Setup Of The Low Temperature AC Susceptibility Measurements And Its Application In Correlated Materials

As an important physical parameter, AC susceptibility has been widely applied in condensed matter physics to probe properties of superconductivity and magnetism. This method can be used in various external conditions, e.g. low temperature, high pressure, and high magnetic field, and can enlarge the parameter space.In correlated electron systems, novel types of quantum states and phenomena are emerging from the many body interactions of electrons. Typical examples include unconventional superconductivity, quantum magnetism, quantum phase transitions etc. One can tune the ground states in correlated materials via non-thermal controlling parameters, e.g. pressure, magnetic field. Magnetic susceptibility is a relatively easy characterization method under these extreme conditions.This thesis is constructed by 3 chapters. Chapter 1 is an introduction. This chapter lists various types of magnetism in solids, overviews recent progress in the iron-based superconductors and heavy fermion materials. Particularly emphasis is put into the superconductivity in the iron-based compounds, phase transitions and non-Fermi liquid in heavy fermion systems.Chapter 2 elaborates the AC susceptibility system in detail. First of all, we explain the difference between the measurements of DC susceptibility and AC susceptibility, the significance of building an AC susceptibility measurement system in extreme conditions and the theoretical basis briefly. Then we describe methods of deriving how the susceptometer transforms magnetic signal of a sample to the voltage quantitatively. In section 4, we describe the designs of susceptometers and a self-made winding machine used in making susceptometers. In section 5, we discuss the cancellation of the AC background offset and compare their advantages and defects. Furthermore, a 3He refrigerator, which provides the low temperature conditions, is introduced in the last section.In Chapter 3, we applied our AC susceptibility measurement to two classes of materials:the Iron-based superconductor Ba(Fe1-xCox)2As2 and the heavy fermion compounds Ce2Ni12-xPdxAs7.With Ba(Fe1-xCox)2As2 we tested the liability of our AC susceptibility measurement system. The derived upper critical field Hc2(Tc) for Ba(Fe0.94Coo.o6)2As2 deviates from WHH (Werthamer-Helfand-Hohenberg) theory and can be fitted by a two-gap BCS model. The heavy fermion material Ce2Ni12-xPdxAs7 is a non-central symmetric compound, which can be a good candidate for studying the quantum critical phenomena. Measurements of the specific heat and DC susceptibility suggest a ferromagnetic transition(TC～5K) in the sample with x=0.5, a possible anti-ferromagnetic transition(TN～2.7K) for x=2.5 while the sample of x=1.5 lies in between. The AC susceptibility measurements reveal a splitting of the magnetic transition in an external magnetic field, which is suppressed with further increasing the magnetic field.