Influence of surface symmetry breaking on the magnetism, collapsing, and three-dimensional dispersion of the Co pnictidesACo2As2(A=Ba, Sr

Abstract

We use angle-resolved photoemission (ARPES) to study the three dimensional (3D) electronic structure of Co pnictides ACo2As2 with A=Ba, Sr, Ca or a mixture of Sr and Ca. These compounds are isostructural to Fe based superconductors, but have one more electron in the Co 3d orbitals. Going from Ba to Ca, they become more and more 3D, eventually forming a "collapsed" tetragonal phase, where the distance between CoAs layers is markedly reduced. We observe with ARPES the periodicity of the electronic structure as a function of kz (i.e. perpendicularly to CoAs layers) and find that it matches in each case that expected from the distance between the planes in the bulk. However, the electronic structure is better fitted by a calculation corresponding to a slab with 2 CoAs layers than to the bulk structure. We attribute this to subtle modifications of the 2D electronic structure induced by the truncation of the 3D dispersion at the surface in the ARPES measurement. We further study how this affects the electronic properties. We show that, despite this distortion, the electronic structure of CaCo2As2 is essentially that expected for a collapsed phase. Electronic correlations produce a renormalization of the electronic structure by a factor 1.4, which is not affected by the transition to the collapsed state. On the other hand, a small shift of the Fermi level reduces the density of states in the eg bands and suppresses the magnetic transition expected in CaCo2As2. Our study evidences that observing the 3D bulk periodicity is not sufficient to ensure bulk sensitivity. It further gives direct information on the role of 3D interactions, mostly governed by Co-As hybridization, among eg and t2g orbitals. It is also useful to better understand the electronic structure of Fe superconductors and the range of validity of ARPES measurements.

Is BaCr2As2 symmetrical to BaFe2As2Richard1, Roekeghem2, Lv3 et al. 2017Phys. Rev. B193120View full textAdd to dashboardCite

“…4 by zooming on the band dispersion near E F , these two bands fit the calculations better after renormalizing the LDA bands by a factor of 1.35. This factor is far from that observed in the Fe-based superconductors and more consistent with the renormalization found in BaCo 2 As 2 [10,11], which corresponds to a d 7 filling. From that perspective, the case of BaCr 2 As 2 is thus not symmetrical to BaFe 2 As 2 .…”

“…It was established theoretically that the strength of the electronic correlations in these materials is tuned by the filling of the 3d shell in presence of a large Hund's coupling [2][3][4][5][6][7]. Accordingly, angle-resolved photoemission spectroscopy (ARPES) studies report that the band renormalization factor 1/Z decreases monotonically upon filling the 3d shell: 1/Z = 3 in BaFe 2 As 2 (d 6 ) [8] (consistent with density functional theory (DFT) + dynamic mean-field theory (DMFT) calculations [9]), 1/Z = 1.4 in BaCo 2 As 2 (d 7 ) [10,11], 1/Z = 1.1 in SrNi 2 As 2 (d 8 ) [12], and 1/Z = 1 in BaCu 2 As 2 (d 10 ) [13]. Within this framework, the electronic correlations should evolve similarly as a function of band filling with respect to the half 3d shell case (d 5 ).…”

Frustrated magnetism in the tetragonal CoSe analog of superconducting FeSe

“…[12][13][14][15] There has been extensive work on the cobalt analogues to "122" iron pnictides, ACo 2 Pn 2 , with various interlayer alkali or alkali-earth cations (A). [16][17][18][19][20][21][22][23] The observed magnetism in these pnictides was largely tuned by size and electronic effects from changing the CoPn layer distances.…”

Electronic structure of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) studied using angle-resolved photoemission spectroscopy and theoretical calculations

“…Moreover, we observe that the hole bands are moved 300-400 meV below the E F compared to the hole bands crossing E F in the AFe 2 As 2 compounds. Here in case of ACo 2 As 2 , the rigid-band-like shift in the band structure results in the appearance of a small electron pocket at the Γ point [49][50][51]. Moreover, we find that the bands in the vicinity of E F are reasonably flat, which are responsible for high intensity peak in the density of states (DOS) at E F and important to understand the physical properties of these ACo 2 As 2 materials.…”

“…Notably, there have been many reports on the structural and magnetic properties of these ACo 2 As 2 compounds [34][35][36][37][38][39][40][41][42][43][44]; however, detailed ARPES studies are very few [49][50][51] and not reported for EuCo 2 As 2 . Therefore, further investigations of the band structure and FSs of ACo 2 As 2 and comparison with the AFe 2 As 2 parent compounds are desired to shed light on the pairing mechanism of Fe-based superconductors.…”