Quantum spin liquids (QSLs) are captivating and enigmatic states of matter that have captured the interest of scientists for many years. Initially suggested by Nobel Prize winner Philip Anderson in the 1970s, these materials defy traditional magnetic behavior by never achieving a stable magnetic state, even near absolute zero temperatures. Instead, the atomic spins within them continuously fluctuate and intertwine, creating a sort of magnetic “liquid.” This peculiar activity occurs due to a phenomenon called magnetic frustration, where opposing forces inhibit the system from reaching a consistent ordered structure. Studying QSLs is notoriously challenging, as they do not exhibit the typical characteristics of magnetic transitions seen in standard magnetic materials, making them difficult to detect and comprehend through conventional methods. Consequently, researchers have found their behavior to be a complex puzzle.
The material β’-EtMe₃Sb[Pd(dmit)₂]₂ is a molecular crystal with a triangular lattice and has been identified as a promising candidate for demonstrating QSL characteristics. The arrangement of spins within it creates innate frustration since the neighboring spin interactions cannot all be simultaneously fulfilled. This configuration seems perfect for a QSL state, but earlier studies raised uncertainty about whether it truly exhibited 2D QSL behavior or if its properties were impacted by dimensional reduction. This uncertainty has been a focal point of current research.
Recent research led by Professor Yasuyuki Ishii from the Shibaura Institute of Technology, along with Yugo Oshima and Hitoshi Seo from RIKEN, Francis L. Pratt from the Rutherford Appleton Laboratory, and Takao Tsumuraya from Kumamoto University, published in the journal Physical Review Letters on December 3, 2024, sheds light on this enigma. Professors Ishii and Oshima had observed signs indicating one-dimensional spin behavior in β’-EtMe₃Sb[Pd(dmit)₂]₂ through muon spin rotation (µSR) and electron spin resonance (ESR) experiments, respectively. These findings diverged significantly from the usual concept of 2D triangular magnets, complicating their interpretation. They then sought theoretical insights from Dr. Seo, Associate Professor Tsumuraya, and their team. Utilizing advanced theoretical modeling, the researchers uncovered that the spin dynamics within this material are predominantly influenced by quasi-one-dimensional (1D) behavior, challenging the conventional understanding of 2D QSLs.
The authors, who are experts in magnetic resonance and unique magnetic phenomena, integrated ESR and µSR techniques with theoretical modeling to investigate β’-EtMe₃Sb[Pd(dmit)₂]₂. “We introduce a different experimental method for analyzing the ground state of β’EtMe3Sb[Pd(dmit)2]2 using ESR and µSR,” stated Professor Ishii, highlighting their research approach.
ESR evaluates spin anisotropy and diffusion by examining how electrons in the material respond magnetically. Meanwhile, µSR illuminates the spin relaxation dynamics and dimensionality of the material by observing the interactions between muon spins and magnetic fields. These experimental methods were enhanced by density-functional theory (DFT) assessments and advanced Hubbard model simulations to gain insights into the electronic structure and magnetic interactions. The results revealed that the spin dynamics in β’-EtMe₃Sb[Pd(dmit)₂]₂ are primarily governed by quasi-1D behavior, rather than the anticipated 2D dynamics. Typically, 1D spin diffusion would arise in the direction of the strongest magnetic interaction; however, the direction highlighted by ESR was thought to represent the weakest interaction based on previous theoretical models. This finding was unexpected, as the material’s 2D nature led researchers to assume it would exhibit 2D spin dynamics. Additionally, muon spin relaxation experiments confirmed these results, where a B-0.5 pattern in spin relaxation appeared, which is indicative of 1D spin diffusion. Supporting this notion, ESR demonstrated anisotropic or direction-dependent spin movement.
“The distinctive features of quantum spin liquids hold potential for future applications in cutting-edge technologies like quantum computers and spintronics devices. This research marks a significant step towards that foundation and paves the way for future technological advancements,” commented co-author Yugo Oshima, explaining the study’s contributions.
Even with these new revelations, questions remain regarding the precise mechanisms of dimensional reduction in this context. The interplay of magnetic frustration, quantum fluctuations, and multi-orbital effects requires additional exploration. Professor Ishii and the team intend to apply their methods to other QSL candidate materials, aiming to identify general principles that apply to these substances. Their work underscores the necessity of employing sophisticated techniques like ESR and µSR to address the challenges involved in studying QSLs. By confirming the existence and dynamic measurement of quantum spin-liquid states, this study brings researchers closer to fully realizing the potential of these extraordinary materials.
