Papers in Physics, vol. 11, art. 110002 (2019) Received: 28 October 2018, Accepted: 29 April 2019 Edited by: A. Goñi, A. Cantarero, J. S. Reparaz Licence: Creative Commons Attribution 4.0 DOI: http://dx.doi.org/10.4279/PIP.110002 www.papersinphysics.org ISSN 1852-4249 Nuclear magnetic resonance on LaFeAsO0.4H0.6 at 3.7 GPa N. Fujiwara,1∗ M. Takeuchi,1 T. Kuwayama,1 S. Nakagawa,1 S. Iimura,2 S. Matsuishi,3 H. Hosono2,3 A prototypical electron-doped iron-based superconductor LaFeAsO1−xHx undergoes an antiferromagnetic (AF) phase for x ≥ 0.49. We have performed nuclear magnetic resonance (NMR) measurements on LaFeAsO0.4H0.6 at 3.7 GPa to investigate the magnetic prop- erties in the vicinity of a pressure-induced quantum critical point (QCP). The linewidth of 1H-NMR spectra broadens at low temperatures below 30 K, suggesting that the spin moments remain ordered at 3.7 GPa. The coexistence of gapped and gapless spin ex- citations was confirmed in the ordered state from the relaxation time T1 of 75As. The pressure-induced QCP is estimated to be 4.1 GPa from the pressure dependence of the gapped excitation. I. Introduction A prototypical electron-doped iron-based pnictide LaFeAsO1−xHx (0 ≤ x ≤ 0.6) exhibits unique elec- tronic properties in a heavily carrier-doped regime: a superconducting (SC) phase with double-domes structure expands in a wide regime (0.05 < x < 0.49) [1] and an antiferromagnetic (AF) phase man- ifests itself by further H doping (0.49 ≤ x) [2–4]. Band calculations show that both Fermi surfaces and nesting vectors change by H doping: the two hole pockets present at Γ point in the lightly H- doped regime almost disappear in the heavily H- ∗E-mail: naoki@fujiwara.h.kyoto-u.ac.jp 1 Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cyo, Sakyo-ku, Kyoto 606-8501, Japan. 2 Institute for Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226- 8503, Japan. 3 Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503, Japan. doped regime [5, 6]. The change in the nesting vec- tors due to H doping would cause a change in wave- vector (q) dependent spin susceptibility χ(q,ω) and would allow for the appearance of two AF phases in the lightly and heavily H-doped regimes. The AF phase in the heavily H-doped regime is strongly suppressed upon applying pressure [7]. We have performed nuclear magnetic resonance (NMR) measurements on LaFeAsO0.4H0.6 at 3.7 GPa, and we have found that the spin excitation gap appear- ing at the AF phase vanishes at around 4.1 GPa. We have investigated the magnetic properties in the vicinity of a pressure-induced quantum critical point (QCP)('4.1 GPa). II. Experimental apparatuses and conditions A pressure of 3.7 GPa was applied using a NiCrAl- hybrid clamp-type pressure cell as shown in Fig. 1 [8]. We have used a mixture of Fluorinert FC-70 and FC-77 as the pressure-transmitting medium. A coil wounded around the powder samples and an optical fiber with the Ruby powders glued on top 110002-1 Papers in Physics, vol. 11, art. 110002 (2019) / N. Fujiwara et al. Figure 1: A NiCrAl-hybrid clamp-type pressure cell [8]. A coil wounded around the powder samples and an op- tical fiber with the Ruby powders were inserted into the sample space. were inserted into the sample space of the pressure cell [8]. The size of the coil was 2.4 mm in di- ameter and 3.5 mm in length, and the number of windings was 18 turns. The pressure was monitored through Ruby fluorescence measurements. The R1 and R2 lines at ambient pressure, 3.0 and 3.7 GPa are shown in Fig. 2. The wavelength of the R1 or R2 peak shifts linearly with respect to pressure. The shift of the