Physicists in Germany have developed a new way of defining the standard unit of electrical resistance. The advantage of the new technique is that because it is based on the quantum anomalous Hall effect rather than the ordinary quantum Hall effect, it does not require the use of applied magnetic fields. While the method in its current form requires ultracold temperatures, an improved version could allow quantum-based voltage and resistance standards to be integrated into a single, universal quantum electrical reference.

Since 2019, all base units in the International System of Units (SI) have been defined with reference to fundamental constants of nature. For example, the definition of the kilogram, which was previously based on a physical artefact (the international prototype kilogram), is now tied to Planck’s constant, h.

These new definitions do come with certain challenges. For example, today’s gold-standard way to experimentally determine the value of h (as well the elementary charge e, another base SI constant) is to measure a quantized electrical resistance (the von Klitzing constant R_K = h/e²) and a quantized voltage (the Josephson constant K_J = 2e/h). With R_K and K_J pinned down, scientists can then calculate e and h.

To measure R_K with high precision, physicists use the fact that it is related to the quantized values of the Hall resistance of a two-dimensional electron system (such as the ones that form in semiconductor heterostructures) in the presence of a strong magnetic field. This quantized change in resistance is known as the quantum Hall effect (QHE), and in semiconductors like GaAs or AlGaAs, it shows up at fields of around 10 Tesla. In graphene, a two-dimensional carbon sheet, fields of about 5 T are typically required.

The problem with this method is that K_J is measured by means of a separate phenomenon known as the AC Josephson effect, and the large external magnetic fields that are so essential to the QHE measurement render Josephson devices inoperable. According to Charles Gould of the Institute for Topological Insulators at the University of Würzburg (JMU), who led the latest research effort, this makes it difficult to integrate a QHE-based resistance standard with the voltage standard.

A way to measure R_K at zero external magnetic field

Relying on the quantum anomalous Hall effect (QAHE) instead would solve this problem. This variant of the QHE arises from electron transport phenomena recently identified in a family of materials known as ferromagnetic topological insulators. Such quantum spin Hall systems, as they are also known, conduct electricity along their (quantized) edge channels or surfaces, but act as insulators in their bulk. In these materials, spontaneous magnetization means the QAHE manifests as a quantization of resistance even at weak (or indeed zero) magnetic fields.

In the new work, Gould and colleagues made Hall resistance quantization measurements in the QAHE regime on a device made from V-doped (Bi,Sb)₂Te₃. These measurements showed that the relative deviation of the Hall resistance from R_K at zero external magnetic field is just (4.4 ± 8.7) nΩ Ω⁻¹. The method thus makes it possible to determine R_K at zero magnetic field with the needed precision — something Gould says was not previously possible.

The snag is that the measurement only works under demanding experimental conditions: extremely low temperatures (below about 0.05 K) and low electrical currents (below 0.1 uA). “Ultimately, both these parameters will need to be significantly improved for any large-scale use,” Gould explains. “To compare, the QHE works at temperatures of 4.2 K and electrical currents of about 10 uA; making its detection much easier and cheaper to operate.”

Towards a universal electrical reference instrument

The new study, which is detailed in Nature Electronics, was made possible thanks to a collaboration between two teams, he adds. The first is at Würzburg, which has pioneered studies on electron transport in topological materials for some two decades. The second is at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, which has been establishing QHE-based resistance standards for even longer. “Once the two teams became aware of each other’s work, the potential of a combined effort was obvious,” Gould says.

Because the project brings together two communities with very different working methods and procedures, they first had to find a window of operations where their work could co-exist. “As a simple example,” explains Gould, “the currents of ~100 nA used in the present study are considered extremely low for metrology, and extreme care was required to allow the measurement instrument to perform under such conditions. At the same time, this current is some 200 times larger than that typically used when studying topological properties of materials.”

