S. Agrestini

Substitution of Sc for Mg in MgB2: Effects on transition temperature and Kohn anomaly

Here we report synthesis and characterization of Mg_{1-x}Sc_{x}B_{2} (0.12<x<0.27) system, with critical temperature in the range of 30>T_{c}>6 K. We find that the Sc doping moves the chemical potential through the 2D/3D electronic topological transition (ETT) in the sigma band where the ``shape resonanceof interband pairing occurs. In the 3D regime beyond the ETT we observe a hardening of the E_{2g} Raman mode with a significant line-width narrowing due to suppression of the Kohn anomaly over the range 0<q<2k_{F}.

The strain of CuO2 lattice: the second variable for the phase diagram of cuprate perovskites

We show the key role of the elastic local strain (or micro-strain) e of the CuO2 lattice in the phase diagram of cuprate superconductors. The superconducting critical temperature Tc(δ, e )i s shown tobe a function of two variables, the doping δ and the microstrain e. PACS number: 74.72.−h (Some figures in this article are in colour only in the electronic version)

Ru K-edge absorption study on the La1-xCexRu2 system

We have measured high-resolution Ru K-edge x-ray absorption spectra for the intermetallic La1-xCexRu2 system. Multiple-scattering calculations are made to explore the origins of different features observed in the experimental x-ray absorption near-edge structure (XANES) and their variation with Ce doping. The experiments and theoretical analysis demonstrate that the Ce doping in La1-xCexRu2 has a direct influence on the states at the Fermi level and hence the hybridization between the f and d states induced by doping.

Charge order, dielectric response, and local structure of La5/3Sr1/3NiO4 system

Charge ordering, dielectric permittivity, and local structure of La5/3Sr1/3NiO4 system have been explored by x-ray charge scattering, complex dielectric impedance spectroscopy, and extended x-ray absorption fine structure (EXAFS) measurements, made on the same single crystal sample. The local structure measured by the temperature-dependent, polarized Ni K-edge EXAFS shows significant distortions in the NiO2 planes. These local distortions could be reasonable cause of high dielectric permittivity of the title system (e≈100 at 5 K) with the charge ordering in this system being a ferroelectriclike second order transition.

LOCAL LATTICE FLUCTUATIONS AS A FUNCTION OF CHEMICAL PRESSURE

Here we report local lattice fluctuations as a function of chemical pressure in the high Tc superconductors at the optimum doping (δ = 0.16). The chemical pressure is determined by the mismatch between the rock-salt M-O (M=Ba, Sr, La) layers and the CuO2 layers. High k-resolution Cu K-edge EXAFS has been used to measure the strain of the Cu-O bond lengths and the amplitude of local lattice distortions probed by the Debye-Waller factor of the Cu-O bonds in different materials under increasing chemical pressure. We find that in the normal metallic state, showing non-Fermi liquid transport behavior, the Debye-Waller factor reaches a maximum at a critical value of the strain ηc of the CuO2 plane. The results provide a direct evidence that at the maximum critical temperature Tc the system has diverging local lattice fluctuations.

TEMPERATURE AND X-RAY ILLUMINATION EFFECTS IN OXYGEN DOPED La2CuO4

We have investigated charge ordering in an oxygen doped La2CuO4.1 crystal by high resolution x-ray diffraction using synchrotron radiation. Thanks to the high brilliance synchrotron radiation it has been possible to record a large number of weak superstructure spots due to charge ordering around the main peaks of the average structure. A study of the charge modulations with stage 3.5, and their behaviour as a function of the temperature and the intensity of the x-ray incident flux is reported. We are able to distinguish a microscopic decomposition in; 1) a stable domain, due to charges self trapped into a crystal of static ordered strings of finite length (~ 145 A) and; 2) bubbles of superstripes showing, as a function of photon dose, a decreasing coherence length at T = 220 K.

ANISOTROPIC THERMAL EXPANSION IN DIBORIDES AS A FUNCTION OF MICRO-STRAIN

Thermal expansion of diborides (AB2), with different intercalated atoms (A) is studied as a function of temperature in the range of 100-370 K by high-resolution x-ray powder diffraction. The results indicate a well defined relationship between the anisotropy of the thermal expansion and the micro-strain of the boron layers, ea = (a-a0)/a0 (where a0 = c/1.08 is the equilibrium a-axis for an unstrained AB2 system), determined by the atomic radius of the intercalated atoms. The thermal expansion is isotropic at ea = 0 (i.e., near c/a = 1.08) while the AB2 system is unstrained, and it gets anisotropic away from the equilibrium. The anisotropy increases with increasing micro-strain in both directions (positive or negative, i.e. tensile or compressive) suggesting that the micro-strain is a key variable to define the state of diborides. As a matter of fact, the MgB2 with the highest Tc shows a tensile micro-strain, ea = 6% and a large thermal expansion anisotropy.

ANISOTROPIC IN PLAIN Cu-O STRAIN AND THE STRIPE QUANTUM CRITICAL POINT IN YBa2Cu3O6+k

Anisotropic Cu-O strain has been measured and a relation between the superconducting critical temperature and the strain is studied in the YBa2Cu3O6+k (Y123). oxygen ordered YBa2Cu3O6+k phases. We show that the critical temperature scales both with the strain along the a axis (ea,) and along the b axis (eb). While undoped, the system is close to the critical strain, ec~ 0.043. The formation of chains with doping gives rise to a decrease of the eb to take the system away from the critical point approaching the dynamical quantum stripe fluctuations (eb ec).

The role of boron lattice expansion in superconducting diborides

Abstract Lattice structure of the diborides has been studied to explore a possible relation between high T c superconductivity and expansion of the boron plane. High purity Mg 1− x A x B 2 diborides have been prepared by direct reaction of the elements and the lattice parameters are determined by X-ray diffraction, providing a measure to the strain ϵ of the B–B distance. The results show that the superconductivity in these intermetallics appears in a critical region of tensile strain and charge density in the boron plane.

Lattice-Charge Stripes in the High- T c Superconductors

We report a 2D plot Tc(δ, η) for the doped perovskites, where δ is the doping and η is the mismatch between the CuO2 layers and the rocksalt layers. We identify for the first time the quantum critical point (QCP) at ηc=0.035 ± 0.005 and δc=0.15 ± 0.03 for the onset of the polaron stripes coexisting with itinerant carriers. The plot Tc(η), for δ=constant, shows the highest Tc at the critical point ηc. The lattice mismatch η drives the system to this QCP of the electron-lattice interaction λ(ηc) for local lattice deformations at metallic densities. The incommensurate superlattice of charge-lattice (polaron) stripes are due to critical fluctuations near this QCP. The solution of the mystery of the anomalous normal phase of high-Tc superconductors implies a solution for the pairing mechanism: the attractive pseudo-Jahn-Teller polaron-polaron interaction and a particular type of critical charge and spin fluctuations (forming a superlattice of quantum stripes tuned at a “shape resonance”) near this QCP play a key role in the pairing mechanism.