Point Contact Josephson Junctions Research Articles

Josephson current in Fe-based superconducting junctions: Theory and experiment

We present theory of dc Josephson effect in contacts between Fe-based and spin-singlet $s$-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for PbIn/Ba$_{1-x}$K$_{x}$(FeAs)$_2$ point-contact Josephson junctions are consistent with theoretical predictions for $s_{\pm}$ symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as $Sr_2RuO_4$ and superconducting topological insulator $Cu_xBi_2Se_3$.

Reversibility and noise sensitivity of Josephson arrays.

We predict that series arrays of point-contact Josephson junctions, shunted by a resistive load, should be unusually sensitive to noise. This behavior stems from a dynamical symmetry which prohibits the existence of in-phase attractors. Simulations verify that breaking the symmetry can radically improve the phase-locking performance of the array.

Shapiro‐Steps in Josephson Point‐Contact of Y‐Ba‐Cu‐O Ceramics

Using point contact Josephson junctions fabricated by Y‐Ba‐Cu‐O high‐Tc superconductors, Shapiro steps (ac Josephson effect) were observed. It was found that condensed pair electrons exist in high‐Tc ceramics. AC Josephson effect was observed at 77 K when the device was fabricated by the samples made by the proposed liquid quenching method.

Investigation of the energy gap of Y-Ba-Cu-O by point-contact Josephson-junction techniques.

Experimental results on stable point-contact Nb/Y-Ba-Cu-O structures are reported. Reproducible current-voltage characteristics showing Josephson coupling are obtained. Measurements of dV/dI give an estimate for the energy-gap value of the high-T/sub c/ superconductor Y-Ba-Cu-O ..delta.. = 19.5 +- 2.0 mV, corresponding to a ratio (2..delta..(0)/k/sub B/T/sub c/) = 4.8 in agreement with previous results obtained by a different technique.

Ac-Josephson effect in YBa2Cu3O7/PbSn point contact junctions

Adjustable YBa2Cu3O7/PbSn point contact Josephson junctions are exposed to a microwave field. The current voltage characteristics of the junctions show microwave-induced constant voltage steps. The power dependence of the current width of the steps is well described by the Bessel function behaviour. This clearly demonstrates the presence of macroscopic quantum effects in the new highTc material.

Josephson effects in the Ba-Y-Cu-O compounds.

A point-contact Josephson junction between a bulk Ba-Y-Cu-O compound (${T}_{c}\ensuremath{\sim}90$ K) and a Nb needle has been made. The $I\ensuremath{-}V$ characteristics under microwave radiation show Shapiro steps, confirming the Josephson nature of the formed contact. Spin-singlet superconductivity in the Ba-Y-Cu-O system is implied from the voltage of the steps. The temperature dependence of the critical currents suggests that the Josephson junction if formed between Nb and another material with much higher ${T}_{c}$. Internal Josephson structure within the bulk ceramics is not found in the Ba-Y-Cu-O system.

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.

Comparison of Nb Thin-Film Point-Contact Josephson Junction Characteristics with the Stewart-McCumber Model

This paper compares the measured characteristics of a three-dimensional Nb thin-film point-contact junction with those predicted by the Stewart-McCumber model. The erratic microwave-induced I-V characteristic we observed at low microwave power levels was also evident in a digital simulation –based on the Stewart-McCumber model–of the time dependence of the junction voltage and the phase space trajectory. Subharmonic steps V=(n/m)(hf/2e) were observed when a 70 GHz signal was applied, but the usual condition ωCRn≃1 for substeps was not fulfilled. Substeps were also observed in the digital simulation even when the current-phase relationship was purely sinusoidal.

Experimental investigation of a new spectrometer comprising a Josephson junction

It is experimentally shown that an Nb-Nb point contact Josephson junction can be used as a very sensitive spectrometer of good frequency resolution in the range of millimetre waves by applying an inverse Hilbert transform on a characteristic deformation of its I/V characteristic caused by the absorbed RF energy.

Characteristics of Nb Thin-Film Point-Contact Josephson Junctions

This paper describes some niobium thin-film point-contact Josephson junctions fabricated by magnetron sputtering and electron-beam lithography. The microwave-induced I-V step characteristics of a junction was expanded at a level of 1.9 mV, showing a step flatness of better than 1 ppm (2 nV) over a current span of 70 µA–the first time such flatness has been observed in thin-film weak links. The junctions are stable and withstand thermal cycling. The distribution of the critical current (Ic) and normal resistance (Rn) at 4.2 K is reported for 32 samples. The temperature dependence of the IcRn product was found to agree with the recent Kulik-Omelyanchuk theory of weak links, but some samples had IcRn products greater than those predicted by weak link theories.

Damping factor measurement of the Riedel singularity in niobium point contact Josephson junctions

A new experimental method is presented to investigate directly the far-infrared frequency dependence of the Josephson ac current amplitude near the superconducting gap. The lowest value of the damping factor δ of the Riedel singularity in niobium contacts at 4 K is found to be δ=(2+2−1)×10−3 and is attributed to quasiparticle damping.

