Overlap Josephson Junctions Research Articles

Path toward manufacturable superconducting qubits with relaxation times exceeding 0.1 ms

As the superconducting qubit platform matures towards ever-larger scales in the race towards a practical quantum computer, limitations due to qubit inhomogeneity through lack of process control become apparent. To benefit from the advanced process control in industry-scale CMOS fabrication facilities, different processing methods will be required. In particular, the double-angle evaporation and lift-off techniques used for current, state-of-the-art superconducting qubits are generally incompatible with modern-day manufacturable processes. Here, we demonstrate a fully CMOS compatible qubit fabrication method, and show results from overlap Josephson junction devices with long coherence and relaxation times, on par with the state-of-the-art. We experimentally verify that Argon milling—the critical step during junction fabrication—and a subtractive-etch process nevertheless result in qubits with average qubit energy relaxation times T1 reaching 70 µs, with maximum values exceeding 100 µs. Furthermore, we show that our results are still limited by surface losses and not, crucially, by junction losses. The presented fabrication process, therefore, heralds an important milestone towards a manufacturable 300 mm CMOS process for high-coherence superconducting qubits and has the potential to advance the scaling of superconducting device architectures.

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.

INFLUENCE OF LENGTH ON THE NOISE DELAYED SWITCHING OF LONG JOSEPHSON JUNCTIONS

The transient dynamics of long overlap Josephson junctions in the frame of the sine-Gordon model with a white noise source is investigated. The effect of noise delayed decay is observed for the case of overdamped sine-Gordon equation. It is shown that this noise induced effect, in the range of small noise intensities, vanishes for junction lengths greater than several Josephson penetration lengths.

Magnetization controlled effects in overlap Josephson junctions coupled with submicron magnetic dots

The effect of submicron ferromagnetic particle on the field-dependent critical current of a short overlap Josephson junction is experimentally studied. Strong reversible variations of the maximum critical current are observed depending on the magnetic state of the particle. Pronounced commensurability effects are detected due to the presence of the additional peaks in the magnetic field induced diffraction pattern. The changes in the Fraunhofer pattern of the Josephson junctions are account for the formation of Abrikosov vortices trapped in the electrodes and inducing a phase inhomogeneity in the junction area.

Commensurability effects in overlap Josephson junctions coupled with a magnetic dots array

Experimental observation of the strong influence of an array of ferromagnetic nanodots on the critical current of a short overlap Josephson junction is reported. Pronounced commensurability effects are detected due to the presence of the additional peaks in the magnetic field induced diffraction pattern. The changes in the Fraunhofer pattern of the Josephson junctions are account for by the formation of Abrikosov vortices trapped in the electrodes which induce a phase inhomogeneity in the junction area. PACS numbers: 74.50.+r; 75.75.+a The role played by inhomogeneities in long Josephson junctions is known. In typical experimental realizations, periodic defects of the barrier thickness provide for a spatial modulation of the critical current density jc which results in sharp peaks of the critical current Ic on variation of the external magnetic field H [1]. These ordered peak structures are observed at fields H for which the number of enclosed magnetic flux quanta in the junction is an integer multiple of the number of defects, which is known as commensurability. Over the last years, Josephson junctions with phase inhomogeneities have been intensively investigated. In earlier studies the properties of Josephson junctions with Abrikosov vortices (AVs) pinned in the vicinity of the barrier were discussed by several authors [2, 3]. Recently investigations of unconventional Josephson junctions with a spatially alternating sign of the critical current have attracted renewed attention. The presence of Josephson phase discontinuities results in an unusual current–phase relation [4], a highly anomalous nonFraunhofer Ic(H) dependence [5], and a spontaneous generation of fractional Josephson vortices [6]. Although regular commensurability peaks similar to the found in [1] are quite expected for unconventional junctions, observing them is difficult due to the effect of randomness in the spatial distribution of jc.

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.

