Physical Properties of Metallic Antiferromagnetic CaCo{1.86}As2 Single Crystals
V. K. Anand, R. S. Dhaka, Y. Lee, B. N. Harmon, Adam Kaminski, D. C. Johnston
备注：
18 pages, 15 figures, 4 tables, 61 references; v2: extended the fits of experimental data by additional electronic structure calculations; published version
本文报道了CaCo{1.86} As_2单晶的研究。电子结构探测角分辨光电子能谱（ARPES）测量的CaCo{1.86} As 2和全电位线性增强平面波计算的超晶胞Ca 8 Co 15 As 16（CaCo{1.88} As 2）。我们的XRD晶体结构精修与先前的X射线和中子粉末衍射数据的组合精修一致，显示了在对应于组成CaCo{1.86} As 2的Co位点上具有7（1）%空位的四重-四重ThCr 2Si 2型结构[D. G. Quirinale等人，B88，174420（2013）]。各向异性磁化率chi（T）数据与Quirianale等人的磁中子衍射数据一致，后者证明了在Neel温度TN = 52（1）K以下存在A型共线反铁磁有序，易磁化轴为四轴。然而，没有明确的证据，从电阻率ρ（T）和热容Cp（T）的数据在TN的磁性转变的观察。从能带计算和rho（T），Cp（T）和ARPES数据中证明了金属基态，自旋极化计算表明A型AFM和FM基态之间存在竞争. Cp（T）数据显示出大的Sommerfield电子系数，反映了费米能级D（EF）处的大态密度，这与能带结构计算一致，该计算也表明Co 3d能带产生大的D（EF）。在1.8K下，H的M（H）数据||c在3.5T的外加磁场下表现出良好的一阶自旋翻转跃迁。在低T下从M（H）数据得到的0.3 μ B/Co的小有序矩、chi的大交换增强以及chi（T）和M（H，T）数据在局部矩Heisenberg模型下缺乏自洽解释，共同表明CaCo{1.86} As 2的磁性是巡游的。
英文摘要：
We report studies of CaCo{1.86}As2 single crystals. The electronic structure is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of CaCo{1.86}As2 and by full-potential linearized augmented-plane-wave calculations for the supercell Ca8Co15As16 (CaCo{1.88}As2). Our XRD crystal structure refinement is consistent with the previous combined refinement of x-ray and neutron powder diffraction data showing a collapsed-tetragonal ThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to the composition CaCo{1.86}As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420 (2013)]. The anisotropic magnetic susceptibility chi(T) data are consistent with the magnetic neutron diffraction data of Quirianale et al. that demonstrate the presence of A-type collinear antiferromagnetic order below the Neel temperature TN = 52(1) K with the easy axis being the tetragonal c axis. However, no clear evidence from the resistivity rho(T) and heat capacity Cp(T) data for a magnetic transition at TN is observed. A metallic ground state is demonstrated from band calculations and the rho(T), Cp(T) and ARPES data, and spin-polarized calculations indicate a competition between the A-type AFM and FM ground states. The Cp(T) data exhibit a large Sommerfield electronic coefficient reflecting a large density of states at the Fermi energy D(EF), consistent with the band structure calculations which also indicate a large D(EF) arising from Co 3d bands. At 1.8 K the M(H) data for H|| c exhibit a well-defined first-order spin-flop transition at an applied field of 3.5 T. The small ordered moment of 0.3 muB/Co obtained from the M(H) data at low T, the large exchange enhancement of chi and the lack of a self-consistent interpretation of the chi(T) and M(H,T) data in terms of a local moment Heisenberg model together indicate that the magnetism of CaCo{1.86}As2 is itinerant.
