We theoretically study the Josephson current in Ising superconductor-half-metal-Ising superconductor junctions. By solving the Bogoliubov-de Gennes equations, the Josephson currents contributed by the discrete Andreev levels and the continuous spectrum are obtained. For very short junctions, because the direct tunneling of the Cooper pair dominates the Josephson current, the current-phase difference relation is independent of the magnetization direction, which is the same as the conventional superconductor-ferromagnet-superconductor junctions. On the other hand, when the length of the half-metal is similar to or greater than the superconducting coherence length, the spin-triplet Josephson effect occurs and dominates the Josephson current. In this case, the current-phase difference relations show the strong magnetoanisotropic behaviors with the period π. When the magnetization direction points to the $\pm$ z directions, the current is zero regardless of the phase difference. However, the current has a large value when the magnetization direction is parallel to the junction plane, which leads to a perfect switch effect of the Josephson current. Furthermore, we find that the long junctions can host both the 0 state and πstate, and the $0$-$π$ transitions can be achieved with the change of the magnetization direction. The physical origins of the switch effect and $0$-$π$ transitions are interpreted from the perspectives of the spin-triplet Andreev reflection, the Ising pairing order parameter and the Ginzburg-Landau type of free energy. In addition, the influences of the chemical potential, the magnetization magnitude, and the strength of the Ising spin-orbit coupling on the switch effect and $0$-$π$ transitions are also investigated. Furthermore, the two-dimensional Josephson junctions are also investigated and we show that the spin-triplet Josephson effect can exist always.
We report on the switching current distributions in nano-hybrid Josephson junctions made of InAs semiconductor nanowires. Temperature dependence of the switching current distribution can be understood by motion of Josephson phase particle escaping from a tilted washboard potential, fitted well to the macroscopic quantum tunneling, thermal activation and phase diffusion models, depending on temperature. Application of gate voltage to tune the Josephson coupling strength enables us to adjust the effective temperature for the escaping process, which would be promising for developing gate-tunable …
An experimental investigation of the critical current noise in underdamped niobium based Josephson junctions by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching using the standard time of flight technique and analyze the data to extract the current noise. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs…
We study the thermal escape problem in the low damping limit. We find that finiteness of the barrier is crucial for explaining the thermal activation results. In this regime low barrier non-equilibrium corrections to the usual theories become necessary. We propose a simple theoretical extension accounting for these non-equilibrium processes which agrees numerical results. We apply our theory to the understanding of switching current curves in underdamped Josephson junctions.
Switching Current Measurements of EuBa2Cu3Oy Intrinsic Josephson Junctions
研究了两种不同各向异性参数γ（=（m*c/m*ab）1/2）的EuBa 2 Cu 3 O y（Eu 123）本征约瑟夫森结（IJJs）的开关动力学.与Bi 2212中的弱耦合IJJ相比，由于Eu 123 IJJ的强耦合性质，在它们的开关电流分布P（I）中观察到了与单堆约瑟夫森结的热激活逃逸模型的显著偏离。这表明在高温液态He中观察到宏观量子隧穿。
Swept bias experiments carried out on Josephson junctions yield the distributions of the probabilities of early switching from the zero voltage state. Kramers' theory of thermally activated escape from a one-dimensional potential is well known to fall short of explaining such experiments when the junctions are at millikelvin temperatures. We propose a simple revision of the theory which is shown to yield extremely good agreement with experimental data.
