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Register a new user$0$-$pi$ transitions in a Josephson junction of irradiated Weyl semimetal
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We propose a setup for the experimental realization of unexpected and anisotropic $0$-$pi$ transitions of the Josephson current, in a junction whose link is made of irradiated Weyl semi-metal (WSM), due to the presence of chiral nodes. We show using a Greens function technique that the Josephson current through a time-reversal symmetric WSM has anisotropic periodic oscillations as a function of $k_0L$, where $k_0$ is the (relevant) separation of the chiral nodes and $L$ is the length of the sample. This is unexpected, for time-reversal symmetric systems, although it has been shown earlier for time-reversal symmetry breaking WSMs. We then show that the effective value of $k_0$ can be tuned with precision by irradiating the sample with photons resulting in $0$-$pi$ transitions of the critical current. Finally, we discuss the robustness of our setup.
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One of the consequences of Cooper pairs having a finite momentum in the interlayer of a Josephson junction, is $pi$-junction behavior. The finite momentum can either be due to an exchange field in ferromagnetic Josephson junctions, or due to the Zeeman effect. Here, we report the observation of Zeeman effect induced 0-$pi$ transitions in Bi$_{1-x}$Sb$_x$, 3D Dirac semimetal-based Josephson junctions. The large g-factor of the Zeeman effect from a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0- to $pi$- regimes. This is revealed by sign changes in the modulation of the critical current by applied magnetic field of an asymmetric superconducting quantum interference device (SQUID). Additionally, we directly measure a non-sinusoidal current-phase relation in the asymmetric SQUID, consistent with models for ballistic Josephson transport.
Two superconductors coupled by a weak link support an equilibrium Josephson electrical current which depends on the phase difference $varphi$ between the superconducting condensates [1]. Yet, when a temperature gradient is imposed across the junction, the Josephson effect manifests itself through a coherent component of the heat current that flows oppositely to the thermal gradient for $ varphi <pi/2$ [2-4]. The direction of both the Josephson charge and heat currents can be inverted by adding a $pi$ shift to $varphi$. In the static electrical case, this effect was obtained in a few systems, e.g. via a ferromagnetic coupling [5,6] or a non-equilibrium distribution in the weak link [7]. These structures opened new possibilities for superconducting quantum logic [6,8] and ultralow power superconducting computers [9]. Here, we report the first experimental realization of a thermal Josephson junction whose phase bias can be controlled from $0$ to $pi$. This is obtained thanks to a superconducting quantum interferometer that allows to fully control the direction of the coherent energy transfer through the junction [10]. This possibility, joined to the completely superconducting nature of our system, provides temperature modulations with unprecedented amplitude of $sim$ 100 mK and transfer coefficients exceeding 1 K per flux quantum at 25 mK. Then, this quantum structure represents a fundamental step towards the realization of caloritronic logic components, such as thermal transistors, switches and memory devices [10,11]. These elements, combined with heat interferometers [3,4,12] and diodes [13,14], would complete the thermal conversion of the most important phase-coherent electronic devices and benefit cryogenic microcircuits requiring energy management, such as quantum computing architectures and radiation sensors.
The phase-dependent bound states (Andreev levels) of a Josephson junction can cross at the Fermi level, if the superconducting ground state switches between even and odd fermion parity. The level crossing is topologically protected, in the absence of time-reversal and spin-rotation symmetry, irrespective of whether the superconductor itself is topologically trivial or not. We develop a statistical theory of these topological transitions in an N-mode quantum-dot Josephson junction, by associating the Andreev level crossings with the real eigenvalues of a random non-Hermitian matrix. The number of topological transitions in a 2pi phase interval scales as sqrt(N) and their spacing distribution is a hybrid of the Wigner and Poisson distributions of random-matrix theory.
Recently Baselmans et al. [Nature, 397, 43 (1999)] showed that the direction of the supercurrent in a superconductor/normal/superconductor Josephson junction can be reversed by applying, perpendicularly to the supercurrent, a sufficiently large control current between two normal reservoirs. The novel behavior of their 4-terminal device (called a controllable PI-junction) arises from the nonequilibrium electron energy distribution established in the normal wire between the two superconductors. We have observed a similar supercurrent reversal in a 3-terminal device, where the control current passes from a single normal reservoir into the two superconductors. We show theoretically that this behavior, although intuitively less obvious, arises from the same nonequilibrium physics present in the 4-terminal device. Moreover, we argue that the amplitude of the PI-state critical current should be at least as large in the 3-terminal device as in a comparable 4-terminal device.
We study the interaction between elliptically polarized light and a three-dimensional Luttinger semimetal with quadratic band touching using Floquet theory. In the absence of light, the touching bands can have the same or the opposite signs of the curvature; in each case, we show that simply tuning the light parameters allows us to create a zoo of Weyl semimetallic phases. In particular, we find that double and single Weyl points can coexist at different energies, and they can be tuned to be type I or type II. We also find an unusual phase transition, in which a pair of Weyl nodes form at finite momentum and disappear off to infinity. Considering the broad tunability of light and abundance of materials described by the Luttinger Hamiltonian, such as certain pyrochlore iridates, half-Heuslers and zinc-blende semiconductors, we believe this work can lay the foundation for creating Weyl semimetals in the lab and dynamically tuning between them.
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