Exploration Of New Superconductors By Using Intercalation/De-intercalation Method

In 1911,Onnes as the first person observed the superconductivity and opened up the new era of research on superconductivity.In 1986,Bednorz and Muller found high-temperature superconductivity in cuprates,which triggered a wave of reseach on high-temperature superconductors.In 2008,Hosono found high-temperature super-conductivity in iron pnictides,which give rise to a new family of high temperature superconductors,iron-based superconductors.Both the cuprates and iron-based su-perconductors are lay erred compounds,with the superconducting CuO2 planes and FeAs/FeSe layers,respectively.By replacing the layered structures between their su-perconducting layers,numerous new superconductors have been obtained.For the FeSe-based superconductors,due to the electrical neutrality and weak Van der Waals interaction of FeSe layers,the usually-used synthesizing method of solid-phase sin-tering at high temperature is not quite suitable for the FeSe-based superconductors.Therefore,the intercalation/de-intercalation method,dominated by liquid-phase reac-tions at relatively lower temperature,has become an important way to explore FeSe-based superconductors or other transition-metal layered compounds with unexpected properties.This thesis will introduce several new iron-based superconductors and two new vandium-based layered compounds which are synthesized by this method,as well as their physical properties.Among them some were discovered by us.In Chapter 1,the basic concepts of superconductivity are introduced at first.Sec-ondly,the research history of superconductivity is introduced from two aspects:mate-rials and theories.And the research progress of iron-based superconductors specially emphasized.Thirdly,we give the derivation of the Hall resistance and magnetoresis-tance under the multi-band model from the Boltzmann formula.In Chapter 2,we mainly introduce some synthesizing methods ever used in the synthesis of our samples,such as solid-state method,flux method and hydrothermal method.And we record the detailed process of synthesizing some iron-based super-conductor crystals.In addition,we give a brief introduction to some methods and devices/instruments that are used for structure analysis,component analysis,magneti-zation measurement,resistivity measurement and so on.In Chapter 3,by using the intercalation/deintercalation method,we successfully synthesized high-quality single crystals of the superconductor Li1-xFexOHFeSe and study the magnetic properties and the transport properties at different currents and magnetic fields.It is found that the critical current density of Li1-xFexOHFeSe is one order of magnitude higher than that of the phase separated KxFe2-ySe2,which clearly indicates bulk superconductivity in it.Furthermore,Tc of Li1-xFexOHFeSe changes barely with increasing current but is slightly suppressed with increasing magnetic field,which indicates that its superconductivity is very robust.In Chapter 4,by using the deintercalation method,we successfully extracted the K ions from KxFe2-yS2 and got the large FeS single crystals for the first time.We measured the physical properties of FeS single crystal samples.The resistivity under different magnetic field directions shows that FeS single crystal has a large anisotropy,which is about 5.8,and a strong magnetoresistance effect.The maximum magnetore-sistance can reach 290%(H=9T,T=10K).In addition,we observed a nonlinear Hall effect,indicating the multi-band effect.Using the two-band model,we calculated the basic parameters of the hole band and electron band.We found that these two bands make balanced contributions to electric conduction.In Chapter 5,by using the intercalation/deintercalation method,we successful-ly synthesized high quality LiOHFeS superconducting single crystals,by extracting K from KxFe2-yS2and inserting LiOH back into it.The XRD data shows that our su-perconducting LiOHFeS has two phases with different c-axis lattice constants.One of the two phases has c=8.91A,which is close to c=8.96A of the non superconducting LiOHFeS.While the other one has a smaller c=8.71A,which is possibly responsible for superconductivity.This result implies that the lattice constant c may be directly related to the superconductivity.From the magnetization measurement and transport measurement of superconducting LiOHFeS,we understand that LiOHFeS has a sharp superconducting transition at about 2.8K,and a very small critical field.In Chapter 6,by using the similar intercalation/de-intercalation method which is widely used in the synthesis of iron chalcogenide superconductors,we try to explore other new superconductors.As a result,we got two new layered materials which show no superconductivity.At first,we synthesized a new transition metal layered com-pound CsV2Se2-xO(x = 0,0.5)by solid state reaction.Then,we extracted Cs from the CsV2Se20 in iodine tetrahydrofuran solution,and get another brand-new layered com-pound V2Se2O.The resistivity and magnetization measurements show that CsV2Se20 behaves like a semiconductor,CsV2Se1.5O is a metal,while V2Se2O is a insulator.And a CDW/SDW-like anomaly was found in CsV2Se20 and CsV2Se1.5O at 168 K and 150 K,respectively.Interestingly,the resistivity of V2Se20 is roughly proportional to log(1/T)in the temperature range from 2 K to 300 K.This logarithmic resistivity tem-perature dependence also appears in some parent phases of cuprate high-temperature superconductors,which is related to strong electron correlation.Using Curie's law,we calculated the magnetic moments of these three systems.We found that the magnetic moment per V atom in V2Se2O is larger than that in its parent CsV2Se2-xO,indicating that electrons in V2Se20 system have a stronger localization and correlation effect.We make a summary in the end.