Formation of electron pairs is essential to superconductivity. For conventional superconductors, tunnelling spectroscopy has established that pairing is mediated by bosonic modes (phonons); a peak in the second derivative of tunnel current d2I/dV2 corresponds to each phonon mode. For high-transition-temperature (high-T(c)) superconductivity, however, no boson mediating electron pairing has been identified. One explanation could be that electron pair formation and related electron-boson interactions are heterogeneous at the atomic scale and therefore challenging to characterize. However, with the latest advances in d2I/dV2 spectroscopy using scanning tunnelling microscopy, it has become possible to study bosonic modes directly at the atomic scale. Here we report d2I/dV2 imaging studies of the high-T(c) superconductor Bi2Sr2CaCu2O8+delta. We find intense disorder of electron-boson interaction energies at the nanometre scale, along with the expected modulations in d2I/dV2 (refs 9, 10). Changing the density of holes has minimal effects on both the average mode energies and the modulations, indicating that the bosonic modes are unrelated to electronic or magnetic structure. Instead, the modes appear to be local lattice vibrations, as substitution of 18O for 16O throughout the material reduces the average mode energy by approximately 6 per cent--the expected effect of this isotope substitution on lattice vibration frequencies. Significantly, the mode energies are always spatially anticorrelated with the superconducting pairing-gap energies, suggesting an interplay between these lattice vibration modes and the superconductivity.

Carbon-substituted MgB 2 single crystals, Mg(B 1-x C x ) 2 of 0.3-1.0 mm size were grown for x=0.02-0.15 by a high-pressure technique. The doping dependence of lattice constants studied in a range of x=0.0-0.2 shows a monotonic decrease in a, while the c parameter remains almost invariant. Using X -ray diffraction and Auger electron spectroscopy, the solubility limit of C in MgB 2 was estimated to be about 15±1%, which is substantially larger than that reported for the polycrystalline samples synthesized by encapsulation techniques.Measurements of temperature dependence of magnetization and resistivity showed a dramatic decrease in T c with C -substitution, followed by complete suppression of superconductivity for x>0.125. Resistivity measurements in magnetic fields parallel and perpendicular to the basal plane of the crystals showed a nearly isotropic state in the heavily -doped crystals (x>0.1).

Here we report the growth of sub-millimeter MgB 2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a singlecrystal X-ray diffraction analysis gives lattice parameters a=3.0851 (5) . It was found that as sintering temperature increased, the sample quality was improved, but for the heat treatment above 950 o C the gold started to melt and to adhere strongly to MgB 2. Preparation of high-density bulk MgB 2 materials at a higher temperature (950-1250 o C) and 3.5 GPa was reported in BN crucibles 7) . However, the samples prepared at 1250 o C apparently showed inhomogeneous structure in macroscopic scale, indicating the occurrence of contamination from the crucible wall. PACSThese results are not quite favorable for crystal growth of MgB 2 under high-pressure.In this letter, we report the successful growth of high-quality MgB 2 single crystals under high pressure in the Mg-B-N system. Using the crystals, we carried out structure refinement and measured anisotropic superconducting properties of magnesium diboride. were used for refinement after absorption correction by a ψ-scan. The structure was refined by the full-matrix least-square procedure.The temperature dependence of the magnetization M(T) was measured by a SQUID (MPMS XL, Quantum Design) for 37 crystals of different shape (total weight 0.25mg) aligned with the c-axis perpendicular to the sample holder. In-plane electrical resistivity was measured for several plate-like crystals of typical size of 500x100x20 µm 3 in a four-probe configuration using a low frequency (17.8Hz) ac technique with a voltage resolution of less than 0.3nV. To study the anisotropic superconducting properties, the resistivity measurements were performed in the magnetic fields up to 7Tesla applied parallel and perpendicular to the Mg and B planes.The Mg-B-N system has been extensively studied during last 40 years [8][9][10] . In this system Mg and Mg-containing compounds act as the catalysts for the 4 transformation of boron nitride (BN) from hexagonal to cubic form under high pressure and the formation of MgB 2 and MgB 6 crystals has been observed 11). These compounds were formed as the intermediate phases in Mg-BN system, during the synthesis of cubic boron nitride, but MgB 2 was decomposed after prolong heat treatment.We found that the lower limit for MgB 2 crystal growth in "pressure- , using polycrystalline and powdered sample, refined structural data were previously unavailable. It has been proved to be very difficult to avoid the formation of impurity phases (MgO and MgB 4 ) in polycrystalline samples and thus the precise structure refinement is almost impossible. In this study well-formed single crystals were chosen for structure analysis to minimize of absorption effects. The refined crystallographic data, atomic coordinates, and thermal parameters are listed in Table 1 .In the final part of this letter we briefly characterize the superconducting properties of our MgB 2 single crystals. .In Fig.5(a) we present the resistive supe...

The electronic properties of the carbon substituted MgB2 single crystals are reported. The carbon substitution drops Tc below 2 K. In-plane resistivity shows a remarkable increase in residual resistivity by C-substitution, while the change of in-plane/out-of-plane Hall coefficients is rather small. Raman scattering spectra indicate that the E2g-phonon frequency radically hardens with increasing the carbon-content, suggesting the weakening of electron-phonon coupling. Another striking C-effect is the increases of the second critical fields in both in-plane and out-of-plane directions, accompanied by a reduction in the anisotropy ratio. The possible changes in the electronic state and the origin of Tc-suppression by C-substitution are discussed.

The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal of YBa_{2}Cu_{3}O_{7-delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (lambda_{a};{-2} and lambda_{b};{-2}) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda_{c};{-2}). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly s wave along the c direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.