Atoms on the Edge

From a very basic understanding about science of electricity, we know that electrical conductivity of a material is due to the flow of electrons inside it. In that context, edge effect is a promising phenomenon which happens when more electrons are emitted from the edges of a conducting specimen than from the flat regions due to which the edges appear shiny. This effect enhances the electrical conductivity of materials such as graphene-based resonant antennas and other photonic and plasmonic nanodevices. The edge effect contributes to the surface currents or what are called as eddy currents along the boundaries of a coil and also increases the electric field in a capacitor.

In a recent study published in Nature journal in September this year, physicists at MIT have been able to capture the images of resistance-less motion of cloud of ultracold atoms flowing along the boundary of a ring of laser light of micrometer wavelength for milliseconds.

In this study, the team of researchers corralled about 1 million sodium atoms in a laser-controlled trap, and cooled them down to nanokelvin values of temperature. The atoms were then made to spin and a circular ring of laser light was introduced around them. In the images of the system taken by the team, it could be observed that when the atoms encountered the ring of light, they flowed effortlessly along its edge, in just one direction. This study can serve as a lead to understand the edge states of electrons, which are even more difficult to capture due to far smaller dimensions. If however, we are somehow able to manipulate the flow of electrons to be friction less, this in turn would enable us for more efficient transmission of data and energy without much loss.

The idea of edge states was first invoked by physicists to explain a curious quantum mechanical phenomenon, known as the Quantum Hall effect. In this phenomenon, the current is sent through layered materials in presence of a magnetic field and it is found that electrons rather than flowing straight, tend to accumulate on one side only, in precise quantum portions. This happens at near absolute zero temperatures. The edge states are chiral, meaning that they move in one direction only. Thus edge modes were supposed to occur when electrons flow under a magnetic field. “But to actually see them is quite a special thing because these states occur over femtoseconds, and across fractions of a nanometer, which is incredibly difficult to capture,” said Richard Fletcher, the co-author of this study at MIT. The other co-authors of the study at MIT include Thomas A. Frank, Professor of Physics; graduate students Sungjae Chi and Ruixiao Yao, former graduate students Airlia Shaffer, Biswaroop Mukherjee, and Martin Zwierlein. All these co-authors are members of Research Laboratory of Electronics at the MIT-Harvard Center for Ultracold Atoms.

Mr. Fletcher further said, “I would stress though that, for us, the beauty is seeing with your own eyes physics which is absolutely incredible but usually hidden away in materials and unable to be viewed directly.” Nonetheless, the team has very carefully designed the setup such that the motion of ultracold atoms mimics the dynamics of electrons moving under the influence of a magnetic field.

Typically, electrons move inside the body of a material while experiencing varying degrees of resistance from the constituent atoms, ions or molecules of the substance. The resistance is least in metals or conductors and moderate in semiconductors. However in case of superconductors, electrons move together as a coherent cloud experiencing no resistance whatsoever. This can be sensed from the fact that if current is once set in a wire loop of a superconducting material; it can flow indefinitely without the support of any power sources.

Edge effect can be an added way of generating more electric current. We may not necessarily need to exhaust all superconducting materials. Rather we may choose some other suitable material for electrical devices where electrons could shuttle along the edges and move along a circuit without any loss.

Edge effect shall also enhance our understanding of Quantum Hall effect which is one of the biggest discoveries of Physics in last 40 years. The study of Quantum Hall effect has bagged three Nobel Prizes in the recent past. One in 1985 for the integer quantum Hall effect, another in 1998 for the fractional quantum Hall effect and a third one in 2016 for its connections with topology.

Apart from fundamental physics research, (QHE) has applications in metrology, telecommunications and electronics. QHE plays a role in the development of qubits based on fractional Quantum Hall States, which could be crucial for building robust quantum computers. The QHE provides a way to define a standard of resistance in electrical devices, which is incredibly precise. The resistance values observed in the QHE are quantized, allowing for highly accurate resistance measurements.

Devices working on the basis of Quantum Hall Effect can be used for highly sensitive magnetic field measurements, which can have applications in various scientific and industrial settings.

Dr. Qudsia Gani, Head Dept. of Physics, Govt. Degree College, Pattan Baramulla, J&K