Stacks Of Intrinsic Josephson Junctions Research Articles

Increase of Phase Retrapping Effects in the Switching Rate from the Finite Voltage State of the Underdamped Intrinsic Josephson Junctions

We report a detailed study of the phase switching rate from the first to the fourth switch for a small stack of Bi2Sr2CaCu2Oy intrinsic Josephson junctions (IJJs). Experimental results were analyzed by using the conventional single-junction model including the thermally-activated phase escape and the multiple phase retrapping. It is shown that the phase retrapping effects are more prominent for higher order switches, even for the underdamped IJJs showing a large hysteresis in the current–voltage characteristics. This clearly suggests that the tilted washboard potential representing the phase switch from the finite voltage state in IJJs can be influenced by a rapid oscillation generated in a phase-switched junction.

Three-Dimensional Simulations of the Electrothermal and Terahertz Emission Properties ofBi2Sr2CaCu2O8Intrinsic Josephson Junction Stacks

Stacks of intrinsic Josephson junctions (JJs) in cuprate superconductors are promising emitters of terahertz light, which is sought for many applications in imaging, spectroscopy, and communication. The authors simulate JJ stacks in three dimensions, simultaneously solving a Fickian diffusion equation for heat and coupled sine-Gordon equations for current. They predict, and confirm experimentally, that applying a modest magnetic field to a stack's long side enhances emission significantly---one more step toward efficient devices.

Cavity mode identification for coherent terahertz emission from high-Tc superconductors.

Stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ emit intense and coherent terahertz waves determined by the internal electromagnetic cavity resonance. We identify the excited transverse magnetic mode by observing the broadly tunable emissions from a nearly square stack and simulating the scattering spectrum. We employ a wedge-type interferometer to measure spatially-integrated power independently of the far-field pattern. The simulation results are in good agreement with observed resonance behaviors as a function of frequency.

Vortex Penetrations in Parallel-connected two Stacks of Intrinsic Josephson Junctions

In mesoscopic stacks of intrinsic Josephson junctions (IJJs) in Bi2Sr2CaCu2O8+y (Bi2212), the penetrations of individual vortices are detectable by the measurements of the transport properties, i.e., c-axis resistance or critical current. We have measured the c-axis resistance as a function of magnetic field in samples with two stacks of IJJs connected in parallel by Bi2212 itself to study any interaction of individual vortex penetrations into them. Since the superconducting loop containing two stacks of IJJs is the same geometry as that of superconducting quantum interference device (SQUID), we might expect a periodic resistance (or current) modulation as a function of magnetic field, whose period corresponds to the area in the loop. However, the results were just simple mixing of the resistive changes by the individual vortex penetrations into each of the stacks; behavior like SQUID has not been observed in present samples.

Heating Effect of Mesa-type Intrinsic Josephson Junction Stacks Using Pulse Current Measurement

We experimentally investigated the self-heating effect of large-scale mesa-type intrinsic Josephson junction stacks in the single-crystal Bi-2212 using pulse current measurement techniques. In addition, the effects of contact resistance between the electrodes and top of the mesa were studied. The current-voltage (I-V) characteristics obtained by using the pulse measurement technique showed significantly improved voltage suppression compared to the dc measurement. Furthermore, we proposed a new pulse waveform. Using the proposed waveform, I-V characteristics could be obtained with heat generation at the contact resistance only. It is revealed that the heating at the contact resistance had a significant effect on the I-V characteristics.

Three-terminal stand-alone superconducting terahertz emitter

We report on the electrothermal behavior and the terahertz emission properties of a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack contacted in a three-terminal configuration. One terminal is used as a collective ground while the other two, contacting the stack from its right and left side, allow to vary the current injection profile. At high bias, a hot spot forms in the stack. Its appearance and position can be controlled by varying the ratios of the injected currents. Depending on this ratio, the emitted power can vary by an order of magnitude. Further, for a given total injection current, the device allows to vary the emission frequency on a 10% level by altering the injection profile. The overall tunability of the emission frequency, varying also the total bias current, is on the order of 20%.

Terahertz emission from a stack of intrinsic Josephson junctions in Pb-doped Bi2Sr2CaCu2O8+δ

We observe continuous terahertz-wave emission from a stack of intrinsic Josephson junctions made of slightly Pb-doped Bi2Sr2CaCu2O. We investigate how Pb doping affects the c-axis current–voltage and emission characteristics. The terahertz emission spectra are measured by Fourier-transform infrared spectroscopy and reveal that the emission frequency is remarkably increased by Pb doping, an effect that we attribute to cavity resonance.

