In the 1980s, scientists measured quantized jumps in the energy of ions for the first time. In the 2000s, it was photons’ turn. Now, thanks to a tour de force of device engineering and nanofabrication, researchers at Stanford University in the US are reporting that phonons – mechanical vibrations like those that carry sound – also display this quantum behaviour.

“We could see it by eye [in the data], which was very satisfying,” team leader Amir Safavi-Naeini tells Physics World. “I’ve been fascinated with the interface between the quantum world and the classical world and trying to move that interface closer to daily experience…and now we have mechanical objects.”

Laying the groundwork

The Stanford team’s recipe for observing quantum jumps in phonons begins with thin films of lithium niobate (LiNbO₃). Although bulk samples of this material had been available since the 1950s, the commercialization of thin-film LiNbO₃ a decade ago was a game changer for Safavi-Naeini and his colleagues once they worked out how to process the material.

Because LiNbO₃ is piezoelectric, placing it under stress – from a vibration, say – produces a voltage. This voltage allows vibrational quanta to couple to superconducting qubits, after which the qubit states can be “read out” to determine whether a phonon was present or not.

The problem was that initially, the phonons had lifetimes of only 800 nanoseconds – well short of the millisecond the team needed to observe them falling from an excited state to the ground state. Extending these phonon coherence times “took a lot of work by a number of graduate students in my group,” Safavi-Naeini says.

After probing various sources of dissipation, including material properties that cause quantum excitations to lose coherence, the researchers realized that surface defects were key offenders. They therefore switched from polymer processing, which left a residue, to shadow masks that superimpose one chip over the other and evaporate metal through it. “There were probably three generations of students that worked on getting that four to five orders of magnitude increase in the performance of the devices,” Safavi-Naeini says.

Once they’d extended their phonon lifetimes, the researchers had to find a way of combining their “very clean, long-lived” mechanical and superconducting systems without destroying either. For this, Safavi-Naeini credits past and present students Takuma Makihara and Erik Szakiel, who spent most of their PhDs tackling issues with the mechanical oscillators before finding a way to couple them to the superconducting systems. “That was the last piece,” Safavi-Naeini recalls. “Once we had that, we could do these experiments.”

Spot the missing phonon

Because the superconducting qubits detect whether an even or odd number of phonons is present in the LiNbO₃ (which at very low energies amounts to determining whether a phonon is there at all), they are effectively performing a parity measurement. That could make the Stanford team’s system useful for quantum error correction, where the goal is to find out whether an error has crept in without actually measuring the quantum states, since doing so would destroy their useful quantum characteristics.

One of the most common sources of error in superconducting qubits is the loss of an excitation, such as when a phonon decays. However, if data is encoded into phonons in pairs and the system detects an odd number of them, this loss becomes apparent without measuring all the states. This so-called “binomial” code has already been implemented for microwave photons, and Safavi-Naeini explains that it works best for long-lived particles – a further benefit of the team’s long-lived phonons.

Protein and gravity measurements

The Stanford researchers are now collaborating with Michael Roukes and colleagues at the California Institute of Technology (Caltech) to explore possible applications in high-precision measurements of single protein masses. They are also interested in fundamental science questions such as whether it’s possible to generate entanglement between mechanical systems through gravitational fields.

However, Safavi-Naeini cautions that their phonon lifetimes would need to be much longer – a factor of 10 for error correction and mass spectrometry, and for gravity perhaps as much as six orders of magnitude, with further modifications to the experimental set-up also required – before they can achieve these goals. “The hope is that by figuring out the fundamental sources of loss in these materials we can build our way up to those experiments in the coming decade,” he says. “Every time you improve, you hit some new source of noise, you understand it, then you can deal with it, and then you go to the next one. It’s a fun physics problem.”

The research is published in Science.

The post Sound waves come in quantum jumps appeared first on Physics World.

I have been involved in tribology for most of my working life, but even today it can feel like a well-kept secret. A highly interdisciplinary field with close ties to materials science, tribology is essentially the science and technology of friction, lubrication and wear between any moving surfaces. But tribology encompasses in part one age-old challenge.

