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Physicists create quantum phase battery

Researchers in Spain and Italy have constructed the first-ever quantum phase battery – a device that maintains a phase difference between two points in a superconducting circuit. The battery, which consists of an indium arsenide (InAs) nanowire in contact with aluminium (Al) superconducting leads, could be used in quantum computing circuits. It might also find applications in magnetometry and highly sensitive detectors based on superconductors.

In a classical battery (also known as the Volta pile), chemical energy is converted into a voltage difference. The resulting current flow can then be used to power electronic circuits. In quantum circuits and devices based on superconducting materials, however, current may flow without an applied external voltage, thus dispensing with the need for a classical battery.  

Josephson junction-based device

The concept of a quantum phase battery was studied theoretically in 2015 by Sebastián Bergeret of the Material Physics Center (CFM-CSIC) and Ilya Tokatly at the University of the Basque Country in Donostia-San Sebastián, Spain. Their battery design comprised a combination of superconducting and magnetic materials and was based on a Josephson junction – a non-superconducting region through which the Cooper pairs responsible for superconductivity can tunnel. This semiconducting “weak link” provides a persistent phase difference between the superconductors in the circuit, similar to the way that a classical battery provides a persistent voltage drop in an electronic circuit. Thanks to this phase difference, a superconducting current (that is, a current with zero dissipation) flows when the junction is embedded in the superconducting circuit.

In a follow-up to this work, researchers led by Francesco Giazotto and Elia Strambini at the NEST-CNR Nanoscience Institute in Pisa and the University of Salerno have now identified a suitable material combination for making such a battery. Their new device is made of n-doped InAs nanowires, which form its core (or pile), and Al superconducting leads acting as poles. The surface of the nanowires is “decorated” with magnetic moments originating from surface oxide states of the material.

The anomalous Josephson effect

The battery works using the so-called anomalous Josephson effect. In a normal Josephson junction, a superconducting current flows whenever there is a phase difference between two ends of the junction. Such a phase difference can be induced, for example, by placing the junction in a superconducting loop and applying a magnetic field.

In contrast, a phase battery provides such a phase difference without the need for this external magnetic field. Instead, the difference is induced by a geometrical effect that involves the interplay between three phenomena: superconductivity; spin-orbit coupling (which describes the interaction between the intrinsic spin of an electron in a solid and the magnetic field induced by the motion of electron); and a magnetic exchange field (which acts on magnetic spins just like an ordinary magnetic field).

“The choice of material for making such a battery was not a simple matter,” explains team member Andrea Iorio. “In our device, the Al superconducting leads provide the superconductivity, the spin-orbit coupling is an intrinsic property of the InAs nanowire and the magnetic impurities provide the local exchange field.”

While the battery needs to be charged by polarizing the magnetic impurities with an external magnetic field, the researchers say the phase bias between the poles persists even when they later switch the field off. They can control the value of the phase difference either by changing the direction of the polarization of the magnetic impurities or by modifying the length of the semiconducting wires.

Trying out other material combinations

The researchers, who report their work in Nature Nanotechnology, say they are now exploring other material combinations to improve the device’s performance and their ability to control it, as well as making it easier to integrate into quantum circuits.  

“One idea we are pushing forward is to use a superconducting wire in contact with a ferromagnetic insulator as a battery,” Iorio explains. “The latter will provide a sizeable exchange field that we can accurately control and we are fabricating and characterizing such a system at the moment.”

The Pisa/San Sebastián research teams are also involved in a European project called SUPERTED that aims to engineer ultrasensitive radiation detectors based on superconductor-ferromagnetic insulator structures, and in particular EuS/Al junctions. “Such systems could be adapted to construct phase batteries with better performance,” Iorio tells Physics World.

Cosmonaut builds engineered cartilage aboard the International Space Station

For the first time, 3D human tissue has been assembled in the microgravity of space. Through a magnetic levitation device, researchers in Russia, led by Vladislav Parfenov at the Russian Academy of Sciences and 3D Bioprinting Solutions, enabled a cosmonaut aboard the International Space Station (ISS) to fabricate human cartilage from a few isolated cells. Their work could lead to crucial new techniques for regenerating tissues during long-term spaceflight.

