Electronic structure of ACo₂As₂ (A = Ca, Sr, Ba, Eu) studied using angle-resolved photoemission spectroscopy and theoretical calculations

Understanding the origin of the unconventional pairing mechanism in high-temperature superconductors, cuprates Bednorz86 ; Schilling93 ; Chu93 and iron pnictides Kamihara08 ; Takahashi08 ; XHChen08 ; GFChen08 ; Rotter08 ; Yuan09 ; Cruz08 ; SefatPRL08 ; WangSc11 , is one of the most challenging issues in condensed-matter physics. In particular, the superconductivity (SC) in the iron pnictides has received enormous attention to find its intimate connection with the magnetic ordering Johnston10 ; CanfieldRev10 . For example, the parent compound BaFe₂As₂ shows a phase change from high temperature tetragonal to low temperature orthorhombic structure at $\approx 135$ K along with an associated antiferromagnetic (AFM) spin density wave (SDW) transition. It is well known that the common method to induce high-T_c superconductivity in these materials is to suppress the SDW ground state by different ways such as partial chemical substitution and/or application of external pressure Takahashi08 ; XHChen08 ; GFChen08 ; Rotter08 ; Yuan09 ; Cruz08 ; SefatPRL08 ; Johnston10 ; CanfieldRev10 . One of the most intriguing aspects is to study how the long-range magnetic order and superconductivity are related in these materials Cruz08 ; PrattPRL09 . In this context, investigation of the electronic structure and Fermi surface (FS) topology using angle-resolved photoemission spectroscopy (ARPES) is vital LiuPRB09 ; YangPRL09 ; ZhangPRL09 ; KondoPRB10 ; LiuPRL09 ; YiPRB09 ; DhakaPRL11 ; LiuNP10 . The FS of BaFe₂As₂ consists of a hole (at the $\Gamma$ point) and an electron (at the $X$ point) pockets, and their similar volumes suggest that the material is compensated since the number of electrons and holes are equal. The nesting between the hole and the electron FSs can give SDW ordering SinghPRL08 ; Mazin10 ; MazinPRL08 and play a pivotal role in driving the antiferromagnetic-paramagnetic phase transition Cruz08 ; Mazin10 ; DhakaPRL13 . The FSs of these materials show a remarkable reconstruction at low temperature due to the presence of an AFM SDW phase LiuPRB09 ; YangPRL09 ; ZhangPRL09 ; KondoPRB10 ; LiuPRL09 ; YiPRB09 , and the suppression of magnetic ordering is linked to the onset of the SC dome DhakaPRL11 ; LiuNP10 . Interestingly, when Fe is replaced by Co in BaFe₂As₂, a rigid-band-like change in the band structure occurs and Lifshitz transitions are observed both at the onset and offset of the SC dome LiuNP10 ; LiuPRB11 ; Thirupathaiah ; Brouet ; Sekiba ; Ideta . On the other hand, no significant changes in the FS and band structure were observed for isoelectronic Ru substituted BaFe₂As₂ across the SC dome DhakaPRL11 ; LiuPRB15 .

Therefore, one approach is to completely substitute Co/Ru at the Fe site and investigate how the band structure and FS topology are related to the magnetic ordering as well as the SC in these materials. In this direction, very interesting structural and magnetic properties of ACo₂As₂,(A = Ca, Sr, Ba, Eu) compounds have been reported VivekPRB14 ; BingPRB19 ; SangeethaPRB18 ; SapkotaPRL17 ; DingPRB17 ; JayasekaraPRB15 ; Ying12 ; Cheng12 ; Bishop10 ; Sefat09 ; Pandey13 . For example, powder x-ray diffraction and magnetization measurements on CaCo₂As₂ demonstrate the structure to be collapsed-tetragonal and A-type collinear AFM order is observed below T${}_{\rm N}=$ 52 K, respectively VivekPRB14 . However, the resistivity and specific heat measurements show no evidence of the magnetic transition in CaCo₂As₂ VivekPRB14 . On the other hand, BaCo₂As₂ exhibits paramagnetic behavior and no magnetic ordering is reported down to 1.8 K Sefat09 ; AnandPRB14 . Interestingly, these materials crystallize in the ThCr₂Si₂-type tetragonal structure and the $c/a$ ratio of SrCo₂As₂ is found to be 2.99, which is intermediate to those of normal-tetragonal BaCo₂As₂ ($c/a=$ 3.20) exhibiting ferromagnetic correlations and collapsed-tetragonal-antiferromagnetic CaCo₂As₂ ($c/a=$ 2.58) Pandey13 . Furthermore, the magnetization, NMR, and neutron diffraction measurements of SrCo₂As₂ show no evidence for long-range magnetic ordering above 0.05 K Pandey13 ; Jasper08 ; Jayasekara13 ; BingPRB19 . This is consistent with the isostructural BaCo₂As₂ Sefat09 , but is in contrast to the AFM behavior of CaCo₂As₂ VivekPRB14 ; Ying12 ; Cheng12 . More interestingly, SrCo₂As₂ shows a negative (positive) thermal expansion coefficient along the $c$ ($a$)-axis in the temperature range from $\approx$7 K to 300 K Pandey13 . Also, the structural properties of EuCo₂As₂ indicate a phase transition from tetragonal at ambient pressure to collapsed-tetragonal at high pressure Bishop10 . The magnetization measurements at ambient pressure indicate that the effective paramagnetic moment at high temperatures arises mainly from the Eu spins with an appearance of AFM ordering $\leq 39$ K Ballinger12 .

