Frustrated Magnetic Interactions, Giant Magneto-Elastic Coupling, and Magnetic Phonons in Iron-Pnictides
Taner Yildirim
备注：
New Results: Fe-mangetism is required to explain the ROOM TEMPERATURE phonon DOS (Fig. 21); Jij(R) up to 12 Ang, indicating that J2 is short range while J1 decays as 1/R^3 (Fig. 7) ; A simple spin-Peierles model (i.e. dJ/da) (see Fig. 11-12); Spin-wave spectrum to determine the sign of exchange interaction (see Fig. 8-9)
我们提出了一个详细的第一性原理研究的Fe-磷族化合物，特别强调竞争磁相互作用，结构相变，巨磁弹性耦合及其对声子的影响。对于LaOFeAs，交换相互作用$J_{i,j}(R)$的计算结果为$\approx 12 $ = $. We find that $ J_{i,j}（R）$ has an oscillatory character with an envelop decaying as $ 1/R^3 $ along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic twinning is given. The lattice parameter dependence of the exchange constants, $ dJ_{i,j} /da $ are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal-orthorhombic phase transition. We then discuss giant magneto-elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimetnal values by 1.4 Å$ PDMATHABMU_2 PDMATHABMV_2 $, by 0.4 Å$ PDMATHABMX_2 PDMATHABMV_2 $, and by 0.13 Å$ $。最后，我们表明，铁自旋也需要获得正确的声子能量，特别是作为c-极化和Fe-Fe面内模式。由于将铁作为磁性离子处理总是给出比非磁性离子好得多的结果，并且由于在正常到超导相变期间没有大的c轴减少，因此铁磁矩应该始终存在于铁磷属元素化物中。我们讨论了这些迷人的Fe-磷属元素化物系统的超导机制，我们的结果的影响。
英文摘要：
We present a detailed first principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto-elastic coupling and its effect on phonons. The exchange interactions $J_{i,j}(R)$ are calculated up to $\approx 12 $Å$. We find that $J_{i,j}(R)$ has an oscillatory character with an envelop decaying as $1/R^3$ along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic twinning is given. The lattice parameter dependence of the exchange constants, $dJ_{i,j}/da$ are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal-orthorhombic phase transition. We then discuss giant magneto-elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimetnal values by 1.4 Å$ $ for CaFe$_2$As$_2$, by 0.4 Å$ $ for BaFe$_2$As$_2$, and by 0.13 Å$ $ for LaOFeAs. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe-Fe in-plane modes. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.
Anisotropic magnetic excitations of a frustrated bilinear-biquadratic spin model -- Implications for spin waves of detwinned iron pnictides
阐明自旋激发的性质对于理解铁磷属元素化物的超导电性机制具有重要意义。受最近BaFe $_{2}$ As $_{2}$非弹性中子散射测量的启发，我们研究了具有波矢$(π,0)$的反铁磁相$S=1$阻挫双线性-双二次海森堡模型的自旋动力学.在$SU(3)$表示中使用味波理论以动力学方式处理双二次相互作用。除了磁振子（偶极）激发，双二次相互作用在高能量下产生四极激发。我们发现，四极波显着影响，在能量依赖的方式，自旋激发谱之间的各向异性沿$(π,0)$和$(0,π)$方向在波矢空间。我们的理论结果捕捉到了反孪晶BaFe $_2$ As $_2$的自旋动力学的基本行为。更一般地说，我们的研究结果强调了电子相关效应的铁pnictides的微观物理的重要性。
Self-consistent spin-wave theory for a frustrated Heisenberg model with biquadratic exchange in the columnar phase and its application to iron pnictides
The magnetoresistance of iron pnictide superconductors is often dominated by electron-electron correlations and deviates from the H$^2$ or saturating behaviors expected for uncorrelated metals. Contrary to similar Fe-based pnictide systems, the superconductor LaRu$_2$P$_2$ (T$_c$ = 4 K) shows no enhancement of electron-electron correlations. Here we report a non-saturating magnetoresistance deviating from the H$^2$ or saturating behaviors in LaRu$_2$P$_2$. We have grown and characterized high quality single crystals of LaRu$_2$P$_2$ and measured a magnetoresistance following H$^{1.3}$ up to 22…
Spectrally-resolved femtosecond reflectivity relaxation dynamics in undoped SDW 122-structure iron based pnictides
本文系统地研究了铁基超导体母自旋密度波（SDW）化合物AAs $_{2}$ Fe $_{2}$（A=Ba，Sr和Eu）的光反射率随温度和光谱变化的动力学。确定了两种不同的松弛过程。在包含磁振子的弛豫-瓶颈模型框架下，分析了对磁结构转变非常敏感的慢过程的行为。结果进行了比较，最近的时间分辨角光电发射结果（TR-ARPES）和可能的替代分配较慢的弛豫磁结构序参量松弛进行了讨论。
Longitudinal magnetoresistance in Co-doped BaFe2As2 and LiFeAs single crystals: Interplay between spin fluctuations and charge transport in iron-pnictides
Magnetism and superconductivity in the layered hexagonal transition metal pnictides
本文研究了122（AM $_2$ B $_2$）六方过渡金属磷属元素化合物（A=（Sr，Ca），M=（Cr，Mn，Fe，Co，Ni），B=（As，P，Sb）的电子结构和磁性。发现该材料家族与包括著名的铁基高温超导体在内的四元结构的材料具有关键的相似性。在这两个家族中，次近邻（NNN）有效反铁磁（AFM）交换耦合在铁基材料中达到最大值。虽然在后者中的NNN耦合是已知的负责C型AFM状态，并导致在掺杂后的扩展的s波超导状态，它们导致前者是非常受抑的磁系统，并可能导致一个时间反转对称性破缺$d+id$超导状态掺杂。因此，如果合成具有六方结构的铁基化合物，可以帮助我们确定高温超导性的起源。
The discovery of pressure-induced superconductivity in helimagnets (CrAs, MnP) has attracted considerable interest in understanding the relationship between complex magnetism and unconventional superconductivity. However, the nature of the magnetism and magnetic interactions that drive the unusual double-helical magnetic order in these materials remains unclear. Here, we report neutron scattering measurements of magnetic excitations in CrAs single crystals at ambient pressure. Our experiments reveal well defined spin wave excitations up to about 150 meV with a pseudogap below 7 meV, which can …
We report theoretical and experimental results on the transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP is a three-dimensional superconductor with an anisotropic electronic structure and nonsymmorphic symmetries. On the other hand, magnetoresistance experimental data and the analysis of superconducting fluctuations of the conductivity in external magnetic field indicate a weakly anisotropic three-dimensional superconducting phase.
