We report on $^{75}$As-NMR measurements in single crystalline BaFe$_{2}$(As$_{0.96}$P$_{0.04}$)$_{2}$ for magnetic fields parallel to the orthorhombic $[110]_{\rm o}$ and $[100]_{\rm o}$ directions above the structural transition temperature $T_{\rm S}\simeq121$ K. A large difference in the linewidth between the two field directions reveals in-plane anisotropy of the electric field gradient, even in the tetragonal phase. This provides microscopic evidence of population imbalance between As-$4p_{x}$ and $4p_{y}$ orbitals, which reaches $|n_x-n_y|/|n_x+n_y| \sim 15$\% at $T\rightarrow T_{\rm S}$ and is a natural consequence of the orbital ordering of Fe-3$d_{xz}$ and $d_{yz}$ electrons. Surprisingly, this orbital polarization is found to be already static near room temperature, suggesting that it arises from the pinning of anisotropic orbital fluctuations by disorder. The effect is found to be stronger below $\sim 160$ K, which coincides with the appearance of nematicity in previous torque and photoemission measurements. These results impose strong constraints on microscopic models of the nematic state.
We report a Raman scattering study of nematic degrees of freedom in the iron-based superconductor parent compound BaFe$_2$As$_2$ under tunable uniaxial strain. We demonstrate that the polarization resolved arsenic (As) phonon intensity can be used to monitor the nematic order parameter as a function of both temperature and strain. At low temperature in the nematic ordered phase we use it to track the continuous and reversible orientation of nematic domains under variable strain. At higher temperature, the evolution of the As phonon intensity under strain reflects an enhanced nematic susceptibi…
In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42…
We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe$_2$As$_2$, which has a tetragonal-to-orthorhombic structural distortion at $T_s$ and antiferromagnetic (AF) phase transition at $T_N$ with ordered moments along the orthorhombic $a$-axis below $T_s\approx T_N\approx 136$ K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with $M_a=M_b=M_c$, where $M_a$, $M_b$, and $M_c$ are spin excitations polarized along the $a$, $b$, and $c$-axis directions of the orthorhombic lattice, respectively. On co…
A rich variety of nematic/smectic orders in Fe-based superconductors is an important unsolved problem in strongly correlated electron systems. A unified understanding of these orders has been investigated for the last decade. In this article, we explain the $B_{1g}$ symmetry nematic transition in FeSe$_{1-x}$Te$_x$, the $B_{2g}$ symmetry nematicity in AFe$_2$As$_2$ (A=Cs, Rb), and the smectic state in BaFe$_2$As$_2$ based on the same framework. We investigate the quantum interference mechanism between spin fluctuations by developing the density wave equation. The observed rich variety of nemat…
Synchrotron x-ray diffraction experiments were performed on BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals as a function of temperature and applied magnetic field along the tetragonal $[1 \bar{1} 0]$ direction, complemented by electrical resistivity and specific heat experiments. For a BaFe$_2$As$_2$ crystal with spin-density-wave antiferromagnetic ordering temperature $T_{AF}=132.5$ K and onset of the orthorhombic phase at $T_{o}=137$ K, the magnetic field favors the growth of tetragonal domains that compete with orthorhombic ones for $T \gtrsim T_{AF}$. For a Sr(Fe$_{1-x…
