Development of high energy-resolution inverse photoemission spectroscopy

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It is highly important to study occupied and unoccupied states of electrons in order to understand electronic properties of solids. Photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) are complementary to each other and very powerful experimental methods to study the electronic structure of solids. While PES directly observes the occupied states below the Fermi level, IPES observes the unoccupied states above the Fermi level. Energy resolution of 2 meV has been achieved for PES and, consequently detailed information of occupied states of solids has been obtained. On the other hands, since the energy resolution of IPES is about 200 meV, information of unoccupied states is considerably limited by the poor energy resolution. Understanding of high-Tc cuprates is one of the most challenging problems in the field of solid state physics. The lacking of information about unoccupied states prevents us from the full understanding of their electronic properties. We have developed high energy-resolution IPES technique in order to study unoccupied states of high-Tc cuprates. We made a new IPES machine based on two ideas to improve the energy resolution : off-plane Eagle mounting of the optical system and dispersion matching between incoming electron and outgoing photon. The off-plane Eagle mounting of the optical system has been completed by setting the diffraction grating, the detector, and the test sample. In order to achieve the dispersion matching between the incoming electron beam and outgoing photon, we have made the parallel plate electron source and have investigated whether the electron beam is obtained as expected. Since the parallel plate electron source cannot supply enough current on the sample to measure inverse photoemission spectra, the IPES system is still not completed yet. We will continue the effort to construct the IPES machine.