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Easy vortex motion, the origin of which is intrinsic to the material, can be counteracted by correlated disorder in the form of columnar defects2. Such defects effectively localize vortices1 and expand the range of finite critical current density Jc (marked by the irreversibility line1 in the field-temperature (H-T) diagram)2,3. One possibly technologically relevant way to produce an effective columnar defect structure is by irradiation with energetic (˜0.2-0.8 GeV) protons, which will induce nuclear fission of sufficiently heavy nuclei4. The resulting fission fragments could in turn produce columnar damage tracks5, as has been shown for 20983Bi nuclei6. There is no directionality in the fission process, so the resulting tracks will be highly misaligned (splayed)6. The action of splay is to force the entanglement of vortices7, expected to reduce thermal activation (creep) of vortices out of columnar tracks and to enhance Jc even further.

In any superconductor, the irreversibility line in the H-T space, above which the vortex matter ‘liquefies’ and is very mobile1, is ultimately limited by the transition temperature Tc. So the question arising is whether the fission process can be realized in mercury/copper oxides — materials with the highest Tc found to date8,9. The answer will depend on three factors: the experimentally unknown fission cross-section σ for a 20080Hg fission process launched with fast protons, the as-yet unknown threshold of electronic energy loss rate for columnar track formation5 in mercury/copper oxides by the fission fragments, and the efficacy of splayed tracks in these cuprates.

We exposed polycrystalline samples of HgBa2CaCu2O6+δ(Hg-1212)9 to 0.8-GeV protons (fluence of 1.52x1017 cm-2) at Los Alamos National Laboratory (backed by the Electric Power Research Institute and NSF) using the proton beam from Los Alamos Meson Physics facility at the Weapons Neutron Research branch. The result was the first experimental record of a mercury fission process by which we could install splayed tracks in a Hg-1212 superconductor with transition temperature Tc of roughly 120 K. A cross-sectional transmission electron microscope image of Hg-1212 (Fig. 1a) illustrates the splayed columnar nature of the damage, typical of a fission process.

Figure 1: Transmission electron micrograph of Hg-1212 bulk superconductor prepared as in ref. 9, irradiated with 0.8-GeV protons.
figure 1

Fission track diameter, ˜80-90 Å; defect density, ˜5.9x1010 cm-2;σ≈110 mbarns; Hg density, ˜5.3x1021 cm-3; density of events, 8.9x1013 fissions cm-3. Mean track length implied is ˜6.6 μm, consistent with calculations5,6. Sketch illustrates the fission process. On entering a mercury nucleus, a 0.8-GeV proton collides with nucleons. The nucleus ‘boils off’ protons and neutrons, which carry off about 0.6 GeV energy. The remaining energy is split between the fission daughters. b, Persistent current density J against temperature for Hg-1212 before and after proton exposure, for three applied magnetic fields. Bulk J is above 107 A cm-2. After irradiation J becomes large above 100 K, well above the irreversibility line of unirradiated sample. c, Irreversibility lines of unirradiated and 0.8-GeV proton-irradiated Hg-1212 obtained from the onset of nonlinear current-voltage characteristics. Arrow, ˜25 K expansion at μ0H=2 T on irradiation. Inset, resistance R (normalized to R at Tc) before and after irradiation. In finite fields onset of linear resistance (dissipation) is shifted to higher temperatures and normalized R at any T is significantly reduced (arrow) by proton irradiation.

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The effect on current conduction is shown in Fig. 1b, c . Before irradiation the persistent current density J in high magnetic fields falls off rapidly with temperature — by three orders of magnitude at μ0H=2 tesla and T≈60 K. After irradiation, J is enhanced by orders of magnitude in fields of several tesla and the regime of finite J is extended to T>100 K (higher than in yttrium-, bismuth-, or thallium-based materials2,3). Even for the relatively low proton fluence used here, the large shift in the irreversibility temperature ΔTirr≈25 K is maintained in a field of up to 5.5 tesla (Fig. 1c).

The large shift in Tirr is possibly a result of the renormalization of splay angles1 by sizeable superconductive anisotropy of Hg-1212 (refs 8,9). A uniform splay distribution will not work in a less anisotropic material, such as yttrium-barium-copper oxides, where for splay angles greater than 10° the motion of vortices is enhanced10. Our results imply that the fission process can be extended to higher proton fluences and to mercury/copper oxides with Tc above 130 K, for which a wire and tape fabrication process has recently been developed11.