15:30-16:15

Bands of massless electrons and liquid lattice at the Pb/Si interface

In this talk, I will review noble electronic band structures which we found recently at the interfaces of monolayer Pb on the Si(111) surface. The Pb monolayer has attracted our interest due to its rich phase diagram, which includes a 2D liquid phase at high temperature, a metallic solid phase with various superstructures below room temperature and a superconducting phase at low temperature. The electronic band dispersions of the 2D solid and liquid phases were investigated by angle-resolved photoelectron spectroscopy. For the solid phase, we found a nearly massless electron bands for the surface state of Pb electrons and the Si interfacial (space charge) state [Kim and Yeom, PRL 104, 246803 (2010)].

When the Pb monolayer melts, its 2D band structure drastically changes into characteristic double ring-shape radial bands. These double radial bands could be interpreted as the free electron like band and a radially backscattered band from it. This evidences clearly the coherent radial scattering of electrons in a liquid state and discloses the well defined quasi band structures of a liquid [Kim and Yeom, PRL 107, 136402 (2011)].

Speaker

Prof. Han Woong Yeom, Center for Atomic Wires & Layers, Yonsei University, Korea

Chair

Dr. Tomonobu Nakayama, MANA Principal Investigator, NIMS

16:15-17:00

Quasicrystals: Novel templates for the growth of thin films with exotic- and nano-structures

The discovery of quasicrystals by Dan Shectman in 1982 [1] has opened a new field of research in condensed matter physics and material science. This discovery was awarded the Nobel Prize in chemistry in 2011 [2]. Quasicrystals are intermetallic compounds of specific stoichiometry exhibiting long range without periodicity and showing forbidden symmetry such as fivefold and tenfold rotational axes and they thus differ from conventional solids: crystals and amorphous. Using a recently available icosahedral Ag-In-Yb quasicrystal as a template [3-5], we have succeeded to grow thin films with exotic- and nano-structures such as quasicrystalline thin films of molecules like pentacene and of single elements like Pb, Sb and Bi, which are periodic by nature.

Scanning tunneling microscopy (STM) reveals that Pb grows quasiperiodically up to two atomic layers. Higher coverage yields fivefold-twinned hexagonal Pb islands with (111) surface orientation, as revealed by low energy electron diffraction (LEED). The islands are of specific heights (magic heights), which correspond to the stacking of four-six atomic layers of the bulk Pb, possibly due to quantum confinement of electrons in the islands. Sb yields quasicrystalline monolayer and partial second layer which is also in quasicrystalline order. At high coverage, a disorder film is produced. Bi forms quasicrystalline monolayer, structure of which is slightly different from that of the Pb and Sb film. The structure of all three films can be described in terms of substrate structure and is in good agreement with first-principles calculations.

The finding of quasicrystalline structures of single elements has opened an avenue to study the impact of atomic ordering on the physical properties of a material.

Reference

[1] Shechtman et al., Physical Review Letters 53 (1984) 1951.

[2] Nobelprize.org. http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2011/

[3] Sharma et al., Physical Review B 80 (2009) 12140 (R).

[4] Sharma et al., Physical Review B 81 (2010) 104205.

[5] Nugent et al., Physical Review B 82 (2010) 14201.

Speaker

Prof. Hem Raj Sharma, Department of Physics, the University of Liverpool, UK

Chair

Dr. Jonathan Hill, MANA Scientist, NIMS