Notably, there have been many reports on the structural and magnetic properties of these ACo₂As₂ compounds VivekPRB14 ; BingPRB19 ; SangeethaPRB18 ; SapkotaPRL17 ; DingPRB17 ; JayasekaraPRB15 ; Ying12 ; Cheng12 ; Bishop10 ; Sefat09 ; Pandey13 ; however, detailed ARPES studies are very few DhakaPRB13 ; XuPRX13 ; MansartPRB16 and not reported for EuCo₂As₂. Therefore, further investigations of the band structure and FSs of ACo₂As₂ and comparison with the AFe₂As₂ parent compounds are desired to shed light on the pairing mechanism of Fe-based superconductors.

In this paper, we report a comprehensive ARPES study of ACo₂As₂ (A = Ca, Sr, Ba, Eu) and present the FSs as well as low-energy band structures, which are found to be very different from BaFe₂As₂ at the Fermi level. The experimental FSs and band dispersion data are compared with full-potential linearized augmented-plane-wave (FP-LAPW) calculations, which are found to be in reasonably good agreement. The corresponding band dispersion data show a small electron pocket at the center and large electron pocket at the corner of the Brillouin zone (BZ). This reveals that no obvious FS nesting is present in these compounds, which is in contrast to the parent compounds of Fe-based high-T_c superconductors. 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 AFe₂As₂ compounds. Here in case of ACo₂As₂, the rigid-band-like shift in the band structure results in the appearance of a small electron pocket at the $\Gamma$ point DhakaPRB13 ; XuPRX13 ; MansartPRB16 . 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 $A$Co₂As₂ materials. In addition, there are no notable changes in the FS topology of CaCo₂As₂ measured at, below, and above the AFM transition temperature.

High-quality single crystals of ACo₂As₂ (A = Ca, Sr, Eu) were grown with Sn-flux whereas BaCo₂As₂ out of self flux and the details of bulk physical property measurements and x-ray analysis can be found in Refs. Pandey13 ; VivekPRB14 ; AnandPRB14 ; SangeethaPRB18 . We used a Scienta R4000 electron analyzer to perform high-resolution ARPES measurements at beamline 7.0.1 of the Advanced Light Source (ALS), Berkeley, California. All samples were cleaved in situ in an ultrahigh vacuum chamber having $\leq$$4\times 10^{-11}$ mbar pressure, yielding flat mirror-like surfaces in the ab plane. The ARPES data were collected with $\sim 20$ meV and $\sim 0.3^{\circ}$ energy and momentum resolutions, respectively. Several samples from different batches were measured to reproduce the results of the Fermi surfaces and band structure. We use a gold sample to determine the Fermi energy.

