| Unconventional superconductors usually mean the superconductors that cannot be described by BCS theory using electron-phonon coupling as the paring glue.The typical examples of unconventional superconductors are cuprate superconductors,iron-based superconductors,organic superconductors,and heavy fermion superconductors.After nearly half a century of research,scientists now believe the paring mechanism of unconventional superconductors is closely related to magnetic fluctuations,but the exact paring mechanism still lacks a unified and completed description.Therefore,it is necessary to explore new types of unconventional superconductors,which may pro-vide a new platform for the research of unconventional superconducting mechanisms.Exploring new unconventional superconductors can also help us to understand uncon-ventional superconductivity more deeply.As an extreme condition,high pressure can effectively alters the atomic spacing and electron shells.The utilization of high pres-sure can help us discover new materials and new phenomena which are difficult to exist under ambient pressure.This paper introduces some attempts to explore unconvention-al superconductors synthesized under high pressure.In the first chapter,we briefly introduce the discovery of superconductors and the BCS theory.Then we introduce the basic characteristics of cuprate superconductors,iron-based superconductors,organic superconductors and heavy fermion superconduc-tors in order to get some inspiration in exploring new unconventional superconductors.In the second chapter,we focus on the high-pressure techniques.Firstly,we in-troduce the subject of high pressure briefly.Then we illustrate the principle and equip-ment of high-pressure synthesis in detail.At last we briefly review the unconventional superconductors synthesized under high-pressure.In the third chapter,we introduce some other synthesis methods,as well as the experimental and analytical methods to do the structure characterization,electrical/heat transport and magnetic susceptibility measurements.In the fourth chapter,we concentrate on the high temperature superconducting like behaviors in potassium doped p-terphenyl.Firstly,we fabricate potassium dope-d p-terphenyl sample at abmient pressure.The temperature dependence of magnetic susceptibility shows no superconducting like behavior in this sample.However,in the high-pressure synthesized samples,the magnetic susceptibilities show the step-like transitions near 125 K clearly.Then we measure the magnetization hysteresis loops(MHLs)from 60 to 150 K and take the MHL at 150 K as the background,i.e.,?M = M(H,T)-M(H,150K).The ?M versus H curves show type-? super-conducting like hysteresis loops below 125 K,and this hysteresis behaviors disappear above 125 K.Based on the magnetic measurements,we conclude that there might be some small fraction of high temperature superconducting phase in the high pressure synthesized potassium-doped p-terphenyl.In the fifth chapter,we introduce the properties of LiPdHx synthesized under high pressure and discuss some reasons why there is no superconductivity in this material.Resistivity and magnetic susceptibility present no superconducting behavior down to about 2 K.The resistances under different high pressures show no superconductivity too.Specific heat measurement shows that the Sommerfeld constant of our sample is a bit smaller than that of the theoretical calculation.As we know,the Sommerfeld constant is proportional to the density of states at Fermi level.The small DOS at Fermi level certainly suppresses the superconducting gap greatly.This may be one reason for the loss of superconductivity in LiPdHx.In addition,we recognize that LiPdHx,is a metastable material and hydrogen will continuously escape from the LiPdH lattice,forming a large number of vacancies.The hydrogen vacancies may change the elec-tronic structure and weaken the electron-phonon coupling,leading to the absence of superconductivity in LiPdHx.At the end of this thesis,we make a full summary. |
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