This experimental research project uses the most advanced methods of synchrotron source high resolution angle resolved photoemission (ARPES) to probe specific aspects of the electronic structure of layered compounds in the near vicinity of the Fermi energy. The materials to be studied are various polytypes of group VB transition-metal dichalcogenides MX2, where M=Nb,Ta,V, and X=S,Se. These materials have strong quasi-two-dimensional (2D) character and relatively simple crystal structures, and yet exhibit very rich physical properties. A variety of charge density waves (CDW's) have been discovered in these materials. The mechanisms of the CDW's in quasi-2D and 1D systems are not well understood and are of current interest. The electronic structure near the Fermi energy plays a central role in the CDW formations and other physical properties. The energy bands of these materials near the Fermi energy involve metal d-electrons and are governed by strong electron-electron correlation. In this project, the detailed electronic structure near the Fermi energy, such as the band dispersions and the Fermi surface topologies in the normal state, and the momentum dependence of the energy gap in the CDW state, will be thoroughly investigated. These studies are timely because of the current interest in highly correlated electron systems, and the availability of ultra-high resolution (energy resolution better than 8 meV, angular resolution better than 0.5 degrees) ARPES facilities. Much of the work will be carried out at the NSF-supported Synchrotron Radiation Center at Stoughton, WI. This work is expected to advance fundamental understanding of the CDW mechanisms, and the electronic structure in high correlated electron systems. More broadly, the results from this basic investigation may include fundamental new phys ical effects and also may provide insight into electron behavior in other systems including semiconductor microelectronic devices. This highly interdisciplinary project involves graduate students who pursue thesis research at both laboratory locations, and receive excellent training beneficial to a future career in industry, government or academia. %%% This experimental research project uses the most advanced methods of surface physics to clarify the electronic band structure of a class of layered materials. These materials to be sutdied, including tantallum diselenide, are important because they are model systems of nearly-two-dimensional form, which exhibit electronic phase transitions to correlated electron states known as Charge Density Waves (CDW's). The research is intended to clarify why these phase transitions occur, as related to the electron bands in the compounds. The electron bands essentially define electrical properties of semiconductors and metals, including such properties as electrical resistance, the speed of carriers in an applied electric field (the mobility, which is very important in making transistors with excellent performance) and others. Electronic band structure in this project will be studied in detail using state-of-the-art photoemission techniques. In this experiment, monochromatic light of chosen wavelength and direction falls on the sample, and leads to the emission of specific electrons, which must have the right energy, speed, and direction in the material. These three properties of the emitted electrons are measured. Systematic measurements of this type are the best experimental method of determining the electron band structure, which as mentioned, governs the electrical properties. The most advanced experiments of this type are carried out a synchrotron light sources such as the NSF- supported Synchrotron Radiation Center at Stoughto n, WI, where this project will be active. Synchrotron sources, which are related to high energy physics accelerators such as cyclotrons, provide extremely intense beams of light including light of very short wavelengths.The results from this basic investigation may include fundamental new physical effects and also may provide insight into electron behavior in other systems including semiconductor microelectronic devices. This highly interdisciplinary project involves graduate students who pursue thesis research at both laboratory locations, and receive excellent training beneficial to a future career in industry, government or academia. ***
