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Why does superconductivity matter?

Dr. Saurabh Basu

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A high-temperature (liquid nitrogen cooled) superconductor levitating above a permanent magnet (TU Dresden). Credit: Henry Mühlpfordt/Wikemedia Commons

Superconductivity was discovered by H. Kamerlingh Onnes on April 8, 1911, who was studying the resistance of solid Mercury (Hg) at cryogenic temperatures. Liquid helium was recently discovered at that time. At T = 4.2K, the resistance of Hg disappeared abruptly. This marked a transition to a new phase that was never seen before. The state is resistanceless, strongly diamagnetic, and denotes a new state of matter. K. Onnes sent two reports to KNAW (the local journal of the Netherlands), where he preferred calling the zero-resistance state ‘superconductivity’’.

There was another discovery that went unnoticed in the same experiment, which was the transition of superfluid Helium (He) at 2.2K, the so-called λ transition, below which He becomes a superfluid. However, we shall skip that discussion for now. A couple of years later, superconductivity was found in lead (Pb) at 7K. Much later, in 1941, Niobium Nitride was found to superconduct below 16 K. The burning question in those days was: what would the conductivity or resistivity of metals be at a very low temperature?

The reason behind such a question is Lord Kelvin’s suggestion that for metals, initially the resistivity decreases with falling temperature and finally climbs to infinity at zero Kelvin because electrons’ mobility becomes zero at 0 K, yielding zero conductivity and hence infinite resistivity. Kamerlingh Onnes and his assistant Jacob Clay studied the resistance of gold (Au) and platinum (Pt) down to T = 14K. There was a linear decrease in resistance until 14 K; however, lower temperatures cannot be accessed owing to the unavailability of liquid He, which eventually happened in 1908.

Heike Kamerlingh Onnes (right), the discoverer of superconductivity. Paul Ehrenfest, Hendrik Lorentz, Niels Bohr stand to his left.

In fact, the experiment with Au and Pt was repeated after 1908. For Pt, the resistivity became constant after 4.2K, while Au is found to superconduct at very low temperatures. Thus, Lord Kelvin’s notion about infinite resistivity at very low temperatures was incorrect. Onnes had found that at 3 K (below the transition), the normalised resistance is about 10−7. Above 4.2 K, the resistivity starts appearing again. The transition is too sharp and falls abruptly to zero within a temperature window of 10−4 K.

Perfect conductors, superconductors, and magnets

All superconductors are normal metals above the transition temperature. If we ask in the periodic table where most of the superconductors are located, the answer throws some surprises. The good metals are rarely superconducting. The examples are Ag, Au, Cu, Cs, etc., which have transition temperatures of the order of ∼ 0.1K, while the bad metals, such as niobium alloys, copper oxides, and 1 MgB2, have relatively larger transition temperatures. Thus, bad metals are, in general, good superconductors. An important quantity in this regard is the mean free path of the electrons. The mean free path is of the order of a few A0 for metals (above Tc), while for good metals (or the bad superconductors), it is usually a few hundred of A0. Whereas for the bad metals (good superconductors), it is still small as the electrons are strongly coupled to phonons. The orbital overlap is large in a superconductor. In good metals, the orbital overlap is small, and often they become good magnets. In the periodic table, transition elements such as the 3D series elements, namely Al, Bi, Cd, Ga, etc., become good superconductors, while Cr, Mn, and Fe are bad superconductors and in fact form good magnets. For all of them, that is, whether they are superconductors or magnets, there is a large density of states at the Fermi level. So, a lot of electronic states are necessary for the electrons in these systems to be able to condense into a superconducting state (or even a magnetic state). The nature of the electronic wave function determines whether they develop superconducting order or magnetic order. For example, electronic wavefunctions have a large spatial extent for superconductors, while they are short-range for magnets.

Meissner effect

The near-complete expulsion of the magnetic field from a superconducting specimen is called the Meissner effect. In the presence of a magnetic field, the current loops at the periphery will be generated so as to block the entry of the external field inside the specimen. If a magnetic field is allowed within a superconductor, then, by Ampere’s law, there will be normal current within the sample. However, there is no normal current inside the specimen. Thus, there can’t be any magnetic field. For this reason, superconductors are known as perfect diamagnets with very large diamagnetic susceptibility. Even the best-known diamagnets (which are non-superconductors) have magnetic susceptibilities of the order of 10−5. Thus, the diamagnetic property can be considered a distinct property of superconductors compared to zero electrical resistance.

A typical experiment demonstrating the Meissner effect can be thought of as follows: Take a superconducting sample (T < Tc), sprinkle iron filings around the sample, and switch on the magnetic field. The iron filings are going to line up in concentric circles around the specimen. This implies the expulsion of the flux lines outside the sample, which makes the filings line up.

