Possible Sources of Trigger Mechanism of Electron-phonon Interactions in Doped CuTl-1223 Superconductors


 This article underlines the important role of copper Cu(3d9) spins at the CuO2 planer sites to comprehend the physics of High temperature Superconductivity . In such studies we have reported the characterization results of samples (Cu0.5Tl0.5)Ba2Ca2Cu3O10-δ , (Cu0.5Tl0.5)(BaCa)(CaMg)Cu1Zn2O10-δ , (Cu0.5Tl0.5)(BaCa)(CaMg) Zn3O10-δ and (AyTl1-y)(BaCa)(CaMg) Zn3O10-δ (A=Ag, K, Ni, Mn; y=0,0.5). Samples were prepared at normal pressure by using two-steps solid state reaction method. The characterization of samples is done via x-ray diffraction (XRD), resistivity (RT) and Fourier Transform Infrared (FTIR) absorption measurements. Intrinsic superconducting parameters and activation energy of all sample is estimated applying two theoretical models, Fluctuation Induced conductivity (FIC) analysis and Mott 3D-VRH. The XRD and FTIR absorption measurements have confirmed the intrinsic doping of K, Ag, Ni, Mn and Zn. Interestingly, samples (Cu0.5Tl0.5)(BaCa)(CaMg) Zn3O10-δ without CuO2 planes have exhibited superconducting behavior above 77K. To verify the role of Cu(3d9) atoms in superconductivity we have synthesized (AyTl1-y)(BaCa)(CaMg) Zn3O10-δ (A=Ag, K, Mn, Ni; y=0,0.5) samples. In aforementioned samples, doping of Ni, Mn, K, Ag ions in Cu1-xTlxBa2O4-δ charge reservoir layer instead of Cu-atoms destroys the superconductivity completely and leads to semiconducting behavior. Key objective to preparing these samples is to investigate the role of two major parameters local moments (spin) of copper atoms and net carrier’s concentration in superconductivity.


Introduction
In 1986 scientists found superconductivity in barium, lanthanum and copper oxide called La 1.85 Ba 0. 15 CuO 4 . The breakthrough awarded the scientists a Nobel Prize in Superconductivity and triggered a surge in condensed-mater physics research. Although similar oxides have revealed superconductors at temperatures as low as 30 K, Tc (transition temperatures) of later cuprates is almost an order of magnitude higher than any previously known superconductor [1][2][3]. In spite 33 years of research, no consent has arisen on causes of superconductivity in copper oxides [4][5]. As a result, main emphasis is on knowing the physical origin of the new Properties of the normal state in the expectation that these may provide signi cant insight into the advent of superconductivity at such high temperature. Cuprates are layered materials with a high anisotropy [6][7]. In oxides, superconductivity is found in CuO 2 planes and these planes are connected with charge reservoir layers which are separated by Ca atoms. In the oxide superconductor, the charge reservoir layer(CRL) sends charges to the superconduting plane (CuO 2 ) .The inter-plane coupling which is important in determining the magnitude of superconductivity arises from the correlation of carriers of different planes. The copper atoms in the CuO 2 planes have a small paramagnetic spin, which interacts with the carriers in conducting bands. The magnetic moments of nearly lled Cu 2+ ( 3d 9 ) have strong antiferromagnetic interactions with neighboring spin-¹⁄ Cu ions and each Cu atom is bound by an O ion [8][9][10]. Chemical doping is often used to change the charge concentration in the CuO 2 planes in order to explore the cuprate's unusual superconductivity. we will frame the discussion on two major parameters local moments( spin) and net carriers concentration. Here two questions arise. (1) Is there any contribution of spin of copper atoms (3d 9 )? (2)Which factors enhance spin-spin interaction ? In current study we have synthesize (Cu 0.5 Tl 0.5 )(Ba 2 Ca 2 )Cu 3 O 10−δ and Cu0.5Tl0.5(BaCa)(MgCa)ZnyCu3-yO10-δ (y = 2, 3) superconductors. In (Cu 0.5 Tl 0.5 )(BaCa) (CaMg)Zn 3 O 10−δ sample we witnessed superconducting behavior even though Zn atoms, non-spin carrying entity with 3d 10 state and mass almost equal to the Copper atom completely replace Cu-atoms in CuO 2 planar sites and make ZnO 2 plane. In the aforementioned sample superconductivity suppresses but it does not disappear, this trend arise a question and that is there any role of spin atom essential for the mechanism of superconductivity. To investigate the role of spin we replace spin carrying copper atom in charge reservoir layer Cu 0.5 Tl 0.5 (BaCa)O 4−δ , by thallium atoms and synthesize (Tl 1.0 )(BaCa)(CaMg)Zn 3 O 10−δ sample in which copper Cu (3d 9 ) atom is transformed from CRL Tl 1.0 (BaCa)O 4−δ . Surprisingly sample turn semiconductor altogether. This result raised many questions.
To answer such questions and to investigate the role of local moments of impurity atoms (spins of copper) and net carriers concentration. We prepared samples by doping the charge reservoir layer with transition metals noble and Alkali metals.
Our technique for getting more knowledge is to replace copper atom Cu (3d 9 ) by others transition metal ions. In periodic table Nickle sit next to copper atom. So We synthesize (Ni 0.5 Tl 0.5 )(BaCa)(CaMg) Zn 3 O 10−δ by replacing Copper atoms in charge reservoir layers This arised other questions to be answered that is there any role of increase or decrease in effective magnetic moment per lattice site that disturbed the superconductivity?. In this scenario, we further prepared two samples (Mn 0.5 Tl 0.5 )(BaCa)(CaMg)Zn 3 O 10−δ and (Ag0.5Tl 0.5 ) (BaCa)(CaMg) Zn 3 O 10−δ by replacing Copper atoms in charge reservoir layers Cu 0.5 Tl 0.5 (BaCa)O 4−δ by transition metals ions Mn (3d 5 ) and Nobel Metal Ag. Astonishing no superconductivity was observed in such samples and it con rmed from results, superconductivity is only maintained in copper doped samples whereas doping with various d-Block elements ( Tl, Ni, Mn, Ag) show semiconducting behavior.
Role of carrier concentration cannot be ruled out to study superconducting behavior. The copper atoms that are only present in CuO 2 planes are de cient in carriers and their spins are aligned antiferromagnetically, while charge reservoir layers have high carriers density. In order to enhance the density of charge carrier we prepared (K 0.5 Tl 0.5 )(BaCa)(CaMg) Zn 3 O 10−δ sample by replacing charge reservoir layers is mixed to get required compounds. The pressure of 3.8 tons/cm 2 is being used to pelletize these powder compounds. To reduce Tl losses, these pellets were individually wrapped in gold capsules and sintered at 860 o C for 10 minutes before being cooled to room temperature. X-ray diffraction was used to identify the material's crystal structure (XRD).The resistivity-temperature measurement is carried using four-probe method. (FTIR) Fourier Transform Infrared Spectroscopy measurements were used at ambient temperature to investigate phonon's modes. which has been revealed in diffraction scan. BaCuO 2 is a well-known stable chemical. It is produced by the decomposition of starting compounds through the decomposition of Ba(NO 3 ) 2 at early stages. The decomposed temperature of Ba(CuO 2 ) is greater than thermally stable temperature of samples (~ 860 o C), thus it stays in nal compound's integral part. The variation of a and c-axis length is attributed to the fact that the ionic radius of Tl ions (1.48 A) is slightly greater than Cu, Ag Mn ions [11] shown in Table 1. In CuTl-1223 low intensity impurity peaks are detected in diffractogram. Structural changes can be seen in the nal compound due to presence of different ionic radii. Table 1 Shows the a-axis, b-axis, c-axis and volume values calculated from XRD analysis of Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3 O 10−δ and Cu 0 . 5 Table 2. As demonstrated in Fig. 2  Various models have been used to characterize electrical properties; the conduction process is guided by charge carriers hopping between levels via different methods. In many disordered carbon forms for charge carriers the Mott variable range hopping (VRH) and thermally assisted hopping are suggested as probable conduction mechanisms [12][13]. Eqn (1) gives the Mott VRH model, which is based on charge carriers hopping between localized states near the Fermi level with the assist of thermal energy. To is the characteristic hopping temperature while factor (ρ o ) is temperature T dependent.