Suppression of magnetic order in CaCo$_{1.86}$As$_{2}$ with Fe substitution: Magnetization, neutron diffraction, and x-ray diffraction studies of Ca(Co$_{1-x}$Fe$_{x}$)$_{y}$As$_{2}$
磁化，中子衍射，高能X射线衍射结果的Sn助熔剂生长的单晶样品的Ca（Co $_{1-x}$ Fe $_{x}$）$_{y}$作为$_{2}$，$0\leq x\leq1$，$1.86\leq y \leq 2$，并显示A型反铁磁秩序，与有序的时刻位于沿$c$轴，持续为$x\lesssim0.12(1)$。随着$x$的增加，反铁磁序被平滑地抑制，有序矩和Néel温度都线性降低。$x\leq0.25$不出现条纹型反铁磁序，$x$也不出现铁磁序，至少达到$x=0.104$，并且发生从CaCo $_{1.86}$ As $_{2}$的四重（cT）相到CaFe $_{2}$ As $_{2}$的四重（T）相的平滑交叉。这些结果表明，空穴掺杂CaCo $_{1.86}$ As $_{2}$对磁性和结构的影响不如Sr取代Ca的空间效应显著。
Crystal and Magnetic Structure of CaCo{1.86}As2 Studied by X-ray and Neutron Diffraction
中子和X射线衍射测量粉末和单晶的CaCo{1.86} As 2。晶体结构是如先前所报道的双列-四列ThCr 2Si 2型结构，但在Co位点上具有7（1）%的空位，对应于组成CaCo{1.86（2）} As 2。从10到300 K，a轴和c轴的热膨胀系数都是正的。单晶的中子衍射测量表明，在低于Neel温度TN = 52（1）K时，A型共线反铁磁有序开始出现，有序磁矩沿四角晶系c轴取向，在ab平面内铁磁排列，沿c轴反铁磁堆叠。
Metamagnetic transition in Ca$_{1-x}$Sr$_x$Co$_2$As$_2$($x$ = 0 and 0.1) single crystals
本文报道了CaCo $_2$ As $_2$和Ca $_{0.9}$ Sr $_{0.1}$ Co $_2$ As $_2$单晶的磁性和输运测量结果。在70 K和90 K左右分别观察到CaCo $_2$ As $_2$和Ca $_{0.9}$ Sr $_{0.1}$ Co $_2$ As $_2$的反铁磁转变。磁性和磁电阻测量表明，这两种材料在低温下从反铁磁态转变为铁磁态，沿c轴的临界磁场分别为3.5T和1.5T。对于沿着ab平面的场，对于Ca $_{0.9}$ Sr $_{0.1}$ Co $_2$ As $_2$，自旋也可以在4.5 T以上的场完全极化。而对于CaCo $_2$和$_2$一样，自旋在高达7 T时不能完全极化。我们提出这两种材料的钴磁矩在ab平面内是铁磁性的，但沿c轴是反铁磁性的（A型AFM）。
Dirac nodal pockets in the antiferromagnetic parent phase of FeAs superconductors
我们发现，以前测得的小费米表面口袋内的反铁磁相SrFe 2 As 2和BaFe 2 As 2是一致的狄拉克色散调制层间跳跃，从而产生一个狄拉克点在k-空间和一个尖点的磁场角度依赖的磁量子振荡频率。这些发现支持了这些材料中节自旋密度波的存在，这可能在保护金属态免受局域化效应方面发挥重要作用。SrFe_2As_2和BaFe_2As_2中狄拉克费米子的速度比石墨烯中慢14-20倍，这表明磷属元素化物为探索强相互作用狄拉克费米子的效应提供了实验室。
通过理论计算了解了CuFe $_2$ Ge $_2$的电子结构和磁性.能带结构显示出费米能级的大电子密度$N(E_F)$，表明磁性具有较强的巡游特征。费米面主要由二维片状结构组成，它们之间可能有很强的嵌套。磁基态沿$a$方向表现为铁磁，沿其它方向表现为反铁磁。结果表明，CuFe $_2$ Ge $_2$是一种反铁磁金属，具有铁基超导体的相似之处，如磁性具有明显的巡游特征，在费米能级处磁序与电子耦合.
