Josephson Transport Research Articles

Tunneling Hamiltonian description of the atomic-scale 0–π transition in superconductor/ferromagnetic-insulator junctions

We show a perturbation theory of the Josephson transport through ferromagnetic insulators (FIs). Recently we have found that the appearance of the atomic scale 0–π transition in such junctions based on numerical calculations. In order to explore the mechanism of this anomalous transition, we have analytically calculated the Josephson current using the tunneling Hamiltonian theory and found that the spin dependent π-phase shift in the FI barrier gives the atomic scale 0–π transition.

A tunneling Hamiltonian theory of 0-π transition in d-wave superconductor/ferromagnetic-insulator heterostructures

We report a theory of the Josephson transport through ferromagnetic insulators (FIs). Recently the appearance of the atomic scale 0-π transition in such junctions has been predicted based on a numerical simulation. In order to understand the physical mechanism of this anomalous phenomena, we have analytically calculated the Josephson current using the tunneling Hamiltonian method and found that the spin dependent π-phase shift in the FI barrier gives the atomic scale 0-π transition. We also show an experimental setup for observing the atomic-scale 0-π transition based on a d-wave junction with a LBCO barrier.

Theory of quantum transport in Josephson junctions with a ferromagnetic insulator

We investigate Josephson transport through ferromagnetic insulators (FIs) taking the band structure of the FIs explicitly into account. Using a recursive Green function method, we find that a π-junction is formed in this type of system. Moreover, an atomic-scale 0–π oscillation is induced by increasing the thickness of an FI and its oscillation period is universal, exactly equal to a single atomic layer. Based on these results, we show that a stable π-state can be realized in junctions based on high-Tc superconductors with a La2BaCuO5 barrier. Such FI-based Josephson junctions may become an element in the architecture of future quantum computers.

Atomic scale [formula omitted] transition in a high- Tc superconductor/ferromagnetic-insulator/high- Tc superconductor Josephson junction

We study the Josephson transport in a high- T c superconductor/ferromagnetic-insulator(FI)/high- T c superconductor numerically. We found the formation of a π -junction in such systems. More remarkably the ground state of such junction alternates between 0- and π -states when thickness of FI is increasing by a single atomic layer. We propose an experimental setup for observing the atomic scale 0 – π transition. Such FI-based π -junctions can be used to implement highly coherent quantum bits.

Thermally induced 0–π phase transition in Josephson junctions through a ferromagnetic oxide film

We investigate the Josephson transport through a ferromagnetic oxide film, e.g., La2BaCuO5, theoretically. Using the recursive Green’s function technique, we found the formation of a π-junction in such systems. Moreover the 0–π 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.

Theory of Josephson transport through spintronics nano-structures

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 π -junction and an atomic-scale 0 – π 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 π -junction behavior. Such FI-based π -junctions can be used to implement highly coherent solid-state quantum bits.

NUMERICAL STUDY OF π-JUNCTION USING SPIN FILTERING BARRIERS

We numerically investigate the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure. By use of the recursive Green's function method, we found the formation of the π junction in the case of the fully spin-polarized FI (FPFI), e.g., La 2 BaCuO 5. Moreover, the 0-π transition is induced by increasing the thickness of FPFI. On the other hand, Josephson current through the Eu chalcogenides shows the π junction behavior in the case of the strong d-f hybridization between the conduction d and the localized f electrons of Eu . Such FI-based Josephson junctions may becomes a element in the architecture of future quantum information devices.

Josephson effect through a quantum dot array

We analyze the ground state properties of an array of quantum dots connected in series between superconducting electrodes. This system is represented by a finite Hubbard chain coupled at both ends to BCS superconductors. The ground state is obtained using the Lanczos algorithm within a low energy theory in which the bulk superconductors are replaced by effective local pairing potentials. We study the conditions for the inversion of the sign of the Josephson coupling ($\pi$-junction behavior) as a function of the model parameters. Results are presented in the form of phase diagrams which provide a direct overall view of the general trends as the size of the system is increased, exhibiting a strong even-odd effect. The analysis of the spin-spin correlation functions and local charges give further insight into the nature of the ground state and how it is transformed by the presence of superconductivity in the leads. Finally we study the scaling of the Josephson current with the system size and relate these results with previous calculations of Josephson transport through a Luttinger liquid.

Phonon Squeezing in a Superconducting Molecular Transistor

Josephson transport through a single molecule or carbon nanotube is considered in the presence of a local vibrational mode coupled to the electronic charge. The ground-state solution is obtained exactly in the limit of a large superconducting gap and is extended by variational analysis. The Josephson current induces squeezing of the phonon mode, which is controlled by the superconducting phase difference and by the junction asymmetry. Optical probes of nonclassical phonon states are briefly discussed.

