The Exploration Of Cr And Mn Based New Superconductors

The study of superconductivity is a main field in the condensed matter physics,especially in finding the unconventional or high Tc superconductors and understanding of their superconducting mechanism.BCS theory have described the microcosmic mechanism of the superconductivity successfully but Failed for Cu based and Fe based superconductors.Physicists have a rough mechanism of the origin about unconventional superconductivity,such as:quantum fluctuation,magnetic fluctuation and quantum critical point?QCP?,however,finding a final superconducting mechanism is a tough work for physicist.Superconductivity is a many-body excited phenomenon,additionally the magnetic fluctuation existing in the unconventional superconductity,so the situations become more complex.The research of superconductivity in unconventional superconductors such as heavy-fermions,high transition-temperature cuprates and iron pnictides has been promoting the development of condensed matter physics in recent decades.The superconductivity in these materials usually emerges in the vicinity of long-range antiferromagnetically ordered state.In addition to doping charge carriers,the application of external pressure is an effective way to induce unconventional superconductivity near a magnetic quantum critical point.Our work is to search more new superconductors and help theoretical physicists to establish the final mechanism of unconventional superconductity.The thesis is organized as follows.The first chapter present an overview of the major events related to the phenomenon of superconductivity.We illustrate several mechanisms of superconductivity in various models.We also review four methods related to experience of discovery of superconductors in history.1.Matthias Rules,2Effects of band overlap,3 Quantum criticality,4 Analogy with same structral unit of known superconductors.In addition,we present the phenomenological theory to predict high Tc superconductors:1.Metalation of?band for enhancement of Tc.2.The gene for high Tc superconductors.The following second chapter describes the experimental details of how to measure the PXRD,the resistivity,magnetic susceptibility and specific heat.The third chapter presents our main works for the discovery of 5 new superconductors.?1?The frist one is CrAs.We summarize the basic physical properties of CrAs,and the discovery of superconductivity in the vicinity of double helical antiferromagnetic order in CrAs via external pressure.In addition,we summarize several follow-up studies on structural,magnetic and superconducting mechanism.Accompanied with discontinuous changes of lattice parameters,a first-order double helical magnetic transition occurs at about 265 K.This magnetic transition can be suppressed under pressure, and superconductivity is found at a maximum transition temperature of 2K when the helical magnetic transition is completely suppressed at 0.8 GPa. Some experimental evidences suggest an unconventional pairing mechanism for CrAs.The observations of a dramatic enhancement of the effective mass and non-Fermi-liquid behavior near critical pressure in the transport measurements are the characteristics of antiferromagnetic quantum critical point.Critical spin fluctuations play an important role in understanding the superconductivity of CrAs.The present finding opens a new way for searching novel unconventional superconductors in the Cr and other transition metal-based systems.?2?The second one is MnP.We report the discovery of superconductivity on the border of magnetic order in the helimagnet MnP via the application of high pressure.Superconductivity with T_sc?1 K emerges and exists merely near the critical pressure Pc?8 GPa,where the long-range magnetic order just vanishes.The present finding makes MnP the first Mn-based superconductor.The close proximity of superconductivity to a magnetic instability suggests an unconventional pairing mechanism. Moreover,the detailed analysis of the normal-state transport properties has proved non-Fermi-liquid behavior and the dramatic enhancement of the quasiparticle effective mass near Pc associated with the magnetic quantum fluctuations.?3?The ternary rare earth chromium nitrides Pr₃Cr₁₀N₁₁ is contaminated by a small amount of Cr₂N.Superconductivity is found at about 5.2K and H_C22 is 12.6T beyond the Pauli limit.Pr₃Cr₁₀N₁₁ is the first superconductor in chromium nitrides compounds.?4?The cystal structure of Sc₃C₄ is a Carbide with C3 unit.It crystallizes in a tetragonal structure with space group P4/mnc.Sc₃C₄ shows a Pauli paramagnetism and a metallic conduction at low temperatures.If we doped with 10%B into the C site,superconductivity transition at about 8K is observed.This may be induced by raising the?or?band above the Fermi level.?5?We discovered the Superconductivity in WP near 0.8K,WP belong to the B8₁ structure,which is same structure with CrAs and MnP.