Lattice instability and soft phonons in single-crystal La2-xSrxCuO4

Bi2CoO2F4-A Polar, Ferrimagnetic Aurivillius Oxide-Fluoride

Aurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi₂ MO₂F₄ (M = Fe, Co, and Ni). Our combined experimental and computational study on Bi₂CoO₂F₄ indicates a low-temperature polar structure of P2₁ ab symmetry (analogous to ferroelectric Bi₂WO₆) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems.

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

A-Site doping with alkali ions, and/or metal substitution at the B and B'-sites, are among the key strategies in the innovative development of A 2BB'X6 halide double perovskite semiconducting materials for application in energy and device technologies. To this end, we have investigated an intriguing series of five halide-based non-toxic systems, A 2AgRhCl6 (A = Li, Na, K, Rb, and Cs), using density functional theory at the SCAN-rVV10 level. The lattice stability and bonding properties emanating from this study of A 2AgRhCl6 matched well with those that have already been synthesized, characterized and discussed [viz. Cs2AgBiX6 (X = Cl, Br)]. Exploration of traditional and recently proposed tolerance factors has enabled us to identify A 2AgRhCl6 (A = K, Rb and Cs) as stable double perovskites. The band structure and density of states calculations suggested that the electronic transition from the top of the valence band [Cl(3p)+Rh(4d)] to the bottom of the conduction band [(Cl(3p)+Rh(4d)] is inherently direct at the X-point of the first Brillouin zone. The (non-spin polarized) bandgap of these materials was found in the range 0.57-0.65 eV with SCAN-rVV10, which were substantially smaller than those computed with hybrid HSE06 and PBE0, and quasi-particle GW methods. This, together with the appreciable refractive index and high absorption coefficient in the region covering the range 1.0-4.5 eV, enabled us to demonstrate that A 2AgRhCl6 (A = K, Rb, and Cs) are likely candidate materials for photoelectric applications. The results of our phonon calculations at the harmonic level suggested that the Cs2AgRhCl6 is the only system that is dynamically stable (no imaginary frequencies found around the high symmetry lines of the reciprocal lattice), although the elastic moduli properties suggested all five systems examined are mechanically stable.

Superconducting pairing and the pseudogap in the nematic dynamical stripe phase of La2-xSrxCuO4

Fully absorption coefficient corrected Raman spectra were obtained in La2-xSrxCuO4. The B1g spectra have a Fleury-Loudon type two-magnon peak (resonant term) whose energy decreases from 3180 cm(-1) (394 meV) to 440 cm(-1) (55 meV) on increasing the carrier density from x = 0 to 0.25, while the B2g spectra have a 1000-3500 cm(-1) (124-434 meV) hump (hill) whose lower-edge energy increases from x = 0 to 0.115 and then stays constant to x = 0.25. The B2g hump is assigned to the electronic scattering (non-resonant term) of the spectral function with magnetic self-energy. The completely different carrier density dependence arises from anisotropic magnetic excitations of spin-charge stripes. The B1g spectra were assigned to the sum of k ∥  and k⊥ stripe excitations and the B2g spectra to k⊥ stripe excitations according to the calculation by Seibold and Lorenzana (2006 Phys. Rev. B 73 144515). The k ∥  and k⊥ stripe excitations in fluctuating spin-charge stripes were separately detected for the first time. The appearance of only k⊥ stripe excitations in the electronic scattering arises from the charge hopping perpendicular to the stripe. This is the same direction as the Burgers vector of the edge dislocation in metal. The successive charge hopping in the Burgers vector direction across the charge stripes may cause Cooper pairs as predicted by Zaanen et al (2004 Ann. Phys. 310 181). Indeed, this is supported by the experimental fact that the superconducting coherent length coincides with the inter-charge stripe distance in the wide carrier density range. The one-directional charge hopping perpendicular to the stripe causes the flat Fermi surface and the pseudogap near (π,0) and (0,π), but the states around (π/2,π/2) cannot be produced. The low-energy Raman scattering disclosed that the electronic states at the Fermi arc around (π/2,π/2) are coupled to the A1g soft phonon of the tetragonal-orthorhombic phase transition. This suggests that the Fermi arc is produced by the electron-phonon interaction. All the present Raman data suggest that Cooper pairs are formed at moving edge dislocations of dynamical charge stripes.

