arXiv subjects

Satoshi Kashiwaya

Publications and source records attributed to Satoshi Kashiwaya.

At least 37 records · Page 2Linked to original sources

arXiv2010

Thermally induced 0-pi phase transition in Josephson junctions through a ferromagnetic oxide film

We investigate the Josephson transport through a ferromagnetic oxide film, e.g., La$_2$BaCuO$_5$, theoretically. Using the recursive Green's function technique, we found the formation of a pi-junction in such systems. Moreover the 0-pi phase transition is induced by increasing the temperature. Such ferromagnetic-oxide based Josephson junctions may become an element in the architecture of future quantum computers.

cond-mat.supr-con↗

arXiv2010

DC Current Driven Critical Current Variation in Sr2RuO4-Ru Junction Proved by Local Transport Measurements

To search for new evidence of the chiral p-wave superconducting domain in Sr2RuO4, we investigated the unconventional local transport characteristics of a microfabricated Sr2RuO4-Ru eutectic junction. We found that the anomalous hysteresis in voltage-current characteristics [as reported in H. Kambara et al.: Phys. Rev. Lett. 101 (2008) 267003.] is strongly affected by dc currents, but not by magnetic fields. This suggests that dc current acts as a driving force to move chiral p-wave domain walls; a domain wall trapped at a pinning potential is forced to shift to the next stable position, thereby forming a larger critical current path and resulting in the anomalous hysteresis.

cond-mat.supr-con↗

arXiv2010

Multi-Junction Switching in Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ Intrinsic Josephson Junctions

We study the dynamics of multi-junction switching (MJS): several intrinsic Josephson junctions (IJJs) in an array switch to the finite voltage state simultaneously. The number of multi-switching junctions ($N$) was successfully tuned by changing the load resistance serially connected to an Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ IJJ array. The independence of the escape rates of $N$ in the macroscopic quantum tunneling regime indicates that MJS is a $successive$ switching process rather than a $collective$ process. The origin of MJS is explained by the gradient of a load curve and the relative magnitudes of the switching currents of quasiparticle branches in the current-voltage plane.

cond-mat.supr-con↗

arXiv2010

Theory of Josephson transport through spintronics nano-structure

We study the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure explicitly. In the case of the fully polarized FIs (FPFIs), we found the formation of a pi-junction and an atomic-scale 0-pi transition induced by increasing the FI thickness. More remarkably, in the Josephson junction through spin-filter materials such as Eu chalcogenides, the orbital hybridization between the conduction d and the localized f electron gives rise to the pi-junction behavior. Such FI-based pi-junctions can be used to implement highly-coherent solid-state quantum bits.

cond-mat.mes-hall↗

arXiv2010

Josephson pi-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 stable pi-state can be realized in junctions based on high-Tc superconductors with La$_2$BaCuO$_5$ barrier.

cond-mat.supr-con↗

arXiv2010

Effect of d-f hybridization on the Josephson current through Eu-chalcogenides

A superconducting ring with a pi junction made from superconductor/ferromagnetic-metal/superconductor (S-FM-S) exhibits a spontaneous current without an external magnetic field in the ground state. Such pi ring provides so-called quiet qubit that can be efficiently decoupled from the fluctuation of the external field. However, the usage of the FM gives rise to strong Ohmic dissipation. Therefore, the realization of pi junctions without FM is expected for qubit applications. We theoretically consider the possibility of the pi coupling for S/Eu-chalcogenides/S junctions based on the d-f Hamiltonian. By use of the Green's function method we found that pi junction can be formed in the case of the finite d-f hybridization between the conduction d and the localized f electrons.

cond-mat.mes-hall↗

arXiv2007

Conductance Spectroscopy of Spin-triplet Superconductors

We propose a novel experiment to identify the symmetry of superconductivity on the basis of theoretical results for differential conductance of a normal metal connected to a superconductor. The proximity effect from the superconductor modifies the conductance of the remote current depending remarkably on the pairing symmetry: spin-singlet or spin-triplet. The clear-cut difference in the conductance is explained by symmetry of Cooper pairs in a normal metal with respect to frequency. In the spin-triplet case, the anomalous transport is realized due to an odd-frequency symmetry of Cooper pairs.

cond-mat.supr-con↗

arXiv2007

Theory of Macroscopic Quantum Tunneling and Dissipation in High-Tc Josephson Junctions

