Planar Oxygen Mode Research Articles

Superconducting properties of Cu0.5Tl0.5(BaCa)(CaMg)1.5Cu4−yCdyO12−δ (y=0,1,2,3) samples

In order to investigate the significance of electron–phonons interaction in the origin of high [Formula: see text] superconductivity, we synthesized Cd-doped Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples and studied the superconducting properties. Onset temperature of the superconductivity Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples suppress with the increased Cd doping ratio in the final. The phonon modes are associated to the vibrations of apical and planar oxygen whereas the CuO2 planar oxygen modes are observed to harden with increasing Cd ratio in sample doping. The phase relaxation time is suppressed for the superconducting charge carriers in Cd-doped samples. The energy amount required to break the Cooper pairs apart increases. Dielectric measurements of Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples have been done.

Infield magnetic measurements of (Cu0.5Tl0.5)Ba2Ca3(Cu4−xTix)O12−δ (x = 0, 0.25, 0.50, 0.75) samples

We have synthesized (Cu0.5Tl0.5)Ba2Ca3(Cu4− xTi x)O12−δ ( x = 0, 0.25, 0.50, 0.75) superconductor samples at 880 °C and studied their superconducting features by XRD analysis, resistivity measurements, ac susceptibility measurements, FTIR absorption spectroscopy, and detailed infield magnetic measurements. By increasing x concentration of Ti doping, the a axis length of the tetragonal unit cell increases, whereas its c axis length decreases. Also, the Tc(onset), Tc( R = 0), and the magnitude of diamagnetism suppresses. The peak position of the apical oxygen modes stay unchanged, however the CuO2/TiO2 planar oxygen modes are softened with increasing x. The critical current density Jc of the samples suppress with an increased field. Suppression of Jc is more steep in doped samples. The doped atoms increase the population of confined defects, which act as pinning cores that increase energy per unit length of flux vortex ɛ. The pinning strength essentially depends on the ɛ; therefore, it also increases with x concentration of Ti doping. The superconductivity enhancement of doped samples is witnessed in increased coherence length ξ c, suppression of the Ginzburg-Landau parameter κ, and the penetration depth λD. The superior infield characteristics of doped samples indicated that Ti atoms act as efficient flux pinning centers whereas due to their presence in the final compound, the phase coherence of Cooper pairs in various unit cells is marginalized, which induces the suppression in Tc's and magnitude of diamagnetism in ac susceptibility measurements.

An-harmonic Oscillations and Superconductivity in (Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) Samples

(Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) samples are synthesized for the investigation of role of an-harmonic oscillations in the mechanism of high Tc superconductivity. In the conducting planes, the carrier density is enhanced by K doping at the charge reservoir layer (Cu0.5Tl0.5)Ba2O4-δ by synthesizing (Cu0.25K0.25Tl0.5)Ba2Ca2Cu3-y-zZnyCdzO10-δ (y = z = 0, 0.25, 0.5, 0.75, 1) samples. The (Cu0.5-xKxTl0.5)Ba2Ca2Cu3-y-z ZnyCdzO10-δ (x = 0, 0.25; y = z = 0, 0.25, 0.5, 0.75, 1) samples were prepared by employing two-step solid-state reaction method accomplished at 860 °C and understand their superconducting properties by resistivity measurements, X-ray diffraction analysis, and FTIR absorption measurements. The orthorhombic unit cell of our samples was confirmed via XRD analysis. The analysis also reflected the increase in c-axis length and the volume of the unit cell with the enhanced dopants (Cd and Zn) concentration. The suppression of Tc(onset) and Tc(R = 0) was found with the increasing concentration of Zn and Cd. An improvement in the Tc(onset) observed in samples doped by K that arises due to increase in the density of free carriers supplied by alkali atoms. With the increasing content of Cd and Zn, the unit cell exhibits the softening of Cu(1)-OA-Cu(2) and Tl-OA-Cu(2) apical oxygen mode and hardening of planar oxygen mode. From FIC analysis, the Tcmf and T* and various cross over temperature were shifted towards lower temperature with the enhanced Zn and Cd dopant concentration. The value of α determining the defect density also increases with the increasing dopant (Zn and Cd) concentration. The numerical values of different parameter like Bc0(T), Bc1(T), Jc(0), and Ebreak showing increasing trend but the phase relaxation time of the carriers τφ points out decreasing trend with the enhanced doping of Cd and Zn. Moreover, these parameters reflected inferior values in K-doped samples relative to K-undoped samples. The value of other parameter like coherence length ξc(0) along c-axis, interlayer coupling J, magnetic field penetration depth λp.d, the Ginzburg Landau (GL) parameter κ, and the Fermi velocity VF of the carriers decreases as doping of Zn and Cd enhanced. These parameters show superior values in K-doped samples relative to K-undoped samples due to increase in the density of free carriers supplied by K atoms. The negative values of real part of dielectric constant suppress in both K undoped and K doped samples with increasing concentration of Cd and Zn. The imaginary part of dielectric constant and ac conductivity showing suppression in their negative values in K undoped samples while showing increase in absolute values in K doped samples with increasing concentration of Cd and Zn.

