强烈相互作用的电子可以产生空间不均匀的电子液晶相的有趣想法已经有十多年的历史了，但是这些系统仍然代表了凝聚态物理的未探索的前沿。一个原因是，由强相互作用产生的多体量子态以及由此产生的电子相的可视化还没有实现。软凝聚态物理被显微镜所改变，显微镜可以对真实空间的结构和图案进行成像。在纯电子系统中获得等效数据的候选技术是光谱成像扫描隧道显微镜（SI-STM）。核心的挑战是检测的多体电子态的脆弱，但“重”的k-空间分量，同时其r-空间成分。Sr 3Ru 2 O 7为解决这些问题提供了一个特别令人兴奋的机会。它具有（i）一个非常强的重正化的d-电子费米液体和（ii）表现出场诱导的电子液晶相的转变。最后，作为层状化合物，它可以被裂解，为SI-STM提供优异的表面。
英文摘要：
The intriguing idea that strongly interacting electrons can generate spatially inhomogeneous electronic liquid crystalline phases is over a decade old, but these systems still represent an unexplored frontier of condensed matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting electronic phases, has not been achieved. Soft condensed matter physics was transformed by microscopies that allowed imaging of real-space structures and patterns. A candidate technique for obtaining equivalent data in the purely electronic systems is Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM). The core challenge is to detect the tenuous but 'heavy' k-space components of the many-body electronic state simultaneously with its r-space constituents. Sr3Ru2O7 provides a particularly exciting opportunity to address these issues. It possesses (i) a very strongly renormalized 'heavy' d-electron Fermi liquid and (ii) exhibits a field-induced transition to an electronic liquid crystalline phase. Finally, as a layered compound, it can be cleaved to present an excellent surface for SI-STM.
We investigate the impurity scattering induced quasiparticle interference in the ($π, 0$) spin-density wave phase of the iron pnictides. We use a five orbital tight binding model and our mean field theory in the clean limit captures key features of the Fermi surface observed in angle-resolved photoemission. We use a t-matrix formalism to incorporate the effect of doping induced impurities on this state. The impurities lead to a spatial modulation of the local density of states about the impurity site, with a periodicity of $\sim 8a_{{\rm Fe}-{\rm Fe}}$ along the antiferromagnetic direction. Th…
Resonant X-ray scattering (RXS) has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of the RXS experiments remains theoretically challenging due to the complexity of the RXS cross-section. Central to this debate is the recent proposal that impurity-induced Friedel oscillations, akin to quasiparticle interference signals observed with the scanning tunneling microscope (STM), can lead to scattering peaks in the RXS experiments. The possibility that quasiparticle properties can be probed in RXS measurements…
Correlating off-stoichiometric doping with nanoscale electronic disorder and quasiparticle interference pattern in high-$T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$
本文提出了一种微观理论，用以解释超导体Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+δ}$中所观察到的电子无序。的基本现象被证明是一致的两种类型的间隙氧掺杂剂的存在：那些主要作为电荷储备和那些接近顶端平面贡献的载体和静电势的CuO $_2$平面。后者的非线性屏蔽产生掺杂空穴浓度的纳米级变化，导致电子不均匀性。基于扩展的t- $J$模型的无限制Gutzwiller近似，我们提供了一个一致的解释观察到的掺杂剂的位置和配对间隙及其空间演化之间的相关性。我们发现，氧掺杂剂的主要原因都对间隙无序和准粒子干涉图案。
One of the key issues in unraveling the mystery of high Tc superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi,Pb)2Sr2CuO6+x cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences and the Fourier transforms uncover the evolution of momentum space topology. More interestingly, we observe nanoscale patches of static charge order with sqrt(2)*sqrt(2) periodicity. Both the dispersive quasiparticle interf…
We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and non-diagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states but …
We predict that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path $(0, 0, k_z)$ at $15$ meV} above and $100$ meV below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at $E=15$ meV and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin-polarized but in opposite directio…
We find that heavy fermion systems can have bulk "Fermi arcs", with the use of the non-Hermitian topological theory. In an interacting electron system, the microscopic many-body Hamiltonian is Hermitian, but the one-body quasiparticle Hamiltonian is non-Hermitian due to the finite quasiparticle lifetime. We focus on heavy electron systems as a stage of finite lifetime quasiparticles with two lifetimes, since quasiparticle lifetimes for f-electrons and c-electrons should be different. Two lifetimes induce exceptional points (EPs) of the non-Hermitian quasiparticle Hamiltonian matrix in momentum…
Field-tuned quasiparticles and electronic structure in heavy-fermion YbNi4P2
University of Bristol（布里斯托大学）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Radboud University（拉德堡德大学）；University College Cork（科克大学）；Technische Universität Braunschweig（布伦瑞克工业大学）；Goethe-Universität Frankfurt（法兰克福歌德大学）
Phase-sensitive measurements of the superconducting gap in Fe-based superconductors have proven more difficult than originally anticipated. While quasiparticle interference (QPI) measurements based on scanning tunneling spectroscopy are often proposed as defnitive tests of gap structure, the analysis typically relies on details of the model employed. Here we point out that the temperature dependence of momentum-integrated QPI data can be used to identify gap sign changes in a qualitative way, and present an illustration for $s_{\pm}$ and $s_{++}$ states in a system with typical Fe-pnictide Fer…
Theory of quasiparticle interference on the surface of a strong topological insulator
强拓扑绝缘体（如Bi 2 Te 3或Bi 1-xSnx）表面的电子形成拓扑保护的螺旋液体，其激发光谱包含奇数个无质量狄拉克费米子。本文对螺旋液体中磁性杂质和非磁性杂质的准粒子散射产生的干涉图样的普遍特征进行了理论分析和分类。我们的研究结果证实了在最近的实验中观察到的非磁性杂质的背散射的情况下，并预测新的干扰功能，独特的螺旋液体的特性，当散射体是磁性的。
Topological Weyl semimetal (TWS) is a metal, where low energy excitations behave like Weyl fermions of high-energy physics. It was recently shown that due to the lower symmetry of condensed matter systems, they can realize two distinct types of Weyl fermions. The type-I Weyl fermion in a metal is formed by a linear crossing of two bands at a point in the crystalline momentum space - Brillouin zone (BZ). The second type TWSs host type-II Weyl points appearing at the touching points of electron and hole pockets, which is a result of tilted linear dispersion. The type-II TWS was predicted to exis…
We report the first observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic imaging scanning tunneling microscopy. A direct link between the observed atomic-scale interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. Furthermore, we show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the spectral weight of the observed coherent surface state.