Tissue engineering has seen a surge of interest in recent years. Traditionally, it involves seeding cells onto biocompatible “scaffolds”, which biodegrade once tissues have assembled themselves into 3D organs. However, more flexible, scaffold-free approaches are also emerging, which enable cells to assemble themselves without the need for structural biomaterials. To do this, researchers use techniques including removable supports, and guiding forces from acoustic and electrostatic fields.

One particularly promising approach involves magnetic levitation, through which strong field gradients can precisely guide tissue cells into place. To achieve strong enough gradients, however, cells must be suspended within a paramagnetic medium containing gadolinium ions. At the concentrations required for the technique to work, these ions are toxic to cells, and can cause dangerous pressure imbalances.

One potential solution to this problem is to perform levitated assemblies in microgravity. Recent studies have shown particular interest in doing this with cartilage – the smooth, elastic tissue found in human joints and intervertebral disks. Currently, it is poorly understood how cartilage is affected during long-term spaceflight, since space-based experiments are extremely expensive and time-consuming. In their study, Parfenov and collaborators designed a magnetic bioassembler for use on the ISS – which would only require a low, non-toxic concentration of gadolinium ions.

To do this, the researchers first fabricated tissue spheroids from human cartilage cells at the Baikonur Cosmodrome in Kazakhstan, which were embedded in heat-reversible hydrogel, then delivered to the ISS in hermetically sealed cuvettes. After gadolinium ions were injected by a cosmonaut aboard the space station, the cuvettes were cooled to release the cells from the hydrogel, then placed in a 37°C chamber within the bioassembler for two days. In these conditions, cell fusion and self-assembly could be sustained, and were recorded on a video camera. Finally, the assembled cartilage was stored at room temperature for two weeks, before being returned to Earth for analysis.

As Parfenov’s team hoped, the self-assembly process captured by the camera showed strong agreement with their mathematical models and computer simulations. The success of their experiment now represents a significant advance in our ability to fabricate 3D human tissues without the need for scaffolds, and with non-toxic levels of gadolinium ions. With the adverse effects of microgravity on human tissue already well known to researchers, their technique could prove critical in maintaining the health of astronauts during future manned space exploration.

The research is described in Science Advances.

Giant exoplanets directly observed orbiting Sun-like star

The first ever direct image of a young Sun-like star accompanied by two giant exoplanets has been captured by astronomers using the Very Large Telescope (VLT) in Chile. The two planets are orbiting the star TYC 8998-760-1, which lies 300 light-years away from Earth. The star has an almost identical mass to our Sun, but is just 17 million years old compared with 4.6 billion years for our Sun.

Perhaps the most striking feature of this system is the large mass of the planets and their giant orbits. The inner planet has 14 times Jupiter’s mass and the outer one six times. They orbit at distances of roughly 160 and 320 times the Earth–Sun distance respectively, which is more than 30 times larger than the orbits of Jupiter and Saturn.

This discovery is a snapshot of an environment that is very similar to our solar system, but at a much earlier stage of its evolution.

Alexander Bohn, Leiden University

“This discovery is a snapshot of an environment that is very similar to our solar system, but at a much earlier stage of its evolution,” says Alexander Bohn from at Leiden University in the Netherlands, who led the new research, published in Astrophysical Journal Letters.

A turbulent past

Bohn told Physics World that this system might represent the lower mass end of multiple star formation. In other words, had a bit more stuff accreted from the protostellar cloud, then this could have resulted in a binary star system, rather than one star and two huge planets.

Alternatively, these giant planets could be the result of several small planetessimals clustering together into cores, which eventually gained enough gravitational pull to accrete gas from the circumstellar disc. This is the favoured scenario for the formation of the largest planets in our own solar system.

“To explain the large separations of our detected planets, however, some kind of outward migration is required,” says Bohn. “This can be performed by gravitationally scattering off of each other or with another, so far undetected, third object in the system.”

To date, more than 4000 exoplanets have been detected. However, the vast majority of these have been spotted using indirect methods, such as observing the dip in starlight as a planet transits between its parent star and our line of sight.

Only a tiny fraction of these planets has been directly observed, and the direct imaging of two or more exoplanets around the same star is even rarer. Only two such systems have been directly observed so far, both around stars markedly different from our Sun.