Effect of Cd intercalation on the superconducting properties of (Cu0.5-yKyTl0.5)Ba2Ca2Cu3-xCdxO10-δ (y = 0, 0.25; x = 0, 0.5, 1.0, 1.5, 2.0) superconductors

(Cu0.5Tl0.5)Ba2Ca2 Cu3-xCdxO10-δ (x = 0, 0.5, 1.0, 1.5, 2.0) samples are synthesized for the investigation of role of an-harmonic oscillations in the mechanism of high Tc superconductivity. The density of carriers in the conducting planes is enhanced with the doping of K at the (Cu0.5Tl0.5)Ba2O4-δ by synthesizing (Cu0.25K0.25Tl0.5)Ba2Ca2Cu3-xCdxO10-δ (x = 0, 0.5, 1.0, 1.5, 2.0) samples. These samples were synthesized by two step solid state reaction method accomplished at 860 °C and studied their superconducting properties by resistivity measurements, X-ray diffraction analysis and FTIR absorption measurements. The planar reflections observed in the XRD analysis of the samples are best fitted to the orthorhombic crystal structure. The c-axis length and the volume of the unit cell increases with the enhanced doping of Cd in the final compound. The onset temperature of superconductivity Tc (onset) and the zero-resistivity critical temperature Tc (R = 0) suppresses with the increased doping of Cd in the final compound. An improvement in the Tc (onset) and Tc (R = 0) is observed in K-doped samples that arises due to increase in the density of free carriers supplied by alkali atoms. A softening of apical oxygen mode of type Cu(1)‒OA‒Cu(2)/Cd is observed with the increased doping of Cd at the CuO2 planar sites. The apical oxygen mode of type Tl‒OA‒Cu(2)/Cd is, however, did not change with the doping of Cd in the final compound. The planar oxygen mode (545 ‒ 570 cm−1) is hardened with the enhanced doping of Cd in the final compound. The excess conductivity analyses of conductivity data have shown a shift in the various cross-over temperatures, Tcmf and T* towards lower temperatures with the enhanced doping of Cd in the final compound. The value of α determining the defect density also increases with the increased doping of Cd in the final compound. The coherence length along the c-axis ξc(0), interlayer coupling J, magnetic field penetration depth λp.d and Ginzburg Landau (GL) parameter κ decreased with the increase doping of Cd. The phase relaxation time of the carriers Ꞇϕ, the Fermi velocity VF of the carriers decreased while Bc0(T), Bc1(T) and Jc (0) are enhanced with increase doing of Cd in the final compound.

Influence of Ti doping on the superconducting properties of YBa2(Cu3−xTix)O7−δ materials

YBa2(Cu3−xTix)O7−δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) superconductors are synthesized at 900 °C by solid state reaction method. The superconducting properties of the samples are studied by X-ray diffraction, resistivity, AC-susceptibility and Fourier transform infrared spectroscopy (FTIR) absorption measurements. These samples have shown orthorhombic crystal structure and the cell parameters slightly increase for higher Ti-contents. The Tc(R = 0), the onset temperature and magnitude of diamagnetism are suppressed with increasing Ti-doping. A new in-chain [Ti–O–Cu(1)] oxygen vibration mode appears around 551 cm−1 in the FTIR spectra which grows in intensity with increased Ti-doping. The peak position of the CuO2 planar oxygen mode is not affected with the doping of Ti showing that Ti is doped at the in-chain-copper Cu(1) sites in the unit cell. The fluctuation induced conductivity analyses reveals that despite an increase in the parameters such as ξc, VF, and inter-layer coupling J, the critical temperature and superconductor volume fraction are suppressed. It shows that the doped Ti-atoms most likely induce anharmonic oscillations which may suppress the density of phonon population and hence the superconductivity. It can also be witnessed in the decreased phase relaxation time of the carriers with increasing Ti-doping. These studies stress on the essential role of soft phonons in the mechanism of high Tc superconductivity in YBa2(Cu3−xTix)O7−δ superconductors.

