The presence of hydrogen in a sample is usually a bad thing in neutron scattering experiments, but now researchers in the US have turned the tables on the lightest element and used it to spot fake antique coins.

The scattering of relatively slow moving neutrons from materials provides a wide range of structural information. This is because these “cold” neutrons have wavelengths on par with the separations of atoms in a materials. However, materials that contain large amounts of hydrogen-1 nuclei (protons) can be difficult to study because hydrogen is very good at scattering neutrons in random directions – creating a noisy background signal. Indeed, biological samples containing lots of hydrogen are usually “deuterated” – replacing hydrogen with deuterium – before they are placed in a neutron beam.

However, there are some special cases where this incoherent scattering of hydrogen can be useful – measuring the water content of samples, for example.

Surfeit of hydrogen

Now, researchers in the US and South Korea have used a neutron beam to differentiate between genuine antique coins and fakes. The technique relies on the fact that the genuine coins have suffered corrosion that has resulted in the inclusion of hydrogen-bearing compounds within the coins.

Led by Youngju Kim at Daniel Hussey at the National Institute of Standards and Technology (NIST) in Colorado, the team fired a parallel beam of neutrons through individual coins (see figure). The particles travel with ease through a coin’s original metal, but tend to be scattered by the hydrogen-rich corrosion inclusions. This creates a 2D pattern of high and low intensity regions on a neutron-sensitive screen behind the coin. The coin can be rotated and a series of images taken. Then, the researchers used computed tomography to create a 3D image showing the corroded regions of a coin.

The team used this neutron tomography technique to examine an authentic 19^th century coin that was recovered from a shipwreck, and on a coin that is known to be a replica. Although both coins had surface corrosion, the corrosion extended much deeper into the bulk of the authentic coin than it did in the replica.

The researchers also used a separate technique called neutron grating interferometry to characterize the pores in the surfaces of the coins. Pores are common on the surface of coins that have been buried or submerged. Authentic antique coins are often found buried or submerged, whereas replica coins will be buried or submerged to make them look more authentic.

Small-angle scattering

Neutron grating interferometry looks at the small-angle scattering of neutrons from a sample and focuses on structures that range in size from about 1 nm to 1 micron.

The team found that the authentic coin had many more tiny pores than the replica coin, which was dominated by much larger (millimetre scale) pores.

This observation was expected because when a coin is buried or submerged, chemical reactions cause metals to leach out of its surface, creating millimetre-sized pores. As time progresses, however, further chemical reactions cause corrosion by-products such as copper carbonates to fill in the pores. The result is that the pores in the older authentic coin are smaller than the pores in the newer replica coin.

The team now plans to expand its study to include more Korean coins and other metallic artefacts. The techniques could also be used to pinpoint corrosion damage in antique coins, allowing these areas to be protected using coatings.

As well as being important to coin collectors and dealers, the ability to verify the age of coins is of interest to historians and economists – who use the presence of coins in their research.

The study was done using neutrons from NIST’s research reactor in Maryland. That facility is scheduled to close in 2026 so the team plans to continue its investigation using a neutron source in South Korea.

The research is described in Scientific Reports.

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By adapting their quantum twisting microscope to operate at cryogenic temperatures, researchers have made the first observations of a type of phonon that occurs in twisted bilayer graphene.  These “phasons” could have implications for the electron dynamics in these materials.

Graphene is a layer of carbon just one atom thick and it has range of fascinating and useful properties – as do bilayer and multilayer versions of graphene. Since 2018, condensed-matter physicists have been captivated by the intriguing electron behaviour in two layers of graphene that are rotated with respect to each other.

As the twist angle deviates from zero, the bilayer becomes a moiré superlattice. The emergence of this structure influences electronic properties of the material, which can transform from a semiconductor to a superconductor.

In 2023, researchers led by Shahal Ilani at the Weizmann Institute of Science in Israel developed a quantum twisting microscope to study these effects. Based on a scanning probe microscope with graphene on the substrate and folded over the tip such as to give it a flat end, the instrument allows precise control over the relative orientation between two graphene surfaces – in particular, the twist angle.

Strange metals

Now Ilani and an international team have operated the microscope at cryogenic temperatures for the first time. So far, their measurements support the current understanding of how electrons couple to phasons, which are specific modes of phonons (quantized lattice vibrations). Characterizing this coupling could help us understand “strange metals”, whose electrical resistance increases at lower temperatures – which is the opposite of normal metals.

