Smaller, faster, more powerful: The demands on microelectronic devices are high and are constantly increasing. However, if chips, processors, and the like are based on electricity, there are limits to miniaturization. Physicists are therefore working on alternative ways of transporting information, such as about spin waves, also called magnons, for example. The advantage would be that they have very little energy loss and can therefore spread over long distances. However, spin waves do not form in just any material, they need certain properties to do so. Hematite, for example, the main component of rust, offers these properties.

New material class for spin wave transport

In an EU project together with the Université Paris-Saclay, Shanghai University, and Université Grenoble Alpes, physicists at Johannes Gutenberg University Mainz (JGU) have now been able to develop a completely new class of materials for transporting spin waves: antiferromagnets with tilted magnetic moments. "These materials have the potential to increase computing speed significantly compared to existing devices and at the same time greatly reduce waste heat," said Felix Fuhrmann of Mainz University. In the antiferromagnets, the spin waves and thus the information stored in them can be transported over long distances – a distance of around 500 nanometers is possible. It may not sound much, but transistors in chips today are usually only about seven nanometers in size, so the range of the spin waves is significantly greater than the distance required. "The transport of information over long distances is crucial for an application in microelectronic devices. With the antiferromagnets, we have found a material class that offers this important property and thus opens up a large pool of materials that can be used for devices," emphasized Fuhrmann.

An external magnetic field as an enabler

The scientists examined the canted antiferromagnet yttrium iron oxide, YFeO₃. Since its crystal structure differs fundamentally from that of the established hematite, the researchers initially asked themselves whether spin waves can still form and propagate – and found out that they definitely can. A little trick makes it possible: the physicists apply an external magnetic field to the material. "Magnons are a collective excitation of the magnetic moments in a magnetically ordered crystal. They can therefore be manipulated by magnetic fields, as we were able to successfully demonstrate," said Fuhrmann.

Professor Mathias Kläui, who initiated the study in his group, emphasized: "The international collaboration with leading groups within a project funded by the European Union was the key to this success."

Japanese scientists have created a Bose-Einstein condensate out of excitons, quasiparticles that combine electrons and positively charged “holes” in a semiconductor. Quasiparticle Bose-Einstein condensates have for six decades been something of a holy grail of low-temperature physics.

Japanese physicists have created the first Bose-Einstein condensate, the mysterious ”‘fifth state” of matter, made from quasiparticles, entities that do not count as elementary particles but that can still have elementary-particle properties like charge and spin. For decades, it was unknown whether they could undergo Bose-Einstein condensation in the same way as real particles, and it now appears that they can. The finding is set to have a significant impact on the development of quantum technologies including quantum supercomputing. They published a paper describing the process of creation of the substance, achieved at temperatures a hair’s breadth from absolute zero.

Bose-Einstein condensates are sometimes described as the fifth state of matter, alongside solids, liquids, gases, and plasmas. Theoretically predicted in the early 20th century, Bose-Einstein condensates, or BECs, were only created in a lab as recently as 1995. They are also perhaps the oddest state of matter, with a great deal about them remaining unknown to science.

BECs occurs when a group of atoms is cooled to within billionths of a degree above absolute zero. Researchers commonly use lasers and “magnet traps” to steadily reduce the temperature of a gas, typically composed of rubidium atoms. At this ultracool temperature, the atoms barely move and begin to exhibit very strange behavior. They experience the same quantum state — almost like coherent photons in a laser — and start to clump together, occupying the same volume as one indistinguishable “super-atom.” The collection of atoms essentially behaves as a single particle.

Currently, BECs remain the subject of much basic research, and for simulating condensed matter systems, but in principle, they have applications in quantum information processing. Quantum supercomputing, still in the early stages of development, makes use of several different systems. But they all depend upon quantum bits, or qubits, that are in the same quantum state.

Most BECs are fabricated from dilute gases of ordinary atoms. But until now, a BEC made out of exotic atoms has never been achieved.

Exotic atoms are atoms in which one subatomic particle, such as an electron or a proton, is replaced by another subatomic particle that has the same charge. Positronium, for example, is an exotic atom made of an electron and its positively charged anti-particle, a positron.

An “exciton” is another such example. When light hits a semiconductor, the energy is sufficient to “excite” electrons to jump up from the valence level of an atom to its conduction level. These excited electrons then flow freely in an electric current — in essence transforming light energy into electrical energy. When the negatively charged electron performs this jump, the space left behind, or “hole,” can be treated as if it were a positively charged particle. The negative electron and positive hole are attracted and thus bound together.

Combined, this electron-hole pair is an electrically neutral “quasiparticle” called an exciton. A quasiparticle is a particle-like entity that does not count as one of the 17 elementary particles of the standard model of particle physics, but that can still have elementary-particle properties like charge and spin. The exciton quasiparticle can also be described as an exotic atom because it is in effect a hydrogen atom that has had its single positive proton replaced by a single positive hole.

Excitons come in two flavors: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole, and paraexcitons, in which the electron spin is anti-parallel (parallel but in the opposite direction) to that of its hole.

Electron-hole systems have been used to create other phases of matter such as electron-hole plasma and even exciton liquid droplets. The researchers wanted to see if they could make a BEC out of excitons.

“Direct observation of an exciton condensate in a three-dimensional semiconductor has been highly sought after since it was first theoretically proposed in 1962. Nobody knew whether quasiparticles could undergo Bose-Einstein condensation in the same way as real particles,” said Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper. “It’s kind of the holy grail of low-temperature physics.”

The researchers thought that hydrogen-like paraexcitons created in cuprous oxide (Cu₂O), a compound of copper and oxygen, were one of the most promising candidates for fabricating exciton BECs in a bulk semiconductor because of their long lifetime. Attempts at creating paraexciton BEC at liquid helium temperatures of around 2 K had been made in the 1990s, but failed because, to create a BEC out of excitons, temperatures far lower than that are needed. Orthoexcitons cannot reach such a low temperature as they are too short-lived. Paraexcitons, however, are experimentally well known to have an extremely long lifetime of over several hundred nanoseconds, sufficiently long to cool them down to the desired temperature of a BEC.

The team managed to trap paraexcitons in the bulk of Cu₂O below 400 millikelvins using a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium and which is commonly used by scientists attempting to realize quantum supercomputers. They then directly visualized the exciton BEC in real space by the use of mid-infrared induced absorption imaging, a type of microscopy making use of light in the middle of the infrared range. This allowed the team to take precision measurements, including the density and temperature of the excitons, that in turn enabled them to mark out the differences and similarities between exciton BEC and regular atomic BEC.

The group’s next step will be to investigate the dynamics of how the exciton BEC forms in the bulk semiconductor and to investigate collective excitations of exciton BECs. Their ultimate goal is to build a platform based on a system of exciton BECs, for further elucidation of its quantum properties, and to develop a better understanding of the quantum mechanics of qubits that are strongly coupled to their environment.

Funding: This research was supported by MEXT, JSPS KAKENHI (Grant Nos. JP20104002, JP26247049, JP25707024, JP15H06131, JP17H06205); by the Photon Frontier Network Program, Quantum Leap Flagship Program (Q-LEAP) Grant No. JPMXS0118067246 of MEXT; and by JSPS through its FIRST Program.