arXiv subjects

Masahiko Machida

Publications and source records attributed to Masahiko Machida.

60 records · Page 4Linked to original sources

arXiv2009

Theory of heterotic SIS Josephson junctions between single- and multi-gap superconductors

Using the functional integral method, we construct a theory of heterotic SIS Josephson junctions between single- and two-gap superconductors. The theory predicts the presence of in-phase and out-of-phase collective oscillation modes of superconducting phases. The former corresponds to the Josephson plasma mode whose frequency is drastically reduced for $\pm$ s-wave symmetry, and the latter is a counterpart of Leggett's mode in Josephson junctions. We also reveal that the critical current and the Fraunhofer pattern strongly depend on the symmetry type of the two-gap superconductor.

cond-mat.supr-con↗

arXiv2008

Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario

We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic \pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like βFermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.

cond-mat.supr-con↗

arXiv2006

Theory for Photon-Assisted Macroscopic Quantum Tunneling in a Stack of Intrinsic Josephson Junctions

We propose a theory for photon-assisted macroscopic quantum tunneling (MQT) in a stack of capacitively-coupled intrinsic Josephson junctions in which the longitudinal Josephson plasma, i.e., longitudinal collective phase oscillation modes, is excited. The scheme of energy-level quantization in the collective oscillatory states is clarified in the $N$-junction system. When the MQT occurs from the single-plasmon states excited by microwave irradiation in the multi-photon process to the uniform voltage state, our theory predicts that the escape rate is proportional to $N^2$. This result is consistent with the recent observation in Bi-2212 intrinsic Josephson junctions.

cond-mat.supr-con↗

arXiv2005

Three-dimensional dynamics of vortex lattice formation in Bose-Einstein condensates

We numerically study the dynamics of vortex lattice formation in a rotating cigar-shaped Bose-Einstein condensate. The study is a three-dimensional simulation of the Gross-Pitaevskii equation with a phenomenological dissipation term. The simulations reveal previously unknown dynamical features of the vortex nucleation process, in which the condensate undergoes a strongly turbulent stage and the penetrating vortex lines vibrate rapidly. The vibrations arise from spontaneous excitation of Kelvin waves on the proto-vortices during a surface wave instability, caused by an inhomogeneity of the condensate density along the elongated axial direction.

cond-mat.other↗

arXiv2003

Ginzburg-Landau calculations of d-wave superconducting dot in s-wave superconducting matrix

We have developed a numerical method that calculated superconducting states and magnetic field distributions for the composite structures of the High-Tc superconductor and the conventional superconductor in arbitrary geometries. We show spontaneous magnetic flux appears at the corner of the boundary of these two superconductors. Also we propose High-Tc superconducting dot embedded in conventional superconductors, which is named as d-dot and show the spatial distribution of superconducting order parameters and the magnetic field.

cond-mat.supr-con↗

arXiv2002

Vibronic mechanism of high-Tc superconductivity

The dispersion of the in-plane Cu-O bond-stretching LO phonon mode in the high-$T_{C}$ superconducting cuprates shows strong softening with doping near the zone boundary. We suggest that it can be described with a negative electronic dielectric function that results in overscreening of inter-site Coulomb interaction due to phonon-induced charge transfer and vibronic electron-phonon resonance. We propose that such a strong electron-phonon coupling of specific modes can form a basis for the phonon mechanism of high-temperature superconductivity. With the Eliashberg theory using the experimentally determined electron dispersion and dielectric function, we demonstrate the possibility of superconductivity with the order parameter of the $d_{k_{x}^{2}-k_{y}^{2}}$ symmetry and the transition temperature well in excess of 100K.

cond-mat.supr-con↗