NMR Study On Kondo Insulator And Iron-based Superconductor

This thesis basically involves two themes:(1)the nuclear magnetic resonance mea-surement(NMR)on Kondo insulator SmB6,the investigation of the electronic inhomo-geneity in doped Kondo insulator Sm1-xYbxB6(0?x?1)by susceptibility,resis-tivity,low temperature heat capacity and NMR measurement,(2)the study of strongly correlated physics and spin fluctuation in iron-based superconductors CsFe2As2 and YFe2Ge2 by NMR measurement.Concretely,the following achievements will be introduced:(1)The demonstration of the in-gap states probed by NMR is induced by disorder or impurities.(2)By an-alyzing the resistivity,magnetic susceptibility,specific heat and NMR data of single crystal Sm1-xYbxB6(0?x?1),we found a novel electronic inhomogeneous state in Sm1-xYbxB6.(3)By site-selective NMR measurement,we observed the break-down of single spin-fluid model in a heavily hole-doped iron-based superconductor CsFe2As2.(4)By conducting a 89Y NMR study on YFe2Ge2 single crystal,we confirm that,through a Hund's coupling induced electronic crossover,the magnetic ground state of YFe2Ge2 becomes close to an itinerant magnetic quantum critical point with A-type spin fluctuations.This thesis is divided into six chapters as follows:1.IntroductionIn this charter,firstly,we briefly introduce the physical properties of the Kondo in-sulator SmB6,the concept of the topological Kondo insulator,and the recent theoretical and experimental progress about SmB6.Then,we also make a brief introduction about the iron-based superconductors and the orbital selective Mott physics in the heavily hole doped iron-based superconductor.2.The basis of the nuclear magnetic resonanceIn this chapter,we give a brief introduction on the basis of NMR techniques and its application to solid state NMR.3.Disorder-induced nuclear spin-lattice relaxation in Kondo insula-tor SmB6In this chapter,the in-gap state of the Kondo insulator SmB6 has been investigated by 11B NMR under magnetic field up to 16 T and low temperature down to 0.245 K.We found clear evidence for the paramagnetic impurity or disorder induced low temperature spin-lattice relaxation 1/T1 in SmB6.The in-gap state exhibits anomalous spin dynamics due to strongly electronic correlation in SmB6.In addition,we also found a electronic crossover behavior in SmB6.Around TK?150 K,both resistivity and Hall coefficient shows electronic crossover behavior,which indicates that the Kondo crossover in a wide temperature region.4.Electronic inhomogeneity in doped Kondo insulator Sm1-xYbxB6 compoundsIn this chapter,we report the electronic inhomogeneity in doped Kondo insulator Sm1-xYbxB6(0 ?x?1).By comprehensively analyzing the resistivity p,magnetic susceptibility ?,specific heat C and NMR data of single crystal Sm1-xYbxB6(0 ?x?1)in a wide temperature range,we found that Sm1-xYbxB6 compounds change grad-ually from a Kondo insulator to a heavy fermion metallic state,and finally to a weakly correlated metal with the doping concentration x increasing from 0 to 1.Interestingly,in middle doping region(0.24 ?x?0.48),a novel electronic inhomogeneous heavy fermion metallic state is observed.Sm1-xYbxB6 compounds provide a good platform to study the novel electronic inhomogeneous state in Kondo insulator system.5.Breakdown of single spin-fluid model in the heavily hole-doped superconductor CsFe2As2In this chapter,we first observed the breakdown of single spin-fluid model in a heavily hole-doped Fe-based superconductor CsFe2As2 by site-selective NMR mea-surement.At high-temperature regime,both Knight shift and nuclear spin-lattice relax-ation at 113Cs and 75As nuclei exhibit distinct temperature-dependent behavior,suggest-ing the breakdown of the single spin-fluid model in CsFe2As2.This is ascribed to the coexistence of both localized and itinerant spin degree of freedom at 3d orbitals,which is consistent with the orbital-selective Mott phase.With decreasing temperature,the single spin-fluid behavior is recovered below T*?75 K due to a coherent state among 3d orbitals.The Kondo liquid scenario is proposed to understand the low-temperature coherent state.6.Approaching itinerant magnetic quantum criticality through a Hund's coupling induced electronic crossover in YFe2Ge2 superconductorIn this chapter,by conducting a 89Y NMR study on YFe2Ge2 single crystal,a uni-versal electronic crossover due to the Hund's coupling induced electronic correlation is revealed,which makes the system approach an itinerant magnetic quantum critical point(QCP)below the crossover temperature T*?75±15 K.During the electronic crossover,both of Knight shift(K)and bulk magnetic susceptibility(?)exhibit a simi-lar nonmagnetic temperature dependence and a so-called "Knight shift anomaly" is also revealed by a careful K-? analysis.Such kind of electronic crossover has been also observed in heavily hole-doped pnictides superconductors AFe2As2(A=K,Rb and Cs),which is ascribed to the Hund's coupling induced electronic correlation.Below T*,the spin-lattice relaxation rate divided by temperature(1/T1T)shows a similar suppression as Knight shift,suggesting a predominant contribution from quasiparticle rather than spin fluctuations.This seems to be in conflict with a predicted magnetic QCP near this system.However,considering a q-dependent "filter" effect on the transferred hyper-fine field,a predominant spin fluctuation with A-type correlation would be perfectly filtered out at 89Y sites,which is consistent with the recent inelastic neutron scattering results.Therefore,our results confirm that,through a Hund's coupling induced elec-tronic crossover,the magnetic ground state of YFe2Ge2 becomes close to an itinerant magnetic QCP with A-type spin fluctuations.In addition,the possible superconduct-ing pairing due to spin fluctuations is also discussed,which also has implications on understanding the superconducting pairing in AFe2As2 superconductors.