Distinction between perfect conductors and superconductors

The distinction between a perfect conductor and a superconductor is brought about by magnetic field-cooled (FC) and zero-field-cooled (ZFc) cases, as shown below in Fig. 1.

In the absence of an external magnetic field, temperature is lowered for both the metal and the superconductor in their metallic states from T > Tc to T < Tc (see left panel for both in Fig. 1). Hence, a magnetic field is applied, which eventually gets expelled owing to the Meissner effect. The field has finally been withdrawn. However, if cooling is done in the presence of an external field, after the field is withdrawn, the flux lines get trapped for a perfect conductor; however, the superconductor is left with no memory of an applied field, a situation similar to what happens in the zero-field cooling case. So, superconductors have no memory, while perfect conductors have memory.

Microscopic considerations: BCS theory

The first microscopic theory of superconductivity was proposed by Berdeen, Cooper, and Schrieffer (BCS) in 1957, which earned them a Nobel Prize in 1972. The underlying assumption was that an attractive interaction between the electrons is possible, which is mediated via phonons. Thus, electrons form bound pairs under certain conditions, such as (i) two electrons in the vicinity of the filled Fermi Sea within an energy range ¯hωD (set by the phonons or lattice). (ii) The presence of phonons or the underlying lattice is confirmed by the isotope effect experiment, which confirms that the transition temperature is proportional to the mass of ions. Since the Debye frequency depends on the ionic mass, it implies that the lattice must be involved. 3 A small calculation yields that an attractive interaction is possible in a narrow range of energy. This attractive interaction causes the system to be unstable, and a long-range order develops via symmetry breaking. In a book by one of the discoverers, namely, Schrieffer, he described an analogy between a dancing floor comprising couples, dancing one with any other couple, and being completely oblivious to any other couple present in the room. The couples, while dancing, drift from one end of the room to another but do not collide with each other. This implies less dissipation in the transport of a superconductor. The BCS theory explained most of the features of the superconductors known at that time, such as (i) the discontinuity of the specific heat at the transition temperature, Tc. (ii) Involvement of the lattice via the isotope effect. (iii) Estimation of Tc and the energy gap. The value of Tc and the gap are confirmed by tunnelling experiments across metal-superconductor (M-S) or metal-insulator-superconductor (MIS) types of junctions. Giaever was awarded the Nobel Prize in 1973 for his work on these experiments. (iv) The Meissner effect can be explained within a linear response regime. (v) Temperature dependence of the energy gap, confirming gradual vanishing, which confirms a second-order phase transition. Most of the features of conventional superconductors can be explained using BCS theory. Another salient feature of the theory is that it is non-perturbative. There is no small parameter in the problem. The calculations were done with a variational theory where the energy is minimised with respect to some free parameters of the variational wavefunction, known as the BCS wavefunction.

Unconventional Superconductors: High-Tc Cuprates

This is a class of superconductors where the two-dimensional copper oxide planes play the main role, and superconductivity occurs in these planes. Doping these planes with mobile carriers makes the system unstable towards superconducting correlations. At zero doping, the system is an antiferromagnetic insulator (see Fig. 2). With about 15% to 20% doping with foreign elements, such as strontium (Sr), etc. (for example, in La2−xSrxCuO4), the system turns superconductivity. There are two things that are surprising in this regard. (i) The proximity of the insulating state to the superconducting state; (ii) For the system initially in the superconducting state, as the temperature is raised, instead of going into a metallic state, it shows several unfamiliar features that are very unlike the known Fermi liquid characteristics. It is called a strange metal.

In fact, there are some signatures of pre-formed pairs in the ‘so-called’ metallic state, known as the pseudo gap phase. Since the starting point from which one should build a theory is missing, a complete understanding of the mechanism leading to the phenomenon cannot be understood. It remained a theoretical riddle.

Dr. Saurabh Basu is Professor at Department of Physics, Indian Institute of Technology (IIT) Guwahati. He works in the area of correlated electron systems with the main focus on bosonic superfluidity in (optical) lattices.

Space & Physics

This Sodium-Fuelled Clean Energy Breakthrough Could Electrify Aviation and Shipping

The innovation offers more than triple the energy density of today’s lithium-ion batteries — potentially clearing a major hurdle for electric-powered aviation, rail, and maritime travel

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An H-cell modified with electrodes and an ion-conducting ceramic membrane. Credits: Gretchen Ertl/MIT News

A new type of fuel cell developed by MIT researchers could represent a pivotal breakthrough in the race to decarbonize heavy transportation. Designed around liquid sodium metal, the innovation offers more than triple the energy density of today’s lithium-ion batteries — potentially clearing a major hurdle for electric-powered aviation, rail, and maritime travel.