We study the dynamics of multi-junction switching (MJS): several intrinsic Josephson junctions (IJJs) in an array switch to the finite voltage state simultaneously. The number of multi-switching junctions ($N$) was successfully tuned by changing the load resistance serially connected to an Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ IJJ array. The independence of the escape rates of $N$ in the macroscopic quantum tunneling regime indicates that MJS is a $successive$ switching process rather than a $collective$ process. The origin of MJS is explained by the gradient of a load curve and the relative m…
Experiments on the distributions of switching currents in Josephson junctions are sensitive probes of the mechanism by which a junction changes abruptly to a finite voltage state. At low temperatures data exhibit smooth and gradual deviations from the expectations of the classical theory of thermal activation over the barrier in the tilted washboard potential. In this paper it is shown that if a very small proportion of the noise energy entering the apparatus at room temperature survives filtering and reaches the sample, it can enhance the escape rate sufficiently to replicate experimental obs…
The combination of superconducting and magnetic materials to create novel superconducting devices has been motivated by the discovery of Josephson critical current (Ics) oscillations as a function of magnetic layer thickness and the demonstration of devices with switchable critical currents. However, none of the hybrid devices have shown any spintronic effects, such as spin-transfer torque, which are currently used in room-temperature magnetic devices, including spin-transfer torque random-access memory and spin-torque nano-oscillators. We have developed nanopillar Josephson junctions with a m…
We present a study of the first and second switches (SWs) with nearly identical switching current $I_{\rm SW}$ in Bi$_2$Sr$_2$Ca$_{0.85}$Y$_{0.15}$Cu$_2$O$_y$ intrinsic Josephson junctions (IJJs). In contrast to early studies where a large difference between $I_{\rm SW}^{\rm 1st}$ and $I_{\rm SW}^{\rm 2nd}$ was observed, a crossover temperature $T_{\rm cr}$ to the quantum phase escapes for the 2nd SW is strongly reduced $(\sim$2.8~K) and agrees with that for the 1st SW. This confirms that the anomalous enhancement of $T_{\rm cr}^{\rm 2nd}$, commonly observed in the IJJs with $I_{\rm SW}^{\rm 1…
Josephson junctions and superconducting wires when probed with current pulses exhibit stochastic switching from superconducting to a stable non-zero voltage state. Electrical current dependence of the switching probability (so called S-curve) or switching current distribution is a fingerprint of the physics governing the escape process. This work addresses the criterion of independent switching event in a series of switching experiments. Treating Josephson junction as an electrical coin with current-tuned switching probability we investigate effect of correlation between switching events on th…
An in-plane magnetic field applied to an Ising superconductor converts spin-singlet Cooper pairs to spin-triplet ones. In this work, we study a Josephson junction formed by two Ising superconductors that are proximitized by ferromagnetic layers. This leads to highly tunable spin-triplet pairing correlations which allow to modulate the charge and spin supercurrents through the in-plane magnetic exchange fields. For a junction with a nonmagnetic barrier, the charge current is switchable by changing the relative alignment of the in-plane exchange fields, and a $π$-state can be realized. Furthermo…
With a combination of simple analytical arguments and extensive numerical simulations, we theoretically propose a Josephson junction with $n+1$ superconductors where the current-phase relation can be toggled in situ between a $\sin(δφ)$ and $\sin(δφ/n)$ shape using an applied magnetic field. Focusing in particular on the case $n=2$, we show that by using realistic system parameters such as unequal interface transparencies, the $\sin(δφ/2)$-shaped solution retains its $2π$-periodicity due to discontinuities at $δφ= \pm π$. Moreover, we demonstrate that as one toggles between the $\sin(δφ)$- and…
We develop a fast Magnetic Josephson Junction (MJJ) - a superconducting ferromagnetic device for a scalable high-density cryogenic memory compatible in speed and fabrication with energy-efficient Single Flux Quantum (SFQ) circuits. We present experimental results for Superconductor-Insulator-Ferromagnet-Superconductor (SIFS) MJJs with high characteristic voltage IcRn of >700 uV proving their applicability for superconducting circuits. By applying magnetic field pulses, the device can be switched between MJJ logic states. The MJJ IcRn product is only ~30% lower than that of conventional junctio…
Analysis of Josephson junctions switching time distributions for the detection of single microwave photons
Università di Salerno（萨莱诺大学）；INFN, Gruppo Collegato Salerno（意大利国家核物理研究所萨莱诺协作组）；INFN, Laboratori Nazionali di Frascati（意大利国家核物理研究所弗拉斯卡蒂国家实验室）；University of Roma Tre（罗马第三大学）；Univ. of Sannio（桑尼奥大学）
We test the effect of an external RF field on the switching processes of magnetic Josephson junctions (MJJs) suitable for the realization of fast, scalable cryogenic memories compatible with Single Flux Quantum logic. We show that the combined application of microwaves and magnetic field pulses can improve the performances of the device, increasing the separation between the critical current levels corresponding to logical '0' and '1'. The enhancement of the current level separation can be as high as 80% using an optimal set of parameters. We demonstrate that external RF fields can be used as …
The authors report on the design and measurement of a reflective single-pole single-throw microwave switch with no internal power dissipation, based on a superconducting circuit containing a single Josephson junction. The data demonstrate the switch operation with 2 GHz instantaneous bandwidth centered at 10 GHz, low insertion loss, and better than 20 dB on/off ratio. The switch's measured performance agrees well with simulations for input powers up to -100 dBm. An extension of the demonstrated circuit to implement a single-pole double-throw switch is shown in simulation.
