• PDF / 296,776 Bytes
• 2 Pages / 585 x 783 pts Page_size
• 115 Downloads / 224 Views

DOWNLOAD

REPORT

---

Ferroelectric-like phase transition observed in a metal

T

he phenomenon of ferroelectricity, in which a compound exhibits a spontaneous electric polarization, is generally thought to be incompatible with metallicity. This seems intuitive, since unbound carriers would quickly act to screen any polarization in a metal. In a strange twist, researchers Youguo Shi of the National Institute for Materials Science in Ibaraki, Japan, and co-workers from China, Japan, and the United Kingdom, now report a ferroelectric-like transition in metallic LiOsO3 at 140 K. Their results open the door to an entirely new class of materials and shed light on fundamental electron behavior in ferroelectrics. Writing in the November issue of Nature Materials (DOI: 10.1038/ NMAT3754; p. 1024), the researchers describe structure refinement studies of

Vanadium oxide metamaterial structure appears to cool as it heats

W

hen an object is heated, it gets hotter and emits more thermal radiation, right? Well, not always. Researchers at Harvard University have developed a thin-film/substrate structure that emits decreasing amounts of thermal radiation when heated over the temperature range of 75–85°C. Viewing this process through an infrared camera, the object appears to be getting colder even though it is really heating up. Mikhail Kats, a graduate student in the group of Federico Capasso at Harvard University, calls this phenomenon “a very unusual situation—almost pathological.” He explains that the amount of radiation emitted at all frequencies from an object at temperature T (Kelvin scale) is proportional to its emissivity times T 4. “So the only way you can see the effect that we saw is if the emissivity goes down faster than T 4 goes up.” This is a huge change in

1002

MRS BULLETIN

•

VOLUME 38 • DECEMBER 2013

•

rhombohedral LiOsO3. Using neutron powder diffraction, they find that this compound forms a LiNbO3-type crystal structure; at room temperature, it can ̅ be described by a centrosymmetric R3c space group, which is not compatible with ferroelectricity. However, upon cooling below 140 K, the structure undergoes a phase transition to a noncentrosymmetric R3c space group, which is compatible with ferroelectricity. The researchers confirmed this symmetry change using convergent-beam electron diffraction. Interestingly, they find that the material remains a metal through the phase change, and they find no associated change in magnetic behavior across the phase transition. This suggests that the transition to ferroelectric behavior in this compound is driven by a displacive, order-disorder-type transition, rather than collective electron motion. The group said that their study may guide other researchers as they seek to

emissivity. The structure that shows such dramatic properties is a thin (150 nm) film of VO2 on a sapphire substrate. “Vanadium oxide is extremely special because the insulating and metallic phases have very different dielectric properties that give you these spectacular changes in the interaction with radiation over ve