Optimization of carriers by self-doping in Cu0. 5Tl0. 5Ba2Ca2Cu3-yMyO10-δ superconductor (original) (raw)
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Low Temperature Physics, 2010
The effect of suppression of antiferromagnetic order and charge stripes formation mechanism, by Zn3d 10 (S = 0) substitution at Cu3d 9 (S = 1/2) sites in CuO 2 planes of Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4 O 12-δ , on superconductivity parameters is demonstrated. If antiferromagnetism and charge stripes have some role in bringing about superconductivity at a particular temperature, the doping of nonmagnetic Zn impurity would destroy it and hence the superconductivity. However we have observed enhanced superconducting properties with the increase of Zn doping in Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4 O 12-δ superconductor. The decreased c-axis length with the increase of Zn doping improves interlayer coupling and hence the three-dimensional conductivity in the unit cell. The role of charge carriers in CuO 2 planes of Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4-y Zn y O 12-δ in bringing about superconductivity has also been explained. PACS: 74.70.-b Superconducting materials other than cuprates; 74.62.Bf Effects of material synthesis, crystal structure, and chemical composition; 74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses). Keywords: Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4-y Zn y O 12-δ superconductors, antiferromagnetism, charge stripes, charge carriers.
Electron Phonon Interactions and Superconductivity
Electron–Phonon Interactions and Superconductivity in (Cu 0.5 Tl 0.5)Ba 2 Ca 3 (Cu 4−y Ti y)O 12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) Abstract We have used two-step solid state reaction method for the synthesis of (Cu 0.5 Tl 0.5)Ba 2 Ca 3 (Cu 4−y Ti y) O 12−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 pla-nar 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 (1.32Å) atoms relative to Cu (1.17Å) 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 superconductiv-ity parameters with Ti doping at the Cu sites shows the essential role of phonon in mechanism of high T c supercon-ductivity 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.
Journal of Superconductivity and Novel Magnetism, 2015
We have synthesized (Cu 0.5 Tl 0.5)Ba 2 Ca 3 (Cu 4−x Ti x)O 12−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 CuO 2 /TiO 2 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
Excess conductivity of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductors
Low Temperature Physics, 2012
Oxide high-T c superconductors (HTSC) are anisotropic in character since the charge carriers have free moment in the conducting CuO 2 planes [1] whereas their motion is impeded by insulating/partially insulating MBa 2 O 4-δ (M = Y, Bi, Hg, Tl, CuTl, etc.) charge reservoir layers. In the transport process the charge carriers have to tunnel across insulating/partially insulating barriers along the c-axis and across the grain boundaries, which promote a fluctuation in the order parameter and in turn to the conductivity of the carriers. The studies of such fluctuation conductivity (FIC) may help in understanding the intrinsic mechanism of superconductivity. Here the electrical resistivity ρ(T) versus temperature data of as-prepared and oxygen post-annealed Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4-y Zn y O 12-δ (y = 0, 0.5, 1.5, 2.5) samples is studied for FIC analyses in the temperature regime well above the critical temperature; such analyses have been carried out by employing Lawrence and Doniach (LD) and Maki-Thompson (MT) models. The coherence length, inter-plane coupling, exponent, dimensionality of fluctuations and the phase relaxation time of the carriers are determined from such analyses. It is observed that the crossover temperature associated with two distinct exponents fits very well with the two-dimensional (2D) and three-dimensional (3D) LD equations. The crossover temperature T 0 is shifted to higher temperatures with enhanced Zn doping. The 3D LD region is shifted to higher temperature with the increased Zn doping. We have elucidated from these analyses that lower Tl content in the final compound may increase the charge carrier's doping efficiency of MBa 2 O 4-δ charge reservoir layer, resulting into an increase in the coherence length along the c-axis and superconductivity parameters. A small decrease in the coherence length along the c-axis ξ c (0) is observed in the samples with Zn doping of y = 1.5 whereas ξ c (0) increases in the samples y = 0.5, 2.5. In comparison with as-prepared samples, the ξ c (0) decreases after post-annealing in oxygen atmosphere. It is most likely that a decrease in the density of charge carrier's is promoted by oxygen diffusion in the unit cell may suppress the ξ c (0). The increase oxygen diffusion is evidenced from the softening of phonon modes after postannealing in oxygen atmosphere. The decreased population of small spins of Cu atoms induced by doping of Zn is viewed in the terms of suppression of spin gap and hence the pseudo-gap in Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4-y Zn y O 12-δ (y = 0, 0.5, 1.5, 2.5) samples.
