Tunnel-type Junctions Research Articles

Fabrication and properties of Nb/MgO/Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ tunnel junctions using crystalline and amorphous MgO films grown by the MBE method

The properties of tunnel-type junctions have been improved by using the Nb/MgO/Au/BSCCO structure. Tunneling spectroscopy was examined in junctions such as Nb/MgO/BSCCO and Nb/MgO/Au/BSCCO. MgO films for tunnel barriers were grown by molecular-beam epitaxy (MBE). In a Nb/MgO/BSCCO junction with no Au layer, the superconducting gap parameters, Delta , of BSCCO single crystals with crystalline and amorphous MgO barriers were estimated to be 25 meV and 38 meV, respectively. Nb/MgO/Au/BSCCO junctions with a 20-nm-thick Au layer indicated a clear gap structure induced by the proximity effect at 30 meV. These junction characteristics were confirmed to be closely related to the existence of interdiffusion between BSCCO and other materials. >

Fabrication and Properties of MgO/Bi2Sr2CaCu2Ox Interfaces Using Crystalline and Amorphous MgO Films Grown by the MBE Method

Tunnel-type junctions with pinhole-free crystalline and amorphous MgO barriers on the order of several nm were successfully fabricated on the cleavage plane of BSCCO single crystals. Both MgO films for the tunnel barriers were grown by the molecular beam epitaxy method. The superconducting gap parameters, Δ, of a BSCCO single crystal deposited with crystalline and amorphous tunnel barriers were estimated to be 25 meV and 38 meV, respectively, for the configuration of a Nb/MgO/BSCCO junction from dI/dV-V characteristics. The possibility of different surface superconductivities for BSCCO single crystals was determined by the characterization of the MgO/BSCCO interface.

Synthesis of high-quality Ba 1−xK xBiO 3 superconductors using the PMQ method

High density and homogeneous Ba 0.6K 0.4BiO x superconductors with Tce of 28.5 K were successfully synthesized by the PMQ method. S/I/N tunnel-type junctions were fabricated using MgO artificial barriers for the first time. In the I–V characteristics of the junctions, non-linearity was clearly observed at 3∼4 mV corresponding to the energy gap of Ba 0.6K 0.4BiO x superconductors.

Characteristics of Y-Ba-Cu-O/Nb and Bi-Sr-Ca-Cu-O/Nb tunnel-type Josephson junctions

The authors fabricated Y-Ba-Cu-O/Nb and Bi-Sr-Ca-Cu-O/Nb tunnel-type junctions and observed the DC and AC Josephson effects. Gold thin films were deposited for an interlayer to protect the reaction between the base electrode and the tunnel oxide before the tunnel barrier formation. In Y-Ba-Cu-O/Au/MgO/sub x//Nb junctions, superconducting current, hysteresis of current-voltage characteristics, and RF-induced voltage steps as high as 0.13 mV in the current-voltage characteristics were observed. Moreover, the superconducting current was modulated by the magnetic field. In Bi-Sr-Ca-Cu-O/Au/MgO/sub x//Nb junctions, superconducting current and RF-induced voltage steps as high as 0.15 mV were also observed.

Temperature dependence of submillimeter wave response in the point-contact Josephson junction

We have measured the magnitude of the first Shapiro step in both bridge-type (Ta-Ta and Ta-Sn) and tunnel-type (Ta-SnOx-Sn) point-contact Josephson junctions in the SMMW ( Submillimeter Wave ) region. By varying the temperature continuously with the frequency of the SMMW fixed (ω=2π/λ, λ = 699, 570 and 469μm) . we determined I_{1}^{\max}/I_{c} minutely as a function of the normalized frequency \omega/\omega_{g} (T), where I_{1}^{\max} is the maximum (half) height of the first Shapiro step as a function of the ac field power, I c is the critical current, and \omega_{g}(T)=4\Delta_{Ta}(T)/h or 2(\Delta_{Ta}(T)+\Delta_{Sn}(T))/h . By fitting the shape of the Riedel peak at \omega/\omega_{g}(T)=I with the theory of the tunnel junction, we have determined the damping factor δ. We obtained δ =0.02 for the Ta-SnOx-Sn tunnel- type junction, and δ=0.05 for both the Ta-Sn and Ta-Ta bridge - type junctions, respectively. At \omega/\omega_{g}(T)>1.5 , we have found that I_{1}^{\max}/I_{c} tends to saturate in both the tunnel-type and the bridge-type junctions as \omega/\omega_{g}(T) increases. For the tunnel-type junction, this agrees with the very slow decrease predicted by the theory of the tunnel junction. But for the bridge-type junction, this slow decrease is in marked contrast to the very rapid decrease found from the previous experiments, where the Shapiro step was measuared for a number of frequencies at a fixed temperature.

Nonlinear response of tunnel-type Josephson junctions at submillimeter-wave frequencies

We have fabricated tunnel-type Josephson junctions with very small junction area by pressing the tip of a sharpened Nb wire onto a thinly oxidized Sn film, and studied their submillimeter-wave responses. It turns out that the junctions have a very high cutoff frequency, and are suitable for the investigation of the response to the submillimeter waves with the frequency higher than 1 THz. From the frequency and laser-power dependence of the Shapiro step, it is found that the tunnel-type junction shows much larger higher-order steps, and accordingly has much stronger nonlinearity than the bridge-type junction in the submillimeter-wave region. The large enhancement of the steps due to the Riedel singularity is also observed in the tunnel-type junction. In addition to the Shapiro steps, photon-assisted tunneling steps are observed clearly, reflecting the large and sharp gap structure in the dc IV curves of the tunnel-type junctions.

Correlation between Types of Junction and Submillimeter-Wave Responses in Point-Contact Josephson Junctions

It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuOx–Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnOx–Sn and Nb–NbOx–NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.