Enhanced Interlayer Coupling of CuO2 Planes Promote the Superconducting Properties of Cu0.5Tl0.5Ba2-YSrYCa2Cu3O10-δ (Y = 0, 0.15, 0.25) Samples

The CuTl-1223 superconducting samples, doping the Sr atom at Ba site, have been synthesized at 860oC pressure. The charge reservoir layer (CRL) of Cu0.5Tl0.5-1223 superconductor is modified by doping Sr atom. The decrease in c-axis length which is most probably due to smaller size of Sr atom as compared to Ba. The substitution of Sr atom at Ba is confirmed by the Fourier Transform Infrared Spectroscopy (FTIR). The critical temperatures i.e., Tc(R=0). Tconset are increased with the Sr content which shows that superconducting magnitude enhanced. The excess conductivity analysis has been done using Aslamazov-Larkin and Lawrance-Donaich models. The crossover temperatures i.e., TCR-3D=TG, T3D-2D and T2D-SWF and c-axis coherence length ξc(0) are slanted to lower values. Moreover, the inter-plane coupling (J) increases due to decrease in c-axis length. From fluctuations induced conductivity, it is found that there is an inverse relationship between critical temperatures and coherence length. *Corresponding author: PMAS Arid Agriculture University Attock Campus, Pakistan, Tel: 920519292122; E-mail: m.usman@uaar.edu.pk Received August 11, 2016; Accepted November 10, 2016; Published November 20, 2016 Citation: Muzaffar MU, Khan NA, Rehman UU, Ali SA (2016) Enhanced Interlayer Coupling of CuO2 Planes Promote the Superconducting Properties of Cu0.5Tl0.5Ba2YSrYCa2Cu3O10-δ (Y=0, 0.15, 0.25) Samples. J Material Sci Eng 5: 298. doi: 10.4172/2169-0022.1000298 Copyright: © 2016 Muzaffar MU, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
The cup-rate intrinsic superconducting parameters are structure dependent.There are two parts i.e., i) MBa 2 O [M=Cu, Tl, Bi, Hg, C] a charge reservoir layer (CRL) and ii) conducting copper oxide planes nCuO 2 [1] of general unit cell in HTSC.In unit cell, the CRL provides the carriers (cooper pairs) to the cooper oxide planes and due to these carriers superconductivity exit [2][3][4].
Hence, the modification in CRL has a vital role in superconducting properties.In order to enhance the magnitude of superconductivity, numerous scientists have tried to modification in CRL [Co, Fe, Al] [ [5][6][7] and in CuO 2 planes [Zn, Ni] [8][9][10].The increase in anisotropy and reduce in inter-plane coupling may be possible due to thicker CRL.Although, in periodic table, both atoms i.e., Sr and Ba lie in the same group but Sr atom is smaller in ionic radius (1.12 Å) as compared to Ba (1.35 Å).It is expected that dopant Sr atom would help to squeeze the CRL for enhanced interlayer coupling and hence improved the efficiency of CRL to the conducting CuO 2 planes.In contrast to the fixed Cu valence (∼2+) in the Tl-bilayer cuprate superconductors, the average formal valence of Cu in the Tl-monolayer compounds TlBa 2 Ca n−1 Cu n O 2n+3 varies as (2+n−1)+.This characteristic is reflected in linear augmented-plane-wave band-structure results for the simplest n=1 member of this Tl-monolayer homologous series, TlBa 2 CuO 5 , where the filling (∼0.16) of the planar Cu(3d)-O(2p) σ* band is reduced well below one-half.It is shown that the 50-50 Ba-La alloy is an appropriate ''parent'' compound for this n=1 phase since the halffilled-band condition is restored.For any member of this Tl-monolayer series, the optimal doping for high-temperature superconductivity should involve a combination of structural and chemical contributions.

Synthesis
The superconducting samples Cu 0.5 Tl 0.5 Ba 2-Y Sr Y Ca 2 Cu 3 O 10-δ (y=0, 0.15, 0.25) were synthesized by using the solid state reaction method.In first step, we prepared the Cu 0.5 Ba 2-Y Sr Y Ca 2 Cu 3 O 10-δ (y=0, 0.15, 0.25) by mixing the Ca(NO 3 ) 2 .4H 2 O, Cu(CN) 2 , SrCO 3 and Ba(NO 3 ) 2 as starting compounds.These compounds were thoroughly mixed for almost 2 hours in mortal and pestle.The chamber furnace at 860℃ is used for heat treatment.After 24 hours continuously firing, the furnace was put off.Repeat the process under the same atmosphere.
In second step, well calculated amount of thallium oxide (Tl 2 O 3 ) was added in precursor material and thoroughly mixed.The material was pelletized and these pellets were sintered for nearly 10 min to get finally Cu 0.5 Tl 0.5 Ba 2-Y Sr Y Ca 2 Cu 3 O 10-δ (y=0, 0.15, 0.25) samples.

Characterizations
To measure the resistivity, we used four-probe method was used.In this method, four uniformly spaced silver paste contacts were applied.The crystal structure of the material was determined by X-ray diffraction (XRD) measurements using Bruker diffractometer at X-ray wavelength of 1.5418 Å.

