Energy dependence of the spin excitation anisotropy in uniaxial-strained ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$

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Energy dependence of the spin excitation anisotropy in uniaxial-strained ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$

Yu Song, Xingye Lu, D. L. Abernathy, David W. Tam, J. L. Niedziela, Wei Tian, Huiqian Luo, Qimiao Si, and Pengcheng Dai

Phys. Rev. B 92, 180504(R) – Published 6 November 2015

Abstract

We use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxial-strained electron-doped iron pnictide ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$ near optimal superconductivity ( $T_{c} = 20 K$ ). Our work has been motivated by the observation of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of electron-underdoped iron pnictides under uniaxial pressure, which has been attributed to a spin-driven Ising-nematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spin-driven Ising-nematic phase, exists for energies below $\sim 60$ meV in uniaxial-strained ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$ . Since this energy scale is considerably larger than the energy splitting of the $d_{x z}$ and $d_{y z}$ bands of uniaxial-strained $Ba {({Fe}_{1 - x} {Co}_{x})}_{2} {As}_{2}$ near optimal superconductivity, spin Ising-nematic correlations are likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.

Received 22 August 2015
Revised 23 September 2015

DOI:https://doi.org/10.1103/PhysRevB.92.180504

Authors & Affiliations

Yu Song¹, Xingye Lu^1,2, D. L. Abernathy³, David W. Tam¹, J. L. Niedziela⁴, Wei Tian³, Huiqian Luo², Qimiao Si¹, and Pengcheng Dai^1,*

¹Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁴Instrument and Source Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

^*pdai@rice.edu

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Vol. 92, Iss. 18 — 1 November 2015

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Figure 1
(a) The phase diagram of ${BaFe}_{2 - x} {Ni}_{x} {As}_{2}$ . In ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$ superconductivity coexists with incommensurate (IC) short-range magnetic order [11]. The mechanical clamp used and the magnetic excitations under uniaxial pressure along the $b$ axis are schematically shown in the inset at the top right. The red squares and dashed line mark $T^{*}$ , a crossover temperature at which the intensity of low-energy magnetic excitations at $(1, 0)$ and $(0, 1)$ in ${BaFe}_{2 - x} {Ni}_{x} {As}_{2}$ under uniaxial pressure merge [31]. (b) Rocking scans of the elastic magnetic peak at 6 K obtained on HB-1A; background measured at 50 K has been subtracted. The inset shows the rocking scans projected into the $[H, K, 0]$ plane. (c) Schematic Fermi surface of ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$ in the paramagnetic state; the arrows mark nesting wave vectors $Q_{1} = (1, 0)$ and $Q_{2} = (0, 1)$ . Fermi surfaces originating from different orbitals are shown in different colors. (d) Schematic splitting of $d_{y z}$ and $d_{x z}$ bands at $X$ and $Y$ in $Ba {({Fe}_{1 - x} {Co}_{x})}_{2} {As}_{2}$ , as found by ARPES [33]. At higher temperatures, the two bands have the same energy (dashed lines) but as the temperature is lowered the $d_{y z}$ band moves up in energy, whereas $d_{x z}$ moves down. (e) Schematic temperature dependence of the orbital splitting in (d); under uniaxial pressure the splitting persists to above the stress-free $T_{N}$ and $T_{s}$ .
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Figure 2
Constant-energy slices symmetrized along $H$ and $K$ axes at $T = 5$ K for energy transfers (a) $E = 4.5 \pm 0.5$ meV ( $E_{i} = 30$ meV), (c) $E = 16 \pm 2$ meV ( $E_{i} = 80$ meV), and (e) $E = 100 \pm 10$ meV ( $E_{i} = 250$ meV). The black boxes indicate regions that contain nonduplicate data due to symmetrizing. Longitudinal cuts along $[H, 0]$ (red circles) and $[0, K]$ (blue diamonds) for energy transfers in (a), (c), and (e) are, respectively, shown in (b), (d), and (f). The solid lines are fits using Gaussian functions and linear backgrounds. $[H, 0] / [0, K]$ scans are obtained by binning $K / H$ in the range (b) $[- 0.15, 0.15]$ , (d) $[- 0.175, 0.175]$ , and (f) $[- 0.3, 0.3]$ after folding along $K / H$ axis.
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Figure 3
Constant-energy slices symmetrized along $H$ and $K$ axes for $E = 4.5 \pm 0.5$ meV ( $E_{i} = 30 meV$ ) at (a) 20 K, (c) 35 K, and (e) 75 K. Corresponding longitudinal cuts along $[H, 0]$ (red circles) and $[0, K]$ (blue diamonds) are respectively shown in (b), (d), and (f). $[H, 0] / [0, K]$ scans are obtained by binning $K / H$ in the range $[- 0.15, 0.15]$ after folding along $K / H$ axis. (g) Temperature dependence of the anisotropy $δ = (I_{10} - I_{01}) / (I_{10} + I_{01})$ for $E = 4.5 \pm 0.5 meV$ . The purple dashed line is a guide to the eye. $T_{c}$ and stress-free $T_{N} / T_{s}$ are marked by vertical dashed lines.
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Figure 4
Energy dependence of anisotropy between $Q_{1}$ and $Q_{2}$ defined as $δ = (I_{10} - I_{01}) / (I_{10} + I_{01})$ for (a) 5 K and (b) 35 K. (c) Energy dependence of $χ_{10}^{″} + χ_{01}^{″}$ at 5 K; $χ_{10}^{″}$ and $χ_{01}^{″}$ are dynamic susceptibilities at $Q_{1} = (1, 0)$ and $Q_{2} = (0, 1)$ . (d) $χ_{10}^{″} - χ_{01}^{″}$ . Data obtained on HB-1A is collected at 6 K, and is plotted together with ARCS data using incident energies $E_{i} = 30$ , 80, 150, and 250 meV. It should be noted that below 10 meV magnetic excitations have significant $L$ -modulation. Since $E$ and $L$ are coupled for our scattering geometry, the strong enhancement in (c) and (d) below 10 meV is in part due to $L$ -modulation. The resonance mode in the superconducting state also contributes to the enhancement below 10 meV. However, given that the anisotropy ( $δ$ ) does not depend on absolute intensity, $δ$ in (a) reflects instrinsic properties of the system regardless of $L$ -modulation.
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Physical Review B

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Energy dependence of the spin excitation anisotropy in uniaxial-strained ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$

Yu Song, Xingye Lu, D. L. Abernathy, David W. Tam, J. L. Niedziela, Wei Tian, Huiqian Luo, Qimiao Si, and Pengcheng Dai

Phys. Rev. B 92, 180504(R) – Published 6 November 2015

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Physical Review B

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Energy dependence of the spin excitation anisotropy in uniaxial-strained BaFe1.9Ni0.1As2

Yu Song, Xingye Lu, D. L. Abernathy, David W. Tam, J. L. Niedziela, Wei Tian, Huiqian Luo, Qimiao Si, and Pengcheng Dai

Phys. Rev. B 92, 180504(R) – Published 6 November 2015

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Energy dependence of the spin excitation anisotropy in uniaxial-strained ${BaFe}_{1.9} {Ni}_{0.1} {As}_{2}$