Research On Physical Properties Of Cu_xNbSe₂ And Tune Properties By Pressure

Since the discovery of graphene,the study of two-dimensional materials has attracted many scholars.Among them,Transition metal dichalcogenides(TMDs)have been attracting significant interest due to stability and complex electrical properties.As a representative of TMDs,there are superconducting transition and charge density waves transition in 2H-NbSe2.Researchers have explored the crystal structure,electromagnetic properties,and energy band of 2H-NbSe2.Its superconducting transition temperature is 7.2 K,which is significantly higher than many other superconductors of TMDs(2-4 K).It is significant to study this superconducting performance.Chemical doping is an effective way to tune the physical properties of materials,and it is great significance for the study of physical properties.Studying the superconductivity of CuxNbSe2 can further explore the superconducting mechanism of 2H-NbSe2.Pressure,as a conventional thermodynamic parameter,is a clean and useful tool to tune the atomic/molecular distance,which can affect the properties of materials without simultaneously introducing any disorders.Pressure can induce electrons phase transitions,structure phase transitions and so on.The physical properties of the superconductor can be tuned by high pressure,which provides a perfect platform for studying superconductivity.In this thesis,CuxNbSe2 single crystals(x=0,0.02,0.045,0.07)were prepared by chemical vapor transport method.The superconductivity of CuxNbSe2 single crystal and Cu0.02NbSe2 under pressure were measured,including resistivity,superconducting transition temperature,and upper critical magnetic field.The main contents are described as follows1.Cu doping inhibits the superconducting transition of CuxNbSe2.When x reaches a certain level?0.055,the superconducting transition temperature stabilizes to the usual value of transition-metal dichalcogenides superconductors,Tc=3.5K.We speculate that this may be due to either the electron doping of the NbSe2 layer,or to the non-magnetic disorder introduced by the Cu doping.2.The temperature T dependence of the upper critical magnetic field Hc2 for CuxNbSe2 was plotted.It shows Hc2 decreases with the increase of Cu doping.Note that there is a linear dependence of Hc2 vs.T at near Tc,With the Werthamer-Helfand-Hohenberg(WHH)expression,we estimated the zero-temperature upper critical fields Hc2(0).It shows that Hc2 decreases with x increases.The coherence length ?GL(0)of CuxNbSe2 was obtained.With the increase of x,the?GL(0)gradually increases.3.With the increasing of pressure,the superconducting transition temperature Tc of Cu0.02NbSe2 single crystal first increases and then decreases.The Tc-P phase diagram in the pressure range is similar to the experimental results of its parent phase NbSe2.The temperature T dependence of magnetization M under different pressures was studyed.With the increasing of pressure,the magnetic measurement transition temperature Tcm increases first and then decreases.It is consistent with the measurement results of electric transport,which further confirmed the effect of pressure on the superconducting transition of Cu0.02NbSe2.4.With the WHH expression,we estimated the zero-temperature upper critical fields Hc2(0).In the pressure range,the Hc2(0)decreases with increasing pressure,which shows a special dependence.5.At T=10 K,it is found that the Hall coefficient RH is always negative under different pressures,and RH is hardly affected by the pressure,RH?-8.4×10-10m3/C.It shows that the charge carrier density and even the shape of the Fermi surface hardly affect its superconductivity,which simplifies the problem of checking the factors that affect superconductivity.