Weak Link Region Research Articles

Electrostatic Control of Phase Slips in Ti Josephson Nanotransistors

The investigation of the switching current probability distribution of a Josephson junction is a conventional tool to gain information on the phase slips dynamics as a function of the temperature. Here we adopt this well-established technique to probe the impact of an external static electric field on the occurrence of phase slips in gated all-metallic titanium (Ti) Josephson weak links. We show, in a temperature range between 20 mK and 420 mK, that the evolution of the phase slips dynamics as a function of the electrostatic field starkly differs from that observed as a function of the temperature. This fact demonstrates, on the one hand, that the electric field suppression of the critical current is not simply related to a conventional thermal-like quasiparticle overheating in the weak-link region. On the other hand, our results may open the way to operate an electrostatic-driven manipulation of phase slips in metallic Josephson nanojunctions, which can be pivotal for the control of decoherence in superconducting nanostructures.

Comparison of magnetisation and transport properties of processed La<SUB align="right">0.67Ca<SUB align="right">0.33MnO<SUB align="right">3 manganites for technological application

A lot of efforts are being made to look for suitable types of colossal magnetoresistance (CMR) materials with specific composition for technological applications. We have synthesised and performed the magnetisation and transport studies on La0.67Ca0.33MnO3 CMR polycrystalline materials prepared using melt and sintering technique. It is observed that the magnetoresistance is enhanced in partially melted sample with increase in insulator-to-metal transition (TMI) temperature enabling its possible applications at room temperature. This indicates the high degree of spin polarisation of the conduction electrons in partially melted sample. It has been observed that grain boundaries disrupt the local magnetic order and alter the electrical conduction of polarised electrons whereas the magnetisation behaviour is not significantly affected except the magnitude of the magnetisation. This suggests that the magnetic spin alignment in the weak link region of the sintered sample increases the absolute magnetisation. The melt processed method is the ideal method to achieve superior magnetic and transport properties for possible major technological application of colossal magnetoresistive devices.

Semiconductor-Nanowire-Based Superconducting Qubit.

We introduce a hybrid qubit based on a semiconductor nanowire with an epitaxially grown superconductor layer. Josephson energy of the transmonlike device ("gatemon") is controlled by an electrostatic gate that depletes carriers in a semiconducting weak link region. Strong coupling to an on-chip microwave cavity and coherent qubit control via gate voltage pulses is demonstrated, yielding reasonably long relaxation times (~0.8 μs) and dephasing times (~1 μs), exceeding gate operation times by 2 orders of magnitude, in these first-generation devices. Because qubit control relies on voltages rather than fluxes, dissipation in resistive control lines is reduced, screening reduces cross talk, and the absence of flux control allows operation in a magnetic field, relevant for topological quantum information.

Persistent currents supported by solitary waves in toroidal Bose-Einstein condensates

We analyze the generation of persistent currents in Bose-Einstein condensates of ultracold gases confined in a ring. This phenomenon has been recently investigated in an experiment [Nature \textbf{506}, 200 (2014)], where hysteresis loops have been observed in the activation of quantized persistent currents by rotating weak links. In this work, we demonstrate the existence of 3D stationary currents with non-quantized angular momentum. They are generated by families of solitary waves that show a continuous variation in the angular momentum, and provide a bridge between different winding numbers. We show that the size of hysteresis loops is determined by the range of existence within the weak link region of solitary waves which configure the energy barrier preventing phase slips. The barrier vanishes when the critical rotation leads winding numbers and solitonic states to a matching configuration. At this point, Landau and Feynman criteria for phase slips meet: the fluid flow reaches the local speed of sound, and stationary vortex lines (which are the building blocks of solitons) can be excited inside the system.

Josephson effect in SIFS tunnel junctions with domain walls in the weak link region

We study theoretically the properties of SIFS type Josephson junctions composed of two superconducting (S) electrodes separated by an insulating layer (I) and a ferromagnetic (F) film consisting of periodic magnetic domains structure with antiparallel magnetization directions in neighboring domains. The two-dimensional problem in the weak link area is solved analytically in the framework of the linearized quasiclassical Usadel equations. Based on this solution, the spatial distributions of the critical current density, J C , in the domains and critical current, I C , of SIFS structures are calculated as a function of domain wall parameters, as well as the thickness, d F , and the width, W, of the domains. We demonstrate that I C (d F , W) dependencies exhibit damped oscillations with the ratio of the decay length, ξ1, and oscillation period, ξ2, being a function of the parameters of the domains, and this ratio may take any value from zero to unity. Thus, we propose a new phys-ical mechanism that may explain the essential difference between ξ1 and ξ2 observed experimentally in various types of SFS Josephson junctions.

