Method to Observe Anomaly of Magnetic Susceptibility for Quantum Spin Systems

We propose a new method for studying the anomaly of magnetic susceptibility $\chi$ that indicates a phase transition for quantum spin systems. In addition, we introduce the fourth derivative $A$ of the lowest energy eigenvalue per site with respect to magnetization, that is, the second derivative of $\chi^{-1}$. To verify the validity of this method, we apply it to an $S=1/2$ XXZ antiferromagnetic chain. The lowest energy of the chain is calculated by numerical diagonalization. As a result, the anomaly of $\chi$ and $A$ exists at zero magnetization. That of $A$ is easier to observe than that of $\chi$, which indicates that the observation of $A$ is a more efficient method to evaluate an anomaly than that of $\chi$. The observation of $A$ reveals an anomaly that shows the high order phase transition, namely, the fourth order phase transition. Our method is helpful for analyzing critical phenomena.