Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose FeGeTe, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin-orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.

In spintronics, two-dimensional van der Waals crystals constitute a most promising material class for long-distance spin transport or effective spin manipulation at room temperature. To realize all-vdW-material–based spintronic devices, however, vdW materials with itinerant ferromagnetism at room temperature are needed for spin current generation and thereby serve as an effective spin source. We report theoretical design and experimental realization of a iron-based vdW material, Fe4GeTe2, showing a nearly room temperature ferromagnetic order, together with a large magnetization and high conductivity. These properties are well retained even in cleaved crystals down to seven layers, with notable improvement in perpendicular magnetic anisotropy. Our findings highlight Fe4GeTe2 and its nanometer-thick crystals as a promising candidate for spin source operation at nearly room temperature and hold promise to further increase Tc in vdW ferromagnets by theory-guided material discovery.

Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.

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The temperature-dependent evolution of the Kondo lattice is a long-standing topic of theoretical and experimental investigation and yet it lacks a truly microscopic description of the relation of the basic f-c hybridization processes to the fundamental temperature scales of Kondo screening and Fermi-liquid lattice coherence. Here, the temperature dependence of f-c hybridized band dispersions and Fermi-energy f spectral weight in the Kondo lattice system CeCoIn5 is investigated using f-resonant angle-resolved photoemission spectroscopy (ARPES) with sufficient detail to allow direct comparison to first-principles dynamical mean-field theory (DMFT) calculations containing full realism of crystalline electric-field states. The ARPES results, for two orthogonal (001) and (100) cleaved surfaces and three different f-c hybridization configurations, with additional microscopic insight provided by DMFT, reveal f participation in the Fermi surface at temperatures much higher than the lattice coherence temperature, T*≈45 K, commonly believed to be the onset for such behavior. The DMFT results show the role of crystalline electric-field (CEF) splittings in this behavior and a T-dependent CEF degeneracy crossover below T* is specifically highlighted. A recent ARPES report of low T Luttinger theorem failure for CeCoIn5 is shown to be unjustified by current ARPES data and is not found in the theory.

Iron oxide is a key compound to understand the state of the deep Earth. It has been believed that previously known oxides such as FeO and Fe2O3 will be dominant at the mantle conditions. However, the recent observation of FeO2 shed another light to the composition of the deep lower mantle (DLM) [1] and thus understanding of the physical properties of FeO2 will be critical to model DLM. Here, we report the electronic structure and structural properties of FeO2 by using density functional theory (DFT) and dynamic mean field theory (DMFT). The crystal structure of FeO2 is composed of Fe 2+ and O 2− 2dimers, where the Fe ions are surrounded by the octahedral O atoms. We found that the bond length of O2 dimer, which is very sensitive to the change of Coulomb interaction U of Fe 3d orbitals, plays an important role in determining the electronic structures. The band structures of DFT+DMFT show that the metal-insulator transition is driven by the change of U and pressure. We suggest that the correlation effect should be considered to correctly describe the physical properties of FeO2 compound. , are also discovered under the high pressure and temperature. Recently FeO 2 , which holds an excessive amount of oxygen, is identified with both first-principles calculation and experiment near 76 GPa [1]. This new iron oxide receives a great attention because it suggests an alternative scernario for describing geochemical anomalies in the lower mantle and the Great Oxidation Event. Thus, it is important to understand the correct electronic and structural properties of FeO 2 .FeO 2 possesses a FeS 2 -type pyrite structure. The crystal structure of FeX 2 (X = O or S) can be obtained by replacing X atom in B1 type FeX with X 2 dimer. FeO and FeS show a spin-state transition accompanied with Mott-type insulator to metal transition under high pressure [4][5][6][7][8]. However, FeS 2 is a non-magnetic compound where the six Fe d electrons occupy the t 2g ground states [8][9][10][11]. NiS 1−x Se x also has a same crystal structure with FeO 2 . It exhibits a complex phase diagram including MIT and magnetic phase transition depending on composition x, temperature, and pressure due to partially filled e g orbital [12,13]. Several previous studies have reported that the p orbitals of S 2 dimer play an important role in describing electronic structures of this compounds [12,13]. So we can expect that O 2 dimer may also be an driving factor for determining electronic and physical properties of FeO 2 .It is well known that standard density functional theory (DFT) fails to reproduce the physical properties and the electronic structures of many TMO compounds because electron correlation effect of d orbitals cannot be described properly. Alternatively, DFT+U which includes the correlation effect of localized orbitals such as 3d gives better results for structural properties, magnetic moments, and electronic structures. Dynamic Mean Field Theory (DMFT) has been believed to be a more advanced technique which deals with local electronic correlatio...

H2O ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy, and first-principles simulations, to investigate the state of hydrogen in the pyrite type FeO2Hx (x ≤ 1) which is a potential H-bearing phase near the coremantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H + (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviors of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation, and hydrogen isotopic mixing in Earth's deep mantle.Hydrogen plays an important role in the deep interior of the Earth 1,2 , where its mobility and bonding properties are altered dramatically from localized to globally itinerant with increasing depth. At shallower depths, hydrogen bonds with oxygen, the most abundant element in Earth, to form hydroxyls which modulate the electrical 3,4 , thermal 5 , and elastic 6 properties of the host minerals, and dictate redox, melting, and isotope partitioning 7 . Properties of hydroxyl groups have been extensively studied during the past half century as a means to locate deep water reservoirs and to monitor water circulation for a broad range of applications in interpretation of large geophysical and geochemical features in depth [8][9][10] . Hydroxyl starts with an asymmetric configuration O-H⋯O in which the hydrogen atom between

Two-dimensional van der Waals (vdW) magnetic materials have emerged as possible candidates for future ultrathin spintronic devices, and finding a way to tune their physical properties is desirable for wider applications. Owing to the sensitivity and tunability of the physical properties to the variation of interatomic separations, this class of materials is attractive to explore under pressure. Here, we present the observation of direct to indirect band gap crossover and an insulator-metal transition in the vdW antiferromagnetic insulator CrPS 4 under pressure through in-situ photoluminescence, optical absorption, and resistivity measurements. Raman spectroscopy experiments revealed no changes in the spectral feature during the band gap crossover whereas the insulator-metal transition is possibly driven by the formation of the high-pressure crystal structure. Theoretical calculations suggest that the band gap crossover is driven by the shrinkage and rearrangement of the CrS 6 octahedra under pressure. Such high tunability under pressure demonstrates an interesting interplay between structural, optical and magnetic degrees of freedom in CrPS 4 , and provides further opportunity for the development of devices based on tunable properties of 2D vdW magnetic materials.