0−πJosephson Tunnel Junctions with Ferromagnetic Barrier

Abstract

We fabricated high quality Nb/Al 2 O 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-π junction, with equal lengths and critical currents of 0 and π parts. The ground state of our 330 µm (1.3λ J ) long junction corresponds to a spontaneous vortex of supercurrent pinned at the 0-π step and carrying ∼ 6.7% of the magnetic flux quantum Φ 0 . The dependence of the critical current on the applied magnetic field shows a clear minimum in the vicinity of zero field. PACS numbers: 74.50.+r, In his classical paper[1] Brian Josephson predicted that the supercurrent through a Josephson junction (JJ) is given by I s = I c sin(µ). Here, µ is the Josephson phase (the difference of phases of the quantum mechanical wave functions describing the superconducting condensate in the electrodes), and I c > 0 is the critical current (maximum supercurrent that one can pass through the JJ). When one passes no current (I s = 0), the Josephson phase µ = 0 corresponds to the minimum of energy (ground state). The solution µ = π corresponds to the energy maximum and is unstable. Later it was suggested that using a ferromagnetic barrier one can realize JJs where. Such junctions obviously have µ = π in the ground state and, therefore, are called π JJs. The solution µ = 0 corresponds to the energy maximum and is unstable.π JJs were recently realized using superconductor-ferromagnet-superconductor (SFS) [3,4,5,6], superconductorinsulator-ferromagnet-superconductor (SIFS) [7] and other [8] technologies. In these junctions the sign of the critical current and, therefore, the phase µ (0 or π) in the ground state, depends on the thickness d F of the ferromagnetic layer and on temperature T [9]. π JJs may substantially improve parameters of various classical and quantum electronic circuits [10,11,12,13,14,15]. To use π JJs not only as a "phase battery", but also as an active (switching) element in various circuits it is important to have a rather high characteristic voltage V c (defined e.g. as V at I = 1.2I c ) and low damping. For example, for classical single flux quantum logic circuits V c defines the speed of operation. For qubits the value of a quasi-particle resistance R qp at V = 0 should be high enough since it defines the decoherence time of the circuits. Both high values of R qp and V c can be achieved by using tunnel SIFS JJs rather than SFS JJs. The dissipation in SIFS JJs decreases exponentially at low temperatures [16], thus, making SIFS technology an appropriate candidate for creating low decoherence quantum circuits, e.g., π qubits. [13,14,15].Actually, the most interesting situation is when one half of the JJ (x < 0) behaves as a 0 JJ, and the other half (x > 0) as a π JJ (a 0-π JJ) [17]: In the symmetric case (equal critical currents and lengths of 0 and π parts) the ground state of such a 0-π JJ corresponds to a spontaneously formed vortex of supercurrent circulating around the 0-π boundary, generating magnetic...

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

“…(17). Nevertheless, these currents should be taken into account in the analysis because they decay significantly slower than I F (ϕ) with increasing L.…”

“…Fig.8 shows the interval of ϕ-state existence, ∆L, in the ideal case of T ≪ T C , γ BM = 0.64 and ε = 0.123. The corresponding set of parameters (17), (35). The solid line is a modulus of the first harmonic amplitude, A, its normal, A N , and ferromagnetic, A F , parts are presented by dashed and dash-dotted lines respectively.…”

Ferromagnetic 0–π Josephson junctions

“…The presence of fractional vortex has been demonstrated experimentally in d-wave superconductor based ramp zigzag junctions [9], in long Josephson 0-π junctions fabricated using the conventional Nb/-Al-Al 2 O 3 /Nb technology with a pair of current injectors [11], in the so-called tricrystal grain-boundary LJJs [8,12,13] or in SFS/SIFS JJs [14,15,16] with stepped ferromagnetic barrier as in Fig. 1.…”

Theoretical Study of Magnetism Induced by Proximity Effect in a Ferromagnetic Josephson Junction with a Normal Metal

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][20][21][22] Here, D F and h ex are the diffusion coefficient and the exchange field in the F, respectively.…”

“…1,2) The proximity effect in s-wave superconductor/ferromagnetic metal (S/F) hybrid junctions provides interesting phenomena which are not observed in S/N hybrid junctions and thus has been extensively studied both theoretically and experimentally. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] In a S/F junction, due to the proximity effect, spin-singlet Cooper pairs (SSCs) penetrate into the F. Because of the exchange splitting of the electronic density of states for up and down electrons, the SSC in the F acquires the finite center-of-mass momentum, and the pair amplitude shows a damped oscillatory behavior with the thickness of the F. [20][21][22] Interesting phenomenon induced by the damped oscillatory behavior of the pair amplitude is a π-state in a S/F/S junction, where the current-phase relation in the Josephson current is shifted by π from that of the ordinary S/I/S and S/N/S junctions (called 0-state). [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]…”