We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered iron-arsenic superconductor, Ba1−xKxFe2As2 and non-supercondcuting BaFe2As2. Our study indicates that the Fermi surface of the undoped, parent compound BaFe2As2 consists of hole pocket(s) at Γ (0,0) and larger electron pocket(s) at X (1,0), in general agreement with full-potential linearized plane wave (FLAPW) calculations. Upon doping with potassium, the hole pocket expands and the electron pocket becomes smaller with its bottom approaching the chemical potential. Such an evolution of the Fermi surface is consistent with hole doping within a rigid band shift model. Our results also indicate that FLAPW calculation is a reasonable approach for modeling the electronic properties of both undoped and K-doped iron arsenites.PACS numbers: 74.25.Jb, Iron-Arsenic based materials comprise a very interesting class of materials with many unusual properties. For example, they have recently been shown to be superconducting with a T c as high as 55K [1,2,3]. This discovery has initiated a frenzy of research activity, which until very recently was limited to studies of only polycrystalline samples. Initial experiments focused on fluorine-doped rare earth oxide based materials (RFeAsOF) [4,5,6]. To date, there is very little photoemission data available on these compounds [7,8,9] with only one angle resolved study [10]. The recent discovery of superconductivity in oxygen free Ba 1−x K x Fe 2 As 2 [11] suggests that the superconductivity is ultimately linked to the electronic properties of the iron arsenic layer(s) with the remaining layers acting as a charge reservoir. This scheme closely resembles the situation found in the cuprates but without oxygen. In both BaFe 2 As 2 and SrFe 2 As 2 there are clear structural phase transitions from a high temperature tetragonal to low temperature orthorhombic phases. [12,13] When potassium is substituted for barium, the temperature at which the structural transition occurs is suppressed and superconductivity emerges [11,12]. Some experiments also point to the existence of a transition into a spin density wave (SDW) state at higher temperature [5,14] and related changes of the electronic structure [15]. Determining the effects of doping on low lying electronic excitations is essential for this study, as they play significant role in determining the normal state and superconducting properties. It is equally important to understand the electronic properties of the parent compound because undoped systems are easier to model theoretically and they represent a basis for higher order approximations. The information about electronic structure and its evolution with doping is deemed essential to formulate a successful model of superconductivity in these fascinating systems.The recent growth of large, high quality single crystals [12] has opened up the possibility of examining the electronic properties of these materials. Here we present data from angle resolved photoemission s...

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility χ, 75 As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient αc is negative from 7 to 300 K, whereas αa (the a-axis thermal expansion coefficient) is positive over this T range. The ρ(T ) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy EF. The band calculations exhibit an extremely sharp peak in the density of states D(E ≈ EF) arising from a flat d x 2 −y 2 band, where the x and y axes are along the a and b axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T → 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T ) does have a large magnitude, but also exhibits a broad maximum at ≈ 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic SrCo2As2 has a gapless quantum spin-liquid ground state.

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature TN = 625 K and a large ordered moment µ = 3.9 µB/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large µ. Ba1−xKxMn2As2 is thus the first metallic ThCr2Si2-type M As-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba1−xKxMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.Superconducting transition temperatures T c > 50 K have been observed for only two classes of materialslayered cuprates and iron arsenides [1,2]. Both classes contain stacked square lattice layers of the transition metal atoms. However, the parent compounds of the two families exhibit divergent physical properties. For example, La 2 CuO 4 is a local magnetic moment antiferromagnetic (AF) insulator [1] while BaFe 2 As 2 is metallic and its AF ordering is widely considered to be best characterized as a spin-density wave arising from conduction carriers [2]. These differences create barriers for a general and comprehensive understanding of the underlying mechanisms of high-T c superconductivity and related phenomena in a broad spectrum of materials. Thus, it is desirable to create a material that can bridge the gap between the cuprates and iron arsenides. Herein we report the synthesis and properties of such a material, Ba 1−x K x Mn 2 As 2 (x = 0.016, 0.05), which shares properties with both classes.The undoped parent compound BaMn 2 As 2 crystallizes in the same body-centered-tetragonal (bct) ThCr 2 Si 2 -type structure as the M Fe 2 As 2 (M = Ca, Sr, Ba) iron arsenide parent compounds do at room temperature [2][3][4]. It is a semiconductor with an activation energy of ∼ 30 meV determined from electrical resistivity ρ(T ) measurements [4,5], consistent with electronic structure calculations that indicate a band gap of ∼ 100-150 meV [5]. Heat capacity C p measurements at low-T yield an electronic linear heat capacity coefficient γ = 0 which is consistent with an insulating ground state [4]. BaMn 2 As 2 orders into a G-type (Néel-or checkerboard-type) AF structure below a Néel temperature T N = 625(1) K with an ordered moment at 10 K of µ = 3.88(4) µ B /Mn aligned along the crystallographic c axis [2, 4, 6]. Since BaMn 2 As 2 is an insulator at low temperatures, these results demonstrate that the antiferromagnetism arises from ordering of local Mn magnetic moments instead of from itinerant current carriers. Both the static and dynamic magnetic properties for T = 4-1000 K are welldescribed by the AF J 1 -J 2 -J c local moment Heisenberg model, with a Mn spin S = 5/2 as expected from the 3d 5 e...