Controllable Josephson diode effect, $0$-$π$ transition and switch effect in the superconductor/two-dimensional Weyl nodal line semimetal/superconductor junctions
Thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis
Comparative study of macroscopic quantum tunneling in Bi_2Sr_2CaCu_2O_y intrinsic Josephson junctions with different device structures
研究了两种器件结构的$_2$ $_2$ O $_y$本征约瑟夫森结（IJJ）的宏观量子隧穿（MQT）.一种是只有两个或三个IJJ的纳米厚小台面结构，另一种是在窄桥结构上堆叠数百个IJJ。实验结果表明，对于单约瑟夫森结，从零电压状态开始的第一次开关事件的开关电流分布与传统理论吻合较好，表明前一种器件结构从热激活到MQT区的交叉温度与后一种器件结构一样高。结合MQT机制中量子化能级之间的多光子跃迁的观察，这些结果强烈地表明，所观察到的MQT行为是高$T_c$铜酸盐中单个IJJ所固有的，与器件结构无关。从第一电阻状态的第二切换事件的切换电流分布，这是仔细区分从第一切换，也比较了两个器件结构之间。尽管在热传递环境的差异，两个设备的第二切换事件被发现显示出类似的温度无关的行为高达比第一切换的交叉温度高得多的温度。我们认为，它不能解释的自加热由于耗散电流后的第一次切换。作为可能的候选者，MQT过程的第二开关和电子温度的有效增加，由于准粒子注入进行了讨论。
It has been proposed that intermittent weak links between spontaneously broken symmetry systems can give rise to a novel type of time crystal. Here a practical construction is analyzed in greater detail. For a film-geometry Josephson junction, an applied magnetic field in the junction region can effectively switch on and off the supercurrent. By tuning experimental parameters, one can possibly let the free energy be arbitrarily close to the equilibrium without compromising the life time. A parallel construction for superfluid $^4$He is also proposed.
We propose to use a time-dependent imaginary potential to describe quantum mechanical tunneling through time-varying potential barriers. We use Gamow solutions for stationary tunneling problems to justify our choice of potential, and we apply our method to describe tunneling of a mesoscopic quantum variable: the phase change across a Josephson junction. The Josephson junction phase variable behaves as the position coordinate of a particle moving in a tilted washboard potential, and our general solution to the motion in such a potential with a time-dependent tilt reproduces a number of features…
We propose theoretically a thermal switch operating by the magnetic-flux controlled diffraction of phase-coherent heat currents in a thermally biased Josephson junction based on a two-dimensional topological insulator. For short junctions, the system shows a sharp switching behavior while for long junctions the switching is smooth. Physically, the switching arises from the Doppler shift of the superconducting condensate due to screening currents induced by a magnetic flux. We suggest a possible experimental realization that exhibits a relative temperature change of 40\% between the on and off …
Josephson current of spin-entangled electrons through the two branches of a SQUID-like structure with two quantum dots exhibits a magnetic-flux response different from the conventional Josephson current. Due to their interference, the period of maximum Josephson current changes from $h/2e$ to $h/e$, which can be used for detecting the Cooper-pair splitting efficiency. The nonlocal spin entanglement provides a quantum mechanical functionale for switching on and off this novel Josephson current, and explicitly a switch is formulated by including a pilot junction. It is shown that the device can …
We study the Andreev levels, supercurrent and tunnelling conductance in a clean Ising superconductor (ISC)/half metal (HM)/Ising superconductor (ISC) Josephson junction with spin-active interfaces using Bogoliubov-de Gennes equations. We theoretically demonstrate the effect of spin mixing, spin flipping processes and spin-orbit coupling (SOC) of the ISC on Andreev Bound States (ABS) spectra, current phase relation (CPR) and tunnelling conductance in transparent and opaque barrier limit. We witness an additional splitting of the Andreev levels due to SOC of the ISC and $0 - π$ transition for di…
