#quantum matter

Solid research leads physicists to propose new state of matter

The term "superfluid quasicrystal" sounds like something a comic-book villain might use to carry out his dastardly plans.

In reality, it's a new form of matter proposed by theoretical physicists at The University of Texas at Dallas in a recent study published in the journal Physical Review Letters. Their study also describes a "recipe" for making the exotic materials in the lab.

Most people are familiar with the three fundamental states of matter: solid, liquid and gas. There are actually several more, including plasma, which is found inside the sun and other stars, and Bose-Einstein condensates, which are very dense and exist only at the most extreme cold temperatures.

A superfluid is an exotic state of matter with unusual properties. It has zero viscosity, which means it can flow across a surface and not slow down—or lose any energy—due to friction with the surface. Superfluids, such as liquid helium, must be cooled to extremely low temperatures for such properties to emerge.

Neutrons reveal the wild Weyl world of semimetals

The observation of an abnormal state of matter in a two-dimensional magnetic material is the latest development in the race to harness novel electronic properties for more robust and efficient next-generation devices.

Neutron scattering at the Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL) helped a multi-institutional team led by Tulane University investigate a graphene-like strontium-manganese-antimony material (Sr1-yMn1-zSb2) that hosts what researchers suspect is a Weyl semimetal phase.

The properties of Weyl semimetals include both magnetism and topological semimetal behavior, in which electrons—or charge carriers—are nearly massless and immune to conduction defects. The team's results were published in the journal Nature Materials.

The neutron scattering measurements at the High Flux Isotope Reactor, a DOE Office of Science User Facility at ORNL, and magnetic field studies at Florida State University's National High Magnetic Field Laboratory uncovered key mechanistic behaviors underpinning the quantum material's relationship between electron transport and magnetism.

Spin current detection in quantum materials unlocks potential for alternative electronics

A new method that precisely measures the mysterious behavior and magnetic properties of electrons flowing across the surface of quantum materials could open a path to next-generation electronics.

Found at the heart of electronic devices, silicon-based semiconductors rely on the controlled electrical current responsible for powering electronics. These semiconductors can only access the electrons' charge for energy, but electrons do more than carry a charge. They also have intrinsic angular momentum known as spin, which is a feature of quantum materials that, while elusive, can be manipulated to enhance electronic devices.

A team of scientists, led by An-Ping Li at the Department of Energy's Oak Ridge National Laboratory, has developed an innovative microscopy technique to detect the spin of electrons in topological insulators, a new kind of quantum material that could be used in applications such as spintronics and quantum computing.

"The spin current, namely the total angular momentum of moving electrons, is a behavior in topological insulators that could not be accounted for until a spin-sensitive method was developed," Li said.

New quantum materials offer novel route to 3-D electronic devices

Researchers have shown how the principles of general relativity open the door to novel electronic applications such as a three-dimensional electron lens and electronic invisibility devices. In a new study funded by the Academy of Finland, Aalto University researchers Alex Westström and Teemu Ojanen propose a method to go beyond special relativity and simulate Einstein's theory of general relativity in inhomogeneous Weyl semimetals. The theory of Weyl metamaterials combines ideas from solid-state physics, particle physics and cosmology and points a way to fabricate metallic designer materials where charge carriers move like particles in curved space-time.

The researchers propose Weyl metamaterials, a generalisation of Weyl semimetals, that enable new types of electronic devices through geometry engineering.

"The systems we introduced offer a route to make the charge carriers move as if they were living in a curved geometry, providing a tabletop laboratory for simulating curved-space quantum physics and certain cosmological phenomena," Alex Westström explains.

Weyl semimetals are an example of recently discovered quantum materials that have received a lot of attention. Charge carriers in these materials behave as if they were massless particles moving at the speed of light.

Heating quantum matter: A novel view on topology

Physicists demonstrate how heating up a quantum system can be used as a universal probe for exotic states of matter

In physical sciences, certain quantities appear as integer multiples of fundamental and indivisible elements. This quantization of physical quantities, which is at the heart of our description of Nature, made its way through the centuries, as evidenced by the antique concept of the atom. Importantly, the discovery of quantized quantities has often been associated with a revolution in our understanding and appreciation of Nature's law, a striking example being the quantization of light in terms of photons, which led to our contemporary (quantum-mechanical) description of the microscopic world.

In an article published in Science Advances, an international team led by Prof. Nathan Goldman -- Faculty of Science, Université libre de Bruxelles -- predicts a novel form of quantization law, which involves a distinct type of physical observable: the heating rate of a quantum system upon external shaking. In order to understand this concept, let us first consider a simpler analogous picture: When an ice cube is placed into a micro-wave oven, the latter excites the water molecules, hence leading to a progressive melting of the ice; during this heating process, the number of molecules that form the ice decreases in time, a process which can be quantified by a heating rate. In the present article, the authors demonstrate how, under specific circumstances, such heating rates must satisfy an elegant and precise quantization law. Specifically, the authors explain that this phenomenon takes place when a physical system, which initially forms an exotic state of matter (a topological phase), is heated up in a controlled manner; upon heating, particles are ejected from the topological phase (in direct analogy with the melting of ice described above) and the corresponding heating rate is shown to satisfy the aforementioned quantization law.

The apparent inner calm of quantum materials

Transitions between different phases of matter are part of our day-to-day lives: when water freezes, for example, it passes from liquid to solid state. Some of these transitions may be of a different kind, resulting from so-called topological excitations that force all the particles to act in unison. Researchers from the University of Geneva (UNIGE) and the CEA,CNRS and UGA have been studying BACOVO -- a one-dimensional quantum material unknown to the general public -- in collaboration with scientists from the neutronic centers ILL and PSI. They have discovered in this material a novel topological phase transition, governed not by a single type of topological excitation, but by two different ones. In addition, they were able to choose which of the two sets would dominate the other. You can read all about their research in the journal Nature Physics.

The researchers drew on the work of the 2016 Nobel Prize for physics awarded to David Thouless, Duncan Haldane, and Michael Kosterlitz. The three physicists predicted that a set of topological excitations in a quantum material is likely to induce a phase transition. Numerous theories have been developed about these topological excitations, including the feasibility of combining two of them in a single material. But is that a real possibility? And if so, what would happen? The teams from UNIGE and CEA, CNRS and UGA were able to provide the first experimental confirmation of the theory predicting the existence of two simultaneous sets of topological excitations and the competition between them. All in all, it is a small revolution in the mysterious world of quantum properties.