Axion mass estimates from resonant Josephson junctions

Recently it has been proposed that dark matter axions from the galactic halo can produce a small Shapiro step-like signal in Josephson junctions whose Josephson frequency resonates with the axion mass [C. Beck, PRL 111, 231801 (2013)]. Here we show that the axion field equations in a voltage-driven Josephson junction environment allow for a nontrivial solution where the axion-induced electric current manifests itself as an oscillating supercurrent. The linear change of phase associated with this nontrivial solution implies the formal existence of a large magnetic field in a tiny surface area of the weak link region of the junction which makes incoming axions decay into microwave photons. We derive a condition for the design of Josephson junction experiments so that they can act as optimum axion detectors. Four independent recent experiments are discussed in this context. The observed Shapiro step anomalies of all four experiments consistently point towards an axion mass of $(110 \pm 2)\mu $eV. This mass value is compatible with the recent BICEP2 results and implies that Peccei-Quinn symmetry breaking was taking place after inflation.

Critical current of SF-NFS Josephson junctions

The properties of SF-NFS sandwiches composed of two superconducting (S) electrodes separated by a weak-link region formed by a normal-metal (N) step with the thickness d N situated on the top of a lower S electrode and a ferromagnetic (F) layer with the thickness d F deposited onto the step and the remaining free surface of the lower electrode have been studied theoretically. It has been shown in the approximation of linearized semiclassical Usadel equations that the two-dimensional problem in the weak-link region can be reduced to two one-dimensional problems in its SFS and SNFS segments. The spatial distributions of the critical current density Jc in the segments as a function of the layer thickness d F have been calculated. The dependences of the critical current Ic of the structure on the magnitude of the magnetization vector M of the ferromagnetic layer have been found for various directions of the magnetization within the junction plane. It has been shown that these dependences are affected considerably by both the orientation of M and the spatial distribution of Jc.

Phase slips and vortex dynamics in Josephson oscillations between Bose-Einstein condensates

We study the relation between Josephson dynamics and topological excitations in a dilute Bose-Einstein condensate confined in a double-well trap. We show that the phase slips responsible for the self-trapping regime are created by vortex rings entering and annihilating inside the weak-link region or created at the center of the barrier and expanding outside the system. Large amplitude oscillations just before the onset of self-trapping are also strictly connected with the dynamics of vortex rings at the edges of the inter-well barrier. Our results extend and analyze the dynamics of the vortex-induced phase slippages suggested a few decades ago in relation to the “ac” Josephson effect of superconducting and superfluid helium systems.

Grain boundary junctions of FeSe0.5Te0.5 thin films on SrTiO3 bi-crystal substrates

Grain boundary junctions were fabricated in the epitaxial FeSe0.5Te0.5 thin films on [001] tilt SrTiO3 bi-crystal substrates with a CeO2 buffer layer. Critical current densities across the junctions with different mis-orientation angles of 4°, 7°, 15°, and 24° were measured at magnetic fields up to 30 T. It was found that the 4° and 7° junctions carry critical current densities comparable to that of the intra-grain film while those of the 15° and 24° junctions were suppressed drastically. A critical mis-orientation angle of around 9° was identified that separates the strong coupling region from the weak link region. We found that the critical current densities across the grain boundary with a 24° mis-orientation angle are modulated by the magnetic field, indicating a Josephson Effect. This junction is estimated to be in the intermediate-size regime with an effective transverse junction width L ∼ 2.6–2.8 μm and a Josephson penetration depth λJ ∼ 1.2 μm.

Gate-tuned Josephson effect on the surface of a topological insulator.

In the study, we investigate the Josephson supercurrent of a superconductor/normal metal/superconductor junction on the surface of a topological insulator, where a gate electrode is attached to the normal metal. It is shown that the Josephson supercurrent not only can be tuned largely by the temperature but also is related to the potential and the length of the weak-link region. Especially, the asymmetry excess critical supercurrent, oscillatory character, and plateau-like structure have been revealed. We except those phenomena that can be observed in the recent experiment.

