⇦Chinese Physics Letters, 2019, Vol. 36, No. 12, Article code 127401 $^{19}$F NMR Study of the Bilayer Iron-Based Superconductor KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ * Yu-Ting Shao (邵钰婷)1,2, Wen-Shan Hong (洪文山)1,2, Shi-Liang Li (李世亮)1,2,3, Zheng Li (李政)1,2**, Jian-Lin Luo (雒建林)1,2,3** Affiliations 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 2School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190 3Songshan Lake Materials Laboratory, Dongguan 523808 Received 22 October 2019, online 25 November 2019 *Supported by the National Key Research and Development Program of China under Grant No 2017YFA0302901, the National Basic Research Program of China under Grant No 2015CB921304, the National Science Foundation of China under Grant Nos 11674375 and 11634015, and the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences under Grant No XDB07020200.
**Corresponding author. Email: Lizheng@iphy.ac.cn; JLLuo@iphy.ac.cn Citation Text: Shao Y T, Hong W S, Li S L, Li Z and Luo J L et al 2019 Chin. Phys. Lett. 36 127401    Abstract We report a $^{19}\!$F nuclear magnetic resonance (NMR) study on single-crystal KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ ($T_{\rm c} \sim 33.3$ K). The $^{19}$F NMR spectral shape of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ is weakly dependent on temperature and the Knight shift is small, which implies weak coupling between the CaF layer and the FeAs layer. The temperature dependence of 1/$^{19}\!T_{1}$ shows a hump below $T_{\rm c}$, however the 1/$^{75}\!T_{1}$ decreases just below $T_{\rm c}$, which implies that there are strong in-plane magnetic fluctuations in the CaF layers than in the FeAs layers. This may be caused by the motion of vortices. The absence of the coherence peak suggests unconventional superconductivity in KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$. DOI:10.1088/0256-307X/36/12/127401 PACS:74.70.Xa, 74.25.nj, 74.78.Fk © 2019 Chinese Physics Society Article Text Iron-based superconductors (SCs) have gradually developed into a diverse group of different structural families since the first case was reported,[1] such as the LaFeAsO$_{1-x}$F$_{x}$ (1111-type), Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ (122-type), LiFeAs (111-type) and $\alpha$-FeSe (11-type).[2–6] Recently, researchers have conducted extensive researches on iron-based superconductors with hybrid structure.[7–11] The 12442-type hybrid structure iron-based SCs are a type of bilayer compounds, which are obtained by intergrowth of 1111-type and 122-type iron-based materials, for example, CaFeAsF and (K,Rb,Cs)Fe$_{2}$As$_{2}$.[8,12] The KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ with a superconducting transition temperature $T_{\rm c}\approx 33.2$ K is the first example of 12442-type iron-based SC.[8] It consists of alternating layers of conducting FeAs layers and insulating CaF layers, as shown in the inset of Fig. 1. First-principle calculations indicate that it has a strong tendency towards magnetism.[13] However, the self-hole doping can suppress the spin density wave state, and then induces superconductivity. The pressure experiments show that $T_{\rm c}$ increases from $\sim$33.5 K at ambient pressure to $\sim$36.5 K around 2 GPa with a slope change of about 8 GPa, and semi-collapsed tetragonal phase is absent below 15 GPa.[14] The $\mathrm{\mu }$SR experiments have shown that KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ is a multiple-gap SC with line nodes.[15] However, the measurements of the ultra-low-temperature thermal conductivity of CsCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ indicates that CsCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ is a multiple-gap nodeless SC.[16] Optical spectroscopy measurements of the CsCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ also show the evidence of isotropic nodeless superconductivity in CsCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$.[17] The anisotropy of upper critical field $H_{\rm c2}$ measured by resistivity is about eight times, which demonstrates the coherence length is much shorter along the $c$ axis than that in $ab$ plane.[18] It is unique for studying the anisotropic superconductivity. Great efforts have been made to study the FeAs layers for iron-based SCs by different experiments. However, insulating layers, such as CaF layers in KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$, is not well studied.[19,20] Nuclear magnetic resonance (NMR) measurement is a microscopic probe and can study the static spin susceptibility and low-energy spin excitation of iron-based SCs.[21,22] It has ability to distinguish different site atoms, which can be used to study both FeAs layers and CaF layers, respectively.[22–24] In this Letter, we present detailed $^{19}$F-NMR experimental studies on a single crystal sample of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$. The $^{19}$F NMR spectral shape of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ is independent of temperature at $\mu_{0}H = 4.2$ T and the Knight shift is small. The temperature dependence of 1/$T_{1}$ shows a hump below $T_{\rm c}$, implying strong magnetic fluctuations in the CaF layers. High-quality single crystals of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ were grown by using KAs as flux. The electrical resistivity measurements were performed on the Physical Property Measurement System (PPMS, Quantum Design) using the conventional four-terminal method. The $^{19}$F NMR measurements ($I = 1/2$, $\gamma _n/2\pi = 40.055$ MHz/T, natural abundance = 100%) were carried out by a standard spin-echo method on a conventional phase-coherent spectrometer with a superconducting magnet. The spin echo intensity was maximized after the $\pi / 2$–$\tau $–$\pi $ pulse sequence and the $^{19}$F NMR spectra were obtained by the Fourier transform of the spin echo. The $^{19}$F NMR spin-lattice relaxation rate 1/$T_{1}$ was measured by a recovery method. The applied magnetic field was fixed at 4.2 T and parallel to the $c$-axis or the $ab$-plane of the sample.
