Development of high energy-resolution inverse photoemission spectroscopy

为了了解固体的电子性质，研究电子的占据态和未占据态是非常重要的。光电发射光谱（PES）和逆光电发射光谱（IPES）是相互补充的，是研究固体电子结构的有力实验方法。PES直接观察费米能级以下的已占据态，而IPES观察费米能级以上的未占据态。PES的能量分辨率达到了2 meV，从而获得了固体占位态的详细信息。另一方面，由于IPES的能量分辨率约为200mev，由于能量分辨率较低，未占用状态的信息受到很大限制。对高tc铜酸盐的理解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，我们无法充分了解它们的电子特性。为了研究高tc铜酸盐的未占据态，我们开发了高能量分辨率IPES技术。为了提高能量分辨率，我们从光学系统的离平面Eagle安装和入射电子与出射光子的色散匹配两方面进行了改进。通过设置衍射光栅、探测器和测试样品，完成了光学系统的离平面老鹰安装。为了实现入射电子束与出射光子的色散匹配，我们制作了平行平板电子源，并对电子束是否达到预期效果进行了研究。由于平行板电子源不能在样品上提供足够的电流来测量逆光发射光谱，因此IPES系统尚未完成。我们将继续努力建设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.