Hot young planets

These latest images were possible due to the SPHERE instrument on the VLT at the European Southern Observatory (ESO) in Chile, which uses a coronagraph to block bright starlight, allowing much fainter planets to be seen. While older planets, such as those seen in our solar system, are too cool to be found with this technique, young planets are hotter, and so glow brighter in infrared light.

This discovery still tells us that there is not just one way to form a planetary system.

Carlo Felice Manara, European Southern Observatory

“This discovery still tells us that there is not just one way to form a planetary system, and the outcome of the process can be very different,” says Carlo Felice Manara, an astronomer based at ESO headquarters in Germany, who was not involved in this latest research. “Why in our solar system we only have planets with mass of Jupiter, or less, and no more massive planets is still an open question.”

Further observations of TYC 8998-760-1 system will enable astronomers to better understand its dynamics. ESO’s Extremely Large Telescope – scheduled to capture its first astronomical images in 2025 – could even detect further planets with Neptune or Saturn masses. Unfortunately, even that instrument is  unlikely to detect rocky planets similar to Earth, as these are just too faint.

Caution required when balancing lung versus heart radiotherapy dose

As treatments for oesophageal cancer improve, patients are living longer, especially those with early-stage localized disease. But longer survival puts oesophageal cancer patients at greater risk of developing serious radiation-induced late toxicities of the heart and lung.

The lack of predictive models, such as those developed for breast cancer patients, has hampered the design of clinically effective radiotherapy treatment plans that also reduce heart and lung toxicity. And because radiation dose to the heart is generally much higher when treating oesophageal cancer, breast cancer models are not automatically transferable.

Planning radiotherapy for oesophageal cancer is further complicated by the fact that reducing radiation dose to the heart increases the potential for lung toxicity. The optimal balance between cardiac and pulmonary toxicities and their influence on overall survival is unknown, according to researchers at the University Medical Center Groningen in the Netherlands and the Osaka Medical Center for Cancer and Cardiovascular Disease in Japan.

To evaluate which clinical and dosimetric parameters are associated with cardiac and/or pulmonary toxicities, the team conducted a retrospective study of 219 oesophageal cancer patients who received 3D conformal radiotherapy. Reporting their results in Radiotherapy and Oncology, the researchers determined that radiation dose to the lungs significantly impacted overall survival. Because of this outcome, they state that reducing cardiac dose at the expense of dose to the lungs “might not always be a good strategy,” especially when using intensity-modulated radiotherapy (IMRT) or volumetric-modulated arc therapy (VMAT).

The study cohort included patients with all stages of oesophageal cancer who underwent radiotherapy at the Osaka Medical Center between 2007 and 2013. Patients ranged in age from 40 to 88 years, were predominantly male (83%), smokers (86%) and drank alcohol (89%). Eighteen percent had a personal (12%) or family (6%) history of cardiovascular disease. Only 12% were diabetics and only 7% were overweight.

Patients were treated using 10 MV photons in 1.8–2.0 Gy daily fractions to a total dose of 50.4–66.0 Gy (median: 60 Gy). They had follow-up visits every three to six months for the first two years, and semi-annually thereafter. With a median follow-up of 27 months, 45% of patients had developed locoregional failures. The median disease-free survival was 64 months.

The researchers measured dose–volume parameters including dose to the whole heart, and substructures of the heart and lungs, and mean doses. They also recorded all newly diagnosed cardiac and pulmonary events. They reported that 28% of patients developed radiation-induced pneumonitis. The primary predictor of radiation pneumonitis was mean lung dose. The patients who developed radiation pneumonitis had significantly worse overall survival compared with other study participants.

Roughly one third of the patients developed pericardial effusion, the build-up of excess fluid in the pericardium that puts pressure on the heart and can lead to heart failure. However, this toxicity did not seem to influence overall survival in the patient cohort.

Multivariable analysis revealed that radiation dose to the lungs (V45, the volume of lung receiving over 45 Gy), but not the dose to the heart, influenced overall survival significantly in these patients. When the team repeated the analyses excluding patients with radiation pneumonitis, the same variables remained significant. In the patients with radiation pneumonitis, the most important predictor for overall survival was the radiation dose to the heart.

