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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 uniquephysical properties.Motivated by a recent work reporting a novel half auxeticeffect in monolayer PdB4 with a hypercoordinated structure, here, weextensively explore similar 2D transition metal boride structures MB4 with Mcovering 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 responseto an applied strain in the other direction, regardless of whether the strainis positive or negative.We show that this special mechanical character isintimately tied to the hypercoordinated structure with the M\c{opyright}B8structural motif. Furthermore, regarding electronic properties, monolayer RhB4is found to be the first example of an almost ideal 2D spin-orbit Dirac pointsemimetal.The low-energy band structure is clean, with a pair of fourfolddegenerate Dirac points robust under spin-orbit coupling located close to theFermi level. These Dirac points are enforced by the nonsymmorphic space groupsymmetry which is also determined by the lattice structure. Our work deepensthe fundamental understanding of structural-property relationship in reduceddimensions. The half auxeticity and the spin-orbit Dirac points will makemonolayer RhB4 a promising platform for nanomechanics and nanoelectronicsapplications.

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