Strain-induced quantum phase transition in the C₃Sc₄ monolayer: towards multiple gapless fermions

Most two dimensional (2D) topological materials host only one kind of fermionic state. However, realizing multiple gapless fermions in a single 2D material is rarely reported. Furthermore, researchers face challenges in regulating various gapless fermion transitions using specific methods. Herein, we perform a study based on the first-principles calculation to investigate the electronic structures and the related fermionic states of strained 2D C3Sc4. C3Sc4 is an ideal system in the ground state with twelve Dirac points. The dynamical, mechanical, and thermal stabilities of the proposed C3Sc4 monolayer are demonstrated in detail. Interestingly, under the condition of 9.5% biaxial tensile strain, gapless and quadratic Weyl fermionic states are observed at the Γ point. A gapless and massless pseudospin-1 fermion appears at the Γ point in the 2D C3Sc4 system under 13% biaxial tensile strain and with hole doping. The Fermi velocity of this massless pseudospin-1 fermion is 2.1 × 105 m s-1, comparable to that of well-known 2D gapless topological materials. The results indicate that 2D C3Sc4 is an ideal playground to explore interesting behaviors of quantum phase transitions and rich gapless fermionic states and also reveal its potential applications in high-speed nano-devices.