Development of high energy-resolution inverse photoemission spectroscopy

为了理解固体的电子性质，研究电子的占据态和未占据态是非常重要的。光电子能谱（PES）和逆光电子能谱（IPES）是研究固体电子结构的两种互补的强有力的实验方法。PES直接观测费米能级以下的占据态，IPES观测费米能级以上的未占据态。能量分辨率为2毫电子伏已实现PES，因此，已获得详细的信息占据状态的固体。另一方面，由于IPES的能量分辨率约为200 meV，未占用状态的信息是相当有限的穷人的能量分辨率。高温超导铜酸盐的研究是固体物理领域中最具挑战性的问题之一。由于缺乏关于未占据态的信息，使得我们不能充分理解它们的电子性质。为了研究高温超导铜氧化物的未占据态，我们发展了高能量分辨的IPES技术。为了提高能量分辨率，我们提出了两种新的IPES装置：离面Eagle安装光学系统和入射电子与出射光子之间的色散匹配。通过安装衍射光栅、探测器和测试样品，完成了光学系统的离面Eagle安装。为了实现入射电子束和出射光子之间的色散匹配，我们制作了平行板电子源，并研究了是否能获得预期的电子束。由于平行板电子源不能向样品提供足够的电流来测量逆光电发射谱，IPES系统还没有完成。我们将继续努力建造IPES机器。

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.