Two-Stage Crossover of Resistivity with Temperature in a Highly Conductive MBene Hf2B2

Abstract

As a class of two-dimensional materials, MBenes have attracted increasing interest due to their high electrical conductivity. Using first-principles calculations combined with the Boltzmann transport equation, we investigate the intrinsic charge transport in orthorhombic 5d-based Hf₂B₂. The room-temperature 2D conductivity reaches 7.41 mS/□, comparable to most known MBenes and MXenes. At 20 K and the high-temperature limit (more than 1500 K), the resistivity follows T^3.7 and T¹ power laws, respectively. The local exponent n(T) = dln ρ_2D/d ln T reveals a two-stage crossover separated by a characteristic temperature T_M = 93 K: below T_M, n(T) decreases rapidly from 3.7 to about 1.5; above T_M, it slowly approaches 1. Our results demonstrate that the rapid decrease originates from the low-frequency phonons below the phonon gap, whereas the slow decrease arises from the additional activation of high-frequency phonons above the phonon gap.