Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and 75As-NMR/NQR

We present the results of 75 As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe 2 As 2 under pressure ( p ). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time ( T 1 ), and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature ( T ∗ ). T ∗ is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3 d orbital-derived bands with the itinerant electron bands. No anomaly in T ∗ is seen at the critical pressure p c = 1.8 GPa where a change of slope of the superconducting (SC) transition temperature T c ( p ) has been observed. In contrast, T c ( p ) seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR 1 / T 1 data, although such a correlation cannot be seen in the replacement effects of A in the A Fe 2 As 2 ( A = K , Rb, Cs) family. In the superconducting state, two T 1 components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short T 1 s indicates a nearly gapless state below T c . On the other hand, the temperature dependence of the long component 1 / T 1 L implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe 2 As 2 under pressure. Disciplines Condensed Matter Physics Authors Paul W. Wiecki, V. Taufour, D. Y. Chung, M. G. Kanatzidis, Sergey Bud’ko, Paul C. Canfield, and Yuji Furukawa This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/ameslab_manuscripts/108 PHYSICAL REVIEW B 97, 064509 (2018) Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and As-NMR/NQR P. Wiecki,1 V. Taufour,1,* D. Y. Chung,2 M. G. Kanatzidis,2,3 S. L. Bud’ko,1 P. C. Canfield,1 and Y. Furukawa1 1Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA 2Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 3Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA (Received 6 November 2017; revised manuscript received 5 February 2018; published 13 February 2018) We present the results of As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe2As2 under pressure (p). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time (T1), and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature (T ∗). T ∗ is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3d orbitalderived bands with the itinerant electron bands. No anomaly in T ∗ is seen at the critical pressure pc = 1.8 GPa where a change of slope of the superconducting (SC) transition temperature Tc(p) has been observed. In contrast, Tc(p) seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR 1/T1 data, although such a correlation cannot be seen in the replacement effects of A in the AFe2As2 (A = K, Rb, Cs) family. In the superconducting state, two T1 components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short T1s indicates a nearly gapless state below Tc. On the other hand, the temperature dependence of the long component 1/T1L implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe2As2 under pressure. DOI: 10.1103/PhysRevB.97.064509