An x-ray scattering study of the structural phase transition in La(2-x)Sr(x)Cu0.99Fe0.01O4 (x = 0.20)

We report a comprehensive x-ray scattering study of the low-temperature orthorhombic (LTO)-high-temperature tetragonal (HTT) structural phase transition in 1% iron-doped La(2-x)Sr(x)CuO(4) (x = 0.2). The superlattice (032) peak intensity and the width are investigated in detail for a wide temperature range. We found that the structural phase transition is not sharp and the tilt ordering of the CuO(6) octahedra persists above the transition temperature T(S) (≈77 K). Even at room temperature, the superlattice peak is still observable. The structural phase transition is identified as an order-disorder type phase transition. We found that the tilt ordering in our iron-doped material is always short-ranged, and in the HTT phase the correlation between the tilts along the b axis is better preserved than that along the a axis. Moreover, we identify the role of the Fe as the nucleation centers of the LTO domains in the structural phase transition.

Structural Studies of La2−xBaxCuO4 Between 11–293 K

Neutron powder diffraction data have been collected from samples of La1.9Ba0.1CuO4 and La1.85Ba0.15CuO4 in the temperature range 11–293 K. The first material undergoes a phase transition from the tetragonal K2NiF4-type structure (space group 14/mmm) to the orthorhombic La2CuO4-type structure (space group Bmab) at about 280 K, and then a second transition to another type of tetragonal (or near-tetragonal) structure with probable space group P42/ncm. The latter transition occurs predominantly between 50–40 K, but about 10% of the orthorhombic phase persists down to 15 K. Rietveld refinement was carried out at selected temperatures in the different structural regions. The differences among the low temperature structures can be cb acterized by tilting of the CuO6 octahedra about different rotation axes on the basis of the simple X-Y model previously proposed by Axe and coworkers. The tilt angle, which is a measure of the long-range order parameter, is about 3.5° at 15 K.

Data sets were also collected from La1.85Ba0.15CuO4 at 293, 90 and 11K. At the two latter temperatures there was some peak broadening consistent with the distortions observed in previous high resolution neutron [Paul and co-workers] and x-ray [Axe and co-workers] powder studies. Rietveld analysis of the data indicated the presence of local octahedral displacements, but it was not possible to determine the nature of the correlations.

Electrical transport in the ferromagnetic state of manganites: small-polaron metallic conduction at low temperatures

We report measurements of the resistivity in the ferromagnetic state of epitaxial thin films of La(1-x)Ca(x)MnO3 and the low-temperature specific heat of a polycrystalline La0.8Ca0.2MnO3. The resistivity below 100 K can be well fitted by rho-rho(0) = Eomega(s)/sinh (2)(hs/ 2k(B)T) with h(omega)(s)/k(B) approximately 80 K and E being a constant. Such behavior is consistent with small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode that is strongly coupled to the carriers. The specific-heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conduction in the ferromagnetic state of manganites.

THE NEW GENERATION HIGH-TEMPERATURE SUPERCONDUCTORS

▪ Abstract  The layered perovskite cuprate materials are a unique class of superconductors with unusual normal-state and superconducting properties. The common physics to all these materials is that of the underlying CuO2 planes. This review provides a survey of and guide to their physical properties as it relates to the superconductivity of this interesting group of conducting oxides.

Neutron scattering: progress and prospects

Over the last decade the unique properties of neutrons have proven useful in a growing number of scientific disciplines. Neutron scattering, traditionally the probe of choice for many magnetic and spectroscopic studies, is now firmly established as an invaluable complement to x-ray scattering for structural and dynamic studies within many other areas of the material sciences, chemistry, and biology. In recent years the instruments and techniques have matured to the point where they are of increasing relevance to the understanding and design of improved practical, everyday materials.

Oxygen Isotope Effect and Structural Phase Transitions in La 2 CuO 4 -Based Superconductors

The oxygen isotope effect on the superconducting transition temperature (alpha(o)) varies as a function of x in La2-xSrxCuO(4) and La2-xBaxCuO(4), with the maximum alpha(o) values (alpha(o) >/= 0.5) found for x near 0.12. This unusual x dependence implies that the isotope effect is influenced by proximity to the Abma --> P4(2)/ncm structural phase transition in these systems. Synchrotron x-ray difaction measurements reveal little change in lattice parameters or orthorhombicity due to isotope exchange in strontium-doped materials where alpha(o) > 0.5, eliminating static structural distortion as a cause of the large isotope effects. The anomalous behavior of alpha(o) in both strontium- and barium-doped materials, in combination with the previously discovered Abma --> P4(2)/ncm structural phase-transition in La(1.88)B(0.12)CuO(4), suggests that an electronic contribution to the lattice instability is present and maximizes at approximately 1/8 hole per copper atom. These observations indicate a dose connection between hole doping of the Cu-O sheets, tilting instabilities of the CuO(6) octahedra, and superconductivity in La(2)CuO(4)-based superconductors.