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critical temperature

A microscope that catches the slightest hints of heat

A superconducting transition-edge sensor (TES) is a device well-known for its extreme sensitivity to photons, the particles of light — so much so that they can count photons one by one. They also have very little noise, which makes their readings quite reliable. TESs are often used in single-photon detectors in quantum communications systems and in cryogenic bolometers (devices that measure infrared radiation) in astronomy. But for these virtues, however, engineers haven’t been able to use TES technology together with scanning-probe optics, where scientists use a physical probe to image surfaces at extremely high resolution. In atomic force microscopy, for example, a very sharp tip is mounted on a flexible cantilever over a surface to measure forces between the tip and the sample at the nanoscale. This technology gap has been important to fill because scanning-probe optics are currently limited by how sensitive detectors are to light fields just a few nanometres big. In other words, the missing piece was a device that married the sensitivity of a TES device with the ability of a scanning probe to access spatial scales of nanometres. A new effort by researchers from Singapore, Switzerland, and the US has offered to fill this gap using a bespoke new technique called bolometric superconducting optical nanoscopy (BOSON). According to the researchers, BOSON integrates a superconducting TES directly into a scanning near-field optical microscope. The findings were published in Physical Review X on July 25.

‘Near field’ has a simple meaning. In conventional microscopy, like the simple light microscope in a high-school biology lab, light from a sample is captured through lenses and eventually sent to the eyes of the observer. This is called far-field microscopy because the light that contains information about the sample under study travels several multiples of its wavelength before interacting with the detecting elements. In near-field microscopy, light travels much less than a single multiple of its wavelength before reaching these elements. For example, if the wavelength of the light is 500 nm, it may travel 5 cm — or 100,000-times its wavelength — before striking the lens. On the other hand, near-field microscopy, also called near-field nanoscopy, captures and analyses light that has travelled much less than 500 nm from the sample. Devices of this kind routinely use junctions made of graphene, semiconductors or metals to translate the properties of the light energy into a measurable electrical current. These technologies demand high optical power, in the milliwatt to sub-milliwatt range, as well as elaborate engineering. They also struggle to detect changes in a sample that produce weak electromagnetic fields, like vibrating atoms in some crystals. Graphene-based devices that reveal temperature changes in a sample by shifting their resistivity are also limited by the fact that graphene’s resistivity changes very weakly with temparature, limiting the devices’ usefulness in bolometry. The team behind the new study thus set about looking for a detector whose resistance would change abruptly with even a small thermal load. This was BOSON.

At the heart of BOSON is a bridge. It’s made of niobium, a metal that becomes a superconductor at very low temperature. It’s also only 200-250 nm wide, a really small size that makes it extremely sensitive to heat. Imagine a single snowflake landing on your finger: even the gentle heat from your body suffices to melt it quickly. Similarly, even a small amount of heat will cause the niobium bridge’s temperature to rise enough to jerk it out of its superconducting state. The bridge sits between wider niobium leads. At the start of the researchers’ experiment, the team passed a constant current through the bridge. Hovering just above the bridge was the small, sharp tip of an atomic force microscope. When an infrared laser struck the probe tip, it concentrated the electromagnetic field onto the bridge. When the tip-induced field raised the electrons’ temperature by only a few millikelvin, a “hot spot” formed on the niobium bridge. In this region, the bridge resisted the flow of current enough for a voltage to register between the leads at the ends of the bridge. This voltage was the ultimate signal of interest, demonstrating that BOSON could reliably detect extremely small changes in temperature.

The researchers also found that BOSON’s resolution is limited not by the size of the atomic force microscope’s tip (around 20 nm tip) but by the lengths across which the energy diffuses into the bridge — under 1 micrometre in the niobium bridge — and the size of the bridge itself. The researchers have written that further narrowing the bridge could further improve its spatial resolution.

