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 16.12.2010   Карта сайта     Language По-русски По-английски
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16.12.2010

Physical chemistry: Model's reputation restored





Journal name:

Nature

Volume:

468,

Pages:

908–909

Date published:

(16 December 2010)

DOI:

doi:10.1038/468908a


Published online







The structure of a mineral has been validated, ending the controversy about its potential usefulness as a model of an unusual magnetic lattice. This model might provide insight into superconductivity.








The mineral herbertsmithite has been hailed as a rare model of an unconventional type of magnetism thought to have a key role in the mechanism of high-temperature superconductivity (a form of superconductivity that occurs above 30 kelvin). But concerns have been raised that chemical disorder in this material could produce defects in the array of magnetic atoms, disturbing or even destroying the properties that make it a useful model. In the Journal of the American Chemical Society, Daniel Nocera and his group1 now report that they have clearly identified the type of disorder present, and have found it to have little influence on the magnetic lattice of the mineral. This represents a crucial step in establishing a simple, clean system to provide unambiguous insight into this important form of magnetism.


A material's magnetism is ultimately derived from unpaired electrons, each of which has a property called spin (S), with a value of ½, that bestows on each of them a magnetic moment. In insulators, such moments or spins are localized on atoms, and commonly interact with their closest neighbours so that specific spin orientations are preferred. In most cases, an antiparallel (antiferromagnetic) configuration of nearest-neighbour spins is favoured so that, when a lattice of such atoms is cooled, their spins usually freeze to form an ordered array (Fig. 1a). However, under certain circumstances — for some magnetic lattices, for instance — quite different behaviour can ensue. One such case is the kagome antiferromagnet, which is formed from corner-sharing triangles (Fig. 1b). In these systems, it is impossible to arrange each near-neighbour pair of spins so that they are all antiparallel, and the system is said to be geometrically frustrated.


Figure 1: Spin arrangements in crystal lattices.

Spin arrangements in crystal lattices.

Unpaired electrons on atoms have a magnetic moment, or spin. These spins adopt preferred alignments (indicated by red arrows) in crystal lattices. a, Antiferromagnetic alignments, in which all neighbouring spins are antiparallel (as in this square lattice) are often favoured. b, In kagome lattices, triangles of atoms are joined at their corners, making a completely antiparallel spin arrangement impossible. c, This can lead to a compromise arrangement in which the spins are oriented at 120° to each other. d, Alternatively, a quantum spin liquid might form as a combination of many states (one of which is depicted here) in which spins pair up, analogous to the pairing of electrons in chemical bonds. e, The mineral herbertsmithite (ZnCu3(OH)6Cl2) contains kagome layers of Cu2+ ions (blue) linked by O2− ions (red), separated by layers of Zn2+ (grey) and Cl ions (not shown). Defects occur in the lattice when Cu2+ ions occupy Zn2+ sites and vice versa (indicated by the double-headed arrow between atoms marked 'X' and 'Y'). Nocera and colleagues1 find that the number of defects in herbertsmithite is smaller than had been thought. This makes the mineral an ideal model for studying spin-liquid states, which have been implicated in high-temperature superconductivity.







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  • Harton Vladislav Vadim  honorary member of ISSC science council

  • Lichtenstain Alexandr Iosif  honorary member of ISSC science council

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  • Yakushev Mikhail Vasilii  honorary member of ISSC science council

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