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Angle-resolved photoemission spectroscopy

Fig. 1. MBE+ARPES system setup.

Fig. 2. Full 3D band structure of 9 atomic layers of silver films grown on Ge(111) substrate measured using ARPES.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for studying the band structure and quasiparticle excitations in solids. This technique is based on the photoelectric effect, where the absorption of photons (typically UV light) causes emission of electrons in the material and allows intrinsic electronic states to be revealed (Fig. 1). Utilizing both energy and momentum conservation, ARPES is capable of directly measuring quasiparticle energy-momentum dispersions in solids, as well as their interaction with other collective excitations. The unique advantage of momentum-energy correspondence makes ARPES a very important tool for studying condensed matter system. An example of APRES spectra is shown in Figure 2, which shows the full 3D electron bands (as a function of electron energy E, in-plane wave vector kx, ky) in 9 atomic layers of silver films grown on a Ge(111) substrate. The circular contours in the Fermi surface are quantum well states formed by confining electrons within the silver films. Such discrete quantum well states, arising from quantization of continuous bulk bands, exhibit parabolic band dispersion, as illustrated in the E vs kx, ky plots. Upon close inspection, additional ripples can be observed near the surface Brillouin zone boundary (M point) of the substrate, indicating strong band folding and hybridization from the incommensurate interface potential from the Ge substrate. In fact, the band structure of such atomic layers shows incommensurate periodicity, akin to band structures in quasicrystals. Nowadays, APRES has become an indispensable tool to study various important topics in condensed matter physics, including high temperature superconductivity, strongly correlated electron systems, topological phases of matter, low dimensional electron systems, etc. At the center, we are particularly interested in combining advanced thin film growth techniques (molecular beam epitaxy) with APRES to create artificial low dimensional correlated electron systems and measure their intrinsic electronic structure. Our goal is to explore the effect of quantum confinement and interface coupling on the electronic correlations and unravel new quantum phases with exotic properties.