Space & Physics
The various avatars of the Hall effect
In this second article of Ed Publica’s series on the Hall effect, Dr. Saraubh Basu examines the physics of the Hall effect variants discovered over the course of the past century.
This is the second article of Ed Publica’s series on the Hall effect, which covers the various manifestations of the Hall effect. You can read the first article here.
The ‘anomalous’ Hall effect
In 1881, just two years after Edwin Hall discovered the eponymous Hall effect, he spotted an anomaly when replicating the effect with ferromagnets.
He had observed a tenfold deflection of electric charges this time around, compared to non-magnetic conductors.
Suspecting the magnetic properties played a role, this avatar of the Hall effect is dubbed the anomalous Hall effect. The word ‘anomalous’ is used owing to the fact that external magnetic field no longer remains as a stringent requirement for the Hall effect; instead, the intrinsic magnetization (for instance, the ferromagnet in the above example) fulfils that criterion.
The physicist Edwin Hall. Credit: Wikimedia
The Hall resistivity in ferromagnets increase steeply under the presence of very weak magnetic fields. However, in stronger magnetic fields, the Hall resistivity doesn’t increase further very much. This saturation is rather strange, for it is in contrast to the classical Hall effect where the Hall resistivity maintains its steady growth.
There are several other effects that play a crucial role in determining the anomalous Hall resistivity, thus making it a complicated phenomenon that physicists lack comprehensive understanding about, in comparison to the various other avatars of the Hall effect.
Quantum avatar(s)
The fact that a simple lab experiment showed how the Hall resistivity can be expressed as an equation that contains merely constants, opened up a a plethora of research to understand the cause of this ‘universality’. For it hinted to the involvement of a very fundamental phenomenon.
In 1980, Klaus von Klitzing discovered the quantum avatar of the Hall effect was detected. He was amidst research at a magnetic facility in Grenoble, France, working to improve electron mobility in metal oxide semiconductor field effect transistors (MOSFET). These are transistors that typically operate at extremely low temperatures and under intense magnetic fields.
von Klitzing observed his sample’s Hall resistivity assuming discretized values. This means the resistivity jumps in steps, by a fixed amount that can be scaled as multiples of an integer number (includes 0 along with whole numbers such as 1,2,3, and so on). This discretization reveals the underlying quantum mechanical behavior that has been unraveled at long last – thus bearing its name – the integer quantum Hall effect. von Klitzing later won the Nobel Prize in Physics for 1985 for this work.
The plot here depicts the transverse and longitudinal Hall resistivity (y-axis) increasing in integer steps as the magnetic field (x-axis) increases. This is due to the integer quantum Hall effect. Credit: Wikimedia
But the quantization isn’t limited to integer multiples. In fact, two years later, the fractional quantum Hall effect was observed in experiments. It was shown there were about 100 fractions, including those that aren’t whole numbers that were now in the formula.
Robert Laughlin, who would later win a share of the 1998 Nobel Prize in Physics, proposed a theory to explain the observations. It boils down to the interaction among electrons, either due to the Coulombic repulsion force or the Pauli exclusion principle.
These interacts would eventually split the degeneracy of these enormously degenerate Landau energy levels. These are quantum states occupied by electrons that complete circular revolutions under the influence of an external magnetic field. Splitting these degeneracies, lead to the opening of an energy gap, for the fractional quantum Hall effect to be observed.
‘Spin’ avatar(s)
Just as there are electric charges in nature, so are there spin currents found in nature. ‘Spin’ is a key property found in quantum particles. Unlike what the name suggests, these quantum particles don’t spin or rotate about any axis passing through them. However, these particles carry an angular momentum as though it does spin.
In 1971, before von Klitzing observed the quantum Hall effect, Mikhail Dyakonov and Vladimir Perel hypothesized the spin Hall effect.
In this avatar of the Hall effect, quantum spins of opposite kinds accumulate at the edges of the sample, orthogonal to the direction in which the charge current passes.
The spin selection can be facilitated by the spin-orbit coupling. This refers to the modified energy levels in an atom when the electron’s motion is under the influence on the magnetic field generated by the nucleus. Strong coupling may be intrinsic to doped semiconductors. The proposal has triggered intense investigation of the phenomenon, with first experimental observations of the spin Hall effect seen in n-doped semiconductors and two-dimensional hole gases.