As well as simplifying access to the constants h and e, Gould says the new work could lead to a universal electrical reference instrument based on the QAHE and the Josephson effect. Beyond that, it could even provide a quantum standard of voltage, resistance, and (by means of Ohm’s law) current, all in one compact experiment.

The possible applications of the QAHE in metrology have attracted a lot of attention from the European Union, he adds. “The result is a Europe-wide EURAMET metrology consortium QuAHMET aimed specifically at further exploiting the effect and operation of the new standard at more relaxed experimental conditions.”

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A recently-discovered class of magnets called altermagnets has been imaged in detail for the first time thanks to a technique developed by physicists at the University of Nottingham’s School of Physics and Astronomy in the UK. The team exploited the unique properties of altermagnetism to map the magnetic domains in the altermagnet manganese telluride (MnTe) down to the nanoscale level, raising hopes that its unusual magnetic ordering could be controlled and exploited in technological applications.

In most magnetically-ordered materials, the spins of atoms (that is, their magnetic moments) have two options: they can line up parallel with each other, or antiparallel, alternating up and down. These arrangements arise from the exchange interaction between atoms, and lead to ferromagnetism and antiferromagnetism, respectively.

Altermagnets, which were discovered in 2024, are different. While their neighbouring spins are antiparallel, like an antiferromagnet, the atoms hosting these spins are rotated relative to their neighbours. This means that they combine some properties from both types of conventional magnetism. For example, the up, down, up ordering of their spins leads to a net magnetization of zero because – as in antiferromagnets – the spins essentially cancel each other out. However, their spin splitting is non-relativistic, as in ferromagnets.

Resolving altermagnetic states down to nanoscale

Working at the MAX IV international synchrotron facility in Sweden, a team led by Nottingham’s Peter Wadley used photoemission electron microscopy to detect the electrons emitted from the surface of MnTe when it was irradiated with a polarized X-ray beam.

“The emitted electrons depend on the polarization of the X-ray beam in ways not seen in other classes of magnetic materials,” explains Wadley, “and this can be used to map the magnetic domains in the material with unprecedented detail.”

Using this technique, the team was able to resolve altermagnetic states down to the nanoscale – from 100-nm-scale vortices and domain walls up to 10-μm-sized single-domain states. And that is not all: Wadley and colleagues found that they could control these features by cooling the material while a magnetic field is applied.

Potential uses of altermagnets

Magnetic materials are found in most long-term computer memory devices and in many advanced microchips, including those used for Internet of Things and artificial intelligence applications. If these materials were replaced with altermagnets, Wadley and colleagues say that the switching speed of microelectronic components and digital memory could increase by up to a factor of 1000, with lower energy consumption.

“The predicted properties of altermagnets make them very attractive from the point of view of fundamental research and applications,” Wadley tells Physics World. “With strong theoretical guidance from our collaborators at FZU Prague and the Max Planck Institute for the Physics of Complex Systems, we realised that our experience in materials development and magnetic imaging positioned us well to attempt to image and control altermagnetic domains.”

One of the main challenges the researchers faced was developing thin films of MnTe with surfaces of a sufficiently high quality that allowed them to detect the subtle X-ray spectroscopy signatures of the altermagnetic order. They hope that their study, detailed in Nature, will spur further interest in these materials.

“Altermagnets provide a new vista of predicted phenomena from unconventional domain walls to unique band structure effects,” Wadley says. “We are exploring these effects on multiple fronts and one of the major goals is to demonstrate a more efficient means of controlling the magnetic domains, for example, by applying electric currents rather than cooling them down.”

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Magnets generally have two poles, north and south, so observing something that behaves like it has only one is extremely unusual. Physicists in Germany and Switzerland have become the latest to claim this rare accolade by making the first direct detection of structures known as orbital angular momentum monopoles. The monopoles, which the team identified in materials known as chiral crystals, had previously only been predicted in theory. The discovery could aid the development of more energy-efficient memory devices.

