Stacks Of Intrinsic Josephson Junctions Research Articles

Circuit models of simultaneously biased intrinsic Josephson junction stacks for terahertz radiations in high-bias regime

Experiment-based circuit models for Josephson plasma emitters (JPEs) made of cuprate superconductor Bi2Sr2CaCu2O8+δ single crystals are proposed. Introducing an LCR resonator shunting an RCSJ parallel circuit allows us to reproduce experimental radiation intensity as a function of device voltage. Mismatch in the resonance frequencies of the total device and the LCR part is related to the wide-range tuning of the radiation frequency of JPE. It is found that coupling capacitances for simultaneously biased operations provide significant increases in radiation intensities, which have been reported as synchronized radiation from multiple mesas in the literature.

Chaotic behavior of a stack of intrinsic Josephson junctions at the transition to branching for overcritical currents

Phase dynamics of a stack of coupled intrinsic Josephson junctions was investigated in the framework of capacitively coupled Josephson junctions with diffusion current model. We study the transition from the current-voltage characteristic specific to Josephson junctions arrays with small dissipation and weak coupling between the junctions to the arrays with strong coupling between the junctions and high dissipation. Low dissipative arrays of Josephson junctions are characterized by the absence of branching for overcritical currents which appears for highly dissipative arrays. Described branching appears due to charging on the superconducting layers and charge traveling waves generation. Arrays of Josephson junctions with intermediate values of coupling and dissipation parameters are characterized by the chaotic behavior, confirmed by positive Lyapunov exponent, and branching on the current voltage characteristic for both sub- and overcritical currents.

Improvement of Non-Uniform Temperature Distributions in Intrinsic Josephson Junction Stacks

Terahertz (THz) electromagnetic (EM) wave emitters have been expected for applications because these waves are suitable for non-destructive inspection, telecommunication technologies, and other uses. Strong and coherent THz EM waves are known to radiate from large-sized intrinsic Josephson junction (IJJ) stacks in which the self-heating effect is considerably larger and the temperatures in the mesa are non-uniformly distributed. In this study, we numerically investigate and discuss the temperature and current distributions in large-sized IJJ stacks because these parameters are difficult to analyze through experimental investigation. The temperature and current distributions can be obtained by self-consistently solving the non-linear diffusion equation in an equivalent circuit of the mesa by considering the temperature dependence of several parameters. As shown in the numerical results, self-heating caused the local temperature of the center in the mesa to exceed the critical temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ). To improve the non-uniform temperature and current distributions, we propose an improvement in which an external heat generation system is placed on the edges of the mesa. By applying this measure, the temperature distribution is improved because the edges of the IJJ mesa are heated externally. Furthermore, the local temperatures throughout the mesa were held below Tc, and the current - voltage characteristics were improved using the proposed improvement.

Mutually Synchronized Macroscopic Josephson Oscillations Demonstrated by Polarization Analysis of Superconducting Terahertz Emitters

We demonstrate mutual synchronization of Josephson oscillations in multiple stacks of intrinsic Josephson junctions of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta }$. Detailed analysis of the full polarization parameters allows the determination of a phase correlation between the stacks: a simultaneous emission state is described by a linear combination of individual emission states with a phase retardation. This proves that the stacks are coupled via a Josephson plasma in a superconducting substrate and the coupling matrices can be extracted from polarization analyses. Our findings suggest a route towards the realization of high-power terahertz sources based on the synchronization of a large number of intrinsic Josephson junctions.

Integrated, Portable, Tunable, and Coherent Terahertz Sources and Sensitive Detectors Based on Layered Superconductors

