Phase competition and impurity effects in the single crystals of (Li,Na)FeAs superconductors

Abstract

Fe-based superconductors are newly-found unconventional superconductors after the famous high T_c cuprate superconductors were discovered. Since the Fe-based superconductors shares many common characteristics with the high T_c cuprates, such as the magnetic ground state of pristine compounds and the superconductivity induced by chemical doping or external pressure, they have attracted lots of interest from many researchers in the condensed matter physics. The spin-fluctuation-mediated pairing mechanism and multi-band nature of the Fe-based superconductors are predicted the result of the s±-wave gap symmetry, which is rather unique among the reported superconductors. Among the Fe-based superconductors, LiFeAs and NaFeAs, so called 111 systems, are exceptional due to their intriguing ground states. At the ambient condition, LiFeAs is known to be a clean limit bulk superconductor without any structural and magnetic transitions while NaFeAs has the coexisting phases of the spin-density-wave ground states and the superconductivity. These unusual ground states will provide better chances to investigate the superconductivity and the phase competition of the Fe-based superconductors. In this thesis, high quality single crystals of LiFeAs and NaFeAs are successfully grown by the flux methods and the crystals with various doping are also prepared to investigate the superconductivity of the 111 systems. At first, the physical properties of the LiFeAs single crystals are investigated. The bulk superconductivity of LiFeAs is examined by the means of the magnetic susceptibility and the specific heat. The upper critical fields, H_c2, near the transition temperature are measured and their anisotropy evidences the electron-electron correlation in the system. The initial slope of the upper critical fields, in addition, imply the LiFeAs is a clean limit superconductor. The doping dependence is also investigated in the Li(Fe1-xCox)As and Li(Fe1-xMnx)As series. The Fermi liquid temperature dependence is found in the resistivity of the Li(Fe1-xCox)As series, and the increase of the Fermi liquid T^2 coefficient indicates the Fermi energy level shift induced by the Co doping with additional electron carriers. The magnetization curves of the Li(Fe1-xMnx)As series indicate that the Mn content introduces local magnetic moments into the system. The suppression of the superconductivity, moreover, by the Mn content is much stronger than one of the Co and Cu content. This result supports the s±-wave gap symmetry and the spin-fluctuation-mediated pairing mechanism of the superconductivity of the LiFeAs system. At last, the complete phase diagram of the (Na1-xLix)FeAs series is realized by the means of the magnetic susceptibility, the electrical resistivity, and the Hall effect measurements. The spin-density-wave ground states is suppressed by the Li substitution, and the superconductivity arises from the phase competition. The superconducting gap structure, furthermore, of the optimal (Na0.95Li0.05)FeAs is investigated. Both results of the lower critical fields and the specific heat measurements indicate that a small gap of ~ 0.5 meV is dominant in the system within the analysis based on the 2-gap model. The underlying nature of the isovalent Li substitution effect resulting the development of the superconductivity in the phase space and the small-gap-dominant superconducting gap structure is discussed.