Monolayer RhB4: half-auxeticity and almost ideal spin-orbit Dirac point semimetal

TL;DR

This paper reports the discovery of monolayer RhB4 exhibiting half-auxetic behavior and hosting an almost ideal spin-orbit Dirac point semimetal state, revealing new insights into 2D materials' structure-property relationships.

Contribution

The study identifies monolayer RhB4 as a stable 2D material with unique half-auxeticity and a robust spin-orbit Dirac point, expanding the understanding of structure-property relationships in 2D materials.

Findings

Monolayer RhB4 exhibits half-auxeticity.

It hosts an almost ideal spin-orbit Dirac point.

The Dirac points are protected by nonsymmorphic symmetry.

Abstract

Structural-property relationship, the connection between materials' structures and their properties, is central to the materials research. Especially at reduced dimensions, novel structural motifs often generate unique physical properties.Motivated by a recent work reporting a novel half auxetic effect in monolayer PdB4 with a hypercoordinated structure, here, we extensively explore similar 2D transition metal boride structures MB4 with M covering 3d and 4d elements.Our investigation screens out one stable candidate, the monolayer RhB4. We find that monolayer RhB4 also shows half auxeticity, i.e., the material always expands in a lateral in-plane direction in response to an applied strain in the other direction, regardless of whether the strain is positive or negative.We show that this special mechanical character is intimately tied to the hypercoordinated structure with the M\c{opyright}B8 structural motif. Furthermore, regarding electronic properties, monolayer RhB4 is found to be the first example of an almost ideal 2D spin-orbit Dirac point semimetal.The low-energy band structure is clean, with a pair of fourfold degenerate Dirac points robust under spin-orbit coupling located close to the Fermi level. These Dirac points are enforced by the nonsymmorphic space group symmetry which is also determined by the lattice structure. Our work deepens the fundamental understanding of structural-property relationship in reduced dimensions. The half auxeticity and the spin-orbit Dirac points will make monolayer RhB4 a promising platform for nanomechanics and nanoelectronics applications.