Traditional electronic devices use the charge of electrons to transfer energy and information. This transfer process is energy-intensive, however, so scientists are looking for alternatives. One possibility is spintronics, which uses the electron’s spin rather than its charge, but more recently another alternative has emerged that could be even more promising. Known as orbitronics, it exploits the orbital angular momentum (OAM) of electrons as they revolve around an atomic nucleus. By manipulating this OAM, it is in principle possible to generate large magnetizations with very small electric currents – a property that could be used to make energy-efficient memory devices.

Chiral topological semi-metals with “built-in” OAM textures

The problem is that materials that support such orbital magnetizations are hard to come by. However, Niels Schröter, a physicist at the Max Planck Institute of Microstructure Physics in Halle, Germany who co-led the new research, explains that theoretical work carried out in the 1980s suggested that certain crystalline materials with a chiral structure could generate an orbital magnetization that is isotropic, or uniform in all directions. “This means that the materials’ magnetoelectric response is also isotropic – it depends solely on the direction of the injected current and not on the crystals’ orientation,” Schröter says. “This property could be useful for device applications since it allows for a uniform performance regardless of how the crystal grains are oriented in a material.”

In 2019, three experimental groups (including the one involved in the latest work) independently discovered a type of material called a chiral topological semimetal that seemed to fit the bill. Atoms in these semimetals are arranged in a helical pattern, which produces something that behaves like a solenoid on the nanoscale, creating a magnetic field whenever an electric current passes through it.

The advantage of these materials, Schröter explains, is that they have “built-in” OAM textures. What is more, he says the specific texture discovered in the most recent work – an OAM monopole – is “special because the magnetic field response can be very large – and isotropic, too”.

Visualizing monopoles

Schröter and colleagues studied chiral topological semimetals made from either palladium and gallium or platinum and gallium (PdGa or PtGa). To understand the structure of these semimetals, they directed circularly polarized X-rays from the Swiss Light Source (SLS) onto samples of PdGa and PtGa prepared by Claudia Felser’s group at the Max Planck Institute in Dresden. In this technique, known as circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES), the synchrotron light ejects electrons from the sample, and the angles and energies of these electrons provide information about the material’s electronic structure.

“This technique essentially allows us to ‘visualize’ the orbital texture, almost like capturing an image of the OAM monopoles,” Schröter explains. “Instead of looking at the reflected light, however, we observe the emission pattern of electrons.” The new monopoles, he notes, reside in momentum (or reciprocal) space, which is the Fourier transform of our everyday three-dimensional space.

Complex data

One of the researchers’ main challenges was figuring out how to interpret the CD-ARPES data. This turned out to be anything but straightforward. Working closely with Michael Schüler’s theoretical modelling group at the Paul Scherrer Institute in Switzerland, they managed to identify the OAM textures hidden within the complexity of the measurement figures.

Contrary to what was previously thought, they found that the CD-ARPES signal was not directly proportional to the OAMs. Instead, it rotated around the monopoles as the energy of the photons in the synchrotron light source was varied. This observation, they say, proves that monopoles are indeed present.

The findings, which are detailed in Nature Physics, could have important implications for future magnetic memory devices. “Being able to switch small magnetic domains with currents passed through such chiral crystals opens the door to creating more energy-efficient data storage technologies, and possibly also logic devices,” Schröter says. “This study will likely inspire further research into how these materials can be used in practical applications, especially in the field of low-power computing.”

The researchers’ next task is to design and build prototype devices that exploit the unique properties of chiral topological semimetals. “Finding these monopoles has been a focus for us ever since I started my independent research group at the Max Planck Institute for Microstructure Physics in 2021,” Schröter tells Physics World. The team’s new goal, he adds, is to “demonstrate functionalities and create devices that can drive advancements in information technologies”.

To achieve this, he and his colleagues are collaborating with partners at the universities of Regensburg and Berlin. They aim to establish a new centre for chiral electronics that will, he says, “serve as a hub for exploring the transformative potential of chiral materials in developing next-generation technologies”.

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