There are different types of phonons, such as acoustic phonons where atoms within the same unit cell oscillate in phase with each other, and optical phonons where they oscillate out of phase. Phasons are phonons involving lattice oscillations in one layer that are out of phase or antisymmetric with oscillations in the layer above.

“This is the one that turns out to be very important for how the electrons behave between the layers because even a small relative displacement between the two layers affects how the electrons go from one layer to the other,” explains Weizmann’s John Birkbeck as he describes the role of phasons in twisted bilayer graphene materials.

For most phonons the coupling to electrons is weaker the lower the energy of the phonon mode. However for twisted bilayer materials, theory suggests that phason coupling to electrons increases as the twist between the two layers approaches alignment due to the antisymmetric motion of the two layers and the heightened sensitivity of interlayer tunnelling to small relative displacements.

Unique perspective

“There are not that many tools to see phonons, particularly in moiré systems” adds Birkbeck. This is where the quantum twisting microscope offers a unique perspective. Thanks to the atomically flat end of the tip, electrons can tunnel between the layer on the substrate and the layer on the tip whenever there is a matching state in terms of not just energy but also momentum too.

Where there is a momentum mismatch, tunnelling between tip and substrate is still possible by balancing the mismatch with the emission or absorption of a phonon. By operating at cryogenic temperatures, the researchers were able to get a measure of these momentum transactions and probe the electron phonon coupling too.

“What was interesting from this work is not only that we could image the phonon dispersion, but also we can quantify it,” says Birkbeck stressing the absolute nature of these quantified electron phonon coupling-strength measurements.

The measurements are the first observations of phasons in twisted bilayer graphene and reveal a strong increase in coupling as the layers approach alignment, as predicted by theory. However, the researchers were not able to study angles smaller than 6°. Below this angle the tunnelling resistance is so low that the contact resistance starts to warp readings, among other limiting factors.

Navigating without eyes

A certain amount of technical adjustment was needed to operate the tool at cryogenic temperatures, not least to “to navigate without eyes” because the team was not able to incorporate their usual optics with the cryogenic set up. The researchers hope that with further technical adjustments they will be able to use the quantum twisting microscope in cryogenic conditions at the magic angle of 1.1°, where superconductivity occurs.

Pablo Jarillo Herrero, who led the team at MIT that first reported superconductivity in twisted bilayer graphene in 2018 but was not involved in this research describes it as an “interesting study” adding, “I’m looking forward to seeing more interesting results from low temperature QTM research!”

Hector Ochoa De Eguileor Romillo at Columbia University in the US, who proposed a role for phason–electron interactions in these materials in 2019, but was also not involved in this research describes it as “a beautiful experiment”. He adds, “I think it is fair to say that this is the most exciting experimental technique of the last 15 years or so in condensed matter physics; new interesting data are surely coming.”

The research is described in Nature.

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Precise control over the generation of intense, ultrafast changes in magnetic fields called “magnetic steps” has been achieved by researchers in Hamburg, Germany. Using ultrashort laser pulses, Andrea Cavalleri and colleagues at the Max Planck Institute for the Structure and Dynamics of Matter disrupted the currents flowing through a superconducting disc. This alters the superconductor’s local magnetic environment on very short timescales – creating a magnetic step.

Magnetic steps rise to their peak intensity in just a few picoseconds, before decaying more slowly in several nanoseconds. They are useful to scientists because they rise and fall on timescales far shorter than the time it takes for materials to respond to external magnetic fields. As a result, magnetic steps could provide fundamental insights into the non-equilibrium properties of magnetic materials, and could also have practical applications in areas such as magnetic memory storage.

So far, however, progress in this field has been held back by technical difficulties in generating and controlling magnetic steps on ultrashort timescales. Previous strategies  have employed technologies including microcoils, specialized antennas, and circularly polarized light pulses. However, each of these schemes offer a limited degree of control over the properties of the magnetic steps they generated.

Quenching supercurrents

Now, Cavalleri’s team has developed a new technique that involves the quenching of currents in a superconductor. Normally, these “supercurrents” will flow indefinitely without losing energy, and will act to expel any external magnetic fields from the superconductor’s interior. However, if these currents are temporarily disrupted on ultrashort timescales, a sudden change will be triggered in the magnetic field close to the superconductor – which could be used to create a magnetic step.

To create this process, Cavalleri and colleagues applied ultrashort laser pulses to a thin, superconducting disc of yttrium barium copper oxide (YBCO), while also exposing the disc to an external magnetic field.

To detect whether magnetic steps had been generated, they placed a crystal of the semiconductor gallium phosphide in the superconductor’s vicinity. This material exhibits an extremely rapid Faraday response. This involves the rotation of the polarization of light passing through the semiconductor in response to changes in the local magnetic field. Crucially, this rotation can occur on sub-picosecond timescales.