Current compact emitter and receiver technologies are generally inefficient and impractical at terahertz (THz) frequencies between 0.1 and 10 THz. Hence, a gap exists between mature microwave and developed optical technologies. On-chip, integrated broadly tunable and powerful quantum sources that coherently radiate THz waves between 0.1 and 11 THz (potentially extendable to 15 THz) and with potential output power of >1 mW can be achieved based on quantum tunneling of electron pairs across the stack of intrinsic Josephson junctions (IJJs) naturally present in a single crystal of the layered high- ${T}_{c}$ superconducting Bi2Sr2CaCu2O $_{8+\delta}$ (BSCCO). Such devices have been found to be especially promising solid-state THz sources capable of bridging the entire THz gap, as their wide-frequency tunability range is superior to that obtained from their semiconducting-based rivals, either single resonant-tunneling diodes (RTDs) or THz-quantum cascade lasers (QCLs). Due to the unique electrodynamics of BSCCO, they can also be operated as switching current detectors, paving the way for the realization of on-chip THz-integrated circuits for applications in ultrahigh-speed telecommunications, quantum information, on-chip spectroscopy, and nondestructive sensing, testing, and imaging. This article reviews the history and recent advances in THz sources and detectors based on IJJs with a focus on the application of IJJ THz devices in THz spectroscopy and various types of THz imaging systems such as reflection, transmission, and computed tomography. We show that compact IJJ THz devices with sub-centimeter-sized modules are easy to use in many applications, as they can be regarded as pocket quantum THz torches.

IV-characteristics and power of emission from stacks of long Josephson junctions with Gaussian spread of critical currents

The creation of sources of coherent emission in the terahertz region of frequencies from stacks of intrinsic Josephson junctions in high-temperature superconductors becomes the recently important problem of applied nanoscience. We investigated numerically IV-characteristics of stacks of three and four long Josephson junctions and power of emission from edges of the stack at different small Gaussian standard deviations of critical currents. We found the split zero-field steps in IV-characteristics. Positions of zero-field steps in the IV-curve can be described by a simple analytical expression in the theory of electrical properties of stacks of Josephson junctions placed in the external magnetic field. Maxima of coherent emission appear at voltages corresponding to the frequencies of the resonance of the in-phase collective modes of the whole stack with modes of geometrical resonances. We found maximal tolerant standard deviations of critical currents at which this resonance exists (from 1 to 10% of averaged critical currents of junctions).

Vortex states in a submicron Bi2212 crystal probed by intrinsic Josephson junctions

To study the pancake-vortex states confined in a submicron Bi2Sr2CaCu2O8+y (Bi2212) crystal, we have measured the c-axis resistance and I-V characteristics of a stack of intrinsic Josephson junctions with a lateral dimension less than 1 µm. Although the stack was accidentally shunted by a parallel resistance of 7.5 kΩ, the I-V characteristics show homogeneous multiple branches after the subtraction of the component. The penetrations of single vortices into the submicron stack were clearly observed in the resistance measurements. A vortex phase diagram was constructed by mapping the c-axis resistance on an H-T plane. Temperature dependence of the first-vortex penetration field is consistent with the theoretical estimation on the formation of a pancake-vortex stack in the center of a superconducting strip.

Spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks

Spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks

Monolithic Superconducting Emitter of Tunable Circularly Polarized Terahertz Radiation

We propose an approach to control the polarization of terahertz (THz) radiation from intrinsic Josephson-junction stacks in single crystalline high-temperature superconductor $Bi_2Sr_2CaCu_2O_{8+\delta}$. By monolithically controlling the surface current distributions in the truncated square mesa structure, we can modulate the polarization of the emitted THz wave as a result of two orthogonal fundamental modes excited inside the mesa. Highly polarized circular terahertz waves with a degree of circular polarization of more than 99% can be generated using an electrically controlled method. The emitted radiation has a high intensity and a low axial ratio (AR<1 dB). The intuitive results obtained from the numerical simulation based on the conventional antenna theory are consistent with the observed emission characteristics.

Josephson emission with frequency span 1\u201311 THz from small Bi2Sr2CaCu2O8+\u03b4 mesa structures

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity.

Self-Mixing Spectra of Terahertz Emitters Based on Bi2Sr2CaCu2O8+δ Intrinsic Josephson-Junction Stacks

Developing technology to exploit the ``THz window'' of the electromagnetic spectrum continues to be a major research theme. Stacks of intrinsic Josephson junctions in a cuprate superconductor are a good source of THz photons; with them, one can even imagine a THz laser on a chip. To better understand the purity of this radiation, the authors study the phenomenon of self-mixing, in which the nonlinearity of the Josephson elements leads to two or more nearby THz emission peaks producing signals at the difference frequencies. These results are helpful for synchronizing thousands of Josephson junctions, and may offer a simple method for evaluating the coherence of their THz emission.

