Hidden orbital polarization in diamond, silicon, germanium, gallium arsenide and layered materials

It was previously believed that the Bloch electronic states of non-magnetic materials with inversion symmetry cannot have finite spin polarizations. However, since the seminal work by Zhang et al. (Nat. Phys. 10, 387–393 (2014)) on local spin polarizations of Bloch states in non-magnetic, centrosymmetric materials, the scope of spintronics has been significantly broadened. Here, we show, using a framework that is universally applicable independent of whether hidden spin polarizations are small (e.g., diamond, Si, Ge and GaAs) or large (e.g., MoS2 and WSe2), that the corresponding quantity arising from orbital—instead of spin—degrees of freedom, the hidden orbital polarization is (i) much more abundant in nature since it exists even without spin–orbit coupling and (ii) more fundamental since the interband matrix elements of the site-dependent orbital angular momentum operator determine the hidden spin polarization. We predict that the hidden spin polarization of transition metal dichalcogenides is reduced significantly upon compression. We suggest experimental signatures of hidden orbital polarization from photoemission spectroscopies and demonstrate that the current-induced hidden orbital polarization may play a far more important role than its spin counterpart in antiferromagnetic information technology by calculating the current-driven antiferromagnetism in compressed silicon. Some non-magnetic substances may exhibit the useful properties of magnetic materials according to scientists in Korea. Spintronics, which uses the spin of an electron for information processing, promises a new generation of low power consumption devices. It has conventionally required magnetic materials, or a restricted class of non-magnetic materials with broken spatial-inversion symmetry. However, the number of material choices available for these applications was expanded with the recent discovery that some other non-magnetic substances also exhibit measurable spin-like effects. Now, Ji Hoon Ryoo and Cheol-Hwan Park from Seoul National University have demonstrated that the previously undiscovered magnetism in non-magnetic materials such as common semiconductor silicon and gallium arsenide arises from so-called hidden orbital polarization, namely coordinated orbital motion of electrons around different atoms, mimicking the spins that determine the characteristics of magnets. Recently, it was found that an electronic state can have hidden, site-dependent spin polarization even in non-magnetic materials with inversion symmetry. Our paper provides a new concept, so far not considered, of hidden orbital polarization and a proof that the hidden spin polarization is not the fundamental physical quantity but a secondary quantity that is completely derived from the hidden orbital polarization proposed in our paper. The hidden orbital polarizations can be important in many physical phenomena, including photoemission, current-induced magnetization, magnetic-field-induced currents, etc. The figure illustrates hidden orbital polarizations of Bloch states in diamond whose spin–orbit coupling is negligible (left) and the current-induced antiferromagnetic magnetization in silicon (right).