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The Invisible Frontier: Low Dimensional Physics

One of the more startling revelations of Physics in the last century has been the idea that Quantum Physics, far from being an exotic theory, is probably the standard description of our world, and that classical physics is merely an approximation at large length scales. Even from a purely geometrical point of view, the properties of materials become interesting as we approach the nanometre scale. The volume of simple shapes decreases faster than their surface area does, as the shape is uniformly shrunk from all directions. The resulting zero-dimensional structures called quantum dots, are not the only way to reduce a material’s size. When a cube’s length and breadth are kept the same but its thickness is reduced to a few atoms in extent, the resulting “nanosheet” has almost no volume, while still having a high surface area. Materials like Graphene, Molybdenum disulphide and Topological Insulators belong to the class of such two-dimensional (2D) materials. Certain other materials such as Silver and Phosphorus in Germanium can be shrunk into so-called ‘nanowires’, in which both width and thickness are reduced to nanometres. These one-dimensional (1D) systems have neither surface area nor volume, but only length.

These nanoscale structures show extraordinary mechanical, thermal and electrical properties, among other unique properties predicted by Quantum Physics. Novel technologies are needed to study these novel materials. Professor Arindam Ghosh's Low Temperature and Nano-Electronics (LTNE) group at the Indian Institute of Science (IISc) is making important advances in fabricating and studying such low-dimensional systems.

Graphene required for the group’s research is produced by peeling layers from a chunk of graphite with scotch tape, until only a one atom-thick layer is left behind. The Nano-electronics lab in the Raman building houses microscopes used to study the structure of 2D and 1D materials, equipment for depositing metallic connections on a 2D structure and an Electron Beam Lithography unit. The latter is used to create patterns on a 2D surface, and also doubles as a Scanning Electron Microscope. The low temperature lab in the Physical Sciences building, which the group light heartedly refers to as “one of the coolest places in India”, houses a Helium-3 refrigerator operating at -272.85 °C and a state-of-the-art dilution refrigerator able to produce some of the coldest temperatures in the country, down to -273.13 °C, just 0.02 °C above absolute zero. The materials that the group works with are extremely delicate and the equipment they use is painstakingly customised to account for this. One such customised measuring device was used to measure, for the first time, the exotic electrical properties of a 2 nm thick gold wire – around 5000 times thinner than the width of a human hair!

The LTNE group, in collaboration with the University of New South Wales, Australia has recently shown that the fundamental time reversal symmetry is broken in a system of elemental semiconductors, Si and Ge, doped with a layer of phosphorous atoms. Recently, the LTNE group also demonstrated the extreme sensitivity of Graphene resistance to a single fluctuating charge by preparing a high quality Graphene-Boron Nitride system.

While the importance of studying low-dimensional systems to reveal fundamental properties of matter cannot be underestimated, they also have applications in future technology. Prof. Ghosh says of his group's work, “We are discovering new physics leading to new standards”. The systems studied in his group promise alternate forms of energy and may even have an impact on the medical field by providing new forms of magnetic sensors.

About the Low Temperature and Nano-Electronics group:

The group is headed by Prof. Arindam Ghosh and currently has 20 members. They perform their experiments in two labs, one of them in the Raman building and another in the new Physical sciences building, inside the IISc campus. More information on the group and their work can be found here.