Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe₄As₄

Abstract

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe 4 As 4 was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at Q res = 1.17(1) Å −1 , corresponding to the (π, π) nesting wave vector in tetragonal notation, evolves below T c . The characteristic energy of the spin resonance E res = 12.5 meV is smaller than twice the size of the superconducting gap (2∆). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe 4 As 4 is the s ± symmetry.Various iron-based superconducting materials with different crystal structures have been investigated to transcend our understanding of the nature of their pairing mechanism, [1][2][3][4] as the crystal structure is believed to have a strong relationship with the superconducting transition temperature T c . 5,6) Recently, new-structure-type iron-based superconducting materials AeAFe 4 As 4 (where Ae = Ca, Sr, or Eu and A = K, Rb, or Cs) were reported. 7-9)The AeAFe 4 As 4 family exhibits T c = 32-37 K, which is relatively high compared with other stoichiometric iron-based superconductors. [10][11][12][13][14] As Ae and A layers stack alternatively between Fe 2 As 2 layers owing to the large difference between their ionic radii, AeAFe 4 As 4 belongs to the P4/mmm space group, which is different from I4/mmm in hole-doped (Ba 1−x K x )Fe 2 As 2 systems. 7, 15) AeAFe 4 As 4 has high T c at stochiometric composition and a unique crystal structure, providing us with a great opportunity to theoretically and experimentally investigate iron-based superconductivity in the absence of substitutional disorder.CaKFe 4 As 4 undergoes superconductivity below T c ≃ 35 K without other phase transitions for 1.8 ≤ T ≤ 300 K. [15][16][17] The magnetic ground state of the normal state in CaKFe 4 As 4 is paramagnetic. 16,18) Various measurements 15,16,[18][19][20][21] indicated that CaKFe 4 As 4 shows behaviors similar to optimally or slightly overdoped (Ba 1−x K x )Fe 2 As 2 . Angle-resolved photoemission spectroscopy (ARPES) measurements supported by density functional theory calculations indicated that the Fermi surface consists of three-hole pockets at the Γ point and two electron pockets at the M point. 22,23) This multiband nature implies multi-gap superconductivity in CaKFe 4 As 4 . Two superconducting gaps which are nodeless and isotropic were, indeed, observed by scanning tunneling microscopy, 20) optical conductivity, 24) penetration depth, 21) muon spin rota- * k iida@cross.or.jp tion, 18) and 75 As nuclear magnetic resonance (NMR) 25) measurements. The largest superconducting gaps (∆ hole = 13 meV and ∆ electron = 12 meV) for quasi-two-dimensional hole and electron p...

Two-gap superconductivity with line nodes in CsCa2Fe4As4F2Kirschner1, Adroja2, Wang3 et al. 2018Phys. Rev. B402180View full textAdd to dashboardCite

“…No such drastic variation is seen in the 1144 family (CaAFe 4 As 4 , A = K, Rb, or Cs [24]) with evidence for multigap s-wave superconductivity and a clear absence of nodes in both experimental and theoretical studies [25][26][27][28][29][30]. Interestingly, this family has a maximum T c = 35 K for A = Rb, the alkali atom with intermediate size.…”

⁷⁵As NMR Studies of Magnetic Properties of the Magnetic Superconductor CaK(Fe_0.967Ni_0.033)₄As₄

“…The parent compound CaKFe 4 As 4 (x = 0) is a superconductor with the transition temperature T c = 35K [17] and a very high upper critical field about 92 T [17] with no other phase transition from 1.8 K to room temperature and shows physical properties similar to those of the optimally doped Ba 1-x K x Fe 2 As 2 . The multiband nature of the compound and s ± nodeless two-gap SC state has been revealed by such as nuclear magnetic resonance (NMR) [19], muon spin relaxation (SR) [20], scanning tunneling microscopy (STM) [21], high-resolution angle-resolved photoemission spectroscopy (ARPES) [22], and neutron scattering [23] measurements as well as density functional theory (DFT) calculations [22]. With Ni substitution for Fe in CaK(Fe 1-x Ni x ) 4 As 4 , T c decreases from 35 K at x = 0 to 10 K at x = 0.049, and the new hedgehog SVC magnetic state appears by x = 0.033 with a Neel temperature (T N ) of 45 K which increases to 52 K at x = 0.049 [16].…”

Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

“…The superconducting gaps are nearly isotropic and different for each of the Fermi surface sheets [4]. The presence of electron and hole sheets supports a spin resonance corresponding to the (π, π) nesting wave vector detected by neutron diffraction [6] and promotes a s ± superconducting pairing symmetry in CaKFe 4 As 4 . Thus, this clean system is a model system for understanding the effect of pairing on its upper critical field.…”

“…CaKFe 4 As 4 is a clean and stoichiometric superconductor with a relatively high T c = 35 K and it belongs to a new family of 1144 iron-based superconductors [5]. This system lacks long-ranged magnetic order or a nematic electronic state at low temperatures [5][6][7][8][9][10][11] but upon doping with Ni a hedgehog magnetic structure is stabilized [9,12,13]. CaKFe 4 As 4 has an exceptionally large critical current density due to the strong point-like defects caused by local structural site effects as well as surface pinning [14][15][16].…”