Determining Hubbard U of VO_(2) by the quasi-harmonic approximation

Vanadium dioxide VO_(2) is a strongly correlated material that undergoes a metal-to-insulator transition around 340 K.In order to describe the electron correlation effects in VO_(2), the DFT+U method is commonly employed in calculations.However, the choice of the Hubbard U parameter has been a subject of debate and its value has been reported over a wide range. In this paper, taking focus on the phase transition behavior of VO_(2), the Hubbard U parameter for vanadium oxide is determined by using the quasi-harmonic approximation(QHA). First-principles calculations demonstrate that the phase transition temperature can be modulated by varying the U values. The phase transition temperature can be well reproduced by the calculations using the Perdew–Burke–Ernzerhof functional combined with the U parameter of 1.5eV. Additionally,the calculated band structure, insulating or metallic properties, and phonon dispersion with this U value are in line with experimental observations. By employing the QHA to determine the Hubbard U parameter, this study provides valuable insights into the phase transition behavior of VO_(2). The findings highlight the importance of electron correlation effects in accurately describing the properties of this material. The agreement between the calculated results and experimental observations further validates the chosen U value and supports the use of the DFT+U method in studying VO_(2).

Experimental realization of honeycomb borophene

We report the successful preparation of a purely honeycomb,graphene-like borophene,by using an Al(11 1) surface as the substrate and molecular beam epitaxy(MBE) growth in ultrahigh vacuum.Scanning tunneling microscopy(STM) images reveal perfect monolayer borophene with planar,non-buckled honeycomb lattice similar as graphene.Theoretical calculations show that the honeycomb borophene on Al(1 1 1) is energetically stable.Remarkably,nearly one electron charge is transferred to each boron atom from the Al(1 1 1) substrate and stabilizes the honeycomb borophene structure,in contrast to the negligible charge transfer in case of borophene/Ag(1 1 1).The existence of honeycomb 2 D allotrope is important to the basic understanding of boron chemistry,and it also provides an ideal platform for fabricating boron-based materials with intriguing electronic properties such as Dirac states.

Recent progress on borophene： Growth and structures

Boron is the neighbor of carbon on the periodic table and exhibits unusual physical characteristics derived from electron-deficient, highly delocalized covalent bonds. As the nearest neighbor of carbon, boron is in many ways similar to carbon, such as having a short covalent radius and the flexibility to adopt sp2 hybridization. Hence, boron could be capable of forming monolayer structural analogues of graphene. Although many theoretical papers have reported finding two-dimensional allotropes of boron, there had been no experimental evidence for such atom-thin boron nanostructures until 2016. Recently, the successful synthesis of single-layer boron （referred to as borophene） on the Ag（lll） substrate opens the era of boron n-nostructures. In this brief review, we will discuss the progress that has been made on borophene in terms of synthetic techniques, characterizations and the atomic models. However, borophene is just in infancy; more efforts are expected to be made in future on the controlled synthesis of quality samples and tailoring its physical properties.