Exploration Of Superconductivity In Strongly Correlated System Under High Pressure

Since the discovery of superconductivity by Onnes in 1911,superconductivity has attracted great attention both in the theory and application aspect.As a core part of con-densed matter physics,strong correlation system is also the focus of attention.Strong correlation system of superconductivity,including cuprate superconductors,iron-based superconductors and heavy fermion superconductors.The mechanism of these systems is far from the expectations of the conventional BCS theory.As a clean way of elec-tronic control,high pressure can not only tune the correlation strength of electronic states,but also is an indispensable and independent variable in the superconducting phase diagram,so that pressure is a powerful tool to explore the relationship between them.On the one hand,the phase diagram of critical temperature and other phase tran-sition temperatures on pressure in the system can be obtained systematically through the pressurization experiment of the known superconducting strong correlation system,which is of great help to understand the superconducting mechanism of the strong cor-relation system;on the other hand,for the non superconducting strong correlation sys-tems under ambient pressure,the attempt to induce superconductivity in these systems can investigate the relationship between the strong correlation and superconductivity,which can help us find more and more unconventional superconducting systems and provide suitable platforms for further exploring the mechanism of superconductivity.The materials studied in this paper include the iron-based,alloy and organic con-ductor systems,and the pressure experiments are carried out.Interestingly,the differ-ent experimental results are obtained,which also proves the rich physical natures of the strong correlation systems under pressure.In chapter One,the history of superconductivity is introduced,and then the re-search status of conventional superconductors and unconventional superconductors is briefly described.The BCS theory,specific heat of conventional superconductors and the rich pressure phase diagrams of various superconductors are introduced as well.Finally,the paper introduces the strong correlation system with fragile superfluidity briefly.In chapter two,the experimental methods used in this paper are introduced,in-cluding sample preparation and high pressure methods.The sample preparation meth-ods include solid state reaction,arc melting and electrochemical reaction methods.The high pressure methods are mainly about piston cylinder pressure method and diamond anvil cell pressure method.In chapter three,the unconventional superconductivity induced by pressure in Cs Fe_4-xSe₄system is introduced.The system has a doping level between Fe Se and Cs_1-xFe_2-ySe₂,and theoretical calculations show that the system should be a metal,but it is insulatorlike by measuring the resistivity.The insulatorlike behavior can be de-scribed by three-dimensional variable-range-hopping model.The magnetization data show a behavior that deviates from the Curie-Weiss law.A sizable residual coefficient can be observed from specific heat measurements at zero temperature limit,indicat-ing that the system may have a quantum spin ground state rather than a simple band insulator.By applying pressure,we induced the superconductivity at 5.1 K,which can be easily suppressed by magnetic field and current.These features show differen-t characteristics from normal iron-based superconductors,but are very similar to the underdoped cuprate superconductors.Therefore,we believe that the induced super-conductivity is formed by the diluted superfluid in the strong correlation background.In chapter four,the discovery of antiferromagnetic state in Zr_0.5Sc_0.5Co and its pressure effect are introduced.and the theoretical explanation is given.By arc melt-ing method,we have successfully synthesized Zr_0.5Sc_0.5Co with the space group of is P m-3m.Both resistance and magnetization measurements show that there is an antiferromagnetic phase transition at about 86 K.The magnetization hysteresis loop measurements in wide temperature region,which reveals a possible canted antiferro-magnetic arrangement.Before and after the phase transition temperature,the resistivity shows a change from Fermi liquid behavior to linear resistance behavior with the in-crease of temperature.The temperature of the phase transition is not changed much by applying pressure up to to 32.1 GPa,and no superconductivity is observed above2 K.The density functional theory calculations suggests that the antiferromagnetic or-der in this material comes from local d electrons,which have no contribution to the conduction.Therefore,in this material,the itinerant and the localized d-electrons are decoupled.In chapter five,the rich magnetic and pressure phase diagram in the organic su-perconductor?-(ET)₂Cu(NCS)₂are studied and strange current effect is also observed.?-(ET)₂Cu(NCS)₂is superconducting with critical temperature of 10.4 k at ambient pressure,and there is a crossover from high temperature incoherent bad metallic state to low temperature coherent Fermi liquid state at around 100 K.When appling pres-sures,superconductivity can be easily suppressed under low pressures,accompanying with the suppression of the metal-insulator crossover.The amorphous transformation of the sample occurs at higher pressures,which results in the complete insulating be-havior in the resistivity curve.By measuring the zero field cooling and field cooling magnetization curve,we get the corresponding magnetic phase diagram,and find that the system exhibits a 2D behavior.Surprisingly,at ambient pressure the superconduc-tivity can be easily killed by increasing the electric current density,and the critical current is proportional to 1-(T/T_c)³.All above properties point to the fragile super-fluidity on the strong pairing background.In chapter six,we summarize the whole thesis.