Quantum Engineering of Metallic Thin Film Structures

STM image of a 2 ML Pb film epitaxially grown on Si(111) substrate The superconductivity is probed using tunneling spectroscopy to resolved the superconducting gap [Science 324, 1314 (2009)].

When an electronic system is confined in one or more dimensions to a length scale comparable to the de Broglie length, quantum confinement occurs.  Such confinements have played a central role in artificially engineered electronic systems such as quantum wells, wires, dots and related superlattices.  For semiconductors, quantum confinement can occur at length scales between 100 and 10 nm.  For metals, however, quantum confinement becomes important at length scales from a few nm down to the sub-nanometer regime.  It has been known for some time that quantum well states (QWS) can contribute to  interesting physical properties such as magnetism.  More recently, our group and others have discovered that QWS can profoundly influence the epitaxial growth of metallic films; this phenomenon has been dubbed “electronic” growth or “quantum” growth.   

Since making this initial discovery, our group has continued to explore the role of quantum confinement in the physical and chemical properties of ultra-thin metallic films.  Recent efforts have been focused on three areas: (a) quantum size effects on surface energy, which is directly relevant to quantum size effects on the growth of ultra-thin metallic films; (b) quantum size effects on work function, which has a strong bearing on surface chemistry; and (c) superconductivity, where the interplay of quantum confinement and superfluidity stiffness at the nanometer length scale give rise to intriguing phenomena. 

One example of this is the group’s recent observation of robust superconductivity in the extreme 2D limit.  Shown in the above figure is the STM image of a thin superconducting Pb film only two atomic layers thick, grown epitaxially on a Si substrate.  The film is atomically flat: the steps observed on the surface are due to the atomic steps of the substrate.  In such a two-atomic layer film, there exists only one transverse channel, due to the quantum confinement of the electronic system; thus, this system represents the ultimate two-dimensional limit of a thin film superconductor. Most surprisingly, its superconductivity is robust, with a Tc at 70% of its bulk value.  This is beyond the expectation of the conventional understanding of 2D superconductivity.

Shih’s group has recently pushed this line of research into creating nanoscale superconductor heterostructures, and has discovered that in some cases the well-established Ginzberg-Landau theory completely fails to describe induced superconductivity due to proximity effects.