Microwave-induced flow of vortices in long Josephson junctions

We report experimental and numerical study of microwave-induced flow of vortices in long Josephson junctions at zero dc magnetic field. Our intriguing observation is that applying an ac-bias of a small frequency $f \ll f_p$ and sufficiently large amplitude changes the current-voltage characteristics ($I$-$V$ curve) of the junction in a way similar to the effect of dc magnetic field, well known as the flux-flow behavior. The characteristic voltage $V$ of this low voltage branch increases with the power $P$ of microwave radiation as $V_{s}\propto P^{\alpha}$ with the index $\alpha \simeq 0.5$. Experiments using a low-temperature laser scanning microscope unambiguously indicate the motion of Josephson vortices driven by microwaves. Numerical simulations agree with the experimental data and show strongly {\it irregular} vortex motion. We explain our results by exploiting an analogy between the microwave-induced vortex flow in long Josephson junctions and incoherent multi-photon absorption in small Josephson junctions in the presence of large thermal fluctuations. In the case of long Josephson junctions the spatially-temporal chaos in the vortex motion mimics the thermal fluctuations. In accordance with this analogy, a control of the intensity of chaos in a long junction by changing its damping constant leads to a pronounced change in the shape of the $I$-$V$ curve. Our results provide a possible explanation to previously measured but not yet understood microwave-driven properties of intrinsic Josephson junctions in high-temperature superconductors.Comment: 8 pages, 13 figure

Quantum escape of the phase in a strongly driven Josephson junction

A quantum mechanical analysis of the Josephson phase escape in the presence of both dc and ac bias currents is presented. We find that the potential barrier for the escape of the phase is effectively suppressed as the resonant condition occurs, i.e. when the frequency $\omega$ of the ac bias matches the Josephson junction energy level separation. This effect manifests itself by a pronounced drop in the dependence of the switching current $I_s$ on the power $W$ of the applied microwave radiation and by a peculiar double-peak structure in the switching current distribution $P(I_s)$. The developed theory is in a good accord with an experiment which we also report in this paper. The obtained features can be used to characterize certain aspects of the quantum-mechanical behavior of the Josephson phase, such as the energy level quantization, the Rabi frequency of coherent oscillations and the effect of damping.Comment: 4 pages, 3 figures, to be published in Physical Review B (Rapid Communication

Incoherent microwave-induced resistive states of small Josephson junctions

We report an experimental and theoretical study of low-voltage resistive states that are observed in small tunnel Josephson junctions under microwave radiation. The studied features emerge from Shapiro steps on the current-voltage characteristics and appear when both thermal fluctuations and high frequency dissipation are strong. In the absence of microwave radiation Josephson junctions display under these conditions a phase diffusion supercurrent branch characterized by a finite small resistance and hysteretic switching to high voltage range. As the microwave radiation is applied, we experimentally observe three different types of resistive states in the currentvoltage characteristics. First, the phase diffusion branch steadily evolves and its maximum reached voltage Vm increases with the microwave power. Another interesting observed feature is a zero-crossing resistive state characterized by a negative resistance. Finally, we find that a low-voltage resistive state can split in numerous hysteretic fine branches resembling incoherent Shapiro-like steps. The appearance of a particular resistive state depends on an interrelation between the Josephson energy EJ, energy of thermal fluctuations kBT, and the frequency of microwave radiation ω. Our theoretical analysis based on an incoherent multi-photon absorption of a junction biased in the Josephson phase diffusion regime, is in a good accord with experimental observations

Observation of breather-like states in a single Josephson cell

We present experimental observation of broken-symmetry states in a superconducting loop with three Josephson junctions. These states are generic for discrete breathers in Josephson ladders. The existence region of the breather-like states is found to be in good accordance with the theoretical expectations. We observed three different resonant states in the current-voltage characteristics of the broken-symmetry state, as predicted by theory. The experimental dependence of the resonances on the external magnetic field is studied in detail.Comment: 7 pages, 8 figure