Still, to highlight BOSON’s optical reach in their study, they overlaid the bridge with a 50-nm thick flake of hexagonal boron nitride (hBN), a material known to contain an unusual kind of wave called hyperbolic phonon-polaritons when illuminated with mid-infrared light. Hyperbolic phonon-polaritons are formed from when photons interact strongly with vibrations in the grid of atoms in a crystal, especially when the vibrations are within a particular frequency range. This interaction allows light to be guided into tracks that are narrower than the diffraction limit — a very desirable ability in microscopes trying to achieve a high resolution. The team shone an infrared laser at the hBN crystal to produce hyperbolic phonon-polaritons, then monitored the niobium bridge. They found that the phonon-polaritons produced an electromagnetic field in the crystal and the bridge was sensitive to changes in this field even when the latter’s power was as feeble as 50 nanowatt — fully four orders of magnitude below the power required to draw the attention of existing near-field microscopes. According to the researchers, this dramatic advance stemmed from operating the detector exactly at its superconducting transition temperature, where the bridge’s sensitivity to temperature changes is highest. BOSON also revealed how the phonon-polaritons dispersed within the hBN crystal, found to be consistent with theoretical predictions. The team said that since the bridge width is the effective detector size, future bridges that are only tens of nanometres wide should be able to study materials like hBN with even more sensitivity.

By combining a superconducting bolometer with a scanning probe, the team has shown that BOSON is a universal, cryogenic nano-optical detector whose sensitivity rivals the best available TES devices. The platform can reportedly detect weak shifts in the energy of a material with nanometre precision while depositing a negligible amount of energy into the sample, a feature that could prove useful in the study of quantum materials, which are typically very fragile. According to the team’s paper, an improved BOSON may in future may be able to detect single polaritons (quasiparticles each made of a photon coupled to an electric dipole) and be sensitive to electromagnetic fields with ultra-high frequencies (in the terahertz range). They’ve also speculated that thinner superconducting bridges and the use of improved techniques to sense voltage across them could make BOSON sensitive to power changes even slighter than nanowatts.

Featured image: A schematic diagram of the experimental setup of BOSON. CP refers to ‘Cooper pairs’, which are the charge carriers in a superconductor. I_bias is the biasing current applied to the niobium bridge. Credit: Phys. Rev. X 15, 031027.

Aug 2, 2025
A stinky superconductor

The next time you smell a whiff of rot in your morning’s eggs, you might not want to throw them away. Instead, you might do better to realise what you’re smelling could be a superconductor (under the right conditions) that’s, incidentally, riled up the scientific community.

The source of excitement is a paper published in Nature on August 17, penned by a group of German scientists, describing an experiment in which the compound hydrogen sulphide conducts electricity with zero resistance under a pressure of 90 gigapascals (about 888,231-times the atmospheric pressure) – when it turns into a metal – and at a temperature of 203.5 kelvin, about -70.5° C. The discovery makes it an unexpected high-temperature superconductor, doubly so for becoming one under conditions physicists don’t find too esoteric.

The tag of ‘high-temperature’ may be unfit for something operating at -70.5° C, but in superconductivity, -70.5° C approaches summer in the Atacama. When the phenomenon was first discovered – by the Dutch physicist Heike Kamerlingh Onnes in 1911 – it required the liquid metal mercury to be cooled to 4.2 kelvin, about -269° C. What happened in those conditions was explained by an American trio with a theory of superconductivity in 1957.

The explanation lies in quantum mechanics, where all particles have a characteristic ‘spin’ number. And QM allows all those particles with integer spin (0, 1, 2, …) to – in some conditions – cohere into one bigger ‘particle’ with enough energy of itself to avoid being disturbed by things like friction or atomic vibrations*. Electrons, however, have half-integer (1/2) spin, so can’t slip into this state. In 1957, John Bardeen, Leon Cooper and Robert Schrieffer proposed that at very low temperatures – like 4 K – the electrons in a metal interact with the positively charged latticework of atoms around them to pair up with each other. These electronic pairs are called Cooper pairs, kept twinned by vibrations of the lattice. The pair’s total spin is 1, allowing all of them to condense into one cohesive sea of electrons that then flows through the metal unhindered.

The BCS theory soon became a ‘conventional’ theory of superconductivity, able to explain the behaviour of many metals cooled to cryogenic temperatures. The German team’s hydrogen sulphide system is also one such conventional scenario – in which the gas had to compressed to form a metal before its superconducting abilities were teased out.

The team, led by Mikhail Eremets and Alexander Drozdov from the Max Planck Institute for Chemistry in Mainz, first made its claims last year, that under heavy pressure hydrogen sulphide becomes sulphur hydride (H₂S → H₃S), which in turn is a superconductor. At the time their experiment showed only one of two typical properties of a superconducting system, however: that its electrical resistance vanished at 190 K, higher than the previous record of 164 K.

