Superconductivity in MgB₂ irradiated with energetic protons

Highlights

• Bulk polycrystalline MgB₂ irradiated with high energy protons.

• High resolution electron microscopy of proton irradiated MgB₂.

• DC susceptibility shows a decrease of Meissner fraction.

• The superconducting screening suggests a defect organization at high fluences

• High field critical current density and pinning energy substantially increase.

Abstract

A series of MgB₂ samples were irradiated with protons of 11.3 and 13.2 MeV. Magnetization data shows an insignificant reduction of the critical temperatures but a continuous decrease of the Meissner fraction with increasing fluence or energy. All samples show a consistent improvement of the critical current density compared to the virgin sample and an increase of the pinning energy at high fields as resulted from relaxation data.

Introduction

Current transport in superconductors is related to the formation of efficient pinning centers that should prevent the motion of flux lines. In the race for creation of artificial pinning centers, particle irradiation was successfully used, mainly in high temperature superconductors, MgB₂ included [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Particle irradiation creates vacancies and interstitials (Frenkel pairs-FP) in excess of equilibrium concentration both in the magnesium and boron sublattices. The FP have a permanent tendency toward recombination and/or aggregation and, eventually, creation of dislocation structures. That tendency is facilitated by the thermal energy which stimulates the migration of defects through the crystal. However, a rather high concentration of vacancies always exists in MgB₂ due to the facile loss of Mg during fabrication [16], [17]. They constitute strongly biased sinks for further absorption of other vacancies. These sinks and the uneven kinetic of interstitials and vacations favor the nucleation of the point defect loops and, further, the growth of dislocations and other defects generated by the release of the associated strains. Actually, only these extended defects are efficient pinning centers in the case of MgB₂ because it has a longer coherence length, ξ ∼ 10 nm. The isolated point defects, which are effective in cuprate superconductors, are practically inactive in this case. However, the defects, no matter the way they are created, have influence on the electronic properties of the material, hence, on the superconducting characteristics. That fact happens because the superconducting MgB₂ phase is close to the structural instability, which depends on electron concentration, a concentration which is easy altered by structural defects [17]. First, disorder reduces the electron mean free path, hence, decreases the coherence length and increases the upper critical field H_c2 [18]. Second, it couples the π-band and the σ-band by interband scattering which diminishes both the superconducting gaps [15] and critical temperature T_c [19].

These aspects were well investigated for neutron irradiated MgB₂ [1], [2], [3], [4], [5], [6] but there is a shortage of data regarding the effect of the damages created by protons irradiation. By our knowledge, the effect of proton irradiation on different superconducting properties were investigated only with protons of energy between 0.4 and 7 MeV [7], [8], [9], [20], [21], [22] and the results are somehow contradictory. For example, using protons up to 6 MeV and fluences below 10¹⁶ p cm⁻², Mezzeti et al. [8], claimed that effects on transport properties are almost negligible whereas significant effects were reported at low temperatures with protons of 2 [7] and 3 MeV [22] energy. However, the sample structures, which are different, put their fingerprint on the field and temperature dependence of the superconducting and transport characteristics.

This paper presents the effect of high energy protons, above 10 MeV, on the superconducting properties of MgB₂. In this way, we extend the range of proton energy which has not been explored yet. The reason is that, at higher energy, the distance traveled by protons, i.e., the range, increases, as well as the energy loss along the range. Therefore, the probability to find concentration of defects along the range, hence, in a certain way, correlated defects, is high though the energy loss is non uniform. Consequently, the longer the range, the higher the capability to fix a vortex line. However, there is a limitation in the energy due to the induced radioactivity. That process usually occurs for protons with energy much higher than 10 MeV but the problem is not critical for materials made of low Z elements like MgB₂ which have short half-lives in order of minutes or hours.