Physical Review Letters

Unexpected: Magnetic regions in metal blow past speed limit

You’re familiar with magnetism, but do you know what it looks like at the smallest scale? Take a block of iron, for example. It’s ferromagnetic, which means if you place it near a permanent magnet – like a refrigerator magnet – the block will also become magnetic to a large extent, larger than materials that aren’t ferromagnetic.

If you zoom in to the iron atoms, you’ll see a difference between areas that are magnetised and areas that aren’t. Every subatomic particle has four quantum numbers, sort of like its Aadhaar or social security ID. No two electrons in the same system can have the same ID, i.e. one, some or all of these numbers differ from one electron to the next. One of these numbers is the spin quantum number, and it can have one of two values, or states, at any given time. Physicists refer to these states as ‘up’ and ‘down’. In the magnetised portions, in the iron block, you’ll see that electrons in the iron atoms will either all be pointing up or all down. This is a defining feature of magnetism.

Scientists have used it to make hard-disk drives that are used in computers. Each drive stores information by encoding it in electrons’ spins using a magnetic field, where, say, ‘1’ is up and ‘0’ is down, so a series of 1s and 0s become a series of ups and downs.

In the iron block, the parts that are magnetised are called domains. They demarcate regions of uniform electron spin in three dimensions in the block’s bulk. For a long time, scientists believed that the ‘walls’ of a domain – i.e. the imaginary surface between areas of uniform spin and areas of dis-uniform spin – could move at up to around 0.5 km/s. If they moved faster, they could destabilise and collapse, allowing a kind of magnetic chaos to spread within the material. They arrived at this speed limit from their theoretical calculations.

The limit matters because it says how fast the iron block’s magnetism can be manipulated, to store or modify data for example, without losing that data. It also matters for any other application that takes advantage of the properties of ferromagnetic materials.

In 2020, a group of researchers from the Czech Republic, Germany, and Sweden found that if you stacked up a layer of ferromagnets, the domain walls could move much faster – as much as 14 km/s – without collapsing. Things can move fast in the subatomic realm, yet 14 km/s was still astonishing for ferromagnetic materials. So scientists set about testing it.

A group from Italy, Sweden, and the US reported in a paper published in Physical Review Letters on December 19 (preprint here) that they were able to detect domain walls moving in a composite material at a stunning 66 km/s – greater than the predicted speed. Importantly, however, existing theories that explain a material’s magnetism at the subatomic scale don’t predict such a high speed, so now physicists know their theories are missing something.

In their study, the group erected a tiny stack of the following elements, in this order: tantalum, copper, a cobalt-iron compound, nickel, the cobalt-iron compound, copper, and tantalum. Advanced microscopy techniques revealed that the ferromagnetic nickel layer (just a nanometre wide) had developed domains of two shapes: some were like stripes and some formed a labyrinth with curved walls.

The researchers then tested the domain walls using the well-known pump-probe technique: a blast of energy first energises a system, then something probes it to understand how it’s changed. The pump here was an extremely short pulse of infrared radiation and the probe was a similarly short pulse of ultraviolet (UV) radiation.

The key is the delay between the pump and probe pulses: the smaller the delay, the greater the detail that comes to light. (Three people won the physics Nobel Prize this year for finding ways to make this delay as small as possible.) In the study it was 50 femtoseconds, or 500 trillionths of a second.

The UV pulse was diffracted by the electrons in nickel. A detector picked up the diffraction patterns and the scientists ‘read’ them together with computer simulations of the domains to understand how they changed.

How did the domains change? The striped walls were practically unmoved but the curved walls of the labyrinthine pattern did move, by about 17-23 nanometres. The group made multiple measurements. When they finally calculated an average speed (which is equal to distance divided by time), they found it to be 66 km/s, give or take 20 km/s.

An image depicting domains (black) in the nickel layer. The coloured lines show their final positions. Source: Phys. Rev. Lett. 131, 256702

The observation of extreme wall speed under far-from-equilibrium conditions is the … most significant result of this study,” they wrote in their paper. This is true: even though the researchers found that the domain-wall speed limit in a multilayer ferromagnetic material is much higher than 0.5 km/s – as the 2020 group predicted – they also found it to be a lot higher than the expected 14 km/s. Of course, it’s also stunning because the curved domain walls moved at more than 10-times the speed of sound in that material – and the more curved a portion was, the faster it seemed to move.

