Magnetism And Superconductivity In Eu-based Compounds

Rare-earth compounds exhibit various and exotic physical phenomena, such as intermediate valence, Kondo effect and heavy fermion behavior, and thus have been the subject of intense re-search in condensed matter physics. The competition between Ruderman - Kittel - Kasuya - Yosida(RKKY) interaction and Kondo effect can be commonly found in these materials. Some Eu-based compounds show fluctuating valence between Eu2+ (4f6) and Eu3+ (4f7) configurations. This kind of valence instability was found to be associated with non-Fermi liquid behavior and exotic superconductivity (SC). An important feature of Eu-based systems is that the valence state of Eu is strongly dependent on temperature. In present dissertation, we mainly focused on the relationship between the Eu valence state, magnetism and SC in three Eu-containing layered compounds.This dissertation covers Eu3Bi2(S,Se)4F4,(Eu,Sr)3Bi2S4F4 and EuPt2As2 three systems. In the former two layered bismuth chalcogenides, we studied the doping effect on the Eu valence state and its relation to SC, while in EuPt2As2, we established a complicated magnetic phase diagram.We synthesized a series of Se-doped Eu3Bi2(S,Se)4F4 polycrystalline samples by a solid-state reaction method and investigated the negative chemical pressure effect induced by Se doping in this system. With Se/S doping, the charge-density-wave-like anomaly in the normal state of resistivity is suppressed, and meanwhile the superconducting transition temperature (Tc) is enhanced. For x =2.0, Tc reaches 3.35 K, which is more than twice the value of Tc in the parent compound. Magnetic-susceptibility and specific-heat measurements reveal that the average Eu valence has an obvious increase with increasing Se content. This means that more charges can be transferred from Eu3+ to the conducting BiS2 layer. Further measurements of Hall-effect confirm the remarkable enhanced charge carrier density (both electron- and hole-type) after Se doping. As iso-valent substitution principally does not introduce extra electrons, we proposed that the Eu valence variations may account for the effect of Se doping in Eu3Bi2S4F4 superconductor.For the Eu3-xSrxBi2S4F4 system, through the partial substitution of Sr for Eu, Tc gradually decreases and superconductivity disappears above 0.3 K when x> 1.0. For the x = 2.0 sample, its resistivity in the normal state shows semiconducting-like behavior, contrast to the metallic behavior in the lower Sr doping concentration. Magnetic-susceptibility and specific-heat measurements reveal that Sr substitution leads to a decrease in both Eu2+ and Eu3+ populations. The decreased Eu3+ population, and the corresponding lower charge carrier density, may be the main origin for the suppression of superconductivity. In addition, we find a significant increase in the Sommerfeld coefficient upon Sr doping, which may be due to the Kondo effect between the magnetic moments(associated to Eu2+ ions) and the conducting electrons. The enhanced electron correlation due to Kondo effect may also partially account for the strongly suppressed Tc.We prepared millimeter-sized EuPt2As2 single crystalline by self-flux method and carefully investigated its physical properties. The electrical resistivity, magnetic susceptibility and specific-heat measurements all reveal the presence of two antiferromagnetic (AFM) transitions (TN1=7.1 K and TN2 = 5.8 K) at low temperatures. Based on the results of magnetic susceptibility measurements, we propose that the Eu2+ moments are likely to align almost collinearly within the ab plane in the temperature range between two transitions, and the spins are canted out toward the c axis below TN2. While TN1 was attributed to the formation of AFM ordering of Eu2+ magnetic moments,TN2 may be associated with a spin reorientation. Finally, we obtained the magnetic phase diagram of EuPt2As2 with magnetic fields along c axis.