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Superconducting device delivers ultrafast changes in magnetic field

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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Ultrashort electron beam sets new power record

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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Earth’s core could contain lots of primordial helium, experiments suggest

Helium deep with the Earth could bond with iron to form stable compounds – according to experiments done by scientists in Japan and Taiwan. The work was done by Haruki Takezawa and Kei Hirose at the University of Tokyo and colleagues, who suggest that Earth’s core could host a vast reservoir of primordial helium-3 – reshaping our understanding of the planet’s interior.

Noble gases including helium are normally chemically inert. But under extreme pressures, heavier members of the group (including xenon and krypton) can form a variety of compounds with other elements. To date, however, less is known about compounds containing helium – the lightest noble gas.

Beyond the synthesis of disodium helide (Na2He) in 2016, and a handful of molecules in which helium forms weak van der Waals bonds with other atoms, the existence of other helium compounds has remained purely theoretical.

As a result, the conventional view is that any primordial helium-3 present when our planet first formed would have quickly diffused through Earth’s interior, before escaping into the atmosphere and then into space.

Tantalizing clues

However, there are tantalizing clues that helium compounds could exist in some volcanic rocks on Earth’s surface. These rocks contain unusually high isotopic ratios of helium-3 to helium-4. “Unlike helium-4, which is produced through radioactivity, helium-3 is primordial and not produced in planetary interiors,” explains Hirose. “Based on volcanic rock measurements, helium-3 is known to be enriched in hot magma, which originally derives from hot plumes coming from deep within Earth’s mantle.” The mantle is the region between Earth’s core and crust.

The fact that the isotope can still be found in rock and magma suggests that it must have somehow become trapped in the Earth. “This argument suggests that helium-3 was incorporated into the iron-rich core during Earth’s formation, some of which leaked from the core to the mantle,” Hirose explains.

It could be that the extreme pressures present in Earth’s iron-rich core enabled primordial helium-3 to bond with iron to form stable molecular lattices. To date, however, this possibility has never been explored experimentally.

Now, Takezawa, Hirose and colleagues have triggered reactions between iron and helium within a laser-heated diamond-anvil cell. Such cells crush small samples to extreme pressures – in this case as high as 54 GPa. While this is less than the pressure in the core (about 350 GPa), the reactions created molecular lattices of iron and helium. These structures remained stable even when the diamond-anvil’s extreme pressure was released.

To determine the molecular structures of the compounds, the researchers did X-ray diffraction experiments at Japan’s SPring-8 synchrotron. The team also used secondary ion mass spectrometry to determine the concentration of helium within their samples.

Synchrotron and mass spectrometer

“We also performed first-principles calculations to support experimental findings,” Hirose adds. “Our calculations also revealed a dynamically stable crystal structure, supporting our experimental findings.” Altogether, this combination of experiments and calculations showed that the reaction could form two distinct lattices (face-centred cubic and distorted hexagonal close packed), each with differing ratios of iron to helium atoms.

These results suggest that similar reactions between helium and iron may have occurred within Earth’s core shortly after its formation, trapping much of the primordial helium-3 in the material that coalesced to form Earth. This would have created a vast reservoir of helium in the core, which is gradually making its way to the surface.

However, further experiments are needed to confirm this thesis. “For the next step, we need to see the partitioning of helium between iron in the core and silicate in the mantle under high temperatures and pressures,” Hirose explains.

Observing this partitioning would help rule out the lingering possibility that unbonded helium-3 could be more abundant than expected within the mantle – where it could be trapped by some other mechanism. Either way, further studies would improve our understanding of Earth’s interior composition – and could even tell us more about the gases present when the solar system formed.

The research is described in Physical Review Letters.

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Frequency-comb detection of gas molecules achieves parts-per-trillion sensitivity

A new technique for using frequency combs to measure trace concentrations of gas molecules has been developed by researchers in the US. The team reports single-digit parts-per-trillion detection sensitivity, and extreme broadband coverage over 1000 cm-1 wavenumbers. This record-level sensing performance could open up a variety of hitherto inaccessible applications in fields such as medicine, environmental chemistry and chemical kinetics.

Each molecular species will absorb light at a specific set of frequencies. So, shining light through a sample of gas and measuring this absorption can reveal the molecular composition of the gas.

Cavity ringdown spectroscopy is an established way to increase the sensitivity of absorption spectroscopy and needs no calibration. A laser is injected between two mirrors, creating an optical standing wave. A sample of gas is then injected into the cavity, so the laser beam passes through it, normally many thousands of times. The absorption of light by the gas is then determined by the rate at which the intracavity light intensity “rings down” – in other words, the rate at which the standing wave decays away.

