As-NMR/NQR Study of CaFe₂As₂under Pressure

Abstract

75 As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe 2 As 2 under pressure. At P = 4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T 1 ) measured at tetragonal phase show no coherence peak just below T c and decrease with decreasing temperature. The superconductivity is of gapless at P = 4.7 kbar but evolves to multiple gaps at P = 10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.KEYWORDS: CaFe 2 As 2 , superconductivity, quantum critical point, NMR/NQR, pressureSince the discovery of high-transition temperature (T c ) superconductivity (SC) in LaFeAsO 1−x F x at T c = 26 K, 1) iron pnictides have become one of the most fascinating research areas in condensed-matter physics. The carrier-doping suppresses structural and magnetic phase transitions in undoped ReFeAsO and SC appears.1) The high-T c SC has also been found in K-doped BaFe 2 As 2 2) and in LiFeAs. 3, 4) Previous nuclear-magnetic resonance (NMR)/nuclear-quadrupole resonance (NQR) measurements on ReFeAsOF, Ba 0.7 K 0.3 Fe 2 As 2 , and Li x FeAs consistently found multiple gap SC. [5][6][7][8] The multiple gap feature is believed to be relevant to their multiple electronic band structure. 9) On the other hand, the mechanism of the Cooper pair formation in iron pnictide is still under debate.The discoveries of pressure(P)-induced SC in RFe 2 As 2 (R = Ca, Sr, and Ba) provide a new route to investigate SC in iron pnictide.10-13) In RFe 2 As 2 , they also show structural transition from a tetragonal (tetra.) to orthorhombic (orth.) structure accompanied by antiferromagnetic order.14-16) In BaFe 2 As 2 and SrFe 2 As 2 , both phase transitions are suppressed by pressure, and SC was found around P c ∼ 50 kbar, with T c ∼ 30 K. 10,13) On the other hand, SC in CaFe 2 As 2 has been observed with lower P c ∼ 5 kbar and T c (P) ∼ 10K.11, 12) The most strikingly different feature in CaFe 2 As 2 is the occurrence of another structural phase transition under P. Above P ∼ 5 kbar, high temperature (T ) tetra. phase changes to a collapsed tetragonal (c-tetra.) phase with drastic reduction in both the unit cell volume (5%) and the c/a ratio (11%).17, 18) Notably, when tetra. phase collapses, P-induced SC disappears. 19)Here we report results of zero-field (ZF) NMR/NQR study in CaFe 2 As 2 under P. P-induced SC in the tetra. phase is successfully confirmed. The T dependences of nuclear spinlattice relaxation rate (1/T 1 ) show no coherence peak just below T c (P). Below T c (P), the T dependences of 1/T 1 indicate the unconventional SC. Present results indicate that electron correlations near a quantum critical point (QCP) play a vital role in inducing unconventional SC. * E-mail address: kawasaki@science.okay...

Stabilization of an ambient-pressure collapsed tetragonal phase in CaFe2As2and tuning of the orthorhombic-antiferromagnetic transition temperature by over 70

“…The peak position in the NQR spectrum for the as-grown sample is higher than the 25 and 30.4 MHz for the tetragonal and collapsed tetragonal phases respectively in CaFe 2 As 2 under high pressure. 26 The combination of no splitting of the NMR lines with the large shift in ν Q are further confirmation that, for the as-grown sample, there is only a structural phase transition without any magnetic phase transition.…”