cpl-36-12-127401-fig1.png
Fig. 1. The temperature dependence of the electrical resistance of the KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ sample at $\mu_{0}H = 0$, 4.2 and 9 T with $H$ parallel to the $c$ axis. The inset shows the crystal structure of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$.
Figure 1 shows the temperature dependence of the electrical resistance of the KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ sample at different externally applied magnetic fields with $H$//$c$-axis. The electrical resistance of the KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ sample shows a clear sharp transition of superconducting with $T_{\rm c} = 33.3$ K at zero field. The narrow transition width indicates the high quality of the single crystal sample. The $T_{\rm c}$ shifts to 31.2 K at 4.2 T and 29.0 K at 9 T. The transition width becomes broader at higher field due to the vortex liquid phase.[18]
cpl-36-12-127401-fig2.png
Fig. 2. $^{19}$F NMR spectra of the KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ sample measured at externally applied magnetic fields $\mu_{0}H = 4.2$ T with (a) $H$//$c$-axis and (b) $H$//$ab$-plane at different temperatures.
Figure 2 shows the $^{19}$F NMR spectra of the KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ sample measured by taking a Fourier transform of the spin echo signal at $\mu_{0} H = 4.2$ T with $H$//$c$-axis and $H$//$ab$-plane, at different temperatures. A single spectrum is observed with a very narrow FWHM of about 60 kHz. The shape of the $^{19}$F NMR spectra is independent of temperature and also not affected by the field direction. The NMR frequency is different when $H$ is parallel to the $c$-axis and parallel to $ab$-plane of the sample, indicating an anisotropy in KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$. From the peak position of the $^{19}$F NMR line shapes, we deduce the temperature dependence of the $^{19}$F Knight shift at $\mu_{0} H = 4.2$ T with $H$//$c$-axis as summarized in Fig. 3. $K$ decreases below $T_{\rm c}$ as expected for spin-singlet pairing. The small magnitude of $K$ implies that spin transfer from the FeAs layers to the CaFe layers is small. Since $^{19}$F has weak coupling with FeAs layers, it can be used to study the properties of CaF insulating layers which separate FeAs conducting layers and results in two-dimensional behaviors.
cpl-36-12-127401-fig3.png
Fig. 3. The temperature dependence of the $^{19}$F Knight shift at $\mu_{0}H = 4.2$ T with $H$//$c$-axis.
cpl-36-12-127401-fig4.png
Fig. 4. (a) Temperature dependence of $^{19}$F NMR spin-lattice relaxation rates at 4.2 T. (b) Temperature dependence of $^{75}$As NMR spin-lattice relaxation rates at 10.5 T with $H$//$c$-axis.