Semicircular Josephson junction embedded in a magnetic field

A semicircular geometry is proposed for overlap Josephson junctions for experimental realization. Analytical and numerical studies on semicircular junctions show that an external static magnetic field applied parallel to the dielectric barrier interacts through the interior of the junction and produces a tilted potential which pushes out trapped fluxons from the interior of the junction, and a flux-free state exists in the junction in the absence of an external bias. Due to the semicircular shape, the effective field at the ends of the junction has opposite polarities which support penetration of opposite polarity fluxons into the junction in the presence of a forward-biased dc current. When the direction of the dc current is reversed, flux penetration is not possible and a flux-free state exists in the junction. Thus this geometry can be used in implementing a fluxon-based diode. The rectification properties of the junction are demonstrated using square-wave signals and sinusoidal ac signals. In the forward-biased state, fluxons and antifluxons enter the junction and move in opposite directions. Using this property, we propose and demonstrate a bidirectional flux-flow oscillator.

Magnetic Field Driven Fluxons in Semicircular Josephson Junctions

A semicircular geometry is considered for overlap Josephson junction and theoretical studies show that an external static, homogeneous magnetic field applied parallel to the dielectric barrier can make transitory motion for both fluxons and antifluxons trapped in the junction without the help of a current fed into the junction. In this geometry, the effective magnetic field has opposite polarities at the ends of the junction and due to this, fluxons and antifluxons can be inserted from opposite ends at sufficiently higher magnetic fields. At low magnetic fields, the applied field produces a tilted potential which drives the trapped fluxons. This phenomena can be highly useful in flux flow oscillators and in bidirectional digital transmission lines.

Soliton Creation and Annihilation in Josephson Junction in the Presence of Periodic Perturbation

The soliton creation and annihilation processes are demonstrated numerically, in the long overlap Josephson junction in the first, second and third Zero-Field Step (ZFS) cases, using the perturbed sine-Gordon (sG) equation in the presence of periodic point-like weak inhomogeneities. In all the ZFS cases, the created soliton is found to be in a bunched (congealed) mode with the other solitons. The I–V characteristics, corresponding to the kink dynamics under a dc bias, at which the creation and annihilation phenomena demonstrated is compared with that of an anti-kink dynamics under the same bias condition.

Nucleation of the phase of a finite Josephson junction

We present the results of a study of thermal and quantum nucleation in an overlap Josephson junction with a finite length. There is a critical length that marks the boundary between nucleation with a uniform phase across the junction and nucleation that is concentrated at one of the ends of the junction. In the thermal activation regime, it is shown to be , while at absolute zero, in the quantum tunnelling regime, it is given by . Here, is the effective Josephson penetration depth. The rates for nucleation at the ends of the junction are given for junctions in the thermal activation regime for all lengths, and for junctions undergoing quantum nucleation at absolute zero temperature for lengths less than or equal to the critical length.

Modulation effect on a perturbed sine-Gordon system with a long overlap Josephson junction

We report numerical simulations on the phase locking of solitons generated by rf excitations. In addition to applying a dc magnetic field and an rf signal, we also employed an ac frequency modulation field on a long overlap Josephson junction. The normalized frequency of the rf signal applied on the boundary of the junction is /spl omega//sub 1/=0.214, while that for the low modulation field is /spl omega//sub 2/=0.002. The analysis indicated that the rf signal applying on both ends of the junction will excite two breather modes, and the total energy per unit length of soliton is mostly embodied in the second harmonic waves of /spl omega//sub 1/, based on the Fourier analysis. The effect of the low-frequency magnetic field will excite low-subband mixing waves with or without the presence of the dc magnetic field.

Multi-fluxon zero-field modes in long Josephson tunnel junctions

The higher order zero field steps of long inline and overlap Josephson junctions have been investigated experimentally by means of low temperature scanning electron microscopy. The results indicate that several stable states are possible when more than one fluxon is present in the junction, corresponding to a different spacing among the fluxons during their propagation. These dynamic states should differ by the spectrum of the emitted radiation. Numerical simulations show that the fluxon interaction with plasma oscillations appears to be responsible for the observed behavior.

Analysis of testing the single-fluxon dynamics in a long Josephson junction by a dissipative spot

A change of the I-V characteristics of a long Josephson junction, operating in the zero-field single-fluxon regime, under the action of a ``hot spot'' (e.g., created by a focused electron beam) is calculated analytically by means of the perturbation theory, and also investigated numerically. The change of the average voltage at a given value of the bias current is calculated as a function of the hot spot-position. The overlap Josephson junction geometry is considered in detail, while the inline one is briefly discussed. A good accord between analytical and numerical results is found. The results are relevant for the interpretation of the low-temperature scanning electron microscopy experiments on imaging the fluxon dynamic states in a long Josephson junction.

