Specific Heat And Nuclear Magnetic Resonance Study Of K₂Cr₃As₃ And KCa₂Fe₄As₄F₂

Unconventional superconductivity is the research frontier and plays an important role in condensed matter physics.Quasi-one-dimensional Cr-based superconductor K₂Cr₃As₃and quasi-two-dimensional Fe-based superconductor KCa₂Fe₄As₄F₂are new unconventional superconductors discovered in recent years.In this dissertation,the superconducting gap function of K₂Cr₃As₃ is studied by low-temperature specific heat measurements and the superconducting properties of 12442-type iron-based superconductor KCa₂Fe₄As₄F₂ are studied by nuclear magnetic resonance?NMR?measurements.We have obtained the following results.We present the detailed low-temperature specific-heat measurements of single-crystal K₂Cr₃As₃ at different magnetic fields.At zero field,the low-temperature specific heat measurements of K₂Cr₃As₃ show the specific heat jump at superconducting transition temperature T_c?6.1 K,with?C/₉)₍?8??2.5,which is larger than predicted value by BCS theory for weak-coupling superconductors and indicates that K₂Cr₃As₃ is in the strong-coupling regime.After subtracting the lattice contribution and Schottky anomaly contribution from the total specific heat data,it is found that the low-temperature electronic specific heat is proportional to T² at different magnetic fields,and proportional toat different temperatures below 2.5 K.The temperature dependence and field dependence of low-temperature electronic specific heat indicate that line nodes are present in the superconducting gap function of K₂Cr₃As₃.We study the nuclear magnetic resonance spectrum of KCa₂Fe₄As₄F₂ for the first time.The ¹⁹F-NMR and ⁷⁵As-NMR experiments of single-crystal KCa₂Fe₄As₄F₂ show that the ¹⁹F-NMR spectra shape of KCa₂Fe₄As₄F₂ is weakly dependent on temperature and the Knight shift is almost unchanged across T_c,which implies there is weak coupling between CaF layer and FeAs layer.The temperature dependence of ¹⁹F spin-lattice relaxation rate 1/¹⁹T₁ shows a hump below T_c.However,1/⁷⁵T₁ decreases just below Tc without Hebel-Slichter coherence peak which implies KCa₂Fe₄As₄F₂ is an unconventional superconductor.The different behavior between 1/¹⁹T₁ and 1/⁷⁵T₁ indicates CaF layer and FeAs layer play different roles on the superconductivity,which may be the cause of the strong anisotropy.