Scaling of Structural Characteristics of Gelled Model Waxy Oils

The structural properties of gelled model waxy oils with different wax concentrations (5–20 wt %) were investigated through a differential scanning calorimetry (DSC) test, rheological measurement, and scaling model for colloidal gels. The wax precipitation curves obtained from the DSC test show that the concentration of precipitated wax crystals φW increases gradually with the decrease of the temperature, and most of the waxes dissolved in the oil phase precipitate at −20 °C. The gelation point increases gradually with an increasing wax concentration from 22 °C at 5 wt % to 26 °C at 10 wt %, then to 32 °C at 15 wt %, and 34 °C at 20 wt %. The structure of gelled waxy oils, similar to the structure of colloidal gels, transits from a strong-link regime to a weak-link regime with the increase of φW, and the value of φW at the transition point is around 1.8 wt % for all of the tested model waxy oils. In the strong-link region, G′E increases while γE decreases with an increasing φW. In the weak-link region, both G′E and γE increase with an increasing φW. The ln G′E ∼ ln φW and ln γE ∼ ln φW relations can be divided into three parts at wax concentrations of ≤10 wt %; at wax concentrations of 15–20 wt %, the relations can be divided into two parts. In the strong-link region, the fractal dimension D of gelled waxy oil was calculated through parameters A and B. In the weak-link region, the fractal dimension D was calculated through parameter B because of the good linear relationship between ln γE and ln φW. The fractal dimension D increases from very small values (less than 1) to high values (approaching 3) with an increasing φW, indicating the continuous development of the microstructure of the gelled waxy oils with an increasing φW.

Studies on the Structural Characteristics of Gelled Waxy Crude Oils Based on Scaling Model

The structural characteristics of three gelled waxy crude oils were studied through DSC analysis, rheological test, and a scaling model for colloidal gels. The total wax content is 8.1 wt % for crude oil A, 11.5 wt % for crude oil B, and 17.3 wt % for crude oil C. The concentration of precipitated wax crystals, φW increases gradually with the decrease of temperature. The gelation point, which increases with the increase of wax content, is 16.5 °C for crude oil A, 26 °C for crude oil B, and 41 °C for crude oil C. The elastic modulus in linear region, GE′ of the gelled crude oils always increases with decreasing temperature while the critical linear elastic strain, γE of the gelled crude oils first decreases with decreasing temperature, then increases with the further decrease of temperature. The microstructure of gelled crude oils, which can be divided into strong-link region and weak-link region, is similar to that of colloidal gels. In the strong-link region, the GE′ increases but the γE decreases with i...

Josephson φ-junctions based on structures with complex normal/ferromagnet bilayer

We demonstrate that Josephson devices with a nontrivial phase difference 0 < φg < π in the ground state can be realized in structures composed of longitudinally oriented normal metal (N) and ferromagnet (F) films in the weak link region. Oscillatory coupling across the F-layer makes the first harmonic in the current–phase relation relatively small, while coupling across the N-layer provides a negative sign of the second harmonic. To derive quantitative criteria for a φ-junction, we have solved the two-dimensional boundary-value problem in the frame of Usadel equations for overlap and ramp geometries of S–NF–S structures. Our numerical estimates show that φ-junctions can be fabricated using up-to-date technology.

Nb Lateral Josephson Junction Induced by Inverse Proximity Effect With NiFe

We utilize the inverse proximity effect in superconductor-ferromagnet bilayer to generate lateral Josephson junctions. The weak link is created by a ferromagnetic NiFe strip across a superconductor Nb bridge due to the inverse proximity effect. The critical currents of the junctions were measured at different temperatures and magnetic fields. The junctions exhibit a modulation of the critical current in perpendicular magnetic field similar to a Fraunhofer interference pattern which proves the dc Josephson effect. The weak link region induced by the NiFe in Nb persists down to 2 K.

