NMR/NQR and Specific Heat Studies of Iron Pnictide Superconductor KFe₂As₂

Abstract

We report the 75 As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2 As 2 (superconducting transition temperature T c 3.5 K) using high quality single crystals. NMR Knight shifts in superconducting state clearly indicate that spin-singlet superconductivity realizes in this material. It is noted that the electronic specific heat coefficient of the material is 93.0 mJ/K 2 mol-fu and that the value is quite heavy for d electron system. KEYWORDS: iron pnictide superconductor, nuclear magnetic resonance, nuclear quadrupole resonance, specific heatMuch attention has been paid after the discovery of superconductivity in F-doped LaFeAsO with a superconducting transition temperature T c = 26 K.1) K-doped BaFe 2 As 2 is the firstly reported oxygen-free iron-pnictide superconductor with T c = 38 K.2) We report the 75 As nuclear magnetic resonance (NMR) measurement of this hole-doped superconductor Ba 1−x K x Fe 2 As 2 with different lattice parameters and different superconducting volume fractions (T c ∼ 38 K). 3)75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. In our samples synthesized by high pressure method, antiferromagnetic (AFM) ordered state remains up to x = 0.4. The internal magnetic field evaluated by zero-field 75 As-NMR is suppressed by the substitution x. However, it is also robust with x. This tendency is quite similar to the pressure dependence of the internal magnetic field in the parent material BaFe 2 As 2 . 4,5) This reflects the first-order nature of this structural phase transition. Hence, the suppression of the structural phase transition rather than carrier doping seems to have a key role in achieving superconductivity in the so-called 122 systems. Recent reports on the pressure-induced superconductivities of BaFe 2 As 2 (T c 13 K) 5) and SrFe 2 As 2 (T c 34 K) 6,7) under hydrostatic pressure and chemical-pressure-induced superconductivity of BaFe 2 (As 1−x P x ) 2 (T c 31 K) 8) are consistent with this notion.KFe 2 As 2 is an end and heavily hole-doped member of Ba 1−x K x Fe 2 As 2 and is superconducting below T c = 3.5 K. 2)We reported the 75 As-NQR and specific heat measurements of this compound using polycrystalline samples.9) The spinlattice relaxation rate 1/T 1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T c . The quasiparticle specific heat C QP /T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T c . The C QP /T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T = 0.4 K T c /10. The temperature dependences of 1/T 1 and C QP /T can be explained by a * E-mail address: hideto@nmr.s.chiba-u.ac.jp multiple nodal superconducting gap scenario rather than by a multiple fully gapped s ± -wave or s ++ -wave scenario determined using simple gap analysis. Recently high-quality single crys...

Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides

“…The decrease of χ(T ) for any orientation of the magnetic field is consistent with the Knight shift measurements in KFe 2 As 2 (Ref. [28]). Finally, from Eq.…”

Critical fields and vortex pinning in overdopedBa0.2K0.8Fe2As2

“…The evolution of pinning with doping mirrors the behavior previously reported in BaFe 2 (As 1-x P x ) 2 [37]. For both families of superconductors the parent compounds, KFe 2 As 2 and BaFe 2 P 2 , form crystals of very high purity displaying residual resistivity ratios of 600 [38] to 1280 [21], and ~85 [39], respectively. Thus, by increasing the doping level the purity increases and, at the time, bulk pinning rapidly decreases.…”

Hydrostatic and uniaxial pressure dependence of superconducting transition temperature of KFe2As2single crystals

“…These results are comparable to the literature data. [36][37][38] Low-field (H = 25 Oe) zero-field-cooled (ZFC) temperature-dependent magnetization data taken under pressures up to ≈11.7 kbar are shown in Fig. 2.…”