Semifluxon Molecule under Control

Andreas Dewes, Tobias Gaber, Dieter Koelle, Reinhold Kleiner, and Edward Goldobin

Phys. Rev. Lett. 101, 247001 – Published 9 December 2008

Abstract

Josephson junctions with a phase drop of $π$ in the ground state allow us to create vortices of supercurrent carrying only half of the magnetic flux quantum $Φ_{0} \approx 2.07 \times 10^{- 15} Wb$ . Such semifluxons have twofold degenerate ground states denoted $↑$ (with flux $+ Φ_{0} / 2$ and supercurrent circulating clockwise) and $↓$ (with flux $- Φ_{0} / 2$ and supercurrent circulating counterclockwise). We investigate a molecule consisting of two coupled semifluxons in a 0- $π$ -0 long Josephson junction. The fluxes (polarities) of semifluxons are measured by two on-chip SQUIDs. By varying the dc bias current applied to the 0- $π$ -0 junction, we demonstrate controllable manipulation and switching between two states, $↑ ↓$ and $↓ ↑$ , of a semifluxon molecule. These results provide a major step towards employing semifluxons as bits or qubits for classical and quantum digital electronics.

Received 5 September 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.247001

Authors & Affiliations

Andreas Dewes^*, Tobias Gaber, Dieter Koelle, Reinhold Kleiner, and Edward Goldobin^†

Physikalisches Institut & Center for Collective Quantum Phenomena, Universität Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen, Germany

^*dewes@uni-tuebingen.de
^†gold@uni-tuebingen.de

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Vol. 101, Iss. 24 — 12 December 2008

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Images

Figure 1
(a) Sketch of the experimental circuit—a 0- $π$ -0 LJJ, posessing two degenerate ground states $↑ ↓$ (red) and $↓ ↑$ (black). The red and black curves were calculated numerically and represent the local magnetic field $\propto d μ (x) / d x$ . The magnetic field located at each phase discontinuity is coupled to and measured by a dc SQUID. (b) Optical microscope image of one of the investigated samples. Two current injector pairs are attached symmetrically to the top superconducting electrode at a distance $a$ (different from sample to sample). The widths $Δ w$ of each injector and the distance $Δ x$ between them is $Δ w \approx Δ x \approx 2 μ m$ . Two dc SQUIDs are placed in front of the phase discontinuities to read out the value of the flux localized at them. Each SQUID is coupled to the long JJ by using a superconducting flux transformer (pickup loop in series with a single turn input coil).Reuse & Permissions
Figure 2
(a) $I_{c \pm} (I_{inj})$ dependences measured experimentally. The injector current of $I_{inj} \approx 3.8 mA$ corresponds to the point $κ = π$ . (b) Dependence of the left SQUID voltage $V_{L}$ on $I_{inj}$ measured experimentally at $I = 0$ . $V_{R}$ (not shown) jumps at the same values of $I_{inj}$ . Both $V_{L, R} (I_{inj})$ are not periodic as there is a little parasitic coupling between $I_{inj}$ and the SQUIDs.Reuse & Permissions
Figure 3
(a) Voltage $V (I)$ across the JJ at $I_{inj} = 3.8 mA$ ( $κ = π$ ) and (b) corresponding dc SQUID voltages $V_{L} (I)$ and $V_{R} (I)$ , measured simultaneously. First, the bias current was swept with overcritical amplitude [light red and light blue reference curves in (b), black curve in (a)] and then with undercritical amplitude [red and blue curves in (b), not shown in (a) as $V (I) = 0$ in this case]. The small interval of bias currents around $I = 0$ for which the JJ was in the Meissner state is cut off from the reference curves in (b). Graphs (c) and (d) show recovered $Φ_{L}^{\exp } (I)$ (red) and $Φ_{R}^{\exp } (I)$ (blue) dependences and their comparison with numerically simulated $Φ_{L}^{sim} (I)$ (gold) and $Φ_{R}^{sim} (I)$ (green) curves.Reuse & Permissions
Figure 4
(a) The $I_{c} (I_{inj})$ dependence measured using a voltage criterion algorithm in the vicinity of $κ = π$ (black symbols). The values of the switching current extracted from experimentally measured $V_{L} (I)$ and $V_{R} (I)$ curves taken at different $I_{inj}$ are shown by red and blue symbols. (b) The critical current (black symbols) and rearrangement currents (stars) calculated numerically for $L = 8.4 λ_{J}$ and $a = 2.8 λ_{J}$ . The region of coexistence of both states (bistability) ( $κ$ , $- κ$ ) and ( $κ - 2 π$ , $- κ + 2 π$ ) around $κ = π$ is shown by magenta color.Reuse & Permissions

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