INTRODUCTION General introduction of superconductors
Superconductivity is a phenomenon of certain materials exhibiting zero electrical resistance and below a critical temperature. It was discovered in 1911 by H. Kamerlingh Onnes in Leiden, three years after he first liquefied Helium. He measured the resistivity of mercury and found the critical temperature TC =4.2K [1,2]. The resistivity behavior as a function of temperature is shown in Fig.

Fig. 1.1: Below the transition temperature, the resistivity drops to zero.
The resistivity of mercury vanished completely below 4.2 K, the transition from normal conductivity to superconductivity occurring over a very narrow range of temperature of the order of 0.05 K.
In 1933 W. Meissner and R. Oschenfeld …show more content…
Superconductivity is another phase of great interest that may arise upon compression of a material. 29 elements superconducts under normal conditions . 23 only under pressure.

On applying high pressure to the material, the electronic structure change so that a new phase comes. For example, Yoo et al. [19] observed a rich phase diagram of Mno under high pressure a paramagnetic (PM)-AFM transition, structural distortion, at 30 GPa; a magnetic transition back to PM, along with a structural transition, at 90 GPa; and a Mott insulator-to-metal transition, as well as a volume collapse, at 100 GPa The first pressure observaation performed on superconductors was in 1925 by Sizoo and Onnes [19], for Sn and Li critical temperature decreases.
Lighter elements are easier to compress than the heavier one therefore they exhibit very different and exotic physical properties at extreme pressure[20]. Feng et al. [21] of superconducting phase for CaLi2 at high pressure followed b experiment gives the evidence of bulk superconductivity in the sample. Conflicting views of has given us all the more reason to study the material under high pressure.

Equation of …show more content…
At room temperature, in the tetragonal phase, the Fe atoms form a square net, and each Fe atom is coordinated to four As atoms. This results in layers of edge sharing, slightly distorted tetrahedra [27]. The Ba atoms are coordinated by eight As atoms in a distorted cubic environment by ionic Ba-As bonds. The lattice parameters at room temperature are ‘a’ = 3.963 Å and ‘c’ = 13.017 Å, with the Ba at the Wyckoff position 2a, Fe at 4d, and As at 4e. The fractional coordinate of arsenic zAs = 0.355[28].

The magnetic structure of BaFe2As2 is stripe type and is shown in figure 1.3. The magnetic moments on the Fe atoms are arranged ferromagnetically along the longer Fe-Fe distance in the rectangular Fe nets (along ‘b’ axis) but afm ordering of the moments can be seen along the ‘a’ and ‘c’ axes [27]. Other properties have been investigated through electronic structure calculation [28]. The afm transition has been found through temperature dependent neutron diffraction measurements [29, 30] and is reflected in the resistivity and magnetization measurements [31]. Figure 1.3:The magnetic moments form ferromagnetic chains along ‘b’ axis which is longer Fe-Fe distance in rectangular Fe nets but are antiferromagnetically ordered along ‘a’ and ‘c’ axes. Figure from Mandrus et al. [28] and references there