Far infrared frequency response of a Josephson junction in a self-pumped mixer

A continuous measurement of the far-infrared frequency response of a point contact Josephson junction is performed between 0.7 and 2 THz. The response at increasing frequencies present three characteristic regions with variations proportional to f−2, f−4, f−6. They illustrate the losses introduced successively at increasing frequencies by the resistively shunted junction model, the junction RC time constant and the attenuation of the Josephson current above the gap frequency.

Niobium thin-film point-contact Josephson junction

A stable, niobium thin-film point-contact Josephson junction can be fabricated using rf magnetron sputtering and electron beam lithography. Constant-voltage steps based on a ac Josephson effect are observed up to a voltage level of 4 mV at 4.2 K when a 9-GHz microwave signal is applied. This voltage level is the highest for any thin-film weak link except for the tunnel junction.

Point-Contact Josephson Voltage Standard

This paper describes a practical Josephson voltage standard used to calibrate transfer standard cells. The standard comprises a niobium point-contact Josephson junction-used as the voltage reference-and a series-parallel combination voltage divider. The construction of this Josephson junction is quite unique, and it is robust. It withstands temperature cycling between room temperature and 4.2 K. The point contact is adjusted at room temperature. The output of the point-contact Josephson junction at 4.2 K with a 9.59-GHz microwave signal applied is about 1.9 mV. The divider ratio is 1/529, and divider ratio stability is better than 0.2 ppm/year at room temperature. Our point-contact Josephson junction was compared with the tunnel junction used to determine the Japanese standard volt. The difference between the two was less than 0.5 ppm.

Damping Factor of the Riedel Peak in Bridge-Type Josephoson Junctions

The Riedel peak in the Josephson current in bridge-type point-contact Josephson junctions has been studied by measuring the frequency dependence of the maximum height of the fundamental ( n =1) Shapiro-step in the energy range near \(\hbar\omega{\sim}4\varDelta_{0}\) with a submillimeter-wave laser, where Δ 0 is the real part of the energy gap. The imaginary part of the energy gap, Δ 2 , has been estimated from the observed rounding of the Riedel singularity. As a result, the damping factor \(\delta{=}\varDelta_{2}/\varDelta_{0}{\gtrsim}0.05\) has been obtained. The value of δ∼0.05 seems to be the lowest value which is intrinsic to bridge-type junctions. It has been also found that the value of δ is large in the junctions which show strong heating effects.

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.

Josephson heterodyne detection at high thermal background levels

Point-contact Josephson junctions with a submicron contact diameter have been used as high-frequency mixers (170–220 GHz) under high thermal background conditions. The junction is formed across a full height (0.56×1.06 mm2) waveguide between a 3–5-μm thick Nb whisker and a 0.2-mm diam Nb center conductor of a coaxial line. The best single-side band mixer-noise temperature that was measured is 165 K (± 25%) with a corresponding single-side band conversion efficiency of 0.36±0.03 (a loss of 4.5 ±0.4 dB) at a signal frequency of 185 GHz, including losses in the coupling horn and the waveguide. The performance was measured with the hot-cold source technique. The effects of nonheterodyne response in that situation were investigated in detail. The junctions have been operated in a coherent receiver that can be used on a telescope with only small adaptations. The instantaneous bandwidth of the receiver is about 20 GHz.

Semimetal-barrier quasi-planar Josephson junction

Niobium-based bismuth-barrier Josephson junctions have been made whose geometrical structure is almost planar. The amplitudes of their zero-voltage currents and microwave-induced constant-voltage current-steps are periodically modulated by applied microwave radiation. Voltage current curves of the junctions have similar energy gap structures to those of point contact Josephson junctions. The value of the parameter S, defined as the ratio of the differential resistance just above the gap to that just below the gap, is about 2.0. The junctions have a barrier length of less than 100 nm, a very low capacitance and a life of several months even at room temperature. A quasi-planar Josephson junction has been turned out a hybrid of other kinds of Josephson junctions.

Frequency measurement at 4.25 THz (70.5μm) using a Josephson harmonic mixer and phase-lock techniques

A direct frequency measurement has been made on the 70.5 mu m wavelength line of the methyl alcohol optically-pumped laser by 43rd-harmonic mixing with a klystron using a point-contact Josephson junction. It was possible to phase-lock the klystron to the laser. The centre frequency of the emitted radiation when the CO2 pump laser (9.68 mu m, P(34)) was tuned near the absorption line centre was found to be 4251 674.0+or-0.7 MHz. This result is about 5 MHz higher than an earlier published result by Petersen et al. (1975). A result is also given for the frequency of the 118-8 mu m wavelength line in methyl alcohol.

Submillimetre and millimetre response of Josephson junctions mounted in a waveguide

Wide-band detection characteristics have been measured for niobium point contact Josephson junctions, mounted in a 1.30 × 0.65 mm 2 rectangular waveguide. Spectral response measurements have been made for different mounting positions in the waveguide as a function of the normal state junction resistance and the backshort position. For the absolute response we measured a minimum detectable external temperature difference (Δ T min) syst ≃ 10mK/Hz 1 2 with a (NEP) syst ≃ 5 × 10 −15 W/Hz 1 2 , at ν ≃ 200GHz.