High magnetic fields induce a pronounced in-plane electronic anisotropy in the tetragonal antiferromagnetic metal CeRhIn$_{5}$ at $H^{*} \gtrsim 30$ T for fields $\simeq 20^{\mathrm{o}}$ off the $c$-axis. Here we investigate the response of the underlying crystal lattice in magnetic fields to $45$ T via high-resolution dilatometry. Within the antiferromagnetic phase of CeRhIn$_{5}$, a finite magnetic field component in the tetragonal $ab$-plane explicitly breaks the tetragonal ($C_{4}$) symmetry of the lattice well below $H^{*}$ revealing a finite nematic susceptibility at low fields. A modest…
Iron-based superconductors are well-known for their intriguing phase diagrams, which manifest a complex interplay of electronic, magnetic and structural degrees of freedom. Among the phase transitions observed are superconducting, magnetic, and several types of structural transitions, including a tetragonal-to-orthorhombic and a collapsed-tetragonal transition. In particular, the widely-observed tetragonal-to-orthorhombic transition is believed to be a result of an electronic order that is coupled to the crystalline lattice and is, thus, referred to as nematic transition. Nematicity is therefo…
Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements
Leibniz-Institut für Festkörper u. Werkstoffforschung (IFW) Dresden（德累斯顿莱布尼茨固体与材料研究所）；Bergische Universität Wuppertal（伍珀塔尔大学）；Kirchhoff Institute for Physics, Heidelberg University（海德堡大学基尔霍夫物理研究所）
The electric field gradient (EFG) tensor at the $^{75}$As site couples to the orbital occupations of the As p-orbitals and is a sensitive probe of local nematicity in BaFe$_2$As$_2$. We use nuclear magnetic resonance to measure the nuclear quadrupolar splittings and find that the EFG asymmetry responds linearly to the presence of a strain field in the paramagnetic phase. We extract the nematic susceptibility from the slope of this linear response as a function of temperature and find that it diverges near the structural transition in agreement with other measures of the bulk nematic susceptibi…
Phonon splitting and anomalous enhancement of infrared-active modes in BaFe$_2$As$_2$
本文对磷属元素化物母体BaFe $_2$ As $_2$的晶格动力学进行了全面的红外光谱研究。在四元结构相中，我们观察到了两个简并的双光子允许的面内红外激活声子模。从四方晶系到斜方晶系的结构转变之后，我们观察到分裂成四个非简并声子模式和显著的声子强度增强。这些详细的数据使我们能够提供一个物理解释的异常声子强度增强的各向异性导电性由于洪德耦合的结果。
Effect of disorder on the resistivity anisotropy near the electronic nematic phase transition in pure and electron-doped BaFe$_2$As$_2$
我们表明，在四重态的代表性欠掺杂的铁砷化物BaFe $_2$作为$_2$，Ba（Fe $_{1-x}$ Co $_x$）$_2$作为$_2$和Ba（Fe $_{1-x}$ Ni $_x$）$_2$作为$_2$的应变诱导电阻率各向异性是独立的无序在很宽的缺陷和杂质浓度范围内。这一结果表明，在顺磁正交状态的平面电阻率的各向异性，这种材料是不是由于弹性散射各向异性缺陷，是最容易理解的电子结构中的固有各向异性。
FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90\,K, and becomes a superconductor below 8\,K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here we employ an advanced \emph{ab-initio} Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same T$_{c}$ as the nematic ph…
We study the iron-pnictide compound BaFe$_2$(As$_{1-x}$P$_x$)$_2$ for x $\sim$0.23, with a doping concentration near quantum criticality and enhanced nematic fluctuating state in the doping-temperature phase diagram. Transport measurements confirm the presence of a magneto-structural transition at 60K from the tetragonal to the orthorhombic phase, followed by a superconducting transition below 16K. The temperature and polarisation dependent Raman spectra reveal that there is a phonon mode at 211 cm$^{-1}$, followed by two broad modes (BM) between 400 and 700 cm$^{-1}$, having an energy differe…
Pressure tuning of Fermi surface topology of optimally doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$
采用高压下的电阻测量和同步辐射X射线衍射测量相结合的方法，研究了电子掺杂BaFe $_{1.9}$ Ni $_{0.1}$ As $_{2}$的超导、输运和结构性质.发现该体系的超导转变温度与$c/a$的轴比随压力的增加而降低的方式相似，但在7.5GPa的临界压力下，$c/a$的超导转变温度为零，此时发生了从四方相到塌缩四方相的同构转变。电阻服从线性温度依赖关系，证明了反铁磁自旋涨落输运机制。压力效应的压力诱导费米表面拓扑结构的修改，其中压力抑制准粒子的有效质量和强度的反铁磁自旋涨落导致超导性的降低，相应的框架内解释。在塌缩的四方相中不存在的超导电性被认为是由于反铁磁自旋涨落的完全抑制。
