Nb Point Contact Research Articles

Series YBCO grain boundary Josephson junctions as a terahertz harmonic mixer

Josephson devices have demonstrated the capability to work as harmonic mixers at terahertz frequencies. Low temperature devices, in particular Nb–Nb point contacts have proven particularly successful. However, practical applications of these devices have been limited by the need of utilizing liquid helium. To overcome this limitation we have investigated the use of high-Tc devices. In this paper, we report mixings up to the 154th harmonic at zero-bias in a series of YBa2Cu3O7−δ (YBCO) grain boundary Josephson junctions. We have integrated a meander series of three bicrystal YBCO Josephson junctions with a log-periodic antenna. When properly operated, this configuration allows to reach a harmonic number much higher than what possible with a single junction. This shows that the mixing performance benefits from the synchronous operation which also results in an improvement of dynamic range. We believe that the integration of a higher number of phase-locked junctions may further improve the mixer’s performance. Future studies should investigate the effect of higher number of junctions on the mixer performances, as well as the effect of the meander length.

Inelastic electron-phonon scattering and excess current in superconducting point contacts with a short coherence length

Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics (I-V curves) and their derivatives for ErNi2B2C point contacts (PC) with d ≥ ξ (where d is the diameter of the PC and ξ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in dV/dI coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For “dirty” NbSe2 and Nb point-contacts with d ≥ ξ, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.

Transport spin polarization inSrRuO3measured through point-contact Andreev reflection

We report a study in which Andreev reflection using a Nb point contact is used to measure the transport spin polarisation of the 4d itinerant ferromagnet SrRuO3. By performing the study in high quality thin films with residual resistivities less than 7micro-ohm-cm, we ensure that the study is done in the ballistic limit, a regime which is difficult to reach in oxide ferromagnets. The degree of transport spin polarisation that we find is comparable to that of the hole doped rare-earth manganites. We conclude that the large transport spin polarisation results mainly from a difference in the Fermi velocities between the majority and minority spin channels in this material.

Carrier reflection at the superconductor-semiconductor boundary observed using a coplanar-point-contact injector.

The Andreev reflection of carriers at the boundary between a superconductor and a semiconductor has been studied buy measurement of the differential resistance of a n-type-Si--Nb point contact with a coplanar structure. The boundary condition of the pair potential in the Si-Nb proximity system is obtained from these measurements. A carrier-concentration dependence of the pair potential at the boundary is observed. The pair potential in the Si at the boundary increases with increasing carrier concentration.

Josephson effect in the unusual superconductors Lu 2Fe 3Si 5, LiTi 2O 4, and UBe 13

A Josephson supercurrent, which can be split by microwave photons of frequency ν into Shapiro steps of spacing ΔV = hν/2 e, is expected between superconductors of like pairing type, either s- or p-wave. A weaker, and possibly unobservable effect, with step spacing Δ V 2 was predicted by Pals and van Haeringen between electrodes of unlike pairing type. Experiments using K-band microwaves and a Nb point contact have been carried out at 1.2 K on the d-band superconductors LiTi 2O 4 and Lu 2Fe 3Si 5. In both cases we find unambiguous indication of s-wave pairing, as in Nb. The same experiment on the heavy-fermion superconductor UBe 13 in the range 0.7–7 K also reveals steps of spacing Δ V. This indicates that UBe 13 has been induced into a state of s-wave superconductivity by proximity with the s-wave superconductors Nb.

Microwave applications of Josephson junction diode and triode

This presentation describes two subjects. The first subject concerns with new effects of quasiparticle injection on microwave irradiated Josephson junctions, which might bring about a new type of Josephson three-terminal device. The second subject concerns with the development of a practical near-millimeter wave Josephson harmonic mixer apparatus using a stable Nb point contact.

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.

Temperature dependence of a Josephson current in point contact junctions, NbNbOxide-ferromagnetic superconductor

We investigate here the temperature dependence of a Josephson critical current I c in the Er 0.58 Ho 0.42 Rh 4 B 4 NbOxide  Nb point contact junctions with lower normal state resistance R n ≈ 0.2–0.8 Ω. The measurements show that the products I c R n have a peak near 2.15 K just above T c2.

