  Tetrahex-carbon is a recently predicted two dimensional (2D) carbon allotrope
which is composed of tetragonal and hexagonal rings. Unlike flat graphene, this
new 2D carbon structure is buckled, possesses a direct band gap ~ 2.6 eV and
high carrier mobility with anisotropic feature. In this work, we employ
first-principles density-functional theory calculations to explore mechanical
properties of tetrahex-C under uniaxial tensile strain. We find that tetrahex-C
demonstrates ultrahigh ideal strength, outperforming both graphene and
penta-graphene. It shows superior ductility and sustains uniaxial tensile
strain up to 20% (16%) till phonon instability occurs, and the corresponding
maximal strength is 38.3 N/m (37.8 N/m) in the zigzag (armchair) direction. It
shows intrinsic negative Poisson's ratio. This exotic in-plane Poisson's ratio
takes place when axial strain reaches a threshold value of 7% (5%) in the
zigzag (armchair) direction. We also find that tetrahex-C holds a direct band
gap of 2.64 eV at the center of Brillouin zone. This direct-band-gap feature
maintains intact upon strain application with no direction-indirect gap
transition. The ultrahigh ideal strength, negative Poisson's ratio and
integrity of direct-gap under strain in tetrahex-C suggest it may have
potential applications in nanomechanics and nanoelectronics.