Josephson effect in superconductor/ferromagnet structures with a complex weak-link region

The critical currents ${I}_{C}$ of SNF-FN-FNS, SN-FN-NS, and SNF-N-FNS Josephson junctions (S---superconductor, F---ferromagnetic, N---normal metal) with complex SNF or SN electrodes (N or NF bilayer are situated under a superconductor) are calculated in the framework of linearized Usadel equations for arbitrary overlap length $d$ of SN interface. We demonstrate that in these geometries, in the case of large resistances of SN interfaces, the critical current can exceed that in ramp-type junctions. Based on these results, the choice of the most practically applicable geometry is discussed. We predict that in a certain parameter range there is single $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition with the increase in the overlap length $d$. This single transition can be realized also in SFN-N-FNS Josephson junctions, where the coherence length in the weak-link region is a real quantity. Further, we predict that in SNF-N-FNS Josephson junctions $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition may take place with increase in distance between superconducting electrodes.

Pinning of spiral fluxons by giant screw dislocations in YBa2Cu3O7 − δ single crystals: Josephson analog of the fishtail effect

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in YBa_2Cu_3O_7 single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting a=3 spiral turnings, the pitch b=18.7 microns and the step height c=1.2nm (the last parameter is responsible for creation of extended weak-link structure around the giant defects). The magnetic field applied parallel to the surface enters winding around the weak-link regions of the screw in the form of the so-called spiral Josephson fluxons characterized by the temperature dependent pitch b_f(T). For a given temperature, a stabilization of the fluxon structure occurs when b_f(T) matches b (meaning an optimal pinning by the screw dislocations) and manifests itself as a pronounced low-field peak in the dependence of the susceptibility on magnetic field (applied normally to the surface) in the form resembling the high-field (Abrikosov) fishtail effect.

AC susceptibility studies of inter-grains in Hg-1223 superconductors

High-Tc ceramics tend to lower its quality by the aging effect. The main cause of the degradation is considered to originate in the weak link region between the superconducting grains. The preservation and recovery of superconductivity by reinforcement of the grain boundary is an important issue for high-Tc application.A quantitative analysis of the contribution due to the grain and the link is necessary and the grained Bean model is proposed, where the superconducting phases are immersed in the matrix link superconductor. Difference of the superconducting characteristics of the grain, the link and grain content factor give a variety of deformation on the AC susceptibility curves. Comparing the observed data with the numerically computed model allows more clear insight on the grain and inter-grain structures.

Transition from the 0 state to the π state in S-FNF-S Josephson structures

The critical current I C of S-FNF-S Josephson structures has been calculated as a function of the distance between the superconducting (S) electrodes, L, via the Usadel semiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between the F and N films, both the typical decrease scale of the critical current and the period of its oscillations to lengths of the scale ξN can be much larger than the respective quantities for the SFS junctions. Moreover, this interaction changes both the magnitude and sign of the critical current. It has been shown that the critical current in a structure with the collinear magnetization vectors of the films can be significantly different from the critical current in a structure with the antiparallel magnetization of the F films.

Effective decrease in the exchange energy in S-(FN)-S Josephson structures

The critical current Ic of S-(FN)-S Josephson structures has been calculated as a function of the distance L between superconducting (S) electrodes using the Usadel quasiclassical equations for the case of specifying the supercurrent in the direction parallel to the interface between the ferromagnetic (F) and normal (N) films of the composite weak-link region. It has been shown that, owing to the interaction between F and N films, both the typical decrease scale Ic(L) and the period of the critical current oscillations can be much larger than the respective quantities for the SFS junctions. The conditions have been determined under which these lengths are on the order of the effective depth ζN of superconductivity penetration to a normal metal.

Transport and interface properties in ramp-type Josephson junctions with a graded Y 1− xPr xBa 2Cu 3O y barrier

High T c ramp-type Josephson junctions have been successfully fabricated using a continually graded barrier Y 1− x Pr x Ba 2Cu 3O y . In these junctions the Josephson coupling occurs at the naturally formed S/N (superconducting/normal) interfaces within the Y 1− x Pr x Ba 2Cu 3O y layer. Such a barrier can lead to a highly transparent S/N boundary hence to greatly enhance the performance of junction. In contrast to most high- T c junctions, the properties of our junctions would be dominated by the barrier material rather than the boundary as such a boundary is absent in the weak link region. A unique feature of this novel barrier structure is that the effective thickness of the barrier can be varied even after fabrication, depending on the measuring temperature and the Pr-concentration gradient. It could make the transport process and the Josephson coupling differ from that of the conventional junctions. The temperature dependences of the barrier thickness and Josephson properties were investigated and compared with those of junctions with a conventional single barrier. Low values of proximity effect parameters found in our junctions show an advantage of this barrier structure.