The temperature dependence of the $^{19}$F NMR spin-lattice relaxation rate 1/$T_{1}$ at 4.2 T is shown in Fig. 4(a). Here 1/$T_{1}$ decreases with the temperature decreasing in the normal state. Below $T_{\rm c}$, the temperature dependence of 1/$T_{1}$ shows a hump. Similar behavior was reported in HgBa$_{2}$CuO$_{4}$, where 1/$T_{1}$ temperature dependence shows a maximum.[25] It was attributed to the strong influence of magnetic-flux line motion. Earlier studies interpreted the enhancement of 1/$T_{1}$ as the effects of thermal motion of flux lines.[26–29] At low temperatures, $^{19}$F NMR spin-lattice relaxation rate decreases fast and follows $1/T_{1} \propto T^3$. As a comparison, we measured the temperature dependence of the $^{75}$As NMR 1/$T_{1}$ at $\mu_{0} H = 10.5$ T with $H$//$c$-axis, as shown in Fig. 4(b). The $^{75}$As nuclear spin-lattice relaxation rate 1/$^{75}\!T_{1}$ are determined from the recovery of the nuclear magnetization, and 1/$^{75}\!T_{1}$ decreases just below $T_{\rm c}$ without Hebel–Slichter coherence peak, which suggests unconventional superconductivity in KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$. There is no hump in 1/$^{75}\!T_{1}$, so the upturn in the temperature dependence of the 1/$^{19}\!T_{1}$ implies that there are strong in-plane magnetic fluctuations in CaF layers, which do not appear in FeAs layers. In summary, we have performed $^{19}$F-NMR experiment in the single crystal of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ with a superconducting transition temperature of $T_{\rm c} \sim 33.3$ K. $^{19}$F NMR spectra and spin-lattice relaxation rates have been measured. Due to weak coupling with FeAs plane, the $^{19}$F NMR spectral shape is independent of temperature and the Knight shift is very small. The temperature dependence of 1/$^{19}\!T_{1}$ shows a hump below $T_{\rm c}$, which is due to in-plane magnetic fluctuations by the vortices motion. The difference between 1/$^{19}\!T_{1}$ and 1/$^{75}\!T_{1}$ indicates that the CaF layers are much different from the FeAs layers, and it will be helpful for studying two-dimensional behaviors and anisotropy in iron-based superconductors. References Iron-Based Layered Superconductor La[O 1- x F x ]FeAs ( x = 0.05−0.12) with T c = 26 KSuperconductivity at 38 K in the Iron Arsenide ( Ba 1 − x K x ) Fe 2 As 2 The superconductivity at 18 K in LiFeAs systemSuperconductivity in the PbO-type structure -FeSeSuperconductivity at 17 K in (Fe 2 P 2 )(Sr 4 Sc 2 O 6 ): a new superconducting layered pnictide oxide with a thick perovskite oxide layerSuperconductivity in iron compoundsNew-Structure-Type Fe-Based Superconductors: Ca A Fe 4 As 4 ( A = K, Rb, Cs) and Sr A Fe 4 As 4 ( A = Rb, Cs)Superconductivity in KCa 2 Fe 4 As 4 F 2 with Separate Double Fe 2 As 2 LayersSuperconductivity and ferromagnetism in hole-doped RbEuFe 4 As 4 Pressure-induced half-collapsed-tetragonal phase in CaKFe 4 As 4 Effects of disorder on the intrinsically hole-doped iron-based superconductor KC a 2 F e 4 A s 4 F 2 by cobalt substitutionCrystal structure and superconductivity at about 30 K in ACa2Fe4As4F2 (A = Rb, Cs)Self-hole-doping–induced superconductivity in KCa 2 Fe 4 As 4 F 2Pressure-induced enhancement of superconductivity and quantum criticality in the 12442-type hybrid-structure superconductor KC a 2 F e 4 A s 4 F 2 Nodal multigap superconductivity in KCa 2 Fe 4 As 4 F 2 Multigap nodeless superconductivity in CsCa 2 Fe 4 As 4 F 2 probed by heat transportBand-selective clean-limit and dirty-limit superconductivity with nodeless gaps in the bilayer iron-based superconductor CsCa 2 Fe 4 As 4 F 2 Single-Crystal Growth and Extremely High H c2 of 12442-Type Fe-Based Superconductor KCa 2 Fe 4 As 4 F 2Review of nuclear magnetic resonance studies on iron-based superconductorsProgress of novel diluted ferromagnetic semiconductors with decoupled spin and charge doping: Counterparts of Fe-based superconductorsThe puzzle of high temperature superconductivity in layered iron pnictides and chalcogenidesTo What Extent Iron-Pnictide New Superconductors Have Been Clarified: A Progress Report F 19 NMR investigation of the iron pnictide superconductor LaFeAsO 0.89 F 0.11 Evidence of a structural phase transition in superconducting SmFeAsO 1− x F x from 19 F NMRMagnetic flux dynamics and structural features in fluorinated HgBa 2 CuO 4 as probed by 19 F NMR19F NMR as a probe of flux-line motion in fluorinated YBa2Cu4O8Flux-line dynamics in YBa 2 Cu 4 O 8 from Y 89 NMRThermal Depinning of Flux Lines in HgBa 2 CuO 4 + δ from 199 Hg Spin-Lattice RelaxationEvidence for the role of fluxoids in enhancing NMR spin-lattice relaxation and implications for intrinsic pinning of the flux lattice in organic superconductors
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