Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of ${\mathrm{RbFe}}_{2}{\mathrm{As}}_{2}$

Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of ${RbFe}_{2} {As}_{2}$

E. Civardi, M. Moroni, M. Babij, Z. Bukowski, and P. Carretta

Phys. Rev. Lett. 117, 217001 – Published 17 November 2016

Abstract

${}^{75}{As}$ , ${}^{87}{Rb}$ , and ${}^{85}{Rb}$ nuclear quadrupole resonance (NQR) and ${}^{87}{Rb}$ nuclear magnetic resonance measurements in a ${RbFe}_{2} {As}_{2}$ iron-based superconductor are presented. We observe a marked broadening of the ${}^{75}{As}$ NQR spectrum below $T_{0} ≃ 140 K$ which is associated with the onset of a charge order in the FeAs planes. Below $T_{0}$ we observe a power-law decrease in the ${}^{75}{As}$ nuclear spin-lattice relaxation rate down to $T^{*} ≃ 20 K$ . Below $T^{*}$ the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in light of an orbital selective behavior expected for the electronic correlations.

Received 11 August 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.217001

Physics Subject Headings (PhySH)

Phase diagrams

Chalcogenides Heavy-fermion systems Pnictides

Nuclear magnetic resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

E. Civardi¹, M. Moroni¹, M. Babij², Z. Bukowski², and P. Carretta¹

¹Department of Physics, University of Pavia-CNISM, I-27100 Pavia, Italy
²Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-422 Wroclaw, Poland

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Vol. 117, Iss. 21 — 18 November 2016

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Figure 1
(a) The ${}^{75}{As}$ NQR spectrum in ${RbFe}_{2} {As}_{2}$ is reported at different temperatures between 5 K and 300 K. The red lines are best fits with one or two (for $T < 130 K$ ) Lorentzians. (b) The merge of the NQR spectra, associated with the $m_{I} = \pm 3 / 2 \to \pm 1 / 2$ transition for ${}^{75}{As}$ and ${}^{87}{Rb}$ and with the $m_{I} = \pm 5 / 2 \to \pm 3 / 2$ transition for ${}^{85}{Rb}$ ( $I = 5 / 2$ ), is shown for $T = 4.2 K$ . The intensity of the three spectra has been rescaled so that all three spectra have similar intensities. (c) The temperature dependence of the full width at half intensity (FWHM) of ${}^{75}{As}$ NQR spectra (red octagons, left scale, for the plot with linear $T$ scale see the Supplemental Material [28]) is shown together with the temperature dependence of ${}^{87}{Rb}$ $ν_{Q}$ (blue squares, right scale). The green solid line tracking the order parameter is a phenomenological fit of the FWHM ( $Δ ν_{Q}$ ) with $Δ ν_{Q} = 300 [1 - (T / T_{0})]^{β} + 170 kHz$ , with $T_{0} = 140 K$ and $β ≃ 0.7$ .
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Figure 2
(a) The recovery of ${}^{75}{As}$ nuclear magnetization $M (τ)$ (measured in NQR) is reported as a function of the delay $τ$ between a saturation radio frequency pulse sequence and the echo readout sequence for different temperatures. The solid lines are the best fits according to Eq. (2) in the text. (b) The frequency dependence of the fraction of fast $A_{f}$ and slow relaxing $A_{s}$ nuclei is reported as a function of the irradiation frequency across the ${}^{75}{As}$ NQR spectrum. The black solid line is the best fit of the spectrum at $T = 4.2 K$ . (c) The temperature dependence of $A_{f}$ (blue) and $A_{s}$ (red) recorded on the low-frequency peak of the ${}^{75}{As}$ NQR spectrum.
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Figure 3
(a) The temperature dependence of ${}^{75}{As}$ NQR $1 / T_{1}$ in ${RbFe}_{2} {As}_{2}$ , for $T \geq 20 K$ , and of the fast relaxation rate $1 / T_{1}^{f}$ , for $T < 20 K$ , are reported for an irradiation frequency centered at the low-frequency peak (blue squares) and for an irradiation frequency centered at the high frequency peak (green circles). The red solid line is a best fit to the data between 20 and 100 K with a power law characterized by an exponent $b = 0.79$ . ${}^{87}{Rb}$ NMR $1 / T_{1}$ (orange circles) in ${RbFe}_{2} {As}_{2}$ is reported between 3.5 and 70 K, for an external magnetic field $H = 7$ Tesla. (b) The temperature dependence of ${}^{75}{As}$ NQR $1 / T_{1}^{f} T$ (blue squares) and $1 / T_{1}^{s} T$ (red circles) in ${RbFe}_{2} {As}_{2}$ is reported. The dashed line at the bottom is the best fit according to an activated behavior with an energy gap $E_{g} = 17 \pm 0.9 K$ . The dashed line at the top is the behavior expected according to Moriya SCR theory (see text). The dashed horizontal line shows schematically the Korringa-like behavior expected for an uncorrelated metal.
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Physical Review Letters

Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of RbFe2As2

E. Civardi, M. Moroni, M. Babij, Z. Bukowski, and P. Carretta

Phys. Rev. Lett. 117, 217001 – Published 17 November 2016

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Superconductivity Emerging from an Electronic Phase Separation in the Charge Ordered Phase of ${RbFe}_{2} {As}_{2}$