Characteristic features of the ac Josephson effect above the gap voltage in niobium point contacts

We have investigated the strength of the ac Josephson effect, I MAX 1/I C, above the gap voltage V g in Nb point contacts. We found that its dependence on voltage is empirically formulated as I MAX 1/I C = exp{- α( V- β)}, where the damping constant α has a strong correlation with the magnitude of V g. From the existence of the threshold at ≈V g in the damping of I MAX 1/I C, we conclude that the exponential damping of the strength of the ac Josephson effect above the gap voltage is caused by the injection of quasiparticles. By finding of the correlation between the gap voltage and the cooling efficiency of the point contact, we confirm that the damping constant α is determined by the cooling efficiency of the point contact.

Singularities of an I-V Curve and Voltage Fluctuations in “Ideal” Superconducting Nb Point Contact

A method of constructing a point contact Josephson device with “ideal” I - V characteristics is described. Radio frequency (RF) outputs are detected with no intentionally applied external RF fields. Enhancements of output RF intensities occur at the gap and sub-gap voltages. Strong sharp peaks of RF outputs of approximately 10 -12 W are observed at lower voltage regions. Effects of thermal and shot noises are discussed in relation to the singularities of an I - V curve.

Comparison of multiple flux quantum transition with the corresponding I-U-characteristic using Nb-point contacts

Comparison of multiple flux quantum transition with the corresponding I-U-characteristic using Nb-point contacts

Niobium point-contact Josephson-junction behavior at 604 GHz

We have measured the ac Josephson effect in Nb point contacts at 604 GHz (496 μm). We find the coupling of the far-infrared radiation to the point contact to depend in a simple manner on the resistance of the contact. The behavior of the high-resistance point contacts (50⩽R⩽200 Ω) is very reproducible, allowing a quantitative comparison of the data to the frequency-dependent Werthamer theory. We also account for the effects of noise and heating and compare these to Tinkham’s heating theory.

Heating effects in high-frequency metallic Josephson devices: Voltage limit, bolometric mixing, and noise

In this paper, we analyze nonequilibrium effects within a simple heating approximation for the case of metallic Josephson weak links which have a favorable three-dimensional cooling geometry. Our principal conclusions are the following: (1) The temperature in the center of the junction with voltage V applied is Tm=[Tb2+3 (eV/2πk)2]1/2, where Tb is the bath temperature, e is the electronic charge, and k is Boltzmann’s constant. This Tm can be as high as 70 K in Nb point contacts on a high Josephson step; thus the device noise temperature TN? (1/2)(Tb+Tm) can greatly exceed Tb. (2) The critical current falls approximately as exp(−P/Po), where P is the power dissipated in the junction and Po is typically 10 μW for Al, Sn, Pb, and Nb, but much smaller for Nb3Sn. This leads to a decrease in the amplitude of Josephson steps at high voltages which is in good agreement with data on the best Sn variable-thickness microbridges and Nb point contacts. In a junction of optimized resistance level, the overall maximum voltage at which microwave-induced steps can still exceed the noise is predicted to be proportional to Tc[Ωξ (0)/ρo]2/7× (ν1/Tc)1/7, where Ω is the solid angle of the three-dimensional cooling, ξ (0) is the extrapolated coherence length at T=0, ρo is the residual resistivity of the metal, and ν1 is the microwave frequency. (3) The thermal response should be fast enough (∼τGL) to allow detection (heterodyne or square law) with bandwidth as large as the energy gap frequency; it is estimated that this bolometric detection effect will dominate true Josephson detection for carrier frequencies above ∼3 THz (i.e., λ≲100 μm) in junctions with resistance 10 Ω or less.

Rf SQUID’s in the nonhysteretic mode: Detailed comparison of theory and experiment

A simple analytic theory for the operation of rf SQUID’s in the regime where the current I in the superconducting ring is a continuous function of the applied flux (nonhysteretic mode) has been extended to include the effects of screening flux LI, where L is the inductance of the ring. A comparison of calculated results with detailed measurements of the response of a rf SQUID using a toroidal Nb ring containing a Nb point contact shows excellent agreement both in cases when screening flux is significant and when it can be neglected.

