Studies Of Physical Properties In A Strong Magnetic Field For Several Correlated Materials

Promoted by the recent developments of experimental techniques under extreme conditions of magnetic field and high pressure, emergent quantum states and macroscopic quantum phenomena are being revealed in the electron correlated materials. In this Disserssation, we systematically studied the physical properties of the heavy fermion antiferromagnet CeRhIn5and several iron pnictides compounds by measuring the transport, magnetic and thermal properties in a magnetic field up to70T and at temperatures down to0.3K. We fouced on the field-induced changes of Fermi surface topology and its relation to the magnetic quantum critical point (QCP) in the heavy fermions, the upper critical field and vortex motion in iron-based superconductors and the origins of magnetism in the parents compounds of iron pnictides superconductors.We performed systematic measurements of the ac specific heat, dHvA oscillations in a pulsed magnetic field up to75T at Los Alamos and the Hall resistivity and magnetization in the45T DC hybrid magnet at Tallahassee for CeRhIn5, which led to the following important discoveries. For the first time, we were able to map the temperature-magnetic field phase diagram of CeRhIn5(both B//a and B//c), which shows a field-induced QCP around Bc0=50T. Second, a field-induced sharp change of the Fermi surface as well as the Hall coefficient are observed around B*≈30T, which is well inside the antiferromagentic (AFM) state. Such a Fermi surface reconstruction is not accompanied by a phase transition in the specific heat and magnetic susceptibility. Based on the dHvA measurements and band structure calculations as well as the comparisons with LaRhIn5and CeCoIn5, evidence for a localized-itinerant transition of the Ce4/-electrons due to breakdown of Kondo effect is provided. These findings point to a field-induced spin-density-wave (SDW) QCP for CeRhhIn5, suggesting that multiple quantum critical points may exist in the pressure-magnetic field phase diagram of the same compound and the Fermi surface can properly characterize the nature of the AF QCPs. Our results will provide new insights to the establishment of a global phase diagram for the heavy fermions.The second part of this Dissertation studies the behavior of upper critical field Bc2(T) for a series of iron based superconductors, including LiFeAs, Tl0.58Rb0.42Fe1.72Se2, Ba(Fe,Co)2As2,(Sr, Na)Fe2As2, by means of measuring the electrical reistivity in a pulsed field up to65T. Even though the iron-based superconductors crystallize in a layered structure as that of the high Tc cuperates, all these compounds we studied show a weak anisotropy of the upper critical field Bc2(T) with decreasing temperature, suggesting the important role of the layer couplings on superconductivity. Furthermore, a large upper critical field is typically observed at zero temperature, i.e., Bc2(0). These findings indicate that the iron-based superconductors not only show rich physics but also point to potential applications.In order to characterize the nature of the magnetic/structural transitions in the parent compounds of the iron-based superconductors, we systematically measured the magnetoresistance and Hall resistivity in a pulsed magnet of65T for BaFe2As2, NaFeAs and LaOFeAs, which show the following common features:(1) The magnetic transition is highly robust against the magnetic field, giving strong evidence that the magnetic ordering is formed by local moments.(2) The magnetoresistance is negligible above the structural phase transition Ts, but becomes significantly enhanced upon cooling down below Ts.(3) The Hall resistivity ρxy(B) is small but linear field dependent for T> Ts. On the other hand, ρxy(B) shows non-linear field dependence below the structural transition. These results indicate that the electronic structure may undergo a pronounced change at Ts, and the magnetic order may originate from the structural transition in iron pnictides. Furthermore, our results also imply the dual character of the3d-electrons in iron pnictides, i.e., the magnetic order is formed by localized moments and the others give rise to complex transport properties through their interaction with the former.