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