Synthesis and Para-conductivity of (Tl $$_{1-x}$$ 1 - x Ti $$_{x}$$ x )Ba $$_{2}$$ 2 Ca $$_{2}$$ 2 Cu $$_{3}$$ 3 O $$_\mathrm{y}$$ y (x $$=$$ = 0, 0.2, 0.4, 0.6, 0.8) Superconductors

We have synthesized Ti-doped (Tl $$_{1-x}$$ Ti $$_{x})$$ Ba $$_{2}$$ Ca $$_{2}$$ Cu $$_{3}$$ O $$_{y}$$ (x = 0, 0.2, 0.4, 0.6, 0.8) superconductors at normal pressure. The samples have shown $$T_\mathrm{c}$$ onset at 117, 118, 113, 110, and 115 K and the $$T_\mathrm{c}$$ $$(R=0)$$ at 103, 96, 94, 97, and 106 K and the onset of diamagnetism around 109, 100, 98, 111, 100 K, respectively. The apical oxygen-related phonon modes are observed around 485 and 540 cm $$^{-1 }$$ , and the planar oxygen modes at 600 cm $$^{-1}$$ . These phonon modes are softened with the increase concentration of Ti in the final compound showing that Ti is incorporated into the (Tl $$_{1-x}$$ Ti $$_{x})$$ Ba $$_{2}$$ Ca $$_{2}$$ Cu $$_{3}$$ O $$_{y}$$ (x = 0, 0.2, 0.4, 0.6, 0.8) samples. The excess conductivity analysis of these samples has shown an enhancement in the values of the parameters such as $$\xi _{\mathrm{c}(0)}$$ , J, and $$V_\mathrm{F}$$ with the doping of Ti. We proposed that the doping of Ti in the $$( {\mathrm{Tl}_{1-{\textit{x}}} \mathrm {Ti}_{{x}} })\mathrm{Ba}_2 \mathrm{O}_{4-\delta } $$ charge reservoir layer facilitates the charge transfer mechanism for conducting CuO $$_{2}$$ planes which in turn enhance the values of aforementioned superconductivity parameters. The values of superconductivity parameters such as $$B_\mathrm{c(0), }$$ $$B_\mathrm{c1(0), }$$ $$B_\mathrm{c2(0)}$$ , and $$J_\mathrm{c}$$ (0) are, however, suppressed with Ti doping. We suggested this to be arising from the weakening of flux pinning character in the final compound. These results have shown that the oxygen defect density in (Tl $$_{1-x}$$ Ti $$_{x})$$ Ba $$_{2}$$ Ca $$_{2}$$ Cu $$_{3}$$ O $$_{y}$$ samples is significantly suppressed with Ti doping.

Electron–Phonon Interactions and Superconductivity in (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y )O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0)

We have used two-step solid state reaction method for the synthesis of (Cu0.5Tl0.5)Ba2Ca3(Cu4−y Ti y ) O12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) superconductors at 880 °C. The oxygen contents in the samples were optimized by carrying out self-doping which is accomplished by post-annealed in flowing oxygen environment at 500 °C for approximately 5 h. The superconducting properties of all the samples were improved, after the self-doping of the carriers in the conducting planes. In the x-ray diffraction scans of the samples, the a-axis length of tetragonal unit cell increases whereas the c-axis decreases with increased Ti doping in the final compound. The Fourier transform infrared spectrometer (FTIR) absorption measurements of these samples have shown that the apical oxygen mode at 548 cm−1 and the planar oxygen mode at 596 cm−1 are softened with increased Ti doping. The origin of softening of planar oxygen mode lies in increase bond lengths of apical oxygen atoms promoted by larger covalent radius of Ti (1.32 A) atoms relative to Cu (1.17 A) atoms that in turn promotes the softening of the apical oxygen modes. Doped Ti (47.90 amu) atoms at the Cu (63.54 amu) sites initiate the an-harmonic oscillations resulting into the suppression of density of phonon modes. The suppression in the values of superconductivity parameters with Ti doping at the Cu sites shows the essential role of phonon in mechanism of high Tc superconductivity and hence the electron–phonons interactions. The excess conductivity analyses (FIC) of conductivity data of oxygen-post-annealed samples have shown decrease in the mean field critical temperature, coherence length along the c-axis, interlayer coupling, and Fermi velocity with increase Ti doping. However, the values of B c, B c1, and J c(0) increase with Ti doping, showing increase in the population of the pinning centers.

