The Exploration Of New Unconventional Superconductors Based On Transition Metals

Unconventional superconductors are superconductors which cannot be understood by conventional BCS theory using phonon as pairing glue, the most typical examples are cuprates, iron-based superconductors, heavy fermion superconductors and organic superconductors. Having studied for nearly half a century, physicists believe that the mechanism of unconventional superconductors may be related to the magnetic fluctu-ations approaching some quantum critical point, however, up to now they still cannot find a unified and self-consistent picture for them. Therefore, it is a very significan-t topic for us to find more unconventional superconductors to provide new platform, and then study their physics to give more clues for the mechanics of unconvention-al superconductors. In this paper, based on some fundamental principles of finding unconventional superconductors, we carry out some interesting trials to explore new superconductors based on transition metal compounds.In chapter one, we firstly summarized conventional superconductivity phenomenon and its physics picture, and then briefly introduced the physics of the four kinds of un-conventional superconductors:cuprates, iron-based superconductors, heavy fermion superconductors and organic superconductors. Then we did an overview of history and sorted the many trials for finding unconventional superconductors, and further tried to present the fundamental principals of finding new superconductors, at last we give an example of designing Co or Ni-based high temperature superconductors.In chapter two, we introduced how we grow and identify crystals, as well as the experimental and analytical methods to do the electrical/heat transport, susceptibility and high pressure measurements.In chapter three, we use LaRu2P2 material as a platform to study the electron correlation effect in Ru-based 122 system. We observed the enhancement of supercon-ducting transition temperature Tc under pressure P in LaRu2P2 single crystal, which cannot be explained by the McMillan’s theory. A self-consistent analysis on the pres-sure induced evolution of normal state resistivity and upper critical field suggests that some extra correlation electron-boson interaction is induced by applying the pressure, which may be the cause of the pressure enhanced superconductivity. Based on First Principle calculation for electron and phonon structure, we proposed possible origin of the enhanced correlation effect, and explained why there is a dome-like structure in Tc-p phase diagram. The present work strongly suggests that LaRu2P2 may have some commonalities as the iron based superconductors in which the spin fluctuations are supposed to play some roles in forming the Cooper pairs, this will stimulate fu-ture studies on finding some correlation-induced unconventional superconductors in Ru-based system.In chapter four, we introduced some of our trials to find new superconductors. Firstly we we studied polycrystalline samples of EuPdAs, through resistivity and mag-netic susceptibility measurements, we observed that EuPdAs shows a phase transi-tion at 180 K. With application of hydrostatic pressure, the transition at 180K is to-tally suppressed under a pressure of 0.48GPa, and the resistivity curve is depressed monotonously upon increasing pressure. However, superconductivity has not been in-duced at low temperature with pressure up to 1.90 GPa. This indicates that the 180 K anomaly is merely a valence change transition and may not relate to the formation of any long range magnetic order. In order to achieve some novel electronic properties, we suggest that tuning the occupation of the 4d orbitals of Pd with electrons would be essential. Secondly, we observed a ferromagnetic transition at about 302 K and a helical magnetic transition below about 61 K for MnP polycrystalline samples. It is found that the magnetization in the low temperature helical magnetic state exhibits hysteresis, while the hysteresis in the ferromagnetic state is almost absent. The appli-cation of a high magnetic field will gradually suppress the helical magnetic state and turn the system into a ferromagnetic one, while the hydrostatic pressure applied on the sample will make the ferromagnetic transition move to lower temperature very slowly. We also discussed a chemical doping experiment from an insulator RuP to a metal CrP, we observed a suspicious quantum critical point which may benefit the emergence of superconductivity at the doping level of Cr60%.In chapter five, we at first introduced some background knowledge for quantum spin liquid state(QSL), then we discussed our chemical doping studies for an unconven-tional superconductors LaRu3Si2 which forms a Kagome lattice. By doping different rare and transition metals on the La and Ru sites respectively, we obtained the whole phase diagram from LaRu3Si2 to CeRu3Si2, and find that all the doped rare and tran-sition metals suppress superconductivity in LaRu3Si2 very slightly. In particular, the suppression effect of doping Ni and Cr magnetic elements on Ru site is much smaller than that by doping Fe, which is also a magnetic element. By fitting and analysing using Curie-Weiss law, We find that the effective magnetic moments for Ni and Cr im-purities are very small, indicating that Ni and Cr actually do not exhibit very strong local magnetic scattering center as Fe ions do, while only act as itinerant magnetic mo-ments.Chapter six is a summary of this paper.