A “Flat/Steep Band” Scenario in Momentum Space

Hydrogen-induced phase stability and phonon mediated-superconductivity in two-dimensional van der Waals Ti2C MXene monolayer

Herein, we report the phase stability of the hydrogenated Ti₂C MXene monolayer using an evolutionary algorithm based on density functional theory. We predict the existence of hexagonal Ti₂CH, Ti₂CH₂, and Ti₂CH₄. The dynamic and energetic stabilities of the predicted structures are verified through phonon dispersion and formation energy, respectively. The electron-phonon coupling is carefully investigated by employing isotropic Eliashberg theory. The T_c values are 0.2 K, 2.3 K, and 9.0 K for Ti₂CH, Ti₂CH₂, and Ti₂CH₄, respectively. The translation and libration adopted by stretch and bent vibrations contribute to the increasing T_c of Ti₂CH₄. The high-frequency hydrogen modes contribute to the critical temperature increase. Briefly, this work not only highlights the effect of H-content on the increments of T_c for Ti₂CH_x, but also demonstrates the first theoretical evidence of the existence of H-rich MXene in the example of Ti₂CH₄. Therefore, it potentially provides a guideline for developing hydrogenated 2D superconductive applications.

Stabilizing superconductivity of ternary metal pentahydride [Formula: see text] via electronic topological transitions under high pressure from first principles evolutionary algorithm

We explored the phase stability of ternary pentahydride [Formula: see text] based on the first principles evolutionary algorithm. Here, we successfully search for a candidate structure up to 500 GPa. As a consequence, the possible stable structure of [Formula: see text] is found be to a monoclinic structure with space group Pm at a pressure of 50 GPa. Moreover, the orthorhombic structure with a space group of Cmcm is found to be thermodynamically stable above 316 GPa. With this, the Kohn-Sham equation plays a crucial role in determining the structural stability and the electronic structure. Therefore, its structural stability is discussed in term of electronic band structure, Fermi surface topology, and dynamic stability. With these results, we propose that the superconducting transition temperature ([Formula: see text]) of Cmcm structure is estimated to be 50 K at 450 GPa. This could be implied that the proposed Cmcm structure may be emerging as a new class of superconductive ternary metal pentahydride. Our findings pave the way for further studies on an experimental observation that can be synthesized at high pressure.

A chemical perspective on high pressure crystal structures and properties

The general availability of third generation synchrotron sources has ushered in a new era of high pressure research. The crystal structure of materials under compression can now be determined by X-ray diffraction using powder samples and, more recently, from multi-nano single crystal diffraction. Concurrently, these experimental advancements are accompanied by a rapid increase in computational capacity and capability, enabling the application of sophisticated quantum calculations to explore a variety of material properties. One of the early surprises is the finding that simple metallic elements do not conform to the general expectation of adopting 3D close-pack structures at high pressure. Instead, many novel open structures have been identified with no known analogues at ambient pressure. The occurrence of these structural types appears to be random with no rules governing their formation. The adoption of an open structure at high pressure suggested the presence of directional bonds. Therefore, a localized atomic hybrid orbital description of the chemical bonding may be appropriate. Here, the theoretical foundation and experimental evidence supporting this approach to the elucidation of the high pressure crystal structures of group I and II elements and polyhydrides are reviewed. It is desirable and advantageous to extend and apply established chemical principles to the study of the chemistry and chemical bonding of materials at high pressure.

“Flat/steep band model” for superconductors containing Bi square nets

The crystal structures of a new family of superconductors containing a Bi square net and their electronic structures around the Fermi level have been reviewed. The structures of these compounds can be viewed as stacked layers denoted by [Bi][(RE)(M 2Bi2)(RE)] RE = rare earth or alkaline earth metal, M = transition metal. Flat/steep band features are shown to exist in all these new superconductors, though the pairing mechanisms may be very different. The Dirac Fermion behavior is reviewed and its implications are discussed.

Superconductivity Induced by Oxygen Doping in Y2 O2 Bi

When doped with oxygen, the layered Y₂ O₂ Bi phase becomes a superconductor. This finding raises questions about the sites for doped oxygen, the mechanism of superconductivity, and practical guidelines for discovering new superconductors. We probed these questions in terms of first-principles calculations for undoped and O-doped Y₂ O₂ Bi. The preferred sites for doped O atoms are the centers of Bi₄ squares in the Bi square net. Several Bi 6p x/y bands of Y₂ O₂ Bi are raised in energy by oxygen doping because the 2p x/y orbitals of the doped oxygen make antibonding possible with the 6p x/y orbitals of surrounding Bi atoms. Consequently, the condition necessary for the "flat/steep" band model for superconductivity is satisfied in O-doped Y₂ O₂ Bi.