Dopant and temperature induced structural phase transitions in La2-xSrxCuO4

Unconventional short-range structural fluctuations in cuprate superconductors

The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La2-xSrxCuO4 and Tl2Ba2CuO6+δ, two cuprate superconductors with distinct point disorder effects and with optimal superconducting transition temperatures that differ by more than a factor of two. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behaviour. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behaviour to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics.

XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La(0.5)Ca(0.5)MnO3

The inclusion of the contribution of Jahn-Teller distortion of MnO(6) units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La(0.5)Ca(0.5)MnO(3) demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La(0.5)Ca(0.5)MnO(3) exhibits insulator character (4-300 K) and an anti-ferromagnetic-ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4-300 K). This is counter-intuitive since large Jahn-Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.

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.

Nanoscale disorder in CaCu3Ti4O12: a new route to the enhanced dielectric response

Significant nanoscale disorder of Cu and Ca atomic substitution is observed in CaCu(3)Ti(4)O(12), based on our integrated study using quantitative electron diffraction and extended x-ray absorption fine structure. Unambiguous identification of this previously omitted disorder is made possible by the unique sensitivity of these probes to valence-electron distribution and short-range order. Furthermore, first-principles-based theoretical analysis indicates that the Ca-site Cu atoms possess partially filled degenerate e(g) states, suggesting significant boost of dielectric response from additional low-energy electronic contributions. Our study points to a new route of enhancing dielectric response in transitional metal oxides by exploiting the strong electronic correlation beyond classical static pictures.

Strain-induced bond buckling and its role in insulating properties of Cr-doped V2O3

Structural transformations around both V and Cr atoms in (V1-xCrx)2O3 across its metal-insulator transition (MIT) at x approximately 0.01 are studied by extended x-ray absorption fine-structure technique. Our new results for Cr made possible by the use of a novel x-ray analyzer that we developed reveal the substitutional mechanism of Cr doping. We find that this system has a buckled structure with short Cr-V and long V-V bonds. This system of bonds is disordered around the average trigonal lattice ascertained by x-ray diffraction. Such local distortions can result in a long range strain field that sets in around dilute Cr atoms in microscopic regions. We suggest that such locally strained regions should be insulating even at small x. The possibility of local insulating regions within a metallic phase, first suggested by Rice and Brinkman in 1972, remains unaccounted for in modern MIT theories.

63Cu NQR evidence for spatial variation of hole concentration in La2-xSrxCuO4

We report experimental evidence for the spatial variation of hole concentration x(hole) in the high T(c) superconductor La2-xSrxCuO4 ( 0.04 < or = x < or = 0.16) via 63Cu NQR using 63Cu isotope enriched samples. We demonstrate that the extent of the spatial variation of the local hole concentration Delta(x)hole is reflected on (63)1/T1 and deduce the temperature dependence. Delta(x)hole increases below 500-600 K, and reaches values as large as Delta(x)hole/x approximately 0.5 below approximately 150 K. We estimate the length scale of the spatial variation in x(hole) to be R(hole) greater, similar 3 nm from an analysis of the NQR spectrum.