Quantum spins don’t really look like the depiction above, which is meant to showcase a fact that particles like electrons do have an intrinsic angular momentum nonetheless. Credit: Karthik / Ed Publica
For more than a decade, studies concerning the spin current and its application to novel spintronics (or spin electronics) have received plethora of attention. This is with regard to efficiently generating, manipulating and detecting spin accumulation in a sample material. Some progress has also occurred from the device fabrication perspective via techniques such as spin injection, among others.
A major advantage in dealing with the spin current lies in the non-dissipative (or very less dissipation) nature which arises owing to the time reversal invariance of the spin current. This presents a non-dissipative scenario (unlike the dissipative effects seen with charged currents), thus making it quite advantageous for spin transport phenomena.
Furthermore, a quantized version of the spin Hall effect exists, with mercury telluride and cadmium telluride quantum well superlattices, showcasing this effect. In 2005, a quantum treatment was proposed by Charles Kane and Eugene Mele, in the form of a tight binding toy model of electrons operating in a two-dimensional honeycomb lattice.
In fact, the ‘wonder material’ graphene, which is a two-dimensional honeycomb lattice constituting carbon atoms, does satisfy some key requirements for the quantum spin Hall effect. However, it lacks a large spin-orbit coupling among other requirements.
Nonetheless, graphene’s ability to entertain the quantum spin Hall effect, makes it a prospective candidate to find applications in next-generation spintronic devices.
Space & Physics
Study Shows Single Qubit Can Outperform Classical Computers in Real-World Communication Tasks
This new research, however, offers compelling evidence of quantum systems’ power in a real-world scenario
Breakthrough Study Shows Quantum Systems Can Outperform Classical Computers in Real-World Communication Tasks
A new study from the S. N. Bose National Centre for Basic Sciences in West Bengal, India, in collaboration with international teams has revealed that even the simplest quantum system, a single qubit, can surpass its classical counterpart in certain communication tasks. This discovery reshapes our understanding of quantum computing and hints at a future where quantum technologies could solve problems that classical computers, even with ample resources, cannot.
Quantum systems have long been seen as the next frontier in computing, with the potential to revolutionize technology. However, proving their superiority over classical systems has been a challenge, as experiments are complex, and limitations often arise that suggest quantum advantage might not be as accessible as once thought. This new research, however, offers compelling evidence of quantum systems’ power in a real-world scenario.
Professor Manik Banik and his team at the S. N. Bose Centre, alongside researchers from the Henan Key Laboratory of Quantum Information and Cryptography, Laboratoire d’Information Quantique, University libre de Bruxelles, and ICFO—the Barcelona Institute of Science and Technology, have demonstrated that a single qubit can outperform a classical bit in a communication task, even when no extra resources, like shared randomness, are available. The theoretical study, published in Quantum, was accompanied by an experimental demonstration featured as an Editors’ Suggestion in Physical Review Letters.
The team’s innovative approach involved developing a photonic quantum processor and a novel tool called a variational triangular polarimeter
The key to this breakthrough lies in the way quantum and classical systems handle communication. Classical communication often relies on shared resources, such as pre-agreed random numbers, to function efficiently. Without these shared resources, the task becomes more challenging. In contrast, the researchers found that a qubit does not require such help and can still outperform a classical bit under the same conditions.
The team’s innovative approach involved developing a photonic quantum processor and a novel tool called a variational triangular polarimeter. This device enabled them to measure light polarization with high precision using a technique known as Positive Operator-Valued Measurements (POVM). These measurements play a crucial role in understanding the behavior of quantum systems, particularly under realistic conditions that include noise.
“This result is particularly exciting because it demonstrates a tangible quantum advantage in a realistic communication scenario,” said Professor Banik. “For a long time, quantum advantage was mostly theoretical. Now, we’ve shown that even a single qubit can outperform classical systems, opening up new possibilities for quantum communication and computing.”