Fabrication and Para-Conductivity Analysis of Oxygen Annealed TlBa_2(Ca_(3-y)Mg_y)Cu_4(O_(12-δ) Superconductors

Oxygen-annealed TlBa_2(Ca_(3-y)Mg_y)Cu_4O_(12-δ) (y = 0; 0:5; 1:0; 1:5; 2) superconductors are synthesized at normal pressure, and the influence of doped Mg-atoms on the superconducting parameters at the microscopic level is investigated by carrying out a para-conductivity analysis. The samples have shown a tetragonal structure, and the unit cell volume decreases with increased Mg-doping. The onset temperature of superconductivity [T_c(onset)] and zero resistivity critical temperature [T_c(R=0)] decreases with Mg doping. The maximum magnitude of diamagnetism is observed for the samples with a Mg concentration of y = 2. The apical oxygen mode of the type Tl-O_A-Cu(2) and CuO_2 planar oxygen modes are softened, as observed in the FTIR (Fourier transform infrared spectroscopy) absorption measurements. The fluctuation induced conductivity (FIC) analysis of conductivity data has shown the enhancement of the inter-plane coupling and coherence length along the c-axis. These studies have shown that there is less suppression of the order parameter of the carriers from its value |ψ|^2 = 1 due to the proximity effect in the CuO_2 planes in the Mg-doped samples.

Significance of Phonon Modes and Excess Conductivity of (Cu0.5Tl0.5)Ba2Ca3(Cu4−x Ti x )O12−δ (x = 0, 0.25, 0.50, 0.75, 1.0) Superconductors

We have synthesized (Cu0.5Tl0.5)Ba2Ca3(Cu4−x Ti x )O12−δ (x = 0, 0.25, 0.50, 0.75, 1.0) superconductors by two-step solid-state reaction method at 880 °C. The a-axis length of tetragonal unit cell increases, whereas the c-axis decreases with doping of Ti in the final compound revealed by X-ray diffraction measurements. Moreover, the magnitude of superconductivity suppresses with increased Ti doping. The Fourier transform infrared spectrometer (FTIR) absorption measurements have shown that the peak position of the apical oxygen modes at 480 and 540 cm −1 remains unchanged, whereas the CuO2/TiO2 planar oxygen modes is softened with increased Ti doping. We explained the softening of planar oxygen mode to be arising due to the difference in the atomic masses of Ti (47.90 amu) and Cu (63.54 amu) atoms. The suppression of superconductivity magnitude is suggested to be originating from an-harmonic oscillations induced by doped Ti atoms which in turn suppress the density of phonon population. These studies show the importance of density of phonon population in mechanism of high- T c superconductivity and hence the electron-phonon interactions. The excess conductivity analyses (FIC) of conductivity data of these samples have shown that with increased Ti doping, the mean field critical temperature T cmf is shifted to lower temperatures. The increase in the coherence length along the c-axis, interlayer coupling, and the Fermi velocity is suggested to be arising from the decrease in the c-axes length. We attribute the suppression in B c, B c1, J c(0), and τ ϕ with Ti doping to the free energy difference between the normal and superconducting state. The idea of suppression in the density of phonon modes induced by Ti doping is supported by excess conductivity analyses.

Synthesis and Excess Conductivity Analyses of Ce-Doped Tl1−x Ce x Ba2Ca2Cu3O10−δ Superconductors

The role of mobile carriers density supplied by TlBa 2O4−δ charge reservoir layer is investigated by doping Ce in TlBa 2Ca 2Cu 3O10−δ superconductors. The T c(onset) and T c(R= 0) versus Ce contents have shown almost a parabolic-shaped phase diagram which is interpreted in terms of hole’s doping. The magnetic measurements showed that magnitude of diamagnetism is gradually suppressed with Ce doping at Tl site. Fourier transform infrared (FTIR) revealed that the phonon modes associated with apical oxygen atoms type Cu(2)–O A–Tl are softened, whereas the planar oxygen modes are hardened with the doping of Ce the unit cell. In the excess conductivity, analyses of conductivity data of Tl 1−xCe xBa 2Ca 2Cu 3O10−δ samples have shown a suppression of T cmf(K) and the width of 2D conductivity regimes, whereas the coherence length along the c-axis ξc(0) and the interlayer coupling J are enhanced with the doping of Ce +4 in the charge reservoir. The increase in ξc(0) is interpreted in the form of homogenization of the carrier’s density of carriers the CuO 2 planes. The process of homogenization is accomplished with the reduction of density of carriers in the outer CuO 2 that consequently promotes increase in the phase relaxation time, τϕ, of the carriers, their Fermi velocity, and the energy required to break apart the Cooper pairs.