Growth of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductor with optimum carriers
Physica C: Superconductivity, 2010
We have tried to vary the carriers concentration in Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4Ày Zn y O 12Àd (y = 0, 1, 1.5, 2, 2.5) superconductor with the help of post-annealing experiments carried out in nitrogen, oxygen and air and to investigate its effects on the superconductivity parameters. The zero resistivity critical temperature [T c (R = 0)], the magnitude of diamagnetism and critical current [I c (H = 0)] are found to increase in Zn free samples after post-annealing in oxygen and air, while these superconducting properties have been suppressed after post-annealing in nitrogen at 550°C for 6 h. The post-annealing of Zn-doped samples in air has marginally increased the superconducting properties, while these properties have been suppressed after post-annealing in nitrogen and oxygen. These studies have led us to the definite conclusion that the Zn-doped material has grown with optimum carriers concentration.
Ti-doped (Cu 0.5 Tl 0.5 )Ba 2 Ca 2 (Cu 3Àx Ti x )O 10Àd (x = 0, 0.25, 0.50, 0.75) superconductors
Keywords: (Cu 0.5 Tl 0.5)Ba 2 Ca 2 (Cu 3Àx Ti x)O 10Àd superconductors Transition metals doping Increased T c Suppressed density of phonons a b s t r a c t Ti-doped (Cu 0.5 Tl 0.5)Ba 2 Ca 2 (Cu 3Àx Ti x)O 10Àd (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 T c (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 T c (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 super-conductivity (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 CuO 2 /TiO 2 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 (47.90 amu) in comparison with Cu (63.54 amu) atoms; the atoms with lighter-mass vibrate at higher frequency. It is most likely that the existence of atoms of different masses in CuO 2 /TiO 2 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 0 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 T c 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 T c superconductivity.
2014
O 10-d samples are synthesized by a solid-state reaction method and characterized by X-ray diffraction, DC resistivity (q), AC susceptibility (v ac ) and Fourier transform infrared absorption spectroscopy. The suppression of zero resistivity critical temperature (T c (R = 0)) and the magnitude of diamagnetism is most likely due to the reduced efficiency with which (Tl 1-x Y x )Ba 2 O 4-d charge reservoir layer supplies the carriers to the CuO 2 planes. The localization at Y 3+ reduces the number of carriers in the CuO 2 planes and thereby promotes antiferromagnetism, which is fatal for superconductivity.
Enhanced Superconductivity in�(Cu0.5Tl0.25Cs0.25)Ba2Ca2Cu3O10-d by Cs Doping
J Low Temp Phys, 2008
The self doping of carriers in CuO 2 planes (accomplished through post annealing in air, N 2 and O 2 atmospheres in previous studies) is replaced by a more efficient alkali metal dopant such as Cs. The doping of Cs in the Cu 0.5 Tl 0.5 Ba 2 O 4−δ charge reservoir of Cu 0.5 Tl 0.5 Ba 2 Ca 2 Cu 3 O 10−δ superconductors has been found to enhance its superconducting properties. Enhanced superconductivity parameters, such as T c , H c and J c , most likely arise from the enhanced doping efficiency of Cu 0.5 Tl 0.25 Cs 0.25 Ba 2 O 4−δ charge reservoir layer; since the alkali metals are known to lose their outermost "s-orbital" electron, which could be supplied to conducting CuO 2 planes. The distance between efficiently doped CuO 2 planes is reduced by Be and Mg doping at the Ca Sites. The quantity of diamagnetism and zero resistivity critical temperature [T c (R = 0)] are suppressed by these substitutions. The postannealing of the Cs doped samples further enhances their superconducting properties; oxygen doping most likely promotes the optimum holes concentration in the superconducting state in the Cs doped samples. These observations have also shown that the free carrier density plays a significant role in the mechanism of superconductivity which was accomplished by synthesizing (Cu 0.5 Tl 0.25 Cs 0.25 )Ba 2 Ca 2 Cu 3 O 10−δ superconductors.