Detailed measurements of the response of an rf SQUID in the regime LI<inf>c</inf>< Φ<inf>0</inf>/2π

Using a superconducting ring containing a Nb point contact inductively coupled to a resonant circuit fed with an rf current source, (typical rf SQUID configuration) we have measured the tank voltage as a function of frequency in the regime LI_{c} . For LI_{c} \ll \Phi_{0}/2\pi , the curves of tank voltage versus drive current for various values of external flux \Phi_{x} through the ring agree with the calculations of Hansma. For higher critical currents, there is an asymmetry in these curves for frequencies above and below ω 0 , the resonant frequency of the circuit. Calculations using an expanded theory including the effects of a conductance G_{o}(1+\alpha \cos \phi) and of screening flux give good agreement with the data. In principle α can be determined from these measurements, however the \cos \phi term and the screening flux produce qualitatively similar effects. A quantitative determination of α requires a precise knowledge of G o /I c . Using values of G o and I c from independent impedance measurements, the data yield a negative value of α in agreement with another experiment.

Josephson junctions at 45 times the energy-gap frequency

Superconductive Nb–Nb point contacts have been studied with 9.5-μm radiation from CO2 lasers. Two models are considered to explain the experiments: one is Werthamer's Josephson junction model and the other is a thermally modulated Josephson junction. The evidence favors Werthamer's model but is not conclusive.

Conversion gain in a Josephson effect mixer

The measured performance of Nb–Nb point contact Josephson junctions used as 36-GHz microwave mixers is in good agreement with the predictions of the resistively shunted junction model. Conversion efficiencies greater than unity are attained, with mixer noise temperatures comparable with the best competing devices.

Conversion gain and noise in a Josephson mixer

Abstract : Fundamental mixing at 36 GHz in non-hysteretic Nb point contacts has been observed with overall conversion efficiencies in the range 0.33 < or = eta < or = 4 and mixer noise temperatures in the neighborhood of ( Taub M) approximately 200K. The observed conversion efficiencies are in good agreement with the Resistively Shunted model. The observed noise can be accounted for by assuming that the Johnson noise in the shunt resistor is the only source of noise driving the junction. Somewhat larger noise is seen from resonantly coupled mixers than is seen when the mixer is driven from an rf current source. Fundamental mixing in hysteretic junctions is limited to a narrow if bandwidth and gives much larger conversion gain but also larger noise. Parametric amplification at the if frequency is seen in hysteretic junctions. Fourth order harmonic mixing with omega sub (LO)/2 pi at 9GHz has been achieved with performance almost comparable to fundamental mixing. Cavity mode mixing appears less promising. (Author)

Absorption spectroscopy via the I-V curve of a Josephson junction

The fact that characteristic features appear on the I-V curves of Josephson junctions coupled electromagnetically to various circuit elements offers the possibility of a type of absorption spectroscopy by coating a small amount of sample on a weak-link and observing the structure induced on the I-V curve. The weak-link then acts as both source and detector. By observing d V/dI and d2V/d I2 vs V for Nb point contacts coated with a material with an absorption at 13.9 cm-1 we have observed features at V = 0.85 mV corresponding to interaction of the sample with the Josephson oscillation. However the I-V curves of point contacts exhibit a profusion of structure not related to the sample. By observing all the structure as a function of temperature it is possible to identify three general types of features : 1. Features that vary like Δ(T), the BCS gap function, and occurring at V = 2 Δ/ne, n an integer, the subharmonic structure. 2. Features that vary linearly with T4 and that correspond to relaxation oscillations of the weak-link coupled to antenna modes of the point contact. 3. Features that do not vary appreciably with temperature and that correspond to interaction with the sample material. All these features can be described using simple lumped parameter models of the weak-link. Thin film bridges appear to offer more promise for useful measurements on sample materials. We are studying various types of weakly linked films, primarily ones formed by implanting Fe into Nb films using an ion accelerator.