I’m talking about the use of bearings, which dates back to at least 4600 BCE, when ancient Egyptians used wood log rollers to move large stone blocks during the construction of their pyramids. In 1500 Leonardo da Vinci used ball bearings in his conceptual helicopter design, while in 1740 the clock maker John Harrison invented caged roller bearings for his marine chronometer.

It was not until the mid-1960s that the word tribology was coined by the British engineer and entrepreneur Peter Jost after discussions with the then editor of the Oxford English Dictionary

The first patent on a ball bearing was granted to Philip Vaughan, an ironmaker from Carmarthen, Wales, bringing tribology firmly into the industrial world. But despite all the history, it was not until the mid-1960s that the word was coined by the British engineer and entrepreneur Peter Jost. After discussions with the then editor of the Oxford English Dictionary, the Greek word “tribos” – meaning rubbing – was suggested as a basis for describing the discipline.

This resonated with Jost, because the terms “tribo-chemistry” and “tribo-physics” were already in use. In an interview with the magazine Machinery Lubrication in 2005, Jost explained how “tribo science and tribo technology” then simply became shortened to tribology. “I felt that because it had a Greek basis, it could easily be adopted into a number of languages by most Western countries,” Jost recalled.

Wear’s the rub?

For the most part, the aim of tribology is to minimize friction and wear between components, which involves a knowledge of both materials and lubrication. Initially, the focus of the field was on moving parts in industrial or mechanical machines, such as bearings, gears, brakes and clutches. Indeed, one reason that the term came about was a conference in Cardiff in 1964 organized by the Iron and Steel Institute and the Institution of Mechanical Engineers (IMechE).

At the conference it became apparent that significant failures in plant machinery, which were being attributed to poor lubrication, were in fact due to a lack of fundamental design knowledge. These findings led to Jost chairing a UK government committee into lubrication education and research and the needs of industry. The resulting study – generally known as the “Jost report” – came out in 1964.

Now all this may seem like ancient history, which I suppose it is, but I was intrigued to discover that delegates to that 1964 conference visited the Llanwern steelworks at nearby Newport. As some of you may recall me describing, I also visited this site during my own student days, where I saw the steel rolling line in action. It was a truly impressive sight.

The total failure was remarkable, underlining just what can happen when moving parts fail without warning

From the viewing platform, I could see a massive volume of red-hot steel thunder along the rollers when, suddenly, there was a bang. Something failed and the steel landed in a crumpled mess all over the plant floor. We were safe but the total failure was remarkable to witness, underlining just what can happen when moving parts fail without warning.

The knock-on effect of this failure would have been costly and sustained. As well as a steel rolling line being out of action, new parts would have had to be sourced and old ones repaired. There’d have been quality concerns to address, safety reviews to be undertaken and the root cause of the problems to be worked out. Quite possibly all those jobs would have needed to be completed before production could restart.

What’s more, the failure of the machine’s moving parts, which depends on tribology, would have damaged the company’s reputation and led to customers annoyed with delays and late deliveries. It is this kind of event that those involved in tribology are trying to prevent, or at least minimize. By understanding tribology, in other words, engineers can design better machines and components.

What it says on the tin

This was made clear in the 1964 Jost report, which said that adopting good tribological practices could save the UK between 1% and 1.4% of its gross domestic product (GDP). In today’s money, that would amount to some £30bn, given that country’s GDP is a shade over £3 trillion. Put simply, designing and maintaining industrial equipment, based on a solid understanding of tribology, yields significant economic benefit.

More recently, a 2017 study by Kenneth Holmberg from the VTT Technical Research Centre of Finland and Ali Erdemir of the Argonne National Laboratory in the US (Friction (5 263) suggests that 20% of the world’s energy consumption originates in tribological contacts. And by the application of best practice, as much as 40% of this could be saved.

More than 60 years after the publication of the Jost report, the influence of tribology continues to grow. Alongside its impact on transportation and manufacturing, tribology is also relevant to extreme environments from aerospace, nuclear power, marine and subsea to space technology, high-voltage electrical systems and even the human body. There are also applications in nano-tribology, cosmetics, food, computational, digital, tribotronics, geotribology, hydrogen and green tribology.

