【作者】 李建；

【导师】 吴涛；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2021， 博士

【摘要】 对电子-电子关联效应的理解是现代凝聚态物理的核心问题和主要任务。伴随电子关联而来的多种自由度间错综复杂的耦合可导致丰富的竞争或合作的有序态,形成复杂多变的相图。本论文以系列铁基超导体作为研究对象,利用脉冲核磁共振(NMR)技术来揭示和研究关联金属体系中出现的新奇物态,并分析了其可能对应的物理模型。首先作为结构最简单的铁基超导体,铁硒(FeSe)展现出了另类的相图演化,其中反常的电子向列序引发大量的研究且至今仍存在不少疑问。为此,我们对FeSe单晶开展了细致的NMR研究。我们合成了高丰度(98%)同位素57Fe的FeSe单晶样品,并首次同时测量了 57Fe与77Se的NMR谱图及自旋-晶格弛豫率。我们发现77Se与57Fe的奈特位移具有明显不同的温度依赖,在向列相中二者的奈特位移及自旋-晶格弛豫率的各向异性随温度的演化也不同。分析可知57Fe原子核可以直接反映Fe位的局域轨道构型,而77Se更多的受到3dxz,3dyz轨道态的影响。我们的实验揭示了 1.除了3ddxz,3dyz轨道的退简并,3dxy轨道在向列序中也发生了重构;2.FeSe具有洪特耦合诱导的轨道选择的电子关联,3dxy轨道的电子态在向列相中随着降温发生非相干到相干的渡越;3.非平庸的自旋-轨道耦合(SOC)效应导致FeSe的向列相中存在不小的局域自旋磁化率各向异性。这些结果表明FeSe中的电子向列相是一个自旋轨道纠缠的电子态,其中不同轨道的电子表现出不同的关联性并随着体系温度变化而出现相干-非相干之间的渡越。FeSe单晶在静水压下演化出了丰富难懂的相图且其超导转变相对于常压可被提高～4倍。另外,其中多种电子型有序间的竞争或合作效应一直是理论与实验关注的焦点,且不同实验手段的测量结果仍存在一些分歧。为此,我们对高丰度57Fe的FeSe单晶样品进行了低压范围内(pmax～2.1 GPa)细致的变压NMR研究。通过比对77Se与57Fe的NMR谱线随静水压的演化我们揭示了长期被遗落的低压下的磁有序预相变过程,而其超导转变与低温低能自旋涨落随静水压的演化表明超导配对机制也发生了相应的变化。另外,基于NMR实验证据,FeSe的电子态随静水压变化也会发生非平庸与磁有序相关的渡越,其中高压下的电子向列序就与FeAs类的具有显著自旋涨落及低温磁有序的向列序相类似。这些结果有助于进一步理解铁基超导体丰富电子性质的起源,并提供了建立统一的物理图像的视角。FeSe及其衍生类材料体系的超导转变具有高度可调性,而常压下FeSe单晶的超导态本身也具有许多非常规的奇异特性。之前的NMR研究由于射频加热效应未能对FeSe单晶的超导态进行完备的表征。为此,我们首次合成了高丰度(50%)同位素77Se的FeSe单晶样品并采用极低功率的射频脉冲对其超导态进行了系统的规避了射频加热效应的NMR测量。我们在所有外场取向下都观测到了与电子自旋磁化率相关的Knight位移的下降,这排除了手征p-波超导配对的可能性。此外,我们在FeSe超导态的磁通晶格中发现了大量的剩余态密度及极度的NMR谱线展宽,这些结果表明FeSe超导态的磁通晶格中出现了十分反常的束缚态。这些实验现象可能与FeSe超导配对处于Bardeen-Cooper-Schrieffer超流机制与Bose-Einstein凝聚(BCS-BEC)渡越区的特征相关,但仍需进一步的理论与实验研究。这些改进的NMR结果为相关理论模型提供了重要的限定及参考。铁基超导体的准二维特征使其十分易于解离、撕薄、插层和形成复杂的共生结构。我们利用NMR的位置选择性对复杂异质结构铁基超导体Ba2Ti2Fe2As4O不同层的物理性质进行了细致的研究。经过系统的角度依赖的NMR谱的测量,我们将之前一直未能确定的发生于～125 K之下的电子相变确认为[Ti2As2O]层中的二维特征的轨道玻璃态。另外,借助NMR的超高分辨率我们首次在该体系中揭示了更低温度下的轨道有序转变及其伴随的结构畸变。类似于电子向列相,其在低温下也出现了相互正交的有序畴区。我们在[Fe2As2]层中还观测到了磁有序与超导的共存。总之,该体系中出现的丰富的电子态使其可作为探索轨道调控及异质结构铁基超导体层间耦合作用物理性质的平台。更多的微观机理仍需大量的理论与实验上的努力。我们也初步的研究了重空穴掺杂的铁基超导体CsFe2As2中Fe位的NMR信号。相关实验证据表明该体系中存在明显的轨道选择的电子关联性以及可能的电子向列序或短程磁有序。另外,我们对系列低超导转变温度的FeSe单晶样品进行了系统的NMR表征。我们发现FeSe单晶的超导态正相关于低温下浮现的强的低能自旋涨落,而其与电子向列序似乎关系不大。这些研究对于厘清FeSe中电子态的本征行为以及主导各电子型有序的关键物理机制具有重要的指导意义。更多还原

【Abstract】 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.更多还原