wavelength ∆λ satisfies the relation P(GPa)=∆λ(nm)/0.365. NMR measurements for the powder samples were acquired using a conventional coherent-pulsed NMR spectrometer. The relaxation rate (1/T1) was measured using a conventional saturation-recovery method for the samples whose FeAs planes are par- allel to the applied field. III. Experimental results i. 1H-NMR spectra 75As(I = 3/2)-NMR spectra broaden due to the nu- clear quadrupole interaction, which makes difficult to investigate the antiferromagnetic (AF) state. However, 1H(I = 1/2) is free from the nuclear quadrupole interaction. Therefore, the 1H signal is narrow at a paramagnetic state, and the broad- ening in the AF phase directly reflects the mag- 500 400 300 200 100 0 In te n si ty ( a rb .u n it s) 700698696694692690 Wave Length (nm) 3.7 GPa 3.0 GPa 0.1 MPa Figure 2: Ruby fluorescence spectra. The smaller and larger peaks correspond to the R2 and R1 transitions, respectively. N M R I n te n si ty ( a rb . u n it ) 10.09.59.08.58.07.57.0 H(kOe) 4.2K 10K 20K 60K 30K 40K 1 H 9 F Figure 3: 1H-NMR spectra for LaFeAsO0.4H0.6 mea- sured at 3.7 GPa and 35.1 MHz. The 9F signal orig- inates from the pressure-transmitting medium, a mix- ture of Fluorinert FC-70 and FC-77. 110002-2 Papers in Physics, vol. 11, art. 110002 (2019) / N. Fujiwara et al. 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 L in e w id th ( k O e ) 140120100806040200 T (K) 0.1 MPa 3.7 GPa T N TN Figure 4: The increase in 1H linewidth due to the or- dered spin moments. TN represents the antiferromag- netic (AF) transition temperature. nitude of the spin moments. Figure 3 shows 1H- NMR spectra measured at 3.7 GPa and 35.1 MHz. The sharp signal of 9F originates from the pressure- transmitting medium mentioned above. The tem- perature dependence of the linewidth is shown in Fig. 4 together with the data at ambient pressure [2, 4]. The onset of the broadening in Fig. 4 cor- responds to the AF transition temperature (TN ). The maximum spin moment is estimated to be 1.80 µB [4]. As seen in Fig. 4, TN is about 100 K at ambient pressure and decreases to 30 K at 3.7 GPa. The pressure-induced QCP is expected at a much higher pressure regime. ii. 1/T1T for 75As The relaxation rate divided by temperature 1/T1T provides a measure of low-energy spin fluctuations. In general, neglecting the wave-number (q) depen- dence of the hyperfine coupling constant, 1/T1T is proportional to the imaginary part of the suscepti- bility: 1/T1T ∝ ΣqImχ(q,ω)/ω where ω represents a NMR frequency. 75As is preferred to 1H for T1 measurements, because FeAs layers are hardly af- fected by the random distrubution of hydrogen in LaO1−xHx layers. Furthermore, owing to the nu- clear quadrupole interaction, one can pick up the 75As signals coming from the powders whose FeAs planes are parallel to the applied field. Figure 5 500 400 300 200 100 0 G a p ( K ) 543210 P (GPa) 5 6 7 8 0.1 2 3 4 5 6 7 8 1 2 3 4 5 1 / T 1 T ( s- 1 K -1 ) 20015010050 T (K) 0.1 MPa 3.0 GPa 3.7 GPa T N T N T N Figure 5: Relaxation rate of 75As divided by tempera- ture, 1/T1T for LaFeAsO0.4H0.6. TN represents the AF transition temperature. The inset shows the pressure dependence of the spin excitation gap ∆ (See Eq. (1)). shows 1/T1T for 75As, and the peaks correspond to TN . The values of TN determined from 1/T1T are consistent with those obtained from the linewidth of 1H. At low temperatures just below TN , 1/T1T is expressed as follows: 1 T1T ∝ e− ∆ T (1) where ∆ represent the spin excitation gap. The pressure dependence of ∆ is shown in the inset to Fig. 5. Assuming that ∆ shows the linear depen- dence, the