Tribology will help to reduce our carbon footprint and counter climate change

As chair of the IMechE’s tribology group committee and a member of the tribology group committee of the Institute of Physics, naturally I am biased. Still, I feel privileged to be part of the tribology community and the amazing things it achieves. Whether it’s making products more energy efficient, creating green lubricants, or making plant-based protein more palatable to meat eaters, tribology will help to reduce our carbon footprint and counter climate change.

So next time you brush your teeth, comb your hair, switch on a light or ride on a bus, spare a thought for tribologists and their dedication to making the world run smoothly. It’s hard to think of anything that doesn’t involve tribology at all.

• The Institute of Physics is hosting the 5th Tribology and Net Zero seminar on 6 November in London co-organized with the Institution of Mechanical Engineers, the Royal Society of Chemistry, and the UK student chapter of the Society of Tribologists and Lubrication Engineers.

The post The secret world of tribology appeared first on Physics World.

Each year, approximately 5000 tonnes of dyes (about 50 times the weight of a blue whale) are discharged into industrial wastewater. These dyes block light from reaching aquatic plants and animals, cause unpleasant odours and pose risks to human health. Congo red, for instance, is notably persistent in the environment. Another dye, methyl orange, is difficult to remove using standard biological or chemical treatments and has been linked to cancer and DNA damage. Researchers in Egypt and the US are devising a new method to clean up such contamination.

One of the well-established routes for cleaning wastewater from dyes is using nanoparticles (of similar size to a small virus) with high surface area, which gives the dye molecules plenty of places to adsorb onto the nanoparticle surface. It’s especially useful if the nanoparticle has magnetic properties – then it’s possible to remove nanoparticles with adsorbed dyes from water using a magnet. The nanoparticles can then be washed and reused, for up to four cycles, enabling the same material to be used again and again instead of being thrown away.

Ideas for breakfast

Lead researcher Hebatullah Hassan Farghal from The American University in Cairo and her team have synthesized magnetite nanoparticles (particles made from a naturally magnetic form of iron oxide) directly in dye-polluted water. As the particles form, they pull the dye molecules out of the water and onto their own surfaces – meaning that the water is left with less dye in it. The researchers then used those same “dirty” nanoparticles to capture two entirely different dyes afterwards. This route, described in RSC Advances, skips the usual “make-wash-dry-treat” sequence. Instead, it offers a more efficient, simpler and greener way to create a working wastewater adsorbent.

Farghal’s previous research work had been on magnetic adsorbents. “The idea for this paper came exactly one or two days after my PhD defence,” she explains. “While I was having breakfast, I realized adsorbent synthesis requires time and cost…I then went to the lab and started my experiments to find that it was successful.”

Catch and catch again

First, Farghal loaded the nanoparticles with two dyes – congo red (CR) and bromocresol green (BCG) – as they formed in the wastewater. More than 90% of both dyes stuck to the particles and the loaded particles grew slightly larger (about 34 nm, compared with 26 nm for plain magnetite). Later, the same “dirty” particles went on to adsorb almost 80% of methylene blue (MB) from a single-dye system after about 3 h, dropping to just over 50% when a second dye, methyl orange (MO), was mixed in.

To reuse the particles after adsorption of MB, the researchers rinsed them in alcohol, which pulled the trapped blue dye molecules back off. They repeated this four times without any drop in performance, and none of the originally loaded dyes leaking out in the process. They note that MB removal worked best at high pH, where the nanoparticles become negatively charged (while MB molecules are positively charged). This electrostatic attraction gave the main mechanism an extra boost.

Simulations second the results

There are several different mechanisms that can happen when a dye molecule attaches to the surface of a nanoparticle. The two most common types are chemisorption, where the molecule forms new chemical bonds with the surface, and physisorption, a weaker attraction more like static cling – no permanent bond is formed, and the molecule can be pulled off relatively easily.

