The interplay between the structural and magnetic phase transitions occurring in the Fe-based pnictide superconductors is studied within a Ginzburg-Landau approach. We show that the magnetoelastic coupling between the corresponding order parameters is behind the salient features observed in the phase diagram of these systems. This naturally explains the coincidence of transition temperatures observed in some cases as well as the character (first or second-order) of the transitions. We also show that magnetoelastic coupling is the key ingredient determining the collinearity of the magnetic ordering, and we propose an experimental criterion to distinguish between a pure elastic from a spin-nematic-driven structural transition.
Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2
我们对Ni-P族化合物超导体BaNi $_2$ As $_2$（$T_\mathrm{c}$ =0.7 K）和SrNi $_2$ P $_2$（$T_\mathrm{c}$ =1.4 K）进行了低温比热$C$和热导率$κ$的测量。这两种化合物的温度依赖性$C(T)$和$κ(T)$与一些s波超导体的结果相似。此外，BaNi $_2$ As $_2$的剩余$κ$的凹场响应排除了费米面上节点的存在.我们假设，完全能隙超导性可能是普遍的镍磷属元素化物超导体。Ba $_{0.6}$ La $_{0.4}$ Ni $_2$ As $_2$的比热数据显示，相对于BaNi $_2$ As $_2$，$T_\mathrm{c}$和$H_\mathrm{c2}$受到轻度抑制。
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。
The interplay of structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity, defined as the breaking of four-fold symmetry triggered by electronic degrees of freedom, is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction (XRD), we reveal a new structural orthorhombic phase with an exceptionally high onset temperature ($T_\mathrm{ort} \sim 250$ K), which coexists with superconductivity ($T_\mathrm{c} = 25$ K), …
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…
We report a systematic crystal growth and characterization of Ca$_{1-y}$La$_{y}$Fe$_{1-x}$Ni$_{x}$As$_{2}$, the newly discovered 112-type iron-based superconductor. After substituting Fe by a small amount of Ni, bulk superconductivity is successfully obtained in high quality single crystals sized up to 6 mm. Resistivity measurements indicate common features for transport properties in this 112-type iron pnictide, suggest strong scattering from chemical dopants. Together with the superconducting transition temperature $T_c$, and the Neel temperature $T_N$ determined by the elastic neutron scatt…
Deviating band symmetries and many-body interactions in a model hole doped iron pnictide superconductor
本文用角分辨光电子能谱研究了最佳掺杂的铁磷属元素化物超导体Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$（T $_c$ = 37 K）的低能能带结构.偏振对比测量被用来识别和跟踪所有三个低能量的类空穴带预测局域密度近似（LDA）计算。光发射的电子揭示了不一致的LDA预测的对称性沿$Γ$ -X高对称动量轴，由于电子动力学的意外强大的旋转各向异性。我们评估的多体效应，如莫特-哈伯德相互作用，可能是异常的基础，并讨论如何观察到的LDA能带结构的偏差影响铁磷属元素化物库珀配对的空穴掺杂制度的能量。
We consider a Cooper pair beam splitter for Iron-Pnictide superconductor and calculate the entangled electron-hole current. We investigate the interplay of various physical parameters such as doping at electron and hole pockets as well as non-zero nesting between the electron and hole pocket. We find that in the absence of magnetic order, the current due to hole pocket and electron pocket add up ordinarily. However in the presence of magnetic ordering the two currents take part in characteristic interference effect to modify the resultant current significantly. This interference effect manifes…
Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, w…
Recent experiments in iron pnictide superconductors reveal that, as the putative magnetic quantum critical point is approached, different types of magnetic order coexist over a narrow region of the phase diagram. Although these magnetic configurations share the same wave-vectors, they break distinct symmetries of the lattice. Importantly, the highest superconducting transition temperature takes place close to this proliferation of near-degenerate magnetic states. In this paper, we employ a renormalization group calculation to show that such a behavior naturally arises due to the effects of spi…
Possibility of Unconventional Pairing Due to Coulomb Interaction in Fe-Based Pnictide Superconductors: Perturbative Analysis of Multi-Band Hubbard Models
Site-dependent NMR Spin-lattice Relaxation in the Superconducting State of an Iron Pnictide Superconductor
在常规超导体中，所有核上的自旋-晶格弛豫速率在低于Tc时应该具有相同的温度依赖性。我们进行了$^{23}$ Na，$^{75}$ As，和$^{59}$ Co NMR研究单晶的NaFe 0.95 Co 0.05 As，并发现自旋晶格弛豫速率显示出非常不同的温度依赖的幂律行为的三个网站。我们认为，这种位置依赖的行为是由于超导体有两个非常不同的大小的差距的事实。每个核的幂律指数受小能隙的超精细耦合强度的影响。我们还发现钴位上的大超导能隙比其它两个位上的小。这表明掺杂剂位置上的超导能隙的局部抑制。
