Physical Properties And Phase Diagram Of Unconventional Superconductors

The search of new superconductors and exploring superconducting(SC)mechanism is one of the advanced subjects in condensed matter physics.Element substitution not only helps to develop new SC systems and leads to the evolution of ground state and physical property,but also reveals the intrinsic factors crucial to superconductivity.This dissertation mainly covers three families of low-dimensional superconductors:(1),The Ce doping effect in LaFePO.Upon Ce doping,SC is suppressed,then single-ion Kondo effect appears,this system finally evolves into a dense Kondo lattice;the coexistence of spin-glass behavior and the Kondo impurity phase is observed.(2),The lower critical magnetic field Hc1 measurements in the optimal F-doped NdOBiS2 single crystals support the single band s-wave paring symmetry in this system.(3),Hole-doping in quasi-one-dimensional Nb2PdS5 enhances Tc and Hc2,and close relationship between the unconventional SC and spin-orbit coupling(SOC)is found in this system.The dissertation consists of five chapters.The first chapter comprises the important theories and an overview of three classes of unconventional superconductors.The second chapter intro-duces the major experimental methods.Next three chapters are the main works of this dissertation,in which the physical properties and the phase diagrams of the superconductors are investigated.The main original results are highlighted as following:(1)We investigated a series of La1-cCexFePO(0?x?1),and constructed a rich phase diagram by transport,magnetic,and thermodynamic measurements.Upon Ce doping,superconductivity is suppressed and disappears near x=0.1.For the broad doping region 0.1?x?<0.7,the Ce moments behave as single-ion Kondo impurities.With further Ce doping,the system evolves into a coherent Kondo lattice.No long-range magnetic ordering is identified on this phase diagram,while short-range spin-glass behavior is suggested to coexist with the single-ion Kondo phase.Within the spin glass phase,the dominant magnetic correlation is found to change from antiferromagnetic-like to ferromagnetic-like as Ce concentration increases.We discuss the results based on the disordered Kondo lattice model,and ascribe the rich phase diagram as the consequence of high frustration of the Ce moments introduced by the superexchange interactions bridged by O and P anions.Our work hence provides an intriguing platform to study the disordered Kondo effect in the high-frustration limit.(2)The superconducting properties of optimal F-doped NdOBiS2 single crystals were studied by performing detailed magnetization and resistivity measurements.The SC transition temperature Tc,critical magnetic field Hc1,Hc290%,Hc250%and Hc20%were extracted from the experimental data.The critical current density J,as a function of temperature has been derived and decreases with the increasing temperature.Linear temperature dependence of both Hc1 and Hc2 was found,which provides an evidence for a single band s-wave paring symmetry.(3)We synthesized a series of Nb2Pd1-xRuxS5(0?x?1)polycrystalline samples by a solid-state reaction method and systematically investigated the Ru-doping effect on superconductivity by transport and magnetic measurements.It is found that SC is enhanced with hole-type Ru doping and is quite robust upon disorder.The Hall coefficient measurements indicate that the charge transport is dominated by hole-type charge carriers similar to the case of Ir doping,suggesting multi-band superconductivity.Upon Ru or Ir doping,Hc2/Tc exhibits a significant enhancement,exceeding the Pauli paramagnetic limit value by a factor of approximately 4.A comparison of Tc and the upper critical field(Hc2)amongst the different doping elements on Pd site,reveals a significant role of SOC.