Spin wave excitations and stability of the ($π,0$) ordered magnetic state are investigated in a minimal two-band itinerant-electron model for iron pnictides. Presence of hopping anisotropy generates a strong ferro-orbital order in the $d_{xz}$ and $d_{yz}$ Fe orbitals. The orbital order sign is as observed in experiments. The induced ferro-orbital order strongly enhances the spin wave energy scale and stabilizes the magnetic state by optimizing the strength of the emergent AF and F spin couplings through optimal band fillings in the two orbitals. The calculated spin-wave dispersion is in quantitative agreement with neutron scattering measurements. Finite inter-orbital Hund's coupling is shown to further enhance the spin wave energies state by coupling the two magnetic sub-systems. A more realistic two-band model with less hopping anisotropy is also considered which yields not only the circular hole pockets, also correct ferro-orbital order and emergent F spin coupling.
We introduce a cluster DMFT (Dynamical Mean Field Theory) approach to study the normal state of the iron pnictides and chalcogenides. In the regime of moderate mass renormalizations, the self-energy is very local, justifying the success of single site DMFT for these materials and for other Hunds metals. We solve the corresponding impurity model with CTQMC (Continuous Time Quantum Monte-Carlo) and find that the minus sign problem is not severe in regimes of moderate mass renormalization.
The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO (R=Ce, Pr, Nd, Sm, Gd and Tb) and RFe2As2 (R=Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magnetic-fiel…
用As核四极共振方法研究了RFeAsO $_{1-x}$ F $_x$（R=La，Sm）铁磷属元素化物中的电荷分布.而未掺杂和最佳掺杂或过掺杂的化合物具有单一的电荷环境，两个电荷环境中检测到的欠掺杂区域。自旋-晶格弛豫测量表明它们在纳米尺度上共存。与掺杂光谱的定量变化一起，它们指向铁层中的局部电子顺序，其中低掺杂和高掺杂样区域将共存。静态磁性和超导性的相互作用的影响进行了讨论。
A magneto-orbital coupling mechanism is proposed to account for the weak energy gap at the Fermi energy in the $(π,0)$ ordered SDW state of a realistic three band model for iron pnictides involving $d_{xz}$, $d_{yz}$, and $d_{xy}$ Fe orbitals. The orbital mixing terms between the $d_{xy}$ and $d_{xz}/d_{yz}$ orbitals, which are important in reproducing the orbital composition of the elliptical electron pockets at $(\pm π,0)$ and $(0,\pm π)$, are shown to play a key role in the energy gap formation in the SDW state.
A three-orbital itinerant-electron model involving d_{xz}, d_{yz} and d_{xy} Fe 3d orbitals is proposed for iron pnictides towards understanding the (π,0) ordered magnetism and magnetic excitations in these materials. It is shown that this model at half filling yields a gapped (π,0) magnetic state with high degree of robustness and stability, and simultaneously reproduces several experimentally observed features such as the electronic structure, spin excitations, as well as the ferro orbital order between the d_{xz} and d_{yz} orbitals.
Exchange Constants and Neutron Spectra of Iron Pnictide Materials
我们证明了Zhao等人（arXiv：0903.2686v1）在CaFe $_2$ As $_2$中观察到的中子散射谱和Han等人（Phys.Rev.Lett. 102，107003（2009）），可以通过假设这些材料中轨道顺序的作用来解释。我们写下一个简单的模型哈密顿与四角对称，其自旋波谱描述所观察到的色散关系。我们进一步认为，这些材料具有轨道选择性莫特转变，这是由相邻的铁原子之间的超交换驱动。我们认为，减少自旋和准一维，而不是挫折是负责减少这些材料的时刻。
We investigate the spin-wave excitations in the spin-density wave state of doped iron pnictides within a five-orbital model. We find that the excitations along ($π, 0$)$\rightarrow$($π, π$) are very sensitive to the doping whereas they do not exhibit a similar sensitivity along ($0, 0$) $\rightarrow$ ($π, 0$). Secondly, anisotropy in the excitations around ($π, 0$) with an elliptical shape grows on moving towards the hole-doped region for low energy, whereas it decreases for the high-energy excitations on the contrary. Thirdly, spin-wave spectral weight shifts towards the low-energy region on …
We investigate the role of Hund's coupling in the spin-wave excitations of the ($π, 0$) ordered magnetic state within a five-orbital tight-binding model for iron pnictides. To differentiate between the roles of intraorbital Coulomb interaction and Hund's coupling, we focus on the self-consistently obtained mean-field SDW state with a fixed magnetic moment obtained by using different sets of interaction parameters. We find that the Hund's coupling is crucial for the description of various experimentally observed characteristics of the spin-wave excitations including the anisotropy, energy-depen…
Considerations of the bad-metal behavior led to an early proposal for a quantum critical point under a P for As doping in the iron pnictides, which has since been experimentally observed. We study here an effective model for the isoelectronically tuned pnictides using a large-$N$ approach. The model contains antiferromagnetic and Ising-nematic order parameters appropriate for $J_1$-$J_2$ exchange-coupled local moments on an Fe square lattice, and a damping caused by coherent itinerant electrons. The zero-temperature magnetic and Ising transitions are concurrent and essentially continuous. The …
We report on systematic excitation-density dependent all-optical femtosecond time resolved study of the spin-density wave state in iron-based superconductors. The destruction and recovery dynamics are measured by means of the standard and a multi-pulse pump-probe technique. The experimental data are analyzed and interpreted in the framework of an extended three temperature model. The analysis suggests that the optical-phonons energy-relaxation plays an important role in the recovery of almost exclusively electronically driven spin density wave order.
