Prominent metallic surface conduction and the singular magnetic response of topological Dirac fermion in three-dimensional topological insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$

TL;DR

This study reveals the dominant surface conduction and unique magnetic response of topological Dirac fermions in a 3D topological insulator, highlighting a sharp resistivity drop and a characteristic paramagnetic peak linked to spin-momentum locking.

Contribution

It demonstrates the temperature-dependent crossover from bulk to surface conduction and observes a rare T^2 dependence in surface resistivity, along with a robust paramagnetic peak associated with topological surface states.

Findings

Surface conduction dominates below crossover temperature.

Surface resistivity follows a T^2 dependence at low temperatures.

Paramagnetic peak in susceptibility linked to spin-momentum locking.

Abstract

We report semiconductor to metal-like crossover in temperature dependence of resistivity ($\rho$) due to the switching of charge transport from bulk to surface channel in three-dimensional topological insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$. Unlike earlier studies, a much sharper drop in $\rho$($T$) is observed below the crossover temperature due to the dominant surface conduction. Remarkably, the resistivity of the conducting surface channel follows a rarely observable $T^2$ dependence at low temperature as predicted theoretically for a two-dimensional Fermi liquid system. The field dependence of magnetization shows a cusp-like paramagnetic peak in the susceptibility ($\chi$) at zero field over the diamagnetic background. The peak is found to be robust against temperature and decays linearly with field from its zero-field value. This unique behavior of $\chi$ is associated with the spin-momentum locked topological surface state of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$. The reconstruction of surface state with time is clearly reflected through the reduction of peak height with the age of the sample.