Upper critical field ofNaFe1−xCoxAs superconductors

Abstract

The upper critical field of NaFe 1−x Co x As was measured from x = 0 to x = 0.08, with the magnetic field applied parallel (H c2 ) and normal (H ⊥ c2 ) to the planes. The data were fitted using one-band and two-band models. The orbital and paramagnetic components of the upper critical field were extracted for H c2 , which we find to be strongly dominated by paramagnetic pair breaking. In the parent compound the paramagnetic limit is equal to the value expected by BCS theory. However, substitution of Fe by Co leads to an enhancement above the BCS limit by a factor of approximately 1.6. In the overdoped region, we observe a significant convex curvature in H ⊥ c2 at low temperatures, which we attribute to the two-band nature of the superconductivity and the asymmetry between the two bands. Additionally, we discuss the behavior of critical field anisotropy, coherence length ξ , and the penetration depth λ.

Two-band and pauli-limiting effects on the upper critical field of 112-type iron pnictide superconductors

“…While, the spin-paramagnetic effect may play an import role in pair breaking in some unconventional superconductors1012141516171819. In our case, we obtained  = 75 T for Ca 0.83 La 0.17 FeAs 2 and  = 71.4 T for Ca 0.8 La 0.2 Fe 0.98 Co 0.02 As 2 .…”

“…For 122-type and 111-type IBSs, μ 0 H c2,c ( T ) exhibits a linear increase down to the lowest temperature, and μ 0 H c2,ab ( T ) also shows a downturn curvature with decreasing temperature. These behaviors can also be interpreted using the two-band theory and spin-paramagnetic effect, respectively1314151617. Furthermore, spin-paramagnetic effect dominates in both of μ 0 H c2,ab ( T ) and μ 0 H c2,c ( T ) in the 11-type IBSs(e.g., Fe 1+y Te 1−x Se x )1819.…”

Robustness of superconductivity to structural disorder inSr0.3(NH2)y(NH3)1Foronda1, Ghannadzadeh2, Sedlmaier3 et al. 2015Phys. Rev. BSelf Cite9040View full textAdd to dashboardCite

“…8). We note that the curve exhibits a concave form that has been observed in multiband high-T c superconductors [22].…”

“…Fits are shown in Fig. 9 for various values of λ 11 , λ 22 , and w = λ 11 λ 22 − λ 12 λ 21 . The Gurevich model was successful in modeling the upward curvature of H c2 , which stems from the difference in diffusivities of the two bands, i.e., [30].…”

Phase separation, competition, and volume-fraction control inNaFe1−xCoxAs

“…2(d), we show both T on N and T on c as functions of field. T N shows a quite significant decrease with field, which can be fitted by the functional form 2 , producing an estimate of the critical field for T on N = 0 to occur at H c ≈ 19 ± 1 T. By contrast, the field-induced decrease of T on c is slower, consistent with the critical field for superconductivity being located at H c2 ≈ 50 T in this system 20 , and suggesting that antiferromagnetism is suppressed at far lower fields than superconductivity. We comment here that such strong field effects on T N are highly unusual in ironbased SC materials, where the in-plane magnetic interactions are normally many tens of meV, and we stress that this result is obtained only for our samples close to optimal doping.…”