T. Wada

University of Tsukuba, Tsukuba, Ibaraki, Japan

Are you T. Wada?

Claim your profile

Publications (11)7.36 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma excitations in the high- Tc superconductor Bi2Sr2CaCu2O8+delta have been investigated in a wide microwave frequency region (9.8-75GHz) ---in particular, in a magnetic field applied parallel to the ab plane of the single crystal. In sharp contrast to the case for magnetic fields parallel to the c axis or tilted from the ab plane, it was found that there are two kinds of resonance modes, which are split in energy and possess two distinctly different magnetic field dependences. One always lies higher in energy than the other and has a shallow minimum at about 0.8kOe , then increases linearly with magnetic field. On the other hand, another mode begins to appear only in a magnetic field (from a few kOe and higher) and has a weakly decreasing tendency with increasing magnetic field. By comparing with a recent theoretical model the higher-energy mode can naturally be attributed to the Josephson plasma resonance mode propagating along the primitive reciprocal lattice vector of the Josephson vortex lattice, whereas the lower-frequency mode is assigned to the novel phase collective mode of the Josephson vortex lattice, which has never been observed before.
    Physical review. B, Condensed matter 01/2005; 72. · 3.77 Impact Factor
  • Source
    I. Kakeya, T. Wada, K Kadowaki
    [Show abstract] [Hide abstract]
    ABSTRACT: We have investigated the Josephson plasma excitations in magnetic fields parallel to the $ab$-plane in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a wide microwave frequency region (9.8 -- 75 GHz). It was found that there are two kind of phase-collective modes: one increases with magnetic fields in higher fields, approaching linear asymptotic dependence with $ck$, which lies well above the inherent plasma frequency $\omega_p$, while the other does not show considerable field dependence. The higher linear mode is attributed to the Josephson plasma mode propagating along the reciprocal lattice vector of the Josephson vortex, whereas the lower one can be ascribed to the oscillation mode of Josephson vortices.
    11/2002;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Josephson plasma resonance has been investigated in Bi2Sr2CaCu2O8+δ single crystals in parallel magnetic fields to the ab-plane. We found two resonance modes; one appears at higher frequency in high fields above the plasma frequency ωp at zero field and absolute zero, and the other lies below ωp and is observed only in magnetic fields without considerable field dependence. Two resonance lines were also found in numerical simulations in a single junction model with randomness of the critical current. The higher frequency mode is attributed to the Josephson plasma mode modified by the periodic structure of Josephson vortices, while the lower frequency mode is interpreted as oscillations of Josephson vortices.
    Physica C Superconductivity 10/2002; · 0.72 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma resonance has been studied in a wide microwave frequency range between 10 and 52 GHz in a magnetic field parallel to the ab-plane in underdoped Bi2Sr2CaCu2O8+δ. Above about 30 GHz two resonance modes were observed: one (LT mode) appears at low temperatures and another (HT mode) at higher temperatures, leaving a temperature gap between two regions. These two resonance modes exhibit a sharp contrast to each other both on temperature and magnetic field dependences and show distinct characters entirely different from the c-axis Josephson plasma resonance. From temperature and field scan experiments at various frequencies it is suggested that the LT mode can be attributed to the coupled Josephson plasma mode with Josephson vortices, while the HT mode is a new plasma mode associated possibly with the periodic array of Josephson vortices.
    Physica C Superconductivity 09/2001; · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Vortex matter phases and phase transitions are investigated by means of Josephson plasma resonance in under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a microwave frequency range between 19 and 70 GHz. Accompanied by the vortex lattice melting transition, a jump of the interlayer phase coherence extracted from the field dependence of the plasma frequency was observed. In the solid phase, the interlayer coherence little depends on field at a temperature region well below $T_c$ while it gradually decreases as field increases toward the melting line up to just below $T_c$. As a result, the magnitude of the jump decreases with increasing temperature and is gradually lost in the vicinity of $T_c$. This indicates that the vortex lines formed in the vortex solid phase are thermally meandering and the phase transition becomes weak especially just below $T_c$.