This research represents more than just an academic milestone; it brings us a step closer to a future where quantum technologies could drastically alter how we process and communicate information. As quantum systems continue to develop, this breakthrough makes the divide between quantum and classical computing not only more fascinating but also more attainable. The study also signals that quantum systems may eventually be able to solve problems that classical computers struggle with, even when resources are limited.
With this discovery, the potential for quantum communication and computation is moving from theoretical to practical applications, making the future of quantum technologies look even more promising.
Space & Physics
IIT Kanpur Unveils World’s First BCI-Based Robotic Hand Exoskeleton for Stroke Rehabilitation
The BCI-based robotic hand exoskeleton utilizes a unique closed-loop control system to actively engage the patient’s brain during therapy
The Indian Institute of Technology Kanpur (IITK) has unveiled the world’s first Brain-Computer Interface (BCI)-based Robotic Hand Exoskeleton, a groundbreaking innovation set to revolutionize stroke rehabilitation. This technology promises to accelerate recovery and improve patient outcomes by redefining post-stroke therapy. Developed over 15 years of rigorous research led by Prof. Ashish Dutta from IIT Kanpur’s Department of Mechanical Engineering, the project was supported by India’s Department of Science and Technology (DST), UK India Education and Research Initiative (UKIERI), and the Indian Council of Medical Research (ICMR).
The BCI-based robotic hand exoskeleton utilizes a unique closed-loop control system to actively engage the patient’s brain during therapy. It integrates three key components: a Brain-Computer Interface that captures EEG signals from the motor cortex to detect the patient’s intent to move, a robotic hand exoskeleton that assists with therapeutic hand movements, and software that synchronizes brain signals with the exoskeleton for real-time feedback. This coordination helps foster continuous brain engagement, leading to faster and more effective recovery.
“Stroke recovery is a long and often uncertain process. Our device bridges the gap between physical therapy, brain engagement, and visual feedback creating a closed-loop control system that activates brain plasticity, which is the brain’s ability to change its structure and function in response to stimuli,” said Prof. Ashish Dutta. “This is especially significant for patients whose recovery has plateaued, as it offers renewed hope for further improvement and regaining mobility. With promising results in both India and the UK, we are optimistic that this device will make a significant impact in the field of neurorehabilitation.”
Traditional stroke recovery often faces challenges, especially when motor impairments stem from damage to the motor cortex. Conventional physiotherapy methods may fall short due to limited brain involvement. The new device addresses this gap by linking brain activity with physical movement. During therapy, patients are guided on-screen to perform hand movements, such as opening or closing their fist, while EEG signals from the brain and EMG signals from the muscles are used to activate the robotic exoskeleton in an assist-as-required mode. This synchronization ensures the brain, muscles, and visual engagement work together, improving recovery outcomes.
Pilot clinical trials, conducted in collaboration with Regency Hospital in India and the University of Ulster in the UK, have yielded impressive results. Remarkably, eight patients—four in India and four in the UK—who had reached a recovery plateau one or two years post-stroke achieved full recovery through the BCI-based robotic therapy. The device’s active engagement of the brain during therapy has proven to lead to faster and more comprehensive recovery compared to traditional physiotherapy.
While stroke recovery is typically most effective within the first six to twelve months, this innovative device has demonstrated its ability to facilitate recovery even beyond this critical period. With large-scale clinical trials underway at Apollo Hospitals in India, the device is expected to be commercially available within three to five years, offering new hope for stroke patients worldwide.
Space & Physics
Obituary: R. Chidambaram, Eminent Physicist and Architect of India’s Nuclear Program
Rajagopala Chidambaram (1936–2025), a man whose work shaped the future of modern India, will always be remembered as the chief architect of India’s nuclear journey.
Rajagopala Chidambaram, a world-class physicist and the chief architect of India’s nuclear program, passed away on January 4, 2025, at the age of 88. Renowned for his unparalleled contributions to India’s nuclear defense and energy security, Chidambaram leaves a profound legacy in both the scientific community and the nation’s strategic defense apparatus.
