Hans-Joachim Grafe

Jeudi 8 Décembre 2016 à 14h00
Amphi Holweck, Esc C
1ème etage

Impurity effects in S=1/2 Heisenberg spin chains as probed by nuclear
magnetic resonance

63Cu NMR measurements on single crystals of SrCuO2 and Sr2CuO3, pristine as
well as doped with Ca, Ni, Zn, Pd and Co will be presented. Both compounds
are known to be good realizations of the antiferromagnetic S=1/2 Heisenberg
chain. The measurements show that both Ca doping on the Sr site as well as
the transition metal doping on the Cu site have a major impact on the magnetic
properties of the spin chains. A line broadening in the low temperature NMR
spectra reveals the existence of an impurity-induced local alternating magnetization,
and exponentially decaying spin-lattice relaxation rates T1 -1 towards low
temperatures indicate the opening of a spin gap. The gap induced by Ca doping
can be well explained within an alternating J model due to structural disorder
[1]. The effect of Ni, Zn and Pd is much stronger and its doping dependence
indicates that the chains are cut into finite-size segments by the defects [2]. On
the other hand, Co doping on the Cu site has a much stronger impact on the
NMR spectra at low temperatures and does not induce a gap-like decrease of
T1 -1 . This can be explained by the different spin of the impurities : whereas Co
is definitely S ̸= 0, Pd, Zn and even Ni are S = 0 impurities in the chains. In all
cases, no essential differences are observed between the single chain compound
Sr2CuO3 and the zigzag chain compound SrCuO2, confirming the irrelevance of
the interchain coupling J′ for the observed effects [3].
[1] F. Hammerath et al., Phys. Rev. Lett. 107, 017203 (2011),
[2] Y. Utz et al., Phys. Rev. B 92, 060405(R) (2015).
[3] F. Hammerath et al., Phys. Rev. B 89, 184410 (2014

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