Superconducting Integrated Terahertz Spectrometers

A superconducting integrated receiver (SIR) comprises all of the elements needed for heterodyne detection on a single chip. Light weight and low power consumption combined with nearly quantum-limited sensitivity and a wide tuning range of the superconducting local oscillator make the SIR a perfect candidate for many practical applications. For the first time, we demonstrated the capabilities of the SIR technology for remote operation under harsh environmental conditions and for heterodyne spectroscopy at atmospheric limb sounding on board a high-altitude balloon. Recently, the SIR was successfully implemented for the first spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks (BSCCO mesa) at frequencies up to 750 GHz; linewidth below 10 MHz has been recorded in the high bias regime. The phase-locked SIR has been used for the locking of the BSCCO oscillator under the test. To extend the operation range of the SIR well above 1 THz, a new technique for fabrication of high-quality SIS tunnel junctions with gap voltage Vg up to 5.3 mV has been developed. Integration of a superconducting high-harmonic phase detector with a cryogenic oscillator opens a possibility for efficient phase locking of the sources with free-running linewidth up to 30 MHz that is important both for BSCCO mesa and NbN/MgO/NbN oscillators.

Tuning the Terahertz Emission Power of an Intrinsic Josephson-Junction Stack with a Focused Laser Beam

Developing a tunable source of high-power terahertz radiation is an ongoing research challenge, with promise for applications including biosensing and high-speed communication. The authors show that the THz emission from a stack of intrinsic Josephson junctions in a cuprate superconductor embedded between two gold layers can be manipulated by a focused laser beam. The output power can be increased by as much as 75% with laser irradiation, and tuned continuously and rapidly as the laser beam is moved along the length of the stack, locally heating different spots of the sample.

Electrothermal behavior and terahertz emission properties of a planar array of two Bi2Sr2CaCu2O8+δ intrinsic Josephson junction stacks

We report on the investigation of the electrothermal behavior and the terahertz (THz) emission properties of two nearby Bi2Sr2CaCu2O (BSCCO) intrinsic Josephson junction stacks, using a combination of electric transport and THz emission measurements plus low temperature scanning laser microscopy. We start with a compact BSCCO stack (placed in a z-shaped structure between two BSCCO electrodes) with lateral dimensions of and height, consisting of about 480 junctions. After characterization, a 200 nm wide slit was introduced by focused ion beam milling, splitting the stack into two halves connected by continuous superconducting electrodes. In a third step, the upper electrode was also split, leading to a structure where the two stacks can be biased separately. In all configurations hot-spot formation was observed. Despite the separation into two stacks only a single hot spot formed, which, depending on the bias condition, could either be located in one of the stacks or extend into both stacks with its center in the slit. In none of the structures it was possible to achieve mutual synchronization of the two stacks, indicating that additional synchronizing elements or the presence of a base crystal as for mesa structures may be necessary for the operation of parallel array structures.

Thermal and electromagnetic properties ofBi2Sr2CaCu2O8intrinsic Josephson junction stacks studied via one-dimensional coupled sine-Gordon equations

We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a 300 $\ensuremath{\mu}\mathrm{m}$ long intrinsic Josephson junction stack consisting of $N=700$ junctions. The junctions in the stack are combined with $M$ segments where we assume that inside a segment all junctions behave identically. Most simulations are for $M=20$. For not too high bath temperatures there is the appearance of a hot spot at high-bias currents. In terms of electromagnetic properties, robust standing-wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to $\ensuremath{\pi}$-kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with $M$, on the basis of simulations with $M$ between 10 and 350.

Oscillatory behavior of vortex-lattice melting transition line in mesoscopic Bi_{2}Sr_{2}CaCu_{2}O_{8+y} superconductors.

The vortex-lattice melting transition of a limited number of vortices confined in mesoscopic square superconductors was studied by c-axis resistance measurements using stacks of intrinsic Josephson junctions in Bi_{2}Sr_{2}CaCu_{2}O_{8+y}. In contrast to the melting transition in bulk crystals, we have first found a clear oscillatory behavior in the field dependence of the melting temperature in small samples of 5-10 μm square. The periods of the oscillations roughly obey the regularity of the matching conditions of square vortex lattices surrounded by a square boundary and the melting temperatures are enhanced around the vortex number of i^{2} (where i is an integer). The results suggest that a confinement effect by the square boundary stabilizes square lattice structures which are realized around i^{2} vortex number even in competition with the favorable Abrikosov triangular lattice in the bulk.

Compact Superconducting Terahertz Source Operating in Liquid Nitrogen

Sources of terahertz radiation (between the far infrared and microwave frequencies) are in high demand for a host of applications. The authors report on what is essentially a ``T-ray flashlight'', a compact and portable source of tunable, continuous terahertz light, employing a stack of intrinsic Josephson junctions in a cuprate superconductor as the emitter. This economical and convenient device can be driven by one everyday 1.5-V battery, and should facilitate airport security, nondestructive evaluation, and remote detection of trace gases, among other applications.

