Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of ${\mathbf{K}}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$

Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of $K_{x} {Fe}_{2 - y} {Se}_{2}$

Jun Zhao, Huibo Cao, E. Bourret-Courchesne, D.-H. Lee, and R. J. Birgeneau

Phys. Rev. Lett. 109, 267003 – Published 28 December 2012

Abstract

The recently discovered K-Fe-Se high-temperature superconductor has caused heated debate regarding the nature of its parent compound. Transport, angle-resolved photoemission spectroscopy, and STM measurements have suggested that its parent compound could be insulating, semiconducting, or even metallic [M. H. Fang, H.-D. Wang, C.-H. Dong, Z.-J. Li, C.-M. Feng, J. Chen, and H. Q. Yuan, Europhys. Lett. 94, 27009 (2011); F. Chen et al., Phys. Rev. X 1, 021020 (2011); and W. Li et al., Phys. Rev. Lett. 109, 057003 (2012)]. Because the magnetic ground states associated with these different phases have not yet been identified and the relationship between magnetism and superconductivity is not fully understood, the real parent compound of this system remains elusive. Here, we report neutron-diffraction experiments that reveal a semiconducting antiferromagnetic (AFM) phase with rhombus iron vacancy order. The magnetic order of the semiconducting phase is the same as the stripe AFM order of the iron pnictide parent compounds. Moreover, while the $\sqrt{5} \times \sqrt{5}$ block AFM phase coexists with superconductivity, the stripe AFM order is suppressed by it. This leads us to conjecture that the new semiconducting magnetic ordered phase is the true parent phase of this superconductor.

Received 13 July 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.267003

Authors & Affiliations

Jun Zhao^1,2,*, Huibo Cao³, E. Bourret-Courchesne⁴, D.-H. Lee^1,4, and R. J. Birgeneau^1,5

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Miller Institute for Basic Research in Science, Berkeley, California 94720, USA
³Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁴Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁵Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

^*zhao@berkeley.edu

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Vol. 109, Iss. 26 — 28 December 2012

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Figure 1
Resistivity of various $K_{x} {Fe}_{2 - y} {Se}_{2}$ samples and $\sqrt{5} \times \sqrt{5}$ superstructure reflections of the insulating $K_{0.8} {Fe}_{1.6} {Se}_{2}$ . The Bragg reflections of insulating $K_{0.8} {Fe}_{1.6} {Se}_{2}$ can be fully described with $\sqrt{5} \times \sqrt{5}$ block AFM order and iron vacancy order. (a) In-plane resistivity as a function of temperature for the insulating sample, the semiconducting sample, and the superconducting sample with $T_{c} = 30 K$ . The resistivity of the superconducting sample is multiplied by 2. (b) Magnetic reflection associated with $\sqrt{5} \times \sqrt{5}$ block AFM order of the insulating sample. (c) Nuclear reflection associated with $\sqrt{5} \times \sqrt{5}$ iron vacancy order of the insulating sample. The lattice parameters of the insulating sample are $a = b = 5.504 (3) Å$ , $c = 14.03 (6) Å$ at 5 K in the orthorhombic notation. The error bars indicate 1 standard deviation throughout the Letter.Reuse & Permissions
Figure 2
Stripe-type magnetic order and rhombus iron vacancy order of semiconducting $K_{0.85} {Fe}_{1.54} {Se}_{2}$ . (a) The three-dimensional stripe-type magnetic structure of iron as determined from our neutron-diffraction data. (b) The in-plane magnetic structure and rhombus iron vacancy order. The red dashed line marks the $2 \times 4$ rhombus iron vacancy order. (c)–(f) A series of temperature-dependent magnetic and nuclear reflections associated with the stripe-type magnetic order and rhombus iron vacancy order. The extra intensities appearing at 5 K are associated with development of long range AFM order. The residual intensities at 300 K are due to rhombus iron vacancy order. (c) Omega scans near $Q = (1 0 1)$ , (d) $Q = (1 0 3)$ , (e) $Q = (1 0 5)$ , and (f) $Q = (3 0 1)$ .Reuse & Permissions
Figure 3
Magnetic order parameter in two different semiconducting $K_{0.85} {Fe}_{1.54} {Se}_{2}$ samples. The temperature dependence of the magnetic Bragg peak intensities displays a second order magnetic phase transition with the Néel temperature $T_{N} = 280 K$ . (a) Temperature dependence of the $Q = (1 0 5)$ magnetic peak of sample A. (b) Temperature dependence of the $Q = (1 0 3)$ magnetic peak of sample A. (c) Temperature dependence of the $Q = (1 0 3)$ magnetic peak of sample B. (d) Temperature dependence of the omega scans of $Q = (1 0 3)$ peak of sample B. The peak exhibits little temperature dependence between 300 and 450 K, indicating that the residual intensity above 280 K is associated with the rhombus iron vacancy order.Reuse & Permissions
Figure 4
$\sqrt{2} \times \sqrt{2}$ Se distortion in the 122 structure of the superconducting phase $K_{0.88} {Fe}_{1.63} {Se}_{2}$ ( $T_{c} = 30 K$ ). (a),(b) Schematic diagram of Se distortion; the arrows indicate the directions of the atom displacements with respect to perfect lattice. (a) Se distortion along the $c$ axis. (b) Se distortion along the $a$ axis. (c)–(f) $\sqrt{2} \times \sqrt{2}$ reflections associated with Se distortion. The Bragg peak intensity increases as increasing $h$ , suggesting it is nuclear in origin. (c) Omega scans of the $Q = (1 0 1)$ Bragg peak display little temperature dependence between 5 to 300 K, indicating that the stripe-type AFM order is completely suppressed. The weak scattering left is associated with Se distortion. (d) Temperature dependence of the $Q = (3 0 1)$ Bragg peak. (e) The $Q = (5 0 1)$ Bragg peak at 5 K. (f) The $Q = (7 0 1)$ Bragg peak at 5 K.Reuse & Permissions

Physical Review Letters

Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of KxFe2−ySe2

Jun Zhao, Huibo Cao, E. Bourret-Courchesne, D.-H. Lee, and R. J. Birgeneau

Phys. Rev. Lett. 109, 267003 – Published 28 December 2012

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Neutron-Diffraction Measurements of an Antiferromagnetic Semiconducting Phase in the Vicinity of the High-Temperature Superconducting State of $K_{x} {Fe}_{2 - y} {Se}_{2}$