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Register a new userNature and impact of stripe freezing in La$_{1.67}$Sr$_{0.33}$NiO$_4$
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La$_{1.67}$Sr$_{0.33}$NiO$_4$ develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below $sim50$ K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.
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Doped antiferromagnets host a vast array of physical properties and learning how to control them is one of the biggest challenges of condensed matter physics. La$_{1.7}$Sr$_{0.3}$NiO$_4$ (LSNO) is a classic example of such a material. At low temperatures holes introduced via substitution of La by Sr segregate into lines to form boundaries between magnetically ordered domains in the form of stripes. The stripes become dynamic at high temperatures, but LSNO remains insulating presumably because an interplay between magnetic correlations and electron-phonon coupling localizes charge carriers. Magnetic degrees of freedom have been extensively investigated in this system, but phonons are almost completely unexplored. We searched for electron-phonon anomalies in LSNO by inelastic neutron scattering. Giant renormalization of plane Ni-O bond-stretching modes that modulate the volume around Ni appears on entering the dynamic charge stripe phase. Other phonons are a lot less sensitive to stripe melting. Dramatic overdamping of the breathing modes indicates that dynamic stripe phase may host small polarons. We argue that this feature sets electron-phonon coupling in nickelates apart from that in cuprates where breathing phonons are not overdamped and point out remarkable similarities with the colossal magnetoresistance (CMR) manganites.
We have successfully grown epitaxial La$_{1.67}$Sr$_{0.33}$NiO$_4$ films with a small crystalline mosaic using pulsed laser deposition. With synchrotron radiation, the x-ray diffraction peaks associated with charge stripes have been successfully observed for relatively thick films. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurements. X-ray scattering provides direct evidence for suppression of the stripe phase in thin samples; the phase disappears for film thicknesses $leqslant$ 2600 ~AA{}. The suppression appears to be a result of shrinking the stripe phase domains. This may reflect the stripe phase progressing from nematic to isotropic.
We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La$_{1.875}$Ba$_{0.125}$CuO$_4$ than in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La$_{1.875}$Ba$_{0.125}$CuO$_4$, $xi^{hole}=255pm 5$ AA whereas for La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$F, $xi^{hole}=111pm 7$ AA. We find that the modulations are temperature independent in La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the low temperature tetragonal phase. In contrast, in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La$_{1.875}$Ba$_{0.125}$CuO$_4$ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude.
Ordering process of stripe order in La{2-x}Sr{x}NiO{4} with x being around 1/3 was investigated by neutron diffraction experiments. When the stripe order is formed at high temperature, incommensurability epsilon of the stripe order has a tendency to show the value close to 1/3 for the samples with x at both sides of 1/3. With decreasing temperature, however, epsilon becomes close to the value determined by the linear relation of epsilon = n_h, where n_h is a hole concentration. This variation of the epsilon strongly affects the character of the stripe order through the change of the carrier densities in stripes and antiferromagnetic domains.
The magnetic excitation spectrums of charge stripe ordered La(2-x)Sr(x)NiO(4) x = 0.45 and x = 0.4 were studied by inelastic neutron scattering. We found the magnetic excitation spectrum of x = 0.45 from the ordered Ni^2+ S = 1 spins to match that of checkerboard charge ordered La(1.5)Sr(0.5)NiO(4). The distinctive asymmetry in the magnetic excitations above 40 meV was observed for both doping levels, but an additional ferromagnetic mode was observed in x = 0.45 and not in the x = 0.4. We discuss the origin of crossover in the excitation spectrum between x = 0.45 and x = 0.4 with respect to discommensurations in the charge stripe structure.
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