† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11674377, 11634015, and 11974405), the National Key R&D Program of China (Grant Nos. 2017YFA0302904 and 2016YFA0300502), and J. Y. also acknowledges support by the Youth Innovation Promotion Association of Chinese Academy of Sciences.
We report 75As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu2As 2, KCa2Fe4As4F2, and A2Cr3As3. In the superconducting state of LaRu2As2, a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/T1 just below Tc, which indicates that LaRu2As2 is a full-gap superperconducor. For KCa2Fe4As4F2, antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate
Transition metal arsenides (TMAs) belong to a big family. The binary TMAs, like TaAs, TaP, NbP, and XP2 (X = Mo, W),[1] are topological Weyl semimetals, whose low energy excitations in the bulk can be viewed as chiral massless Weyl Fermions. The ternary TMAs show rich novel properties, with examples including density wave[2] and superconductivity.[3] The discovery of superconductivity in transition metal arsenide LaFeAsO1–xFx opens a door to another high-temperature superconducting family besides cuprates.[4] More importantly, the physical properties of TMAs can be tuned by chemical substitution,[5,6] doping,[4] or pressure.[7] Therefore, transition-metal arsenides provide a rich material base for exploring exotic physical phenomena.
Among the TMA family, compounds with ThCr2Si2-type layered crystal structure have attracted much attention in condensed matter physics. High temperature superconductivity in AFe2As2 (A = Ca, Sr, Ba, etc.) was induced by doping[8] or pressure.[9] Iron and ruthenium are in the same group. Many Ru-based compounds show unconventional superconductivity.[10,11] Naturally, it is practical to look for unconventional superconductivity in ruthenium-based compounds with ThCr2Si2 structure. Recently, Guo et al. found that LaRu2As2 shows superconductivity with zero resistivity at 6.8 K.[12] LaRu2As2 and LaRu2P2 are isostructural and their physical properties have been studied by ab initio calculations,[13,14] which indicate that the conduction band electrons are mainly contributed from La-5d and Ru-4d orbitals.
KCa2Fe4As4F2 is a newly discovered superconductor with separated double Fe2As2 layers, whose Tc reaches as high as 33.5 K.[15] It can be regarded as the intergrowth of 1111-type CaFeAsF and 122-type KFe2As2.[15] The Fe valence in CaFeAsF and KFe2As2 is +2 and +2.5, respectively. Appointing the insulted compound CaFeAsF as the parent compound, KCa2Fe4As4F2 can be viewed as a self hole-doping system, which is consistent with the Hall effect measurements and electronic structures calculations.[15,16] In the superconducting state, the inverse square penetration depth (
A2Cr3As3 (A = Na, K, Rb, Cs) is the first chromium-based superconducting family under ambient pressure, with Tc ranging from 8.0 K to 2.2 K.[22–25] In the crystal structure, [Cr3As3]∞ chains are separated by the alkaline metal. Owing to the asymmetric distribution of the alkaline metal, there exists two types of As sites. Density function theory (DFT) calculations show that the Fermi surface is formed by one three-dimensional (3D) band γ and two quasi-one-dimensional (1D) bands α and β.[26,27] Experimental results point to unconventional superconductivity in A2Cr3As3.[28–34] In the normal state, ferromagnetic fluctuation (FM) was revealed[29] and further found to be enhanced by small radius alkaline metal replacement.[35] Similar to iron-pnictide superconductors, A2Cr3As3 shows a close relationship between magnetic fluctuations and superconductivity. More recently, nontrivial topological aspects in A2Cr3As3 have been pointed out.[35,36]
In this work, we perform nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3. We investigate the properties of LaRu2As2 and KCa2Fe4As4F2. For A2Cr3As3, we assign the one to one correspondence between the two NQR transition lines and the two As crystallographic sites, by combining with the first-principle calculation.
