engleski

Heat Conduction in Quantum Antiferromagnets

During the last decade we were witnessed to the ascending interest to the field of quantum antiferromagents. Until some twenty years ago, it was accepted that the solution of the Heisenberg hamiltonian in general leads to the long-range ordered magnetic state. Later, it was proven that if only short-range spin correlations exist in a Heisenberg system, a discrete spin-energy spectrum appears due to the finite size of the system. These systems are usually called "spin-gap antiferromagnets". There are several realizations leading to the appearance of the spin correlations of the short range: (a) the spin-Peierls systems, which undergo the structural phase transition due to spin-phonon coupling ; (b) weakly coupled spin dimers ; (c) spin ladders, where spin chains are mutually coupled forming a ladder-like spin coupling structures ; (d) Haldane gap antiferromagnets (i.e., infinite chains made of integer spins) ; and (e) geometrically frustrated systems (i.e., such spin structure where no configuration can not simultaneously satisfy all the magnetic bonds and minimize the energy). The thermal conductivity of spin liquids probes the couplings between phonon and spin subsystems, as well as the phonon and spin contribution to the heat transport. There is a variety of phenomena observed in the thermal conductivity of spin liquids. For example, in various strontium-cuprates a large enhancement of the thermal conductivity is observed due to the opening of the heat channel carried by either magnons or spinons. In spin-Peierls compounds the thermal conductivity exhibits rather unusual double peak features at low temperatures, explained as a fingerprint of the spin-phonon resonance scattering. The thermal conductivity of strongly frustrated systems shows the spin-phonon resonance at low temperatures. We study the thermal transport properties of several spin liquids: (i) of copper-tellurides, quasi-zero dimensional compounds with S=1/2, (b) of a quasi-1D material LiCu_2O_2 (S=1/2), and (c) 2-D S=1 system nickel-telluride. We find that thermal conductivity of these systems exhibit features typical for spin-phonon resonance coupling, gradually decreasing as the dimensionality of the systems increase.

quantum antiferromagents; spin liquids; thermal conductivity

The paper is presented as an invited talk by Ante Bilušić.