Abstract:
(Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun-yCdy)O2n+4-δ (n=3, 4) high Tc superconductors have been
synthesized by solid state reaction method. The samples were characterized by dc-
electrical
resistivity,
ac-magnetic
susceptibility,
XRD,
FTIR,
and
dielectric
measurements. The main objective of these experiments was to investigate the possible
existence of electron-phonon interactions in the mechanism of high Tc superconductivity.
Owing to our previous studies in which we have achieved enhanced superconductivity by
doping
Zn
at
the
Cu-sites
in
(Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun-yZny)O2n+4-δ
superconductors, we have doped heavier Cd atoms at the Cu sites in (Cu0.5Tl0.5)Ba2(Can-1-
xMgx)(Cun-yCdy)O2n+4-δ
superconductors. Since Cd (4d10) and Zn (3d10) have similar
ground state electronic configurations but the atomic mass of Cd is twice as that of Zn; it
was expected that similar enhancement of superconductivity will be observed in Cd-
doped samples if electron-phonon-interactions were not essential for mechanism of
superconductivity in these compounds. The XRD of (Cu0.5Tl0.5)Ba2(Can-1-xMgx)(Cun-
yCdy)O2n+4-δ
samples revealed orthorhombic structure following PMMM space group
along with a decrease in the c-axis length with the enhanced doping of Cd and Mg. From
the resistivity measurements, a sequential suppression of the Tc(R=0) was observed with
the increasing doping concentration. The ac-susceptibility measurements have shown
suppression of the transition temperature from normal to superconducting state and an
overall decrease in the magnitude of diamagnetism with the increasing x and y-contents.
The Tc(R=0) and magnitude of diamagnetism in (Cu0.5Tl0.5)Ba2Ca3(Cu4-yCdy)O12-δ
samples were improved after annealing in oxygen atmosphere. This improvement most
likely arises due to the incorporation of oxygen in the charge reservoir layer which
optimizes the carriers’ density in the superconducting CuO2 planes. In the FTIR
absorption spectra, the apical oxygen modes are systematically changed with the
increasing concentration of both the Cd and Mg which clearly indicate the incorporation
of Cd and Mg in the unit cell. Fluctuation induced conductivity (FIC) analyses of the Mg-
free (Cu0.5Tl0.5)Ba2Ca3(Cu4-yCdy)O12-δ samples were carried out in the neighborhood of Tc
and above by applying the Aslamazov-Larkin (A-L) and the Lawrence-Doniach (L-D)
models. Coherence length along the c-axis (ξc(0)), phase relaxation time (τφ), and the
vii
Fermi velocity of the carriers (VF), were calculated from these analyses. The 3D to 2D
cross-over temperature (T3D-2D), and the temperature at which the resistivity curve starts
deviation from linear behavior (T*), are shifted to lower temperatures while width of the
transition region (∆Tc) is shrunken with the enhanced Cd-concentration. It is most likely
that variation in the apical oxygen modes of vibration are associated with the damped
harmonic oscillations of the heavier Cd-atoms (ions) in the CuO2 planes, which suppress
the population of the desired phonons (required for electron-phonon interactions) from its
optimum level. This reduced population of the desired phonons causes suppression in the
density of the Cooper pairs which ultimately result into reducing the magnitude of
superconductivity in the final compound. In the FIC analyses results, the suppression of
T3D-2D, T*, and the narrowing width of the superconducting transition with the enhanced
Cd-doping strongly support this argument. Further, the temperature and frequency
dependent measurements of the capacitance (C) and conductance (G) were carried out in
the frequency range of 10 KHz–10 MHz at various temperatures between 80 and 300K
for Mg-free samples, from which the dielectric constants (ε′, ε′′) and ac-conductivity (σac)
were calculated. Negative capacitance (NC) was observed for all the samples of the
series. Large values of NC were observed at lower temperature and frequencies which
may be attributed to the reduced thermal vibrations and higher polarizability of the
material. The polarization of the samples most likely arises due to the displacement of
mobile charges in the CuO2/CdO2 planes by the applied electric field relative to the static
charges at Ba2+, Tl3+, and Cu2+ sites in the charge reservoir layer. But with the increasing
concentration of Cd, the NC, dielectric constant and the ac-conductivity are
systematically decreased at low temperatures and frequencies. The decrease in the values
of ε′ and σac are most likely related to the induced anharmonic oscillations of the heavier
Cd atoms in CuO2/CdO2 planes reducing the polarizability of the final compound and
eventually suppresses the dielectric properties. All these studies strongly suggest that the
role of electron-phonon interactions is essential for the mechanism of high Tc
superconductivity in oxide superconductors.