Long Josephson junctions with uniform dc and rf driving currents: General results for the phase-locked regimes

We investigate the dynamics of a long overlap Josephson junction in the presence of constant and sinusoidally time-varying currents on a finite-length spatial interval imposing open circuit boundary conditions at the ends. We scan the behavior of the system in parameter plane regions where the phase locking between the fluxon oscillations in the junction and the external time-varying field is possible. This scanning is performed for different values of the dc driving term and for lengths of the spatial interval ranging from 3.8 up to 15 and dissipation parameters ranging from 0.025 to 0.25. In spite of the complexity of the system and of the broad range of parameters investigated we find that the response to the rf excitation presents general features that can be well understood in terms of current-voltage characteristics of the junctions (and related experimental parameters). The results are compared with a recent theoretical model based on a perturbation approach of the sine-Gordon Hamiltonian.

Flux-flow steps in overlap Josephson junctions - imaging of different states by low temperature scanning electron microscopy (LTSEM)

Modelling a long Josephson junction with the perturbed sine-Gordon equation one possible solution represents a continous, unidirectional motion of magnetic flux quanta (fluxons). In the dc I-V curve this fluxon motion manifests itself as a current step. Using LTSEM we investigated flux-flow behavior of different types of Josephson junctions. The electron beam acts as a local perturbation influencing the “fluxon chain” in the Josephson junction. For different bias points on the same flux-flow step we observed different patterns inside the junction.

ZERO FIELD STEPS IN AN ANNULAR SYMMETRIC JOSEPHSON JUNCTION

A numerical study of a perturbed sine-Gordon equation (SGE) in an annularly symmetric Josephson junction by considering the cosΦ-term is carried out in this paper. The numerical results show that in the absence of external applied field there are two branches of the zero field steps (ZFS) in the I-V characteristic; the lower voltage ZFS corresponds to the single soliton excitation (1-ZFS) and the higher voltage one corresponds to three solitons exicitation (3-ZFS). In our calculations for the definite size and parameters of the annularly symmetric Josephson junction, the two solitons excitation is unstable. The three solitons excitation is similar to the 3-ZFS with the symmetric mode in the one-dimensional overlap Josephson junction except that there exist amplitude molulation for the time behaviour in both of the voltages 〈φτ〉ρ and φτ(ρ1,τ).

Experimental evidence for supersoliton modes in a long modulated Josephson junction

We report experimental observation of steps in multi-fluxon current-voltage characteristics of a long overlap Josephson junction with a periodic modulation of the critical current density. For the period of the fluxon chain incommensurable with the spatial period of the modulation we observed a low-voltage step in the current-voltage characteristic with the voltage approximately proportional to the incommensurability parameter. This effect is explained in terms of so-called “supersoliton” excitations in spatially modulated Josephson junctions.

Resonance interaction between fluxon and plasma waves in long Josephson junction

The first zero-field step (ZFS1) on the current-voltage characteristic (CVC) of the long overlap Josephson junction with periodically modulated critical current density (CCD) distribution has been investigated analytically and numerically within the framework of perturbed sine-Gordon equation. A set of peaks on ZFS1 has been observed at fixed voltages dependent on the spatial period of CCD distribution. These peaks have been shown to be due to the resonance interaction of a soliton and plasma waves in the junction provided the following condition holds: mW sol = W p , where m is an integer.

An approximate analysis of the I– V characteristic of zero-field singularities in long Josephson junctions

Fluxon propagation in long overlap Josephson junctions in the presence of losses and bias is analyzed. It is shown that losses determine a lower limit of the fluxon contraction. For a linear loss analytical expressions for the energy, the momentum, and the current-voltage relation are given.

Comparison between experiment and perturbation theory for solitons in Josephson junctions

Experiments have been made on long inline and overlap Josephson junctions at various temperatures and current densities. The junctions had parameters such that the recently developed perturbation theory for soliton motion according to the modified sine-Gordon equation should be applicable. A comparison showed that this is the case, and the damping constant was derived as a function of the temperature. In addition, results were obtained for the soliton-antisoliton annihilation process. A fine structure in the zero-field steps at low temperatures is interpreted as being due to plasma oscillations in connection with soliton reflections at the boundaries.