Disorder Induced Effects on the Critical Current Density of Iron Pnictide BaFe_1.8 Co_0.2 As_2 single crystals
研究无序在超导体中的作用是表征基本超导性质以及评估材料潜在应用的重要组成部分。在大多数情况下，缺陷矩阵上的信息是穷人，使这样的研究困难，但这种情况可以通过引入缺陷，在一个可控的方式，如中子辐照提供改善。在这项工作中，我们分析了中子辐照对Ba（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$单晶的影响。我们主要集中在磁性能，这是由磁强计测定。通过中子辐照引入无序导致对可逆和不可逆磁性的显著影响，例如转变温度、上临界场、各向异性和临界电流密度。通过与未辐照状态下的性质进行比较，对结果进行了详细讨论。
Emergence of novel magnetic order at finite temperature in overdoped pnictides
我们研究了在两种不同的掺杂物：空位和磁性杂质的存在下，受抑海森堡$J_1-J_2$模型中磁有序的温度依赖性。我们表明，无论他们的磁性比，杂质的引入淬火无序选择机制与伊辛样相变在低温下的顺序，并让位给$90^\circ$（反共线）秩序。掺杂剂的存在引发了一个非平凡的竞争之间的熵选择的状态（共线）和大力青睐的（反共线）在稀释和温度的依赖。而在磁性杂质的情况下，在整个温度和掺杂范围内观察到感兴趣的磁相，在磁性杂质的情况下，在$12\%$稀释度以上的所有温度下，每个磁序都被破坏。在固定的低温和调整掺杂，我们显示了一级相变导致重新进入的伊辛类秩序与渗透的岛屿的$90^\circ$秩序。在固定的掺杂和不同的温度下，我们观察到的过渡从反共线的共线相的辅助下，在磁性杂质的存在下，一个新出现的磁性相，而在空位的情况下，这种过渡的特点是由一个共存的区域。此外，通过调节杂质的磁矩，可以实现类伊辛序的完全崩溃。这与超导Ba（Fe $_{1-x}$ Ir $_x$）$_2$ As $_2$ with $x<0.047$中Ir掺杂原子的观察结果一致。
Flux-flow and vortex-glass phase in iron pnictide BaFe$_{2-x}$Ni$_x$As$_2$ single crystals with $T_c$ $\sim$ 20 K
我们分析了三种晶体BaFe $_{2-x}$ Ni $_x$ As $_2$与$T_c$ $\sim$ 20 K三种不同的几何形状相对于所施加的磁场和晶体的c轴之间形成的角度的等场磁电阻率数据的磁通流区域。场依赖的活化能，$U_0$，从TAFF和修改后的涡旋玻璃模型，这是比较从磁通蠕变文献中获得的$U_0$的值。我们观察到由TAFF模型得到的$U_0$在不同的晶体之间显示出偏差，而由涡旋玻璃模型得到的相应的玻璃线几乎是一致的。它示出的数据是很好地解释了修改后的涡旋玻璃模型，允许提取值的$T_g$，玻璃化转变温度，和$T^*$，温度与平均场临界温度$T_c(H)$的规模。所得到的玻璃线服从各向异性的金兹伯格-朗道理论，并很好地拟合在文献中发展的理论，考虑到无序的影响。
Electronic nematic correlations in the stress free tetragonal state of BaFe$_{2-x}$Ni$_{x}$As$_{2}$
利用输运和中子散射方法研究了电子掺杂的BaFe $_{2-x}$ Ni $_x$ As $_2$铁磷族化合物在无外应力退孪晶状态下的电子、结构和磁性。使用一个专门设计的原位机械退孪装置，我们表明，在单轴应变四重态BaFe $_{2-x}$ Ni $_x$作为$_2$低于温度$T^\ast$，以前确定为电子退孪相的签名，也存在于应力自由四重态下$T^{\ast\ast}$（$<T^\ast$）的平面电阻率各向异性。通过对BaFe $_2$ As $_2$和BaFe $_{1.97}$ Ni $_{0.03}$ As $_2$的中子散射测量，我们认为铁磷属元素化物无应力四重态的电阻率各向异性是由与反铁磁序有关的磁弹耦合引起的。这些结果表明，局域晶格畸变和自旋关联是造成铁磷属元素化物四重态电阻率各向异性的原因。