Unlike traditional batteries that require time-consuming recharging, this system operates like a fuel cell that can be refueled quickly using liquid sodium — a cheap, abundant substance derived from salt. The technology, which uses air as a reactant and a solid ceramic electrolyte to facilitate the reaction, was tested in lab prototypes and demonstrated energy densities exceeding 1,500 watt-hours per kilogram — a level that could enable regional electric flight and clean shipping.

“We expect people to think that this is a totally crazy idea,” said Professor Yet-Ming Chiang, lead author and Kyocera Professor of Ceramics, in a media statement. “If they didn’t, I’d be a bit disappointed because if people don’t think something is totally crazy at first, it probably isn’t going to be that revolutionary.”

Chiang explained that current lithium-ion batteries top out at around 300 watt-hours per kilogram — far short of the 1,000 watt-hours needed for electric aircraft to become viable at scale. The new sodium-based cell meets that benchmark, which could enable 80% of domestic flights and drastically reduce aviation’s carbon footprint.

Moreover, the sodium-fueled system offers environmental benefits beyond zero emissions. Its chemical byproduct, sodium oxide, reacts spontaneously in the atmosphere to capture carbon dioxide and convert it into sodium bicarbonate — better known as baking soda — which may help counteract ocean acidification if it ends up in marine environments.

“There’s this natural cascade of reactions that happens when you start with sodium metal,” Chiang said. “It’s all spontaneous. We don’t have to do anything to make it happen, we just have to fly the airplane.”

The team has already created two functioning lab-scale prototypes: one vertical and one horizontal model. In both, sodium gradually reacts with oxygen from air to generate electricity, and a moist air stream improves the process by allowing liquid byproducts to be expelled more easily.

Karen Sugano, one of the MIT doctoral students on the project, noted, “The key was that we can form this liquid discharge product and remove it easily, as opposed to the solid discharge that would form in dry conditions,” she said in a media statement.

The researchers have founded a startup, Propel Aero, housed in MIT’s startup incubator The Engine, to scale the technology. Their first commercial goal: a brick-sized fuel cell capable of powering a large agricultural drone — expected to be ready within a year.

Chiang emphasized the economic and safety benefits of using sodium, which melts just below 100°C and was once mass-produced in the U.S. for leaded gasoline production. “It reminds us that sodium metal was once produced at large scale and safely handled and distributed around the U.S.,” he said.

Critically, the fuel cell design also avoids many safety concerns of high-energy batteries by physically separating the fuel and oxidizer. “If you’re pushing for really, really high energy density, you’d rather have a fuel cell than a battery for safety reasons,” Chiang said.

By reviving and reimagining sodium-metal chemistry in a practical, scalable form, the MIT team may have lit the path toward clean, electrified transportation systems — from the skies above to the oceans below.

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Space & Physics

Is Time Travel Possible? Exploring the Science Behind the Concept

Subtle forms of time travel — such as time dilation — do occur and have practical implications in science and technology.

Veena M A

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Everyone is, in a way, a time traveller. Whether we like it or not, we are constantly moving through time — one second per second. From one birthday to the next, we travel through time at a steady pace, just like walking one foot per footstep. However, when we talk about “time travel,” we often imagine something much more dramatic — traveling faster (or even backward) through time, as seen in science fiction movies and novels. But is such a thing truly possible?

From Fiction to Science

The concept of time travel first gained widespread attention through literature, particularly with H.G. Wells’ 1895 novel The Time Machine. In it, time is described as the fourth dimension, akin to space, and the protagonist travels forward and backward in time using a specially built machine. Interestingly, this idea predates Albert Einstein’s theory of relativity, which would later reshape how we understand space and time.

Image credit: Wikimedia Commons

Einstein’s Contribution: Relativity and Time Dilation

In the early 20th century, Albert Einstein introduced a revolutionary idea through his theory of relativity. He proposed that space and time are interconnected, forming a four-dimensional continuum called space-time. According to his theory, the speed of light (186,000 miles per second) is the ultimate speed limit in the universe. But how does this relate to time travel?
Einstein’s theory states that as you move faster — especially at speeds approaching the speed of light — time slows down relative to someone who is stationary. This phenomenon, known as time dilation, has been proven through various experiments. One famous example involved two synchronized atomic clocks — one placed on Earth and the other onboard a high-speed jet. When the plane returned, the onboard clock showed slightly less time had passed compared to the one on the ground. This demonstrates that, at very high speeds, time passes more slowly.

