Thin-film Josephson Junctions Research Articles

Dynamic transition of vortices into phase slips and generation of vortex-antivortex pairs in thin film Josephson junctions under dc and ac currents

We present theoretical and numerical investigations of vortices driven by strong dc and ac currents in long Josephson junctions described by a nonlinear integro-differential equation which takes into account nonlocal electrodynamics of films, vortex bremsstrahlung, and Cherenkov radiation amplified by the attraction of vortices to the edges of the junction. This work focuses on the dynamics of vortices in Josephson junctions in thin films where the effects of Josephson nonlocality are essential but London screening is negligible. We obtained an exact solution for a vortex driven by an arbitrary time-dependent current in an overdamped junction where the vortex turns into a phase slip if the length of the junction is shorter than a critical length which depends on current. Our analytical and numerical results show that the dynamic behavior of vortices depends crucially on the ohmic damping parameter. In overdamped junctions vortices expand as they move faster and turn into phase slips as current increases. In underdamped junctions vortices entering from the edges produce Cherenkov radiation generating cascades of expanding vortex-antivortex pairs, which ultimately drive the entire junction into a resistive phase slip state. Simulations revealed a variety of complex dynamic states of vortices under dc and ac currents which can manifest themselves in hysteretic current-voltage characteristics with jumps and regions with negative differential resistance resulting from transitions from oscillating to ballistic propagation of vortices, their interaction with pinning centers, and standing nonlinear waves in the junction.

Microwave resonant activation in hybrid single-gap/two-gap Josephson tunnel junctions

Microwave resonant activation is a powerful, straightforward technique to study classical and quantum systems, experimentally realized in Josephson junction devices cooled to very low temperatures. These devices typically consist of two single-gap superconductors separated by a weak link. We report the results of the first resonant activation experiments on hybrid thin film Josephson junctions consisting of a multi-gap superconductor (MgB2) and a single-gap superconductor (Pb or Sn). We can interpret the plasma frequency in terms of theories both for conventional and hybrid junctions. Using these models, we determine the junction parameters including critical current, resistance, and capacitance and find moderately high quality factors of Q0∼ 100 for these junctions.

Effect of current injection into thin-film Josephson junctions

New thin-film Josephson junctions have recently been tested in which the current injected into one of the junction banks governs Josephson phenomena. One thus can continuously manage the phase distribution at the junction by changing the injected current. A method of calculating the distribution of injected currents is proposed for a half-infinite thin-film strip with source-sink points at arbitrary positions at the film edges. The strip width $W$ is assumed small relative to $\Lambda=2\lambda^2/d$, $\lambda$ is the bulk London penetration depth of the film material, $d$ is the film thickness.

Manipulating Josephson junctions in thin-films by nearby vortices

It is shown that a vortex trapped in one of the banks of a planar edge-type Josephson junction in a narrow thin-film superconducting strip can change drastically the dependence of the junction critical current on the applied field, Ic(H). When the vortex is placed at certain discrete positions in the strip middle, the pattern Ic(H) has zero at H=0 instead of the traditional maximum of ‘0-type’ junctions. The number of these positions is equal to the number of vortices trapped at the same location. When the junction–vortex separation exceeds ∼W, the strip width, Ic(H) is no longer sensitive to the vortex presence. The same is true for any separation if the vortex approaches the strip edges.

Frequency Characteristics of Misaligned Tl2Ba2CaCu2O8 Thin-Film Intrinsic Josephson Junction Arrays Irradiated by Millimeter-Wave

We have investigated the millimeter wave irradiation characteristics of misaligned Tl2Ba2CaCu2O8 thin-film intrinsic Josephson junction arrays at a liquid nitrogen temperature in an antenna system by both simulation and experiments. The dielectric substrate was regarded as a dielectric resonance antenna to improve high-frequency electromagnetic coupling between the intrinsic Josephson junction array and a horn antenna. A useful model for simulating the microwave system was devised to demonstrate and analyze the optimization of the irradiation in the antenna system. The electric field distribution in the antenna system was computed and displayed. Also, the near-field and far-field frequency characteristics of the receiving antenna and transmitting antenna were calculated and analyzed to study the mechanism of the irradiation. In the experiment, by detecting the suppressed critical current of the IJJA, an optimum transmission frequency range was measured and the frequency characteristic was studied in the transmitting and receiving antenna system. The critical current of the IJJA was suppressed to 37 % and the optimum coupling effect was achieved at 74.8 GHz. And the electromagnetic simulation matched up well with the experimental result. Potential reasons of the acceptable nuances between the simulation and experiment results were also taken into account. The influence the size of the microbridge had on the frequency characteristic of the dielectric antenna were elaborately discussed.

