Glassy slowing of stripe modulation in(La,Eu,Nd)2

Abstract

63 Cu and 139 La nuclear quadrupole resonance and Zeeman perturbed nuclear magnetic resonance experiments are performed on the striped phase of the high temperature superconductors La2−xBaxCuO4 and La2−x−y(Nd,Eu)ySrxCuO4. The first goal of the present study is to utilize the fact that ordered Cu magnetic moments exert a static hyperfine field on the 63 Cu and 139 La nucleii to deduce the charge density and ordered moment within the CuO2 planes. A hyperfine broadened NQR lineshape is observed in both La2−xBaxCuO4 and La1.80−xEu0.20SrxCuO4 for x ≈ 1 8 . Detailed numerical analysis of the 63 Cu NQR lineshape establishes that widely accepted models of periodic sinusoidal or square-well shaped modulations of spin density waves with maximum moment ∼ 0.3µB , as inferred from elastic neutron scattering and µSR measurements, can not account for the NQR lineshape unless we assume a relatively small ordered moment ∼ 0.15µB with a comparably large distribution. The second goal of the present work is to establish the temperature dependence of the fluctuation frequency scale of stripes. We find that the the fraction of missing 63 Cu NQR intensity below charge ordering temperature T charge accurately tracks the temperature dependence of the charge order parameter as measured by scattering methods. By fitting a single model to the temperature dependences of the wipeout fraction F (T ) for 63 Cu and 139 La NQR, the spin order parameter measured by elastic neutron scattering, and the µSR data, we deduce the spatial distribution of the spin fluctuation frequency scale Γ and its temperature dependence. These results indicate that as soon as charge dynamics slow down, the spin fluctuations begin to slow dramatically with spin stiffness 2πρ ef f s ∼ 200 K. By extending the analysis to other hole concentrations, we demonstrate a qualitative difference in the spatial variation of electronic states induced by slowing charge dynamics above and below x = 1 8 .

Ferromagnetic insulating state in manganites:55MnNMR study

“…12 is the ac susceptibility of La 1−δ MnO 3 sample. The frequency dependence of the real and imaginary part of the susceptibility as well as their absolute values are indicative of the cluster glass behaviour similarly as reported by Ghivelder et al The situation may be similar to that observed in the lanthanum cuprates, where slowing down of the spin and/or charge fluctuations results in a strong peak in the spinlattice relaxation for the La nuclei and in a loss of signal on the Cu nuclei and it is ascribed to the glassy magnetic state [23][24][25]. It is interesting that for both Cu and La signals stretched exponential like spin-echo decay is observed in the slow fluctuation limit similarly to the one for FMI signals in manganites, while in the motionally narrowed limit, below a transition temperature T charge , corresponding to the localization of charge-ordered stripes, a Lorentzian single-exponential spin-echo decay is detected [24,26,27].…”

Spin freezing in the disordered pyrochlore magnet NaCaCo2F7 : NMR studies and Monte Carlo simulations

“…23 Na spectra broaden substantially at the same temperature range where ac susceptibility data show a broad maximum [4]. Such a spectrum proves a quasi-static and spatially disordered field distribution in NaCaCo 2 F 7 .…”

“…23 Na NMR spectra Figure 2(a,b) shows the angle dependent, with respect to the external magnetic field, 23 Na NMR field-sweep spectra from [1 1 1] to [1 0 0] direction at 80 K and at representative temperatures for [1 0 0] direction, respectively. The spectra can be described by diagonalizing the full NMR static Hamiltonian.…”

“…Analysis of the 23 Na NMR spectra 23 Na NMR spectra were taken on the geodesic between (1 1 1) and (1 0 0) directions. To describe the data, a physical model is constructed, by taking into account the crystal structure and the nearest neighbors environments.…”

NMR Search for the Spin Nematic State in a LaFeAsO Single Crystal

“…The reason NMR and neutron scattering detect different onset temperatures of the SDW is that each experimental probe has a different characteristic measurement time scale [28,29]. Elastic neutron scattering measurements would consider the SDW 'static' when fluctuations slow down to below the instrument resolution of ∼ 1meV.…”