Polycrystalline LaRu2As2 and KCa2Fe4As4F2 samples were grown by conventional solid state reaction, as previously reported in Refs. [12,15]. The single crystals of A2Cr3As3 were synthesized by high-temperature solution method with A = Cs, Rb, K, Na0.75K0.25 or ion-exchanged reaction with A = Na, the details of synthesis can be found in Refs. [22–25]. The Tc of LaRu2As2 and KCa2Fe4As4F2 were determined by AC susceptibility using an in-situ NMR coil. The electric field gradient (EFG) of K2Cr3As3 was calculated by the all electron full-potential linear augmented plane wave (FLAPW) method implemented in Hiroshima Linear-Augmented-Plane-Wave (HiLAPW) code with generalized gradient approximation including spin–orbit coupling.[37] The NMR and NQR spectra were obtained by scanning RF frequency and integrating spin-echo intensity at a fixed magnetic field H0. The spin-lattice relaxation time T1 was measured by the saturation-recovery method. The T1 was obtained by fitting the nuclear magnetization M(t) to 1–M(t)/M0 = exp(–3t/T1) in NQR case and 1–M(t)/M0 = 0.1exp(–t/T1)+0.9exp(–6t/T1) in NMR case, where M(t) and M0 are the nuclear magnetization at time t after the single comb pulse and at thermal equilibrium, respectively.
Figure
Figure
![](cpb_29_6_067402/cpb_29_6_067402_ieqn3.gif)
Figure
![](cpb_29_6_067402/cpb_29_6_067402_ieqn4.gif)
Figure
Figure
After subtracting the second order perturbation effect according to Eq. (
To further study the nature of the AFM spin fluctuations, we compare the 1/T1 for H0 parallel to c direction and perpendicular to c direction. In this compound, the principle axis of EFG is along c direction. Therefore, T1 measured in the NMR central peaks with θ = 90° corresponds to
![](cpb_29_6_067402/cpb_29_6_067402_ieqn10.gif)
![](cpb_29_6_067402/cpb_29_6_067402_ieqn11.gif)
Finally, we discuss the property in the superconducting state. Figure
![](cpb_29_6_067402/cpb_29_6_067402_ieqn13.gif)
![](cpb_29_6_067402/cpb_29_6_067402_ieqn14.gif)
![](cpb_29_6_067402/cpb_29_6_067402_ieqn15.gif)
![](cpb_29_6_067402/cpb_29_6_067402_ieqn16.gif)
Next, we turn to A2Cr3As3 (A = Na, Na0.75K0.25, K, Rb, Cs). Figure
![]() | Fig. 9. (a) 75As NQR spectra of A2Cr3As3 (A = Na, Na0.75K0.25, K, Rb, Cs) measured at T = 150 K. The two peaks correspond to two As sites, namely, As1 site and As2 site. The data for the right peak of K2Cr3As3 was referred from Ref. [28]. (b) Surrounding environments of As1 site and As2 site, respectively. (c) Temperature dependence of νQ normalized by its value at 4.2 K. The original data of K2Cr3As3 and Cs2Cr3As3 were referred from Refs. [28,63]. |
![]() | Fig. 10. EFG principle axes (VZZ) for K2Cr3As3, which are shown by the blue and red sticks for As1 site and As2 site, respectively. |
![]() | Table 1. Comparison of experimental and theoretical results of νQ for K2Cr3As3. The experimental data were obtained at 300 K. In the EFG calculation, the lattice constant at 300 K which was referred from Ref. [22] was used. The units of Vαα = ∂V2/∂α2 (α = X, Y, Z) and νQ are 1019 V/m2 and MHz, respectively. . |
Before closing, we note that the three transition-metal arsenides reported here show quite different normal-state properties and the superconducting gap symmetry in these compounds is also different. LaRu2As2 shows superconductivity with a full gap, whose origin maybe electron–phonon coupling.[13,14] On the other hand, unconventional superconductivity is caused by AFM spin fluctuations in KCa2Fe4As4F2 and FM spin fluctuation in A2Cr3As3.[29,35] Future issues include clarifying whether spin triplet pairing is realized by the FM spin fluctuation in A2Cr3As3.
We have performed NMR and NQR measurements on three types of transition metal arsenides, LaRu2As2, KCa2Fe4As4F2, and A2Cr3As3. In LaRu2As2, different from Fe-based superconductors with the same crystal structure, a coherence peak in the temperature dependence of 1/T1 appears just below Tc, indicating that the superconducting gap is fully open. In double Fe2As2 layers compound KCa2Fe4As4F2, the strength of antiferromagnetic spin fluctuations is found to be similar to that in Ba0.45K0.55Fe2As2, indicating that the stoichiometric compound KCa2Fe4As4F2 is in the moderately hole-overdoped region. In fact, the anisotropy of 1/T1,
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