Summary of Research Activity

  EPR and Field Dependent Microwave Absorption Studies     

        I have been working on high  Tc   super  conducting  materials  since 1987.  My doctoral thesis is on Electron  Paramagnetic  resonance  (EPR) and  Low  Field  Dependent  Microwave  Absorption  in  High  Tc    Cuprate Superconductors (HTCS).  EPR is a very sensitive  tools  to  characterize the presence of impurity phases in HTCS and it also gives some clue about the localized charge on the copper atoms and their interaction with their environments.

As a probe of the material properties, a microwave measurement is a useful complement to low frequency susceptibility and resistivity measurements for at least two  reasons.   Firstly, it is contact less measurement. Secondly, it permits the study  of  the  resistive  behavior below super  conducting  transition  temperature  Tc .  Additionally,  the changes in the micro structure of the materials are reflected in the  low field dependent microwave absorption (LFDMA). 

We prepared the polycrystalline samples YBaCuO by  different methods and characterized them by resistivity measurements, XRD, EPR and SEM.  LFDMA was studied in details in these well characterized  samples as a function of temperature, modulation field, microwave power and grain size.   All HTCS give intense LFDMA at and below T_c. The derivative signal of LFDMA shows peak and hysteresis.  Peak position (H_m ) of derivative signal of  LFDMA gives the measure of grain size of HTSC materials.

The LFDMA signal arises due to the microwave loss in weak links forming  Josephson  junctions  in  granular  superconductors.   In polycrystalline samples  there  are  a  large  number  of  inter-granular Josephson junctions (JJ) of different sizes and oriented  randomly.   All these  junctions  can be approximated by a single representative resistively shunted JJ. Microwave loss was calculated in the single representative JJ on the basis of the RSJ model. The calculated results agree with that of the experimental results qualitatively. From this we conclude that microwave loss in the polycrystalline samples is due to the intergranular JJ.

We have also studied the field cooling (FC) and field exposure  (FE) effect on the LFDMA  in  YBaCuO & TlBaCaCuO. The measurement of hysteresis area under field cooling and field exposure gives the measure of  lower critical field of inter granular junction (Hc1j ) and that of grain (Hc1g ).  From above measurements  it  has  been  concluded  that TlBaCaCuO    have lower pinning force than YBaCuO. Thus the above measurements gives the measure of relative pinning strength of the materials.

 

Substitutional  studies

 

We have also measured the resistivity and ac susceptibility of Zn doped LaBaCaCu₃O_7-d systems having Zn content of .5%, 1.0%, !.5%,2.0%, 2.5% and 3.0% at wt. From above studies following conclusions have been drawn about the role of Zn in LBCCO samples. (1) The conduction mechanism of electrons appears to follow a crossover from the purely metallic regime to localization regime due to either weak localization or electron-electron interaction effects after about 1.5% Zn. (2) The superconducting transition as revealed by resistivity vs temperature curves or susceptibility vs temperature curves becomes sharper with zn-content