We have investigated macroscopic quantum tunneling (MQT) in in-plane high-Tc superconductor Josephson junctions and the influence of the nodal-quasiparticle and the zero energy bound states (ZES) on MQT. We have shown that the presence of the ZES at the interface between the insulator and the superconductor leads to strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably suppressed in comparison with the c-axis junctions in which ZES are completely absent.

cond-mat.supr-con↗

arXiv2006

Macroscopic quantum dynamics of pi-junctions with ferromagnetic insulators

We theoretically investigate the macroscopic quantum dynamics of a pi junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a pi-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 pi junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small.

cond-mat.mes-hall↗

arXiv2006

Anomalous Josephson effect in p-wave dirty junctions

The Josephson effect in p-wave superconductor / diffusive normal metal / p-wave superconductor junctions is studied theoretically. Amplitudes of Josephson currents are several orders of magnitude larger than those in s-wave junctions. Current-phase (J-ϕ) relations in low temperatures are close to those in ballistic junctions such as J\propto\sin(ϕ/2) and J\proptoϕeven in the presence of random impurity potentials. A cooperative effect between the midgap Andreev resonant states and the proximity effect causes such anomalous properties and is a character of the spin-triplet superconductor junctions.

cond-mat.supr-con↗

arXiv2005

Decoherence of Phase Qubit using High-Tc Superconductor

We discuss how to make use of high-Tc d-wave Josephson junctions in the construction of a phase qubit. We especially focus on the effect of the quasiparticle dissipation and the zero energy bound state on the macroscopic quantum tunneling which corresponds to the final measurement process of the d-wave phase qubit.

cond-mat.mes-hall↗

arXiv2005

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 in compared with the c-axis junctions in which the ZES are completely absent.

cond-mat.supr-con↗

arXiv2004

Quasiparticle Dissipation in d-wave Superconductor Phase Qubit

Recent phase sensitive experiments on high Tc superconductors, e.g., YBa_2Cu_3O_7 single crystals, have established the d-wave nature of the cuprate materials. Here we discuss how to make use of d-wave Josephson junctions in the construction of a phase qubit. We especially focus on the effect of quasiparticle dissipation on the macroscopic quantum tunneling which corresponds to the measurement process for the d-wave phase qubit.

cond-mat.mes-hall↗

arXiv2004

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 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 compared with the finite twist angle cases.

cond-mat.supr-con↗

arXiv2004

Anomalous charge transport in mesoscopic triplet superconductor junctions

Charge transport properties of a diffusive normal metal /triplet superconductor (DN/TS) junction are studied based on the Keldysh-Nambu quasiclassical Green's function formalism. % Contrary to the unconventional singlet superconductor junction case, the mid gap Andreev resonant state (MARS) at the interface of the TS is shown to enhance the proximity effect in the DN. % The total resistance of the DN/TS junction is drastically reduced and is completely independent of the resistance of the DN in the extreme case. % Such anomalous transport accompanies a giant zero-bias peak in the conductance spectra and a zero-energy peak of the local density of states in the DN region. % These striking features manifest the presence of novel proximity effect peculiar to triplet superconductor junctions.

cond-mat.supr-con↗

arXiv2003

A phenomenological theory of zero-energy Andreev resonant states

A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal / superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.

cond-mat.supr-con↗

arXiv2003

Nature of carriers in one-dimensional correlated electron systems

Electron densities at the edge site in one-dimensional Hubbard model and t-J model are studied by using the Bethe ansatz solutions and exact diagonalization method. It is found that the boundary is electron-attractive, or equivalently hole-repulsive, near half-filling. We propose a new criterion to determine whether the carriers in a strongly correlated system are electron-like or hole-like depending on the electronic behavior at the boundary.

cond-mat.str-el↗

arXiv2003

Electronic states around a vortex core in high-Tc superconductors based on the t-J model

Electronic states around vortex cores in high-Tc superconductors are studied using the two-dimensional t-J model in order to treat the d-wave superconductivity with short coherence length and the antiferromagnetic (AF) instability within the same framework. We focus on the disappearance of the large zero-energy peak in the local density of states observed in high-Tc superconductors. When the system is near the optimum doping, we find that the local AF correlation develops inside the vortex cores. However, the detailed doping dependence calculations confirm that the experimentally observed reduction of the zero-energy peak is more reasonably attributed to the smallness of the core size rather than to the AF correlation developed inside the core. The correlation between the spatial dependence of the core states and the core radius is discussed.

cond-mat.supr-con↗