Unusual suppression of a spin resonance mode by magnetic field in underdoped ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}$: Evidence for orbital-selective pairing

Unusual suppression of a spin resonance mode by magnetic field in underdoped ${NaFe}_{1 - x} {Co}_{x} As$ : Evidence for orbital-selective pairing

Yu Song, Guotai Tan, Chenglin Zhang, Rasmus Toft-Petersen, Rong Yu, and Pengcheng Dai

Phys. Rev. B 98, 064507 – Published 13 August 2018

Abstract

We use inelastic neutron scattering to study the fate of the two spin resonance modes in underdoped superconducting ${NaFe}_{1 - x} {Co}_{x} As (x = 0.0175)$ under applied magnetic fields. While an in-plane magnetic field of $B = 12$ T only modestly suppresses superconductivity and enhances static antiferromagnetic order, the two spin resonance modes display disparate responses. The spin resonance mode at higher energy is mildly suppressed, consistent with the field effect in other unconventional superconductors. The spin resonance mode at lower energy, on the other hand, is almost completely suppressed. Such dramatically different responses to applied magnetic field indicate distinct origins of the two spin resonance modes, resulting from the strongly orbital-selective nature of spin excitations and Cooper pairing in iron-based superconductors.

Received 2 May 2018
Revised 13 July 2018

DOI:https://doi.org/10.1103/PhysRevB.98.064507

Physics Subject Headings (PhySH)

Antiferromagnetism Superconductivity

Pnictides Single crystal materials

Inelastic neutron scattering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yu Song^1,2,*, Guotai Tan³, Chenglin Zhang¹, Rasmus Toft-Petersen⁴, Rong Yu⁵, and Pengcheng Dai^1,3,†

¹Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
²Department of Physics, University of California, Berkeley, California 94720, USA
³Center for Advanced Quantum Studies and Department of Physics, Beijing Normal University, Beijing 100875, China
⁴Helmholtz-Zentrum Berlin für Materialen und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
⁵Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials and Micro-nano Devices, Renmin University of China, Beijing 100872, China

^*yusong@berkeley.edu
^†pdai@rice.edu

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Vol. 98, Iss. 6 — 1 August 2018

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Figure 1
(a) Schematic Fermi surface of underdoped ${NaFe}_{1 - x} {Co}_{x} As$ , with dominant orbital contributions marked by different colors, based on ARPES measurements [49, 50]. (b) Schematic of momentum-dependent superconducting gaps in underdoped ${NaFe}_{1 - x} {Co}_{x} As$ , adapted from previous work [34]. The in-plane zone center is $Γ$ or $Z$ depending on $k_{z}$ , and the in-plane zone corner is $M$ or $A$ depending on $k_{z}$ . (c) Schematic of the $[H, 0, L]$ scattering plane. The magnetic field $B$ is along $K$ , perpendicular to the scattering plane. (d) Magnetic field dependence of the magnetic order parameter measured at $Q = (1, 0, 0.5)$ . A constant background has been subtracted. The solid lines are guides to the eye.
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Figure 2
(a) Background-subtracted constant- $Q$ scans at $Q = (1, 0, 0.5)$ . (b) Temperature dependence at $Q = (1, 0, 0.5)$ and $E = 3.25$ meV under $B = 0$ T and 12 T, with background subtracted. (c) Background-subtracted constant- $Q$ scans at $Q = (1, 0, 1)$ . (d) Temperature dependence at $Q = (1, 0, 0.5)$ and $E = 6.25$ meV under $B = 0$ T and 12 T, with background subtracted. The solid lines are guides to the eye. See Appendix for details on background subtraction.
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Figure 3
(a) Color-coded and interpolated magnetic field dependence of low-energy magnetic fluctuations at $Q = (1, 0, 0.5)$ and $T = 2$ K, with background subtracted. The circles correspond to points where measurements were taken. Magnetic field dependence of magnetic fluctuations at $Q = (1, 0, 0.5)$ and $T = 2$ K for (b) $E = 2.5$ meV, (c) $E = 3.25$ meV, (d) $E = 3.75$ meV, and (e) $E = 4.5$ meV. The flat red lines are from fits at 20 K shown in Fig. 2, which do not show magnetic field dependence. See Appendix for details on background subtraction.
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Figure 4
(a) Background-subtracted constant-energy scans along $(H, 0, 0.5)$ for $E = 3.25$ meV, (b) along $(1, 0, L)$ for $E = 3.25$ meV, (c) along $(H, 0, 0.5)$ for $E = 2.5$ meV, and (d) along $(1, 0, L)$ for $E = 4.5$ meV. Solid lines in (a) and (c) are fits to Gaussian peaks, and solid lines in (b) and (d) are fits to lattice Lorentzian peaks. See Appendix for details on background subtraction.
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Figure 5
Orbital-selective destruction of the superconducting pairing amplitudes by the applied magnetic field in a five-orbital $t − J$ model. Shown are the leading pairing channels with $s \pm$ symmetry in $x z / y z$ and $x y$ orbitals. The horizontal axis is the ratio of applied magnetic field $H$ and the next-nearest-neighbor exchange coupling $J_{2}$ .
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Figure 6
(a) Constant- $Q$ scans at $Q = (1, 0, 0.5)$ , without background subtraction. Background measured at $Q = (0.8, 0, 0.902)$ is shown for comparison, and the solid line is an empirical fit to the background. (b) Background-subtracted constant- $Q$ scans at $Q = (1, 0, 0.5)$ for $T = 20$ K under different applied fields. (c) Constant- $Q$ scans at $Q = (1, 0, 1)$ , without background subtraction. Background measured at $Q = (0.8, 0, 1.25)$ is shown for comparison, and the solid line is an empirical fit to the background. (d) Background-subtracted constant- $Q$ scans at $Q = (1, 0, 1)$ for $T = 20$ K under different applied fields.
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Physical Review B

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Unusual suppression of a spin resonance mode by magnetic field in underdoped ${NaFe}_{1 - x} {Co}_{x} As$ : Evidence for orbital-selective pairing

Yu Song, Guotai Tan, Chenglin Zhang, Rasmus Toft-Petersen, Rong Yu, and Pengcheng Dai

Phys. Rev. B 98, 064507 – Published 13 August 2018

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Physical Review B

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Unusual suppression of a spin resonance mode by magnetic field in underdoped NaFe1−xCoxAs: Evidence for orbital-selective pairing

Yu Song, Guotai Tan, Chenglin Zhang, Rasmus Toft-Petersen, Rong Yu, and Pengcheng Dai

Phys. Rev. B 98, 064507 – Published 13 August 2018

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Unusual suppression of a spin resonance mode by magnetic field in underdoped ${NaFe}_{1 - x} {Co}_{x} As$ : Evidence for orbital-selective pairing