In the multiband systems, Hund's coupling ($J$) plays a significant role in the spin and charge excitations. We study the dependence of electronic anisotropy on $J$ in terms of Drude-weight along different directions as well as the orbital order in the four-fold symmetry broken spin-density wave state of iron pnictides. A robust behavior of the Drude-weight anisotropy within a small window around $J \sim 0.25U$ with $U$ as intraorbital Coulomb interaction is described in terms of orbital-weight distribution along the reconstructed Fermi surfaces. We also find that the ferro-orbital order incre…
The large in-plane anisotropy of the resistivity is a hallmark of the nematic state of the iron pnictides. Solving the Boltzmann transport equation, we show that the prominent doping dependence as well as the large values of the anisotropy can be well explained by momentum-dependent spin-fluctuation scattering without assuming anisotropic impurity states. Due to the forward-scattering corrections, the hot spots contribute to the resistive anisotropy even in the case of strong spin fluctuations, which makes large values of the anisotropy possible. The ellipticity of the electron pockets plays a…
Antiferromagnetic order in multi-band Hubbard models for iron-pnictides
利用Gutzwiller变分理论研究了${\rm La O Fe As}$中3个铁离子$t_{2g}$带和2个铁离子3 $d$带的多带哈伯德模型.我们对顺磁基态的分析表明，无论是Hartree-Fock平均场论还是有效自旋模型都不能充分描述这些系统。与Hartree-Fock型方法相反，Gutzwiller理论预测反铁磁秩序需要大量的本地Hund规则交换相互作用值。对于三带模型，反铁磁矩适合广泛的相互作用参数的实验数据。然而，对于更合适的五带模型，铁$e_g$电子取代了$t_{2g}$电子，它们对有序矩有很大贡献。
In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity …
Sr$_2$VO$_3$FeAs: A Nanolayered Bimetallic Iron Pnictide Superconductor
在铁磷属元素化物超导体中，对于磁有序机制和非常规的序参量特征，统一的概念之一是近似嵌套的电子和空穴费米面。利用密度泛函方法对SrFe $_2$ P $_2$中的费米面进行了正确的预测，我们发现最近报道的超导SrPDMATHABMM VO $_3$ FeAs（$T_c$ =37 K，磁性和超导电性之间没有明显的竞争）具有与以前的铁磷属元素化物不同的费米面几何形状和性质.中间层（V双层）产生穿过费米能级的能带。耦合到FeAs层是小的，除了沿区域边界的相互作用，但是，耦合退化的费米面嵌套。Sr $_2$ VO $_3$ FeAs，其开放的壳层原子的交替层，值得进一步仔细研究，这将有助于理解铁磷属化合物的性质的起源。
In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal non-uniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microsc…
Our comprehensive study on EuFe$_2$As$_2$ reveals a dramatic reduction of magnetic detwinning fields compared to other AFe$_2$As$_2$ (A = Ba, Sr, Ca) iron pnictides by indirect magneto-elastic coupling of the Eu$^{2+}$ ions. We find that only 0.1T are sufficient for persistent detwinning below the local Eu$^{2+}$ ordering; above $T_\text{Eu}$ = 19K, higher fields are necessary. Even after the field is switched off, a significant imbalance of twin domains remains constant up to the structural and electronic phase transition (190K). This persistent detwinning provides the unique possibility to s…
The electronic structure of iron pnictides is topologically nontrivial, leading to the appearance of Dirac cones in the band structure for the antiferromagnetic phase. Motivated by the analogy with Dirac cones in graphene, we explore the possible existence of topologically protected surface states. Surprisingly, bands of surface states exist even in the paramagnetic state. A realistic five-orbital model predicts two such bands. In the antiferromagnetic phase, these surface bands survive but split. We obtain the bulk and surface dispersion from exact diagonalization of two- and five-orbital mod…
Electron correlations produce a rich phase diagram in the iron pnictides. Earlier theoretical studies on the correlation effect demonstrated how quantum fluctuations weaken and concurrently suppress a $C_2$-symmetric single-Q antiferromagnetic order and a nematic order. Here we examine the emergent phases near the quantum phase transition. For a $C_4$-symmetric collinear double-Q antiferromagnetic order, we show that it is accompanied by both a charge order and an enhanced nematic susceptibility. Our results provide understanding for several intriguing recent experiments in hole-doped iron ars…
X-ray emission and absorption spectroscopies are jointly used as fast probes to determine the evolution as a function of doping of the fluctuating local magnetic moments in K- and Cr- hole-doped BaFe2As2. An increase in the local moment with hole-doping is found, supporting the theoretical scenario in which a Mott insulating state that would be realized for half-filled conduction bands has an influence throughout the phase diagram of these iron-pnictides.
