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doi:10.1038/nindia.2015.70 Published online 26 May 2015

Evidence for new quantum matter uncovered

K. S. Jayaraman

The Indian researchers Yogesh Singh (left) and Kavita Mehlawat.
© IISER
Two researchers from Indian Institute of Science Education and Research (IISER) Mohali are part of an international team that has for the first time found direct experimental evidence for a novel quantum matter called "Kitaev matter".

Named after Alexei Kitaev, a physics professor at Caltech, the new state of matter has opened innovative physics in the quantum realm and laid the foundation for designing materials with exotic physical properties. It may even lead to the realisation of robust quantum computers which will exploit "spin" states to store information on an atomic size scale.

Elementary particles like electrons have, in addition to charge and mass, a fundamental property called spin. In many ways, spins are like tiny magnets the particles carry; they align when a magnetic field is applied leading to magnetism, They also "talk" to other spins nearby that may affect them. In Kitaev matter, an electron spin talks to another spin in a way that depends on the direction (orbital state) of the spin. This coupling of spin and orbital states of the electron manifests in many novel types of quantum matter.

Scientists believe that "Kitaev matter" can possibly host special particles called Majorana Fermions, which can be used for robust quantum computation. Therefore, realisation of Kitaev model is much sought after in real materials. However, until now Kitaev matter has either been difficult to prepare artificially or all its evidence has been indirect. Now for the first time, the researchers have found direct evidence to ‘Kitaev matter’ in the honeycomb-like structure of sodium iridate (Naa2IrO3).

For this study, Yogesh Singh and his graduate student Kavita Mehlawat prepared the high-quality samples on which Kitaev physics could be investigated. To probe Kitaev matter, the researchers used a technique called "diffuse magnetic X-ray scattering" which looks at the interactions between magnetic moments at short distances from each other. "This spin- and real-space entanglement directly uncovers the bond-directional nature of these interactions, thus providing a direct connection between honeycomb iridates and Kitaev physics," the researchers conclude.

“We have shown direct evidence that interactions between magnetic moments on a lattice can depend on the direction of the neighbours. Such bond-directional interactions on the honeycomb lattice are basic ingredients for the Kitaev model," Singh says.

Spin–orbit entanglement drives a system of particles such as atoms, photons and electrons towards a spin-liquid phase. The spin liquid which is an exotic quantum state of densely packed spins and having many possible configurations is an analog of liquid helium. But unlike Helium, it does not solidify down to the lowest temperature due to strong quantum fluctuations. The latest study employing honeycomb-like structure of sodium iridate may have, incidentally, found a novel recipe to achieve a ‘spin liquid’ state in real materials for the first time, the researchers believe.


References

1. Chun, S. H. et al. Direct evidence for dominant bond-directional interactions in a honeycomb lattice iridate Na2IrO3. Nat. Phys. (2015) doi: 10.1038/nphys3322