Their August 17 paper reports that the second property has since been observed, too: that pressurised hydrogen sulphide doesn’t allow any external magnetic field to penetrate beyond its surface. This effect, called the Meissner effect, is observed only in superconductors. For Eremets, Drozdov et al, this is the full monty: a superconductor functioning at temperatures that actually exist on Earth. But for the broader scientific community, the paper marks the frenzied beginning of a new wave of experiments in the field.

Given the profundity of the findings – of a hydrogen-based high-temperature superconductor – they won’t enter the canon just yet but will require independent verification from other teams. A report by Edwin Cartlidge in Nature already notes five other teams around the world working on replicating the discovery. If and when they succeed, the implications will be wide-ranging – for physics as well as historical traditions of physical chemistry.

The BCS theory of superconductivity provided a precise mechanism of action that allowed scientists to predict the critical temperature (T_c) – below which a material becomes superconducting – of all materials that abided by the theory. Nonetheless, by 1957, the highest T_c reached had been 10 K despite scientists’ best efforts; so great was their frustration that in 1972, Philip Warren Anderson and Marvin Cohen predicted that there could be a natural limit at 30 K.

However, just a few years earlier – in 1968 – two physicists, Neil Ashcroft and Vitaly Ginzburg, refusing to subscribe to a natural limit on the critical temperature, proposed that the T_c could be very high in substances in which the vibrations of the atomic latticework surrounding the electrons was pretty energetic. Such vigour is typically found in the lighter elements like hydrogen and helium. Thus, the Ashcroft-Ginzburg idea effectively set the theoretical precedent for Eremets and Drozdov’s work.

But between the late 1960s and 2014, when hydrogen sulphide entered the fray of experiments, two discoveries threw the BCS theory off kilter. In 1986, scientists discovered cuprates, a class of copper’s compounds that were superconductors at 133 K (at 164 K under pressure) but didn’t function according to the BCS theory. Thus, they came to be called unconventional superconductors. The second discovery was of another class of unconventional superconductors, this time in compounds of iron and arsenic called pnictides, in 2008. The highest T_c among them was less than that of the cuprates. And because cuprates under pressure could muster a T_c of 164 K, scientists pinned their hopes on them of breaching the room-temperature barrier, and worked on developing an unconventional theory of superconductivity.

But for those choosing to persevere with the conventional order of things, there was a brief flicker of hope in 2001 with the discovery of magnesium diboride superconductors: they had a T_c of 39 K, an important but not very substantial improvement on previous records among conventional materials.

The work of Eremets & Drozdov was also indirectly assisted by a group of Chinese researchers in 2014, who were able to anticipate hydrogen sulphide’s superconducting abilities using the conventional BCS theory. According to them, hydrogen sulphide would become a metal under the application of 111 gigapascals of pressure, with a T_c between 191 K and 204 K. And once it survives independent experimental scrutiny intact, the Chinese theoretical work will prove valuable as scientists confront their next big challenge: pressure.

The ultimate fantasy would be to have a T_c is in the range of ambient temperatures. Imagine leagues of superconducting cables radiating out from coal-choked power plants, a gigawatt of power transmitted for a gigawatt of power produced**, or maglev trains running on superconducting tracks at lower costs and currents, or the thousands of superconducting electromagnets around the LHC that won’t have to be supercooled using jackets of liquid helium. Sadly, that Eremets & Drozdov have (probably) achieved a T_c of 203.5 K doesn’t mean that the engineering is accessible or affordable. In fact, what allowed them to fetch 203.5 K is what the barrier is for the tech to be ubiquitously used, making their feat an antecedence of possibilities rather than a demonstration itself.

It wasn’t possible until the 1970s to achieve pressures of a few gigapascals in the lab, and similar processes today are confined to industrial purposes. A portable device that’d sustain that pressure across large areas is difficult to build – yet that’s when metallic sulphur hydride shows itself. In their experiment, Eremets and Drozdov packed a cold mass of hydrogen sulphide against a stainless steel gasket using some insulating material like teflon, and then sandwiched the pellet between two diamond anvils that pressurised it. The diameter of the entire apparatus was a little more than a 100 micrometers across. Moreover, they also note in their paper that the ‘loading’ of the hydrogen sulphide between the anvils needs to be done at a low temperature – before pressurisation – so that the gas doesn’t decompose before the superconducting can begin.

These are impractical conditions if hydrogen sulphide cables have to be handled by a crew of non-specialists and in conditions nowhere near controllable enough as the insides of a small steel gasket. As an alternative, should independent verification of the Eremets & Drozdov experiment happen, scientists will use it as a validation of the Chinese theorists’ calculations and extend that to fashion a material more suited to their purposes.