Millimeter-Wave Radiation From Misaligned $ \hbox{Tl}_{2}\hbox{Ba}_{2}\hbox{CaCu}_{2}\hbox{O}_{8}$ Thin-Film Intrinsic Josephson Junctions

We have measured the current-voltage characteristics of misaligned Tl2Ba2CaCu2O8 thin-film intrinsic Josephson junctions (IJJs) embedded in a quasi-optical resonator at 78.5 K. The substrate is regarded as a dielectric resonator to improve high-frequency electromagnetic coupling between the IJJs. The experimental result is fitted well with the electromagnetic simulation. Radiation power larger than 10 pW has been detected at 75.6 GHz. Moreover, the phenomenon of jump voltage is explored, and a possible explanation is discussed.

Radiation from Intrinsic Josephson Junctions in Misaligned Tl2Ba2CaCu2O8 Thin Films

We measure the current-voltage (I-V) curves of intrinsic Josephson junctions (IJJs) in misaligned Tl2Ba2CaCu2O8 thin films embedded in a Fabry-P,rot (F-P) resonator at liquid nitrogen temperature. Regarding the substrate as a dielectric resonator, electromagnetic coupling between the intrinsic Josephson junctions is improved. By adjusting the location of the substrate in the F-P resonator, the critical current of the IJJs under microwave irradiation (75.6 GHz) is suppressed almost to zero. Two pairs of symmetrical radiation (75.6 GHz) peaks are detected. Possible explanations for this experimental result are discussed.

Niobium dc SQUIDs with Nanobridge Junctions

Abstract DC SQUIDs with outer loop size ∼10 μmx 10 μm have been fabricated with nanobridge Josephson junctions in thin Niobium films (25, 100 and 200 nm) using either electron beam or focused ion beam techniques. Such SQUIDs are ideally suited to incorporate into the centre strip of a superconducting coplanar waveguide. The current-voltage characteristics and magnetic field modulation of the SQUIDs are compared for the two methods. The temperature dependence of the critical current is quite linear for the e-beam devices whilst the focused ion beam devices fit an exponential. Here we also investigate trimming methods to reduce the critical currents of the nanojunctions fabricated in the thicker films, in order to improve the SQUID parameters at lower temperatures.

Internal degrees of freedom, long-range interactions and nonlocal effects in perturbed Klein–Gordon equations

We investigate different mechanisms for the excitation of soliton internal degrees of freedom and for the existence of long-range interactions between solitons. We will study a nonlocal Klein–Gordon equation that is used as a model for Josephson junctions in thin films. We will show the connections between nonlocality, nonlinearity, internal degrees of freedom, long-range interactions and power-law behaviors.

Intrinsic Josephson Junctions in Bi-2212 Thin Films Fabricated by Metal-Organic Decomposition

We fabricated a mesa <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10\ \mu{\rm m}\times 10\ \mu{\rm m}$</tex></formula> in area and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim$</tex></formula> 150 nm in height on a single grain in a 240-nm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{8+\delta}/{\rm MgO}$</tex></formula> thin-film grown by metal-organic decomposition (MOD) with face-to-face annealing. A multi-branch structure peculiar to intrinsic Josephson junctions (IJJs) was observed in the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I-V$</tex> </formula> characteristics of the fabricated mesa, and this structure was characterized by approximately 100 quasiparticle branches with critical current densities of 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm kA/cm}^{2}$</tex> </formula> and voltage-jump intervals of 20 mV. Moreover, the temperature dependence of the critical current was in good agreement with a theoretical result based on <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$c$</tex> </formula> -axis Josephson tunneling in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$d$</tex></formula> -wave superconductors. These results imply that the thin films grown by MOD have sufficient quality to fabricate IJJs for practical terahertz applications.