Synthesis and Para-conductivity Analyses of TlBa2(Ca 3 − y Mg y )Cu4 O 1 2 − δ Superconductors

TlBa 2(Ca 3−yMg y)Cu 4O12−δ (y=0,0.5,1.0, 1.5,2) superconductors are synthesized at normal pressure, and the influence of doped Mg atoms on the superconductivity parameters at the microscopic level is investigated by carrying out para-conductivity analyses of conductivity data. The samples have shown tetragonal structure, and the unit cell volume decreases with increased Mg doping. The onset temperature of superconductivity [ Tc(onset)] and zero resistivity critical temperature [ Tc(R=0)] decreases with Mg. Maximum magnitude of diamagnetism is observed in the samples with Mg of y=2. The apical oxygen mode of the type Tl-O A-Cu(2) and CuO 2 planar oxygen modes are softened as observed in fourier transform infrared spectrometer (FTIR) absorption measurements. The fluctuation-induced conductivity (FIC) analyses of conductivity data have shown the enhancement of inter-plane coupling and coherence length along the c-axis. These analyses have shown an increase in the coherence length along the c-axis and the inter-layer coupling J. The enhancement of inter-plane may possibly be arising due to a small decrease in the value of the order parameter of the carriers from ∣ψ∣ 2=1 in the CuO 2 planes in Mg-doped samples. The values of Bc0(T), Bc1(T), Jc(0) are suppressed with the increased incorporation of Mg, which most likely arises due to the weak coupling of the grains induced by Mg doping.

Synthesis and excess conductivity analysis of TlBa2(Ca3−y Be y )Cu4O12−δ superconductors

TlBa2(Ca3−yBey)Cu4O12−δ (y = 0, 0.5, 1.0, 1.5, 2) samples are synthesized at normal pressure and the influence of doped Be-atoms on the superconductivity parameters at the microscopic level is investigated by carrying out excess conductivity analyses of conductivity data. The samples have shown tetragonal structure and the unit cell volume decreases with increased Be-doping. The onset temperature of superconductivity [Tc(onset)] and zero resistivity critical temperature [Tc(R = 0)] decrease with increased Be, however, the magnitude of diamagnetism is enhanced with Be (except for Be-doping of y = 1.0). The apical oxygen mode of the type Tl–OA–Cu(2) and CuO2 planar oxygen mode are softened as observed in FTIR absorption measurements. The FIC analyses of conductivity data have shown an increase in the coherence length along the c-axis and inter-plane coupling. The values of Bc0(T), Bc1(T), Jc(0), τφ are improved with the doping of Be. These observations suggested that due to the proximity effect there is less suppression in the value of the order parameter of the Cooper pairs from |ψ|2 = 1 value in the CuO2 planes in Be-doped samples that maintains the density of carriers in the conducting CuO2 planes [since the |ψ|2 = n/2] which promotes enhancement in the magnitude of superconductivity.

Enhancement of superconductivity of TlBa2Ca3Cu4O12−δ superconductors by Be doping

A reproducible synthesis and characterization of the TlBa2(Ca3−yMy)Cu4O12−δ (M=0, Be; y=0, 0. 5, 1.0, 1. 5, 2) superconductors have been studied by using x-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. The x-ray diffraction (XRD) studies of these samples have shown a tetragonal structure in which the length of cell parameters and volume of the unit cell have been found to decrease with increased Be-doping. The zero resisitivity critical temperature [Tc(R=0)] have been suppressed with Be doping. A significant increase in the onset temperature of superconductivity have been observed in Be doped O2 annealed samples. The magnitude of diamagnetism of as prepared samples is higher for particular level of doping. In comparison with as-prepared samples, an improvement in the magnitude of diamagnetism could be witnessed in all oxygen post-annealed samples. It is most likely that oxygen optimization in the unit cell which promotes increase the superconducting properties of TlBa2(Ca3-yBey)Cu4O12−δ (y=0, 0.5, 1.0, 1.5, 2) samples in terms of increased magnitude of diamagnetism. The softening of apical oxygen mode of the type Tl-OA-Cu(2) and CuO2 planar oxygen mode have been observed providing unequivocal evidence of the enhanced inter-plane coupling. In comparison with Be as-prepared samples, the magnitude of diamagnetism is significantly suppressed in O2 annealed samples. It is most likely that oxygen optimization in the unit cell suppresses the density of the carriers and hence the superconducting properties of the final compounds. The doped Be atoms suppress the density of anti-ferromagnetic aligned moments existing in the inner CuO2 planes of TlBa2(Ca3−yMy)Cu4O12−δ [M=Be] which enhance the magnitude of superconductivity in the final compound.

