Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and As75 -NMR/NQR

Abstract

We present the results of 75 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 orbital-derived 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 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.Recent NMR measurements have pointed out a possible d-electron heavy fermion behavior in the AFe 2 As 2

Dominant in-plane symmetric elastoresistance in CsFe2As2

“…The inflection point of the resistance provides an estimate of the coherenceincoherence crossover temperature T * . In these samples, lower T * is associated with increases in both the Sommerfeld coefficient γ and ∂γ/∂p [19], and increased spinfluctuations [20,22,23]. As the resistance data in Fig.…”

“…The Sommerfeld coefficient, γ, which characterizes such correlations, amounts to 180 mJ/mol/K 2 in CsFe 2 As 2 , a value comparable to some heavy-fermion compounds. Furthermore, resistivity, magnetization, NMR and thermal expansivity measurements all found evidence for a crossover between a low-temperature heavy Fermi-liquid regime and a high temperature regime of reduced quasiparticle coherence [18][19][20][21][22][23][24]. These properties are signatures of heavy-Fermion behavior, which, uncommonly, emerges in these d-electron systems.…”

“…Since the elastoresistance coefficient m A1g measures the in-plane symmetric strain derivative of the resistance, their proportionality provides evidence that the resistance, being dominated by electronic scattering, is a good gauge for the electronic entropy in this system. Indeed, the resistance is known to be sensitive to the coherence-incoherence crossover in the AFe 2 As 2 (A = K, Rb, Cs) compounds [22,23]. The residual resistivity reaches very high values of near to 1000 in these compounds, due to the high scattering rate in the hightemperature incoherent state [18].…”

“…As the resistance data in Fig. Rb [22], Cs) crystals, as well as KFe2As2 under 5 GPa hydrostatic pressure [23,41] are also shown for comparison. Stars represent the inflection point of the resistance, which is a measure of the coherence-incoherence crossover temperature T * .…”

Pressure-induced magnetism in the iron-based superconductors AFe2As2 ( A=K , Cs, Rb)

“…Given the near degeneracy between different pairing states, it is conceivable that the pairing symmetry of certain Fe-SCs might be tuned by pressure [7][8][9][10][11][12][13]. Such an idea leads, in particular, to the observation of a V-shaped T -P phase diagram in AFe 2 As 2 (A = K, Cs, and Rb), where the transition temperature T c decreases initially as a function of pressure, then at a critical pressure P c , it suddenly changes direction and increases [7][8][9] or remains almost constant at least up to pressures ∼4 GPa [10,[13][14][15]. The constancy of the Hall coefficient through P c rules out a change in the Fermi surface [7,8,11].…”