We analyze how thermal fluctuations near a finite temperature nematic phase transition affect the spectral function $A({\bf k},ω)$ for single-electron excitations in a two-dimensional metal. Perturbation theory yields a splitting of the quasi-particle peak with a d-wave form factor, reminiscent of a pseudogap. We present a resummation of contributions to all orders in the Gaussian fluctuation regime. Instead of a splitting, the resulting spectral function exhibits a pronounced broadening of the quasi-particle peak, which varies strongly around the Fermi surface and vanishes upon approaching the Brillouin zone diagonal. The Fermi surface obtained from a Brillouin zone plot of $A({\bf k},0)$ seems truncated to Fermi arcs.
Non-Fermi liquids are strange metals whose physical properties deviate qualitatively from those of conventional metals due to strong quantum fluctuations. In this paper, we report transport measurements on the FeSe$_{1-x}$S$_x$ superconductor, which has a quantum critical point of a nematic order without accompanying antiferromagnetism. We find that in addition to a linear-in-temperature resistivity $ρ_{xx}\propto T$, which is close to the Planckian limit, the Hall angle varies as $\cot θ_{\rm H} \propto T^2$ and the low-field magnetoresistance is well scaled as $Δρ_{xx}/ρ_{xx}\propto \tan^2 θ…
相关电子流体可以表现出一系列令人惊讶的复杂相，其中一个更令人惊讶的是电子自旋，一个自发产生的空间各向异性的几何不变的金属相。经典向列相通常出现在棒状分子的液体中;考虑到电子是点状的，考虑电子向列相的最初理论动机来自于将电子流体视为量子熔化的电子晶体，而不是费米气体的强相互作用产物。1999年在超净量子霍尔系统中进行的戏剧性传输实验和2007年在强磁场中的Sr 3Ru 2 O 7中进行的戏剧性传输实验证实了这种相在自然界中的存在。在本文中，我们简要地回顾了理论上的考虑，管理的顺序，总结量子霍尔和Sr 3Ru 2 O 7实验，明确地建立了这个阶段的存在，并调查了一些目前的证据，这样一个阶段的铜酸盐和铁基高温超导体。
The electronic structure of the enigmatic iron-based superconductor FeSe has puzzled researchers since spectroscopic probes failed to observe the expected electron pocket at the $Y$ point in the 1-Fe Brillouin zone. It has been speculated that this pocket, essential for an understanding of the superconducting state, is either absent or incoherent. Here, we perform a theoretical study of the preferred nematic order originating from nearest-neighbor Coulomb interactions in an electronic model relevant for FeSe. We find that at low temperatures the dominating nematic components are of inter-orbit…
Quenched nematic criticality separating two superconducting domes in an iron-based superconductor under pressure
电子态及其相关的临界涨落已经成为各种非常规超导体中超导配对的潜在候选者。然而，在大多数材料中，它们与其他磁有序相的共存对确定它们的重要性提出了重大挑战。在这里，通过结合化学和流体静力学的物理压力在FeSe $_{0.89}$ S $_{0.11}$，我们提供了一个独特的访问一个干净的量子相变的情况下的远程磁秩序。我们发现在超导相变附近，高温下电阻率的非费米液体行为在最低温度下演变为费米液体行为。从高磁场中的量子振荡，我们跟踪费米面和电子关联的演化作为施加压力的函数。我们检测实验的Lifshitz过渡，分离两个不同的超导区域：一个新兴的电子相位与一个小的费米面和强电子相关性和其他一个大的费米面和弱相关性，促进嵌套和稳定的磁有序相在高压下。质量发散的缺乏表明临界涨落被晶格的强耦合淬灭。这表明在没有磁序的情况下，超导性在量子相变时不会增强。
我们发展了一个微观理论的电子相变接近各向同性费米液体在二维和三维。得到了有序状态下小振幅集体激发的显式表达式;值得注意的是，除了沿对称性所决定的特殊方向外，Goldstone模式（directorwave）是过阻尼的。在量子临界点，我们发现一个动力学指数的$z=3$，这意味着稳定的高斯不动点。过阻尼Goldstone模的主导微扰效应导致费米液体理论在α相的崩溃和费米表面周围强烈的角度依赖的电子自能。其它金属液晶相，{ \it} e. g.}一个量子六角体，表现类似。
Unconventional symmetry breaking due to nonlocal order parameters has attracted considerable attention in many strongly correlated metals. Famous examples are the nematic order in Fe-based superconductors and the star-of-David charge density order in kagome metals. Such exotic symmetry breaking in metals is a central issue of modern condensed matter physics, while its theoretical foundation is still unclear in comparison with the well-established theory of superconductivity. To overcome this difficulty, here we introduce the "form factor" that generalizes the nonlocal order parameter into the …
p-wave Superfluidity by Spin-Nematic Fermi Surface Deformation
研究了具有强自旋相关各向异性的色散关系的正方晶格上相互吸引的费米子。由此产生的费米表面失配抑制了s波BCS型不稳定性，为非常规类型的秩序扫清了道路。使用Diagrammatic Monte Carlo方法对费曼图解系列进行无偏采样，揭示了中间耦合强度区域的丰富相图。代替所提出的库珀对玻色金属相[A. E. Feiguin和M. P. A. Fisher，Phys. Rev. Lett. 103，025303（2009）]我们发现了强各向异性下的无公度密度波和中等各向异性下具有非常规对称性的两种不同的p波超流态。
Simultaneous emergence of superconductivity, inter-pocket scattering and nematic fluctuation in potassium-coated FeSe superconductor
超导性起源于电子配对。因此，费米面上的配对沟道和配对胶是理解超导体的两个关键问题。最近，在电子掺杂的FeSe超导体中发现了超过40 K的高温超导性，包括K $_x$ Fe $_{2-y}$ Se $_2$、Li $_{0.8}$ Fe $_{0.2}$ OHFeSe和单层FeSe薄膜。然而，它们的配对机制仍然存在争议。本文研究了包钾FeSe单晶电子结构的系统演化。通过在样品表面上原位蒸发钾来精确控制掺杂水平。我们发现当费米面的Lifshitz跃迁使两个电子袋之间的袋间散射打开时，超导性就出现了。在相同的掺杂浓度下，电子有序度显著降低，在较宽的掺杂范围内，电子有序度仍有较大的涨落。我们的研究结果表明，在电子掺杂的FeSe超导体的超导电性，袋间散射，和涨落之间的潜在的相关性。