The researchers concluded that “additional mechanisms are required to fully understand these effects” – as well as that they could be “important” to explain “ultrafast phenomena in other systems such as emerging quantum materials”.

This is my second recent post about scientists finding something they didn’t expect to, but in settings more innocuous than in the vast universe or at particle smashers. Read the first one, about the way paint dries, here.

You can do worse than watching paint dry – ask physics

I live in Chennai, a city whose multifaceted identity includes its unrelenting humidity. Its summers are seldom hotter than those in Delhi but they are more unbearable because it leaves people sweaty, dehydrated, and irritated. Delhi’s heat doesn’t have the same effect because when people sweat there, the droplets evaporate into the air, whose low relative humidity allows it to ‘accommodate’ moisture. But in Chennai, the air is almost always humid, more so during the summer, and the sweat on people’s skin doesn’t evaporate. Yet their bodies continue to sweat because it’s one of the few responses they have to the heat.

Paint, fortunately, has a different story to tell. Fresh paint on a wall doesn’t dry faster or slower depending on how humid the air is. This is because pain is made of water plus some polymers whose molecules are much larger than those of water. At first, water does begin to escape the paint and evaporate from the surface. This pulls the polymer molecules to the surface in a process called advection. On the surface, the polymer molecules form a dense layer that prevents the water below from interacting directing with the air, or its humidity. So the rate of evaporation slows until it reaches a constant low value. This is why, even in dry weather, paint takes its time to dry.

Scientists have verified that this is the case in a new study, in which they also reported that their findings can be used to understand the behaviour of little respiratory droplets in which viruses travel through the air. (Some studies – like this and this – have suggested that a virus’s viability may depend on the relative humidity and how quickly the droplet dries, among other factors. Since the relative humidity varies by season, a link could explain why some viral outbreaks are more seasonal.)

Generally, the human skin – as the largest outer-organ of the human body – is responsible for making sure the body doesn’t lose too much water through evaporation. Scientists think that it can adjust how much sweat is released on the skin by modifying the mix of lipids (fatty substances) in its outermost layer. If it did, it would be an example of an active process – a dynamic response to environmental and biological conditions. Paint drying, on the other hand, is a non-active process: the rate of evaporation is limited by the polymer molecules at the surface and their properties.

In 2017, a chemical engineer at the University of Bordeaux named Jean-Baptiste Salmon predicted that an active process may not be needed at all to explain humidity-independent evaporation because it arises naturally in solutions like that of paint. The new study tested the prediction of Salmon et al. using a non-active polymer solution, i.e. one that’s incapable of developing an active response to changes in humidity.

They filled a plastic container with polyvinyl alcohol, then drilled a small hole near the bottom and fit a glass tube there with an open end. The liquid could flow through the tube and evaporate from the end. To prevent the liquid from evaporating from its surface, they coated it with an oily substance called 1-octadecene. They placed this container on a sensitive weighing scale and the whole apparatus inside a sealed box with adjustable humidity. The researchers adjusted the humidity from 25% to 90% and each time studied the evaporation rate for more than 16 hours.

They found that Salmon et al. were right: the evaporation rate was higher for around three hours before dropping to a lower value. This was because polymer molecules had accumulated at the layer where the liquid met the air. But in these three hours, the rate of evaporation didn’t drop even when the humidity was increased. In other words, humidity-independent evaporation begins earlier than Salmon et al. predicted.

The researchers also reported another divergence: the evaporation rate wasn’t affected by a relative humidity of up to 80% – but beyond that, the rate fell if the humidity increased further. So what Salmon et al. said was at play but it wasn’t the full picture; some other forces were also affecting the evaporation.

The researchers ended their paper with an idea. They took a closer look at the open end of the tube, where the polyvinyl alcohol evaporated, with a microscope. They found that the polymer layer was overlaid with a stiffer semisolid, or gel-like, layer. Such layers are known to form when there is a compressive stress, and further block evaporation. The researchers found that their equations to predict the evaporation rate roughly matched the observed value when they were modified to account for this stress. They also found that a sufficiently thick gel layer could form within one second – a short time span considering the many hours over which the rate of evaporation evolves.

