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Register a new userTunneling spectroscopy of Na$_{x}$CoO$_2$ and Na$_{x}$CoO$_2$$cdot$ yH$_2$O
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The single electron tunneling spectroscopy on superconductor Na$_{x}$CoO$_2$$cdot$ yH$_2$O and its starting compound Na$_{x}$CoO$_2$ has been studied with point-contact method. The spectra of Na$_{x}$CoO$_2$ have two types of distinct shapes at different random locations, this is attributed to the non-uniformly distributed sodium escaped from the inner part of the sample. While all the measured spectra of the superconducting samples Na$_{x}$CoO$_2$$cdot$ yH$_2$O have a good spatial reproducibility, and show a remarkable zero bias conductance depression appearing below an onset temperature which associates very well with the resistance upturn at around 45 K. The latter behavior resembles in some way the pseudogap feature in high-T$_c$ cuprate uperconductors.
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We report on first investigation of the lattice dynamics in the novel superconducting material Na$_{0.35}$CoO$_2$$cdot$1.3H$_2$O and the non-hydrated parent compound Na$_{0.7}$CoO$_2$ by inelastic x-ray scattering. The measured phonon dispersion along the $Gamma-M$ direction show a marked softening with hole doping of two optical phonon branches close to the Brillouin zone boundary. The phonon spectra, dispersion, and softening are well reproduced by first-principle calculations. The calculations indicates that the soft branches are mainly composed of Co-vibration modes. The estimation of the critical temperature based on electron-phonon coupling mechanism undisputedly points to a non-conventional superconducting state in this material.
A weak magnetic order was found in a non-superconducting bilayered-hydrate Na$_{x}$CoO$_{2}cdot y$H$_{2}$O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ shows a prominent peak at 5.5 K, below which a Co-NQR peak splits due to an internal field at the Co site. From analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be $sim$ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as $sim 0.015$ $mu_B$ using the hyperfine coupling constant reported previously. It is shown that the NQR frequency $ u_Q$ correlates with magnetic fluctuations from measurements of NQR spectra and $1/T_1T$ in various samples. The higher-$ u_Q$ sample has the stronger magnetic fluctuations. A possible phase diagram in Na$_{x}$CoO$_{2}cdot y$H$_{2}$O is depicted using $T_c$ and $ u_Q$, in which the crystal distortion along the c-axis of the tilted CoO$_2$ octahedron is considered to be a physical parameter. Superconductivity with the highest $T_c$ is seemingly observed in the vicinity of the magnetic phase, suggesting strongly that the magnetic fluctuations play an important role for the occurrence of the superconductivity.
It has often been suggested that correlation effects suppress the small e_g Fermi surface pockets of NaxCoO_2 that are predicted by LDA, but absent in ARPES measurements. It appears that within the dynamical mean field theory (DMFT) the ARPES can be reproduced only if the on-site energy of the eg complex is lower than that of the a1g complex at the one-electron level, prior to the addition of local correlation effects. Current estimates regarding the order of the two orbital complexes range from -200 meV to 315 meV in therms of the energy difference. In this work, we perform density functional theory calculations of this one-electron splitting Delta= epsilon_a1g-epsilon_e_g for the full two-layer compound, Na2xCo2O4, accounting for the effects of Na ordering, interplanar interactions and octahedral distortion. We find that epsilon a_1g-epsilon e_g is negative for all Na fillings and that this is primarily due to the strongly positive Coulomb field created by Na+ ions in the intercalant plane. This field disproportionately affects the a_1g orbital which protrudes farther upward from the Co plane than the e_g orbitals. We discuss also the secondary effects of octahedral compression and multi-orbital filling on the value of Delta as a function of Na content. Our results indicate that if the e_g pockets are indeed suppressed that can only be due to nonlocal correlation effects beyond the standard DMFT.
We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $qparallel$[100] in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q rightarrow 0$ limit and, hence, sensitive to the tetragonal-to-orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$ [F. Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of $C_{66}$ deduced from our experiments and those derived from measurements of the Youngs modulus in Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ at optimal doping. These results provide an upper limit of about 50 {AA} for the nematic correlation length for the optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where $C_{66}$ is not accessible by volume probes, as well as the C4 tetragonal magnetic phase.investigated
We have studied the superconducting phase diagram of NaHspace as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state. Our findings are consistent with a recent report that intercalation of oxpspace ions into Na$_{x}$CoO$_{2}$, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal Tcspace is achieved through a Co valence range of 3.24 - 3.35, while Tcspace decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO$_{2}$ sheets as Co valence is varied into the optimal Tcspace region is further evidence for this criticality.
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