New insights into unconventional superconductors Using an instrument to characterise the superconductivity of iron pnictides (BaFe2(As0.7P0.3)2), EU-funded scientists made important steps toward unravelling the mystery surrounding high-temperature superconductors. Industrial Technologies © Thinkstock Except for cuprates, iron pnictides are another known family of recently discovered high-temperature superconductors whose superconductivity origin is not yet clearly understood. Addressing the question of how superconductivity arises in these high-temperature iron-based superconductors is one of the most important issues in condensed matter physics. EU-funded scientists on the project LOWT-MFM-OF-TIS (Low temperature magnetic force microscopy study of topological insulators) used magnetic force microscopy (MFM) to measure an important length parameter in superconductivity. Known as the London penetration depth (λ), this quantity describes the depth to which an external magnetic field can penetrate the superconductor. In addition, it is an important parameter determining the critical temperature in high-temperature semiconductors. In this critical value, the metal electrical resistivity drops to zero, marking the metal transition to superconducting state. Previous research has unearthed a startling relationship linking λ with the doping level of BaFe2(As0.7P0.3)2. In particular, these superconductors were found to exhibit a peak in the penetration depth at optimal doping. Hitherto research has not pinpointed another peak in λ. Through use of MFM where a sharp magnetised tip scans the material sample, scientists demonstrated the existence of a peak at the optimal doping. Measurements also showed that the critical temperature dependence on λ is similar for different λ values close to the peak. Scientists also found an inverse relationship between λ and the critical temperature near the edge of the underdoped region. This finding was a clear indication of mixing between a spin density wave – low-energy ordered states of solids at low temperatures – and superconductivity. Magnetic imaging of superconducting vortices enabled scientists to observe twin boundaries inside the superconducting dome region. Such topological defects are believed to drive phase transitions and offered yet another indication of coexistence between spin density waves and superconductivity. LOWT-MFM-OF-TIS made groundbreaking progress in understanding the unconventional superconductivity in these promising high-temperature semiconductors that hold great potential for novel high-speed devices. Keywords Superconductors, iron pnictides, high-temperature superconductors, magnetic force microscopy, topological insulators