“These discrepancies motivate the search for extra physics beyond Salmon et al., which may again relate to a gelled polymer skin at the air-solution interface,” they concluded in their paper, published in the journal Physical Review Letters on December 15.

The journal’s part in a retraction

This is another Ranga Dias and superconductivity post, so please avert your gaze if you’re tired of it already.

According to a September 27 report in Science, the journal Nature plans to retract the latest Dias et al. paper, published in March 2023, claiming to have found evidence of near-room-temperature superconductivity in an unusual material, nitrogen-doped lutetium hydride (N-LuH). The heart of the matter seems to be, per Science, a plot showing a drop in N-LuH’s electric resistance below a particular temperature – a famous sign of superconductivity.

Dias (University of Rochester) and Ashkan Salamat (University of Nevada, Las Vegas), the other lead investigator in the study, measured the resistance in a noisy setting and then subtracted the noise – or what they claimed to be the noise. The problem is apparently that the subtracted plot in the published paper and the plot put together using raw data submitted by Dias and Salamat to Nature are different; the latter doesn’t show the resistance dropping to zero. Meaning that together with the noise, the paper’s authors subtracted some other information as well, and whatever was left behind suggested N-LuH had become superconducting.

A little more than a month ago, Physical Review Letters officially retracted another paper of a study led by Dias and Salamat after publishing it last year – and notably after a similar dispute (and on both occasions Dias was opposed to having the papers retracted). But the narrative was more dramatic then, with Physical Review Letters accusing Salamat of obstructing its investigation by supplying some other data as the raw data for its independent probe.

Then again, even before Science‘s report, other scientists in the same field had said that they weren’t bothering with replicating the data in the N-LuH paper because they had already wasted time trying to replicate Dias’s previous work, in vain.

Now, in the last year alone, three of Dias’s superconductivity-related papers have been retracted. But as on previous occasions, the new report also raises questions about Nature‘s pre-publication peer-review process. To quote Science:

In response to [James Hamlin and Brad Ramshaw’s critique of the subtracted plot], Nature initiated a post-publication review process, soliciting feedback from four independent experts. In documents obtained by Science, all four referees expressed strong concerns about the credibility of the data. ‘I fail to understand why the authors … are not willing or able to provide clear and timely responses,’ wrote one of the anonymous referees. ‘Without such responses the credibility of the published results are in question.’ A second referee went further, writing: ‘I strongly recommend that the article by R. Dias and A. Salamat be retracted.’

What was the difference between this review process and the one that happened before the paper was published, in which Nature‘s editors would have written to independent experts asking them for their opinions on the submitted manuscript? Why didn’t they catch the problem with the electrical resistance plot?

One possible explanation is the sampling problem: when writing an article as a science journalist, the views expressed in the article will be a function of the scientists that I have sampled from within the scientific community. In order to obtain the consensus view, I need to sample a sufficiently large number of scientists (or a small number of representative scientists, such as those who I know are in touch with the pulse of the community). Otherwise, there’s a nontrivial risk of some view in my article being over- or under-represented.

Similarly, during its pre-publication peer-review process, did Nature not sample the right set of reviewers? I’m unable to think of other explanations because the sampling problem accounts for many alternatives. Hamlin and Ramshaw also didn’t necessarily have access to more data than Dias et al. submitted to Nature because their criticism emerged in May 2023 itself, and was based on the published paper. Nature also hasn’t disclosed the pre-publication reviewers’ reports nor explained if there were any differences between its sampling process in the pre- and post-publication phases.

So short of there being a good explanation, as much as we have a scientist who’s seemingly been crying wolf about room-temperature superconductivity, we also have a journal whose peer-review process produced, on two separate occasions, two different results. Unless it can clarify why this isn’t so, Nature is also to blame for the paper’s fate.

Is Dias bringing the bus back?

So Physical Review Letters formally retracted that paper about manganese sulphide, in the limelight for having been coauthored by Ranga P. Dias, yesterday. The retraction notice states: “Of the authors on the original paper, R. Dias stands by the data in Fig. 1(b) and does not agree to retract the Letter.” Figure 1(b) is reproduced below.