Resonant Modes in a Stack of Intrinsic Josephson Junctions

In order to study the origin of coherent emission of intrinsic junctions in high-temperature superconductors, we investigated numerically ac power of emission at zero-field steps in IV-characteristics of a stack of two inductively coupled long Josephson junctions as well as emission from the solitary long junction. We found that zero-field steps appeared due to noise, random distribution of critical currents or due to normal edges of junctions in the stack. Due to the formation of collective excitations in the stack of coupled junctions, the zero-field step was split into two steps, which correspond to two normal modes of excitations. We investigated the dependence of heights of steps on the length of the stack and found that there existed two different threshold values of the length, so above the threshold length the zero-field step existed and below the threshold length it vanished. We calculated the dependence of ac power on the length of the stack and found the optimal value of the length for larger emission.

Dynamic Stimulation of Superconductivity With Resonant Terahertz Ultrasonic Waves

An experiment is proposed to stimulate a superconducting thin film with terahertz (THz) acoustic waves, which is a regime not previously tested. For a thin film on a piezoelectric substrate, this can be achieved by coupling the substrate to a tunable coherent THz electromagnetic source. Suggested materials for initial tests are a niobium film on a quartz substrate, with a BSCCO intrinsic Josephson junction (IJJ) stack. This will create acoustic standing waves on the nm scale in the thin film. A properly tuned standing wave will enable electron diffraction across the Fermi surface, leading to electron localization perpendicular to the substrate. This is expected to reduce the effective dimensionality, and enhance the tendency for superconducting order parallel to the substrate, even well above the superconducting critical temperature. This enhancement can be observed by measuring the in-plane critical current and the perpendicular tunneling gap. A similar experiment may be carried out for a cuprate thin film, although the conduction electrons might be more responsive to spin waves than to acoustic waves. These experiments address a novel regime of large momentum transfer to the electrons, which should be quite distinct from the more traditional regime of large energy transfer obtained from direct electromagnetic stimulation. The experiments are also motivated in part by novel theories of the superconducting state involving dynamic charge-density waves and spin-density waves. Potential device applications are discussed.

Dynamics of First and Second Switches in Bi$_{2}$ Sr$_{2}$CaCu $_{2}$O$_{8+\delta }$ Intrinsic Josephson Junction Stacks Measured by Specifically Designed Electronics

We measure switching probability distributions for the first and the second switches of stacks of Bi 2 Sr 2 CaCu 2 O 8+δ intrinsic Josephson junctions. To measure switching probability distributions, we design a current source and introduced a microcontroller. The resolution and accuracy are sufficient for measuring switching probability distributions of intrinsic Josephson junctions. For the first switch, crossover temperatures between thermally activated escape and macroscopic quantum tunneling are in the range between 0.8 and 2.2 K. The crossover temperatures of the second switch are approximately 8.0 and 6.8 K for the samples with critical densities of 1.0 and 2.0 kA/cm 2 , respectively. We analyze critical current density dependence of the crossover temperature of the second switch. The crossover temperature decreases with increasing critical current density. This anticorrelation can be explained neither by the single-junction model nor by the heating effect. The anticorrelation is a peculiar phenomenon observed in the second switch. We consider that the anticorrelation is attributed to the capacitive coupling. The strength of the capacitive coupling is weaker in IJJs with higher critical current density because the charge screening length is shorter due to increase of carrier density.

Engineering and characterization of a packaged high-Tc superconducting terahertz source module

We present an effective engineering technique for compactly packaging high-Tc superconducting continuous-wave terahertz source modules. A terahertz-emitting device, which consists of stacks of intrinsic Josephson junctions in single crystalline Bi2Sr2CaCu2O, bias electrodes, a collimating lens, and other components, is packaged into a single finger-sized assembly. The rigid and stable structure used for the packaging guarantees physical and chemical stability with good thermal contact, and provides reproducible characteristics with a high yield rate. The coherent terahertz waves can be emitted from the back side of the base crystal without significant screening. The intuitive results obtained from the numerical simulation are consistent with the observed thermal properties. The modules are easy to use, and thus intended for all users unfamiliar with superconducting electronic devices.