In their experiments, researchers monitored changes to the polarization of an ultrashort “probe” laser pulse passing through the semiconductor shortly after they quenched supercurrents in their YBCO disc using a separate ultrashort “pump” laser pulse.

“By abruptly disrupting the material’s supercurrents using ultrashort laser pulses, we could generate ultrafast magnetic field steps with rise times of approximately one picosecond – or one trillionth of a second,” explains team member Gregor Jotzu.

Broadband step

This was used to generate an extremely broadband magnetic step, which contains frequencies ranging from sub-gigahertz to terahertz. In principle, this should make the technique suitable for studying magnetization in a diverse variety of materials.

To demonstrate practical applications, the team used these magnetic steps to control the magnetization of a ferrimagnet. Such a magnet has opposing magnetic moments, but has a non-zero spontaneous magnetization in zero magnetic field.

When they placed a ferrimagnet on top of their superconductor and created a magnetic step, the step field caused the ferrimagnet’s magnetization to rotate.

For now, the magnetic steps generated through this approach do not have the speed or amplitude needed to switch materials like a ferrimagnet between stable states. Yet through further tweaks to the geometry of their setup, the researchers are confident that this ability may not be far out of reach.

“Our goal is to create a universal, ultrafast stimulus that can switch any magnetic sample between stable magnetic states,” Cavalleri says. “With suitable improvements, we envision applications ranging from phase transition control to complete switching of magnetic order parameters.”

The research is described in Nature Photonics.

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Researchers at the SLAC National Accelerator Laboratory in the US have produced the world’s most powerful ultrashort electron beam to date, concentrating petawatt-level peak powers into femtosecond-long pulses at an energy of 10 GeV and a current of around 0.1 MA. According to officials at SLAC’s Facility for Advanced Accelerator Experimental Tests (FACET-II), the new beam could be used to study phenomena in materials science, quantum physics and even astrophysics that were not accessible before.

High-energy electron beams are routinely employed as powerful probes in several scientific fields. To produce them, accelerator facilities like SLAC use strong electric fields to accelerate, focus and compress bunches of electrons. This is not easy, because as electrons are accelerated and compressed, they emit radiation and lose energy, causing the beam’s quality to deteriorate.

An optimally compressed beam

To create their super-compressed ultrashort beam, researchers led by Claudio Emma at FACET-II used a laser to shape the electron bunch’s profile with millimetre-scale precision in the first 10 metres of the accelerator, when the beam’s energy is lowest. They then took this modulated electron beam and boosted its energy by a factor of 100 in a kilometre-long stretch of downstream accelerating cavities. The last step was to compress the beam by a factor of 1000 by using magnets to turn the beam’s millimetre-scale features into a micron-sized long current spike.

One of the biggest challenges, Emma says, was to optimise the laser-based modulation of the beam in tandem with the accelerating cavity and magnetic fields of the magnets to obtain the optimally compressed beam at the end of the accelerator. “This was a large parameter space to work in with lots of knobs to turn and it required careful iteration before an optimum was found,” Emma says.

Measuring the ultra-short electron bunches was also a challenge. “These are typically so intense that if you intercept them with, for example, scintillating screens (a typical technique used in accelerators to diagnose properties of the beam like its spot size or bunch length), the beam fields are so strong they can melt these screens,” Emma explains. “To overcome this, we had to use a series of indirect measurements (plasma ionisation and beam-based radiation) along with simulations to diagnose just how strongly compressed and powerful these beams were.”

Beam delivery

According to Emma, generating extremely compressed electron beams is one of the most important challenges facing accelerator and beam physicists today. “It was interesting for us to tackle this challenge at FACET-II, which is a facility designed specifically to do this kind of research on extreme beam manipulation,” he says.

The team has already delivered the new high-current beams to experimenters who work on probing and optimising the dynamics of plasma-based accelerators. Further down the line, they anticipate much wider applications. “In the future we imagine that we will attract interest from users in multiple fields, be they materials scientists, strong-field quantum physicists or astrophysicists, who want to use the beam as a strong relativistic ‘hammer’ to study and probe a variety of natural interactions with the unique tool that we can provide,” Emma tells Physics World.

The researchers’ next step will be to increase the beam’s current by another order of magnitude. “This additional leap will require the use of a different plasma-based compression technique, rather than the current laser-based approach, which we hope to demonstrate at FACET-II in the near future,” Emma reveals.

The present work is described in Physical Review Letters.

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