2020同步年報

Physics and Materials Science 009 Control of Nanoscale Transport with a Novel Size Effect From an emergence of quantization on a nanometer scale, the QSE allows flexible control of mat- ter and is a rich source of advanced functionalities. A complete picture of the QSE with a new class of size effects can dominate the nanoscale transport in systems with metallic surface states, typical- ly topological materials. Fig. 1 : Direct observation of a QSE-induced metal-insulator transition. (a,b) Schematic of the metal-insulator transition in a Bi nanofilm. (c) Bulk and sur- face Brillouin zone of Bi in direction [111]. (d) Bi bulk band structures calculated around the hole and electron pockets with a tight-binding meth- od. (e) Band structures obtained from a tight-binding calculation for a Bi(111) slab (14 BL). (f,g) Experimental Fermi surfaces and band structures measured on a Bi(111) film (14 BL) grown on a Ge(111) substrate. Shaded areas in (e) and solid curves in (g) show the calculated bulk projections. (h,i) Experimental band structures magnified inside dashed and solid boxes in (g), respectively, for each thickness. [Reproduced from Ref. 1] Q uantized electronic states generated by the quantum size effect (QSE) in nano-confined systems enable a unique tun- ability for a wide range of phenomena such as superconductivity, light-matter interaction and non-equilibrium carrier dynamics. Modulations of the bandgap and the density of states further improve functionalities in catalysts and information devices. A QSE-induced transition into an insulating phase in semi-metallic nanofilms was predicted for bismuth a half cen- tury ago and has reignited interest with regard to its surface states exhibiting non-trivial electronic topology. In the case of a system having metallic surface states as typically observed in topological materials, the transition is marked by the disappear- ance of conducting channels in the film interior; thereafter, electric current flows only through the surfaces. This transition was first predicted a half century ago for bismuth (Bi). A Bi single crystal is a typical semimetal with small carrier pockets and three-dimensional Dirac dispersions. Moreover, because of a large spin-orbit coupling, Bi surfaces host spin-polarized metallic states that have been intensively examined in the context of electronic topology. The QSE-driven metal-insulator transition in Bi nanofilms originally received great attention as a nanoscale path to achieve a substantial thermoelectric figure of merit, and is now of interest to enhance surface-state-induced exotic phenomena. Evidence of the metal-insulator transition in Bi films was obtained only in this decade through transport measurements on epitaxially grown samples. A recent angle- resolved photoemission spectroscopy (ARPES) measurement on Bi films furnishes a clear contrast to the transport results showing only the interior-insulating phase below a threshold thickness. Although this strange contradiction between metal- lic and insulating signatures observed in completely the same system implies the presence of an intriguing mechanism, essen- tial quantization information was lacking in previous experiments.