P Doping Effect In EuFe₂(As_1-xP_x)₂ Single Crystals

As two different kinds of ordered phenomena in condensed matter physics, both superconductivity (SC) and ferromagnetism (FM) are macroscopic manifestations of microscopic quantum effect. Owing to an orbital mechanism and a paramagnetic effect for the spin-singlet SC, SC is incompatible with magnetism. Moreover, superconductivity will be suppressed by the magnetic field and finally disappear.Earlier experiments revealed the possible coexistence of SC and FM under narrow regimes of temperature and external field, such as ErRh4B4 and Ho1.2Mo6S8. After this, the coexistence of SC and FM was discovered in many other kinds of new materials. The discovery of Fe-based superconductors in 2008 brought new findings on the interplay of SC and FM. As a unique "122" compound, EuFe2As2 shows both SC coming from 3d electrons of the FeAs-layers and long-range magnetic ordering in the Eu sub-lattice because of the 4f electrons upon appropriate doping. The object studied in this thesis is EuFe2(As1-xPx)2 system, which shows the coexistence of SC and FM.The thesis consists of four chapters. The first chapter presents the background of Fe-based superconductors. In this chapter, first we presented the crystal structure of the iron-based superconductors, then discussed the methods to gain SC, including electron or hole doping, physical pressure and chemical pressure. The second chapter is a summary of the experimental techniques we used, where the x-ray diffraction (XRD), photolithography, coating technology, resistivity, magnetic susceptibility measurements and preparation of Hall electrode were presented in detail. The fourth chapter is the summary and expectation.The third chapter presents the experimental results and discussion. XRD analysis for a range of EuFe2(As1-xPx)2 single crystalline samples with different nominal compositions, along with other structure analysis, suggests that P was successfully doped into As-site. The temperature-dependent resistivity shows the suppression of spin-density-wave (SDW) and the appearance of SC with P-doping. As we can see in magnetic susceptibility measurements, the diamagnetic behavior in the EuFe2(As1-xPx)2 single crystal system was very different from that of the usual superconductors. The system is also in possession of ferromagnetism, very fascinating, which calls for further study. The Hall-effect measurement for EuFe2(Aso.55Po.45)2 single crystal indicates the dominance of electron-type charge carriers. Through the magneto-resistance measurements in different directions, we gain the upper critical field (HC2) for EuFe2(As0.65P0.35)2 single crystalline sample. When the current is in the basal ab plane and the applied magnetic field parallel to the ab plane, μ0Hc2 (0)= 26.1T, while the applied magnetic field is perpendicular to the ab plane, μ0HC2 (0)= 17.1T. The upper critical field (HC2) obtained through the magneto-resistance measurements suggests the obvious anisotropy of EuFe2(As0.55P0.45)2 single crystalline sample.Similar results were also obtained for x=0.3 and x=0.4 samples.