Astronaut Twins and Time

A notable example of time dilation involved twin astronauts Scott and Mark Kelly. Scott spent 520 days aboard the International Space Station, while Mark spent only 54 days in space. Due to the effects of time dilation, Scott aged slightly less than Mark — by about 5 milliseconds. Though this difference is minuscule, it is real and measurable, showing that time can indeed “bend” under certain conditions.

The GPS Example

Surprisingly, even GPS satellites experience time differently than we do on Earth. These satellites orbit at altitudes of about 20,200 kilometers and travel at speeds of roughly 14,000 km/h. Due to both their speed (special relativity) and weaker gravitational pull at high altitudes (general relativity), time ticks slightly faster for the satellites than for devices on Earth. This discrepancy is corrected using Einstein’s equations to ensure precise positioning. Without these adjustments, GPS systems could be off by several miles each day.

Science Fiction vs. Scientific Reality

Science fiction has long explored imaginative time travel — characters jumping into machines and traveling decades into the future or past. Stories often depict them altering historical events or witnessing the far future. However, there is no scientific evidence that anyone has travelled backward or forward in time in such a dramatic way.

Renowned physicist Stephen Hawking addressed this idea humorously in 2009. He hosted a party for time travellers — but only announced it afterward, reasoning that if time travel were possible, people from the future would show up. No one came. Hawking took this as a tongue-in-cheek sign that backward time travel may not be feasible.

Could Wormholes Be the Key?

Theoretical physics does suggest possibilities like wormholes — shortcuts through space-time. According to Einstein’s equations, these could, in theory, connect distant places and times. A wormhole might allow someone to enter at one point in space and exit at another, potentially in a different time. However, this remains purely speculative. The extreme gravitational forces within black holes or wormholes could destroy anything attempting to pass through.
Moreover, the idea of backward time travel introduces major paradoxes — such as the classic “grandfather paradox,” where someone goes back in time and prevents their own existence. Such contradictions challenge our understanding of causality and logic.

The Limitations of Current Science

At present, building a time machine capable of transporting people backward or forward in time by centuries remains outside the realm of scientific possibility. It’s a concept best enjoyed in novels and films for now. However, subtle forms of time travel — such as time dilation — do occur and have practical implications in science and technology.

While we may not have DeLoreans or TARDISes at our disposal, time travel — at least in small, measurable ways — is a part of our reality. The interplay of speed, gravity, and time demonstrates that our universe is far more flexible than it appears. And who knows? In some distant corner of the cosmos, nature might already be bending time in ways we are only beginning to imagine.

Until then, we’ll keep moving forward — one second per second.

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Space & Physics

MIT Physicists Capture First-Ever Images of Freely Interacting Atoms in Space

The new technique allows scientists to visualize real-time quantum behavior by momentarily freezing atoms in motion and illuminating them with precisely tuned lasers

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Image: Sampson Wilcox

In an intriguing advancement for quantum physics, MIT researchers have captured the first images of individual atoms freely interacting in space — a feat that until now was only predicted theoretically.

The new imaging technique, developed by a team led by Professor Martin Zwierlein, allows scientists to visualize real-time quantum behavior by momentarily freezing atoms in motion and illuminating them with precisely tuned lasers. Their results, published in Physical Review Letters, reveal how bosons bunch together and fermions pair up in free space — phenomena crucial to understanding superconductivity and other quantum states of matter.

“We are able to see single atoms in these interesting clouds of atoms and what they are doing in relation to each other, which is beautiful,” said Zwierlein in a press statement.

Using their method — called “atom-resolved microscopy” — the team was able to trap atom clouds with a loose laser, briefly immobilize them with a lattice of light, and then image their positions via fluorescence. This approach allowed the researchers to observe quantum behaviors at the level of individual atoms for the first time.

The MIT group directly visualized sodium atoms (bosons) bunching together in a shared quantum wave — a vivid confirmation of the de Broglie wave theory — and lithium atoms (fermions) pairing up despite their natural repulsion, a key mechanism underlying superconductivity.

“This kind of pairing is the basis of a mathematical construction people came up with to explain experiments. But when you see pictures like these, it’s showing in a photograph, an object that was discovered in the mathematical world,” said co-author Richard Fletcher in a press statement.

Two other research teams — one led by Nobel laureate Wolfgang Ketterle at MIT, and another by Tarik Yefsah at École Normale Supérieure — also reported similar quantum imaging breakthroughs in the same journal issue, marking a significant moment in the experimental visualization of quantum mechanics.

The MIT team plans to expand the technique to probe more exotic quantum behaviors, including quantum Hall states. “Now we can verify whether these cartoons of quantum Hall states are actually real,” Zwierlein added. “Because they are pretty bizarre states.”

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