Mg-doped TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) superconductors

Ti-doped TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) samples are synthesized at 860 °C by two step solid state reaction method. All these samples have shown metallic variations of resistivity from room temperature down to onset of superconductivity. The room temperature resistivity systematically increases with increased Mg contents in the final compound. The onset temperature of superconductivity [Tc(onset)], as observed in the resistivity and susceptibility measurements, and the zero resistivity critical temperature are found to increase for Mg-doping of x = 0.05 and 0.15; however, the values of these parameters are suppressed for Mg-doping of x = 0.25 and 0.35. Magnitude of the superconductivity also increases for Mg-doping of x = 0.05 and 0.15, whereas its values decrease for Mg-doping of x = 0.25 and 0.35. All TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) samples have shown orthorhombic crystal structure and the c-axis lattice parameter is systematically suppressed with the increase of Mg-doping. In the phonon modes assignment as observed from Fourier transform infrared spectrometer absorption measurements, the planar oxygen mode is softened, whereas the apical oxygen mode of type Tl-OA-Cu(2)/Ti is hardened with increased Mg-doping. The Oδ mode of oxygen atoms increases in intensity with increased Mg incorporation in the final compound. It is most likely that the density of phonon modes increases till mg-doping of x = 0.15, but for x = 0.25 and 0.35 the c-axis length decreases further, which may enhance the population of an-harmonic modes of oscillations. Such competitive modes of oscillations are most likely produced due to the presence of two different masses existing in the conducting CuO2/TiO2 planes, i.e., Ti (47.90 amu) and Cu (63.54 amu). Since the distance between the planes decreases with increased Mg-doping, the equilibrium population of the phonon modes in CuO2/TiO2 planes may also change and hence the Tc and magnitude of superconductivity. These observations strongly suggest the existence of electron-phonon interactions essential for the mechanism of high Tc superconductivity.

Study of Mg-doped (Cu0.5Tl0.5)Ba2(Ca2−yMgy)(Cu2.5Cd0.5)O10−δ (y = 0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) superconductors

(Cu0.5Tl0.5)Ba2(Ca2−yMgy)(Cu2.5Cd0.5)O10−δ (y=0, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0) superconductors have been synthesized at normal pressure. The c-axis length (except for the samples with y=0.05) decreases with the increased Mg-doping, showing that the distance between CuO2/CdO2 planes decreases with increased Mg-doping. The onset of superconductivity in the resistivity and ac-susceptibility measurements is suppressed with the Mg-doping. Tc(R=0) and magnitude of diamagnetism are also suppressed with the increasing Mg contents. Incorporation of Mg at Ca sites in the unit cell can be evidenced from the modification of the FTIR absorption spectra. The CuO2/CdO2 planar oxygen modes are not significantly altered, but the apical oxygen modes around 480 and 530cm−1 are softened with the doping of Mg at the Ca sites. It is most likely that softening of the apical oxygen modes is induced due to better inter-plane coupling produced by Mg-doping. Due to the presence of heavier Cd atoms in the CuO2 planes, the phonon population is suppressed. The influence of an-harmonic oscillations is further enhanced due to enhancement of inter-CuO2/CdO2-plane coupling which further suppresses the phonon density and hence the superconductivity. These studies show the pivotal role of electron–phonon interactions in the mechanism of high Tc superconductivity.

