The temperature dependence of the upper, HC2, and lower, HC1, critical fields was deduced from Hall-probe magnetization measurements of high-quality MgB2 single crystals along the 2 main crystallographic directions. It was shown that γ(HC2) = HC2-ab/HC2-C and γ(HC1) = HC1-C/HC1-ab differed significantly at low temperatures (being approximately 5 and approximately 1, respectively) and had opposite temperature dependences. It was suggested that MgB2 could be described by a single field-dependent anisotropy parameter, γ(H) (= λC/λab = χab/χC), that increased from γ(HC1) at low field to γ(HC2) in high fields.

Anisotropies of the Lower and Upper Critical Fields in MgB2 Single Crystals. L.Lyard, P.Szabo, T.Klein, J.Marcus, C.Marcenat, K.H.Kim, B.W.Kang, H.S.Lee, S.I.Lee: Physical Review Letters, 2004, 92[5], 057001 (3pp)