Magnetic-Field-Induced Superconductivity in Ultrathin Pb Films with Magnetic Impurities

Abstract

It is well known that external magnetic fields and magnetic moments of impurities both suppress superconductivity. Here, we demonstrate that their combined effect enhances the superconductivity of a few atomic layer thick Pb films grown on a cleaved GaAs(110) surface. A Ce-doped film, where superconductivity is totally suppressed at a zero field, actually turns superconducting when an external magnetic field is applied parallel to the conducting plane. For films with Mn adatoms, the screening of the magnetic moment by conduction electrons, i.e., the Kondo singlet formation, becomes important. We found that the degree of screening can be reduced by capping the Pb film with a Au layer, and observed the positive magnetic field dependence of the superconducting transition temperature.In 1914, Kamerlingh Onnes reported the destructive effect of a magnetic field on the superconductivity of Pb and Sn [1]. Today, it is conventional wisdom that external magnetic fields, as well as magnetic impurities, break the time-reversal symmetry of Cooper pairs and tend to suppress superconductivity. There are two mechanisms responsible for this: the orbital effect (OE) and the paramagnetic effect (PE) [2,3]. In the case of a superconductor containing magnetic elements, localized magnetic moments are also affected by the magnetic field. This should lead to an additional effect on superconductivity while it is usually obscured by OE and/or PE. For atomically thin or layered materials, OE can be eliminated by setting the magnetic field direction parallel to the conducting plane. Recently, it has been demonstrated that the reduction of the superconducting transition temperature T c due to the parallel magnetic field H is extremely small (∼ 1% for 10 T) in ultrathin Pb films grown on a cleaved GaAs(110) surface [4]. The suppression of PE was explained in terms of the spin precession due to the Rashba field [5,6], which allows nonmagnetic scattering by defects to mix the spinup state and spin-down state [7]. In this study, we employ ultrathin Pb films as the host superconductor to minimize OE and PE, which act directly on conduction electrons, and investigate the magnetic-field effect on superconductivity related to magnetic impurities. For films with Ce, we observe pronounced positive H dependence of T c . Furthermore, in a Pb-Ce(10 at%) alloy film, H actually induces a quantum phase transition from the normal state to the superconducting state. These results are consistent with a recent theory by Kharitonov and Feigelman [8]. In the case of deposition of 3d transition metals, the exchange coupling is strong and the localized moment of an adatom is expected to be screened by conduction electrons, forming the Kondo singlet state [9]. We find that the exchange coupling is weakened on a Pb film covered with a Au layer and observe the positive H dependence of T c for Mn deposition.The films were grown by vapor deposition onto a nondoped insulating GaAs single-crystal substrate, which was cooled down to liquid helium temperatures to avoid g...

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