Symmetry-broken electronic structure and uniaxial Fermi surface nesting of untwinned CaFe${}_{2}$As${}_{2}$

Symmetry-broken electronic structure and uniaxial Fermi surface nesting of untwinned CaFe $_{2}$ As $_{2}$

Q. Wang, Z. Sun, E. Rotenberg, F. Ronning, E. D. Bauer, H. Lin, R. S. Markiewicz, M. Lindroos, B. Barbiellini, A. Bansil, and D. S. Dessau

Phys. Rev. B 88, 235125 – Published 26 December 2013

Abstract

We used angle-resolved photoemission spectroscopy to make direct measurements of the electronic structure of the untwinned uniaxial state of CaFe $_{2}$ As $_{2}$ , the parent compound of an iron-based superconductor. The very small photon beam size, combined with the relatively large single-domain area on the crystal surfaces, allowed us to obtain the intrinsic symmetry-broken dispersions and Fermi surface (FS) geometries along the orthogonal Fe-Fe bond directions without any mechanical or magnetic detwinning processes. Comparing the optimized local density approximation calculations, an orbital-dependent band shifting is introduced to obtain better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Our results indicate strong electronic anisotropy in CaFe $_{2}$ As $_{2}$ and put strong constraints on theories for the iron-pnictide system.

Received 23 June 2013
Revised 27 November 2013

DOI:https://doi.org/10.1103/PhysRevB.88.235125

Authors & Affiliations

Q. Wang^1,2, Z. Sun^1,3, E. Rotenberg⁴, F. Ronning², E. D. Bauer², H. Lin⁵, R. S. Markiewicz⁵, M. Lindroos⁵, B. Barbiellini⁵, A. Bansil⁵, and D. S. Dessau^1,*

¹Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
²Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
³National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People's Republic of China
⁴Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Physics Department, Northeastern University, Boston, Massachusetts 02115, USA

^*dessau@colorado.edu

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Vol. 88, Iss. 23 — 26 December 2013

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Images

Figure 1
(a) The schematic of in-plane crystal structure of $A$ Fe $_{2}$ As $_{2}$ type ( $A$ = Ba, Sr, or Ca) iron pnictides. The solid red and green boxes indicate the in-plane unit cell for the nonmagnetic tetragonal state and the AFMO state, respectively. In the AFMO phase, the directions of the spins are shown as red arrows on top of the Fe atoms. (b) and (c) The 3D BZ in the tetragonal and the orthorhombic state, respectively. (d)–(e) The measured FS of high-temperature tetragonal state (180 K) and low-temperature AFMO state (20 K), respectively, with in-plane BZ boundaries at Γ/γ (dashed boxes) or $Z$ / $z$ (solid boxes) planes. (f)–(j) Intensity maps near the zone center at different BE of a low-temperature AFMO state (20 K). All data were taken with 80 eV photons.
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Figure 2
(a)–(i) Schematics of the three different experimental configurations. Symmetries with respect to the gray-shaded mirror plane are shown. (b)–(j) FS and (c)–(k) intensity plots along the high-symmetry directions, as indicated by white solid cuts in (b)–(j), respectively. (d)–(l) Dispersions along the high-symmetry cuts obtained from (c)–(k), with allowable symmetries determined by the experimental geometries indicated. All data were taken with 80 eV photons at 20 K.
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Figure 3
Comparison between (a) and (b) the experimental data and (c) the optimized theoretical calculations. Experimental orbital symmetry information is color coded according to the polarization-dependent experiments. The dashed lines indicate multiple states that are consistent with the polarization data. The agreement with experiment is improved by shifting the $y z$ bands up about 0.1 eV [green arrows in panel (f)] in calculations, which is indicative of orbital ordering.
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Figure 4
(a) and (b) FS taken with 80 and 99 eV photons at $T$ = 20 K. The incommensurate FS nesting vector ${\vec{q}}_{1}$ is labeled on both plots, ${\vec{q}}_{0}$ represents the AFM nesting vector. (c) and (d) $k_{z}$ dispersions of the Fermi crossings along the blue and yellow cuts of (a) and (b). The dashed lines are guides for the eyes of the $k_{z}$ dispersions, while the blue solid lines indicate the persistence of the nesting vector ${\vec{q}}_{1}$ in momentum space. (e) The measured FS of Ca(Fe $_{1 - x}$ Co $_{x}$ ) $_{2}$ As $_{2}$ sample with $x$ ∼ 3.5% at 40 K. (f) Second-derivative image of the FS topology shown in panel (e). Possible nesting along ZY′ direction with nesting vector ${\vec{q_{1}}}^{'}$ is indicated by the white dashed lines. (g) Dispersions along the high-symmetry cuts (ZX′ and ZY′) of Co-doped sample.
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