NMR Studies On High-temperature Superconductor NaFe_1-xCo_x As And Weyl Semimetal TaAs

Since the discovery of Cuprate superconductors,researches on the high-temperature superconductivity have continued to be research hotspots in con-densed matter physics.The discovery of iron-based superconductor impregnates new activities for this field.The anti-ferromagnetic quantum critical point and its fluctuations are considered to be important clues for understanding the non-Fermi liquid behaviors in the normal states and the mechanism of high-T_csuperconduc-tivity.In iron pnictides,in addition to the magnetic order,there also exists an electronic nematic order that breaks rotation symmetry and preserves the trans-lational symmetry.Previous studies indicate that the nematic fluctuations are ubiquitous above the superconducting dome and suggest the existence of a ne-matic quantum critical point?QCP?.However,these researches are all carried out in the normal states and a direct evidence for a nematic QCP inside the super-conducting dome is still lacking.In most iron pnictides,the putative magnetic QCP and electronic nematic QCP are usually close by or even indistinguishable,which hindered the progress of experimental investigations on the later.On the other hand,topological materials have inspired a new research upsurge in condensed matter physics for their rich physical connotation and promising ap-plication over the last decade.In 2015,the TaAs family was predicted to be a kind of Weyl semimetals that broke the inversion symmetry.Although the Fermi arc surface states have been confirmed by surface sensitive technology ARPES and STM,it can't be a sufficient condition to recognize a Weyl semimetal for the inability to determine the Fermi level precisely.Therefore,it is vital to directly measure the bulk states.In point of the bulk measurements,the negative magne-toresistance caused by chiral anomaly,another hallmark of Weyl semimetal,has also been observed.However,we note that the experimental interpretation for the negative magnetoresistance is still controversial.Firstly,current jetting,a non-intrinsic effect,may lead to negative magnetoresistance.Secondly,theoreti-cal study indicates that the negative magnetoresistance can be induced by trivial bands which are not related to Weyl bands.So far,an unambiguous experimental signature from the bulk bands is still lacked.As a local and space selectable microscopic tool,Nuclear Magnetic Reso-nance?NMR?has unique advantages in the research of condensed matter physics.In this thesis,I first introduce the research status and problems of superconduc-tivity and topological semimetals,and then I introduce the basic principle and technology of NMR.Lastly,I report the results of NMR study on iron-based superconductor Na Fe_1-xCo_xAs system and topological Weyl semimetal TaAs.For Na Fe_1-xCo_xAs system,through the measurements of²³Na NMR spec-tra,we found two sharp peaks in the Co doping dependence of zero-temperature London penetration depth,one at x_c1=0.027 and the other at x_c2=0.032.This indicates two quantum critical points exist in this system.We study the spin fluctuations of the system via⁷⁵As spin-lattice relaxation rate and the result indi-cates x_c1=0.027 is an anti-ferromagnetic QCP.x_c2=0.032 is around the position where the nematic transition temperature extrapolates to zero and the electrical resistivity we measured shows a T-linear variation from the superconducting crit-ical temperature up to T=110 K.These results indicate that a nematic quantum critical point lies beneath the superconducting dome.For TaAs,we first investigate the microscopic magnetism via⁷⁵As Knight shift K_c.K_cis found to be negative at low magnetic fields and has a strong field?B?dependence as ln B at T=1.56 K.Such nonlinear field-dependent magneti-zation can be well accounted for by Landau diamagnetism arising from the 3D linearly dispersed bands,and thus is a fingerprint of topological semimetals.We further study the low-energy excitations by the⁷⁵As spin-lattice relaxation rateis a conspiracy of anomalous orbital hyperfine coupling and E²dependence of the density of states N?E?of Weyl Fermions.At high field B=12.95 T,1/T₁T exhibits the similar T²behavior,scaling with K_c²?T²,which indicates that the Korringa relation also holds for a Weyl semimetal.Analyzing the Korringa ratio reveals that the energy range of the linear bands is about 250 K in TaAs.