arXiv subjects

Takeo Kato

Publications and source records attributed to Takeo Kato.

97 records · Page 6Linked to original sources

arXiv2003

Temperature Dependence of Zero-Bias Resistances of a Single Resistance-Shunted Josephson Junction

Zero-bias resistances of a single resistance-shunted Josephson junction are calculated as a function of the temperature by means of the path-integral Monte Carlo method in case a charging energy $E_{\rm C}$ is comparable with a Josephson energy $E_{\rm J}$. The low-temperature behavior of the zero-bias resistance changes around $α=R_{\rm Q}/R_{\rm S}=1$, where $R_{\rm S}$ is a shunt resistance and $R_{\rm Q}=h/(2e)^2$. The temperature dependence of the zero-bias resistance shows a power-law-like behavior whose exponent depends on $E_{\rm J}/E_{\rm C}$. These results are compared with the experiments on resistance-shunted Josephson junctions.

cond-mat.supr-con↗

arXiv2001

Local Inhomogeneity Effects on Nucleation Process in a High External Bias

Quantum nucleation processes in the presence of local moderate inhomogeneities are studied theoretically at high biases. The quantum nucleation rate Gamma is calculated for one-dimensional systems in a form Gamma = A e^(-B/hbar) by using the `bounce' method. The bias-dependence of the exponent B is shown to be changed by inhomogeneities. This change is explained by the reduction of the effective spatial dimension of the system. By studying the system-size dependence of the prefactor A, the condition for the appearance of inhomogeneity effects is evaluated. Nucleation rates in thermal activation regimes are also calculated, and compared with quantum tunneling regimes. For higher-dimensional systems, it is shown that the local approximation of inhomogeneity does not hold, and that spatial profiles of inhomogeneity become important.

cond-mat.stat-mech↗

arXiv2000

Dimensional Crossover by a Local Inhomogeneity in Soliton-Pair Nucleation

Soliton-pair nucleation rates Gamma = A exp(-B) are studied in highly-biased sine-Gordon systems with a local inhomogeneity for both a thermal activation regime and a quantum tunneling regime. It is found that the local inhomogeneity strongly affects the nucleation rates by modifying the bias-dependence of the exponent B. This change in the exponent B is explained as a dimensional crossover caused by the local inhomogeneity. It is also shown that there is another crossover, at which A becomes independent of the system size.

cond-mat.supr-con↗

arXiv1999

Quantum Effects in Resistance-Shunted Josephson Junctions

Thermodynamics and transport properties of resistance-shunted Josephson junctions are studied theoretically in the tight-binding limit E_C/E_J<<1, where E_C and E_J are a charging energy and a Josephson coupling energy respectively. Based on a phenomenological harmonic-oscillator model, weak coupling region K=R_Q/R<<1 is analytically studied, where R and R_Q=h/(2e)^2 are a shunted resistance and the quantum resistance. In addition to the effective bandwidth, we find that this multi-level system genuinely has a novel crossover at lower energy below which the density of states becomes strongly degenerate. These two energy scales control the linear DC responses, optical responses, and nonlinear I-V characteristics. The lower energy crossover indicates the existence of a new class of strongly-correlated phenomena beyond the framework of the Kondo problem.

cond-mat.supr-con↗

arXiv1998

Thermodynamics and Optical Conductivity of a Dissipative Carrier in a Tight Binding Model

Thermodynamics and transport properties of a dissipative particle in a tight-binding model are studied through specific heat and optical conductivity. A weak coupling theory is constituted to study the crossover behavior between the low-temperature region and the high-temperature region analytically. We found that coherent part around zero frequency in the optical conductivity disappears for 0<s<2, where s is an exponent of a spectral function of the environment. Detailed calculation is performed for ohmic damping (s=1). In this case, the specific heat shows an unusual $T$-linear behavior at low temperatures, which indicates that the environment strongly influences the particle motion, and changes the low-energy states of the dissipative particle. The optical conductivity σ(ω) takes a non-Drude form even at zero temperature, and the high-frequency side behaves as ω^(2K-2), where K is a dimensionless damping strength. The high frequency side of the optical conductivity is independent of temperatures, while the low frequency side depends on the temperature, and behaves as T^(2K-2) at high temperatures. We also comment on the application of this model to the description of incoherent motion in correlated electron systems.

cond-mat.stat-mech↗

arXiv1997

Vortices and Quantum tunneling in Current-Biased 0-π-0 Josephson Junctions of d-wave Superconductors

We study a current-biased 0-π-0 Josephson junction made by high-T_c superconductors, theoretically. When a length of the πjunction is large enough, this junction contains a vortex-antivortex pair at both ends of the πjunction. Magnetic flux carried by the vortices is calculated using the sine-Gordon equation. The result shows that the magnetic flux of the vortices is suppressed to zero as the distance between the vortices is reduced. By applying an external current, the orientation of the vortices is reversed, and a voltage pulse is generated. The current needed for this transition and generated pulse energy are calculated. Macroscopic quantum tunneling (MQT) in this transition is also studied. The tunneling rate has been evaluated by an effective Hamiltonian with one degree of freedom.

cond-mat.supr-con↗

arXiv1996

Macroscopic Quantum Tunneling of a Fluxon in a Long Josephson Junction

Macroscopic quantum tunneling (MQT) for a single fluxon moving along a long Josephson junction is studied theoretically. To introduce a fluxon-pinning force, we consider inhomogeneities made by modifying thickness of an insulating layer locally. Two different situations are studied: one is the quantum tunneling from a metastable state caused by a single inhomogeneity, and the other is the quantum tunneling in a two-state system made by two inhomogeneities. In the quantum tunneling from a metastable state, the decay rate is estimated within the WKB approximation. Dissipation effects on a fluxon dynamics are taken into account by the Caldeira-Leggett theory. We propose a device to observe quantum tunneling of a fluxon experimentally. Required experimental resolutions to observe MQT of a fluxon seem attainable within the presently available micro-fabrication technique. For the two-state system, we study quantum resonance between two stable states, i.e., macroscopic quantum coherence (MQC). From the estimate for dissipation coefficients due to quasiparticle tunneling, the observation of MQC appears to be possible within the Caldeira-Leggett theory.

cond-mat↗