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Register a new userUnidirectional Diagonal Order and 3D Stacking of Charge Stripes in Orthorhombic Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4
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The interplay between crystal symmetry and charge stripe order in Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these crystals are orthorhombic. The corresponding distortion of the NiO2 planes is found to dictate the direction of the charge stripes, similar to the case of diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular, diagonal stripes seem to always run along the short a-axis, which is the direction of the octahedral tilt axis. In contrast, no influence of the crystal symmetry on the charge stripe ordering temperature itself was observed, with T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees of freedom allows one to produce macroscopic samples with unidirectional stripe order. In samples with stoichiometric oxygen content and a hole concentration of exactly 1/3, charge stripes exhibit a staggered stacking order with a period of three NiO2 layers, previously only observed with electron microscopy in domains of mesoscopic dimensions. Remarkably, this stacking order starts to melt about 40K below T_CO. The melting process can be described by mixing the ground state, which has a 3-layer stacking period, with an increasing volume fraction with a 2-layer stacking period.
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The influence of high electric fields on the charge stripe order in Nd1.67Sr0.33NiO4 was studied by means of simultaneous hard x-ray diffraction and electrical transport experiments. Direct measurements of the charge stripe satellite peaks in zero and high electric fields provide no evidence for a deformation or a sliding of the stripe lattice, which contradicts previous indications from non-linear conductance effects. By using the order parameter of a structural phase transition for instant sample temperature measurements, non-linear transport effects can be attributed to resistive heating. Implications for the pinning of stripes in the nickelates are discussed.
For a doped antiferromagnet with short-range spin stripe correlations and long-range charge stripe order we find that the manifestation of charge order changes abruptly as a function of momentum along the Fermi surface. The disorder averaged local density of states is almost perfectly homogeneous when integrated only over states which contribute to the ``nodal spectral weight whereas it displays long range charge stripe order when integrated only over states which contribute to the ``antinodal spectral weight. An effectively two dimensional nodal liquid can thus coexist with static charge stripes provided there is no static spin order. We also study commensurate spin and charge stripe ordered systems where the Fermi surface consists of a nodal hole pocket and an open ``stripe band section. Due to the stripe order the relation between hole density and size of a pocket will be reduced compared to a paramagnet by a factor of two for even charge period and four for odd charge period and we find an estimated upper limit on the area fraction of a hole pocket of 1.6% for charge period four and 4% for charge period five. We also discuss why electron pockets are not expected for a stripe ordered system and show that the open Fermi surface section may be electron like with a negative Hall coefficient.
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.
The high-temperature superconducting cuprates are governed by intertwined spin, charge, and superconducting orders. While various state-of-the-art numerical methods have demonstrated that these phases also manifest themselves in doped Hubbard models, they differ on which is the actual ground state. Finite cluster methods typically indicate that stripe order dominates while embedded quantum cluster methods, which access the thermodynamic limit by treating long-range correlations with a dynamical mean field, conclude that superconductivity does. Here, we report the observation of fluctuating spin and charge stripes in the doped single-band Hubbard model using a quantum Monte Carlo dynamical cluster approximation (DCA) method. By resolving both the fluctuating spin and charge orders using DCA, we demonstrate for the first time that they survive in the doped Hubbard model in the thermodynamic limit. This discovery also provides a new opportunity to study the influence of fluctuating stripe correlations on the models pairing correlations within a unified numerical framework.
Stripe order where electrons self-organize into alternating periodic charge-rich and magnetically-ordered charge-poor parallel lines was proposed as a way of optimizing the kinetic energy of holes in a doped Mott insulator. Static stripes detected as extra peaks in diffraction patterns, appear in a number of oxide perovskites as well as some other systems. The more controversial dynamic stripes, which are not detectable by diffraction, may be universally present in copper oxide superconductors. Thus it is important to learn how to detect dynamic stripes as well as to understand their influence on electronic properties. This review article focuses on lattice vibrations (phonons) that might show signatures of the charge component of dynamic stripes. The first part of the article describes recent progress in learning about how the phonon signatures of different types of electronic charge fluctuations including stripes can be distinguished from purely structural instabilities and from each other. Then I will focus on the evidence for dynamic stripes in the phonon spectra of copper oxide superconductors.
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