FeSe has a unique ground state in which superconductivity coexists with a nematic order without long-range magnetic ordering at ambient pressure. Here, to study how the pairing interaction evolves with nematicity, we measured the thermal conductivity and specific heat of FeSe$_{1-x}$S$_x$, where the nematicity is suppressed by isoelectronic sulfur substitution. We find that in the whole nematic ($0\leq x \leq 0.17$) and tetragonal ($x=0.20$) regimes, the application of small magnetic field causes a steep increase of both quantities. This indicates the existence of deep minima or line nodes in …
Raman scattering can detect spontaneous point-group symmetry breaking without resorting to single-domain samples. Here we use this technique to study $\mathrm{BaFe_2As_2}$, the parent compound of the "122" Fe-based superconductors. We show that an applied compression along the Fe-Fe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature $T_{\mathrm{s}}$ into a crossover that spans a considerable temperature range above $T_{\mathrm{s}}$. Even in crystals that are not subject to any applied force, a distribution of subs…
Mechanical strain is a powerful technique for tuning electronic structure and interactions in quantum materials. In a system with tetragonal symmetry, a tunable uniaxial in-plane strain can be used to probe nematic correlations in the same way that a tunable magnetic field is used to probe magnetic correlations. Here, we present a new spectroscopic scanned probe technique that provides atomic-resolution insight into the effect of anisotropic strain on the electronic structure. We use this technique to study nematic fluctuations and nematic order across the phase diagram of a prototypical iron-…
Ferroquadrupole order associated with local $4f$ atomic orbitals of rare earth ions is a realization of electronic nematic order. However, there are relatively few examples of intermetallic materials which exhibit continuous ferroquadrupole phase transitions, motivating the search for additional materials that fall in to this category. Furthermore, it is not clear a priori whether experimental approaches based on transport measurements which have been successfully used to probe the nematic susceptibility in materials such as the Fe-based superconductors, will be as effective in the case of $4f…
Uniaxial strain detwinning of CaFe2As2 and BaFe2As2: optical and transport study
砷化铁超导体的母体化合物$A$ Fe $_{2}$ As $_{2}$（$A$ =Ca，Sr，Ba）在温度$T_{\mathrm{TO}}$为135至205 K（取决于碱土元素）时经历四方晶系到正交晶系的结构转变。在$T_{\mathrm{TO}}$下方，独立式晶体分裂成相等分布的结构域，其掩盖了材料的固有的、面内的、各向异性的性质。在这里，我们演示了一种方法，机械解孪晶的CaFe $_{2}$ As $_{2}$和BaFe $_{2}$ As $_{2}$。偏振光成像和同步辐射$X$ -射线测量表明，脱孪晶几乎完全，并且是可逆的，应变释放后孪晶图案恢复。在孪晶和去孪晶状态的电阻率测量表明，电阻率，$ρ$，沿正交$a_{o}$ -轴减小，但沿正交$b_{o}$ -轴在两种化合物中增加。紧接在$T_{\mathrm{TO}}$之下，对于Ca和Ba化合物，比率$ρ_{bo}/ ρ_{ao}$分别= 1.2和1.5。与CaFe $_{2}$ As $_{2}$相反，BaFe $_{2}$ As $_{2}$在名义上的四元相中显示出各向异性，这表明在BaFe $_{2}$ As $_{2}$中波动在$T_{\mathrm{TO}}$以上起的作用比在CaFe $_{2}$ As $_{2}$中更大，或者存在更高的温度交叉或相变。
In several Fe-based superconductors, slight $C_4$ symmetry breaking occurs at $T^*$, which is tens of Kelvin higher than the structural transition temperature $T_S$. In this "hidden" nematic state at $T_S<T<T^*$, the orthorhombicity is tiny [$ϕ=(a-b)/(a+b) \ll 0.1$%], but clear evidences of bulk phase transition have been accumulated. To explain this long-standing mystery, we propose the emergence of antiferro-bond (AFB) order with the antiferro wavevector ${\bf q}=(0,π)$ at $T=T^*$, by which the characteristic phenomena below $T^*$ are satisfactorily explained. This AFB order originates from …