“Cardiac dose–volume parameters predicted the risk of pericardial effusion and pulmonary dose–volume parameters predicted the risk of radiation pneumonitis,” write the authors. “However, in this patient cohort, pulmonary DVH parameters (V45) were more important for overall survival than cardiac DVH parameters.”

Because worse overall survival can be caused by radiation dose to both organs-at-risk, the authors suggest that oesophageal cancer patients are treated with proton therapy, because it can reduce both the radiation dose to the heart and to the lungs. First author Jannet Beukema tells Physics World that the University Medical Center Groningen is now utilizing proton therapy for preoperative oesophageal cancer patients. They are currently conducting more research to determine heart and lung toxicity in this patient group.

China successfully launches Tianwen-1 Mars probe

China has launched its first independent mission to Mars with the uncrewed craft expected to look for clues of water-ice, past life and study the evolution of our neighbouring planet. At 12:41 p.m. Beijing time today, Tianwen-1 (“heavenly questions”) took off atop a Long March 5 rocket from Wenchang Satellite Launch Center in southern China. The craft contains an orbiter as well as a lander and rover and is expected to reach Mars in February.

[Tianwen-1 is] a milestone mission for China’s aerospace to go deeper into space

Yansheng Wu

China Aerospace Science and Technology Corporation (CASC) – the country’s main space contractor – said that the launch went smoothly and about 45 minutes later the probe entered the planned Earth–Mars transfer orbit. “[Tianwen-1 is] a milestone mission for China’s aerospace to go deeper into space,” noted mission deputy commander Yansheng Wu in a statement.

Onboard Tianwen-1 are 13 scientific instruments – seven on the orbiter and six on the rover – that will aim to obtain comprehensive, first-hand data about the planet including its magnetosphere and atmosphere as well as surface and subsurface geological structures. Some instruments on Tianwen-1 are similar to those used in previous NASA missions, such as a surface component detector that fires a laser to vaporize rock surfaces and measure chemical elements as well as ground-penetrating radar to detect global water-ice distribution from orbit.

Confidence boost

Landing on Mars will be China’s next challenge. Despite its recent successes with a Moon landing, China has never landed through a thin atmosphere like that on Mars. To do so, Tianwen-1 will use a parachute and a “retro-engine” to slow down its descent. At about 100 m above the surface, Tianwen-1 will briefly pause firing its retro engines to take snapshots of the landing area to calculate the best landing spot. To manage this aspect of the landing, the craft will use an autonomous obstacle avoidance technology, which was originally developed for China’s lunar missions. It is hoped that the lander and rover will touch down in a region called Utopia Planitia, which is a largely flat impact basin but scientifically interesting with potential water-ice underneath.

China’s first attempt to reach Mars was in 2011 when an orbiter piggybacked on a Russian mission to the Martian moon Phobos. However, the launch failed and China began to come up with a new mission. The successful launch of Tianwen-1 greatly boosts confidence in the Long March 5 rocket – China’s biggest heavy-lifting launcher – after it experienced a major failure in 2017 due to a first-stage engine problem. This success has now paved the way for a sample-return mission to the Moon by the end of the year as well as the launch of the core module of the Chinese space station early next year.

Tianwen-1 is one of three missions heading to Mars this month. On Monday, the United Arab Emirates successfully launched its Mars-bound Hope orbiter from Japan while on 30 July NASA is expected to launch its Mars 2020 mission, which includes the Perseverance rover and a Mars helicopter called Ingenuity.

Fluid flow and stress cause tiny marine organisms to emit light

New research by Raymond Goldstein and colleagues at the University of Cambridge has revealed how fluid motion and direct mechanical pressure triggers the emission of bioluminescent light by tiny marine organisms.

Bioluminescence occurs in a wide array of organisms including dinoflagellates, which are single-celled plankton that play a crucial role in aquatic food chains. Despite their global importance, scientists do not have a full understanding of how these organisms emit green-blue light in response to fluid motion in their environments.

Goldstein’s team looked at how the light intensity from dinoflagellates correlates with various mechanical stressors. They did this by exposing the tiny organisms to two types of stimuli mimicking the mechanical attack of a predator and swimming in a densely populated wave.

