arXiv subjects

Keiji Yada

Publications and source records attributed to Keiji Yada.

59 records · Page 4Linked to original sources

arXiv2006

Renormalization of Coulomb interactions in s-wave superconductor Na$_x$CoO$_2$

We study the renormalized Coulomb interactions due to retardation effect in Na$_x$CoO$_2$. Although the Morel-Anderson's pseudo potential for $a_{1g}$ orbital $μ^*_{a1g}$ is relatively large because the direct Coulomb repulsion $U$ is large, that for interband transition between $a_{1g}$ and $e_g'$ orbitals $μ^*_{a1g,eg'}$ is very small since the renormalization factor for pair hopping $J$ is square of that for $U$. Therefore, the s-wave superconductivity due to valence-band Suhl-Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na$_x$CoO$_2$.

cond-mat.supr-con↗

arXiv2006

Robustness of s-wave superconductivity against Coulomb interactions in Na$_x$CoO$_2$

We study the depairing effect due to Coulomb interactions in Na$_x$CoO$_2$. We consider the electron-phonon coupling and the Coulomb interactions, and determine $T_c$ for s-wave superconductivity by solving the linearized Eliashberg equation. When we consider shear phonons as well as breathing phonons, $T_c$ is enhanced by Suhl-Kondo (SK) mechanism. Since SK mechanism is strong against Coulomb interactions, $T_c$ remains finite even if strong Coulomb interactions cancel out the attractive force due to breathing phonons. The orbital degree of freedom is important to understand the mechanism of superconductivity in Na$_x$CoO$_2$.

cond-mat.supr-con↗

arXiv2006

Electron-Phonon mechanism for Superconductivity in Na$_{0.35}$CoO$_2$: Valence-Band Suhl-Kondo effect Driven by Shear Phonons

To study the possible mechanism of superconductivity in Na$_{0.35}$CoO$_2$, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and $t_{2g}(a_{1g}+e_g')$-electrons of Co-ions, and study the transition temperature for a s-wave superconductivity. The obtained $T_{\rm c}$ is very low when the $e_g'$-valence-bands are far below the Fermi level. However, $T_{\rm c}$ is strongly enhanced when the top of the $e_g'$-valence-bands is close to the Fermi level (say -50meV), thanks to interband hopping of Cooper pairs caused by shear phonons. This ``valence-band Suhl-Kondo mechanism'' due to shear phonons is significant to understand the superconductivity in Na$_{0.35}$CoO$_2$. By the same mechanism, the kink structure of the band-dispersion observed by ARPES, which indicates the strong mass-enhancement ($m^\ast/m\sim3$) due to optical phonons, is also explained.

cond-mat.supr-con↗

arXiv2005

Spin fluctuations and weak pseudogap behaviors in Na_{0.35}CoO_2:renomarization of band structure

We analyze the normal electronic states of Na$_{0.35}$CoO$_2$ based on the multi-orbital Hubbard model using the FLEX approximation. The fundamental electronic property of this system is drastically changed by the presence or absence of the small hole pockets associated with the $e_g'$ orbital. This change of the Fermi surface topology may be caused by the crystalline electric splitting due to the trigonal distortion. When small hole pockets are absent, the weak pseudogap bahaviors appear in the density of states and the uniform spin susceptibility, which are observed by recent experiments. We estimate the mass enhancement factor of quasiparticle $m^*/m\simeq 1.5\sim 1.8$. This result suppports recent ARPES measurements.

cond-mat.str-el↗

arXiv2005

Origin of the Weak Pseudo-gap Behaviors in Na_{0.35}CoO_2: Absence of Small Hole Pockets

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the effective d-p model with full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. They sensitively depend on the topology of the Fermi surfaces, which changes as the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300K, assuming that six small hole-pockets predicted by LDA band calculations are absent. When they exist, on the contrary, then ``anti-pseudo-gap behaviors'' should inevitably appear. Thus, the present study strongly supports the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent ARPES measurements. A large Fermi surface around the Γ-point would account for the superconductivity in water-intercalated samples.

cond-mat.str-el↗