Anisotropic Spin-Superfluid Magnet as a Long Josephson Junction

Lei Jiang, Yong Xu, Chuanwei Zhang

Atomtronics has the potential for engineering new types of functional devices, such as Josephson junctions (JJs). Previous studies have mainly focused on JJs whose ground states have zero or $\ensuremath{\pi}$ superconducting phase difference across the junctions, while arbitrary phase-tunable JJs may have important applications in superconducting electronics and quantum computation. Here we show that a phase-tunable JJ can be implemented in a spin-orbit-coupled cold atomic gas with the magnetic tunneling barrier generated by a spin-dependent focused laser beam. We consider the JJ confined in either a linear harmonic trap or a circular ring trap. In the ring trap, the magnetic barrier induces a spontaneous mass current for the ground state of the JJ, demonstrating the magnetoelectric effects of cold atoms.

Solid State Physics

Benedikt J. P. Pernack, M. V. Fistul, Ilya Eremin

We study theoretically a variety of collective quantum phases occurring in frustrated sawtooth chains of Josephson junctions embedded in a dissipationless transmission line. The basic element of a system, i.e., the triangular superconducting cell, contains two 0 and one $\ensuremath{\pi}$ Josephson junctions characterized by ${E}_{J}$ and $\ensuremath{\alpha}{E}_{J}$ Josephson energies, accordingly. In the frustrated regime the low-energy quantum dynamics of a single cell is determined by anticlockwise or clockwise flowing persistent currents (vortex or antivortex). The direct embedding of $\ensuremath{\pi}$ Josephson junctions in a transmission line allows us to establish a short or long-range interaction between (anti)vortices of well-separated cells. By making use of the variational approach, we map the superconducting circuit Hamiltonian to an effective $XX$ spin model with an exchange spin-spin interaction decaying with the distance $x$ as ${x}^{\ensuremath{-}\ensuremath{\beta}}$, and the local ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\ensuremath{\sigma}}}_{x,n}$ terms corresponding to the coherent quantum beats between vortex and antivortex in a single cell. We obtain that, in long arrays such as $N\ensuremath{\gg}{\ensuremath{\ell}}_{0}\ensuremath{\simeq}\sqrt{C/{C}_{0}}$, where $C$ and ${C}_{0}$ are capacitances of a 0 Josephson junction and a transmission line, respectively, the amplitude of quantum beats is strongly suppressed. By means of exact numerical diagonalization, we study the interplay between the coherent quantum beats and the exchange spin-spin interaction leading to the appearance of various collective quantum phases such as the paramagnetic ($P$), compressible superfluid ($CS$), and weakly compressible superfluid ($w\text{\ensuremath{-}}CS$) states.

伸 高木

V. V. Ryazanov, V. V. Bol’ginov, Danila S. Sobanin +4 more

The lack of dense, fast, energy efficient memory has been the main detractor for multiple superconducting digital projects in the past. Recently, fundamental physics research in superconductor-ferromagnet thin-film tunnel structures created a new opportunity to solve this long-standing problem. Superconductivity and ferromagnetism, two deeply antagonistic electronic properties, can co-exist in form of Magnetic Josephson Junctions (MJJs). The superconducting-ferromagnetic MJJs are electrically compatible with traditional superconductor-insulator-superconductor (SIS) Josephson junctions (JJs) used for digital energy-efficient single flux quantum (eSFQ/ERSFQ) circuits. Both MJJ and JJ circuits have similar fabrication process and can be integrated on a single chip. As a result, a combination of MJJs and JJs can be used to form addressable memory cells, energy-efficient memory periphery circuits and programmable logic elements. In this paper, we present the test results of superconductor-insulator-ferromagnet-superconductor (SIFS) MJJs showing their applicability for superconducting spintronic memory and digital circuits.