A new type of 3D quantum spin liquid has been identified near a member of the langbeinite family. The unique crystal structure of this material and its corresponding magnetic interactions lead to a peculiar behavior linked to what is described as an “island of liquidity.” This discovery was made by a global team utilizing experiments at the ISIS neutron source and theoretical modeling on a nickel-langbeinite sample.
A new type of 3D quantum spin liquid has been identified near a member of the langbeinite family. The unique crystal structure of this material and its corresponding magnetic interactions lead to a peculiar behavior linked to what is described as an “island of liquidity.” This discovery was made by a global team utilizing experiments at the ISIS neutron source and theoretical modeling on a nickel-langbeinite sample.
In a crystal lattice where spins cannot align to minimize energy, a phenomenon known as magnetic frustration occurs. When this frustration is significant, the spins keep fluctuating chaotically, even as temperatures drop to near absolute zero, resulting in the material acting as a quantum spin liquid. Quantum spin liquids (QSLs) are known for their extraordinary properties, including topologically protected effects that might be useful for developing extremely stable qubits in the future. Although quantum spin liquids were primarily investigated in two-dimensional settings, they can also appear in 3D configurations, albeit with less frequency.
Exploring Frustration
An international team has now showcased this behavior in a fresh category of 3D materials: Langbeinites, which are rare sulfate minerals found in nature. Altering one or two components within their chemical formula leads to various types that fall under this category.
For their research, the scientists created synthetic langbeinite crystals with the formula K2Ni2(SO4)3. Nickel, the magnetic element in this study, is crucial as the nickel ions create two interconnected trillium lattices. This configuration results in the necessary magnetic frustration, which intensifies further when an external magnetic field is introduced. Consequently, the magnetic moments of the nickel ions cannot all align favorably, leading to fluctuations that generate a quantum spin liquid.
Neutron Data and Theory: A Near-Perfect Match
The research team, headed by Ivica Živković from EPFL, successfully measured the magnetic fluctuations using the British neutron source ISIS in Oxford. The samples exhibited behavior consistent with quantum spin liquids, not just at very low temperatures but also at a “lukewarm” temperature of 2 Kelvin.
Meanwhile, the team led by HZB theorist Johannes Reuther was able to interpret the measurements using various theoretical approaches. “Our theoretical phase diagram even identifies an ‘island of liquidity’ at the center of a strongly frustrated tetratrillium lattice,” explains Matias Gonzalez, the study’s lead author and a postdoctoral researcher in Reuther’s group. PhD student Vincent Noculak calculated the spin interactions using a method based on Feynman diagrams, which Reuther developed years ago (pseudo-fermion function renormalisation group, PFFRG). The correlation between the experimental data and theoretical outcomes is impressively strong. “Despite the complex interactions, we can accurately model this system,” notes Reuther.
Potential QSL Candidates in Langbeinites
Langbeinites comprise a vast yet largely unexplored category of materials. This study highlights that the quest for quantum behavior within these materials is promising. A team led by HZB physicist Bella Lake has already synthesized new members of this category, which could also qualify as 3D quantum spin liquids. “Although this remains primarily fundamental science,” Johannes Reuther emphasizes, “in light of the increasing interest in novel quantum materials, langbeinite compounds could be very relevant for applications in quantum information.”