We calculate the Fermi surfaces and band dispersions of $A$Co₂As₂ with the FP-LAPW method using the local density approximation Perdew92 . We use $R_{\rm MT}\times{\it k}_{\rm max}=$ 8 or 9 to find the self-consistent charge density, where the smallest muffin tin (MT) radius is multiplied by the maximum $k$ value in the plane wave expansion basis. Here, the MT radii of 2.1, 2.3, 2.5, 2.5, 2.1, and 2.1 a.u. were taken for Ca, Sr, Ba, Eu, Co, and As, respectively. Note that 828 $k$-points were selected in the irreducible BZ to perform the calculations untill we reached the total energy convergence criterion of 0.01 mRy/ primitive cell. Also, we relaxed the As atom z-axis position to find a minimum total energy, which gave z${}_{\rm As}=$ 0.3622, 0.3515, 0.3441, and 0.3611 for $A=$ Ca, Sr, Ba, and Eu, respectively. To compute the FSs, we divide the $-2\pi/a\leq(k_{\it x}$, $k_{\it y})\leq 2\pi/a$ ranges of $k_{\it x}$, $k_{\it y}$ planes with different $k_{\it z}$ values into 200$\times$200 meshes.

In order to understand how the low-energy band structure changes between different members of $A$Co₂As₂ family, in Figs. 1(a–d) we present the photoemission intensity maps of ACo₂As₂ (A = Ca, Sr, Ba, Eu) at E_F measured at k${}_{\rm z}\approx 2\pi/c$. To plot the FS maps, we have integrated the intensity of the photoelectrons within $\pm 10$ meV about $E_{\rm F}$. Interestingly, we find that the shape of the FSs of ACo₂As₂ is more complicated DhakaPRB13 ; XuPRX13 than those of the parent $A$Fe₂As₂ compounds of the 122-family DhakaPRL11 . In particular, the FS map of CaCo₂As₂ [Fig. 1(a)] clearly exhibits small elliptical pockets centered around the corner ($X$ point) of the BZ. Additionally, we also observe four small pockets around the center ($\Gamma$ point) of the BZ. In the case of SrCo₂As₂ [Fig. 1(b)], the FS topology is slightly different in the sense that the elliptical pockets at the $X$ point become straight and the four small pockets around the $\Gamma$ point are now less visible. The FS of BaCo₂As₂ in Fig. 1(c) is significantly different where the pockets at $X$ point break into small segments, and the four small pockets around the $\Gamma$ point are become smaller in size and change in shape. More interestingly, we observe a small circular pocket at the $\Gamma$ point in the FS map of EuCo₂As₂. At the corner ($X$ point) of the BZ, long straight segments of intensity pocket for the $A=$ Sr, Ba, and Eu samples are clearly different in shape as compared to CaCo₂As₂. In the case of $A=$ Eu, the intensity of the four small electron pockets (along the $k_{x}$, $k_{y}$ directions) around the center of the BZ almost disappeared as compared to CaCo₂As₂, see Figs. 1(a, d).

Interestingly, we observe a significant change in the FS topology of $A$Co₂As₂ family by changing the element at $A$ site (for example in the present study $A=$ Ca, Sr, Ba, and Eu) DhakaPRB13 ; XuPRX13 ; Pandey13 , as compared to the other related parent compounds $A$Fe₂As₂ DhakaPRL11 ; DhakaPRBrc14 . Moreover, the appearance of a small intensity pocket at the $\Gamma$(0,0) point at $E_{\rm F}$ in EuCo₂As₂ [see Fig. 1(d)] clearly suggests a shift in the Fermi level, which may be different from the other $A$Co₂As₂ ($A=$ Ca, Sr, Ba) compounds.

Therefore, to further understand the electronic structure of these samples, we performed theoretical calculations for all the samples at the same ${\it k}$_z points and compared the calculated FS maps in Figs. 1(e–h), respectively.

In case of CaCo₂As₂, four patches of intensity (along the $k_{x}$, $k_{y}$ directions) around the $\Gamma$ point and large elliptical pockets across the $X$ points are clearly seen in Fig. 1(e). Overall, the theoretically-calculated FS topology clearly shows significant changes between the Ca, Sr, Ba, and Eu members, see Figs. 1(e–h). Also, the FS maps of ACo₂As₂ (A = Ca, Sr, Ba, Eu) are very different from the other related parent compounds of the 122 family such as CaFe₂As₂ and BaFe₂As₂ DhakaPRBrc14 ; KondoPRB10 . However, if we see the FS plotted about 400 meV below $E_{\rm F}$ (discussed later) DhakaPRB13 , it is very similar to that of BaFe₂As₂, where the FSs are roughly circular in shape and are roughly similar in size KondoPRB10 . This clearly indicates a shift of about 400 meV in the Fermi energy by completely replacing Fe with Co in these systems DhakaPRB13 ; XuPRX13 . It is important to note here that the overall shape of the measured FSs in Figs. 1(a–d) is in relatively good agreement with the respective calculations in Figs. 1(e–h).

