Intrinsic Charge Dynamics in High- Tc AFeAs(O,F) Superconductors

Abstract

We report the first determination of the in-plane complex optical conductivity of 1111 high-T_{c} superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at T_{c} and experiences an anomalous T-linear softening and narrowing below T^{*}≈110 K≫T_{c}. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2Δ/k_{B}T_{c}≈7 in both materials, indicating strong pairing.

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

“…A small 1 3.5 ± 0.5 meV gap was found not only to be very robust, but also to agree well with the results of microwave penetration depth and quasiparticle conductivity measurements [19]. Additional structures, in the form of conductance maxima or shoulders, could be interpreted as being due to one or possibly two larger gaps, whose amplitudes lie in the energy ranges 6.0-7.5 meV (in agreement with optical conductivity measurements [20]) and 8-10 meV. The later values are quite large and would correspond to gap ratios 2 /k B T c of the order of 9.…”

“…Based on the spectra shown in Fig. 2, and consistently with the results of penetration depth [19], critical field [19], infrared spectroscopy [20] and NMR [25] measurements, as well as with electronic band-structure calculations for 1111 compounds, from now on we will assume PrFeAs(O,F) to be a multiple-gap s±-wave superconductor. of temperature.…”

“…Additional features that can take the form of maxima, shoulders, or slope changes are present at about ±7.5 mV (dashed vertical lines). These two values are particularly interesting because a full SC gap with an amplitude 1.6k B T c = 3.5 meV was observed in underdoped PrFeAs(O,F) single crystals by microwave penetration depth and by quasiparticle conductivity measurements [19], while a gap of 3.5k B T c 7.5 meV was detected by optical conductivity measurements [20]. Other structures whose occurrence is apparently less systematic can be observed at higher energies (arrows).…”

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

“…A small ∆ 1 ≃ 3.5 ± 0.5 meV gap was found not only to be very robust, but also to agree well with the results of microwave penetration depth and quasiparticle conductivity measurements 19 . Additional structures, in the form of conductance maxima or shoulders, could be interpreted as being due to one or possibly two larger gaps, whose amplitudes lie in the energy ranges ≃ 6.0-7.5 meV (in agreement with optical conductivity measurements 20 ) and ≃ 8-10 meV. The latter values are quite large and would correspond to gap ratios 2∆/k B T c of the order of 9.…”

Reciprocity between local moments and collective magnetic excitations in the phase diagram of BaFe2(As1−xPx)2

“…3 The persistence of spin excitations along the superconducting dome was previously observed in both electron-and hole-doped FePns but with different effects on the energy of these modes. In hole-doped Ba 1−x K x Fe 2 As 2 , the spin excitations soften upon doping due to the increase of the electronic correlations as demonstrated by theoretical calculations and accurate measurements of the Sommerfeld constant [29][30][31] . In electron-doped FePns a different behaviour has been detected with the spin excitations being unaffected by doping in their bandwidth but with a decrease of spectral weight 29,32 .…”