A new high-temperature quantum spin liquid with polaron spins (1704.06450v1)

Abstract: The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realisation turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave (CDW) state of 1T-TaS$_2$. In this system, the charge excitations have a well-defined gap of $\sim 0.3$ eV, while nuclear magnetic quadrupole resonance and muon spin relaxation experiments reveal that the spins show gapless quantum spin liquid dynamics and no long range magnetic order down to 70~mK. Canonical $T^{2}$ power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200~K to $T_f= 55$ K. Below this temperature we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.

A New High-Temperature Quantum Spin Liquid with Polaron Spins

The paper presents a comprehensive experimental paper on 1T-TaS$_2$, evidencing the realization of a quantum spin liquid (QSL) state in this material's low-temperature phase. The work stands out for identifying a new gapless spin state, enriched with disorder from what is traditionally expected from a QSL, extending the current understanding of spin-liquid behavior and magnetic phase transitions in strongly correlated electron systems.

Findings

1. Quantum Spin Liquid State: The authors identify a nearly ideal QSL state in 1T-TaS$_2$, featuring polaronic spins arranged on a triangular lattice within a charge-density wave (CDW) phase. Crucially, no long-range spin ordering is detected, which is a haLLMark feature of QSLs. Experimental methods, such as muon spin relaxation ($\mu^+$SR) and nuclear quadrupole resonance (NQR), substantiate this conclusion.
2. Gapless Spin Dynamics: Through extensive $\mu^+$SR and NQR measurements between 70 mK and 210 K, gapless spin dynamics are observed, characterized by a canonical $T^2$ temperature dependence. This behavior substantiates the QSL nature down to low temperatures, affirming previous theoretical predictions while filling a longstanding void in experimental validation.
3. Emergent Spin State: Below around 55 K, 1T-TaS$_2$ undergoes a further transition to a distinct spin state. This state retains gapless dynamics yet exhibits reduced spin excitations and a high degree of local disorder. The paper posits the emergence of a new quantum spin order, although its exact nature remains a subject for further inquiry.

Implications and Future Directions

The findings have significant theoretical and practical implications for quantum magnetism and correlated materials research. The documented behavior within 1T-TaS$_2$, with its transitioning from a QSL to a yet-unexplored quantum spin ordered phase, might prompt reevaluation of current models that struggle to describe such complex electron correlation effects.

Furthermore, this paper opens avenues for exploring QSL properties and phenomena in other transition-metal dichalcogenides. Specifically, detailed exploration of the interplay between charge ordering, lattice geometry, and spin dynamics could unveil novel functionalities or applications, such as in quantum computation or high-temperature superconductivity. Also, understanding the mechanisms driving the transition to an emergent quantum spin ordered phase can guide synthetic efforts towards designing materials with tunable quantum phases.

The research elucidates how fundamental quantum states can be realized and manipulated, paving the way for not only theoretical advancements but also progress in material science applications. As a forward-looking agenda, pursuing variable chemical compositions or structural modifications in TaS$_2$ and similar compounds could unlock more insights into underlying physical principles or even utility in applied domains like electronics or sensing technologies.

Conclusion

The paper marks a key step in bridging theoretical conceptions of QSLs with experimental realities, particularly in the context of low-dimensional systems with inherently strong correlation effects. The thoroughness with which the behavior, particularly in 1T-TaS$_2$, is examined offers valuable foundational data for future studies and potentially groundbreaking applications in industry. This not only reaffirms the importance of quantum spin liquids in contemporary physics but also inspires subsequent studies to unravel further complexity in quantum magnetic states and lattice-driven phenomena.