Ti-doped (Cu0.5Tl0.5)Ba2Ca2 (Cu3−xTix)O10−δ (x = 0, 0.25, 0.50, 0.75) superconductors

Ti-doped (Cu0.5Tl0.5)Ba2Ca2 (Cu3−xTix)O10−δ (x=0, 0.25, 0.50, 0.75) samples have been synthesized at 860°C by following two step solid state reaction methods and studied their superconducting properties by X-ray diffraction, resistivity, ac-susceptibility, FTIR absorption measurements. Ti-doped samples have shown orthorhombic crystal structure and the cell parameters increase with the increase of Ti-contents. The Tc(R=0) and onset of diamagnetism increases with increase Ti-doping in the final compound. The magnitude of superconductivity, however, increases for Ti-doping of y=0.25,0.75. The numbers of carriers in the conducting planes have been optimized by carrying out post-annealing of the samples in oxygen atmosphere. After the post-annealing the Tc(R=0) is improved in un-doped samples whereas it is suppressed in all Ti-doped samples. After post annealing in oxygen atmosphere, the magnitude of superconductivity (measured in emu/gm.Os) remains unchanged in un-doped samples whereas it is suppressed in all Ti-doped samples. In FTIR absorption measurements, the CuO2/TiO2 planar oxygen modes are slightly hardened in Ti-doped samples. The most likely reason for the hardening of the planar oxygen modes is decrease in the mass of Ti (47.90amu) in comparison with Cu (63.54amu) atoms; the atoms with lighter-mass vibrate at higher frequency. It is most likely that the existence of atoms of different masses in CuO2/TiO2 planes may suppress the density of phonon population due to setting-in of an-harmonic oscillations. The phonons of harmonic oscillation with wave vector q interact with q′ of an-harmonic oscillation and produce a new phonon with wave vector Q. The phonons with wave vector Q do not contribute in the Cooper-pair formation. It is most likely that a decrease in the population of desired phonons required for higher Tc occurs that in turn suppress the density of the Cooper-pair which in turn promote a decrease in the magnitude of superconductivity. These studies indicate the essential role of electron–phonons-interactions in the mechanism of high Tc superconductivity.

Single and multi-walled carbon nanotubes doped (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ superconductors

Carbon nanotubes (CNTs) added polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) superconductor samples have been synthesized at normal pressure. The X-ray diffraction (XRD) studies of these samples have shown an orthorhombic crystal structure following PMMM space group. The added CNTs have been found to diffuse at the inter-grain sites and act as cementing material. SEM images show that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the CuTl-1223 compounds improve the electrical connection between superconducting grains which in turn increase the Jc of the final compound. The room temperature resistivity of as-prepared samples decreases with the addition of CNT. The onset temperature of superconductivity [Tc(onset)] and zero resistivity critical temperature [Tc(R=0)] are found to decrease with the CNT additions. Except for the samples with CNT additions of 0.25%, the density [mass/volume] of final compound after CNT addition is decreased. Magnitude of diamagnetism is also suppressed with the addition of CNT which suggests that some of carbon atoms diffuse into the unit cells at the intra-grain sites. The Tc(R=0) becomes quite stable after post-annealing in oxygen atmosphere; the Tc(R=0) varies between 98 and 96K in polycrystalline (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples. The out of phase component of magnetic susceptibility observed as a broad peak in the as-prepared samples, is transformed to a sharp peaks after post-annealing of the samples in oxygen atmosphere. This shows that oxygen diffusion converts the inter-grain material to respective oxides and carbonates that increase the surface areas of the current flow around inter-grain loops that reduce the hysteresis losses. The electron-microscopy of these samples has shown that inter-grain voids population is suppressed with the addition of CNTs. FTIR absorption measurements of (CNT)y(Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (y=0, 0.25%, 1.0% and 7%) samples have shown apical and planar oxygen modes at their usual positions.

Influence of Ce Substitution at Sr-Site on Superconducting Fluctuation Behavior and Infrared Absorption of Tl0.9Bi0.1Sr2−x Ce x Ca0.9Y0.1Cu1.99Fe0.01O7−δ (x=0–0.20) Ceramics