*The foundation for this section of QM was laid by Satyendra Nath Bose, and later expanded by Albert Einstein to become the Bose-Einstein statistics.

**But not a gigawatt of power consumed, thanks to power thefts to the tune of Rs.2.52 lakh crore.

Aug 19, 2015
“Maybe the Higgs boson is fictitious!”

That’s an intriguing and, as he remarks, plausible speculation by the noted condensed-matter physicist Philip Warren Anderson. It appears in a short article penned by him in Nature Physics on January 26, in which he discusses how the Higgs mechanism as in particle physics was inspired by a similar phenomenon observed in superconductors.

According to the Bardeen-Cooper-Schrieffer theory, certain materials lose their resistance to the flow of electric current completely and become superconductors below a critical temperature. Specifically, below this temperature, electrons don’t have the energy to sustain their mutual Coulomb repulsion. Instead, they experience a very weak yet persistent attractive force between them, which encourages them to team up in pairs called Cooper pairs (named for Leon Cooper).

If even one Cooper pair is disrupted, all Cooper pairs in the superconductor will break, and it will cease to be a superconductor as well. As a result, the energy to break one pair is equivalent to the energy necessary to break all pairs – a coercive state of affairs that keeps the pairs paired up despite energetic vibrations from the atoms in the material’s lattice. In this energetic environment, the Cooper pairs all behave as if they were part of a collective (described as a Bose-Einstein condensate).

This transformation can be understood as the spontaneous breaking of a symmetry: the gauge symmetry of electromagnetism, which dictates that no experiment can distinguish between the laws governing electricity and magnetism. With a superconductor, however, the laws governing electricity in the material become different below the critical temperature. And when a gauge symmetry breaks, a massive¹ boson is formed. In the case of BCS superconductivity, however, it is not an actual particle as much as the collective mode of the condensate.

In particle physics, a similar example exists in the form of electroweak symmetry breaking. While we are aware of four fundamental forces in play around us (strong, weak, electromagnetic and gravitational), at higher energies the forces are thought to become unified into one ‘common’ force. And on the road to unification, the first to happen is of the electromagnetic and weak forces – into the electroweak force. Axiomatically, the electroweak symmetry was broken to yield the electromagnetic and weak forces, and the massive Higgs boson.

Anderson, who first discussed the ‘Higgs mode’ in superconductors in a paper in 1958, writes in his January 26 article (titled Higgs, Anderson and all that),

… Yoichiro Nambu, who was a particle theorist and had only been drawn into our field by the gauge problem, noticed in 1960 that a BCS-like theory could be used to create mass terms for massless elementary particles out of their interactions. After all, one way to describe the energy gap in BCS is that it represents a mass term for every point on the Fermi surface, mixing the particle with its opposite spin and momentum antiparticle. In 1960 Nambu and Jona-Lasinio developed a theory in which most of the mass of the nucleon comes from interactions — this theory is still considered partially correct.

But the real application of the idea of a superconductivity-like broken symmetry as a source of the particle spectrum came with the electroweak theory — which unified the electromagnetic and weak interactions — of Sheldon Glashow, Abdus Salam and Steven Weinberg.

What is fascinating is that these two phenomena transpire at outstandingly different energy scales. The unification of the electromagnetic and weak forces into the electroweak force happens beyond 100 GeV. The energy scale at which the electrons in magnesium diboride become superconducting is around 0.002 eV. As Terry Pratchett would have it, the “aching gulf” of energy in between spans 12 orders of magnitude.

At the same time, the parallels between superconductivity and electroweak symmetry breaking are more easily drawn than between other, more disparate fields of study because their occurrence is understood in terms of the behavior of fundamental particles, especially bosons and fermions. It is this equivalence that makes Anderson’s speculative remark more attractive:

If superconductivity does not require an explicit Higgs in the Hamiltonian to observe a Higgs mode, might the same be true for the 126 GeV mode? As far as I can interpret what is being said about the numbers, I think that is entirely plausible. Maybe the Higgs boson is fictitious!

To help us along, all we have at the moment is the latest in an increasingly asymptotic series of confirmations: as reported by CERN, “the results draw a picture of a particle that – for the moment – cannot be distinguished from the Standard Model predictions for the Higgs boson.”

¹Massive as in having mass, not as in a giant boson.

Feb 7, 2015