Zeeman-limited superconductivity in crystalline Al films

We report the evolution of the Zeeman-mediated superconducting phase diagram (PD) in ultrathin crystalline Al films. Parallel critical field measurements, down to 50 mK, were made across the superconducting tricritical point of films ranging in thickness from 7 to 30 ML. The resulting phase boundaries were compared with the quasiclassical theory of a Zeeman-mediated transition between a homogeneous BCS condensate and a spin-polarized Fermi liquid. Films thicker than $\sim 20\mathrm{ML}$ showed good agreement with theory, but thinner films exhibited an anomalous PD that cannot be reconciled within a homogeneous BCS framework.

Figure 1

(a) In situ STM image of a 10-ML-thick epitaxial Al film, which shows atomically flat plateaus interspersed with 1 ML-deep pits. (b) Profile trace along the white dash line which crosses over a pit. (c) AFM image of a 10-ML Al film capped by its native oxide.

Figure 2

The transition temperature of the epitaxial Al films used in this study as a function of film thickness and sheet resistance.

Figure 3

Hysteretic parallel critical field transition of a 17 ML epitaxial Al film at 90 mK. The arrows depict the magnetic field sweep direction. Inset: Perpendicular critical field transition of a 12-ML epitaxial Al film and a comparably thick quench-condensed Al film.

Figure 4

Zeeman-limited phase diagrams of epitaxial Al films of varying thickness. The symbols represent the superheating (upward triangles) and supercooling (downward triangles) critical fields as a function of reduced temperature. Note that the abscissa field scale is the same for each panel. The superconducting gap was determined from the transition temperature via the BCS relation ${\mathrm{\Delta }}_0=1.74k{T}_c$. The lines are the theoretic phase boundaries as obtained from QCST by varying $G^0,b$, and $c$. The best fit values of these parameters are listed in the panel legends. The tricritical point is defined by the temperature at which the parallel critical field transition becomes hysteretic.

Figure 5

Reduced tricritical point temperature as a function of film thickness. The triangles were obtained from the QCST fits and the circles from the critical field measurements. (Inset) The antisymmetric Fermi liquid parameter obtained from the QCST fits as a function $T_c$.