The team built their study upon previous work by Michael Latz and colleagues at the Scripps Institution of Oceanography in the US. Latz’s team had used macroscopic contracting fl0ws, Couette flow, microfluidic flows, and atomic force microscopy to study dinoflagellates.

“Next logical step”

“Our work is the next logical step that builds on [Latz’s] work, by using micropipette technology to hold individual cells and apply quantified cell wall deformations, all with high-speed imaging,” explains Goldstein.

The bioluminescence of dinoflagellates is unique from many perspectives. The biochemical and cellular cascade leading to a light flash begins when the cell membrane is subject to mechanical stress, which activates a receptor in the membrane. This mechanical signal is translated into an electrical signal by increasing the concentration of calcium ions.

As a result, membranes of inner organelles depolarize and generate an action potential that propagates along the membranes. Ultimately, the pH of the environment decreases, promoting luciferin oxidation catalyzed by luciferase, which is accompanied by a photon release.

Ideal experimental subjects

To investigate the underlying biophysical processes, Goldstein and his team held single cells of the dinoflagellate Pyrocystis lunula on a glass micropipette using suction. Measuring 30–1000 μm, they are ideal experimental subjects thanks to their robustness and reduced motility. According to Goldstein, the team often manipulated each cell up to 20 times without any obvious damage.

The cells were mechanically stimulated either by directing a submerged jet of fluid (growth medium) at a single cell or by squeezing a single cell between two pipettes with submicron precision. The cell wall deformation was monitored by direct imaging with a high-speed camera.

The results showed that the cells flash only after the fluid pressure is high enough to induce significant cell wall deformation. The flash occurs and decays within fractions of a second. The bioluminescence intensity eventually decreases, most probably due to exhaustion of the luciferin. Interestingly, bioluminescence tends to occur within the central region around the cell’s nucleus.

For mechanical stimuli, light intensity was low when the deformation was small (whatever the speed), or when the deformation rate was small (whatever the deformation). However, the bioluminescence flash was large when both the deformation and its rate were large. These results suggest that the membrane and calcium channels have viscoelastic properties – which was confirmed by a phenomenological model.

“Welcome contribution”

According to Latz, “this study provides a welcome contribution to our knowledge of mechanosensory responsiveness in the dinoflagellate bioluminescence system. I applaud their use of controlled and quantified modes of mechanical stimulation using sophisticated analytical techniques.”

“One of our challenges is the discrepancy between sensitivity in macroflows and microstimulation, based on the as-yet unknown distribution of mechanosensors within the cell and how their activity is integrated into a whole cell response. As I am now retired, I hope that it will inspire others to continue the work,” Latz adds.

Goldstein adds “Michael Latz has shown, already a number of years ago, that dinoflagellates can be used as local sensors for fluid shear. We are now very interested in the role of dinoflagellates in algal blooms, in extending our understanding of cell wall mechanics, and in force and light distribution over the entire cell body.”

The research is described in Physical Review Letters.

Nanostructured topological insulator growth speeds up

A supportive “mattress” of material has made it possible to grow nanoribbons of a previously hard-to-synthesize topological insulator. The technique, which is based on conventional chemical vapour transport, produces crystalline nanostructures of zirconium pentatelluride (ZrTe5) with a high carrier mobility, and could be used to produce ultrathin and even monolayer ZrTe5 in the future.

Topological insulators are materials that act as electrical insulators in their bulk, while conducting electricity extremely well on their surfaces (or edges) via special, topologically protected, electronic states. Within these states, electrons can only travel in one direction and do not backscatter. This remarkable behaviour allows the materials to carry electrical current with near-zero dissipation – meaning that they could someday be used to make electronic devices that are far more energy-efficient than any that exist today.

Even within this exotic family of materials, ZrTe5 stands out thanks to its wide variety of topological phases. Depending on how it is configured, it can behave as a weak or a strong insulator, a Dirac semimetal or a 3D quantum Hall state. When thinned down to a monolayer, it is also predicted to be a large band gap quantum spin Hall insulator.

