Abstract
Chromium dioxide CrO$_2$ belongs to a class of materials called ferromagnetic half-metals, whose peculiar aspect is to act as a metal in one spin orientation and as semiconductor or insulator in the opposite one. Despite numerous experimental and theoretical studies motivated by technologically important applications of this material in spintronics, its fundamental properties such as momentum resolved electron dispersions and Fermi surface have so far remained experimentally inaccessible due to metastability of its surface that instantly reduces to amorphous Cr$_2$O$_3$. In this work, we demonstrate that direct access to the native electronic structure of CrO$_2$ can be achieved with soft-X-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr$_2$O$_3$ layer. For the first time the electronic dispersions and Fermi surface of CrO$_2$ are measured, which are fundamental prerequisites to solve the long debate on the nature of electronic correlations in this material. Since density functional theory augmented by a relatively weak local Coulomb repulsion gives an exhaustive description of our spectroscopic data, we rule out strong-coupling theories of CrO$_2$. Crucial for the correct interpretation of our experimental data in terms of the valence band dispersions is the understanding of a non-trivial spectral response of CrO$_2$ caused by interference effects in the photoemission process originating from the non-symmorphic space group of the rutile crystal structure of CrO$_2$.
Highlights
Among the transition metal dioxides with rutile structure, chromium dioxide (CrO2) is the only one possessing a ferromagnetic conducting phase
We demonstrate that direct access to the native electronic structure of CrO2 can be achieved with soft-x-ray angle-resolved photoemission spectroscopy whose large probing depth penetrates through the Cr2O3 layer
Our study bears a number of new perspectives on the spectroscopic and theoretical side: (1) We have illustrated the spectroscopic power of SX-Angle-resolved photoemission spectroscopy (ARPES) whose enhanced probing depth gives us access to the k-resolved electronic structure of materials, where intrinsic perturbations of crystallinity or stoichiometry in the surface region hinder application of conventional VUV-ARPES
Summary
Among the transition metal dioxides with rutile structure, chromium dioxide (CrO2) is the only one possessing a ferromagnetic conducting phase. Its ground-state Fermi surface (FS) is composed of 100% spin-polarized electrons, resulting from the so-called “half-metallic” nature of CrO2. For almost 30 years, the half-metallicity has been correctly predicted within density functional theory (DFT) using the local spin-density approximation (LSDA) to electron exchange correlation [1,2]. A clear experimental demonstration was obtained by point-contact Andreev reflection, showing a spin polarization of the conductive electrons. The half-metallicity of CrO2 finds important practical application in spintronics. The exciting possibility to inject a spin-triplet supercurrent into CrO2 has been demonstrated [5], which sets up interesting connections between spintronics and superconductivity. One of the biggest limitations to fully exploit the device potential of the half-metallicity of CrO2 is its dramatic spin depolarization with temperature, which is considered as a property restricted to the ground state. Several depolarization mechanisms have been suggested [6,7], including those where electronic correlations might play an important role [7,8]
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