Tricritical point in intrinsic magnetic topological insulator MnBi 4 Te 7

Abstract Tricriticality is a fundamental and attractive topic in condensed-matter physics, which reveals competition and balance among multiple phases and interactions. It has been verified that the tricritical point can be obtained in the tricritical Ising model. Here, we study the prototype of the metamagnetic tricriticality, MnBi$_4$Te$_7$. We demonstrate that the phase diagram can be modelled using a two-order-parameter $\phi^6$ theory. We obtain the tricritical point (12.5 K,600 Oe) and the critical exponents ($\beta_t = 0.26(7), \gamma_t = 0.94(9)$, and $\delta_t = 4.6(1)$) which are confirmed by the scaling equation and Widom scaling law. The critical exponents belong to the universality class of Landau $\phi^6$ mean field theory, which indicates the tricritical phenomenon at the boundary of the first-order and second-order phase transitions under an external magnetic field. As an intrinsic magnetic topological insulator, MnBi$_4$Te$_7$ features a tricritical point in its phase diagram and the MnBi$_{4-x}$Sb$_x$Te$_7$ family has been proposed as a key experimental relevance, new chiral universality, and space-time supersymmetry (SUSY).