Inhomogeneous Low Frequency Spin Dynamics inLa1.65Eu0.2Sr0.15

Abstract

We report Cu and La nuclear magnetic resonance (NMR) measurements in the title compound that reveal an inhomogeneous glassy behavior of the spin dynamics. A low temperature peak in the La spin lattice relaxation rate and the "wipeout" of Cu intensity both arise from these slow electronic spin fluctuations that reveal a distribution of activation energies. Inhomogeneous slowing of spin fluctuations appears to be a general feature of doped lanthanum cuprate. PACS 74.72.Bk, 75.30.Ds, 75.40.Gb Lanthanum cuprate, the prototypical single layer high temperature superconductor, has been extensively studied for several years to understand the origin of its unusual normal state behavior as well as the mechanism for superconductivity. Rare earth co-doped lanthanum cuprate has received attention recently because elastic neutron scattering experiments have revealed ordering of doped holes into charged stripes that constitute antiphase domain walls producing incommensurate antiferromagnetic (AF) order in the intervening undoped domains [1]. Charge stripe order is likely intimately related to the high temperature superconductivity [2][3][4][5]. Isostructural lanthanum nickelate demonstrates clear stripe order [6], and it has been shown there that both the charge order and the magnetic order are glassy [6,7]. It is also known that the magnetic order associated with charge ordering in lanthanum cuprate is glassy [8,9], but this situation is more difficult because the charge superlattice peaks are very hard to observe, presumably because the stripes tend to be dynamic. As a consequence little detail is known about the glassy behavior. Hunt et al. have observed suppression of the Cu NQR signal intensity ("wipeout") with decreasing temperature that they attribute to charge stripe order [10].NMR provides information complementary to neutron scattering because the nuclei are sensitive to the local magnetic field and the dynamic behavior of the electronic system without requiring spatial correlations. Chou et al., first proposed that the very strong peak in the 139 La nuclear spin relaxation rate displays an activated temperature dependence [13]. Furthermore, these data demonstrate a distribution P (E a ) of activation energies E a centered at E a /k B T ∼ 50 K and with a width comparable this center value indicating strongly inhomogeneous magnetic properties [13]. To understand if this inhomogeneity arises from disorder due to, e.g., substitutional dopants, we have applied this analysis to several lanthanum cuprate systems exhibiting AF order at low temperatures to allow us to explore the effect of varying the density and character of the disorder: in-plane doping by Li substitution for Cu, variation of doping density in LTT phase La 1.8−x Eu 0.2 Sr x CuO 4 : 0.01 ≤ x ≤ 0.15. Remarkably, we find that the character of the inhomogeneity, that is, the distribution of activation energies is essentially unchanged in all these cases and very similar to lightly doped La 2−x Sr x CuO 4 [11], suggesting that this inhomogeneity is intrinsi...

Cu nuclear magnetic resonance study of charge and spin stripe order inLa1.875Ba0.125CuO4

“…Namely, the crossover from a Gaussian decay in the high-temperature phase to exponential below T CO we see here has been reported for other 214 cuprates 21 and YBCO 8 , but with somewhat incomplete explanations. It is known that the Gaussian decay in cuprates is generally caused by a strong CuCu transferred coupling 43,44 , and an unlike-spin coupling has been invoked to explain the crossover to exponential relaxation 21 . Yet such an explanation requires the existence of a significant fraction of wiped-out, quickly relaxing nuclei which cause the exponential echo decay of the observable nuclei.…”

“…fect already related to charge order in YBCO [8] and 214 cuprates 21 . The spin-spin relaxation time T 2 abruptly decreases below T CO , and continues to decrease as the charge order develops.…”

“…Yet without access to the disappeared signal (except at very low temperatures 14,15 ) it was hard to pin down the physical origin of the wipeout and its connection to charge stripes. The fraction of wiped-out nuclei has been phenomenologically related to the charge stripe order parameter 17,18 , and wipeout mechanisms based on spin fluctuations have been put forward 14,21,24 . To add to the controversy, subsequently a marked difference between wipeout fractions in powder and single crystals was observed 14,17,19,25 , as well as differences between Cu and O NMR and NQR 8,10,20 .…”

“…However, charge stripes in the 214 family are difficult to study directly by in-plane copper NMR/NQR, due to an effect known as wipeout. It has been known for a long time that in stripe-ordered lanthanum cuprates such as LBCO and LESCO the copper NMR signal diminishes -is wiped out -in the charge-ordered phase 14,[17][18][19][20][21][22] . In fact, after the initial discovery of charge stripes 23 in Ndcodoped LSCO by neutron scattering, the Cu wipeout effect served as a first indication of the existence of stripes in other compounds such as LBCO and LESCO 14,17,18 .…”

NMR studies of pseudogap and electronic inhomogeneity in Bi2Sr2CaCu2OCrocker1, Dioguardi2, apRoberts-Warren3 et al. 2011Phys. Rev. BSelf Cite

“…Indeed, competition between a tendency for the doped carriers to phase segregate in a Mott insulator with long range Coulomb interactions can lead to glassy order in the charge and spin degrees of freedom. 44,45 The O NMR linewidths measured here confirm the presence of this static inhomogeneity, but we cannot infer any details about the spatial dependence.…”