Critical fluctuations and quenched disordered two-dimensional charge stripes in La(5/3)Sr(1/3)NiO4

Measurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures

We study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations.

Direct Visualization of Spatial Inhomogeneity of Spin Stripes Order in La1.72Sr0.28NiO4

In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. La1.72Sr0.28NiO4 is a prototypical strongly correlated perovskite showing spin stripes order. In this work we present the spatial distribution of the spin order inhomogeneity by applying micro X-ray diffraction to La1.72Sr0.28NiO4, mapping the spin-density-wave order below the 120 K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with negligible spin-density-wave order. The present work highlights the complex spatial nanoscale phase separation of spin stripes in nickelate perovskites and opens new perspectives of local spin order control by strain.

XMaS @ the ESRF

This paper describes the motivation for the design and construction of a beamline at the European Synchrotron Radiation Facility (ESRF) for the use of UK material scientists. Although originally focused on the study of magnetic materials, the beamline has been running for 20 years and currently supports a very broad range of science as evidenced by the research topics highlighted in this article. We describe how the beamline will adapt to align with the ESRF's upgrade to a diffraction limited storage ring. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Direct observation of dynamic charge stripes in La2-xSrxNiO4

The insulator-to-metal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2-xSrxNiO4 and La2-xBaxCuO4, the doped charge carriers can segregate into periodically spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of charge-stripe fluctuations has been lacking. Here we report the detection of critical lattice fluctuations, driven by charge-stripe correlations, in La2-xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the charge-stripe melting temperature. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly correlated systems, including high-temperature superconductors such as La2-xSrxCuO4.

Evidence for short-range-ordered charge stripes far above the charge-ordering transition in La1.67Sr0.33NiO4

The temperature evolution of structural effects associated with charge order (CO) and spin order in La1.67Sr0.33NiO4 has been investigated using neutron powder diffraction. We report an anomalous shrinking of the c/a lattice parameter ratio that correlates with T(CO). The sign of this change can be explained by the change in interlayer Coulomb energy between the static-stripe-ordered state and the fluctuating-stripe-ordered state or the charge-disordered state. In addition, we identify a contribution to the mean-square displacements of Ni and in-plane O atoms whose width correlates quite well with the size of the pseudogap extracted from the reported optical conductivity, with a non-Debye-like component that persists below and well above T(CO). We infer that dynamic charge-stripe correlations survive to T∼2T(CO).

Ultrafast charge localization in a stripe-phase nickelate

Self-organized electronically ordered phases are a recurring feature in correlated materials, resulting in, for example, fluctuating charge stripes whose role in high-TC superconductivity is under debate. However, the relevant cause-effect relations between real-space charge correlations and low-energy excitations remain hidden in time-averaged studies. Here we reveal ultrafast charge localization and lattice vibrational coupling as dynamic precursors of stripe formation in the model compound La(1.75)Sr(0.25)NiO4, using ultrafast and equilibrium mid-infrared spectroscopy. The opening of a pseudogap at a crossover temperature T* far above long-range stripe formation establishes the onset of electronic localization, which is accompanied by an enhanced Fano asymmetry of Ni-O stretch vibrations. Ultrafast excitation triggers a sub-picosecond dynamics exposing the synchronous modulation of electron-phonon coupling and charge localization. These results illuminate the role of localization in forming the pseudogap in nickelates, opening a path to understanding this mysterious phase in a broad class of complex oxides.

Jahn-Teller mechanism of stripe formation in doped layered La(2-x)Sr(x)NiO4 nickelates

We introduce an effective model for e(g) electrons to describe quasi-two-dimensional layered La(2-x)Sr(x)NiO(4) nickelates and study it using correlated wavefunctions on 8 × 8 and 6 × 6 clusters. The effective Hamiltonian includes the kinetic energy, on-site Coulomb interactions for e(g) electrons (intraorbital U and Hund's exchange J(H)) and the coupling between e(g) electrons and Jahn-Teller distortions (static modes). The experimental ground state phases with inhomogeneous charge, spin and orbital order at the dopings x = 1/3 and 1/2 are reproduced very well by the model. Although the Jahn-Teller distortions are weak, we show that they play a crucial role and stabilize the observed cooperative charge, magnetic and orbital order in the form of a diagonal stripe phase at x = 1/3 doping and a chequerboard phase at x = 1/2 doping.

Shear modulus and plasticity of a driven charge density wave

We have probed the effects of transverse variations in pinning strength on charge-density-wave (CDW) structure in NbSe3 by x-ray micro-beam diffraction. In ribbonlike crystals having a large longitudinal step in thickness, the CDW first depins on the thick side of the step, causing rotations of the CDW wave vector. By measuring these rotations as a function of position and electric field, the corresponding shear strains are determined, allowing the CDW's shear modulus to be estimated. These results demonstrate the usefulness of x-ray microdiffraction as a tool in studying collective dynamics in electronic crystals.

Commensurate-incommensurate crossover of charge stripe in La2-xSrxNiO4 (x approximately 1/3)

The temperature (T) dependence of the charge-stripe order in La2-xSrxNiO4 has been investigated in the vicinity of x approximately 1/3 by synchrotron radiation x-ray diffraction measurements. With decreasing T, a prominent commensurate-incommensurate (C-IC) crossover is observed in the x<1/3 region, while for the x>1/3 region the IC order is dominant over the whole T range. Such a C-IC crossover is interpreted as the entropy-driven self-doping of the charge stripes, and its x dependence indicates the clear electron-hole asymmetry with the x=1/3 compound as the Mott insulator.

Spin correlations among the charge carriers in an ordered stripe phase

We have observed a diffuse component to the low-energy magnetic excitation spectrum of stripe-ordered La(5/3)Sr(1/3)NiO4 probed by neutron inelastic scattering. The diffuse scattering forms a square pattern with sides parallel and perpendicular to the stripe directions. The signal is dispersive, with a maximum energy of approximately 10 meV. Probed at 2 meV, the scattering decreases in strength with increasing temperature, and is barely visible at 100 K. We argue that the signal originates from dynamic, quasi-one-dimensional, antiferromagnetic correlations among the stripe electrons.

Freezing of a stripe liquid

The existence of a stripe-liquid phase in a layered nickelate, La(1.725)Sr(0.275)NiO(4), is demonstrated through neutron scattering measurements. We show that incommensurate magnetic fluctuations evolve continuously through the charge-ordering temperature, although an abrupt decrease in the effective damping energy is observed on cooling through the transition. The energy and momentum dependence of the magnetic scattering are parametrized with a damped-harmonic-oscillator model describing overdamped spin waves in the antiferromagnetic domains defined instantaneously by charge stripes.

First time determination of the microscopic structure of a stripe phase: low temperature NMR in La(2)NiO(4.17)

The experimental observations of stripes in superconducting cuprates and insulating nickelates clearly show the modulation in charge and spin density. However, these have proven to be rather insensitive to the harmonic structure and (site or bond) ordering. Using (139)La NMR in La(2)NiO(4+delta) with delta = 0.17, we show that in the 1/3 hole doped nickelate below the freezing temperature the stripes are strongly solitonic and site ordered with Ni(3+) ions carrying S = 1/2 in the domain walls and Ni(2+) ions with S = 1 in the domains.