Quantum phase transitions in an Yb-based semiconductor YbCuS$_{2}$ with an effective spin-1/2 zigzag chain

In rare-earth-based semiconductors or insulators, bond dependent superexchange interaction mediated by nonmagnetic anions plays the leading role in ordering of localized 4$f$ moments. In case the magnetic cations with effective spin of 1$/$2 form a geometrically frustrated lattice, the magnetic order is suppressed and the quantum fluctuations are enhanced, leading to a quantum spin liquid (QSL) state. Possible QSL states have been extensively studied in spin-1$/$2 triangular, kagome, pyrochlore, and Kitaev magnets [1]. Another example hosting geometrical frustration is a zigzag spin chain. A rich variety of nontrivial quantum phases could manifest themselves under magnetic fields, e.g., spin dimerization, 1$/$3 magnetization plateau, and vector chirality [2,3].

In the present work, we focus on an Yb-based semiconductor YbCuS$_{2}$ crystallizing in an orthorhombic structure, where the edge sharing of the YbS$_{6}$ octahedra arranges the Yb ions in a zigzag chain along the a-axis [4]. The Curie-Weiss behavior of the magnetic susceptibility indicates the trivalent state of an Yb ion [5]. The negative paramagnetic Curie temperature of ${\theta}_{\rm p}$ $=$ 48 K suggests an antiferromagnetic (AFM) interaction.

We have synthesized polycrystalline samples of YbCuS$_{2}$ and studied the magnetic properties [6]. The trivalent Yb state was confirmed by the hard x-ray photoemission spectroscopy. The specific heat shows a sharp peak at $T_{\rm o}$ $=$ 0.95 K, indicating a first-order phase transition. The magnetic entropy at $T_{\rm o}$ is only 20\% of $R$ln2 expected for the Kramers doublet ground state. Therefore, the phase transition must be strongly suppressed by the magnetic fluctuations. In the $B$–$T$ phase diagram, $T_{\rm o}$ is robust against magnetic fields for $B$ $<$ 4 T, and $T_{\rm o}$($B$) increases to higher temperatures for 4 $<$ $B$ $<$ 7 T but decreases for $B$ $>$ 7 T. The magnetic-field induced transitions exist at 4, 9.1, and 15.6 T and the 1$/$3 magnetization plateau-like behavior appears at $T$ $<$ $T_{\rm o}$. The peculiar phase diagram suggests nontrivial quantum phases arising from the geometrical frustration inherent in the Yb zigzag chain. Moreover, we have performed neutron powder diffraction experiments with the WISH diffractometer installed at ISIS, RAL. On cooling for $T$ $<$ $T_{\rm o}$, we observed magnetic Bragg peaks, indicating an antiferromagnetic long-range order. A magnetic structure is proposed by analyzing the $Q$-dependent intensity of the magnetic reflections to discuss relationship between the spin alignments and the magnetic frustration in the Yb zigzag chain.

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