Year: 2026
Type: preprint
Abstract: Extreme stiffness contrasts between loading modes usually require architected metamaterials with compliant hinges, whereas covalent crystals rarely exhibit such behavior intrinsically. Here, density functional theory calculations of the complete in-plane elastic tensors of four tetragonal carbon monolayers reveal a more than 50-fold spread in shear stiffness, despite less than a twofold variation in axial stiffness and the same symmetry-allowed tensor form. Twin T-graphene shows the strongest contrast: its shear constant, $C_{66}=3.03$~N m$^{-1}$, is 47 times smaller than in graphene and yields a shear Kelvin eigenvalue 29 times below the two normal-strain eigenvalues, while the Born stability criteria hold. This shear constant revises an earlier inference of elastic isotropy based on normal strains alone. Constrained relaxations and symmetry-resolved mode analysis identify the mechanism as a $B_{2g}$ counter-torsion of the carbon sublayers about the mid-plane bonds, which removes 96.3% of the clamped-ion shear stiffness with bond-length changes below 0.15%. An internal-coordinate model quantifies this cancellation, identifying internal relaxation, rather than weak bonding, as the origin of the soft channel. The resulting anisotropy predicts transverse sound velocities of 1.95 km s$^{-1}$ along [100] and 10.62 km s$^{-1}$ along [110], a testable acoustic signature. Phonon calculations, ab initio molecular dynamics at 300 K, and large-deformation simulations indicate that the soft channel is dynamically stable, preserves the local bonding, and persists beyond the harmonic regime. Our results show how bonding topology enables extreme mode-selective elasticity through internal relaxation in a mechanically stable covalent crystal.
Source: arXiv (Cornell University)
Authors: González, J. W., Pacheco, M., Correa, J. D.
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7222326147