Critical magnetic fields of ultrathin bismuth films

Critical transverse magnetic fields were measured in bismuth films 70 to 18 Å thick. It was found that for the temperature range (5–1.8°K) and fields investigated the relation HC(T) is linear in bismuth layers freshly condensed on a liquid-helium-cooled substrate. The slope dHc/dT increases as the film thickness decreases, attaining a value of 95,000 Oe/deg for the thinnest films. This increase is correlated with the increase in film resistivity, i.e., with the reduction in the mean free path of the conduction-band electrons. The value of the factor A obtained from the value of the critical magnetic field for a film about 20 Å thick (Hc = A·104Tc) is 9.5. Thus, in ultrathin bismuth films the critical magnetic field is much higher than the paramagnetic limit. This behavior is explained by the large spin-orbit scattering in freshly-deposited ultrathin bismuth films. When the thickness of the bismuth film decreases from 40 to 10 Å, the density of states falls off by approximately a factor of two.

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