Josephson current through a correlated quantum level: Andreev states andπjunction behavior

The Josephson transport and the electronic properties of a quantum dot characterized by a single level coupled to superconducting leads is analyzed. Different approximations are used and compared: the mean field approximation, the second order perturbation theory in the Coulomb interaction and the exact diagonalization in the zero band-width limit. The system exhibits a rich behavior as a function of the relevant parameters. We discuss in detail the conditions for the observation of Pi-junction behavior and the effect of Coulomb interactions on the Andreev states.

Josephson Transport through a Hubbard Impurity Center

We investigate the Josephson transport through a thin semiconductor barrier containing impurity centers with the on-site Hubbard interaction u of an arbitrary sign and strength. We find that in the case of the repulsive interaction the Josephson current changes sign with the temperature increase if the energy of the impurity level epsilon (measured from the Fermi energy of superconductors) falls in the interval (-u,0). We predict strong temporal fluctuations of the current if only a few centers are present within the junction. In the case of the attractive impurity potential (u<0) and at low temperatures, the model is reduced to the effective two level Hamiltonian allowing thus a simple description of the nonstationary Josephson effect in terms of pair tunneling processes.

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).

Josephson properties of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ bicrystal junctions grown by a sequential deposition technique using molecular beam epitaxy

We report Josephson and crystallographic properties of the Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ junctions fabricated on MgO(100) bicrystal substrates by a molecular-beam-epitaxy method incorporating co-evaporation and sequential deposition techniques. With the sequential deposition technique which has the advantage of promoting the surface diffusion of adatoms, we obtained the highly growth-controlled films without precipitation of any second impurity phases. During the film growth, the sharp reflection high-energy electron diffraction (RHEED) patterns were also observed. However, the RHEED patterns showed the a-b twin structures due to the lattice mismatch, which influenced the Josephson transport properties at the junction boundary. The normal resistance of the bicrystal junction was 1.5 /spl Omega/ and the I/sub c/R/sub n/ product was 0.75 mV at 4.2 K. The Shapiro steps under millimeter-wave irradiation were clearly observed up to 65 K. The Josephson microwave self-radiation spectra at receiving frequency f/sub REC/=22 GHz. The observed Josephson transport properties are discussed in relation to the microscopic crystallographic properties.

Scattering theory of superconductive tunneling in quantum junctions

A consistent theory of superconductive tunneling in single-mode junctions within a scattering formulation of Bogolyubov-de Gennes quantum mechanics is presented. The dc Josephson effect and the dc quasiparticle transport in the voltage-biased junctions are considered. Elastic quasiparticle scattering by the junction determines the equilibrium Josephson current. The origin of Andreev bound states in tunnel junctions and their role in equilibrium Josephson transport are discussed. In contrast, quasiparticle tunneling in voltage-biased junctions is determined by inelastic scattering. A general expression for inelastic scattering amplitudes is derived and the quasiparticle current is calculated at all voltages with emphasis on a discussion of the properties of subgap tunnel current and the nature of subharmonic gap structure.

Josephson transport in Tl-cuprate bicrystal weak links

Superconducting epitaxial Tl2Ba2CaCu2O8 (Tl-2212) and Tl2Ba2CuO6 (Tl-22O1) thin films are synthesised on single crystal and bicrystal LaAlO3 substrates by an ex-situ process. Tl-2212 films were optically smooth, with T-c's from 105 to 108K and Delta T-c <2 K, I-c reaching 2.10(6) A/cm(2) at 77 K. Tl-2201 films exhibited tetragonal or orthorhombic symmetry depending on the thallium content. The T-c, of the as-synthesised Tl-2201 films was in the range 40-55 K. By oxygen depletion, T-c was increased by 15 - 20 K. Properties of weak links in Tl-2212 and Tl-2201 thin films are reported.

Josephson transport in complex mesoscopic structures

A universal spectral equation is derived for Andreev bound states in superconducting quantum junctions, relating bound state energies with the normal electron scattering amplitudes. The equation is applied to calculation of d.c. Josephson effect in mesoscopic S-2DEG-S junctions.

Superfluid helium Josephson transport

Using electro-mechanical engineering anaologies where voltage goes to pressure and current goes to mass flow, the superfluid Josephson transport of mass through a constricted region has been discussed by Anderson. Here we discuss the conjugate Josephson transport of canonical momentum.