As discussed above, in case of CaCo₂As₂ sample, the magnetization measurements show an AFM transition at 52 K VivekPRB14 . Also, it is important to note here that the ARPES studies of Ba(Fe_1-xRu_x)₂As₂ and Ba(Fe_1-xCo_x)₂As₂ show significant changes in the band structure with sample temperature DhakaPRL13 ; Brouet13 . Moreover, it was shown that temperature-dependent FS nesting may play an important role in driving the AFM-paramagnetic phase transition in these materials LiuPRL09 ; KondoPRB10 ; DhakaPRL11 ; LiuNP10 . Therefore, to get more insights for understanding whether there are changes in the band structure of CaCo₂As₂ with temperature across the AFM transition temperature (T_N), we measured the FSs with 135 eV photon energy using a synchrotron radiation source. In Fig. 2, the FSs are shown at different sample temperatures from 20 to 240 K, which confirm that there are no notable differences in the FS topology measured below [Fig. 2(a)] and above [Figs. 2(b–d)] ($T_{\rm N}$). Further, as the FS of $A$Fe₂As₂ below $T_{\rm N}$ shows a reconstruction LiuNP10 , which is clearly absent in the FS of CaCo₂As₂ measured at 20 K, see Fig. 2(a). These results indicate that the electron correlations are moderate in the $A$Co₂As₂ family, as the overall mass enhancement of the Co 3$d$ electrons is found to be smaller MaoPRB18 as compared to the Fe 3$d$ electrons in $A$Fe₂As₂ based compounds LiuPRL09 ; KondoPRB10 ; DhakaPRL11 ; LiuNP10 .

In order to compare the FS topology at different binding energies, we show the $(k_{\it x},k_{\it y})$ plots from 0 to 700 meV below E_F for all the samples in Fig. 3. For the CaCo₂As₂, we observe a decrease in the intensity of the small pockets around the zone center along with a shape change at the corner of the BZ, see panel (a) in Fig. 3. At 400 meV below E_F, a small pocket is clearly seen at the center of the BZ, which grows in size with further increase in the binding energy. At the same time, the shape of the pocket at the corner is very similar to that of the electron pocket observed in $A$Fe₂As₂ samples. Similar changes are observed for the SrCo₂As₂ sample, as shown in panel (b) except that the center pocket appeared at around 300 meV below E_F, and the size of both the pockets at the center and at the corner of the BZ is almost similar at 400 meV. For the BaCo₂As₂ sample, a small intensity of the central pocket is visible at 200 meV, see panel (c), and again the size of both pockets is similar at 300-400 meV below E_F. Interestingly, in case of the EuCo₂As₂, the central pocket is already visible at E_F, see panels (d, e), measured at 135 eV and 115 eV photon energies, respectively. Further, the size and shape of both pockets at the center and corner of the BZ look similar for all the samples at around 400 meV below E_F except for EuCo₂As₂. More carefully, if we compare the size of the pocket at the BZ center at 700 meV below E_F for all the samples, it qualitatively increases from Ca to Sr to Ba to Eu. These $(k_{\it x},k_{\it y})$ plots of $A$Co₂As₂ further motivated us to investigate the low-energy band structure in detail by plotting the band dispersion and comparing them with those calculated theoretically along different momentum $(k_{\it x},k_{\it y})$ directions.

