Half moons and pinch points: Same physics, different energy

Pinch points and half moons are one and the same.

Left panels: Spin (atomic magnet) configurations respecting (lower panel) and violating (upper panel) the conservation law. Right panels: The corresponding neutron scatterings for the two situations: 3D structure of the neutron scattering pattern (mid panel) and the constant energy cross sections of the pinch point (lower panel) and half moon (upper panel). The two patterns corresponding to the two spin configurations on the left. Credit: Theory of Quantum Matter Unit
Left panels: Spin (atomic magnet) configurations respecting (lower panel) and violating (upper panel) the conservation law. Right panels: The corresponding neutron scatterings for the two situations: 3D structure of the neutron scattering pattern (mid panel) and the constant energy cross sections of the pinch point (lower panel) and half moon (upper panel). The two patterns corresponding to the two spin configurations on the left. Credit: Theory of Quantum Matter Unit

At the point when physicists send neutrons shooting through a frustrated magnet, the particles spray out the opposite side in signature patterns. The structures seem on the grounds that, even at low temperatures, particles in a baffled metal sway in time with one another.

One particular example, known as a “pinch point,” looks like a necktie and holds wide recognition in the realm of spin liquids. Squeeze focuses are frequently joined by unsung bow designs called “half moons,” however the physics connecting the wonders have never been elucidated.

Now, scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) have uncovered that pinch points and half moons are one and the equivalent – just marks of similar material science at various vitality levels.

Han Yan, a graduate student in the Theory of Quantum Matter Unit at OIST and first author of the study said, “The theory itself is kind of simple. From the same theory that gives you the pinch point at lower energy, you can calculate what happens at higher energy — and you get a pair of half moons.”

When focusing on a frustrated magnet, each atom making up the material seems to spin erratically. In all actuality, notwithstanding, these molecules partake in a flawlessly organized move, turning in time with one another so their magnetic pulls eventually counterbalance. This artful dance is hard to watch straightforwardly, so all things being equal, physicists scan for obvious pieces of information that the execution is occurring.

Scientists used a technique called neutron scattering to hunt for the clues. Neutrons carry no electric charge, but they do act as a localized source of magnetism. Each atom also acts as tiny magnets, complete with their own north and south poles. When sent whizzing through a material, a neutron’s speed and direction are thrown off by the atoms it passes, and thus it is “scattered.”

The pattern of the scattering tells physicists how atoms are behaving inside a material. For instance, if neutrons scatter helter-skelter, physicists infer that the atoms within a material are aligned randomly. If neutrons scatter in a hallmark bow-tie, they infer that the atoms are twirling in tandem, as they would in a frustrated magnet.

Pinch points show up when an equivalent number of atomic magnets, or spins” are pointing “out” as pointing “in” in any area of the baffled magnet. This balance renders the material non-magnetic and keeps up it at a negligible level of vitality.

Half moons show up when a baffled magnet has vitality past this minimal level, and along these lines disregards the neighborhood protection law which requires an equivalent number of spins be called attention to as in.

Generally, half moons are squeeze focuses set on a bend. The more noteworthy the ebb and flow, the more grounded the infringement, the more vitality the framework is utilizing. The OIST analysts revealed this relationship in their figurings and later put it under serious scrutiny.

Scientists tested their hypothesis in a simulated system where pinch points and half moons can be observed together, known as a Heisenberg antiferromagnet on a kagome lattice. They also applied their equations to recent observations of the frustrated magnet Nd2Zr2O7 and found that their theory explained the appearance of the two patterns in an application, as well.

Yan said, “Pinch points and half moons come from the same underlying physics — one from respecting the local conservation law and the other from violating it. When you put them together, they form a whole picture of the overall phenomenology.”

“From a certain point of view, each condensed matter system is unto itself a different universe. It’s a great intellectual curiosity to find these universes, with their own strange laws of nature, but it also relates to daily life. People are trying to identify the particularly useful laws in these mini-universes so we might use them to our advantage.”

Their unified theory, published October 12, 2018, as a Physical Review B Rapid Communication, is the first to explain the underlying physics driving the often paired phenomena.

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