We report a significant enhancement of the upper critical field H c2 of different MgB 2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of H c2 increase as ρ increases. One particular high resistivity film had zero-temperature H c2 (0) well above the H c2 values of competing non-cuprate superconductors such as Nb 3 Sn and Nb-Ti. Our high-field transport measurements give record values H c2 ⊥ (0) ≈ 34T and H c2 || (0) ≈ 49 T for high resistivity films and H c2 (0) ≈ 29 T for untextured bulk polycrystals. The highest H c2 film also exhibits a significant upward curvature of H c2 (T), and temperature dependence of the anisotropy parameter γ(T) = H c2 || / H c2⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(T c ) ≈ 2 to γ(0) ≈ 1.5. This remarkable H c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB 2 , which offers special opportunities for further H c2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of H c2 (T) observed suggest that MgB 2 can be made into a versatile, competitive high-field superconductor.

High-energy milling of magnesium diboride ͑MgB 2 ͒ prereacted powder renders the material largely amorphous through extreme mechanical deformation and is suitable for mechanically alloying MgB 2 with dopants including carbon. Bulk samples of milled carbon and MgB 2 powders subjected to hot isostatic pressing and Mg vapor annealing have achieved upper critical fields in excess of 32 T and critical current density approaching 10 6 A/cm 2 . 6 The measured H c2 in this case is the higher parallel critical field H c2 ʈ ͑0 K͒, rather than the lower, perpendicular critical field that controls the bulk J C . One immediate question is how thin films and bulk differ. A partial answer was given by Braccini et al.1 who noted that the c-axis parameter of the highest H c2 films was expanded relative to C-doped bulk samples, which have lower H c2 values.Several groups 7-10 have also produced wires containing SiC, the latter finding that H c2 could attain values as high as the CVD filaments of Wilke et al. 6 In fact there is increasing suspicion that one of the beneficial effects of SiC addition occurs by C doping of the MgB 2 . In general there is good agreement that C doping of MgB 2 is a useful means of alloying MgB 2 , even if the best bulk H c2 values are only about half those of the best thin films. The present Letter discusses the ex situ synthesis of alloyed MgB 2 powder using high energy ball milling of MgB 2 with C. Since a major goal of MgB 2 technology is the fabrication of high critical current density, multifilament wire suitable for magnet applications, we need a scalable bulk process capable of producing carbon-doped precursor powder. One such method is provided by this paper.The composition of carbon-doped MgB 2 is commonly given using the format Mg͑B 1−X C X ͒ 2 . Alfa-Aesar prereacted MgB 2 powder was mixed with powdered graphite in the following proportions: 0.1C + 0.9MgB 2 ͑nominal X = 0.0525͒ and 0.3C + 0.7MgB 2 ͑nominal X = 0.17͒. The mixtures were high energy ball milled in a Spex 8000M mixer/mill with a WC milling container and milling media. Powders were milled until no graphite peak was discerned in the x-ray diffraction ͑XRD͒ pattern, which took 10 h for X = 0.0525 and 25 h for X = 0.17. The milled powders were then cold isostatic pressed to form pellets that were welded into evacuated stainless steel tubes and hot isostatic pressed ͑HIP͒ at 1000°C and Ͼ30 ksi for 200 min. These HIP-treated pellets were cut apart and examined. They were black, lusterless, and lacked mechanical strength. These features were attributed to incomplete reaction due to a deficiency of Mg with respect to ͑B+C͒ resulting from adding C to the ex situ powder without adding Mg. Accordingly, portions of the HIP-treated pellets were welded into evacuated stainless steel tubing with a quantity of Mg metal that was about half the volume of the MgB 2 pellet. MgB 2 pellets and Mg metal were arranged in such a way that the sample would be exposed to Mg vapor, while limiting exposure to liquid Mg, as described by Braccini et al. 11 ...

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We have noticed the following misprints in our paper:2. Page 2, right column, 8th line from the bottom: it should be 2⍀ ± = + + ⌫ + ± ⍀ 0 instead of ⍀ ± = + + ⌫ + ± ⍀ 0 .

We have studied structural and superconducting properties of MgB 2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing metalorganic precursor bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, T c decreases, and the upper critical field increases dramatically as compared to clean films. The selffield J c in the carbon doped film is lower than that in the clean film, but J c remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB 2 materials for high magnetic-field applications. a)Electronic address: avp11@psu.edu