The Electrical Transport Properties Of FeSe Superconducting Thin Films

Among all iron-based high-temperature superconductors,the tetragonal ?-FeSe has the simplest composition and crystal structure.Therefore,it has become one of the research hotspots of new iron-based superconducting materials since its discovery.The FeSe superconductors have an inverse PbO type structure that composed of Fe-Se layers.In a single FeSe layer,Fe atoms and Se atoms are covalently bonded to each other,as the inter-layer interaction is the weaker van der Waals interaction.So,rich physical phonomena can be observed in FeSe thin films.At present,there are still many new physical phenomena which remains unexplained in FeSe thin films.In this paper,a series of FeSe films are deposited on MgO(100)substrates by magnetron sputtering,and their microstructures and electrical transport properties are systematically studied.Firstly,we explored the optimal deposition conditions for FeSe thin films deposited by magnetron sputtering to form a series of single-oriented tetragonal FeSe thin films,and studied the influence of preparation conditions on the micro structure and superconducting properties of these thin films.When the film thickness is?300 nm,the FeSe film exhibits complete superconductivity,and the superconducting transition temperature is?8 K.As the film thickness is reduced to?60 nm,miscellaneous phases begins to emerge,and the grain distribution become uneven.Consequencely,the superconductivity disappears,and the electrical conductivity of the insulating film increases.The superconducting-insulator transition is by changing the disorder of the layered epitaxial FeSe thin film,which can be regarded as a two-dimensional system.The normal sheet resistance of the FeSe thin film in the superconductor-insulator transition critical region,is 6.26 k?,which is consistent with the reported threshold sheet resistance 6.45 kQ for crystalline superconducting thin films or superconducting granular films.The superconductivity of FeSe thin films under a magnetic field are studied.The thermal activated magnetic flux creep(TAFF)theory is used to analyze the broadening of the superconducting transition region under a magnetic field.The magnetic flux pinning behavior of FeSe thin film undergoes a transition from single magnetic flux jump to collective magnetic flux creeping with increasing magnetic field,and the corresponding magnetic field for the transition is about 2T.Secondly,this paper studies the two-dimensional superconducting properties of epitaxial FeSe films with complete superconductivity.By measureing the current-voltage relationship(I-V)characteristic of FeSe film under different magnetic fields,we find that under zero magnetic field,as I?0,the ?-? curve follows an exponential relationship:V ? I^?.When T=4.7 K,?=3,and this means the FeSe film undergoes a Berezinskii-Kosterlitz-Thouless(BKT)transition at?4.7 K.In addition,near transition temperature T_BKT,the temperature dependence of the sheet resistance satisfies the Halperin-Nelson formula,and T_BKT=5.0 K is obtained by fitting with it.The T_BKT obtained by the two methods are nearly the same,and the above results prove that the FeSe film undergo a BKT transition.Finally,we have investigated the low temperature isothermal current-voltage characteristics of FeSe epitaxial films with thickness of?360 nm at different magnetic fields parallel to the c axis.When a magnetic field with magnitude 0.1 T??₀H ?6 T is applied,the low temperature isothermal current-voltage data can be scaled onto two different branches by using vortex scaling theory,When comparing with the scaling theory,the dimensionality D can only be taken as 2,indicating a vortex-liquid phase to quasi-2D vortex-glass phase transition occurs in the FeSe film.