Superfluid density of Ba(Fe1-xMx)2<

Abstract

The temperature dependence of the ab-plane optical reflectivity of Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.95Ni0.05)2As2 single crystals is measured in a wide spectral range. Upon entering the superconducting regime, the reflectivity in both compounds increases considerably at low frequency, leading to a clear gap in the optical conductivity below 100 cm −1 . From the analysis of the complex conductivity spectra we obtain the penetration depth λ(T ) = (3500 ± 350)Å for Ba(Fe0.92Co0.08)2As2 and (3000 ± 300)Å for Ba(Fe0.95Ni0.05)2As2. The calculated superfluid density ρs of both compounds nicely fits Homes' scaling relation ρs = (125 ± 25)σ dc Tc. . Here we present a comprehensive optical study on optimal electron-doped Ba(Fe 0.92 Co 0.08 ) 2 As 2 (T c =25 K) and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 (T c =20 K) single crystals. The superconducting gap is clearly observed in reflectivity spectra for T < T c . The spectral weight analysis on optical conductivity provides information on the penetration depth and the superfluid density.Single crystals of Ba(Fe 0.92 Co 0.08 ) 2 As 2 and Ba(Fe 0.95 Ni 0.05 ) 2 As 2 were synthesized using selfflux method [3]. The platelets with a typical size of 2 mm × 2 mm × 0.1 mm have naturally flat and shiny surfaces. The resistivity and susceptibility evidence sharp superconducting transitions at 25 K and 20 K, indicating that samples are doped uniformly [4]. The temperature dependence of optical reflectivity was measured in a wide frequency range from 20 to 37 000 cm −1 using a coherentsource spectrometer in the THz range, infrared Fourier transform spectrometers (Bruker IFS 66v/s and IFS 113v) and a Woollam spectroscopic ellipsometer extending up to the ultraviolet. The lowfrequency extrapolation was done according to the dc conductivity measured on the same crystals by standard four-probe method. The optical conductivity was calculated from the reflectivity spectra using Kramers-Kronig analysis. Fig. 1 shows the temperature dependent reflectivity and conductivity of Ba(Fe 1−x M x ) 2 As 2 . The R(ω) spectra show a good metallic behavior above T c as the reflectivity goes towards unity at low frequencies and increases by cooling. Upon entering the superconducting regime, the reflectivity starts to increase rapidly with a change in curvature below 70 cm −1 and 60 cm −1 , respectively. This yields

FeTe0.55Se0.45: A multiband superconductor in the clean and dirty limit

“…In our previous study of this material, we noted that it fell on the general scaling line originally observed for the high-temperature superconductors [36,37], recently demonstrated for some of the iron-based materials [70], ρ s0 /8 4.4 σ dc T c , where σ dc is measured just above T c . A natural consequence of the BCS theory in the dirty limit is the emergence of a similar scaling line [37,71] ρ s0 /8…”

Looking at the superconducting gap of iron pnictides

“…(b) Reflectivity measurements performed by different groups yield a very similar behavior at low temperatures. The solid line was obtained by Wu et al[29,30] on Ba(Fe 0.92 Co 0.08 ) 2 As 2 at T = 9 K. Kim et al[31] investigated Ba(Fe 0.935 Co 0.065 ) 2 As 2 at T = 5 K (blue dashed curve). The green dotted curve corresponds to a measurement of van Heumen et al[32] on Ba(Fe 0.93 Co 0.07 ) 2 As 2 at T = 10 K. Nakajima et al[21] investigated Ba(Fe 0.94 Co 0.06 ) 2 As 2 (dot-dashed red line) and Ba(Fe 0.92 -Co 0.08 ) 2 As 2 (dot-dot-dashed orange line) single crystals with T c = 25 and 20 K, respectively.…”

“…The behavior of Ba(Fe 0.95 Ni 0.05 ) 2 As 2 is very similar to the optimally doped Co samples [29,30]. The reflectivity turns up around 60 cm −1 and approaches unity in way that the optical conductivity becomes gapped with 2∆ (1) 0 /hc = 35 cm −1 .…”

Do organic and other exotic superconductors fail universal scaling relations?

“…(1), to include the SC critical temperature T c , Only data points obtained from optical spectroscopies (IR and MW SI) are included in the plot. The data points are from: cuprates ab-plane [2,11,12], cuprates c-axis [2], pnictides [13,14], elements [2], TiN [15], Ba1−xKxBiO3 [16], MgB2 [17,18], organic SC [19][20][21], fullerenes [22], heavy fermion CeCoIn5 [23], negative-U induced SC TlxPb1−xTe [24] and Y2C2I2 [25].…”