Kiejin Lee

Spin-polarized quasiparticle tunnel injection in a YBa2Cu3Oy/Au/Co junction

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by injection of spin-polarized quasiparticles (QP) using a cobalt ferromagnetic injector. The injection of spin-polarized QP generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The observed current gain depends on the thickness of Au interlayer (dAu) and is directly related to the nonequilibrium magnetization due to spin relaxation effects. For dAu=15 nm, the tunnel characteristic a YBCO/Au/Co junction exhibited a zero bias conductance peak, which may be interpreted by spin scattering processes at a ferromagnetic/d-wave superconductor junction.

Nonequilibrium Microwave Emission from DC-Biased High Tc YBa2Cu3O7-y Junctions

We report the observation of microwave emission from dc-biased high Tc Au/insulator/YBa2Cu3O7-y junctions, which provides a novel method of generating broad-band microwaves. The microwave intensity is typically of the order of subpicowatts, and increases almost linearly with the increase of junction current and vanishes as the bath temperature approaches Tc. The observed effect is not similar to the Josephson effect and may be interpreted in terms of the nonequilibrium dynamics involving Cooper pairs and quasiparticles.

Josephson transport and crystallographic properties of Bi2Sr2CaCu2Oy junctions on (100) MgO bicrystal substrates

Abstract We report the Josephson effects of the grain boundary Bi 2 Sr 2 CaCu 2 O y (2212) bicrystal junctions formed on (100)MgO bicrystal substrates with a misorientation angle of 24° by in situ coevaporation growth technique using molecular beam epitaxy (MBE). It is found that the 2212 film on MgO bicrystal substrate consists of the a − b twin structures by reflection high-energy electron diffraction (RHEED) observation. The atomic force microscope (AFM) observation at the 2212 bicrystal junction interface suggests that the actual junction boundary strongly deviates from the straight line geometry. These microscopic geometry effects are discssed with the observed Josephson effects.

Intrinsic BSCCO Josephson junctions on off-axis substrates

Anisotropic transport and microwave properties of intrinsic Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ (2212) Josephson junctions fabricated on off-axis substrates were studied. The twin-free thin film 2212 Josephson junctions have been prepared by molecular beam epitaxy (MBE) on LaAlO/sub 3/ [001] substrates tilted 6/spl deg/ toward the [110] direction, and were patterned into a line shape along [110] LaAlO/sub 3/ with 20 /spl mu/m in width. We found that the anisotropy parameter /spl gamma/ was increased from 76 to 120 by the annealing at temperature of 320-350/spl deg/C in the MBE chamber. The coherent Josephson radiation from stacked 2212 intrinsic junctions and their modulation under applied magnetic field were observed at frequency f/sub REC/=1.7 GHz by using a nonresonant detection method. These results confirmed the occurrence of the mutual phase locking of the stacked series array of intrinsic junctions along the c-axis.

Crystallography and microwave properties of intrinsic Bi2Sr2CaCu2Oy Josephson junctions on off-axis substrates

Abstract Untwinned thin-film intrinsic Bi 2 Sr 2 CaCu 2 O y (BSCCO) Josephson junctions have been prepared on tilted SrTiO 3 (001) and LaAlO 3 (001) substrates with the tilt angle of 6° toward the [110] azimuth by molecular beam epitaxy (MBE) with the sequential deposition technique. We found that the highly a - b in-plane aligned BSCCO junctions exhibited regular steps with several heights of half a unit cell and had a stacked series array of intrinsic junctions. The observation of coherent Josephson radiation at frequency ƒ REC = 1.7 GHz by using a non-resonant detection method from the stacked BSCCO intrinsic junctions and the periodic critical current modulation under applied magnetic field proved the mutual phase-locking of the stacked series array of intrinsic junctions along the c -axis.

Millimeter-wave self-radiation from high-Tc Josephson junction arrays

We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUIDs geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.

Observation of Nonequilibirum Microwave Emission from dc-Biased High Tc YBa2Cu3O7-y Junctions

We report the observation of microwave emission from dc-biased high Tc Au/insulator/YBa2Cu3O7-y junctions, which provides a novel method to generate broad-band microwaves. The microwave intensity increases nearly linearly with the increase of junction current and diminishes away as the bath temperature approaches Tc. The observed effect is not of Josephson nature and may be interpreted by the nonequilibrium dynamics involving Cooper pairs and quasiparticles.

Anisotropic Transport Properties in Twin-Free Bi2Sr2CaCu2Ox Thin Films on Tilted LaAlO3 (001) Substrates

Anisotropic transport properties of twin-free Bi2Sr2CaCu2Ox (BSCCO) thin films were studied. Twin-free BSCCO thin films were prepared by molecular beam epitaxy (MBE) on LaAlO3 (001) substrates tilted 6° toward the [110] direction. By postannealing in high vacuum (HV) of 10-8 Torr at a temperature of 300–400°C, the carrier density per CuO2 plane was reduced from 0.5 to 0.3. The anisotropy parameter (γ) was determined to be 38 in the HV-annealed sample.

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.

An inhomogeneous Josephson junction array model in HTSC granular bridges

Experimental and theoretical studies are reported of the electrodynamic behaviours, such as current - voltage characteristics (IVCs), microwave self-radiations and dynamic resistances, dV/dI, of granular high- superconducting (HTSC) (YBCO) and (BSCCO) film bridges. We show that the granular structure of bridges can be understood as series-connected independent and inhomogeneous resistively shunted junction (RSJ) arrays.