Thermal transport in low dimensional quantum spin systems: scattering processes for magnetic excitations
Zusammenfassung der Projektergebnisse
The main goal of this project was to experimentally study the magnetic heat transport of different low dimensional quantum spin systems in order to identify and analyze the various scattering processes of the magnetic excitations as one basic step to achieve a fundamental understanding of this novel transport phenomenon. In one part of the project we focused on the systematic variation of material parameters for analyzing magnetic scattering processes. Here, the most important findings concern the S = 1/2 Heisenberg chain systems Sr2CuO3, SrCuO2 and CaCu2O3, because the underlying spin model predicts ballistic, i.e. divergent magnetic heat conductivity. In fact, after fabricating the former two materials in an unprecedented clean single crystalline form we were able to show that the magnetic heat conductivity of these materials is extremely enhanced. The analysis of the data yield very large spinon mean free paths in the order of 1µm and provide very good evidence that the intrinsic heat transport in S = 1/2 antiferromagnetic Heisenberg chains is indeed ballistic. This surprising and unexpected finding at the beginning of the project led us to study the effect of extrinsic scattering processes which can be induced by various doping schemes in deep detail. For the different types of disorder we obtained very surprising and counterintuitive results: (i) Bond disorder, i.e., a weak modulation of the magnetic exchange by isovalently substituting Ca for Sr leads to a very strong suppression of the magnetic heat conductivity and induces a spin gap, (ii) Non-magnetic site disorder (dilution of magnetic copper sites by Mg dopants) has virtually no effect on the heat transport in the double chain compound SrCuO2 but cuts the chains and leads to an according decrease in the single chain compound Sr2 CuO3 and (iii) Magnetic S = 1 site disorder (substitution of Ni for Cu) causes a strong suppression of the magnetic heat conductivity, strongly enhances the phonon heat conductivity and induces a solid spin gap. These findings may trigger further research on two aspects. Firstly, the large observed magnetic mean free path of the order 1µm is comparable to typical transport lengths in spin transport experiments. Thus, the extremely pure materials may open a new route to study spin information transport by magnetic excitations without charge transport. Secondly, the observed effects of the various disorder types on both the magnetic ground states as well as on the magnetic heat transport are far from being understood and require further theoretical work. The second part of the project focused on the investigation of the magnetic heat conductivity of cuprate materials at high temperature, i.e. at T > 300 K. This work is motivated by the fact that the magnetic exchange in many of these materials is as large as J/kB ∼ 2000 K. Thus, the usually studied temperature range T < 300 K covers only the low-energy scale of the underlying spin systems, while sizeable magnetic transport is to be expected at high temperatures as well, due to the large exchange coupling. Moreover, in this higher energy regime the thermal conductivity becomes better treatable for theoretical approaches. In this project we have therefore successfully set up a high temperature measurement technique (laser flash method) and, for the first time, managed to obtain high temperature data for various low dimensional quantum spin systems, in particular the spin chain compounds SrCuO2 and CaCu2O3 . The results reveal a new scattering process for the latter compound which is currently under closer investigation. In conclusion, this project revealed a number of new, unexpected aspects of not only the magnetic heat conductivity in low dimensional quantum antiferromagnets but also the general physics of such systems.
Projektbezogene Publikationen (Auswahl)
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Heat conduction in low-dimensional quantum magnets, Euro. Phys. J - Special Topics, 151, 73-83, (2007)
C. Hess
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Ballistic heat transport of quantum spin excitations as seen in SrCuO2 , Phys. Rev. B 81, 020405R (2010)
N. Hlubek, P. Ribeiro, R. Saint-Martin, A. Revcolevschi, G. Roth, G. Behr, B. Büchner, and C. Hess
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Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr, Phys. Rev. B 81, 144428 (2010)
N. Hlubek, M. Sing, S. Glawion, R. Claessen, S. van Smaalen, P. H. M. van Loosdrecht, B. Büchner, and C. Hess
