Two-Dimensional Superconducting State of Monolayer Pb Films Grown on GaAs(110) in a Strong Parallel Magnetic Field

Abstract

Two-dimensional (2D) superconductivity was studied by magnetotransport measurements on single-atomic-layer Pb films on a cleaved GaAs(110) surface. The superconducting transition temperature shows only a weak dependence on the parallel magnetic field up to 14 T, which is higher than the Pauli paramagnetic limit. Furthermore, the perpendicular-magnetic-field dependence of the sheet resistance is almost independent of the presence of the parallel field component. These results are explained in terms of an inhomogeneous superconducting state predicted for 2D metals with a large Rashba spin splitting.PACS numbers: 73.20.At,Superconductivity in ultrathin films has been studied for a long time. In Ref.[1], superconductivity was observed even for a few-monolayer thickness in quenchcondensed films of Bi and Pb deposited on a glazed alumina substrate coated with amorphous Ge. Very recently, it has been revealed that superconductivity can occur in single atomic layers of Pb and In grown epitaxially on a Si(111) substrate [2,3]. A single-atomic-layer metal film on an insulating substrate is an interesting system for studies of superconductivity, not only because it is a complete two-dimensional (2D) system but also because of the broken spatial inversion symmetry. The asymmetry of the confining potential in the direction perpendicular to the 2D plane, combined with atomic spin-orbit coupling, is expected to cause the Rashba effect, which lifts the spin degeneracy of the 2D electronic states [4,5]. Actually, angle-resolved photoelectron spectroscopy measurements showed a large Rashba spin splitting of the order of 100 meV on the surfaces of heavy elements, such as Au [6], W [7], and Bi [8], and those of lighter elements, such as Si and Ge, covered with a monolayer of heavy elements, such as Bi [9][10][11].In this Letter, we report magnetotransport measurements on superconducting monolayer Pb films produced by quench condensation onto a cleaved GaAs(110) surface. Here, we focus on the effect of the magnetic field applied parallel to the surface. While the perpendicular component H ⊥ of the magnetic field strongly affects the orbital motion of electrons in the 2D plane, the parallel component H is expected to couple only to the spin degree of freedom. We show that the reduction of the superconducting transition temperature T c is very small even in strong parallel magnetic fields (H ⊥ = 0), which are much larger than the Pauli paramagnetic limit. Furthermore, the H ⊥ dependence of the sheet resistance at low temperature is found to be almost independent of the presence of H . These results are explained by assuming an inhomogeneous superconducting state predicted for Rashba spin-split 2D systems.In order to measure the sheet resistance R sq of ultrathin films, we apply the experimental procedure developed for studies on adsorbate-induced surface inversion layers on InAs [13][14][15] and InSb [16]. In this work, we used a nondoped insulating GaAs single-crystal substrate so as not to create conduction channels in the substrate...

Thickness-Dependent Oscillations of the Critical Magnetic Field Induced by the Quantum Size Effect in Superconducting Nanofilms

“…It is shown that the critical field oscillates as a function of the nanofilm thickness which is explained in terms of the electron energy quantization induced by the confinement in the direction perpendicular to the film. It is found that the zero-temperature critical field in the nanofilms tends to be higher than the ClogstonChandrasekhar paramagnetic limit which is in agreement with recent experiments [15]. The validity of the CC formula can be restored by the use of the thickness-dependent energy gap.…”

“…Such observation for the Pb nanofilms has been recently reported in Ref. [15] in which the experimentally measured critical field H C,|| has been much higher than the paramagnetic limit H CC . The discrepancy of the calculated critical field and H CC results from the deviation of the energy gapΔ from its bulk value.…”

Effect of Parallel Magnetic Field on Superconductivity of Ultrathin Metal Films Grown on a Cleaved GaAs Surface

“…14, R N is lower and T c0 is higher for Pb and Bi. 10) We attribute this to the atomically flat surface of the cleaved GaAs substrate. Films of In and Pb show superconductivity even in the monolayer regime (d 0.3 nm).…”

“…10) We used a nondoped insulating GaAs single-crystal substrate so as not to create conduction channels in the substrate. Current and voltage electrodes were prepared at room temperature by the deposition of gold films onto noncleaved surfaces.…”

“…Very recently, the present authors studied the effect of a parallel magnetic field H on the superconductivity of monolayer Pb films on GaAs(110). 10) Superconductivity was found to occur even for H = 14 T, which is much higher than the Pauli paramagnetic limiting field H P . 11,12) The observed weak H dependence of the superconducting transition temperature T c is well reproduced by…”