Josephson junctions with centered step and local variation of critical current density

Superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson tunnel junctions based on Nb\Al2O3\Ni\Cu\Nb stacks with a thickness step in the metallic NiCu interlayer were fabricated. The step height of a few 0.1 nm was defined by optical lithography and controlled etching of both Nb and NiCu layers. Experimentally determined junction parameters by current-voltage characteristics and Fraunhofer pattern indicate a uniform NiCu thickness and similar interface transparencies for etched and non-etched parts. The critical current diffraction pattern was calculated and measured for stepped junctions having the same ground phase difference but different critical current densities in both halves. The measured data show a good agreement with simulations.Comment: slight modification

Fabrication of high quality ferromagnetic Josephson junctions

We present ferromagnetic Nb/Al2O3/Ni60Cu40/Nb Josephson junctions (SIFS) with an ultrathin Al2O3 tunnel barrier. The junction fabrication was optimized regarding junction insulation and homogeneity of current transport. Using ion-beam-etching and anodic oxidation we defined and insulated the junction mesas. The additional 2 nm thin Cu layer below the ferromagnetic NiCu (SINFS) lowered interface roughness and ensured very homogeneous current transport. A high yield of junctional devices with jc spreads less than 2% was obtained.Comment: 5 pages, 6 figures; VORTEX IV conference contribution; Submitted to Physica

Optimization of Al/AlOx/AlAl/AlO_x/Al-Layer Systems for Josephson Junctions from a Microstructure Point of View

$Al/AlO_x/Al$-layer systems are frequently used for Josephson junction-based superconducting devices. Although much work has been devoted to the optimization of the superconducting properties of these devices, systematic studies on influence of deposition conditions combined with structural analyses on the nanoscale are rare up to now. We have focused on the optimization of the structural properties of $Al/AlO_x/Al$-layer systems deposited on Si(111) substrates with a particular focus on the thickness homogeneity of the $AlO_x$-tunnel barrier. A standard high-vacuum electron-beam deposition system was used and the effect of substrate pretreatment, different Al-deposition temperatures and Al-deposition rates was studied. Transmission electron microscopy was applied to analyze the structural properties of the $Al/AlO_x/Al$-layer systems to determine the thickness homogeneity of the $AlO_x$ layer, grain size distribution in the Al layers, Al-grain boundary types and the morphology of the $Al/AlO_x$ interface. We show that the structural properties of the lower Al layer are decisive for the structural quality of the whole $Al/AlO_x/Al$-layer system. Optimum conditions yield an epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness variation of only 1.6 nm over more than 10 $\mu m$ and large lateral grain sizes up to 1 $\mu m$. Thickness fluctuations of the $AlO_x$-tunnel barrier are minimized on such an Al layer which is essential for the homogeneity of the tunnel current. Systematic variation of the Al-deposition rate and deposition temperature allows to develop an understanding of the growth mechanisms

Observation of Josephson coupling through an interlayer of antiferromagnetically ordered chromium

The supercurrent transport in metallic Josephson tunnel junctions with an additional interlayer made up by chromium, being an itinerant antiferromagnet, was studied. Uniform Josephson coupling was observed as a function of the magnetic field. The supercurrent shows a weak dependence on the interlayer thickness for thin chromium layers and decays exponentially for thicker films. The diffusion constant and the coherence length in the antiferromagnet were estimated. The antiferromagnetic state of the barrier was indirectly verified using reference samples. Our results are compared to macroscopic and microscopic models.Comment: Phys. Rev. B (2009), in prin

Correlating the nanostructure of Al-oxide with deposition conditions and dielectric contributions of two-level systems in perspective of superconducting quantum circuits

This work is concerned with Al/Al-oxide(AlO$_{x}$)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlO$_{x}$ tunnel barrier. The study is focused on the correlation of the fabrication conditions, nanostructural and nanochemical properties and the occurrence of two-level tunneling systems with particular emphasis on the AlO$_{x}$-layer. Electron-beam evaporation with two different processes and sputter deposition were used for structure fabrication, and the effect of illumination by ultraviolet light during Al-oxide formation is elucidated. Characterization was performed by analytical transmission electron microscopy and low-temperature dielectric measurements. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the layer systems and the properties of two-level tunneling systems. Based on the understanding of the observed structural characteristics, routes are derived towards the fabrication of Al/AlO$_{x}$/Al-layers systems with improved properties.Comment: 28 pages, 4 figure

