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TRANSDYN Report Summary

Project ID: 510543
Funded under: FP6-MOBILITY
Country: Greece

Final Activity Report Summary - TRANSDYN (Transport and dynamics of novel materials and nanostructures)

A new, very efficient, mode of thermal conduction was discovered, transport by magnetic excitations in quasi-one dimensional insulating materials. The magnetic conduction is highly anisotropic, it dwarfs the usual lattice contribution and it has mostly been studied in transition metal oxides as the Sr2CuO3, SrCuO2 spin chain or (La,Sr,Ca)14Cu24O41 'ladder' compounds. These materials, described by strongly interacting electronic / magnetic systems, belong to the class of 'novel materials'.

This line of experimental research was promoted by a prediction (by the author and collaborators) of dissipation-less transport in one dimensional quantum integrable many body systems. In particular, it was shown that prototype models commonly used to describe 1D materials imply ideal transport properties even at high temperatures. This phenomenon is the quantum analogue of transport by nondecaying pulses (solitons) in 1D classical nonlinear 'integrable' systems, an exemplary development in physics that lead, a quarter of century later, to impressive technological applications e.g. in optical fibres communications.

Regarding technological applications the key idea is that the magnetic heat conduction, often characterized by energies of the order of electron-volt, is:
(i) as efficient as the metallic conduction at high temperatures;
(ii) it occurs in insulating materials; and
(iii) it is highly directional.
Thus these compounds, in an appropriate form as thin films, can be promoted for technological applications, e.g. as substrates for carrying away heat in electronic devices that can also be switched - manipulated by light or magnetic fields.

As the thermal transport due to magnetic excitations described by the one dimensional Heisenberg spin-1/2 model was predicted to be ballistic, the observed finite, albeit very high, thermal conductivity is caused by spin-phonon or spin-impurity scattering. Thus the experiments pose the problem of assessing the role of spin-phonon scattering in limiting the thermal conduction.

In this direction, we have first studied, the classical Heisenberg model coupled to phonons. State of the art molecular dynamics simulations on large systems indicated (the surprising result) that coupling to acoustic phonons is insufficient to induce normal transport, the thermal conductivity diverges. Coupling to phonons with substrate (optical phonons), with or without anharmonicity, is necessary to obtain a finite conductivity. Besides these studies on classical systems, we investigated, in the framework of the memory function approach, the thermal conductivity of the spin-1/2 XY and isotropic Heisenberg model coupled to optic and acoustic phonons.

Another major theoretical issue in this field, is the role of conservation laws (macroscopic in integrable systems) in determining the 'Drude weight' - the measure of ballistic transport. In this direction we have shown that only by including all conservation laws - even their nonlinear combinations - the full magnitude of the 'Drude weight' can be accounted for.

Finally, we should emphasise that the quasi-one dimensional compounds we are considering, besides the extraordinary thermal conductivity they also show unconventional - ballistic - electrical and spin transport (as it has been probed e.g. in NMR experiments). Thus, in the longer run, they will also be of interest in nanoelectronics or the emerging priority field of 'spintronics'.


Xenophon ZOTOS
Tel.: +30-2810-394226
Fax: +30-2810-394301
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