Recent experiments suggest that besides of antiferromagnetic fluctuations nematic fluctuations may contribute to the occurrence of superconductivity in iron pnictides. Motivated by this observation we study superconductivity from nematic fluctuations in a minimal two-band model. The employed band parameters are appropriate for iron pnictides and lead to four pockets for the Fermi line. It is shown that low-energy, long-wavelength nematic fluctuations within the pockets give rise to strong-coupling superconductivity whereas the large momenta density fluctuations between pockets are rather irrel…
The interplay between strong correlations and non-trivial topology in twisted moiré systems can give rise to a rich landscape of ordered states that intertwine the spin, valley, and charge degrees of freedom. In this paper, we investigate the properties of a system that displays long-range valley-polarized nematic order. Besides breaking the threefold rotational symmetry of the triangular moiré superlattice, this type of order also breaks twofold rotational and time-reversal symmetries, which leads to interesting properties. First, we develop a phenomenological model to describe the onset of t…
We analyze the scattering rate for 2D fermions interacting via soft nematic fluctuations. The ground state is an s-wave superconductor, but other pairing channels are almost equally attractive. This strongly alters the scattering rate: At energies beyond the pairing gap $Δ$, it is renormalized by contributions from all pairing channels. At energies of order $Δ$, it is determined by the competition between scattering into a gapped continuum and dispersing nematic resonance. The outcome is a "peak-peak-dip-hump" spectrum, similar, but not identical, to the "peak-dip-hump" structure in the cuprat…
Compton scattering provides invaluable information on the underlying Fermi surface (FS) and is a powerful tool complementary to angle-resolved photoemission spectroscopy and quantum oscillation measurements. Here we perform high-resolution Compton scattering measurements for La_{2-x}Sr_{x}CuO_{4} with x=0.08 (Tc=20K) at 300K and 150K, and image the momentum distribution function in the two-dimensional Brillouin zone. We find that the observed images cannot be reconciled with the conventional hole-like FS believed so far. Instead, our data imply that the FS is strongly deformed by the underlyin…
The instability of a Fermi surface against Ising nematic order destroys the quasiparticle character of the low-energy degrees of freedom. Therefore, observables exhibit deviations from Fermi liquid behavior which gives rise to the term Ising nematic quantum critical metal. To obtain a theoretical description we use a finite-temperature version of Eliashberg theory which allows to treat the strong coupling between quantum and thermal fluctuations in the absence of well-defined quasiparticles. Here, we use this self-consistent, diagrammatic approach to compute, in particular, the nematic suscept…
Quasi-particle evidence for the nematic state above $T_{\rm{c}}$ in Sr$_x$Bi$_2$Se$_3$
在电子自旋态，电子系统的对称性低于同一系统的晶体结构。在各种非常规超导体中已经观察到了电子双态，例如铜酸盐和铁基超导体、重费米子超导体和拓扑超导体。向列性与超导电性的关系是凝聚态物理中尚未解决的一个重要问题。通过角分辨比热测量，我们报道了拓扑超导体Sr $_x$ Bi $_2$ Se $_3$中向列性的体准粒子证据。比热表现出明显的2重对称性，尽管6重对称晶格。最重要的是，在超导转变温度以上的正常状态下出现了2重对称性。这是解释的角度依赖塞曼效应，由于各向异性的态密度在α相。这些结果突出了向列性和非常规超导性之间的相互关系。
Isoelectronic substitution is an ideal tuning parameter to alter electronic states and correlations in iron-based superconductors. As this substitution takes place outside the conducting Fe planes, the electronic behaviour is less affected by the impurity scattering experimentally and relevant key electronic parameters can be accessed. In this short review, I present the experimental progress made in understanding the electronic behaviour of the nematic electronic superconductors, FeSe1-xSx. A direct signature of the nematic electronic state is in-plane anisotropic distortion of the Fermi surf…
We propose an SU(4) spin-valley-fermion model to investigate the superconducting instabilities of twisted bilayer graphene (TBG). In this approach, bosonic fluctuations associated with an emergent SU(4) symmetry, corresponding to combined rotations in valley and spin spaces, couple to the low-energy fermions that comprise the flat bands. These fluctuations are peaked at zero wave-vector, reflecting the "ferromagnetic-like" SU(4) ground state recently found in strong-coupling solutions of microscopic models for TBG. Focusing on electronic states related to symmetry-imposed points of the Fermi s…