The problem with the second plot is that its curves reportedly resemble some in Dias’s doctoral thesis from 2013, in which he had examined the same properties of germanium tetraselenide, a different kind of material. Curves can look the same to the extent that they can have the same overall shape; it’s a problem when they also reproduce the little variations that are a result of the specific material synthesised for a particular experiment and the measurements made on that day.

That Dias is the only person objecting to the retraction is interesting because it means one of his coaouthors, Ashkan Salamat, agreed to it. Salamat heads a lab in the University of Nevada, Las Vegas, that’s been implicated in the present controversy. Earlier this year, well after Physical Review Letters said it was looking into the allegations against the manganese sulphide paper, Scientific American reported:

Salamat has since responded, suggesting that even though the two data sets may appear similar, the resemblance is not indicative of copied data. “We’ve shown that if you just overlay other people’s data qualitatively, a lot of things look the same,” he says. “This is a very unfair approach.”

Physical Review Letters also accused Salamat of attempting to obstruct its investigation after it found that the raw data he claimed to have submitted of the group’s experiments wasn’t in fact the raw data. Since then, Salamat may well have changed his mind to avoid more hassle or in deference to the majority opinion, but I’m still curious if he could have changed his mind because he no longer thought the criticisms to be unfair.

Anyway, Dias is in the news because he’s made some claims in the past about having found room-temperature superconductors. A previous paper was retracted in September 2022, two years after it was published and independent researchers found some problems in the data. He had another paper published in March this year, reporting room-temperature but high-pressure superconductivity in nitrogen-doped lutetium hydride. This paper courted controversy because Dias et al. refused to share samples of the material so independent scientists could double-check the team’s claim.

Following the retraction, The New York Times asked Dias what he had to say, and his reply seems to bring back the bus under which principal investigators (PIs) have liked to throw their junior colleagues at signs of trouble in the past:

[He] has maintained that the paper accurately portrays the research findings. However, he said on Tuesday that his collaborators, working in the laboratory of Ashkan Salamat, a professor of physics at the University of Nevada, Las Vegas, introduced errors when producing charts of the data using Adobe Illustrator, software not typically used to make scientific charts.

“Any differences in the figure resulting from the use of Adobe Illustrator software were unintentional and not part of any effort to mislead or obstruct the peer review process,” Dr. Dias said in response to questions about the retraction. He acknowledged that the resistance measurements in question were performed at his laboratory in Rochester.

He’s saying that his lab made the measurements at the University of Rochester and sent the data to Salamat’s lab at the University of Nevada, where someone else (or elses) introduced errors using Adobe Illustrator – presumably while visualising the data, but even then Illustrator is a peculiar choice – and these errors caused the resulting plot to resemble one in Dias’s doctoral thesis. Hmm.

The New York Times also reported that after refusing in the past to investigate Dias’s work following allegations of misconduct, the University of Rochester has now launched an investigation “by outside experts”. The university doesn’t plan to release their report of the findings, however.

But even if the “outside experts” conclude that Dias didn’t really err and that, honestly, Salamat’s lab in Las Vegas was able to introduce very specific kinds of errors in what became figure 1(b), Dias must be held accountable for being one of the PIs of the study – a role whose responsibilities arguably include not letting tough situations devolve into finger-pointing.

Gerald Guralnik (1936-2014)

Of the six scientists who came up with the idea of a Higgs boson in the mid-1960s, independently or in collaboration with others, I’ve met all of one. Tom Kibble was at the Institute of Mathematical Science, Chennai, in January 2013 for a conference. He was 80 years old then, and looked quite frail. Every time somebody tapped his shoulder before taking a photograph, he would break into a self-effacing smile. It was clear he was surprised by the attention he was receiving. Kibble thought he didn’t deserve it.

