Magnetic excitations in infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelates brings us tantalizingly close to a material class that mirrors the cuprate superconductors. We measured the magnetic excitations in these nickelates using resonant inelastic x-ray scattering at the Ni L3-edge. Undoped NdNiO2 possesses a branch of dispersive excitations with a bandwidth of approximately 200 milli–electron volts, which is reminiscent of the spin wave of strongly coupled, antiferromagnetically aligned spins on a square lattice. The substantial damping of these modes indicates the importance of coupling to rare-earth itinerant electrons. Upon doping, the spectral weight and energy decrease slightly, whereas the modes become overdamped. Our results highlight the role of Mottness in infinite-layer nickelates.

A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics

We use first-principles methods to study doped strong ferroelectrics (taking BaTiO3 as a prototype). Here, we find a strong coupling between itinerant electrons and soft polar phonons in doped BaTiO3, contrary to Anderson/Blount’s weakly coupled electron mechanism for "ferroelectric-like metals”. As a consequence, across a polar-to-centrosymmetric phase transition in doped BaTiO3, the total electron-phonon coupling is increased to about 0.6 around the critical concentration, which is sufficient to induce phonon-mediated superconductivity of about 2 K. Lowering the crystal symmetry of doped BaTiO3 by imposing epitaxial strain can further increase the superconducting temperature via a sizable coupling between itinerant electrons and acoustic phonons. Our work demonstrates a viable approach to modulating electron-phonon coupling and inducing phonon-mediated superconductivity in doped strong ferroelectrics and potentially in polar metals. Our results also show that the weakly coupled electron mechanism for "ferroelectric-like metals” is not necessarily present in doped strong ferroelectrics.

Imaging the coupling between itinerant electrons and localised moments in the centrosymmetric skyrmion magnet GdRu2Si2

Magnetic skyrmions were thought to be stabilised only in inversion-symmetry breaking structures, but skyrmion lattices were recently discovered in inversion symmetric Gd-based compounds, spurring questions of the stabilisation mechanism. A natural consequence of a recent theoretical proposal, a coupling between itinerant electrons and localised magnetic moments, is that the skyrmions are amenable to detection using even non-magnetic probes such as spectroscopic-imaging scanning tunnelling microscopy (SI-STM). Here SI-STM observations of GdRu2Si2 reveal patterns in the local density of states that indeed vary with the underlying magnetic structures. These patterns are qualitatively reproduced by model calculations which assume exchange coupling between itinerant electrons and localised moments. These findings provide a clue to understand the skyrmion formation mechanism in GdRu2Si2.

Influence of hydrogenation conditions on the structural, optical, and magnetic properties of degenerated Ni/Al co-doped anatase nanocomposite

In the present work, pristine and Ni2+/Al3+- codoped anatase (TiO2) nanocomposite powders were synthesized by the thermal co-precipitation method. The samples were characterized by several techniques. The X‐Ray diffraction (XRD) was used for structural characterization and the optical absorption spectroscopy was used for the optical characterization. The structural/optical studies proved the formation of a substitutional solid solution (SSS). The magnetization measurements were performed to investigate the magnetic properties of the synthesized samples. In the present work, nickel (Ni2+) dopant ions were used to introduce stable ferromagnetic (FM) properties into the synthesized anatase, while Al3+ dopant ions were utilized to supply itinerant electrons necessary to support and boost the created FM properties. The roadmap of the present work was to establish the hydrogenation conditions necessary to create ferromagnetic (FM) properties in the host codoped samples and study the effect of hydrogenation temperature on the parameters of the created FM properties, especially the magnetic energy (Umag) and saturation magnetization (Msat). It was found that the created FM properties in Ni/Al-doped TiO2 nanocomposite powder by using the hydrogenation effect could be enhanced and controlled via the temperature of the hydrogenation (TH). The experimental results revealed that Umag of TiO2:Ni:Al system increased by ~241% and the saturation magnetization by ~140% with increasing of TH by 100 oC (from 400 oC to 500oC). The obtained saturation magnetization (Msat) of 1.09 emu/g and magnetic energy (Umag) of 42.6 erg/g were higher than the previously obtained values for created Ni-doped TiO2 by ~50 times. Such great novel results were obtained due to dealing with two factors; the Al3+ ions as co-dopant, which can supply an excess of itinerant electrons that boost the S.S Heisenberg interactions in addition to choosing a suitable temperature of hydrogenation.

Optical Properties of Metals

Human beings have been using metals since the bronze age because of their unique optical, mechanical and physical properties, which originate from itinerant electrons. In this article, we introduce basic models to describe itinerant electrons in metals. We also introduce optical spectroscopy techniques and spectrum analysis methods that can be used to study the optical properties of metals. We hope that our article will be helpful for researchers using optical spectroscopy techniques in the field of metals and anyone who is interested in the optical properties of metals.