Intrinsic Josephson Junctions Research Articles

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.

Modeling of Intrinsic Josephson Junctions in High Temperature Superconductors under External Radiation in the Breakpoint Region

The current-voltage (IV) characteristics of the intrinsic Josephson junctions in high temperature superconductors under external electromagnetic radiation are calculated numerically in the parametric resonance region. We discuss a numerical method for calculation of the Shapiro step width on the amplitude of radiation. In order to accelerate computations we used parallelization by task parameter via Simple Linux Utility for Resource Management (SLURM) arrays and tested it in the case of a single junction. An analysis of the junction transitions between rotating and oscillating states in the branching region of IV-characteristics is presented.

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.

Fluctuation induced magneto-conductivity of Y3Ba5Cu8O18±x and YBa2Cu3O7−d

A comparative study of the fluctuation magneto-conductivity in [Formula: see text] (noted [Formula: see text]) and [Formula: see text] (noted [Formula: see text]) polycrystalline samples was carried out. Samples were synthesized in oxygen atmosphere using a standard solid state reaction technique. Phases and microstructure have been systematically investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magneto-conductivity was measured in a magnetic field ranging from 0 T to 7 T applied perpendicular to the current path direction. The magneto-conductivity data were analyzed in terms of the temperature derivative of the resistivity and the logarithmic temperature derivative of the conductivity [Formula: see text], where [Formula: see text] is the fluctuation conductivity. Analyses of the magneto-conductivity data reveal that [Formula: see text] possesses a better quality of intrinsic Josephson junction compared to [Formula: see text] one while in the former one the depression in superconducting temperature is more pronounced. The applied magnetic field narrowed substantially the three dimensions (3D) Gaussian regime and for [Formula: see text] sample this regime is vanished for high magnetic field ([Formula: see text][Formula: see text]1 T). Results are discussed in relation with the difference in the structure and microstructure [Formula: see text] and [Formula: see text]. The upper critical and the irreversibility magnetic fields were estimated and were found to be higher in the [Formula: see text] sample.

Staircase Structure of Shapiro Steps

We investigate IV-characteristics of coupled Josephson junctions which model the intrinsic Josephson junctions in high temperature superconductors under external electromagnetic radiation. A staircase structure of Shapiro steps is found in the branching region. Its origin is related to the coupling between junctions and their switching from rotating to oscillating states. This conclusion is tested by detailed analysis of the IV-characteristics as for total stack and for each junction in the stack. IV-curves of junctions in the stack are compared with the average of time derivative of phase difference. Experimental observation of this staircase structure would give us a proof of coupling between junctions and a way for precise measurement of its value. Such investigations would be also useful for a diagnostic of Josephson junctions in the stack.

Efficient Fabrication of Intrinsic-Josephson-Junction Terahertz Oscillators with Greatly Reduced Self-Heating Effects

The hunt is on for sources of photons to fill the so-called ``terahertz gap'' in the electromagnetic spectrum, for applications spanning medical diagnosis, imaging, security, ultrahigh-speed communication, and biotechnology. The authors suppress the inevitable self-heating of intrinsic Josephson junctions in Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$, realizing an emitter with strongly expanded tunability and a range from 0.3 to 1.63 THz---and potentially extending to 15 THz, which would cover almost the entire gap with one device.

Possible ways to synchronize phase dynamics in intrinsic Josephson junctions for terahertz radiation

Coherent terahertz (THz) radiations based on intrinsic Josephson junctions (IJJs) from a Bi2Sr2CaCu2O[Formula: see text] (BSCCO) mesa under a bias voltage were observed in 2007. Later on, a novel [Formula: see text] phase kink state was proposed which can explain the key experimental results. However, how to drive the system into the [Formula: see text] phase kink state efficiently still remains an open problem. In this work, we propose several ways to synchronize the system to the [Formula: see text] phase kink state. We start from random and typical regular initial phase configurations. It is found that starting from some initial phase configurations, the system can be driven efficiently into the [Formula: see text] phase kink state. The time spent before entering the [Formula: see text] phase kink states for these initial phase configurations is shorter when the initial phase configuration is further away from the uniform phase configuration.

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.

A high-Tc intrinsic Josephson junction emitter tunable from 0.5 to 2.4 terahertz

Strong, monochromatic, coherent and continuous terahertz (THz) radiation was generated from the intrinsic Josephson junctions in a cylindrical stand-alone mesa sandwich structure fabricated from a single crystal of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. By varying the base temperature and the dc bias current-voltage characteristic (IVC) points, the emission frequency is tunable from 0.5 to a record high 2.4 THz observed on two inner IVC branch points. Strong emission power peaks were observed at 1.0 THz and 1.6 THz. This device is hence an excellent candidate to fill the “THz gap” between ∼1.4 and 2.0 THz.

Millimeter wave characteristics of intrinsic Josephson junctions with planar equiangular spiral antenna in misaligned Tl2Ba2CaCu2O8 thin film

An intrinsic Josephson junctions (IJJs) microbridge with planar equiangular spiral antenna (PESA) is proposed and studied by simulation and experiment. This IJJs circuit is simulated firstly to obtain the minimum of reflection coefficient and pattern. Secondly, IJJs with PESA are fabricated on misaligned Tl2Ba2CaCu2O8 superconducting film. The millimeter wave characteristics are investigated by a Febry–Pérot resonator, which consists of a spherical mirror antenna and a plane mirror. At 37.4GHz, the IJJs and the millimeter wave show an optimum coupling, which deviate from the simulation for only 0.004GHz. In addition, the extent of the coupling between IJJs and the resonator is discussed at different angle for the polarization direction of the spherical mirror antenna with the microbridge. The result shows that the strongest coupling is obtained at 0–30° angle. Good conformance of measurements and simulations illustrate the effectiveness of our design in strong coupling between sample and resonator.

C-axis electrical resistivity of PrO1−aFaBiS2 single crystals

The high anisotropy in RO1−aFaBiS2 (R denotes a rare-earth element) superconductors demonstrates their potential use as intrinsic Josephson junctions, considering the weak coupling among BiS2–PrO(F)–BiS2 (superconducting–normal–superconducting) layers along the c-axis. We grew PrO1−aFaBiS2 single crystals using CsCl/KCl flux. The superconducting anisotropies of the grown single crystals were estimated to be approximately 40–50 from the effective mass model. The c-axis transport properties were characterized using single-crystal s-shaped intrinsic Josephson junctions with a focused ion beam. Along the c-axis, the crystals showed zero resistivity at 2.7 K and a critical current density of 1.33 × 103 A/cm2 at 2.0 K. The current–voltage curve along the c-axis displayed hysteresis. The c-axis transport measurements under a magnetic field parallel to the ab-plane revealed a “lock-in” state due to the Josephson vortex flow, indicating that BiS2 superconductors are promising candidates for intrinsic Josephson junctions.

Enhancement of the critical current of intrinsic Josephson junctions by carrier injection

We present a study of the doping effect by carrier injection of high-Tc superconducting Bi-based whiskers. The current was injected in the c-axis direction, i.e., perpendicular to the superconducting planes. