A new theoretical advance enables us to understand how the magnetic properties of a class of magnets called antiferromagnets respond to a magnetic field. The theory describes the magnetic behaviors of both collinear antiferromagnets, in which adjacent magnetic moments point in opposite directions from atom-to-atom, and noncollinear antiferromagnets, where the magnetic moments rotate from one atom to the next.
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The Helfand and Werthamer theory developed in 1960s predicts the magnetic field at which a superconductor turns into a normal metal if certain details of the electronic structure are known. When new superconductors are discovered, their upper critical field is usually analyzed using this theory, even though it has a well-known shortcoming — it assumes that the electronic properties of the superconductor are the same in all directions and lead to an isotropic upper critical field.
Magnetism behaves very strangely in compounds of lanthanum, strontium, cobalt and oxygen, and researchers have just attained new insight into the decades-old question of why. Pure LaCoO3 is a non-magnetic, narrow-gap semiconductor at low temperatures, but it acquires magnetic properties as the temperature is raised – in contrast with most materials, which tend to lose magnetism at higher temperatures. With strontium doping the magnetic properties become more prominent until, at 18% Sr, the compound becomes metallic and ferromagnetic, like iron.
Light, combined with a novel rhodium catalyst, enables greener production of chemical feedstocks from biorenewables. A key challenge in the utilization of biomass for fuels and fine chemical applications is the control of oxygen and nitrogen-containing functional groups.Unfortunately, current routes such as gasification also generate unwanted by-products such as carbon dioxide and carbonaceous material.
Minute chemical substitutions are used to induce superconductivity in many materials, but the precise role of these dopants in iron-pnictide superconductors is an ongoing debate. In semiconductors, doping allows charge carrier concentrations to be controlled enabling electronic devices to be created. However, dopants in iron-arsenide superconductors do not simply impact charge carrier concentrations.
Stepping on a sample, particularly in high heels, is not something most researchers would do. However, by applying pressure comparable to that generated under a stiletto heel, researchers discovered that the low temperature properties of an iron-arsenide-based superconductor where cobalt is substituted for some of the iron, Ca(Fe1-xCox)2As2, can be changed from antiferromagnetic, to superconducting to non-magnetic in a highly controllable manner.
Designing methods to slow down electromagnetic signals just got easier with a new model that predicts how light will absorb and scatter from devices made from metamaterials. Metamaterials are built from small, engineered structures that, in some ways, mimic the role of atoms, yet can manipulate light in ways not seen in conventional materials. Slowing down light can arise in metamaterials through a process known as electromagnetically induced transparency, when destructive coupling occurs between a bright resonator and a dark resonator.
Researchers have developed a new way to track gold nanorods as they move around and re-orient themselves on metal surfaces, with significantly improved spatial resolution and speed compared with existing methods.
Researchers have found a way to enhance the force of light on matter. Most of the time the momentum of light and the associated forces are too small to notice, but at the nanoscale the effect can be quite large, and researchers have used these forces to dynamically manipulate optical waveguides at the nanoscale. Optical forces decay significantly, however, as the distance between the waveguides increases and become too small for all-optical device actuation at larger separations.
Gold atoms can be the key to making new materials with fascinating and frequently beautiful arrangements of atoms. For example, materials made from gold, sodium and gallium contain gold atoms arranged into tetrahedra, rods of hexagonal stars, or diamond-like three-dimensional frameworks. For certain gold concentrations, gold interacts in a novel way with sodium and stabilizes the formation of icosahedra.