source:physics world
“Quantum spin liquids are the exception,” says Gang Chen, Professor of Physics at Fudan University in China. He is describing the theory Soviet theoretical physicist Lev Landau developed to characterize the ferromagnetic or antiferromagnetic ordering adopted by spins in a magnet when they get too cold to keep fluctuating thermally. Quantum spin liquids shirk this theory. “The spins in quantum spin liquids do not order even down to absolute zero temperature. It is a very exotic quantum phase of matter and cannot be understood in the framework by Landau.”
First proposed in the 1970s, interest in these materials was further piqued in the 1980s at the suggestion that they were the “mother state” for the high-temperature superconductivity in cuprates. More recent reports of a possible quantum spin liquid state in YbMgGaO4 that is robust against weak disorder has rekindled interest. Now in Chinese Physics Letters Chen, alongside Hechang Lei, Qingming Zhang, Xiaoqun Wang and colleagues in China report a whole diverse family of previously unknown potential quantum spin liquids, opening up a range of opportunities, such as the possibility of tuneable charge gaps and variable exchange coupling, as well as settling some of the confusion around the origin of these exotic states.
Disorder and disagreement
Although gauge theory now explains some of the behaviour of quantum spin liquids, a number of aspects of these materials are still not understood. The recent report of stable quantum liquid spin characteristics in YbMgGaO4 roused conflicting theories over the cause of this state. Many experts in the field concluded that the stability of the disordered spin or quantum spin liquid state results from disorder in the magnesium and gallium atoms, while others believed it to be intrinsic to the material. Despite the availability of large single crystal YbMgGaO4 samples, which allowed neutron scattering, muon spin relaxation, and electron spin resonance investigations, extensive characterizations failed to dispel the contention. The existence of quantum spin liquid states in the large family of rare-earth chalcogenides as reported by Chen, Lei and Zhang would seem to settle this debate.
“These [rare-earth chalcogenide] materials do not have the Ga/Mg charge disorder,” explains Chen. “Since the spin liquid phenomena are now believed to be present in these new systems, it is thus thought that the quantum spin liquid physics in YbMgGaO4 may be intrinsic and have little to do with the Ga/Mg charge disorder.”
Quantum spin liquid triangulation
Philip Warren Anderson first introduced the idea of quantum spin liquids in the 1970s as the result of geometrically “frustrated spins” unable to find a way to align in a triangular lattice. It was also Anderson who suggested the link with high-temperature superconducting cuprates in the 1980s. “Although we understand now that the spins on a triangle could find a way out, the triangular geometry remains to be a good place to search for quantum spin liquids due to the frustrated nature of the geometry,” Chen tells Physics World.
The researchers further narrowed their search to rare-earth chalcogenides – which have the formula AReCh2, where A is for alkali or monovalent ions, Re is rare earth, and Ch is O, S, Se – on account of the spin-orbit coupling properties in rare earth elements. “The effective interaction is anisotropic in effective spin space and also depends on the bond orientation,” says Chen. “Such a special and novel interaction is almost impossible for 3d transition metal ions, and thus provides new mechanisms for the physical realization of spin liquids.”
Digging out family traits
Lei in the Department of Physics and Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices at Renmin University and Zhang at the Chinese Academy of Sciences and Lanzhou University are both experimental physicists. Following their discovery of the spin liquid candidate YbMgGaO4, Zhang’s group began to explore other rare-earth based magnetic triangular materials. Here the researchers systematically synthesized different rare earth chalcogenides and then characterized them according to their thermodynamic properties. They measured the temperature dependence of the magnetic susceptibility with a superconducting quantum interference device (SQUID) magnetometer, and the heat capacity using PPMS (Quantum Design Physical Property Measurement System). Chen and Wang as theoretical physicists applied their expertise to providing the physical understanding of the data and theoretical support.
Michael Baenitz and researchers in Germany, UK and Switzerland have highlighted the putative spin liquid properties of NaYbS2, but as Chen points out, all the rare-earth atoms have similar chemical properties, allowing substitution of one with the other. “Thus one could obtain a wide range of new models and model parameters,” explains Chen. “The charge gaps in these materials are smaller than the one in YbMgGaO4, hence one may vary the charge gap and may eventually access the metallic side by applying pressures. This could be an interesting direction to understand the quantum phase transition from Mott insulating spin liquid to the metallic phases.”
The researchers also highlight the availability of single-crystal samples as a “major milestone in materials science since it allows elastic and inelastic neutron scattering measurements. “These are data-rich experiments, and would provide a lot more useful information for us to identify the actual ground states for these materials” says Chen. “We are looking forward to these potentially exciting outcomes.”