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Register a new userCharge-Stripe Ordering From Local Octahedral Tilts: Underdoped and Superconducting La2-xSrxCuO4 (0 < x < 0.30)
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E. S. Bozin
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The local structure of La2-xSrxCuO4, for 0 < x < 0.30, has been investigated using the atomic pair distribution function (PDF) analysis of neutron powder diffraction data. The local octahedral tilts are studied to look for evidence of [110] symmetry (i.e., LTT-symmetry) tilts locally, even though the average tilts have [010] symmetry (i.e., LTO-symmetry) in these compounds. We argue that this observation would suggest the presence of local charge-stripe order. We show that the tilts are locally LTO in the undoped phase, in agreement with the average crystal structure. At non-zero doping the PDF data are consistent with the presence of local tilt disorder in the form of a mixture of LTO and LTT local tilt directions and a distribution of local tilt magnitudes. We present topological tilt models which qualitatively explain the origin of tilt disorder in the presence of charge stripes and show that the PDF data are well explained by such a mixture of locally small and large amplitude tilts.
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We present local structural evidence supporting the presence of charge inhomogeneities in the CuO2 planes of underdoped La2-xSrxCuO4. High-resolution atomic pair distribution functions have been obtained from neutron powder diffraction data over the range of doping 0 < x < 0.30 at 10 K. Despite the average structure getting less orthorhombic we see a broadening of the in-plane Cu-O bond distribution as a function of doping up to optimal doping. Thereafter the peak abruptly sharpens. Complementary evidence is also evident from the observation of octahedral tilt disorder in the PDF at higher atomic separation. This suggests a crossover from a charge inhomogeneous state at and below optimal doping to a homogeneous charge state above optimal doping. The strong response of the local structure to the charge-state implies a strong electron-lattice coupling in these materials.
We present a volume-sensitive high-energy x-ray diffraction study of the underdoped cuprate high temperature superconductor La2-xSrxCuO4 (x = 0.12, Tc=27 K) in applied magnetic field. Bulk short-range charge stripe order with propagation vector q_ch = (0.231, 0, 0.5) is demonstrated to exist below T_ch = 85(10) K and shown to compete with superconductivity. We argue that bulk charge ordering arises from fluctuating stripes that become pinned near boundaries between orthorhombic twin domains.
We report an anomalous local structural response in the CuO2 planes associated with the appearance of charge inhomogeneities at low temperature in underdoped but superconducting La2-x(Sr,Ba)xCuO4. We used pair distribution function analysis of neutron powder diffraction data. The increase in the Cu-O bond length distribution at low temperature has an onset temperature which correlates with observations of charge and spin freezing seen by other probes.
The cuprate superconductors exhibit ubiquitous instabilities toward charge-ordered states. These unusual electronic states break the spatial symmetries of the host crystal, and have been widely appreciated as essential ingredients for constructing a theory for high-temperature superconductivity in cuprates. Here we report real-space imaging of the doping-dependent charge orders in the epitaxial thin films of a canonical cuprate compound La2-xSrxCuO4 using scanning tunneling microscopy. As the films are moderately doped, we observe a crossover from incommensurate to commensurate (4a0, where a0 is the Cu-O-Cu distance) stripes. Furthermore, at lower and higher doping levels, the charge orders occur in the form of distorted Wigner crystal and grid phase of crossed vertical and horizontal stripes. We discuss how the charge orders are stabilized, and their interplay with superconductivity.
The individual kparallel and kperp stripe excitations in fluctuating spin-charge stripes have not been observed yet. In Raman scattering if we set, for example, incident and scattered light polarizations to two possible stripe directions, we can observe the fluctuating stripe as if it is static. Using the different symmetry selection rule between the B1g two-magnon scattering and the B1g and B2g isotropic electronic scattering, we succeeded to obtain the kparallel and kperp strip magnetic excitations separately in La2-xSrxCuO4. Only the kperp stripe excitations appear in the wide-energy isotropic electronic Raman scattering, indicating that the charge transfer is restricted to the direction perpendicular to the stripe. This is the same as the Burgers vector of an edge dislocation which easily slides perpendicularly to the stripe. Hence charges at the edge dislocation move together with the dislocation perpendicularly to the stripe, while other charges are localized. A looped edge dislocation has lower energy than a single edge dislocation. The superconducting coherence length is close to the inter-charge stripe distance at x le 0.2. Therefore we conclude that Cooper pairs are formed at looped edge dislocations. The restricted charge transfer direction naturally explains the opening of a pseudogap around (0, {pi}) for the stripe parallel to the b axis and the reconstruction of the Fermi surface to have a flat plane near (0, {pi}). They break the four-fold rotational symmetry. Furthermore the systematic experiments revealed the carrier density dependence of the isotropic and anisotropic electronic excitations, the spin density wave and/or charge density wave gap near ({pi}/2, {pi}/2), and the strong coupling between the electronic states near ({pi}/2, {pi}/2) and the zone boundary phonons at ({pi}, {pi}).
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