Josephson Flux Flow Research Articles

Josephson flux-flow oscillator: The microscopic tunneling approach

We elaborate a theoretical description of large Josephson junctions which is based on the Werthamer's microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is particularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the time integrals at each time step while taking into account all the memory effects. To validate the obtained microscopic model of large Josephson junction we focus our attention on the Josephson flux flow oscillator. The proposed microscopic model of flux flow oscillator does not involve the phenomenological damping parameter, rather, the damping is taken into account naturally in the tunnel current amplitudes calculated at a given temperature. The theoretically calculated current-voltage characteristics is compared to our experimental results obtained for a set of fabricated flux flow oscillators of different lengths. Our theoretical calculation agrees well with the obtained experimental results, and, to our knowledge, is the first where theoretical description of Josephson flux flow oscillator is brought beyond the perturbed sine-Gordon equation.

The effect of bias feed profile on the linewidth of noisy Josephson flux flow oscillator

For creation of a noisy non-stationary spectrometer operating in the sub-THz frequency range, the emitting power and spectral characteristics of a flux-flow oscillator based on a long Josephson junction are considered. The effect of bias current profiles on the spectral linewidth of a long overlap Josephson junction is investigated and compared with the inline junction geometry. It has been demonstrated that the spectral linewidth can be increased by a factor of three with a moderate reduction of the emitted power.

Josephson effect between a two-band superconductor withs++ors±pairing symmetry and a conventionals-wave superconductor

In this work, we investigate the Josephson effect between a two-band superconductor either with the $s++$ (two energy gaps have the same sign and are fully gapped) pairing symmetry or $s\pm$ (two energy gaps have $\pi$ phase difference and are fully gapped) pairing symmetry and a conventional s-wave superconductors. The ground state, critical current, plasma modes, flux flow dynamics, and response to external ac electric field, possible soliton solutions are investigated. For junctions with the charge neutrality breaking, we find a new plasma mode for junctions, which gives rise to new resonance peaks in the Josephson flux flow region. Because of the frustrated interaction in junctions with $s\pm$ pairing symmetry, time-reversal-symmetry (TRS) can be broken if the frustration is optimized. In the TRS broken (TRSB) state, there is a non-trivial phase difference between the two Josephson tunnelling channels, which results in a non-trivial interference. Furthermore, we find a novel massless plasma mode at the TRSB transition for junctions with the charge neutrality breaking. In the TRSB state, a spontaneous magnetic flux appears where there is a spatial inhomogeneity in the Josephson coupling, thus provides a possible smoking-gun evidence for the underlying pairing symmetry.

Spectral characteristics of noisy Josephson flux flow oscillator

The current-voltage and spectral characteristics of a flux flow oscillator (FFO) based on a long Josephson junction are studied. The investigations are performed in the range of small bias currents and magnetic fields where the FFO radiates a quasi-chaotic signal with extremely large radiation linewidth, and the displaced linear slope (DLS) is observed at the current-voltage characteristic. Using direct numerical simulation of the sine-Gordon equation, it is shown that for large lengths of the Josephson junction or in the case of partial matching of the FFO with external waveguide system, the DLS with extremely large linewidth is transformed into Fiske steps with very narrow linewidth. As for Fiske steps, the appearance of regime of chaotic oscillations can be explained by multiple reflections of the traveling waves from junction ends rather than simply by excitation of the internal oscillation modes in the “soft” fluxon chain at weak magnetic fields.

RF responses and possible applications of intrinsic Josephson junctions in different vortex states

We report various RF induced effects on Josephson vortex flow of Bi2Sr2CaCu2O8+y intrinsic Josephson junctions in vortex states being different with existence of pancake vortices. For pancake free state, RF induced zero-crossing steps, which had been found in ordinary Josephson flux flow oscillators, were first observed at voltage V=NΦ0f for N-stacked intrinsic Josephson junctions. In contrast the RF irradiations to the junctions in mixed state with pancakes those step responses changed features similar to Shapiro steps at Vn=nNΦ0f with order n. The former evidently demonstrates that electromagnetic waves propagate in the intrinsic Josephson junctions interacting with moving vortices. The later indicates that Josephson vortices move at the same speed coherently.

Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semi-fluxon oscillations of Josephson flux-flow (JFF) in a Bi-2212 stacked structures near $T_c$ as a probe of melting of Josephson vortex lattice (JVL). We found that oscillations exist above 0.5T. The amplitude of oscillations is found to decrease gradually with temperature and to turn to zero without any jump at $T=T_0$, 3.5K below the resistive transition temperature $T_c$ indicating the phase transition of the second order. This characteristic temperature $T_0$ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, $T_{BKT}$, in elementary superconducting layers of Bi-2212 at zero magnetic field. On the base of these facts we infer that melting of triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and anti-vortices. The $B-T$ phase diagram of the BKT phase found out from our experiment is consistent with theoretical predictions.

Externally phase-locked flux flow oscillator for submm integrated receivers: achievements and limitations

A Josephson flux flow oscillator (FFO) is the most developed superconducting local oscillator for integration with an SIS mixer in a single-chip submm-wave receiver. Recently, using a new FFO design, a free-running linewidth /spl les/10 MHz has been measured in the frequency range up to 712 GHz, limited only by the gap frequency of Nb. This enabled us to phase lock the FFO in the frequency range 500-712 GHz where continuous frequency tuning is possible; resulting in an absolute FFO phase noise as low as -80 dBc at 707 GHz. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated SIS harmonic mixer. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field has been studied in order to further optimize the FFO design. A new approach with a self-shielded FFO has been developed and experimentally tested.

Shapiro step response in the vortex state of Bi2Sr2CaCu2O8+$delta;

We report the first observation of Shapiro step effect in the Josephson vortex state of Bi 2 Sr 2 CaCu 2 O 8+ δ in high magnetic field (H⩾1 T ) . We observed phase locking between external microwave radiation and Josephson oscillation in whole stack of N intrinsic Josephson junctions at fields tilted with respect to the c -axis. Steps were observed in Josephson flux-flow (JFF) on the I – V characteristic at voltage V st = pNhν /2 e , with p integer, ν =45– 142 GHz . The amplitude of both Shapiro steps and Josephson critical current increase sharply with tilting angle and then decrease. We attribute this behavior to the interaction between Josephson and pancake vortices.

Phase-locked Josephson flux flow local oscillator for sub-mm integrated receivers

The Josephson flux flow oscillator (FFO) has proven to be one of the best on-chip local oscillators for heterodyne detection in integrated sub-mm receivers based on SIS mixers. Nb–AlOx–Nb FFOs have been successfully tested from about 120 to 700 GHz (gap frequency of Nb) providing enough power to pump an SIS mixer (about 1 μW at 450 GHz). Both the frequency and the power of the FFO can be dc-tuned. Extensive measurements of the dependence of the free-running FFO linewidth on the differential resistances associated with both the bias current and the control-line current (applied magnetic field) have been performed. The FFO line is Lorentzian both in the resonant regime, on Fiske steps (FSs), and on the flux flow step (FFS). This indicates that internal wide-band noise is dominant. A phenomenological noise model can account for the FFO linewidth dependence on experimental parameters. The narrow free-running FFO linewidth achieved, in combination with the construction of a wide-band phase-locked loop (PLL) system, enable us to phase lock the FFO in the frequency range 490–712 GHz where continuous frequency tuning is possible.

Line width of Josephson flux flow oscillators

A combination of wide-band electronic tunability and moderate free-running line width makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated submillimeter-wave SIS receivers. The possibility of FFO phase locking at all frequencies of interest has to be proven before one initiates real FFO applications. To achieve this goal a comprehensive set of line width measurements of the FFO operating in different regimes has been performed. FFOs with tapered shape have been successfully implemented in order to avoid the superfine resonant structure with voltage spacing of about 20 nV and extremely low differential resistance, recently observed in the IVC of the standard rectangular geometry. The obtained results have been compared with existing theories and FFO models in order to understand and possibly eliminate excess noise in the FFO. The intrinsic line width increases considerably at voltages above the boundary voltage because of the abrupt increase of the internal damping due to Josephson self-coupling. The influence of FFO parameters, in particular the differential resistances associated both with the bias current and with the applied magnetic field on the radiation line width, has been studied. Possible means of decreasing the free-running FFO line width will be discussed.

Shapiro step response in the coherent Josephson flux flow state of Bi2Sr2CaCu2O8+delta.