Discussion And Results
Due to the randomization in the conduction channels, carrier transport is actually proceeds via three-dimensional (3D) variable range hopping process. As a result using the Mott 3D VRH type conduction mechanism (equation (1) Table 3.  semiconducting behavior which supports our thesis that in the absence of a copper atom, electron spins ip on varying degree that causes scattering waves asymmetrically that in turn disturb coherence in wave function. This process disrupts phase coherence, resulting in destructive interference, which leads to an increase in resistivity [17][18][19]. Aslamazov and Larkin [24] used a microscopic technique to calculate excess conductivity where the uctuation are modest. They derived σ /σ300 = Aε λ (1) with reduced temperature ε = (T -Tmf)/Tmf ( mean eld temperature Tmf is calculated from peak of dρ/dT against T plot). The Lawrence and Doniach (LD) model for the polycrystalline samples, is as follow [25]. ln(σ) was plotted against ln(ε) in Fig. 4(a,b,c) to analyze the excess conductivity applying AL model. This can be seen, each plot is categorized into three distinct regions. The different sections of the plots are linearly tted and exponent values 'λ' is derived from the slopes to relate the experimental results with theoretical values. The λcr (critical exponent ) corresponds to the critical region has a value of 0.33, λ 3D values ranging from -0.48 to -0.564 showing 3D behavior, λ 2D values ranging -0.92 to -1.0089 signifying 2D behavior see in Table 4. In afore mentioned samples Mg-atoms are substituted at Ca sites, in prior research Ca by Mg atoms were successfully replaced, that suppresses c-axis length and rise critical current density [27]. indicate that these defects are not behaving as pinning centers. Table 5 shows that the London penetration depth λ p.d and the GL parameter increase in all doped samples.   The phonon modes are observed in order to understand the process of high temperature superconductivity because electron-phonon interactions are probably important for superconductivity's mechanism. Oxygen, have highest vibrational amplitude due to its smaller atomic size, is responsible for causing such electron phonon interactions. In the CuTl-1223 unit cell, oxygen phonon modes have been