Impact of the two Fe unit cell on the electronic structure measured by ARPES in iron pnictides

Abstract

In all iron pnictides, the positions of the ligand alternatively above and below the Fe plane create 2 inequivalent Fe sites. This results in 10 Fe 3d bands in the electronic structure. However, they do not all have the same status for an ARPES experiment. There are interference effects between the 2 Fe that modulate strongly the intensity of the bands and that can even switch their parity. We give a simple description of these effects, notably showing that ARPES polarization selection rules in these systems can not be applied by reference to a single Fe ion. We show that ARPES data for the electron pockets in Ba(Fe 0.92 Co 0.08 ) 2 As 2 are in excellent agreement with this model as well as with direct calculation of the spectral weight. We observe both the total suppression of some bands and the parity switching of some other bands. Once these effects are properly taken into account, the structure of the electron pockets, as measured by ARPES, becomes very clear and simple. By combining ARPES measurements in different experimental configurations, we clearly isolate each band forming one of the electron pockets. We identify a deep electron band along one ellipse axis with the d xy orbital and a shallow electron band along the perpendicular axis with the d xz /d yz orbitals, in good agreement with band-structure calculations. We show that the electron pockets are warped as a function of k z as expected theoretically, but that they are much smaller than predicted by the calculation.

ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

“…We find number of carriers reduced by ∼10-20% compared to calculations and compatible with charge neutrality. We reported the 3D electronic structure of Co8 in [30] and just recall the results in Fig. 4(e).…”

“…Fourth and fifth columns : number of carriers n per Fe in each band, after integration over kz, from theory and experiment. In this configuration, polarization selection rules [8,30] select orbitals even with respect to the plane containing this axis and the normal to the sample surface and odd with respect to the plane perpendicular to the previous one. Note that the band parity may be different from its main orbital character, because there are 2 inequivalent Fe in the unit cell [30,31].…”

“…In this configuration, polarization selection rules [8,30] select orbitals even with respect to the plane containing this axis and the normal to the sample surface and odd with respect to the plane perpendicular to the previous one. Note that the band parity may be different from its main orbital character, because there are 2 inequivalent Fe in the unit cell [30,31]. The photon energy selects a particular k z value, which we estimate through the free electron final state approximation, already heavily used in iron pnictides [24,32,33].…”

“…2(b). We have explained in [30], which bands can be observed in this experimental configuration and why. Along (a1), we observe the shallow d xz /d yz band, while along (a2), only the deeper d xy band is detected.…”

Consequences of Broken Translational Symmetry inFeSexTe1−x

“…For instance, if a given crystal structure is slightly modified by a perturbing potential, the initial states are forced to follow a new reduced Brillouin zone (BZ), whereas the ARPES intensity is hardly affected, and essentially follows the pristine periodicity [1]. Moreover, single-site orbitals are unfit to describe the electronic structure in a multisite unit cell [2]. As a result, not just an intensity modulation but clearly distinct spectral features may be observed in ARPES among the different BZs.…”