We report the first observation of Shapiro steps in the Josephson flux flow state of Bi2Sr2CaCu2O8+delta stacked junctions. When the junction is irradiated at microwave frequencies in a strong parallel magnetic field ( H> or =1 T), a steep increase of the current is observed in the dc current-voltage characteristics when the external microwave frequency and the Josephson frequency are harmonically related. The existence of Shapiro steps in the Josephson flux flow state is strong evidence of coherent motion of the Josephson vortex lattice through the whole thickness of the stack.

Novel features of Josephson flux flow in Bi-2212: contribution of in-plane dissipation, coherent response to mm-wave radiation, size effect

We studied Josephson flux flow (JFF) in Bi-2212 stacks fabricated from single crystal whiskers by focused ion beam technique. For long junctions with the in-plane sizes 30×2 μm 2, we found considerable contribution of the in-plane dissipation to the JFF resistivity, ρ Jff, at low temperatures. According to recent theory [A. Koshelev, Phys. Rev. B 62 (2000) R3616] that results in quadratic type dependence of ρ Jff( B) with the following saturation. The I– V characteristics in the JFF regime also can be described consistently by that theory. In the JFF regime we found Shapiro-step response to the external mm-wave radiation. The step position is proportional to the frequency of applied microwaves and corresponds to the Josephson emission from all the 60 intrinsic junctions of the stack being synchronized. That implies the coherence of the JFF over the whole thickness of the stack and demonstrates the possibility of synchronization of intrinsic junctions by the magnetic field. We also found a threshold character of the appearance of the JFF branch on the I– V characteristic with the increase of magnetic field, the threshold field B t being scaled with the junction size perpendicular to the field L (L=30–1.4 μ m), as B t≈ Φ 0/ Ls, where s is the interlayer spacing. On the I– V characteristics of small stacks in the JFF regime we found Fiske-step features associated with resonance of Josephson radiation with the main resonance cavity mode in transmission line formed by stacks.

Towards a phase-locked superconducting integrated receiver: prospects and limitations

Presently a Josephson flux flow oscillator (FFO) appears to be the most developed superconducting on-chip local oscillator for integrated submillimeter-wave SIS receivers. The feasibility of phase locking the FFO to an external reference oscillator at all frequencies of interest has to be proven for practical FFO implementation in radio astronomy and other spectral applications. A linewidth of a phase-locked FFO as low as 1 Hz has been measured relative to an external reference oscillator in the frequency range 270–440 GHz on steep Fiske steps in the low damping regime. The increase of the intrinsic linewidth at higher voltages due to an abrupt increase of the internal damping considerably complicates phase locking of the FFO. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated harmonic SIS mixer. Results on FFO linewidth and spectral line profile have been compared to theory in order to optimize the FFO design. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field, has been studied. Two integrated receiver concepts with phase-lock loop have been developed and experimentally tested.

Properties of Tl 2Ba 2CaCu 2O 8 thin film intrinsic Josephson junctions in an in-plane magnetic field

We have performed transport measurements on intrinsic Josephson junctions in misaligned thin films of Tl 2Ba 2CaCu 2O 8. This has been done in both zero field and in-plane aligned magnetic fields of up to 5 T. We have compared wet-etched and ion-milled devices and find that the more precise structure produced by ion-milling gives superior results. In the case of ion-milled devices we are able to individually switch on up to 50 junctions. For an n-plane field of 3 T there is complete suppression of the critical current. We have seen displaced branches for in-plane fields above 1.5 T. This branching is due to Josephson flux flow. For in-plane fields greater than 2 T the relationship between the maximum flux-flow voltage and the applied field is linear.

Superfine resonant structure on IV-curves of long Josephson junction and its influence on flux flow oscillator linewidth

The Josephson Flux Flow Oscillator (FFO) has proven to be a perfect on-chip local oscillator for integrated submm receivers; a noise temperature (DSB) below 100 K has been achieved at 500 GHz. Recently a FFO linewidth as low as 1 Hz has been measured in the frequency range 270 - 440 GHz. A new technique for both linewidth measurements and phase locking of the FFO is developed; this method employs an offchip harmonic multiplier. By measuring the frequency of the FFO radiation emission, its IV-curve (IVC) can be reconstructed with an accuracy better than 1 nV. A superfine resonant structure with a voltage spacing of about 20 nV and extremely low differential resistance has been observed in the FFO IVCs. This resonant structure modifies the performance of the FFO compared to the one expected from the "averaged IVC". The influence of this resonant structure on phase locking is discussed. Also results of FFO phase noise measurements are presented.