Using a realistic ten-orbital tight-binding model Hamiltonian fitted to the angle-resolved photoemission (ARPES) data on LiFeAs, we analyze the temperature, frequency, and momentum dependencies of quasiparticle interference (QPI) to identify gap sign changes in a qualitative way, following our original proposal [Phys. Rev. B 92, 184513 (2015)]. We show that all features present for the simple two-band model for the sign-changing $s_{+-}$-wave superconducting gap employed previously are still present in the realistic tight-binding approximation and gap values observed experimentally. We discuss…
We use scanning tunneling microscopy to investigate Bi2Sr2Ca2Cu3O10+δ trilayer cuprates from the optimally doped to overdoped regime. We find that the two distinct superconducting gaps from the inner and outer CuO2 planes both decrease rapidly with doping, in sharp contrast to the nearly constant Tc. Spectroscopic imaging reveals the absence of quasiparticle interference in the antinodal region of overdoped samples, showing an opposite trend to that in single- and double-layer compounds. We propose that the existence of two types of inequivalent CuO2 planes and the intricate interaction betwee…
It is a long-standing important issue in heavy fermion physics whether $f$-electrons are itinerant or localized when the magnetic order occurs. Here we report the {\it in situ} scanning tunneling microscopy observation of the electronic structure in epitaxial thin films of CeRhIn$_5$, a prototypical heavy fermion compound with antiferromagnetic ground state. The conductance spectra above the Néel temperature $T_N$ clearly resolve the energy gap due to the hybridization between local 4$f$ electrons and conduction bands as well as the crystal electric field excitations. These structures persist …
The electronic structure of the Kondo lattice CeIn3 has been studied by on-resonant angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. A weakly dispersive quasiparticle band has been observed directly with an energy dispersion of 4 meV by photoemission, implying the existence of weak hybridization between the f electrons and conduction electrons. The hybridization is further confirmed by the formation of the hybridization gap revealed by temperature-dependent scanning tunneling spectroscopy. Moreover, we find the hybridization strength in CeIn3 is much we…
Quasiparticle interference in the spin-density wave phase of iron-based superconductors
我们提出了一个解释最近在母体铁基超导体[T.- M. Chuang等人，Science 327，181（2010）]。我们认为，在准粒子干涉（QPI）中确定的准一维纳米结构是磁（$π$，0）自旋密度波（SDW）顺序与底层电子结构相互作用的结果。我们表明，QPI峰的演变在很大程度上反映了准粒子之间的散射参与SDW形成的电子带。由于电子袋的椭圆率和只有一个电子袋参与SDW的事实，所得到的QPI具有明显的一维结构。我们进一步预测，QPI跨越到二维的能量尺度上，设置的SDW间隙，我们估计从中子散射数据约为90毫电子伏。
The superconducting order parameter is directly related to the pairing interaction, with the amplitude determined by the interaction strength, while the phase reflects the spatial structure of the interaction. However, given the large variety of materials and their rich physical properties within the iron-based high-Tc superconductors, the structure of the order parameter remains controversial in many cases. Here, we introduce Defect Bound State Quasi Particle Interference (DBS-QPI) as a new method to determine the superconducting order parameter. Using a low-temperature scanning tunneling mic…
Weyl semimetals are gapless three-dimensional topological materials where two bands touch at an even number of points in the bulk Brillouin zone. These semimetals exhibit topologically protected surface Fermi arcs, which pairwise connect the projected bulk band touchings in the surface Brillouin zone. Here, we analyze the quasiparticle interference patterns of the Weyl phase when time-reversal symmetry is explicitly broken. We use a multi-band $d$-electron Hubbard Hamiltonian on a pyrochlore lattice, relevant for the pyrochlore iridate R$_2$Ir$_2$O$_7$ (where R is a rare earth). Using exact di…
Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. Here, we show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the $z$ axis that are contained within the experimental data. By using a three-dimensional tight binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimenta…
