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Normalized magnetization vs. temperature measured for Bi powder at various magnetic fields in the ZFC regimes, showing the occurrence of superconducting transition at Tc = 8.7 K. The right inset demonstrates the invisibility of the Meissner fraction verified in field cooled on cooling (FCC) measurements. The left inset gives the temperature dependence of the resistance R(T).
The low-field portions of M(H) isotherms obtained in ZFC regime after subtraction of the irrelevant here orbital diamagnetic signal. The magnetization values were multiplied by factors (in brackets) to better visualize the data. The lower critical field Hc 1(T) was determined as exemplified for M(H) obtained at T = 2 K (the straight line corresponds to the Meissner behavior).
(a) Magnetization hysteresis loop M(H) measured at T = 2 K; (b) the same M(H) after subtraction of the diamagnetic background magnetization M = −χH, where |χ| = 0.0151 mG/Oe. The results demonstrate that the measured Bi powder is a type-II superconductor with a strong vortex pinning.17
Lower critical field Hc 1(T) obtained from the data of Fig. 2. The line corresponds to the two-fluid-model equation Hc 1(T) = Hc 1(0)[1−(T/Tc )4] with Hc 1(0) = (129 ± 1) Oe, and Tc = (8.75 ± 0.05) K.
Reduced ZFC magnetization measured for virgin Bi powder and after the sample annealing at T = 300 °C during 24 h in Ar atmosphere.
False color optical image of Bi polycrystalline sample studied in Ref. 4 and the present work.
Four nonsuperconducting contacts (Ag) placed on the surface of Bi polycrystalline sample consisting of single crystalline blocks of size ∼1 × 1 mm (Fig. 6) in the plane perpendicular to the trigonal c axis.
Reduced resistance measured for bulk polycrystalline Bi sample (Fig. 7) at zero and various applied magnetic fields: four-probe R 1 = V 23/I 14 (a) and two-probe R 2 = V 14/I 14 (b) measurements. Arrow in (a) marks the zero-field superconducting transition temperature Tc (0) = (7.3 ± 0.1) K determined at the maximum of the derivative dR 1(T)/dT.
The upper critical field Hc 2(T) obtained from the data of Fig. 8a. The linear fit gives the slope dHc 2/dT ≅ 0.6 kOe/K.
Contact resistances Rc for six measured polycrystalline Bi samples. The sample with slightly higher Rc loses the superconductivity. For the definition of internal and external contacts see text and Fig. 7.
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