Topological nanoribbons

Previous studies of ZrTe5 mainly focused on its 3D bulk form. For real-world applications, however, nanowires or nanoribbons may be more useful, as surface states play a more important role when the surface-to-volume ratio is higher. New electronic properties could also emerge. Research on other types of nanostructured topological insulators has already revealed unexpected phases, and novel phenomena called “Majorana zero modes” (which could be used as bits in quantum computers) were observed experimentally for the first time when indium antimonide (InSb) nanowires were combined with superconductors.

Until the present work, efforts to grow high-quality nanostructures of ZrTe5 had foundered for lack of a reliable way to produce them quickly or in large-enough quantities. The new technique developed by Xiaosong Wu and colleagues of Peking University and South University of Science and Technology of China in Shenzhen is based on conventional chemical vapour transport. Here, source materials such as zirconium and tellurium react in their gas phases with the help of a transport agent (iodine, in this case) and crystallize. “The only difference between our technique and the conventional method is the introduction of silicon, which allows for the growth of nanostructured ZrTe5,” Wu explains.

Silicon plays an important role in growth

The ZrTe5 nanostructures do not grow directly on the silicon substrate, Wu adds. Instead, they develop out of a “mattress” that mainly consists of Zr and Te in a ratio of around 1:3.

Wu thinks this ZrTe3 mattress may provide a more suitable substrate for ZrTe5 to grow on. Certainly, when he and his colleagues looked closer at the mattress’ edge, they observed etches in the silicon substrate. “We know that iodine reacts with silicon and forms SiI4, which is in the gaseous phase at the growth temperature we employed,” says Wu. “We found that silicon plays an important role in the reaction but we are as yet unsure on how it affects the growth process exactly.”

The researchers say they can also grow millimetre-long single crystals in around two hours – a huge improvement on conventional methods, which require weeks.

Growing thinner nanostructures

According to the team, aberration-corrected transmission electron microscopy (TEM) images of the as-grown ZrTe5nanoribbons suggest that the structures are highly crystalline. Quantum transport measurements also show that they contain few impurities and that they have a high carrier mobility of over 30,000 cm2/Vs – meaning that they conduct electricity extremely well.

“Our method shows great promise for growing high-quality ultra-thin nanostructures of ZrTe5 and even monolayers,” Wu tells Physics World. “Being able to do so would allow us to make this large band gap quantum spin Hall insulator in the laboratory and study its fundamental physical properties in experiments.”

The researchers, who report their work in Chinese Physics B, say they are now busy improving their technique so they can grow thinner nanostructures.

A flat-Earth fight, an inconsistent Hubble constant, and carbon atoms at a graphene ‘watering hole’

This episode of the Physics World Weekly podcast explores two very different scientific debates: one about the shape of the Earth, and the other about the expansion of the universe.

Scientifically speaking, the “debate” about the Earth’s shape was settled hundreds (perhaps even thousands) of years ago: our planet is round, an “oblate spheroid” squashed at its poles and bulging at its equator. But that hasn’t deterred some people from believing it’s a flat disc, and their numbers seem to be on the rise. According to the science writer Rachel Brazil, whose feature article “Fighting flat-Earth theory” appears in the July issue of Physics World, modern flat-Earthism is driven by a wider tendency towards conspiracy-mindedness, with considerable crossover between flat-Earth beliefs and other, unrelated conspiracy theories such as climate-change denial and the anti-vaccination movement. Because of this, Brazil suggests that “arguing the physics” may not actually be the best way to change minds. “With people who are so untrusting, they will just see everything as a hoax,” she tells Physics World editor-in-chief Matin Durrani.

As for the expansion of the universe, this debate is, mercifully, less heated. It centres on the fact that the Hubble constant, which characterizes the speed of the universe’s expansion, has been measured in two ways. The first, “local” method relies on measuring the intrinsic brightness of certain stellar objects (known as “standard candles”). The second, “global” method is based on measurements of the cosmic microwave background radiation. As the science writer Keith Cooper explains, these methods produce different – and incompatible – values for how fast the universe is expanding. You can find out more about this discrepancy (and how to resolve it) in Cooper’s feature “Finding a consistent constant”, and in a related video.

