Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to ∼50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004672.pdf

Emission of Coherent THz-Radiation from Superconductors.

The discovery of superconductors with high critical temperatures has led to a considerable effort to fabricate Josephson junctions operating at temperatures approaching, or even exceeding, 77 K for both scientific investigations and potential applications. Superconductor - normal - superconductor (SNS) devices, with noble or oxide metals as normal interlayers, are perhaps the most widely explored high- junction type at present. Although demonstrations of individual high- SNS devices exhibiting excellent current - voltage characteristics, high critical current - resistance products, and low noise behaviour have been made, reproducible devices suitable for electronic applications are elusive. It is therefore important to ask how well these nominally SNS high- junctions are understood. We review the available data, with emphasis on junction critical currents, and conclude that there is little evidence supporting a conventional proximity effect interpretation in the majority of reported high- devices. The strongest candidates for SNS behaviour are junctions in which N is a superconductor above its transition temperature. We discuss the present experimental and theoretical understanding of SNS junctions with emphasis on the implications for future research and development of these devices.

https://iopscience.iop.org/article/10.1088/0953-2048/9/4/001/pdf

Stationary properties of high-critical-temperature proximity effect Josephson junctions

Some recent developments in the fabrication of intrinsic Josephson junctions (IJJ) and their application for studying high-temperature superconductors are discussed. The major advantages of IJJ and unsolved problems are outlined. The feasibility of three-terminal devices based on the stacked IJJ is briefly evaluated.

http://iopscience.iop.org/article/10.1088/0953-2048/13/8/201/pdf

Intrinsic Josephson junctions: recent developments

The recent growing interest in terahertz (THz) and sub-THz science and technology is due to its many important applications in physics, astronomy, chemistry, biology and medicine, including THz imaging, spectroscopy, tomography, medical diagnosis, health monitoring, environmental control, as well as chemical and biological identification. We review the problem of linear and nonlinear THz and sub-THz Josephson plasma waves in layered superconductors and their excitations produced by moving Josephson vortices. We start by discussing the coupled sine-Gordon equations for the gauge-invariant phase difference of the order parameter in the junctions, taking into account the effect of breaking the charge neutrality, and deriving the spectrum of Josephson plasma waves. We also review surface and waveguide Josephson plasma waves. The spectrum of these waves is presented, and their excitation is discussed. We review the propagation of weakly nonlinear Josephson plasma waves below the plasma frequency, ωJ, which is very unusual for plasma-like excitations. In close analogy to nonlinear optics, these waves exhibit numerous remarkable features, including a self-focusing effect and the pumping of weaker waves by a stronger one. In addition, an unusual stop-light phenomenon, when ∂ω/∂k ≈ 0, caused by both nonlinearity and dissipation, can be observed in the Josephson plasma waves. At frequencies above ωJ, the current-phase nonlinearity can be used for transforming continuous sub-THz radiation into short, strongly amplified, pulses. We also present quantum effects in layered superconductors, specifically, the problem of quantum tunneling of fluxons through stacks of Josephson junctions. Moreover, the nonlocal sine-Gordon equation for Josephson vortices is reviewed. We discuss the Cherenkov and transition radiations of the Josephson plasma waves produced by moving Josephson vortices, either in a single Josephson junction or in layered superconductors. Furthermore, the expression for the Cherenkov cone of the excited Josephson plasma waves is derived. We also discuss the problem of coherent radiation (superradiance) of the THz waves by exciting uniform Josephson oscillations. The effects reviewed here could be potentially useful for sub-THz and THz emitters, filters and detectors.

https://iopscience.iop.org/article/10.1088/0034-4885/73/2/026501/pdf

Terahertz Josephson plasma waves in layered superconductors: spectrum, generation, nonlinear and quantum phenomena

We demonstrate the injection of spin-polarized electrons into paramagnetic Au nanowires by driving an electric current from a ferromagnetic permalloy (Py) electrode. The nonequilibrium spin accumulation in Au results in a difference between the chemical potentials for spin-up and spin-down electrons that is detected as a field-dependent voltage signal using a second Py electrode. The magnitude of the voltage contrast (>10%) and its coincidence with the magnetic switching of the Py electrodes attest to the spin-sensitive origin of the signals. By increasing the separation of the Py injector and detector, we observe an exponential decay of the spin signals. The measurements yield a spin-diffusion length of 63±15nm and an injected spin polarization of 3% in Au at 10 K.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-003678.pdf