Journal of Alloys and Compounds, 2009
Cadmium doped Cu 0.5 Tl 0.5 Ba 2 Ca 3 Cu 4−y Cd y O 12−ı (y = 0, 0.25, 0.5, 0.75, 1.0) samples have been synthesized and their superconducting properties are studied using X-ray diffraction (XRD), resistivity, ac-susceptibility and Fourier Transform Infrared (FTIR) absorption measurements. These samples have shown tetragonal structure, their unit cell volume is decreased with the increased Cd doping. The zero resistivity critical temperature [T c (R = 0)] decreases and the magnitude of diamagnetism is suppressed with the increased concentration of Cd in superconductors. The T c (R = 0) and the magnitude of diamagnetism in these compounds are improved after post-annealing in oxygen atmosphere. It is most likely that the increased concentration of oxygen in Cu 0.5 Tl 0.5 Ba 2 O 4−ı charge reservoir layer optimizes the density of carriers in the conducting planes to the desired level consequently increasing superconductivity in the final compound. Changes in the shape of FTIR absorption spectra after Cd doping have shown incorporation of Cd in the unit cell of the final compounds. The FTIR absorption measurements of these samples have shown hardening of apical oxygen modes of type Cu(1)-O A-Cu(2)/Cd y (y = 0, 0.25, 0.5, 0.75, 1.0) with increased Cd doping. It is most likely that hardening of apical oxygen modes of vibration are associated with the damped harmonic oscillations of heavier Cd atoms in the CuO 2 planes which suppress the phonons population from a desired level reducing the magnitude of superconductivity in the final compound.
Doping of the CuO2 planes of Cu1−xTlxBa2Ca2Cu3O10−y superconductor via light and heavier ions
Physica C: Superconductivity, 2003
The superconducting properties of Cu 1Àx Tl x Ba 2 Ca 2 Cu 3 O 10Ày thin films prepared by amorphous phase epitaxial (APE) method have been studied, by resistivity measurements, X-ray diffraction, electron microscopy and infrared spectroscopy. The main emphasis of this research work is on the doping of the charge reservoir layer, which ultimately controls carrier concentration in the CuO 2 conducting planes. The phenomena of the superconductivity, takes place in the conducting CuO 2 planes, however, we can control it via charge concentration in the CuO 2 planes. We have done this by carrying out annealing experiments in different conditions i.e. in air, nitrogen atmosphere and in vacuum. In these annealing experiments the doping is carried out by intercalation of the light O 2 in the CuO chains and by the removal of heavy thallium from the CuO chain axis. The removal of thallium at 717°C from the CuO chains shifted the T c (R ¼ 0) to higher values and the doping of oxygen and nitrogen at 450°C increased the semiconductivity and shifted the T c (onset) to lower values. The removal of oxygen from CuO chains by vacuum annealing at 450°C has also increased the degree of semiconductivity and shifted the T c (onset) to lower temperatures. The XRD of the samples of Cu 1Àx Tl x -1223 phase showed the predominant c-axis oriented material with c-axis lattice constant 15.53 A A. The electron micrograph of these samples showed elongated grains with the grains ends fused together. The surface roughness of the samples was below 0.2 lm. The infrared absorption measurements have shown the softening of 451 cm À1 mode after removal of oxygen from the unit cell, which shows that oxygen doping controls the carrier concentration in the charge reservoir layer and hence the superconductivity in the material.