Chaos induced by coupling between Josephson junctions

It is found that, in a stack of intrinsic Josephson junctions in layered high temperature superconductors under external electromagnetic radiation, the chaotic features are triggered by interjunction coupling, i.e., the coupling between different junctions in the stack. While the radiation is well known to produce chaotic effects in the single junction, the effect of interjunction coupling is fundamentally different and it can lead to the onset of chaos via a different route to that of the single junction. A precise numerical study of the phase dynamics of intrinsic Josephson junctions, as described by the CCJJ+DC model, is performed. We demonstrate the charging of superconducting layers, in a bias current interval corresponding to a Shapiro step subharmonic, due to the creation of a longitudinal plasma wave along the stack of junctions. With increase in radiation amplitude chaotic behavior sets in. The chaotic features of the coupled Josephson junctions are analyzed by calculations of the Lyapunov exponents. We compare results for a stack of junctions to the case of a single junction and prove that the observed chaos is induced by the coupling between the junctions. The use of Shapiro step subharmonics may allow longitudinal plasma waves to be excited at low radiation power.

Enhanced Macroscopic Quantum Tunneling in Capacitively Coupled BiPb2201 Single-Layered Intrinsic Josephson Junctions

Macroscopic quantum tunneling (MQT) in an intrinsic Josephson junction (IJJ) stack of Bi1.9Pb0.1Sr1.39La0.63CuO6+δ (BiPb2201) has been investigated. For the first switch, from superconducting to the first resistive branch in current–voltage characteristics, the crossover between MQT and thermal activation (TA) takes place at 0.6 K. On the other hand, for the second switch, the MQT-TA crossover temperature is increased to 2.0 K. This result is interpreted as follows: the MQT rate of the second switch is enhanced by the charge coupling between adjacent IJJs as well as in Bi2Sr2CaCu2O8+δ. We consider that the enhancement of the MQT rate is a common feature among bismuth-cuprates with single and double CuO2 layers in their crystal structures.

Size Dependence of Individual Vortex Penetration into Intrinsic Josephson Junction Stacks of Bi2Sr2CaCu2O8+y

By c-axis transport measurements in an intrinsic Josephson junctions (IJJs) stack of Bi2Sr2CaCu2O8+y in magnetic fields parallel to c-axis, a penetration of individual vortices can be detected as a sudden jump or drop of c-axis resistance or critical current, which has already been reported in a sample of small in-plane area (<2μm2). We explored the individual vortex penetrations in IJJs stacks with much larger in-plane areas and successfully observed the same phenomenon even in a sample as large as 100μm2. Behavior of the c-axis resistance and critical current caused by the individual vortex penetrations are investigated through the sample-size dependence in various temperatures and magnetic fields.

Imaging of local temperature distributions in mesas of high-Tc superconducting terahertz sources

Stacks of intrinsic Josephson junctions in high-Tc superconductors are a promising source of intense, continuous, and monochromatic terahertz waves. In this paer, we establish a fluorescence-based temperature imaging system to directly image the surface temperature on a Bi2Sr2CaCu2O8+δ mesa sample. Intense terahertz emissions are observed in both high- and low-bias regimes, where the mesa voltage satisfies the cavity resonance condition. In the high- bias regime, the temperature distributions are shown to be inhomogeneous with a considerable temperature rise. In contrast, in the low-bias regime, the distributions are rather uniform and the local temperature is close to the bath temperature over the entire sample.

Phase dynamics modeling of parallel stacks of Josephson junctions

The phase dynamics of two parallel connected stacks of intrinsic Josephson junctions (JJs) in high temperature superconductors is numerically investigated. The calculations are based on the system of nonlinear differential equations obtained within the CCJJ + DC model, which allows one to determine the general current-voltage characteristic of the system, as well as each individual stack. The processes with increasing and decreasing base currents are studied. The features in the behavior of the current in each stack of the system due to the switching between the states with rotating and oscillating phases are analyzed.

Structured Chaos in 1-D Stacks of Intrinsic Josephson Junctions Irradiated by Electromagnetic Waves

We investigate the effect of the capacitive and diffusive coupling on a novel type of structured chaos that was recently reported in numerical simulations of the IV characteristics of a single underdamped irradiated Josephson junction. A detailed characterization of the synchronization patterns in 1-D stacks of junctions is made through high-resolution (with respect to steps in the dc bias current) numerical simulations within the CCJJ+DC model. We show that the inclusion of coupling can result in complete synchronization of all the chaotic signals from the different junctions in the stack, a phenomenon that occurs when all the phases remain range-bound relative to that of the applied external radiation.

Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks with improved cooling: Coherent emission above 1 THz

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.