We have performed high-resolution angle-resolved photoemission spectroscopy on FeSe superconductor (Tc ~ 8 K), which exhibits a tetragonal-to-orthorhombic structural transition at Ts ~ 90 K. At low temperature we found splitting of the energy bands as large as 50 meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T ~ 110 K, slightly above Ts, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives r…
We investigate coherent phonon oscillations of BaFe$_2$As$_2$ using optical pump-probe spectroscopy. Time-resolved optical reflectivity shows periodic modulations due to $A_{1g}$ coherent phonon of $c$-axis arsenic vibrations. Optical probe beams polarized along the orthorhombic $a$- and $b$-axes reveal that the initial phase of coherent oscillations shows a systematic deviation as a function of temperature, although these oscillations arise from the same $c$-axis arsenic vibrations. The oscillation-phase remains anisotropic even in the tetragonal structure, reflecting a nematic response of Ba…
Electronic Nematicity Revealed by Torque Magnetometry in Iron Arsenide EuFe$_2$(As$_{1-x}$P$_x$)$_2$
电子向列相是一种电子取向的有序性，它打破了铁磷属元素化物超导体的旋转对称性。然而，这一阶段的掺杂依赖性的普遍性和它的关系，其他的破缺订单（如超导）在不同的家庭的铁pnictides，仍然是悬而未决的问题。本文用转矩磁法研究了EuFe $_2$（As $_{1-x}$ P $_x$）$_2$在不引入外压的情况下的转动对称破缺。发现在最佳掺杂下，超导相在结构转变上方增殖并持续进入超导状态，之后它变得不存在或非常弱，与BaFe $_2$（As $_{1-x}$ P $_x$）$_2$中观察到的行为形成鲜明对比。这些测量结果表明，在超导圆顶下，接近最佳掺杂的假定量子跃迁。
The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom [1-4]. The nematic state observed in many of these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion [5-7]. A more subtle and much less explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space …
We report results of Raman scattering experiments on twin-free BaFe$_{2}$As$_{2}$ with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the $E_g$ modes and the As $A_{1g}$ mode. Each of the two $E_g$ phonons in the tetragonal phase is observed to split into a $B_{2g}$ and a $B_{3g}$ mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10\,cm$^{-1}$ and less than 5\,cm$^{-1}$ for the low- and the high-energy $E_g$ mode, respectively. The detailed study…
The elastoresistivity tensor $m_{ij,kl}$ relates changes in resistivity to strains experienced by a material. As a fourth-rank tensor, it contains considerably more information about the material than the simpler (second-rank) resistivity tensor; in particular, for a tetragonal material, the $B_{1g}$ and $B_{2g}$ components of the elastoresistivity tensor ($m_{xx,xx}-m_{xx,yy}$ and $2m_{xy,xy}$, respectively) can be related to its nematic susceptibility. Previous experimental probes of this quantity have focused exclusively on differential longitudinal elastoresistance measurements, which dete…
We use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at $x=0.3$. However, we show that the tetragonal-to-orthorhombic structural ($T_s$) and paramagnetic to antiferromagnetic (AF, $T_N$) transitions in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ are always coupled and approach t…
The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. The tetragonal cuprate HgBa$_2$CuO$_{4+δ}$, with only one CuO$_2$ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility within the CuO$_2$ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy…
A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe$_2$As$_2$, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature $T_s$. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature $T_N$ ($<T_s$) and create in-plane resistivity anisotropy above $T_s$. Here we use neutron polarization analysis to …