We propose and analyze a mesoscopic Josephson junction consisting of two ferromagnetic insulator-superconductors (FI-Ss) coupled through a normal metal (N) layer. The Josephson current of the junction is non-trivially affected by the spin-splitting field induced by the FIs in the two superconductors. In particular, it shows sizeable enhancement by increasing the amplitude of the exchange field ($h_{ex}$) and displays a switchable current-phase relation which depends on the relative orientation of $h_{ex}$ in the FIs. In a realistic EuS/Al-based setup this junction can be exploited as a high-re…
We study the voltage noise properties including the switching rates and statistics of phase-slips in moderately damped Josephson junctions using a novel efficient numerical approach combining the matrix continued-fraction method with the full counting statistics. By analyzing the noise results obtained for the RCSJ model we identify different dominating components, namely the thermal noise close to equilibrium (small current-bias regime), the shot noise of (multiple) phase-slips in the intermediate range of biases and the switching noise for yet higher bias currents. We extract thus far inacce…
Josephson junctions containing ferromagnetic layers have generated interest for application in cryogenic memory. In a junction containing both a magnetically hard fixed layer and soft free layer with carefully chosen thicknesses, the ground-state phase difference of the junction can be controllably switched between 0 and π by changing the relative orientation of the two ferromagnetic layers from antiparallel to parallel. This phase switching has been observed in junctions using Ni fixed layers and NiFe free layers. We present phase-sensitive measurements of such junctions in low-inductance sym…
In this work we give a characterization of the RF effect of memory switching on Nb-Al/AlOx-(Nb)-Pd$_{0.99}$Fe$_{0.01}$-Nb Josephson junctions as a function of magnetic field pulse amplitude and duration, alongside with an electrodynamical characterization of such junctions, in comparison with standard Nb-Al/AlOx-Nb tunnel junctions. The use of microwaves to tune the switching parameters of magnetic Josephson junctions is a step in the development of novel addressing schemes aimed at improving the performances of superconducting memories.
We predict that the amplitude of the Josephson current through an antiferromagnetic weak link changes by several orders of magnitude upon rotation of the Néel order parameter characterizing the staggered magnetic order. This occurs due the presence of spin-orbit coupling arising from structural inversion asymmetry which makes the band gap in the antiferromagnet highly sensitive to the staggered order parameter direction. We also demonstrate that when phase-biasing the junction, magnetization dynamics is induced which switches the direction of the Néel vector in the antiferromagnet. These resul…
We study the Josephson current in two types of lateral junctions with spin-orbit coupling and an exchange field. The first system (type 1 junction) consists of superconductors with heavy metal interlayers linked by a ferromagnetic bridge, such that the spin-orbit coupling is finite only at the superconductor/heavy metal interface. In the second type (type 2) of system we assume that the spin orbit coupling is finite in the bridge region. The length of both junctions is larger than the magnetic decay length such that the Josephson current is carried uniquely by the long-range triplet component …
We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a λ/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitativ…
We study analytically and numerically the influence of the quasiparticle charge imbalance on the dynamics of the asymmetric Josephson stack formed by two inequivalent junctions: the fast capacitive junction $JJ_{1}$ and slow non-capacitive junction $JJ_{2}$. We find, that the switching of the fast junction into resistive state leads to significant increase of the effective critical current of the slow junction. At the same time, the initial switching of the slow junction may either increase or decrease the effective critical current of the fast junction, depending on ratio of their resistances…
