[seminar] 18.05.2017. Hans-Joachim Grafe: Spin chains probed by nuclear magnetic resonance:I. SrCuO2 and Sr2CuO3, II. LiCuSbO4

[Damir Pajic]

Drage kolegice i kolege,
Drago mi je da mogu najaviti sljedeć‡i Seminar Fizičkog odsjeka:

Dear colleagues,
I would like to invite you to the following Seminar of the Department of 
Physics:

četvrtak/Thursday  18.05.2017. u 10.30 h
room F201 (floor II)

Spin chains probed by nuclear magnetic resonance:
I. SrCuO 2 and Sr 2 CuO 3
II. LiCuSbO 4

Dr. Hans-Joachim Grafe
Leibniz Institute for Solid State and Materials Research, Dresden, Germany


In the first part of the talk, $^{63}$Cu NMR measurements on the 
antiferromagnetic $S=1/2$ Heisenberg chains SrCuO$_{2}$ and 
Sr$_{2}$CuO$_{3}$ doped with Ni, Zn, Pd, and Co will be presented. A 
line broadening of the NMR spectra for Ni, Zn, and Pd doping reveals the 
existence of an impurity-induced local alternating magnetization, and 
exponentially decaying spin-lattice relaxation rates $T_{1}^{-1}$ 
towards low temperatures indicate the opening of a spin gap. The doping 
dependence indicates that the chains are cut into finite-size segments 
by the defects [1]. On theother hand, Co doping has a much stronger 
impact on the NMR spectra at low temperatures and does not induce a 
gap-like decrease of $T_{1}^{-1}$. This can be explained by the 
different spin of the impurities: whereas Co is definitely $S \ne 0$, 
Pd, Zn and even Ni are $S = 0$ impurities in the chains.

In the second part of the talk, $^{7}$Li NMR measurements on the 
strongly frustrated $S=1/2$ Heisenberg chain LiCuSbO$_{4}$ in magnetic 
fields up to 16 T will be presented. In this compound, frustration of 
the ferromagnetic nearest neighbor  exchange with the antiferromagnetic 
next nearest neighbor exchange leads to different field induced 
multipolar states at low temperatures. These states can be identified by 
different temperature dependencies of the spin lattice relaxation rate 
$T_{1}^{-1}$: at low fields a quasi long-range ordered spin density wave 
phase is more and more stabilized, leading to a power law like increase 
of $T_{1}^{-1}$. At higher fields above 13 T, a gapped behavior 
dominates the $T$ dependence of $T_{1}^{-1}$, consistent
with a spin-nematic state with quadrupolar correlations. The nematic 
scenario is supported by a microscopic theoretical approach using the 
DMRG technique.

[1] Y. Utz et al., Phys. Rev. B 92, 060405(R) (2015).
[2] H.-J. Grafe et al., arXiv:1607.05164 (2016).


Cjeloviti oglas je u prilogu./Formatted announcement is attached.

Lijep pozdrav / Best regards
Damir Pajić



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