Shiro Kawabata

Macroscopic quantum tunneling and quasiparticle dissipation in d-wave superconductor Josephson junctions

We examine the macroscopic quantum tunneling (MQT) in high-Tc superconductor Josephson junctions with a d-wave order parameter. Using a microscopic Hamiltonian and the functional integral method, we analytically obtain the MQT rate (the inverse lifetime of the metastable state ) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is strongly suppressed in comparison with the finite twist angle cases.

Test of Bell's inequality using the spin filter effect in ferromagnetic semiconductor microstructures

A theoretical proposal for testing Bells inequality in mesoscopic systems is presented. We show that the entanglement of two electron spins can be detected in the spin filter effect in the mesoscopic semiconductor / ferromagnetic semiconductor / semiconductor junction. In particular, we show how to test Bells inequality via the measurement of the current-current correlation function in this setup. We also discuss the influence of an imperfect spin filter and derive the condition to examine the violation of Bells inequality experimentally.

Macroscopic quantum dynamics of π junctions with ferromagnetic insulators

We theoretically investigate the macroscopic quantum dynamics of a π junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a π-junction qubit (a S-FI-S superconducting quantum interference device ring) is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency. We also investigate macroscopic quantum tunneling (MQT) in current-biased S-FI-S π junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small.

Josephson π State in a Ferromagnetic Insulator

We predict anomalous atomic-scale 0-pi transitions in a Josephson junction with a ferromagnetic-insulator (FI) barrier. The ground state of such junction alternates between 0 and pi states when thickness of FI is increasing by a single atomic layer. We find that the mechanism of the 0-pi transition can be attributed to thickness-dependent phase shifts between the wave numbers of electrons and holes in FI. Based on these results, we show that a stable pi state can be realized in junctions based on high-T{c} superconductors with a La2BaCuO5 barrier.

Effect of zero-energy bound states on macroscopic quantum tunneling in high-Tc superconductor junctions

The macroscopic quantum tunneling MQT in the current biased high-Tc superconductor Josephson junctions and the effect of the zero-energy bound states ZES on the MQT are theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/ superconductor interface leads to a strong ohmic quasiparticle dissipation. Therefore the MQT rate is noticeably inhibited compared with the c-axis junctions in which the ZES are completely absent.

Theory of macroscopic quantum tunneling in high-Tc c-axis Josephson junctions

given by cos 2 and cos 2, respectively. We also show that a dissipative effect resulting from the nodal quasiparticle excitation on MQT is negligibly small, which is consistent with recent MQT experiments using Bi2Sr2CaCu2O8+ intrinsic junctions. These results indicate that MQT in c-axis twist junctions becomes a useful experimental tool for testing the OPS of high-Tc materials at low temperature, and suggest high potential of such junctions for qubit applications.

Switching Dynamics of Bi2Sr2CaCu2O8+δIntrinsic Josephson Junctions: Macroscopic Quantum Tunneling and Self-Heating Effect

The switching dynamics of current-biased Bi

Robustness of Spin-Triplet Pairing and Singlet–Triplet Pairing Crossover in Superconductor/Ferromagnet Hybrids

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Efficient electron refrigeration using superconductor/spin-filter devices

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Experimental study of macroscopic quantum tunnelling in Bi2212 intrinsic Josephson junctions

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