Several lab measurements, including infrared spectroscopy of the particle surface, already hinted that physisorption is the main mechanism responsible for binding CR, BCG and MB molecules to nanoparticles. This weak binding is also why the ethanol wash could strip the dye back off without damaging the particle.

To confirm this idea, the team also ran a computational method called density functional theory (DFT) on a single dye molecule sitting on a magnetite surface. All dyes bound with similar strength. The simulations also showed electrons flowing in opposite directions depending on the dye’s charge – from the negatively charged CR and BCG toward the surface, and from the surface toward the positively charged MB – the same charge attraction seen in the lab.

The researchers point out that DFT and experimental outcomes cannot be compared quantitatively, only qualitatively, since the first one examines a perfect molecule on an ideal surface and the second one concerns real-world conditions. They note that the DFT analysis was carried out by Ahmed A Abokifa and Mohamed S Mohamed from the University of Illinois Chicago, with Mayyada El-Sayed supervising the wider study.

Outside of the lab

In standard tap water or saline (3% NaCl), the story gets more complicated. The researchers increased the adsorbent dose by roughly 15 times – from 0.67 to 10 g/l – to try to compensate for real-world conditions. This worked in saline, but removal in tap water was only about half of what it was in distilled water, likely because calcium, magnesium and other substances naturally present in tap water compete with the dyes for space on the particle’s surface.

“Though the dose significantly increased in tap water and saline, this dose is still applied in the literature,” Farghal says, adding that her future investigation will try to close that gap and reduce costs.

Still, one thing works in the material’s favour: making the particles is cheap. Producing 1 kg costs around $3730 – roughly eight times cheaper than other nanomaterials sometimes used for the same purpose, such as cobalt ferrite (which costs roughly $30,100 per kilogram).

Farghal says her next research will involve “circular economy approaches that will be outside the box.” Circular economy is an approach in which waste from one process becomes a resource for the next – that waste doesn’t have to be wasted. This study is itself an example of that thinking: dye-polluted water became the raw material for a new, useful adsorbent.

The post Dirty magnetite nanoparticles keep cleaning up dyes appeared first on Physics World.

The performance of optoelectronic materials and devices relies on carrier transport, recombination, ion migration and phase segregation. They unfold on length scales from nanometers to micrometers and timescales from picoseconds to hours. Conventional characterization averages over this heterogeneity and rarely captures these events in operational devices.

Optical microscopy and spectroscopy can recover this hidden picture. They convert local material function directly into optical contrast, and that contrast can be resolved across the length and time scales where the function happens.

In this webinar, Dr Sudipta Seth will talk about advanced optical microscopy and spectroscopy that interrogate halide perovskite materials and corresponding optoelectronic devices under relevant in situ and operando conditions. This approach uncovers the local fields, transport bottlenecks, and defect chemistry that set solar cell performance. He will show how these local optical signatures translate into a mechanistic understanding of device operation, and how that understanding feeds back into the design of better probes, interfaces, defects and devices.

The talk is aimed equally at materials scientists curious about what happens inside their samples and at microscopists and spectroscopists looking for new problems suited to their tools.

Certificate of attendance will be awarded upon completion of the webinar.

Sudipta Seth is a research associate at KU Leuven, Belgium, where he conducts advanced research at the intersection of materials chemistry, optoelectronic devices and spectroscopy through the development of innovative microscopy methodologies. He leads the nanoscale device spectroscopy research line at Hofkens Laboratory. He completed his PhD at the University of Hyderabad and subsequently worked as a postdoctoral fellow at Lund University and as a visiting junior fellow at the Tokyo Institute of Technology. His work integrates single-particle spectroscopy, super-resolution and nanoscale microscopy and ultrafast spectroscopy to investigate fundamental photophysics in semiconductor materials and optoelectronic devices. He has received several academic fellowships, including INSPIRE-SHE (India), Wenner-Gren Postdoctoral Fellowship (Sweden), FWO Research Stay Abroad (Belgium), and Marie Sklodowska-Curie Postdoctoral Fellowship (European Commission).

The post In situ and operando optical microscopy and spectroscopy: investigating perovskite optoelectronic devices at work appeared first on Physics World.