[seminar] Seminar FO, Hammerath

[hbuljan at phy.hr]

Poštovani kolege,

podsjećam na današnji seminar.

Srdačan pozdrav
Hrvoje Buljan

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SEMINAR FIZIČKOG ODSJEKA
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Vrijeme:   utorak, 15. 11. 2011., 14:15 sati (točno)
Mjesto:    Fizički odsjek, Bijenička c. 32, predavaonica F201

Unusual NMR Line Broadening and Spin Gap in the Zigzag
S=1/2 Spin Chain Compound Sr1-xCaxCuO2

              Franziska Hammerath
Leibniz Institute for Solid State Research, Dresden Germany

We present 63Cu Nuclear Magnetic Resonance (NMR) measurements on undoped
SrCuO2 and Ca-doped Sr0:9Ca0:1CuO2 single crystals. The crystal structure
contains one-dimensional CuO2 double chains that are magnetically
decoupled due to frustration. The system orders magnetically only below _2
K. Nevertheless, the Cu NMR spectra broaden already at temperatures below
100K and show an anomalous two peak structure at low temperatures, which
are still far above the magnetic ordering temperature. For the Ca-doped
sample, this broadening is reduced. The reason for this unusual broadening
is not known, but a similar broadening has been reported for the single
chain compound Sr2CuO3.
The Cu NMR spin-lattice relaxation rate, T1-1 , is temperature independent
for SrCuO2, as it is expected for a one-dimensional S=1/2 Heisenberg spin
chain. Doping with nonmagnetic, isovalent Ca takes place on the Sr sites
outside the spin chains, and should not a_ect the magnetic properties of
the compound. It is therefore very surprising that we do observe a
decrease of T1-1 in Ca-doped Sr0.9Ca0.1CuO2 for temperatures below 90K
that clearly evidences the opening of a gap in the spin excitation
spectrum. Density Matrix Renormalization Group (DMRG) calculations of the
J1-J2 Heisenberg model are presented to discuss the origin of this spin
gap.

[1] F. Hammerath et al., Phys. Rev. Lett. 107, 017203 (2011).

				Voditelj seminara FO
				Hrvoje Buljan, hbuljan at phy.hr
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