Cryogenic innovation: ramping up research productivity at ultralow temperatures

ICEoxford, as the name suggests, is in the business of keeping things cold – ultracold to be precise. As such, the UK-based designer and manufacturer of specialist cryogenic systems supports fundamental and applied research efforts across a diverse customer base – from neutron and X-ray beamline scientists to quantum computing start-ups; from scanning-probe microscopy specialists to physicists studying the fundamental properties of high-temperature superconductors. “Generally, our customers want to get cold, get cold as quickly as possible, plus many want the option of studying their samples within a magnetic field,” explains Paul Kelly, Chief Technical Officer at ICEoxford.

With those requirements in mind, Kelly and his colleagues in Oxfordshire have pioneered the DRY ICE 1.5K 100 mm, a closed-loop cryostat that enables users to carry out experiments in the temperature range from 1.3 K to 325 K while ensuring tight tolerance on temperature stability (±10 mK below 10 K), fast cool-down times (<30 min to 1.4 K) and minimal vibration in the sample space (as little as ±100 nm). There’s also the option to add 3He and dilution-fridge inserts to achieve even lower temperatures (300 mK and 15 mK respectively).

Kelly, for his part, is keen to emphasize the win-win of a 100 mm diameter sample space matched with the cryostat’s high-cooling-power capability (>30 mW at 1.75 K). “This is the largest available sample space and highest cooling power on the market for a variable-temperature-insert [VTI] cryostat,” he claims. Equally significant is the fast-track cool-down cycle – and particularly so for scientists allocated time on the beamlines of big-science facilities like the ISIS Neutron and Muon Source or the Diamond Light Source (both also in Oxfordshire).

“Beam time is expensive and scientific users only get a fixed slot to complete their experiments – typically just a few days,” adds Kelly. Smaller laboratories also benefit, with the rapid cool-down translating into enhanced productivity versus research output (e.g. fast sample pre-testing in the cryogenic systems of quantum computers).

Good vibrations

Given the varied, often unique, requirements of ICEoxford’s end-users, the vendor sets a high bar for its design, engineering and manufacturing teams. Their task, says Kelly, is to bring a versatility of approach to all customer relationships, with a relentless focus on delivering the best system to do the job. “We work closely with the customer to fully understand their requirements and will tailor the product design to meet their exact specifications,” he notes.

Paul Kelly
Paul Kelly: “This is the largest available sample space and highest cooling power on the market for a VTI cryostat.” (Courtesy: ICEoxford)

The DRY ICE 1.5K 100 mm provides a case study in versatility, not least in terms of its vibration performance. The system can utilize two types of cryocooler: a Gifford-McMahon (GM) set-up (which relies on a mobile piston for compression and expansion of the helium fluid in the cold-head) or pulse-tube refrigerator (PTR) technology (with no moving parts in the low-temperature region). “Most users don’t see vibration as a show-stopper,” says Kelly, “but those who do are likely to choose PTR cooling, as the vibration is approaching an order of magnitude lower than GM.”

Further damping comes with the use of so-called “ICE sock” technology, a common method of vibration isolation across a range of ICEoxford products. “Put simply,” explains Kelly, “we build a container called a sock to surround the cryocooler. We then load the cooler into the sock and hold it away from any mechanical parts on a bellows.” A range of anti-vibration frames are also available to suit bespoke laboratory requirements, with product R&D ongoing to achieve even lower vibration levels (<50 nm at the sample space).

Versatile product design is further evidenced in ICEoxford’s patented Dual-Cool technology, which enables the DRY ICE 1.5K 100 mm system to switch between static and dynamic exchange-gas mode for sample cooling. Essentially, dynamic mode yields higher cooling powers, and in turn faster cool-down, after the sample probe is loaded into the cryostat at room temperature. After this initial cooling, however, most users want the sample to be in a more stable vacuum or low-pressure, static-gas environment for their experiments, swapping from dynamic to static mode once the sample approaches target temperature.

Magnetic attractions

Cooling aside, high-field superconducting magnets are designed to be an integral part of the DRY ICE 1.5K 100 mm system, with ICEoxford offering a range of solenoid, split-pair and vector-rotate magnets up to 18 T field strength. While studies of magnetic properties at ultralow temperatures are a given, many scientists also want to carry out simultaneous optical investigations of their materials – not so easy if the sample is sat within the large coil of a solenoid magnet.