pressure-induced QCP is estimated to be 4.1 GPa. IV. Discussion The activated spin excitation as shown in Eq. (1) originates from a spin density wave (SDW). How- ever, 1/T1T also shows Curie-Weiss behavior below 110002-3 Papers in Physics, vol. 11, art. 110002 (2019) / N. Fujiwara et al. TN . The behavior is not observed at ambient pres- sure and it is characteristic of the critical behav- ior near the pressure-induced QCP. The coexistence of the gapped and gapless excitations are specific to this system. In this system, major Fermi sur- faces are electron pockets with a square-like shape in two dimensional k space. Some parts of the elec- tron pockets would contribute to the nesting and the SDW formation. The critical behavior would originate from the other parts of the Fermi sur- faces. The nesting condition becomes worse and the bandwidth becomes broader with increasing pres- sure. Owing to these effects, the activated behavior shown in Eq. (1) would disappear at the pressure- induced QCP. V. Conclusions We performed NMR measurements on LaFeAsO0.4H0.6 at 3.7 GPa to investigate the magnetic properties in the vicinity of the pressure- induced QCP. We have found that the SDW ordered state still remains at 3.7 GPa. The pressure-induced QCP is estimated to be 4.1 GPa from the pressure dependence of the spin excitation gap. The gapless excitation observed as the Curie-Weiss behavior of 1/T1T coexists with the gapped excitation, implying that each excitation originates from different parts within the Fermi surfaces. Acknowledgements - This work is supported by JSPS KAKENHI Grant Number JP18H01181, and a grant from Mitsubishi Foundation. We thank H. Kontani and H. Takahashi for discussion. [1] S Iimura, S Matsuishi, H Sato, T Hanna, Y Muraba, S W Kim, J E Kim, M Takata, H Hosono, Two-dome structure in electron-doped iron arsenide superconductors, Nat. Commun. 63, 943 (2012). [2] N Fujiwara, S Tsutsumi, S Iimura, S Mat- suishi, H Hosono, Y Yamakawa, H Kon- tani, Detection of antiferromagnetic order- ing in heavily doped LaFeAsO1−xHx pnic- tide superconductors using nuclear-magnetic- resonance techniques, Phys. Rev. Lett. 111, 097002 (2013). [3] M Hiraishi, S Iimura, K M Kojima, J Ya- maura, H Hiraka, K Ikeda, P Miao, Y Ishikawa, S Torii, M Miyazaki, I Yamauchi, A Koda, K Ishii, M Yoshida, J Mizuki, R Kadono, R Kumai, T Kamiyama, T Otomo, Y Murakami, S Matsuishi, H Hosono, Intro- duction to solid state physics, Nat. Phys. 10, 300 (2014). [4] R Sakurai, N Fujiwara, N Kawaguchi, Y Ya- makawa, H Kontani, S Iimura, S Matsuishi, H Hosono, Quantum critical behavior in heav- ily doped LaFeAsO1−xHx pnictide supercon- ductors analyzed using nuclear magnetic res- onance, Phys. Rev. B 91, 064509 (2015). [5] Y Yamakawa, S Onari, H Kontani, N Fujiwara, S Iimura, H Hosono, Phase diagram and su- perconducting states in LaFeAsO1−xHx based on the multiorbital extended Hubbard model, Phys. Rev. B 88, 041106(R) (2013). [6] S Iimura, S Matsuishi, M Miyakawa, T Taniguchi, K Suzuki, H Usui, K Kuroki, R Ka- jimoto, M Nakamura, Y Inamura, K Ikeuchi, S Ji, H Hosono, Switching of intra-orbital spin excitations in electron-doped iron pnictide su- perconductors, Phys. Rev. B 88, 060501(R) (2013). [7] N Fujiwara, N Kawaguchi, S Iimura, S Matsu- ishi, H Hosono, Quantum phase transition un- der pressure in a heavily hydrogen-doped iron- based superconductor LaFeAsO, Phys. Rev. B 96, 140507(R) (2017). [8] N Fujiwara, T Matsumoto, K K Nakazawa, A Hisada, Y Uwatoko, Fabrication and effi- ciency evaluation of a hybrid NiCrAl pressure cell up to 4 GPa, Rev. Sci. Instrum. 78, 073905 (2007). 110002-4