Spontaneous creation of discrete breathers in Josephson arrays

We report on the experimental generation of discrete breather states (intrinsic localized modes) in frustrated Josephson arrays. Our experiments indicate the formation of discrete breathers during the transition from the static to the dynamic (whirling) system state, induced by a uniform external current. Moreover, spatially extended resonant states, driven by a uniform current, are observed to evolve into localized breather states. Experiments were performed on single Josephson plaquettes as well as open-ended Josephson ladders with 10 and 20 cells. We interpret the breather formation as the result of the penetration of vortices into the system.Comment: 5 pages, 5 figure

Giant persistent photoconductivity in monolayer MoS2 field-effect transistors

Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD-based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (λ = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large number of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis for the defect-based engineering of the electronic and optical properties of TMDs for device applications

Quasiparticle and Cooper Pair Tunneling in the Vortex State of Bi-2212

From measurements of the c-axis I-V characteristics of intrinsic Josephson junctions in Bi_2Sr_2CaCu_2O_{8+delta} (Bi-2212) mesas we obtain the field dependence (H || c) of the quasiparticle (QP) conductivity, sigma_q(H,T), and of the Josephson critical current density, J_c(H,T). The quasiparticle conductivity sigma_q(H) increases sharply with H and reaches a plateau at 0.05 T <H< 0.3 T. We explain such behavior by the dual effect of supercurrents around vortices. First, they enhance the QP DOS, leading to an increase of sigma_q with H at low H and, second, they enhance the scattering rate for specular tunneling as pancakes become disordered along the c-axis at higher H, leading to a plateau at moderate H.Comment: 4 pages, 4 figure

Wave Packet Spreading with Disordered Nonlinear Discrete-Time Quantum Walks

We use a novel unitary map toolbox - discrete-time quantum walks originally designed for quantum computing - to implement ultrafast computer simulations of extremely slow dynamics in a nonlinear and disordered medium. Previous reports on wave packet spreading in Gross-Pitaevskii lattices observed subdiffusion with the second moment m2∼t1/3 (with time in units of a characteristic scale t0) up to the largest computed times of the order of 108. A fundamental and controversially debated question - whether this process can continue ad infinitum, or has to slow down - stands unresolved. Current experimental devices are not capable to even reach 1/104 of the reported computational horizons. With our toolbox, we outperform previous computational results and observe that the universal subdiffusion persists over an additional four decades reaching astronomic times 2×1012. Such a dramatic extension of previous computational horizons suggests that subdiffusion is universal, and that the toolbox can be efficiently used to assess other hard computational many-body problems. © 2019 American Physical Society

Observation of a collective mode of an array of transmon qubits

Arrays of transmon qubits coupled to a λ/2 superconducting coplanar waveguide resonator have been studied by microwave spectroscopy. The emergence of a collective mode has been discovered for a cluster of N &gt; 5 qubits, whose coupling constant to the electromagnetic field in the resonator is √N times greater compared to a single qubit. In addition, the emergence of collective multiphoton transitions exciting higher levels of a qubit cluster has been demonstrated and the interaction of an individual qubit with such a cluster has been investigated

Circuit quantum electrodynamics of granular aluminum resonators

Granular aluminum (grAl) is a promising high kinetic inductance material for detectors, amplifiers, and qubits. Here we model the grAl structure, consisting of pure aluminum grains separated by thin aluminum oxide barriers, as a network of Josephson junctions, and we calculate the dispersion relation and nonlinearity (self-Kerr and cross-Kerr coefficients). To experimentally study the electrodynamics of grAl thin films, we measure microwave resonators with open-boundary conditions and test the theoretical predictions in two limits. For low frequencies, we use standard microwave reflection measurements in a low-loss environment. The measured low-frequency modes are in agreement with our dispersion relation model, and we observe self-Kerr coefficients within an order of magnitude from our calculation starting from the grAl microstructure. Using a high-frequency setup, we measure the plasma frequency of the film around 70 GHz, in agreement with the analytical prediction. © 2018, The Author(s