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Register a new userObservation of vortex structure in MgB$_2$ single crystals by Bitter decoration technique
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We report the observation of superconducting vortices in pure and lightly Al doped MgB$_2$ single crystals. Low field experiments allow for the estimation of the London penetration depth, lambda ~ 1900 $AA$ for T$sim$6 K. Experiments in higher fields (e.g. 200 Oe) clearly show a triangular vortex lattice in both real space (13 $mu$m by 13 $mu$m Bitter decoration image of over 1000 vortices) and reciprocal space.
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The effect of electron correlation (EC) on the electronic structure in MgB$_2$, AlB$_2$ and ZrB$_2$, is studied by examining the partial density of states (PDOS) of B-2$psigma$ and $ppi$ orbitals using the polarization dependence of x-ray emission and absorption spectra. The discrepancies between observed and calculated PDOSs cannot be attributed to EC effects. The present results suggest that the EC effect is less than the experimental error ($sim$ 0.2 eV), which indirectly supports a scenario that electron-phonon interaction plays an essential role in the occurrence of superconductivity.
The electronic properties of the carbon substituted MgB$_2$ single crystals are reported. The carbon substitution drops T$_c$ 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 E$_{2g}$-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 T$_c$-suppression by C-substitution are discussed.
In strong-coupling superconductors with a short electron mean free path the self-energy effects in the superconducting order parameter play a major role in the phonon manifestation of the point-contact spectra at above-gap energies. We compare the expressions for the nonlinear conductivity of tunnel, ballistic, and diffusive point-contacts and show that these expression are similar and correspond to the measurements of the phonon structure in the point-contact spectra for the $pi$-band of MgB$_{2}$.
Magnetic flux structure on the surface of EuFe$_2$(As$rm_{1-x}$P$rm_x$)$_2$ single crystals with nearly optimal phosphorus doping levels $x=0.20$, and $x=0.21$ is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with $x=0.21$ in the temperature range between the critical temperatures $T_{rm SC}=22$ K and $T_{rm C}=17.7$ K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below $T_{rm C}$. The nature of this structure is discussed.
We have performed local tunneling spectroscopy on high quality Mg$_{1-x}$Al$_x$B$_2$ single crystals by means of Variable Temperature Scanning Tunneling Spectroscopy (STS) in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional $Delta_pi$ as a function of Al content. Temperature and magnetic field dependences of the conductance spectra were studied in S-I-N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured $T_C$ down to 24K for x=0.2. On the other hand, we have found that the gap amplitude shows a maximum value $Delta_pi= 2.3$ meV for x=0.1, while the $Delta_pi / T_C$ ratio increases monotonously with doping. The locally measured upper critical field was found to be strongly related to the gap amplitude, showing the maximum value $H_{c2}simeq3T$ for x=0.1 substituted samples. For this Al concentration the data revealed some spatial inhomogeneity in the distribution of $Delta_pi$ on nanometer scale.
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