Gapless quantum spin liquid ground state in the two-dimensional spin-1/2 triangular antiferromagnet YbMgGaO$_4$ (1506.06992v2)

Abstract: Quantum spin liquid (QSL) is a novel state of matter which refuses the conventional spin freezing even at 0 K. Experimentally searching for the structurally perfect candidates is a big challenge in condensed matter physics. Here we report the successful synthesis of a new spin-1/2 triangular antiferromagnet YbMgGaO$_4$ with R$\bar{3}$m symmetry. The compound with an ideal two-dimensional and spatial isotropic magnetic triangular-lattice has no site-mixing magnetic defects and no antisymmetric Dzyaloshinsky-Moriya (DM) interactions. No spin freezing down to 60 mK (despite $\Theta$$_w$ $\sim$ -4 K), the low-T power-law temperature dependence of heat capacity and nonzero susceptibility suggest that YbMgGaO$_4$ is a promising gapless ($\leq$ $|$$\Theta$$_w$$|$/100) QSL candidate. The residual spin entropy, which is accurately determined with a non-magnetic reference LuMgGaO$_4$, approaches zero ($<$ 0.6 \%). This indicates that the possible QSL ground state (GS) of the frustrated spin system has been experimentally achieved at the lowest measurement temperatures.

• The paper demonstrates that YbMgGaO maintains a gapless quantum spin liquid state with no magnetic ordering down to 60 mK despite antiferromagnetic interactions.
• It utilizes magnetization and heat capacity measurements alongside a nonmagnetic LuMgGaO reference to accurately isolate magnetic properties, unveiling near-zero residual spin entropy below 0.3 K.
• An unexpected field-induced quantum state, marked by a susceptibility plateau between 1.6 and 2.8 T, suggests novel quantum fluctuations and dynamics.

Overview of YbMgGaO as a Gapless Quantum Spin Liquid Candidate

The paper presents the synthesis and characterization of a novel two-dimensional (2D) spin-1/2 triangular antiferromagnet, YbMgGaO, which is proposed as a candidate for a gapless quantum spin liquid (QSL). This material addresses significant challenges related to intrinsic structural disorders, offering a promising realization of QSL states in condensed matter physics.

Structural and Magnetic Properties

YbMgGaO possesses an ideal 2D triangular-lattice structure with R3m symmetry, which avoids complications associated with site-mixing, antisymmetric Dzyaloshinsky-Moriya (DM) interactions, and interlayer exchange couplings typically seen in similar systems. The material’s structural purity is ensured by the large chemical distinction between magnetic Yb³⁺ ions and nonmagnetic ions, minimizing the presence of magnetic defects.

Magnetization and heat capacity measurements indicate a lack of magnetic ordering down to 60 mK, despite the presence of antiferromagnetic (AF) nearest-neighbor interactions characterized by a Weiss temperature (θw) of approximately -4 K. The observed power-law temperature dependence of the heat capacity and nonzero susceptibility implies minimal excitation gaps from the ground state (GS), endorsing YbMgGaO as a potential gapless QSL.

Experimental Findings

The experimental investigations confirmed an almost zero residual spin entropy below 0.3 K under magnetic fields ranging from 0 to 9 T, suggesting that the GS is non-degenerate, disordered, and possibly a gapless QSL. The use of a nonmagnetic reference compound, LuMgGaO, enabled the precise subtraction of lattice heat capacities, enhancing the accuracy of magnetic property assessments.

One unexpected discovery is the field-induced quantum state at 0.5 K, evidenced by an anomalous susceptibility plateau under external magnetic fields between 1.6 and 2.8 T. This indicates an unconventional behavior that does not align with typical paramagnetic states and necessitates further exploration.

Implications and Future Directions

The results indicate that YbMgGaO could effectively serve as a model system for studying quantum spin liquids, potentially overcoming the limitations posed by structural defects and anisotropic interactions in other candidates. The existence of a field-induced quantum state within this material also suggests new domains for investigating the dynamics of quantum fluctuations and entropic phenomena at low temperatures.

Theoretically, these findings challenge numerical predictions that often suggest long-range Néel order in spin-1/2 triangular Heisenberg antiferromagnets (THAFs). The discrepancies highlight the importance of considering bond randomness, next-nearest-neighbor interactions, and multisite exchange processes, which may stabilize QSL phases against AF ordering.

Thus, YbMgGaO presents valuable opportunities to refine theoretical models of spin-liquid systems and expand understanding of QSL behaviors in geometrically frustrated lattices. Subsequent research, employing large single crystals and improved analytical frameworks, is crucial for exploring these complex quantum states further.