He, Carl Hagen and Gerald Guralnik comprised one of the three teams that conceived the mechanism to explain how some fundamental particles acquired mass in the early universe, over time making possible chemical reactions, stars, life, and many things besides. The other two teams comprised Francois Englert and Robert Brout, and Peter Higgs; Higgs’ name has today become attached to the name of the mechanism. For their work, Higgs and Englert were awarded the 2013 Nobel Prize in physics. Brout couldn’t receive the prize because he had died in 2011. Kibble, Hagen and Guralnik were left out because of limits on how many people the prize could be awarded to at a time.

Fair share of obstacles

On April 26, 2014, Gerald Guralnik died of a heart attack in Rhode Island after delivering a lecture at Brown University. He was 77. In those seven decades, he had become one of the world’s leading experts on theoretical particle physics, which, through the 1960s, was entering its boom time as the world would later discover. In this period, he co-scripted one of the most enduring quests in modern physics research.

Before I started writing this, I visited the Wikipedia page for the Physical Review Letters papers published by the three groups that first called the world’s attention to their findings. In the second line, Peter Higgs is mentioned as having worked with Satyen Bose – undoubtedly the consequence of a grave misapprehension that pervaded India when the 2013 Nobel Prizes were announced. Many believed Satyen Bose had been neglected for his work, but he just hadn’t worked on the Higgs boson, only on the underlying theory that controls the lives and times of all bosons. If such are the facile issues that concern some misguided Indians today, Guralnik tackled more than a fair share in his time.

For a few years after Kibble, Hagen and Guralnik published their paper, their work wasn’t taken seriously. Guralnik wrote in Huffington Post in August 2012 that, in the summer of 1965, Werner Heisenberg – the originator of the notorious uncertainty principle – thought Guralnik’s ideas were junk. The New York Times wrote that Robert Marshak, a famous theoretical physicist, told Guralnik that if he wished to survive in physics, he “must stop thinking about this sort of problem and move on,” advice that Guralnik “wisely obeyed”. According to Kibble, however, Marshak later admitted that he had been misguided.

Deference over primacy

Nevertheless, some other scientists had starting working on Guralnik & co.’s theories. By the 1970s, Sheldon Glashow, Abdus Salam and Steven Weinberg had succeeded in ironing out many of its inconsistencies and won the Nobel Prize for physics in 1979 for their work… even though it would be 50 more years to prove via experiment that the Higgs mechanism was for real. This is because there was no disputing that the implications of the work of Kibble, Hagen, Guralnik, Higgs, Brout and Englert were revolutionary, at least among those who were willing to accept it.

To this end, the 1979 prizewinners and the ‘Higgs Six’ were aware of and deferential toward the contributions of others to the development of this new theory. In fact, Higgs, who has often wound up being the centre of attention when talk of his eponymous mechanism comes up, has said that he’d rather call it the ABEGHHK’tH mechanism (A denoted Phillip Warren Anderson; ‘tH, Gerardus ‘t Hooft).

But others were less considerate, which didn’t go down well with Guralnik. As Kibble wrote in his obituary in Nature, “Guralnik came to feel that our early paper was often unfairly neglected. He gave talks and wrote papers pointing out our distinctive contribution, of which he was justifiably proud, and in which he was unquestionably the prime mover.” This doesn’t mean he went on to become a sour, old bat, of course, but only that Guralnik seemed to appreciate the gravitas of his work much more than others at the time. When Higgs and Englert shared the 2013 Nobel Prize in physics, Guralnik told Brown Daily Herald that he was “a little hurt”, but happier for the recognition that his peers – and by extension his work – had received.

(It is, in fact, hard to say if he is as celebrated as Higgs is today, physicists notwithstanding. Such are the consequences of asymmetric recognition, a sort of ceiling effect that silences avant garde advancements until the world is ready to hear them. This is also a complaint I’ve heard from far too many Indian scientists and whose efforts to remedy it I don’t begrudge them even if it only seems like an infantile squabble over primacy.)

In fact, after his work in establishing the theoretical foundations of the Higgs mechanism, which itself is a cornerstone of a unified theory that describes both the electromagnetic and weak nuclear forces of nature, Guralnik proceeded to make a lot of other contributions. He worked on computational approaches to quantum field theory, quantum chromodynamics (i.e., the theory of the strong nuclear force), the application of chaos theory to particle physics, and string theory. His was a versatile genius, in part combative and in part pliant. Rest in peace.