NMR Investigation On Novel Electronic States In Iron-based Superconductors

Understanding the physical consequences of electron-electron correlation is the key problem and main task of modern condensed matter physics.Intricate and exquisite couplings among multiple degrees of freedom(d.o.f.)accompanied with the electron-electron correlation give rise to abundant either cooperative or competitive ordering states,which result in complicated and variable phase diagrams.Through applying pulsed NMR spectroscopic methods on a series of iron-based superconductors,we suc-cessfully reaveal and carefully study several novel elelctonic states,which emerge in these correlated metallic systems.We also propose possible corresponding physical models by detail analysis on these experimental datas.First,the structurally simplest iron-based superconductor FeSe exhibits exotic phase diagram.For example,the special electronic nematicity have arouse great atten-tions and large amount of studies in recent years,while still remains many questions.We conducted detailed NMR studies on FeSe single crystals.We synthesized 57Fe en-riched FeSe single crystals and take comprehensive NMR measurements on 57Fe and 77Se nuclei simultaneously for the first time.We find that the Knight shift of 57Fe and 77 Se show distinctly different temperature dependence.The anisotropy of the Knight shift and spin-lattice relaxation rate of these two sites also show different temperature dependence in the electronic nematic state.Through analyzing the detail form of the hyperflne coupling of 57Fe and 77Se,we know that 57Fe can directly reflect the local orbital configurations on Fe site,while 77Se is more affected by the 3dxz and 3dyz or-bital.Our experiments reveal three key points of the temperature-dependent evolution of the electronic state in FeSe:1.Except for the lifting degeneracy of the 3dxz and 3dyz orbital,the 3dxy orbital also reconstruct in the nematic states.2.FeSe possesses the Hund's coupling induced orbital-selective electronic correlations,and the 3dxy orbital exhibits an incoherence to coherence crossover with lowering temperature.3.Nontriv-ial spin-orbit interaction leads to a sizable spin-space anisotropy in the electronic ne-matic state of FeSe.These experimental results suggest that the nematic state of FeSe is a spin-orbital intertwined electronic state,in which different orbital exhibit different electronic correlations and coherent-incoherent crossover with varying temperature.Diverse and elusive phases are introduced in FeSe with applying hydrostatic pres-sures.Its' superconducting transition temperature can be enhanced about 4 times com-paring with the value at ambient pressure.In addition,the competitive or cooperative effects between these various electronic orders has been the focus of the theoretical and experimental studies,and the results of different experiments appearing some dis-cripances.To this end,we conducted detailed NMR studies on 57Fe-enriched single crystal in the low hydrostatic pressure range(pmax?2.1 GPa).By comparing the evo-lution of NMR lines of 77Se and 57Fe with varying hydrostatic pressure,we unravel the incipient magnetic ordering at low-pressure range which is unreported in previous stud-ies.The evolution of the superconducting transition temperature and low-energy spin fluctuations at low temperatures with varying pressure indicate that the superconducting pairing mechanism may also change with varying pressure.Based on the NMR experi-mental evidences,the electronic states of FeSe also exhibit an intriguing crossover with applying hydrostatic pressure.For example,the electronic nematicity at high pressures is most likely the one in FeAs type materials.These results contribute to the further understanding on the origin of the abundant electronic properties of iron-based super-conductors,and provide valuable perspectives to build a unified physical picture.The superconducting transition of FeSe and its' derived systems is highly tunable.Besides,the superconducting state of FeSe single crystal under ambient pressure has al-ready been recognized as remarkably unconventional,which exhibit many exotic prop-erties.Due to the radio-frequency(RF)heating effects,previous NMR studies failed to fully characterize the intrinsic superconducting properties of FeSe.To this end,we synthesized 77Se-enriched FeSe single crystals and conducted systematic NMR mea-surements on the superconducting state of FeSe with sufficiently low-power RF pulses to avoid the RF heating effects.The decline of the spin susceptibility leads to Knight-shift reductions under all the orientations of the external magnetic fields which exclude the possibility of the chiral p-wave paring symmetry.Besides,a large amount of residual density of states and the extremely broad NMR lines with prominent anisotropy were observed deep inside the vortex-lattice state in FeSe,which indicate the existence of a rather abnormal boundary states.These results may be relate to the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate(BCS-BEC)crossover character of the super-conducting paring of FeSe.Thus,the revised NMR results provide important constraints and references for the relevant theoretical models.The two-dimensional nature of iron-based superconductors makes them easy to exfoliate,intercalate and form complex structures.Utilizing the site-resolve abil-ity of NMR spectroscopy,we successfully conduct detailed studies on the electronic properties of the two different layers in the intercalated iron-based superconductor Ba2Ti2Fe2As40.After systematic angle-dependent NMR spectra measurements,we recognize the previously undefined?125 K phase transition as the formation of the 2D orbital glass state in the[Ti2As2O]layer.In addition,with the extremely fine energy resolution of NMR,we also revealed an orbital ordering transition at lower tempera-tures,which is accompanied with structural distortions.The accompained formation of mutually orthogonal ordered domains is also observed for the first time in this system.We also find coexistence of magnetic order and superconductivity in[Fe2As2]layer at low temperatures.In a word,the diversity of the electronic order in this material make it an attractive system to study the orbital engineering as well as the exact form of microscopic interactions which may shed light on other heterostructure materials.We also take a primitive 57Fe NMR characterizations on heavily-hole-doped iron-based superconductors CsFe2As2.The experimental results confirm the prominent orbital-selective correlations and yield the possibility of the existence of electronic ne-maticity.Besides,we systematically studied the series of low-Tc FeSe single crystals with 77Se NMR spectroscopy.We find that Tc is positively related to the low-energy spin fluctuations at low temperatures,while it seems irrelevant to the electronic nematic-ity.All these studies provide important indications and help to figure out key physical mechanism of each electronic order in these correlated metallic systems.