Possible Superconductivity above 25 K in Single-Crystalline Co-Doped BaFe₂As₂

Abstract

We present the superconducting properties of single-crystalline Ba(Fe 0:9 Co 0:1 ) 2 As 2 by measuring its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal a fish-tail hysteresis loop at high temperatures and a relatively high critical current density above J c ¼ 10 5 A/cm 2 at low temperatures. The upper critical field determined by resistive transition is anisotropic with an anisotropic parameter of $3:5. Hall effect measurements indicate that Ba(Fe 0:9 Co 0:1 ) 2 As 2 is a multiband system and that the mobility of electrons is dominant. Magneto-optical imaging reveals a prominent Bean-like penetration of vortices, although there is a slight inhomogeneity in a sample. Moreover, we observe distinct superconductivity above 25 K, which leads us to speculate that a higher transition temperature can be realized by fine-tuning the Co-doping level.Since the discovery of the high-T c iron-based oxypnictide superconductor LaFeAsO 1Àx F x with T c $ 26 K, 1) other ironbased superconductors have been sought for to obtain a higher transition temperature. In rare-earth-substituted iron oxypnictides RFeAsO 1Àx F x (R = rare earth), transition temperature has been increased up to 55 K. 2) In these iron oxypnictides, superconductivity is induced by introducing electrons in the (FeAs) À layers by substituting F for O. Following these discoveries, oxygen-free iron-arsenide AFe 2 As 2 (A ¼ Ba, Sr, Ca) was discovered. These materials show superconductivity with the substitution of alkali metals, such as Na, K, and Cs, for A resulting in the introduction of holes in the (FeAs) À layers. 3-6) In hole-doped oxygen-free iron arsenides, transition temperature is increased to $38 K. 3,4) On the other hand, very recent studies revealed that the electron doping by the substitution of Co or Ni, which have one or two excess d electrons compared with Fe in conducting layers, induces superconductivity in oxygen-free iron arsenide. 7,8) In fact, nuclear magnetic resonance measurements have revealed that Co atoms donate electrons without generating localized moments. 9) Although the highest transition temperature in electrondoped BaFe 2 As 2 is reported to be $23 K, 10) which is lower than that in hole-doped BaFe 2 As 2 , the fact that the substitution of transition metals leads to the induction of superconductivity contrasts strongly with a drastic suppression of T c in cuprates. 11) However, detailed study of transition temperature as a function of Co-doping level is limited. 10) It is an open question whether the highest T c in electron-doped BaFe 2 As 2 can be increased further.In this study, we prepare a single-crystalline sample of Ba(Fe 0:9 Co 0:1 ) 2 As 2 and present its superconducting properties by studying its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. We address the possibility of further enhancing transition temperature by fine-tuning Co-doping level.Single-crystalline samples with a nominal comp...

Josephson effect between electron-doped and hole-doped iron pnictide single crystals

“…1͑a͔͒ showed that the superconducting transitions were very sharp and the T C s were approximately 17 K for SrFe 1 ͒ single crystals confirms that the transport is dominated by electrons ͑holes͒, which is consistent with the dopant type of the compound and in qualitative agreement with previous reports. 12,13 In both types of single crystals, all Hall resistivity curves display a strong linear dependence in magnetic fields up to 7 T. As an example, Fig. 1͑c͒ shows the Hall resistivity curves obtained at 35 K.…”

Transport and magnetic properties of Co-doped BaFe2As2epitaxial thin films grown on MgO substrate

“…The slopes of H c2 along the c-and ab-directions at T c are -28.2 kOe/K and -51.3 kOe/K, respectively. For comparison, the H c2 (t) in a Ba(Fe 0.93 Co 0.07 ) 2 As 2 single crystal (data taken from Nakajima et al [19]) is also shown in figure 1(b) (open symbols). The values of H c2 at T = 0 K estimated using the Werthamer-Helfand-Hohenberg formula, 0 0.69 |d d ⁄ | , were ~360 and 650 kOe along the c-and ab-directions, respectively.…”

Possible Sign-Reversings-Wave Superconductivity in Co-Doped BaFe2As2Proved by Thermal Transport Measurements

“…6 The substituted Co atoms donate extra electrons to FeAs layers as itinerant carriers without creating localized moments. 7 For thermal transport measurements, two different crystals with dimensions of 1 × 0.4 × 0.04 mm 3 were used.…”