transfer gap $E_{gap}$ are calculated in the framework of the three-band model. Fitting values of $J$ and $E_{gap}$ to the experimental data allows to narrow the uncertainty region of the three-band model parameters. As a result, the $t/J$ ratio of the t-J model is fixed in the range $2.4 \div 2.7$ for holes and $2.5 \div 3.0$ for electrons. Formation of the Frenkel exciton is justified and the main features of the charge-transfer spectrum are correctly described in the framework of this approach.