Spin injection, diffusion and detection in lateral spin valves

The dependences of critical current on the temperature and magnetic field and the current-voltage characteristics in response to incident microwave radiation are measured in the direction of the c-axis of the 80 K superconducting phase (Bi, Pb)2Sr2CaCu2Oy bulk single crystal. Josephson junctionlike behavior is successfully observed, and it shows that the single crystal itself is composed of Josephson junctions along the c-direction and that its electrical transport in this direction is limited by intrinsic tunnel barriers existing in the single crystal.

https://iopscience.iop.org/article/10.1143/JJAP.31.L829/pdf

Observation of josephson junctionlike behavior in single-crystal (bi, pb)2sr2cacu2oy

We present a study of the critical currents of small sized intrinsic Josephson junction stacks in external magnetic fields applied parallel to the ${\mathrm{CuO}}_{2}$ layers. The stacks consisting of 5--12 junctions were realized as mesa structures patterned on top of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ single crystals. Lateral mesa dimensions ranged from $10\ifmmode\times\else\texttimes\fi{}10$ to $0.8\ifmmode\times\else\texttimes\fi{}0.8\ensuremath{\mu}{\mathrm{m}}^{2}.$ For the largest structures a complex magnetic field dependence of the critical currents was found. With decreasing mesa dimensions the critical currents systematically approached a Fraunhofer-type magnetic field dependence. Measurements agree well with the results of numerical calculations using coupled sine-Gordon equations.

Critical currents of small Bi 2 Sr 2 CaCu 2 O 8 + x intrinsic Josephson junction stacks in external magnetic fields

We have studied the microwave response of an array of intrinsic Josephson junctions in the c-axis direction in large Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 7/ (BSCCO) single crystals indicating multiple resistive branches on the I-V characteristics. Under the 8-10 GHz microwave irradiation, they showed three types of responses depending on its power P. For relatively low P, the Shapiro steps were observed, whose voltage separations were two order magnitude larger than that calculated from /spl Delta/V=hf/2e, indicating that about one hundred intrinsic junctions in the crystal are phase-locked. For high P, two types of RF-induced steps were observed. In the case of resonance mode, the I-V curve showed the pronounced constant voltage step together with subharmonic ones. On the other hand, for off-resonance mode the broad step was observed. The voltages of these steps increased lineally with p/sup 1/2/. These results suggest that the fluxon motions in intrinsic Josephson junctions interact with external RF field. >

Microwave response of intrinsic Josephson junctions in BSCCO single crystals

The phase dynamics of Josephson junctions (JJs) under external electromagnetic radiation is studied through numerical simulations. Current-voltage characteristics, Lyapunov exponents, and Poincare sections are analyzed in detail. It is found that the subharmonic Shapiro steps at certain parameters are separated by structured chaotic windows. By performing a linear regression on the linear part of the data, a fractal dimension of D = 0.868 is obtained, with an uncertainty of ±0.012. The chaotic regions exhibit scaling similarity, and it is shown that the devil's staircase of the system can form a backbone that unifies and explains the highly correlated and structured chaotic behavior. These features suggest a system possessing multiple complete devil's staircases. The onset of chaos for subharmonic steps occurs through the Feigenbaum period doubling scenario. Universality in the sequence of periodic windows is also demonstrated. Finally, the influence of the radiation and JJ parameters on the structured chaos is investigated, and it is concluded that the structured chaos is a stable formation over a wide range of parameter values.

Structured chaos in a devil's staircase of the Josephson junction

The experimental evidence of the breakpoint on the current-voltage characteristics (IVCs) of the stacks of intrinsic Josephson junctions (IJJs) is presented. The influence of the capacitive coupling on the IVCs of Bi2Sr2CaCu2Oy IJJs has been investigated. At 4.2K, clear breakpoint region is observed on the branches in the IVCs. It is found that due to the coupling between junctions, the hysteresis observed on the IVC is small compared to that expected from the McCumber parameter. Measurements agree well with the results predicted by the capacitively coupled Josephson junction model including the diffusion current.

Akinobu Irie

Papers

Observation of josephson junctionlike behavior in single-crystal (bi, pb)2sr2cacu2oy

Critical currents of small Bi 2 Sr 2 CaCu 2 O 8 + x intrinsic Josephson junction stacks in external magnetic fields

Microwave response of intrinsic Josephson junctions in BSCCO single crystals

Structured chaos in a devil's staircase of the Josephson junction

Experimental manifestation of the breakpoint region in the current-voltage characteristics of intrinsic Josephson junctions