Electronic nematics are exotic states of matter where electronic interactions break a rotational symmetry of the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Intriguingly such phases appear in the copper- and iron-based superconductors, and their role in establishing high-temperature superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of nematic character …
An SDW antiferromagnetic (SDW-AF) low temperature phase transition is generally observe and the AF spin fluctuations are considered to play an important role for the superconductivity paring mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transp…
The large anisotropy in the electronic properties across a structural transition in several correlated systems has been identified as the key manifestation of electronic nematic order, breaking rotational symmetry. In this context, FeSe is attracting tremendous interest, since electronic nematicity develops over a wide range of temperatures, allowing accurate experimental investigation. Here we combine angle-resolved photoemission spectroscopy and theoretical calculations based on a realistic multi-orbital model to unveil the microscopic mechanism responsible for the evolution of the electroni…
We investigated the out-of-plane transport properties of parent and chemically substituted BaFe$_{2}$As$_{2}$ for various types of substitution. Based on the studies of Hall coefficient and chemical-substitution effect, we have clarified the origin for the unusual temperature dependence of out-of-plane resistivity $ρ_c(T)$ in the high-temperature paramagnetic-tetragonal phase. Electron (hole) carriers have an incoherent (coherent) character, which is responsible for non-metallic (metallic) $ρ_c(T)$. Although both of electron and hole contributions are almost comparable, a slightly larger contr…
A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($T_{\rm c}$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe overlap with each other, the effects of two orders on superconductivity remain perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($T$) against pressure ($P$) and isovalent S-su…
Nematic phase transitions in high-temperature superconductors have a strong impact on the electronic properties of these systems. BaFe$_2$As$_2$,\@ with an established nematic transition around 137 K induced by magnetic fluctuations, and BaNi$_2$As$_2$,\@ a non-magnetic analog of BaFe$_2$As$_2$ with a structural transition in the same temperature range,\@ share a common tetragonal aristotype crystal structure with space-group type $I4/mmm$.\@ In contrast to BaFe$_2$As$_2$ where collinear stripe magnetic order is found for the orthorhombic low-$T$ phase, a unidirectional charge density wave tog…
The main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$ by electronic transport, angle-resolved photoemission spectroscopy (ARPES) and inelastic neutron scattering (INS) measurements. Previous neutron diffraction and transport measurements suggested that the collinear antiferromagnetism persists to $x=0.8$, with similar Néel temperature $T_N$ and structural transition temperature $T_s$ around …
Electronic nematicity, the breaking of the crystal lattice rotational symmetry by the electronic fluid, is a fascinating quantum state of matter. Recently, BaNi$_2$As$_2$ has emerged as a promising candidate for a novel type of nematicity triggered by charge fluctuations. In this work, we scrutinize the electronic nematicity of BaNi$_2$(As$_{1-x}$P$_x$)$_2$ with $0 \leq x \leq 0.10$ using electronic transport measurements under strain. We report a large $B_{1g}$ elastoresistance coefficient that is maximized at a temperature slightly higher than the first-order triclinic transition, and that c…
Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well-separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural tran…
