Enhanced superconducting transition temperature in hyper-interlayer-expanded FeSe despite the suppressed electronic nematic order and spin fluctuations

Matevž Majcen Hrovat, Peter Jeglič, Martin Klanjšek, Takehiro Hatakeda, Takashi Noji, Yoichi Tanabe, Takahiro Urata, Khuong K. Huynh, Yoji Koike, Katsumi Tanigaki, and Denis Arčon

Phys. Rev. B 92, 094513 – Published 23 September 2015

Abstract

The superconducting critical temperature, $T_{c}$ , of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a ${}^{77}{Se}, {}^{7}{Li}$ , and ${}^{1}H$ nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded ${Li}_{x} {(C_{2} H_{8} N_{2})}_{y} {Fe}_{2 - z} {Se}_{2}$ with a nearly optimal $T_{c} = 45$ K. The absence of any shift in the ${}^{7}{Li}$ and ${}^{1}H$ NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent ${}^{7}{Li}$ and ${}^{1}H$ spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, the strong temperature dependence of ${}^{77}{Se}$ NMR shift and spin-lattice relaxation rate, $1 /^{77} T_{1}$ , is attributed to the holelike bands close to the Fermi energy. $1 /^{77} T_{1}$ shows no additional anisotropy that would account for the onset of electronic nematic order down to $T_{c}$ . Similarly, no enhancement in $1 /^{77} T_{1}$ due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence $1 /^{77} T_{1} \propto T^{4.5}$ still complies with the Cooper pairing mediated by spin fluctuations.

Received 3 August 2015

DOI:https://doi.org/10.1103/PhysRevB.92.094513

Authors & Affiliations

Matevž Majcen Hrovat¹, Peter Jeglič¹, Martin Klanjšek¹, Takehiro Hatakeda², Takashi Noji², Yoichi Tanabe³, Takahiro Urata³, Khuong K. Huynh³, Yoji Koike², Katsumi Tanigaki^3,4, and Denis Arčon^1,5,*

¹Jožef Stefan Institute, Condensed Matter Physics Department, Jamova cesta 39, SI-1000 Ljubljana, Slovenia
²Department of Applied Physics, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
³Department of Physics, Graduate School of Science, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
⁴World Premier Institute - Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8578, Japan
⁵Faculty of Mathematics and Physics, University of Ljubljana, Jadranska cesta 19, SI-1000 Ljubljana, Slovenia

^*denis.arcon@ijs.si

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Vol. 92, Iss. 9 — 1 September 2015

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Images

Figure 1
Schematic crystal structure of the intercalated FeSe-based ${Li}_{x} {(C_{2} H_{8} N_{2})}_{y} {Fe}_{2 - z} {Se}_{2}$ compound with the hyperexpanded distance between neighboring Fe layers of $d = 10.37 Å$ and enhanced superconducting critical temperature $T_{c} = 45$ K. Here, red spheres represent Fe, gray spheres Se, orange spheres Li, blue spheres N, brown spheres C, and green spheres H. The position of impurity Fe atoms in the interlayer space is unknown.
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Figure 2
Temperature evolution of (a) ${}^{7}{Li}$ and (b) ${}^{1}H$ NMR spectra of i-FeSe powder. Please note the absence of shift and the similar line broadening for both nuclei. Dotted vertical lines indicate the corresponding Larmor frequencies.
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Figure 3
(a) Temperature dependencies of ${}^{1}H$ (orange triangles), ${}^{7}{Li}$ (violet upside-down triangles), and ${}^{77}{Se}$ (red circles) spin-lattice relaxation rates, $1 /^{n} T_{1}$ $(n = 1, 7, 77)$ . (b) Temperature dependence of the ratio, $R$ , of ${}^{1}H$ to ${}^{7}{Li},^{1} T_{1} /^{7} T_{1}$ (blue upside-down triangles), and of ${}^{1}H$ to ${}^{77}{Se},^{1} T_{1} /^{77} T_{1}$ (green triangles), spin-lattice relaxation times. Dotted vertical lines indicate the superconducting critical temperature $T_{c} = 45$ K of the i-FeSe sample.
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Figure 4
(a) Temperature evolution of the ${}^{77}{Se}$ NMR spectra measured in i-FeSe (red line) and in FeSe (blue line) powders. Note the presence of a second weak peak attributed to the presence of nonintercalated FeSe impurity regions in the spectra of i-FeSe. (b) Temperature dependencies of the ${}^{77}{Se}$ NMR shifts for i-FeSe (red circles) and FeSe (blue squares). Dotted vertical lines indicate the superconducting critical temperature $T_{c} = 45$ K for i-FeSe (red) and $T_{c} = 8$ K for FeSe (blue). The solid vertical blue line marks an FeSe structural phase transition at $T_{s} = 91$ K.
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Figure 5
Temperature dependencies of (a) the ${}^{77}{Se}$ spin-lattice relaxation rates, $1 /^{77} T_{1}$ , divided by temperature and (b) of the stretching exponent $α_{s}$ for i-FeSe (red circles) and FeSe (blue squares). Insets to (a) and (b) show the temperature dependencies of $1 /^{77} T_{1} T$ and $α_{s}$ in the reduced $T / T_{c}$ scale. Dotted vertical lines indicate the superconducting critical temperature $T_{c} = 45$ K for i-FeSe (red) and $T_{c} = 8$ K for FeSe (blue). Dotted vertical black lines in the insets mark the onset of superconductivity where $T / T_{c} = 1$ .
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Figure 6
Temperature dependence of ${}^{77}{Se}$ spin-lattice relaxation rate, $1 /^{77} T_{1}$ , below the i-FeSe superconducting critical temperature $T_{c} = 45$ K. The dashed line shows the expected temperature dependence for a power law $1 /^{77} T_{1} \propto T^{5}$ compatible with the $s^{\pm}$ superconductivity. Solid red line is a fit to $1 /^{77} T_{1} = A + B {(T / T_{c})}^{n}$ with $A = 0.24 s^{- 1}, B = 3.39 s^{- 1}$ , and $n = 4.5$ . Inset: ${}^{1}H$ spin-lattice relaxation rate, $1 /^{1} T_{1}$ , is nearly temperature independent below $T_{c}$ . Thin dotted vertical lines mark $T_{c}$ .
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