Born on November 11, 1936, in India, Dr. Chidambaram was an alumnus of Presidency College, Chennai, Tamil Nadu, and the Indian Institute of Science, Bengaluru, Karnataka. His academic background, coupled with his innate curiosity and vision, led him to become one of India’s foremost scientific minds. Throughout his illustrious career, Dr. Chidambaram played an instrumental role in shaping India’s nuclear capabilities, overseeing both the Pokhran-I (1974) and Pokhran-II (1998) nuclear tests, which cemented India’s position as a nuclear power on the world stage.
As a physicist, Dr. Chidambaram’s groundbreaking research in high-pressure physics, crystallography, and materials science greatly advanced the understanding of these fields. His pioneering work laid the foundation for modern materials science research in India, contributing to the nation’s scientific progress in multiple areas. His expertise in these complex disciplines not only bolstered India’s nuclear research but also advanced its technological prowess.
In addition to his work in nuclear weapons development, Dr. Chidambaram made significant strides in nuclear energy, ensuring that India remained at the forefront of scientific and technological advancements. As Director of the Bhabha Atomic Research Centre (BARC) and later as Chairman of the Atomic Energy Commission of India, he was integral to India’s peaceful nuclear energy initiatives. As Principal Scientific Adviser to the Government of India, Dr. Chidambaram guided national policies on defense, energy, and nuclear research, shaping the future of India’s scientific endeavors.
He was a vital member of the team that conducted India’s first nuclear test, Smiling Buddha, at Pokhran in 1974. His leadership during the Pokhran-II tests in 1998, which confirmed India’s nuclear deterrent, was a defining moment in the nation’s history. Chidambaram’s steadfast commitment to India’s defense and scientific advancement earned him respect both at home and abroad.
A visionary leader, Dr. Chidambaram believed in the power of science and technology to drive national development. His efforts were instrumental in championing key initiatives in energy, healthcare, and strategic self-reliance. He steered numerous projects that significantly advanced India’s science and technology landscape. Notably, he played a central role in the indigenous development of supercomputers and was the driving force behind the conceptualization of the National Knowledge Network, which connected research and educational institutions across India.
Dr. Chidambaram was also an ardent advocate for the application of science and technology to improve societal conditions. He established the Rural Technology Action Groups and the Society for Electronic Transactions and Security, among other programs. His emphasis on “Coherent Synergy” in India’s scientific efforts helped foster collaboration across various disciplines, accelerating the country’s scientific growth.
On the global stage, Dr. Chidambaram served as the Chairman of the Board of Governors of the International Atomic Energy Agency (IAEA) in 1994-1995 and contributed to several high-level international nuclear discussions. His expertise was sought worldwide, and in 2008, he was appointed to the Commission of Eminent Persons by the IAEA to assess the agency’s role in nuclear governance.
He was a vital member of the team that conducted India’s first nuclear test, Smiling Buddha, at Pokhran in 1974
In recognition of his exceptional contributions to science and national development, Dr. Chidambaram received several prestigious accolades, including the Padma Shri in 1975 and the Padma Vibhushan in 1999. He was also awarded honorary doctorates from several universities and was a fellow of several eminent Indian and international scientific academies.
Dr. Chidambaram’s passing marks the end of an era for India’s nuclear program and the global scientific community. His legacy as a scientist, visionary leader, and architect of India’s nuclear journey will continue to inspire future generations. His contributions to national security, energy, and technological innovation have left an indelible mark on India’s scientific and strategic landscape.
Rajagopala Chidambaram’s profound impact on India’s nuclear and scientific trajectory will be remembered for generations to come. His work in advancing both national defense and the peaceful use of nuclear energy stands as a testament to his vision of a self-reliant, scientifically empowered India.
“Deeply saddened by the demise of Dr Rajagopala Chidambaram. He was one of the key architects of India’s nuclear programme and made ground-breaking contributions in strengthening India’s scientific and strategic capabilities. He will be remembered with gratitude by the whole nation and his efforts will inspire generations to come,” Prime Minister Narendra Modi wrote on X.
Dr. Ajit Kumar Mohanty, Secretary, Department of Atomic Energy, in a statement issued, said, “Dr. Chidambaram was a doyen of science and technology whose contributions furthered India’s nuclear prowess and strategic self-reliance. His loss is an irreparable one for the scientific community and the nation.”
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