Researchers have identified a model system of quantum critical points for better understanding new materials. This model system can help understand unusual behaviors in materials near to a quantum critical point and may be used in comprehending entanglement and quantum computing.

Silicon and other well-studied materials are understood with the help of well-established frameworks, such as the widely used density functional theory. However, new materials like transition metal oxides, manganates, ruthinates, and iridates are difficult to understand using these frameworks. The unique properties of these materials, such as sensitivity to small perturbations, make them promising for advanced applications in devices like sensors, GPS, and memory RAM.

Prof. N. S. Vidhyadhiraja from Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), an autonomous unit of the Department of Science and Technology (DST) recently led a team of researchers to conduct a study focused on a specific situation in quantum physics, called "local quantum criticality" that occurs in certain materials. Their study, published in Physical Review B on 1 May 2023, was supported by the Science and Engineering Research Board (SERB), an attached institution of the Department of Science and Technology (DST). SERB has now been subsumed into ANRF.

Prof. Vidhyadhiraja says that their research in quantum many-body physics is primarily focused on the concept of emergence in condensed matter. He notes, “Drawing parallels with the organization of bees, birds, and ants, we can say that the behavior of electrons in materials cannot be predicted by studying individual electrons but depends on their collective interactions. Environmental conditions, such as temperature, are important in determining the final order in materials.”

Earlier research focused on vanadium oxide, a material that undergoes drastic transition from insulator to metal and vice versa when subjected to small changes in pressure or temperature. Beyond its conductivity shift, vanadium oxide also alters its optical properties, becoming opaque or transparent, in response to small changes in temperature, pressure, doping, and magnetic fields.

Specifically, the present study focuses on quantum critical metal-insulator transitions which are local in nature and occur at zero kelvin. Imagine having a material where some electrons move around and are influenced by a critical point in their phase diagram. The quantum critical point, which is sitting at zero kelvin, and which cannot be accessed experimentally, has consequences on the properties of the system at finite temperatures and finite pressures. So, in this study, the researchers found a model system that contains a whole range of quantum critical points.

The theoretical model that studies this critical behavior is called the “modified periodic Anderson model (MPAM).” It has been observed that in some cases, the way electronic energy levels are distributed changes dramatically at the critical point.

Researchers discovered a distinct energy distribution pattern called a "soft-gap spectrum" in the MPAM, which is, in fact, a three-orbital lattice model. This unique energy distribution emerges at a critical point when the material transitions from being a metal to an insulator. A specific parameter which describes the relationship between temperature and the energy levels, changes in a peculiar way and becomes temperature-independent precisely at the critical point. This leads to the formation of the soft-gap spectrum.

Prof. Vidhyadhiraja concludes, "These findings can help in characterizing quantum criticality and understanding unusual behaviors in materials proximal to a quantum critical point. The potential of this study lies in understanding entanglement and possibly quantum computing.”

Publication link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.107.205104

Illustration depicting quantum computing and entanglement

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