Results and Discussion
The crystal structure of Cu 0.5 Tl 0.5 (Ba 2-y Sr y )Ca 2 Cu 3 O 10-δ where; y=0, 0.15 and 0.25 superconducting samples have been determined from the x-ray diffraction data, shown in Figure 1.The dimensions of unit cell were calculated from check cell.All the samples have orthorhombic structure with PMMM space group.It can be seen from Figure 1 that there is a slight shift in peak positions to higher 2 theta values with the increase of Sr content, which is most probably due to the decrease in the c-axis length of the unit cell.The overall contraction of the unit cell can be seen from the decrease of both a and c axis with the Sr content (Figures 2a and 2b).Due to smaller in size, Sr atom prompt such modifications in the unit cell.
The resistivity versus temperature measurements of Cu 0.5 Tl 0.5 (Ba 2-Y Sr Y )Ca 2 Cu 3 O 10-δ (y=0, 0.15, 0.25) superconducting samples are in Figure 3a and inset is shown the variation of ρ290 K(Ω-cm) versus Sr content.These samples have shown metallic behaviour from room temperature down to onset of superconductivity.From resistivity analysis, it is observed that these samples have shown Tc (onset)  Where, C is dimensional exponent and its values vary with dimension of fluctuations i.e., 0.5, 1, 2 for 3D, 2D and 0D fluctuations respectively [11][12][13].
Moreover, C mf mf C T T T is the reduced temperature, Tc is usually referred as mean field critical temperature [14,15] and C is the fluctuation amplitude.
Where, e, d and ξc(0) are electronic charge, inter-layer thickness and ξc(0) coherence length respectively.Since Lowerence Donich is a modified form of Aslamsov-Larkin theory and it explained the fluctuations from 2D to 3D regimes.In the light of Lowerence Donich (LD model), the excess conductivity and cross over temperature is given below, The other parameters are given below, can be calculated by using TG and NG equations and GL theory.Where, ρ is the resistivity measured experimentally and ρn is the extra-plotted normal state resistivity.The fluctuation induced conductivity analysis (FIC) has been done employing the above cited models (Aslamazov-Larkin and Lawrance-Donaich).The extracted superconducting parameters are given in Tables 1 and 2. The graphs are plotted between ln(Δσ) and ln(ɛ) of Cu 0.5 Tl 0.5 Ba 2-Y Sr Y Ca 2 Cu 3 O 10-δ (y=0, 0.15, 0.25) samples are shown in Figures 5a-5c.It can be seen from Table 1 that all the crossover temperatures i.e., T CR-3D , T 3D-2D , T 2D- SW have been suppressed with the Sr doping which shows that there is an inverse correlation between the crossover temperatures and superconductivity transition temperatures (Tc,0 and Tc onset ).The zero temperature coherence length, interlayer coupling strength, electronphonon coupling parameter and critical magnetic field calculated from the excess conductivity analysis are given in Table 2.It is observed that the interlayer coupling strength has been increased with the increase of Sr in the charge reservoir layer (CRL).
The values of parameters such as B c0 (T ), B c1 (T ), and J c(0) are increased with the Sr content.These parameters appreciably dependent on thermodynamic critical magnetic field B c and it is related to the free energy difference at the interface of normal and superconducting electrons.So in our case, the dopant atom seems to support in the difference of free energy and as a result, these parameters increases.The coherence length along the c-axis ξc(0) and the Fermi velocity vF of superconducting carriers are decreased with Sr content in Cu 0.5 Tl 0.5 (Ba 2-y Sr y )Ca 2 Cu 3 O 10-unit cell.Since K F =(32N/V)1/3; [n=N/V], ξc=ħ2K F /2m∆ and V F .These parameters are dependent on density of carriers and doping of Sr atom, increases the density of carriers which suppresses the order parameter's values.It confirms that Sr atom promote the efficiency of transfer the charge carriers to the CuO 2 planes.

Conclusion
The present work has been resulted in the following conclusions as stated by the above study.Using solid state reaction method, the Sr-doped Cu 0.5 Tl 0.5 1223 (y=0, 0.15, 0.25) samples were synthesized at ambient pressure.The XRD analysis shows that samples have orthorhombic crystal structure.The fluctuation induced conductivity analysis (FIC) has been done employing the above cited models (Aslamazov-Larkin and Lawrance-Donaich).Three vibrational modes are witnessed around 613.4, 483.7 and 421.2 cm -1 in un-doped sample.However, with Sr doping in CRL, the former two modes are related to apical oxygen atoms of nature Tl-O A -Cu (2) , Cu (1) -O A -Cu (2) are softened from 483.7 to 482.0 cm -1 and 421.2 to 415.1 cm -1 whereas the third mode related to the planar oxygen atoms of nature Tl-O P -Cu (2) , are hardened from 613.4 to 623.4 cm -1 .The substitution of Sr at Ba site in the charge reservoir layer decreases the lattice parameters including c-axis, as a result, promotes the enhanced interlayer coupling.The magnitude of the superconductivity is notably increased with the inclusion of Sr which shows that dopant Sr atom at Ba site in Cu 0.5 Tl 0.5 Ba 2 O 10-(CRL) promotes efficiency of transfer the carriers to the conducting CuO 2 planes.From FTIR analysis, it has been observed that the planar and apical phonon modes are hardening and softening respectively.The library of FTIR spectra available in this laboratory includes more than 9400 spectra of organic, polymeric, and inorganic materials.These spectra are compared to the unknown sample spectra using computer software to identify the "best match".The changing in position of these phonon modes provided evidence of Sr atom in the unit cell.The FIC analysis shows that there is an inverse correlation between the superconductivity critical temperature Tc onset and the crossover related to the fluctuations in the order parameter.