Physical properties of new delafossite triangular-lattice compounds TlErSe$_2$ and TlTmSe$_2$

TL;DR

This study investigates the structural and magnetic properties of two new delafossite compounds, TlErSe$_2$ and TlTmSe$_2$, revealing long-range magnetic order in one and none in the other, highlighting their potential for exotic quantum states.

Contribution

The paper reports the synthesis, structural characterization, and magnetic behavior analysis of TlErSe$_2$ and TlTmSe$_2$, identifying their phase purity and contrasting magnetic properties, which is novel for these compounds.

Findings

TlErSe$_2$ exhibits long-range magnetic order below 0.42 K.

TlTmSe$_2$ shows no long-range magnetic order.

Distinct magnetic behaviors are explained by ab initio calculations.

Abstract

Delafossite compounds containing rare-earth ions have been proven to be an ideal platform to investigate frustrated magnetic ground states. Here, we discuss two triangular-lattice antiferromagnets, TlErSe$_2$ and TlTmSe$_2$, as potential candidates for hosting exotic quantum states. Powder X-ray diffraction data analysis of the black-color polycrystalline Tl$RE$Se$_2$ ($RE$: Er and Tm) samples confirms the phase purity. Both materials crystallize in the trigonal $\alpha$-NaFeO$_2$ structure ($R\overline{3}m$) with lattice parameters $a$ = 4.1070(4) \r{A} and $c$ = 23.1472(1) \r{A} for the erbium compound and $a$ = 4.0916(1) \r{A} and $c$ = 23.1483(2) \r{A} for the thulium compound. Magnetic susceptibility measurements show an effective moment of $\mu_{\text{eff}} = 9.6(2) \mu_B$/f.u. ($7.5(1) \mu_B$/f.u.) for TlErSe$_2$ (TlTmSe$_2$) for temperatures above 200 K. While $^3$He specific-heat measurements reveal long-range magnetic order below $T_N = 0.42 $K for TlErSe$_2$, no sign of long-range magnetic order was observed for TlTmSe$_2$. Based on our results, we map out the T-H phase diagram for polycrystalline TlErSe$_2$ and discuss the striking difference in the magnetic behavior of TlTmSe$_2$ based on our ab initio quantum chemical calculations.