Resonant Cherenkov radiation in Josephson flux flow oscillators with integrated meander-shaped transmission line

A novel Cherenkov flux flow oscillator (CFFO) is considered. The system consists of a long Josephson junction with an overlaying meander-shaped stripline. The traveling wave in the line is exited at each intersection point by the chain of fluxons moving along the junction. At certain resonance frequencies the radiation delivered to the stripline from all intersections points interfere constructively and the generated power reaches its maximum value. To first order the resonance condition is given by the distance between the fluxons, and is independent of the system boundaries. So one can change the resonance frequency by varying the external magnetic field. This is radically different from oscillators relying on conventional internal junction resonances such as the Fiske resonance, and allows for design of a powerful CFFO which can be electronically tuned. The existence of resonant interactions with both forward and backward waves is shown, and the analogy to the Cherenkov synchronism is demonstrated. Numerical simulations confirm the analytical predictions and allow us to investigate the strongly nonlinear case. The proposed CFFO is ideally suited for realization in HTSC technology.

Laminar phase flow for an exponentially tapered Josephson oscillator

Exponential tapering and inhomogeneous current feed were recently proposed as means to improve the performance of a Josephson flux flow oscillator. Extensive numerical results backed up by analysis are presented here that support this claim and demonstrate that exponential tapering reduces the small current instability region and leads to a laminar flow regime where the voltage wave form is periodic giving the oscillator minimal spectral width. Tapering also leads to an increased output power. Since exponential tapering is not expected to increase the difficulty of fabricating a flux flow oscillator, we suggest that this feature should be incorporated in future designs.

Linewidth and phase locking of Josephson flux-flow oscillators

We report on measurements of the linewidth of the emitted radiation from Josephson Flux Flow Oscillators (FFOs). Frequency and phase locking to an external 10 MHz reference oscillator are demonstrated experimentally in the frequency range 270–440 GHz. A linewidth as low as 1 Hz (as determined by the resolution bandwidth of the spectrum analyzer) has been measured. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The damping mechanisms are discussed and related to the self-excitation of quasiparticles. Narrow linewidth, wide-band tunability and low noise are important for radio astronomy and air- and space-borne spectroscopy for atmospheric research and environmental monitoring.

Phase locked 270–440 GHz local oscillator based on flux flow in long Josephson tunnel junctions

The combination of narrow linewidth and wide band tunability makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated sub-mm wave receivers for, e.g., spectral radio astronomy. The feasibility of phase locking the FFO to an external reference oscillator is demonstrated experimentally. A FFO linewidth as low as 1 Hz (determined by the resolution bandwidth of the spectrum analyzer) has been measured in the frequency range 270–440 GHz relative to a reference oscillator. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The results of residual FFO phase noise measurements are also presented. Finally, we propose a single-chip fully superconductive receiver with two superconductor–insulator–superconductor mixers and an integrated phase-locked loop.

Intergranular transport properties of polycrystalline Sm 1.82Ce 0.18CuO 4− y under low applied magnetic fields

We report on intergranular transport properties of polycrystalline samples of Sm 1.82Ce 0.18CuO 4− y exhibiting the double resistive superconducting transition. These samples are comprised of small islands which undergo a superconducting transition at a higher temperature T ci∼21 K and the long-range phase coherence is achieved through Josephson coupling at a lower temperature T cJ∼10 K. We have performed several transport measurements including magnetic field and temperature dependence of electrical resistance, R( T, H), and I– V characteristics in these samples. Such measurements were taken under low applied magnetic fields 0≤ H≤200 Oe and at temperatures T< T cJ∼10 K in order to investigate the phase correlations of the superconducting order parameter throughout the material. The results of magnetoresistance R( T, H) enabled us to conclude that dissipation due to Josephson flux flow is governed by an intergranular activation energy given by U J( T, H)∝(1− T/ T cJ)( H −0.6). In addition, we have found an irreversible behavior of R( H) curves, which shows a crossover field H= H*∼30 Oe. We also observed that H* is excitation current independent and that I cJ( H*)≈0. The combination of these results has indicated that the phase of the superconducting order parameter is depressed at intergranular regions or more appropriately at the grain boundaries in these superconducting granular materials.