Therefore, to elucidate the character of these FS pockets, we extracted in-plane dispersions for all the samples and shown the energy-momentum intensity plots in Figs. 4 and 5 along the four different cuts #1, #2, #3 and #4. These cuts are from $(k_{\it x},k_{\it y})=$ ($-$2$\pi$, 0 to 2$\pi$, 0), (2$\pi$, $\pi$ to $-$2$\pi$, $-\pi$), (0, $-$2$\pi$ to 2$\pi$, 0), ($-$2$\pi$, 2$\pi$ to 2$\pi$, $-$2$\pi$), respectively, as marked in Fig. 1(a). The band dispersions measured with 135 eV photon energy are shown in Fig. 4 for the $A$Co₂As₂ ($A=$ Ca, Sr) samples. The photoelectron intensity along the cut #1 at the center of the BZ ($\Gamma$ point) is not very clear for the these samples; however, we clearly observe two small electron pockets centered at the Fermi momenta of $k_{\rm F\it x}\approx\pm 0.7\pi/a$. Interestingly, the top of the hole pocket at the center of the BZ is clearly seen at about 400 meV below $E_{\rm F}$. Moreover, we find a large electron pocket with $k_{\rm F\it y}=\pm 0.65\pi/a$ in the band dispersion plotted along the cut #2 at the corner of the first BZ at ${\it X}=(\pi,\pi)$. In the cut #3, the data show a large electron pocket with $k_{\rm F}=\pm 0.65\pi/a$ and high intensity peak at the $X$ point. For comparison, we have also plotted the data along the cut #4, which again shows large electron pockets at the $X$ points. To get more insights, it is important to compare the experimental results with theoretical calculations, and therefore, we present the calculated band dispersions in the lower panels for Fig. 4 along all four different cuts. We find a reasonable qualitative agreement between the theoretical and measured data.

Moreover, in Fig. 5, we show the low-energy band structure of BaCo₂As₂ and EuCo₂As₂ samples to understand the character of the FS pockets. We note here that the band dispersions of BaCo₂As₂ are very similar to those of SrCo₂As₂ except that the small-intensity electron-like bands are visible at the center of the BZ for BaCo₂As₂, which are found to be slightly stronger when measured with $h\nu=$ 85 eV in Ref. DhakaPRB13 . Also, the slope of the hole bands is more slanting in $A$Co₂As₂ ($A=$ Sr, Ba) in comparison to CaCo₂As₂, see for example cuts # 1 and #4 in Figs. 4 and 5.

It is intriguing to see the band structure of EuCo₂As₂, which is found to be significantly different from the other $A$Co₂As₂ ($A=$ Ca, Sr, Ba) samples, see Figs. 4 and 5. We observe that the intensity of two smaller electron pockets at Fermi momenta $k_{\rm F\it x}=\pm 0.9\pi/a$ reduced significantly and the top of the hole band at the center of the BZ ($\Gamma$ point) is about 300 meV below $E_{\rm F}$, see along the cut #1. This indicates that in the case of EuCo₂As₂, the rigid band shift at the center of the BZ is smaller ($\approx$300 meV) than in the other $A$Co₂As₂ ($A=$ Ca, Sr, Ba) compounds (400 meV). Therefore, the Fermi momentum of the electron pocket at the $\Gamma$ point $k_{\rm F\it x}=\pm 0.15\pi/a$ is also slightly smaller than in BaCo₂As₂ DhakaPRB13 . At the corner of the first BZ [$X=(\pi,\pi)$], we observe a larger electron pocket for $A$Co₂As₂ with $k_{\rm F}=\pm 0.65\pi/a$ (along both the cuts #2 and #3) as compared to the AFe₂As₂ compounds DhakaPRL11 . The data through cut #4 again confirm the presence of large electron pockets at the $X$ points and a smaller electron pocket at the $\Gamma$ point. The band structure plots obtained from the FP-LAPW calculations, shown in the lower panels of Fig. 5, are in reasonable agreement with the experimentally-observed band dispersions.

Now we discuss the broad comparison of the band structure and FS topology between ACo₂As₂ and AFe₂As₂ compounds. Overall, the electronic DOS($E$) of the ACo₂As₂ samples are found to be similar to those of the Fe-based compounds. However, we observe a band shift of 300–500 meV below E_F due to extra $d$ electron in Co as compared to Fe DhakaPRB13 ; Pandey13 ; Sefat09 ; Singh09 , which may reflect the high intensity peak at E_F. Note that the Co $d_{x^{2}-y^{2}}$ orbital has a larger bandwidth (between $-5$ eV and 2 eV) than the Fe $d$ orbitals (from $-2$ eV to 2 eV) SinghPRL08 . Therefore, a complex multi-band Fermi surface is observed in the ARPES measurements on the $A$Co₂As₂ compounds along with the large DOS at E_F with no apparent nesting DhakaPRB13 ; XuPRX13 . In this scenario, the band structure of ACo₂As₂ MaoPRB18 appears significantly different at E_F with respect to that of BaFe₂As₂. Also, as noted above, CaCo₂As₂ shows magnetic ordering at low temperature; however, APRES measurements across the magnetic transition indicate no significant changes in the band structure. Similarly, a complex FS structure of EuRh₂As₂ and BaNi₂As₂ has been reported using ARPES, but no signature of band folding due to magnetic ordering was observed, which indicates a weak coupling between layers PalczewskiPRB12 ; Zhou11 . Moreover, a significant decrease in electronic correlation is reported in BaCr₂As₂ RichardPRB17 ; NayakPNAS17 . Note that in the $A$Co₂As₂ compounds a considerable decrease in interlayer distance and z_As results in different correlation strengths MaoPRB18 when compared with iron pnictides YinNM11 .