Ce substituted Tl0.9Bi0.1Sr2−xCexCa0.9Y0.1Cu1.99Fe0.01O7−δ (x=0–0.20) samples were synthesized to determine the effects of the higher valence ion substitution on superconductivity and structure of the Fe-doped Tl1212 derivatives. The normal state behavior for x=0 showed semiconductor-like behavior which gradually turned to metallic behavior with increasing Ce at x=0.05–0.15. However, further substitution of Ce for x>0.15 turned the normal state to insulating behavior. The zero critical temperature, Tc zero increased from 65.4 K (x=0.05) to 71.0 K (x=0.10), but slightly decreased for x>0.10 indicating the optimum value of average copper valence was achieved at x=0.10. Excess conductivity analysis using the Aslamazov Larkin, AL and Lawrence–Doniach, LD models revealed two-dimensional, 2D to three-dimensional, 3D transition of superconducting fluctuation behavior, SFB with the highest transition temperature, \(T_{\mathrm{2D}\mbox{-}\mathrm{3D}}\) at x=0.10. FTIR analysis in conjunction with XRD results showed softening of FeO2/CuO2 planar oxygen mode from 610.5 cm−1(x=0) to 605 cm−1(x=0.20) which is suggested to be related to possible increase of inter plane coupling, J and this is supported by computed results based on the LD model. The enhanced J increases superconducting coherence length along c-axis, ξc(0), and hence lowers anisotropy, γ resulting in enhanced superconducting properties.

Superconducting fluctuation behavior and infrared absorption properties of Tl1−Cu Sr1.6Yb0.4CaCu2O7 and Tl0.5Pb0.5Sr2−Mg Ca0.8Yb0.2Cu2O7− ceramics

Abstract In this paper, the effects of Cu substitution at Tl site in Tl1−xCuxSr1.6Yb0.4CaCu2O7−δ (x = 0–0.5) ceramics and Mg substitution at Sr site in Tl0.5Pb0.5Sr2−yMgyCa0.8Yb0.2Cu2O7−δ ceramics, respectively on superconducting and structural properties are reported. Excess conductivity analyses based on Asmalazov–Larkin (AL) theory revealed 2D–3D transition and showed Cu substitution affects the A3D constant and increases c-axis coherence length, ξc(0). FTIR analysis for Tl1−xCuxSr1.6Yb0.4CaCu2O7−δ (x = 0–0.5) showed CuO2 planar oxygen mode observed around 578.7 cm−1 in the x = 0 sample was softened to 570 cm−1 with increased Cu substitution indicating enhanced coupling between CuO2 planes. On the other hand, Tl0.5Pb0.5Sr2−yMgyCa0.8Yb0.2Cu2O7−δ (y = 0–1.0) showed increasing semiconductor-like normal state behavior and gradual suppression of Tc with Mg. Excess conductivity analyses of Tl0.5Pb0.5Sr2−yMgyCa0.8Yb0.2Cu2O7−δ (y = 0) showed exclusively 1D–2D transition behavior. FTIR absorption data indicate possible tilt of oxygen atoms in the CuO2 plane as a result of unequal bond lengths of Pb–OA–Cu(2) and Tl–OA–Cu(2) and enhanced CuO2 inter-plane coupling. Superconductivity in both series is explained in terms interplay between the effect of increased inter-plane coupling on ξc(0) and hole concentration in CuO2 planes.

Cd-Doped Cu0.5Tl0.5Ba2Ca2Cu3−y Cd y O10−δ (y=0,0.5,1.0,1.5,2.0) Superconductors

Superconducting properties of cadmium doped Cu0.5Tl0.5Ba2Ca2Cu3−yCdyO10−δ (y=0,0.5,1.0,1.5,2.0) samples have been studied using X-ray diffraction, resistivity, ac-susceptibility and FTIR absorption measurements. In X-ray diffraction studies these samples have shown to have tetragonal structure. The zero resistivity critical temperature and magnitude of diamagnetism are suppressed with the increased incorporation of Cd in the final compound. A change in the shape of FTIR absorption spectra, after doping, has shown the incorporation of Cd in the unit cell. A systematic hardening of the apical oxygen modes and softening of the CuO2 planar modes of vibration with increased Cd doping have shown that it is incorporated in the unit cell of Cu0.5Tl0.5Ba2Ca2Cu3−yCdyO10−δ (y=0,0.5,1.0,1.5,2.0) superconductors. The FTIR absorption measurements of these samples have shown that hardening of the apical oxygen modes of types Cu(1)–O(2)–Tl and Cu(1)–O(2)–Cu(2)/Cdy (y=0,0.5,1.0,1.5,2.0) increases with the increase of Cd doping in the samples. A softening of the CuO2 planar oxygen mode Cu(2)–O–Cu(2) is also observed with the increased Cd doping in the final compound. It is most likely that hardening of the apical oxygen modes and the softening of the planar modes of vibration are associated damped harmonic oscillations produced by heavier Cd atoms in the CuO2 planes, which suppress the phonon population from a desired level, reducing the magnitude of superconductivity in the final compound.