Talk 2: 11:45–

EPR in doped AF S = 1/2 quantum spin
chains: quantum critical phenomena,
Oshikawa-Affleck theory and new magnetic
oscillations
S.V. Demishev
Low Temperatures and Cryogenic Engineering Department, A.M. Prokhorov General
Physics Institute of RAS, Vavilov street, 38, 119991 Moscow, Russia

We report results of high frequency (60-360 GHz) EPR study of the quasi one-dimensional
magnet CuGeO3 doped with magnetic impurities Co, Fe, Mn, which substitute cooper inS = 1/2 AF Cu2+ chains. In all cases studied a magnetic impurity modifies the collective
EPR line whereas no specific impurity resonances related with the doping spins S = 3/2(Co2+ ), S = 2 (Fe2+ ) or S = 5/2 (Mn2+ ) are observed. The first effect of doping consists inseparation of the quantum spin chains in two kinds. In the first group all types of the longrange magnetic order (spin-Peierls or Neel) are suppressed down to the lowest temperatures
studied and the ground state is the Griffiths phase. In the second group the dimerization
with the same (or slightly reduced) temperature as in pure single crystal is conserved. It isworth noting that a relatively low concentration of a magnetic impurity, namely x = 0.01
(Fe) or x = 0.02 (Co), is sufficient for the complete damping of dimerization. In this case themagnetic system consists of the spin clusters having different coupling constants J and low
temperature magnetic susceptibility follows power law χ ∼ 1/T ξ with the exponent ξ < 1.
For the quantitative description of the Griffiths phase magnetic properties we have suggested
a simple model with a dispersion of Neel temperatures TN ∼ J ≤ Tmax in clusters with theprobability w(TN ) ∼ TN−ξ . In this approach the calculation of ξ(T ) at arbitrary temperature
is possible, which leads to a correct determination of ξ and Tmax from the experimental data.
It is found that for all impurities studied Tmax ∼ 120 K is about exchange integral along
Cu2+ chains in undoped case, whereas critical exponent strongly depends on dopants andequals ξ = 0.3 (Fe), ξ = 0.7 (Mn) and ξ = 0.8 (Mn). The second effect of doping can beunderstood in the framework of the Oshikawa-Affleck theory describing ESR in S = 1/2
AF quantum spin chains. Doping of CuGeO3 with magnetic impurities leads to onset ofthe staggered field in the sample, which together with exchange anisotropy controls theEPR line width. As long as in presence of the staggered field the line width W and gfactor are connected by the universal relation [1], the quantitative analysis of the ESR canbe done by computing the Oshikawa-Affleck function [2]: fOA = W (T )/∆g(T )Ṫ (here ∆gdenotes the g-factor shift). It is found that the temperature dependences of the line widthand g-factor are formed as a result of the competition between interchain antiferromagnetic
interactions and staggered Zeeman energy. We argue that the b axis, which corresponds tothe strongest interchain exchange direction, is an easy direction for the staggered field,i.e. the alignment of the external field B along b increases the magnitude of the staggeredmagnetization. Experimental data also suggest that in doped CuGeO3 antiferromagnetic
interaction increases with magnetic field. The third recently observed effect of doping havebeen deduced from the anomalous polarization characteristics of the magnetic resonance inCuGeO3 doped with 2% of Co impurity, which accompanies in this system the collective EPR
on Cu2+ chains [3]. For the Faraday geometry the new mode is damped for the microwave
field Bω aligned along a certain crystallographic direction showing that the character ofmagnetic oscillation differs from the standard spin precession and can be described by linear
oscillation of the magnetization. This effect seems to be closely connected with the impurity
driven staggered magnetization and argued to correspond to an unknown before, collectivemode of magnetic oscillations in an S = 1/2 AF quantum spin chain. This work is supportedby the Programme Strongly Correlated Electrons of Russian Academy of Sciences.
[1] S.V. Demishev et al., Europhys. Lett., 63, 446 (2003)[2] S. Demishev et al., Progress of Theoretical Physics Supplement No. 159, 387 (2005)[3] S.V. Demishev et al., Pis’ma v ZhETF, 84, 305 (2006)