2002/07/10 by J. Karpinski, J. Karpiński, M. Angst +19 · 2 citations
Materials Science · Physics and Astronomy · #Boron and Carbon Nanomaterials Research #Metallurgical and Alloy Processes #Superconductivity in MgB2 and Alloys #cond-mat.supr-con
paper · pdf · doi:10.1088/0953-2048/16/2/317
published as Supercond. Sci. Technol. 16, 221 (2003) · 17 pages pdf only, 15 figures integrated (higher resolution photographs available on request); submitted to Supercond. Sci. Technol. (Proceedings of Boromag conference)
arxiv created 2002/07/10 · openalex publication_date 2003/01/03 · arxiv updated 2009/11/30 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/30
Single crystals of MgB 2 with a size up to 1.5 × 0.9 × 0.2 mm 3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB 9 . Single crystals of BN and MgB 2 grow simultaneously by a peritectic decomposition of MgNB 9 . Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ∼0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H // c c2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H // ab c2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H // ab c2 / H // c c2 was found to be temperature dependent (decreasing from γ ≅ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γ eff , as calculated from reversible torque data near T c , is field dependent (increasing roughly linearly from γ eff ≅ 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB 2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H // c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H // c c2 .