Researchers have used this method with frequency comb lasers to probe the absorption of gas samples at a range of different light frequencies. A frequency comb produces light at a series of very sharp intensity peaks that are equidistant in frequency – resembling the teeth of a comb.

Shifting resonances

However, the more reflective the mirrors become (the higher the cavity finesse), the narrower each cavity resonance becomes. Due to the fact that their frequencies are not evenly spaced and can be heavily altered by the loaded gas, normally one relies on creating oscillations in the length of the cavity. This creates shifts in all the cavity resonance frequencies to modulate around the comb lines. Multiple resonances are sequentially excited and the transient comb intensity dynamics are captured by a camera, following spatial separation by an optical grating.

“That experimental scheme works in the near-infrared, but not in the mid-infrared,” says Qizhong Liang. “Mid-infrared cameras are not fast enough to capture those dynamics yet.” This is a problem because the mid-infrared is where many molecules can be identified by their unique absorption spectra.

Liang is a member of Jun Ye’s group in JILA in Colorado, which has shown that it is possible to measure transient comb dynamics simply with a Michelson interferometer. The spectrometer entails only beam splitters, a delay stage, and photodetectors. The researchers worked out that, the periodically generated intensity dynamics arising from each tooth of the frequency comb can be detected as a set of Fourier components offset by Doppler frequency shifts. Absorption from the loaded gas can thus be determined.

Dithering the cavity

This process of reading out transient dynamics from “dithering” the cavity by a passive Michelson interferometer is much simpler than previous setups and thus can be used by people with little experience with combs, says Liang. It also places no restrictions on the finesse of the cavity, spectral resolution, or spectral coverage. “If you’re dithering the cavity resonances, then no matter how narrow the cavity resonance is, it’s guaranteed that the comb lines can be deterministically coupled to the cavity resonance twice per cavity round trip modulation,” he explains.

The researchers reported detections of various molecules at concentrations as low as parts-per-billion with parts-per-trillion uncertainty in exhaled air from volunteers. This included biomedically relevant molecules such as acetone, which is a sign of diabetes, and formaldehyde, which is diagnostic of lung cancer. “Detection of molecules in exhaled breath in medicine has been done in the past,” explains Liang. “The more important point here is that, even if you have no prior knowledge about what the gas sample composition is, be it in industrial applications, environmental science applications or whatever you can still use it.”

Konstantin Vodopyanov of the University of Central Florida in Orlando comments: “This achievement is remarkable, as it integrates two cutting-edge techniques: cavity ringdown spectroscopy, where a high-finesse optical cavity dramatically extends the laser beam’s path to enhance sensitivity in detecting weak molecular resonances, and frequency combs, which serve as a precise frequency ruler composed of ultra-sharp spectral lines. By further refining the spectral resolution to the Doppler broadening limit of less than 100 MHz and referencing the absolute frequency scale to a reliable frequency standard, this technology holds great promise for applications such as trace gas detection and medical breath analysis.”

The spectrometer is described in Nature.

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New transfer arm moves heavier samples in vacuum

Vacuum technology is routinely used in both scientific research and industrial processes. In physics, high-quality vacuum systems make it possible to study materials under extremely clean and stable conditions. In industry, vacuum is used to lift, position and move objects precisely and reliably. Without these technologies, a great deal of research and development would simply not happen. But for all its advantages, working under vacuum does come with certain challenges. For example, once something is inside a vacuum system, how do you manipulate it without opening the system up?

Heavy duty: The new transfer arm
Heavy duty: The new transfer arm. (Courtesy: UHV Design)

The UK-based firm UHV Design has been working on this problem for over a quarter of a century, developing and manufacturing vacuum manipulation solutions for new research disciplines as well as emerging industrial applications. Its products, which are based on magnetically coupled linear and rotary probes, are widely used at laboratories around the world, in areas ranging from nanoscience to synchrotron and beamline applications. According to engineering director Jonty Eyres, the firm’s latest innovation – a new sample transfer arm released at the beginning of this year – extends this well-established range into new territory.

“The new product is a magnetically coupled probe that allows you to move a sample from point A to point B in a vacuum system,” Eyres explains. “It was designed to have an order of magnitude improvement in terms of both linear and rotary motion thanks to the magnets in it being arranged in a particular way. It is thus able to move and position objects that are much heavier than was previously possible.”

The new sample arm, Eyres explains, is made up of a vacuum “envelope” comprising a welded flange and tube assembly. This assembly has an outer magnet array that magnetically couples to an inner magnet array attached to an output shaft. The output shaft extends beyond the mounting flange and incorporates a support bearing assembly. “Depending on the model, the shafts can either be in one or more axes: they move samples around either linearly, linear/rotary or incorporating a dual axis to actuate a gripper or equivalent elevating plate,” Eyres says.