Finally, we compare the FSs and low-energy band structures, in Fig. 6 of $A$Co₂As₂ ($A=$ Ca, Eu, Sr) measured at 90 K with 100 eV photon energy. The intensity of photoelectrons along ($k_{\it x}$, $k_{\it y}$) is plotted at $E_{\rm F}$ in Figs. 6(a–c) and at 400 meV below $E_{\rm F}$ in Figs. 6(d–f). The FS plots at $E_{\rm F}$ show a long segment of intensity across the $X$ points (corners of the BZ). The FS plots at 400 meV below $E_{\rm F}$ show clear changes in the FS shape at the $X$ point, which changes from a large segment to an oval–shape pocket and approximately similar size as the pocket at the center of the BZ ($\Gamma$ point). These results demonstrate the rigid band-like shift of 300-400 meV (depending on $A$) below $E_{\rm F}$ for $A$Co₂As₂ as compared to $A$Fe₂As₂. Figures 6(g–i) show the band dispersion data plotted across the corner of the BZ ($X$ point) i.e. from (0, $-$2$\pi$) to (2$\pi$, 0) and the corresponding energy dispersive curves (EDCs) are shown in Figs. 6(j–l). Again, for all these compounds we observe a large electron pocket at the $X$ point and the bottom of this pocket is about 500 meV below $E_{\rm F}$. More interestingly, we reveal a rather large flat band, driven by the Co 3$d_{x^{2}-y^{2}}$ orbital, in close vicinity of $E_{\rm F}$ with strong intensity near the $X$ point in both the experimental and calculated data of $A$Co₂As₂ compounds DhakaPRB13 , as shown in Fig. 4 [cuts #2, #3 and #4 of panel (a)] as well as in Fig. 6. In ARPES measurements on the Fe_1.03Te_0.94S_0.06 superconductor, Starowicz $etal.$ observed a flat band at the Fermi level, which gives rise to high density of states at $E_{\rm F}$ Starowicz13 . This was explained in terms of as a Van Hove singularity, which is believed to play an important role in the emergence of superconductivity in Fe-based compounds Starowicz13 . The presence of a large density of states near $E_{\rm F}$ due to a nearly flat band at the corner of the BZ is crucial to understand the physical properties of the $A$Co₂As₂ compounds.

We have presented a comprehensive study of the electronic properties of the $A$Co₂As₂ ($A=$ Ca, Sr, Ba, and Eu) compounds using ARPES and theoretical FP-LAPW calculations. The FSs of these compounds are different from those of the parent compounds of FeAs-based high-temperature superconductors. The band dispersion data show a small electron pocket at the center and large electron pockets at the corner of the Brillouin zone. The experimental data agree reasonably well with the theoretical calculations. The absence of the FS nesting in $A$Co₂As₂ is in contrast to the $A$Fe₂As₂ compounds. However, the top of the hole bands is found to be moved 300–400 meV below E_F (depending on $A$) resulting an appearance of a small electron pocket at the center of the BZ. More interestingly, we observe large flat bands near E_F, which result in a large density of states and could be responsible for the interesting physical properties of these materials. Furthermore, no significant changes are observed in the FS topology of CaCo₂As₂ between 20 and 300 K across the AFM transition. We discuss similarities and differences in the electronic properties of $A$Co₂As₂ with respect to the parent AFe₂As₂ compounds of 122 family.

We thank Aaron Bostwick and Eli Rotenberg for excellent support at the ALS. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. The Advanced Light Source is supported by the Office of Basic Energy Sciences, U. S. Department of Energy under Contract No. DE-AC02-05CH11231. RSD also acknowledges the support by BRNS through a DAE Young Scientist Research Award with Project Sanction No. 34/20/12/2015/BRNS.