In this case, one option is to load the sample into the bore of a split-pair magnet. “The DRY ICE system can be designed with a number of optical windows to allow, for example, laser spectroscopy experiments in transmission or reflection mode,” explains Kelly. The use of two- or three-way vector-rotate magnets provides further flexibility, with the latter able to generate a magnetic field in three discrete directions. In this way, it is possible to keep the sample stationary while the magnetic field is varied around it – a key feature when studying a specific plane within a crystal lattice or if the heat generated by sample rotation is a source of interference for small-scale electrical conductivity measurements.

One other notable feature of the DRY ICE portfolio is the high degree of automation, with LabVIEW-based software used to control and monitor temperature in the DRY ICE 1.5K 100 mm system. “This has been developed with the customer in mind,” concludes Kelly, “to reduce the set-up and turnaround time of the system while increasing laboratory productivity.” What’s more, it’s possible to add features to the software such as integrated control for the superconducting magnet as well as automating the top-load and probe cool-down.

Variants of the DRY ICE 1.5K cryostat are also available in configurations with 30, 50 or 70 mm diameter sample space.

Weak electric currents track tiny magnetic skyrmions

Magnetic skyrmions – quasiparticles with a vortex-like structure – show much promise as storage bits for next-generation computer memories and logic devices, but tracking their motion is no easy task. Researchers at the RIKEN Center for Emergent Matter Science in Japan have now succeeded in tracing them using small pulses of electric current – a development that brings real-world devices based on skyrmions a step closer to reality.

Skyrmions can be thought of as 2D knots (or “spin textures”) in a material in which the magnetic moments rotate 360° within a plane. They exist in many structures, notably magnetic thin films and multilayers, and are very small, measuring just tens of nanometres across.

Because skyrmions are much smaller than the magnetic domains used in modern disk drives, and also robust against external perturbations, they are considered ideal building blocks for future magnetic data storage technologies such as “racetrack” memories. In these devices, spin currents (which consist of electrons with opposite spins moving in opposite directions) are used to “push” a magnetic domain past magnetic read/write heads. A skyrmion could replace a magnetic domain and serve as a substitute information carrier (a “1” in the binary system of 1s and 0s)

There is a drawback, however, in that it is difficult to measure skyrmions without using large electric currents. This is because of their small size, which makes them crystallize in an energetically stable close-packed hexagonal lattice that is hard to budge.

Making use of a helimagnet

In their experiments, a team of researchers led by Xiuzhen Yu studied thin films of iron germanide (FeGe). This material is magnetic, but not in the usual ferromagnetic sense of having all the magnetic moments within each domain aligned in the same direction. Instead, it is helimagnetic, meaning that the magnetic moments within its domains are arranged in spiral or helical patterns. These unusual patterns make it easier to manipulate the skyrmions in the magnet, as the magnetic moments follow the spiral patterns.

Yu and colleagues began by etching a square notch in their FeGe film to generate localized spin current around the corner of the notch. To create and then erase single skyrmions and skyrmion clusters in the film, the researchers applied pulses of electric currents first in one direction relative to their notched films and then in the opposite direction. By then recording the results using a technique called Lorentz transmission electron microscopy, they found a point in the material at which they could isolate individual skyrmions (measuring just 80 nm in size) and record how they moved based on known processes such as the topological Hall effect. This effect occurs when a voltage is applied along a thin conducting sheet of material while a magnetic field is applied perpendicular to its surface.

Relatively low electric current

The electric currents that the researchers applied to create and move the skyrmions in FeGe are very weak, around 109 A/m2, which is a thousand times lower than that required for traditional magnetic domain wall hard drives.

“Our result demonstrates that it is possible to manipulate and track individual skyrmions and skyrmion crystals (which contain a number of vortices and are thus difficult to move) using a relatively low electric current,” Yu says. “Such detection is an important step towards real-world applications based on these particles because it would correspond to the reading procedure in a skyrmion-based logic device.”

Members of the team, who report their work in Science Advances, say they are now developing a bilayer system of FeGe films that could host combined skyrmions, which might be easier to detect. “We are also eager to track skyrmions in one-dimensional structures such as nanowires (which would mimic a racetrack structure) at room temperature,” Yu tells Physics World. “The challenges here will be nanofabrication and device design.”

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