Tuning THz emission properties of Bi2Sr2CaCu2O8+δ intrinsic Josephson junction stacks by charge carrier injection

We report on doping and undoping experiments of terahertz (THz) emitting intrinsic Josephson junction stacks, where the change in charge carrier concentration is achieved by heavy current injection. The experiments were performed on stand-alone structures fabricated from a Bi2Sr2CaCu2O single crystal near optimal doping. The stacks contained about 930 intrinsic Josephson junctions. On purpose, the doping and undoping experiments were performed over only a modest range of charge carrier concentrations, changing the critical temperature of the stack by less than 1 K. We show that both undoping and doping is feasible also for the large intrinsic Josephson junction stacks used for THz generation. Even moderate changes in doping introduce large changes in the THz emission properties of the stacks. The highest emission power was achieved after doping a pristine sample.

High-resolution terahertz spectroscopy with a noise radiation source based on high-Tc superconductors

Stochastic fields can play a ‘constructive’ role in their interaction with quantum systems. In this paper, we demonstrate that the phase-diffusion field (PDF) in the terahertz range (THz) induces macroscopic polarization in molecular gas. We explain the observed effect using a simple model in which the PDF is treated as a series of ultrashort pulses of a regular signal, resulting in transient absorption in molecular gas. The experimental investigation of this effect is carried out using ammonia, and the PDF is generated by an oscillator based on stacks of Bi2Sr2CaCu2O8 intrinsic Josephson junctions. These radiation sources do not require a phase-lock loop system to provide high resolution, which simplifies the spectrometer considerably. The PDF radiation sources open up new horizons for the development of compact high-resolution THz spectrometers and applications thereof.

A power-adjustable superconducting terahertz source utilizing electrical triggering phase transitions in vanadium dioxide

We report a practical superconducting terahertz (THz) source, comprising a stack of Bi2Sr2CaCu2O8 intrinsic Josephson junctions (IJJs) and a vanadium dioxide (VO2) tunable attenuator with coplanar interdigital contacts. The electrical triggering phase transitions are observed not only at room temperature, but also at low temperatures, which provides a proof of the electrical triggering. Applying this, the VO2 attenuator is implemented for the independent regulations on the emission powers from the IJJ THz emitter, remaining frequencies and temperatures unchanged. The attenuation can be tuned smoothly and continuously within a couple of volts among which the maximum is, respectively, −5.6 dB at 20 K or −4.3 dB at 25 K. Such a power-adjustable radiation source, including the VO2 attenuator, can further expand its practicability in cryogenic THz systems, like superconducting THz spectrometers.

Chimera States in an Intrinsically Coupled Stack of Josephson Junctions

Numerical simulations indicate that a stack of intrinsic Josephson junctions, as may be realized in a high temperature superconductor, can exhibit breathing chimera states. Previously, such states had only been reported for systems containing two populations, for which the coupling strength between oscillators in one population differed from that between the two populations. This is the first time that breathing chimera states have been observed in simulations of identical oscillators that are also identically coupled. Moreover, in a mechanical sense, the nature of the coupling in the present system corresponds to a non-trivial inertial coupling, which has not been explored previously.

Effect of Al and Ca co-doping, in the presence of Te, in superconducting YBCO whiskers growth.

High-Tc superconducting cuprates (HTSC) such as YBa2Cu3O7 - x (YBCO) are promising candidates for solid-state THz applications based on stacks of intrinsic Josephson junctions (IJJs) with atomic thickness. In view of future exploitation of IJJs, high-quality superconducting YBCO tape-like single crystals (whiskers) have been synthesized from Ca-Al-doped precursors in the presence of Te. The main aim of this paper is to determine the importance of the simultaneous use of Al, Te and Ca in promoting YBCO whiskers growth with good superconducting properties (Tc = 79-84 K). Further, single-crystal X-ray diffraction (SC-XRD) refinements of tetragonal YBCO whiskers (P4/mmm) are reported to fill the literature lack of YBCO structure investigations. All the as-grown whiskers have also been investigated by means of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Our results demonstrate that the interplay of Ca, Te and Al elements is clearly necessary in order to obtain superconducting YBCO whiskers. The data obtained from SC-XRD analyses confirm the highly crystalline nature of the whiskers grown. Ca and Al enter the structure by replacing the Y and the octahedral coordinated Cu1 site, respectively, as in other similar orthorhombic compounds, while Te does not enter the structure of whiskers but its presence in the precursor is essential to the growth of the crystals.