    Physica C Superconductivity 09/2001; · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Vortex matter phases and phase transitions are investigated by means of Josephson plasma resonance in under-doped Bi2Sr2CaCu2O8+δ single crystals in a microwave frequency range between 19 and 70 GHz. Accompanied by the vortex lattice melting transition, a jump of the interlayer phase coherence extracted from the field dependence of the plasma frequency was observed. In the solid phase, the interlayer coherence little depends on field at a temperature region well below Tc while it gradually decreases as field increases toward the melting line up to just below Tc. As a result, the magnitude of the jump decreases with increasing temperature and is gradually lost in the vicinity of Tc. This indicates that the vortex lines formed in the vortex solid phase are thermally meandering and the phase transition becomes weak especially just below Tc.
    Physica C Superconductivity 01/2001; 362(1):234-238. · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma resonance has been studied in a wide microwave frequency range between 10 and 52 GHz in a magnetic field parallel to the ab-plane in underdoped Bi2Sr2CaCu2O8+δ. Above about 30 GHz two resonance modes were observed: one (LT mode) appears at low temperatures and another (HT mode) at higher temperatures, leaving a temperature gap between two regions. These two resonance modes exhibit a sharp contrast to each other both on temperature and magnetic field dependences and show distinct characters entirely different from the c-axis Josephson plasma resonance. From temperature and field scan experiments at various frequencies it is suggested that the LT mode can be attributed to the coupled Josephson plasma mode with Josephson vortices, while the HT mode is a new plasma mode associated possibly with the periodic array of Josephson vortices.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2001; 362(1):71-77.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma resonance of under-doped Bi2Sr2CaCu2O8+δ single crystals in magnetic fields parallel to the ab-plane has been studied at microwave frequencies ν between 10 and 52 GHz. Two resonance lines are discovered with distinctly different characters as functions of different physical parameters such as magnetic fields, microwave frequencies, temperatures, etc. Those two resonances behave entirely differently from the ones for H∥c and are attributed to the mixed resonance plasma modes with collective motion of the Josephson vortices.
    Physica C Superconductivity 01/2001; 357:611-613. · 0.72 Impact Factor
  • Source
    K Kadowaki, T. Wada, I. Kakeya
    [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma resonance has been studied in a wide microwave frequency range between 10 and 52 GHz in a magnetic field parallel to the $ab$-plane in under-doped $\BI$. Above about 30 GHz two resonance modes were observed: one (LT mode) appears at low temperatures and another (HT mode) at higher temperatures, leaving a temperature gap between two regions. These two resonance modes exhibit a sharp contrast each other both on temperture and magnetic field dependences and show distinct characters different entirely from the c-axis Josephson plasma resonance. From temperature and field scan experiments at various frequencies it is suggested that the LT mode can be attributed to the coupled Josephson plasma mode with Josephson vortices, while the HT mode is a new plasma mode associated possibly with the periodic array of Josephson vortices.
    11/2000;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Vortex matter phases and phase transitions are investigated by means of Josephson plasma resonance in under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a microwave frequency range between 19 and 70 GHz. Accompanied by the vortex lattice melting transition, a jump of the interlayer phase coherence extracted from the field dependence of the plasma frequency was observed. In the solid phase, the interlayer coherence little depends on field at a temperature region well below $T_c$ while it gradually decreases as field increases toward the melting line up to just below $T_c$. As a result, the magnitude of the jump decreases with increasing temperature and is gradually lost in the vicinity of $T_c$. This indicates that the vortex lines formed in the vortex solid phase are thermally meandering and the phase transition becomes weak especially just below $T_c$. Comment: 5pages and 4 figures. Submitted to Physica C (Proceedings of Plasma2000, Sendai)
    09/2000;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Josephson plasma resonance measurements under magnetic fields parallel to the CuO_2 layers as functions of magnetic field, temperature, and microwave frequency have been performed in Bi_2Sr_2CaCu_2O_{8+\delta} single crystals with doping range being from optimal to under-doped side. The feature of the resonance is quite unique and cannot be explained by the conventional understandings of the Josephson plasma for H \parallel c, that requires a new theory including coupling effect between Josephson vortex lattice and Josephson plasma.
    07/2000;