Continual development, review and improvement

While similar devices are already on the market, Eyres says that the new product has a significantly larger magnetic coupling strength in terms of its linear thrust and rotary torque. These features were developed in close collaboration with customers who expressed a need for arms that could carry heavier payloads and move them with more precision. In particular, Eyres notes that in the original product, the maximum weight that could be placed on the end of the shaft – a parameter that depends on the stiffness of the shaft as well as the magnetic coupling strength – was too small for these customers’ applications.

“From our point of view, it was not so much the magnetic coupling that needed to be reviewed, but the stiffness of the device in terms of the size of the shaft that extends out to the vacuum system,” Eyres explains. “The new arm deflects much less from its original position even with a heavier load and when moving objects over longer distances.”

The new product – a scaled-up version of the original – can move an object with a mass of up to 50 N (5 kg) over an axial stroke of up to 1.5 m. Eyres notes that it also requires minimal maintenance, which is important for moving higher loads. “It is thus targeted to customers who wish to move larger objects around over longer periods of time without having to worry about intervening too often,” he says.

Moving multiple objects

As well as moving larger, single objects, the new arm’s capabilities make it suitable for moving multiple objects at once. “Rather than having one sample go through at a time, we might want to nest three or four samples onto a large plate, which inevitably increases the size of the overall object,” Eyres explains.

Before they created this product, he continues, he and his UHV Design colleagues were not aware of any magnetic coupled solution on the marketplace that enabled users to do this. “As well as being capable of moving heavy samples, our product can also move lighter samples, but with a lot less shaft deflection over the stroke of the product,” he says. “This could be important for researchers, particularly if they are limited in space or if they wish to avoid adding costly supports in their vacuum system.”

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Inverse design configures magnon-based signal processor

For the first time, inverse design has been used to engineer specific functionalities into a universal spin-wave-based device. It was created by Andrii Chumak and colleagues at Austria’s University of Vienna, who hope that their magnonic device could pave the way for substantial improvements to the energy efficiency of data processing techniques.

Inverse design is a fast-growing technique for developing new materials and devices that are specialized for highly specific uses. Starting from a desired functionality, inverse-design algorithms work backwards to find the best system or structure to achieve that functionality.

“Inverse design has a lot of potential because all we have to do is create a highly reconfigurable medium, and give it control over a computer,” Chumak explains. “It will use algorithms to get any functionality we want with the same device.”

One area where inverse design could be useful is creating systems for encoding and processing data using quantized spin waves called magnons. These quasiparticles are collective excitations that propagate in magnetic materials. Information can be encoded in the amplitude, phase, and frequency of magnons – which interact with radio-frequency (RF) signals.

Collective rotation

A magnon propagates by the collective rotation of stationary spins (no particles move) so it offers a highly energy-efficient way to transfer and process information. So far, however, such magnonics has been limited by existing approaches to the design of RF devices.

“Usually we use direct design – where we know how the spin waves behave in each component, and put the components together to get a working device,” Chumak explains. “But this sometimes takes years, and only works for one functionality.”

Recently, two theoretical studies considered how inverse design could be used to create magnonic devices. These took the physics of magnetic materials as a starting point to engineer a neural-network device.

Building on these results, Chumak’s team set out to show how that approach could be realized in the lab using a 7×7 array of independently-controlled current loops, each generating a small magnetic field.

Thin magnetic film

The team attached the array to a thin magnetic film of yttrium iron garnet. As RF spin waves propagated through the film, differences in the strengths of magnetic fields generated by the loops induced a variety of effects: including phase shifts, interference, and scattering. This in turn created complex patterns that could be tuned in real time by adjusting the current in each individual loop.

To make these adjustments, the researchers developed a pair of feedback-loop algorithms. These took a desired functionality as an input, and iteratively adjusted the current in each loop to optimize the spin wave propagation in the film for specific tasks.

This approach enabled them to engineer two specific signal-processing functionalities in their device. These are a notch filter, which blocks a specific range of frequencies while allowing others to pass through; and a demultiplexer, which separates a combined signal into its distinct component signals. “These RF applications could potentially be used for applications including cellular communications, WiFi, and GPS,” says Chumak.

While the device is a success in terms of functionality, it has several drawbacks, explains Chumak. “The demonstrator is big and consumes a lot of energy, but it was important to understand whether this idea works or not. And we proved that it did.”

Through their future research, the team will now aim to reduce these energy requirements, and will also explore how inverse design could be applied more universally – perhaps paving the way for ultra-efficient magnonic logic gates.

The research is described in Nature Electronics.

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