Abstract
In this dissertation, the physical properties of heavy Fermion system CeAl2 and superconductors of Bi4O4S3 and K2Fe4Se5 have been studied by specific heat, magnetic susceptibility and electrical resistivity under high pressure. Two kind of high pressure cells were used, including cylinder Be-Cu cell and diamond anvil cell. The former is used for magnetic susceptibility measurements. The later is used for specific heat and electrical resistivity measurements. CeAl2 fermion system is antiferromagnetism ordering at Neél temperature TN= 3.8 K with Kondo temperature TK=5 K. Under high pressure TN increases initially and then decreases around P = 0.3 GPa. Since the antiferromagnetism originates from RKKY interactions and the exchange constant is a cosine function of the distance among magnetic Ce3+ ions, thus causing the variation of TN and with a maximum of TN = 4.34 K near 0.3 GPa. Moreover, the valence of Ce ions decreases from 3.058 to 3.004 as the pressure increases, this result might be explained by the scenario that the Ce 4f1 energy level is moved to lower energy, the further away from the Fermi level causes a less valence fluctuation with a more localization of the Ce ions. From the resistivity data and the fitting to, the coefficient A is decreased by applied pressure. Since Kondo temperature TK is linearly proportional to, thus and enhanced Kondo temperature is observed. From above results we anticipate that quantum critical point may appear at P= 3 GPa at which TN approaches to 0. K2Fe4Se5, is an interesting and complicated iron-based superconductor system. The superconductivity was observed at 33 K in K1.9Fe4.1Se5 through 750oC annealing. Based on BCS theory and specific data, ΔC/γTc is estimated to be 0.148 and the superconducting energy gap is calculated to 6.39 meV. In high pressure susceptibility measurements of K2Fe4Se5 with 300oC annealing, instead of the Verwey transition neat 120 K, no superconductivity was observed. The temperature of Verwey transition is slightly reduced. as pressure increases up to 1 GPa, Finally, the system Bi4O4S3 is a recently discovered superconductivity with transition temperature at 5 K in 2010. The superconductivity temperature Tc is reduced from 5 K to 4 K by applied pressure. Due to the entropy estimation from integrated from the peak of superconductivity in specific heat is not clear; in order to understand the system further work is needed.