We present a systematic study on the heavily Cr doped iron pnictides BaFe$_{1.9-x}$Ni$_{0.1}$Cr$_{x}$As$_{2}$ by using elastic neutron scattering, high-resolution synchrotron X-ray diffraction (XRD), resistivity and Hall transport measurements. When the Cr concentration increases from $x=$ 0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Néel temperature $T_N$ coincides with the tetragonal-to-orthorhombic structural transition temperature $T_s$, and both of them keeps around 35 K. The magnetic ordered moment, …
Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides
通过比较第一性原理计算和非弹性中子散射（INS）测量结果，研究了122-铁磷族化合物超导体中既没有静态磁相也没有结构相变的BaFe 1.85 Co 0.15 As 2和BaFe 1.91 Ni 0.09 As 2样品的自旋激发谱的对称性.在正常和超导（SC）状态下，光谱缺乏围绕（1/2 1/2 L）轴的42/m螺旋对称性，这是由I4/mmm空间群所暗示的。这在正常和SC状态自旋动力学的面内各向异性和自旋共振模式的面外色散中都是明显的。我们发现，这种效果源于较高的对称性的磁性Fe子晶格相对于晶体本身，因此INS信号继承了对称性的展开布里渊区（BZ）的Fe子晶格。面内各向异性是温度无关的，并且可以在正常状态密度泛函理论计算中定性地再现，而不调用先前提出的作为对这种效应的解释的破缺（“破缺”）基态。在SC转变之下，磁共振模式Er的能量以及其强度和SC自旋间隙继承了沿着面外方向L的正常态强度调制，其周期比体心四面体BZ对称性所预期的大两倍。这种调制的幅度在更高的掺杂降低，提供了一个类比，在双层铜酸盐的偶和奇谐振模式之间的分裂。结合我们和以前的数据，我们表明，在奇数L的一个普遍的线性关系Er=4.3*kB*Tc持有所有研究的铁基超导体，独立于他们的载体类型。它的有效性下降到最低的掺杂水平是一致的，与较弱的电子相关性在铁磷族化合物相比，欠掺杂的铜酸盐。
Understanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and in-plane resistivity anisotropy in uniaxial pressure detwinned BaFe$_2$As$_2$ and SrFe$_2$As$_2$, the parent compounds of iron pnictide superconductors. While BaFe$_2$As$_2$ exhibits weakly first order tetragonal-to-orthorhombic structural and antiferromagnetic (AF) phase transitions below $T_s > T_N\approx 138$ K, SrFe$_2$As$_2$ has strongly c…
Recent studies in heavily hole-doped iron-based superconductor RbFe$_2$As$_2$ have suggested the emergence of novel electronic nematicity directed along the Fe-As direction, 45$^\circ$ rotated from the usual nematicity ubiquitously found in BaFe$_2$As$_2$ and related materials. This motivates us to study the physical properties of Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$, details of which remain largely unexplored. Here we report on the normal-state charge transport in Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$ superconductors by using high-quality single crystals in the range of Rb concentration $0.14\le x…
Raman scattering experiments on LaFeAsO with splitted antiferromagnetic (T_AFM = 140 K) and tetragonal-orthorhombic (T_S = 155 K) transitions show a quasi-elastic peak (QEP) in B2g symmetry (2 Fe tetragonal cell) that fades away below ~T_AFM and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the T-dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between T_S and T_AFM in LaFeAsO compared with their coincidence in BaFe2As2 manifests itself in slower …
Microscopic origin of pressure-induced phase transitions in iron-pnictide $AFe_2As_2$ superconductors: an {ab initio} molecular-dynamics study
用{ \it ab initio}分子动力学方法研究了加压下$A$ Fe $_2$ As $_2$（$A$ =Ca，Sr，Ba）的电子和晶格结构.我们发现，结构相变（正交到四面体对称性）总是伴随着一个磁相变在所有的化合物，而过渡的性质是不同的三个系统。计算结果解释了CaFe $_2$ As $_2$中存在塌缩的四方相，而BaFe $_2$ As $_2$中不存在塌缩的四方相的原因.我们认为，费米面嵌套功能的变化主导的相变压力下，而不是自旋挫折或近藤的情况。超导电性的后果进行了讨论。
We report results of low-temperature heat capacity, magnetocaloric effect and neutron diffraction measurements of TmVO$_{4}$, an insulator that undergoes a continuous ferroquadrupolar phase transition associated with local partially-filled $4f$ orbitals of the thulium (Tm$^{3+}$) ions. The ferroquadrupolar transition, a realization of Ising nematicity, can be tuned to a quantum critical point using a magnetic field oriented along the $c$-axis of the tetragonal crystal lattice, which acts as an effective transverse field for the Ising-nematic order. In small magnetic fields, the thermal phase t…