Differential Conductance Measurements of ${\rm MgB}_{2}$-Based Josephson Junctions Below 1 Kelvin

Magnesium diboride has many intriguing characteristics, including its relatively high critical temperature and two-band nature. Most prior studies of MgB2 thin film Josephson junctions have been conducted above 2 Kelvin. We report results of sub-1 Kelvin experiments of MgB2/insulator/Pb junctions whose a-b plane is exposed for electron tunneling. By measuring differential conductance at low temperature, new details in the structure of the sigma- and pi-band gaps are observed in this data, consistent with theoretical predictions.

Millimeter wave irradiation characteristics of high temperature superconductor bicrystal Josephson junction embedded in a Fabry-Perot resonator

The millimeter wave irradiation characteristics of a Tl-2212 high-temperature superconducting thin film bicrystal Josephson junction embedded in a hemispherical Fabry-Perot resonator was studied. The coupling characteristics of the bicrystal Josephson junction with the Fabry-Perot resonator have been investigated experimently, showing that it was greatly affected by the factors such as the location of the sample in the open resonator, the angle between them, the pattern of the junction and the distance between the two reflecting mirrors of the resonator. The optimized coupling between Josephson junction and applied microwave was achieved by refined adjustment of the factors. In this case, 9 orders distinct Shapiro steps were observed. Taking advantage of the Fabry-Perot resonator technique, the coupling problem between high-temperature superconducting thin film Josephson junction and applied millimeter wave was well solved, which is of significance for the research on radiation and detection of millimeter wave and terahertz wave of high-temperature superconducting Josephson junction.

Narrow Josephson junctions in thin films

We consider the edge-type Josephson junctions in thin films, for which the stray fields significantly affect the screening and tunneling currents. It is demonstrated that the spatial distribution of the phase difference ϕ across thin-film Josephson junctions is nonlocal. We find that in the limit of weak tunneling and short junctions the phase difference ϕ is a universal function. This function is proportional to the applied field H an depends only on the junction geometry. In the case of narrow thin strips we find this dependence analytically. Using this universal function we demonstrate that the maximum supercurrent across narrow junctions in thin films decays as 1/, that is much slower than 1/H for bulk junctions.

Thin-film Josephson junctions with alternating critical current density

We study the field dependence of the maximum current ${I}_{m}(H)$ in narrow edge-type thin-film Josephson junctions with alternating critical current density. ${I}_{m}(H)$ is evaluated within nonlocal Josephson electrodynamics taking into account the stray fields that affect the difference of the order-parameter phases across the junction and therefore the tunneling currents. We find that the phase difference along the junction is proportional to the applied field, depends on the junction geometry, but is independent of the Josephson critical current density ${g}_{c}$, i.e., it is universal. An explicit form for this universal function is derived for small currents through junctions of the width $W⪡\ensuremath{\Lambda}$, the Pearl length. The result is used to calculate ${I}_{m}(H)$. It is shown that the maxima of ${I}_{m}(H)\ensuremath{\propto}1/\sqrt{H}$ and the zeros of ${I}_{m}(H)$ are equidistant but only in high fields. We find that the spacing between zeros is proportional to $1/{W}^{2}$. The general approach is applied to calculate ${I}_{m}(H)$ for a superconducting quantum interference device with two narrow edge-type junctions. If ${g}_{c}$ changes sign periodically or randomly, as it does in grain boundaries of high-${T}_{c}$ materials and superconductor-ferromagnet-superconductor heterostructures, ${I}_{m}(H)$ not only acquires the major side peaks, but due to nonlocality the following peaks decay much slower than in bulk junctions.

Edge-type Josephson junctions in narrow thin-film strips

We study the field dependence of the maximum current ImH in narrow edge-type thin-film Josephson junctions. We calculate ImH within nonlocal Josephson electrodynamics taking into account the stray fields. These fields affect the difference of phases of the order parameter across the junction and therefore the tunneling currents. We find that the phase difference along the junction is proportional to the applied field, depends on the junction geometry, but is independent of the Josephson critical current density, i.e., it is universal. An explicit formula for this universal function is derived and used to calculate ImH. It is shown that the maxima of ImH1 /H and the zeros of ImH are equidistant only in high fields. We find that the spacing between the zeros is proportional to 1 /w 2 , where w is the width of the junction. The general approach is applied to calculate ImH for a superconducting quantum interference device SQUID with two narrow edge-type junctions.