Memory cell based on a φ\varphi Josephson junction

The $\varphi$ Josephson junction has a doubly degenerate ground state with the Josephson phases $\pm\varphi$. We demonstrate the use of such a $\varphi$ Josephson junction as a memory cell (classical bit), where writing is done by applying a magnetic field and reading by applying a bias current. In the "store" state, the junction does not require any bias or magnetic field, but just needs to stay cooled for permanent storage of the logical bit. Straightforward integration with Rapid Single Flux Quantum logic is possible.Comment: to be published in AP

Phase retrapping in aφJosephson junction: onset of the butterfly effect

We investigate experimentally the retrapping of the phase in a φ Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile U 0 ( ψ ) in φ JJ is a 2 π periodic double-well potential with minima at ψ = ± φ mod 2 π , the question is at which of the two minima − φ or + φ the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the ± φ wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures

Visualizing supercurrents in ferromagnetic Josephson junctions with various arrangements of 0 and \pi segments

Josephson junctions with ferromagnetic barrier can have positive or negative critical current depending on the thickness $d_F$ of the ferromagnetic layer. Accordingly, the Josephson phase in the ground state is equal to 0 (a conventional or 0 junction) or to $\pi$ ($\pi$ junction). When 0 and $\pi$ segments are joined to form a "0-$\pi$ junction", spontaneous supercurrents around the 0-$\pi$ boundary can appear. Here we report on the visualization of supercurrents in superconductor-insulator-ferromagnet-superconductor (SIFS) junctions by low-temperature scanning electron microscopy (LTSEM). We discuss data for rectangular 0, $\pi$, 0-$\pi$, 0-$\pi$-0 and 20 \times 0-$\pi$ junctions, disk-shaped junctions where the 0-$\pi$ boundary forms a ring, and an annular junction with two 0-$\pi$ boundaries. Within each 0 or $\pi$ segment the critical current density is fairly homogeneous, as indicated both by measurements of the magnetic field dependence of the critical current and by LTSEM. The $\pi$ parts have critical current densities $j_c^\pi$ up to 35\units{A/cm^2} at T = 4.2\units{K}, which is a record value for SIFS junctions with a NiCu F-layer so far. We also demonstrate that SIFS technology is capable to produce Josephson devices with a unique topology of the 0-$\pi$ boundary.Comment: 29 pages, 8 figure

Manifestation of the odd-frequency spin-triplet pairing state in diffusive ferromagnet / superconductor junctions

Using the quasiclassical Green's function formalism, we study the influence of the odd-frequency spin-triplet superconductivity on the local density of states (LDOS) in a diffusive ferromagnet (DF) attached to a superconductor. Various possible symmetry classes in a superconductor are considered which are consistent with the Pauli's principle: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity (OSO) state. For each of these states, the pairing state in DF is studied. Particular attention is paid to the study of spin-singlet s-wave and spin-triplet p-wave superconductors as the examples of ESE and ETO superconductors. For spin-singlet case the magnitude of the OTE component of the pair amplitude is enhanced with the increase of the exchange field in DF. When the OTE component is dominant at low energy, the resulting LDOS in DF has a zero energy peak (ZEP). On the other hand, in DF / spin-triplet p-wave superconductor junctions LDOS has a ZEP in the absence of the exchange field, where only the OTE paring state exists. With the increase of the exchange field, the ESE component of the pair amplitude induced in DF is enhanced. Then, the resulting LDOS has a ZEP splitting. We demonstrate that the appearance of the dominant OTE component of the pair amplitude is the physical reason of the emergence of the ZEP of LDOS.Comment: 9 pages, 9 figure

0-pi Josephson tunnel junctions with ferromagnetic barrier

We fabricated high quality Nb/Al_2O_3/Ni_{0.6}Cu_{0.4}/Nb superconductor-insulator-ferromagnet-superconductor Josephson tunnel junctions. Using a ferromagnetic layer with a step-like thickness, we obtain a 0-pi junction, with equal lengths and critical currents of 0 and pi parts. The ground state of our 330 microns (1.3 lambda_J) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-pi step and carrying ~6.7% of the magnetic flux quantum Phi_0. The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field.Comment: submitted to PR