CeCuAs tief 2

Abstract

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersKondo insulators and heavy fermion metals are strongly correlated materials that have been investigated for more than 40 years. In these materials, the hybridization between conduction bands and localized states, typically arising from a rare earth element with open 4f shell, leads to the formation of a nonmagnetic ground state with heavy quasiparticles. If the rare earth atoms form a regular lattice, the hybridization with the conduction electrons leads to the opening of a gap. Then, depending on whether the Fermi energy is situated in the gap or within one of the hybridized bands, a Kondo insulator or a heavy fermion metal results. Questions of current interest include: Can one transform these two states into each other? How to understand Kondo materials that are neither metals nor insulators, but have semimetallic properties? Is topology playing an important role in such semimetals? To address these questions we explored the tetragonal intermetallic compound CeCuAs2. Published electrical resistivity data on polycrystalline samples suggest that it may be a Kondo semimetal. Very recently, the group of Arumugam Thamizhavel at the Tata Institute of Fundamental Research in India succeeded to grow single crystals of this compound and provided them to us for the initial physical property characterization. In this master thesis, electrical transport, magnetization, and specific heat measurements were performed on single crystalline samples from several growth batches. The primary goal was to distinguish between intrinsic properties of the compound and effects related to defects, for instance in the form of a deficiency in As. Indeed, the examined samples display either an "insulator-like" increase of the electrical resistivity with decreasing temperature or a "metal-like" decrease; this goes along with pronounced differences in the magnetorestistance and Hall effect. The "insulator-like" behavior is seen in more stoichiometric samples and is thus identified as the intrinsic behavior. For all samples a pronounced anisotropy is observed in the magnetization, with no evidence of magnetic ordering down to 0.4 K. From an entropy estimation of the specific heat data a Kondo temperature of about 4 K is extracted. Thus, the most stoichiometric CeCuAs2 samples available to date classify as Kondo semimetal. To address these questions we explored the tetragonal intermetallic compound CeCuAs2. Published electrical resistivity data on polycrystalline samples suggest that it may be a Kondo semimetal. Very recently, the group of Arumugam Thamizhavel at the Tata Institute of Fundamental Research in India succeeded to grow single crystals of this compound and provided them to us for the initial physical property characterization. In this master thesis, electrical transport, magnetization, and specific heat measurements were performed on single crystalline samples from several growth batches. The primary goal was to distinguish between intrinsic properties of the compound and effects related to defects, for instance in the form of a deficiency in As. Indeed, the examined samples display either an "insulator-like'' increase of the electrical resistivity with decreasing temperature or a "metal-like'' decrease; this goes along with pronounced differences in the magnetoresistance and the Hall effect. The "insulator-like'' behavior is seen in more stoichiometric samples and is thus identified as the intrinsic behavior. For all samples a pronounced anisotropy is observed in the magnetization, with no evidence of magnetic ordering down to 2 K. A Kondo temperature of about 4 K is extracted from entropy, estimated from the specific heat measurements. Thus, the most stoichiometric CeCuAs2 samples available to date can be classified as Kondo semimetals.7