Improvement of the homo-biepitaxial YBCO film fabrication process on Yttrium Stabilized Zirconia

We present the results of the application of the dc magnetron sputtering modulation mode to fabricate YBCO homo-biepitaxial thin film Josephson junctions on Yttrium Stabilized Zirconia substrate. A 45° in-plane rotation of the grown YBCO films with respect to the substrate orientation is realized by using a thin YBCO buffer layer deposited at a temperature (∼600 °C) lower than the optimal value (800 °C). The control of the in-plane orientation allows to form Josephson junctions between two YBCO superconducting region of the film with and without buffer layer. We obtained that the modulated deposition mode improves the crystalline in-plane orientation of YBCO on buffer layer/YSZ substrate, increases the grain size and decreases the film stress. The improved homo-epitaxial process was applied for fabrication of dc SQUID devices.

High Temperature Superconducting Thin Film Josephson Junctions

High-temperature superconducting YBa2Cu3Ox (YBCO) thin films (120 nm thick) were used to prepare YBCO/B/YBCO Josephson ramp-type junctions with tunneling barrier (B) performed by Ar ion beam modification of the ramp area surface. After the sample transportation into a high vacuum deposition chamber the amorphised ramp surface was annealed and then the YBCO counter electrode was sputtered. The area of the ramp-type YBCO junction was determined by a photoresist mask and Ar ion beam etching. Current voltage characteristics and current steps during irradiation of the samples by microwaves confirmed the presence of Josephson effect in the made structures.

Sub-micron thin film intrinsic Josephson junctions

We have fabricated sub-micron intrinsic Josephson junctions in thin films of Tl-Ba-Ca-Cu-O using two differing techniques suited to different applications. By using lateral focussed ion-beam milling we have created arrays of intrinsic junctions in c-axis oriented films. Such arrays, with areas as low as 0.25 /spl mu/m/sup 2/, display large hysteresis comparable to that observed in single-crystal intrinsic junctions. By using normal focussed ion-beam milling we have created arrays in mis-aligned films grown on vicinal substrates. In arrays of area less than 0.4 /spl mu/m/sup 2/ we observe Josephson phase diffusion and a suppressed critical current, showing that charging effects may be significant in these junctions.

Far-infrared broadband measurements with Hilbert spectroscopy

A principal possibility of broadband far-infrared (FIR) spectral measurements with Hilbert-transform technique, based on ac Josephson effect, has been experimentally studied. High-pressure 200 W mercury arc lamp was used as a source of FIR broadband radiation. The grain-boundary YBaCuO thin-film Josephson junctions (JJ) with the normal-state resistance R n =1–3 Ω and the I c R n-product up to 1 mV were used in the experiments. Broadband spectra with different FIR mesh-type filters have been measured by this technique. First results on the detection of gas absorption spectra in FIR range with Hilbert spectroscopy (HS) were obtained. Several lines of CO absorption spectrum in the range from 300 up to 1000 GHz have been resolved, when broadband blackbody radiation passed through a gas cell.

Nonlocal electrodynamics of Josephson junctions in thin films and fractional vortices

The phase difference φ across a Josephson junction is considered for a film with a thickness d ≪ λ, where λ is the London penetration depth in the superconducting banks. Special attention is given to the case of a critical current density jc varying along the junction. It is shown that a nonlinear integro-differential equation determines the spatial distribution of φ for d ≪ λ. Josephson properties of grain boundaries in thin-YBCO films are treated for the case of jc alternating along these boundaries. It is shown that if the typical amplitude of alternations of jc is high compared to the average value of jc, then a spontaneous flux and two types of fractional Josephson vortices can be observed. The fractional Josephson vortices keep magnetic fluxes ϕ1 and ϕ2, where ϕ1 + ϕ2 = ϕ0, ϕ0 is flux quantum, and ϕ1 < ϕ0/2, ϕ2 > ϕ0/2. We demonstrate that these fractional vortices can be observed in thin-YBCO films under conditions of appearance of the spontaneous magnetic flux. A method is proposed to extract the fractional vortices from the experimental flux patterns. Propagation of an electromagnetic wave along a grain boundary with an alternating critical current density is treated as an example of an application of the integro-differential equation for the phase difference φ.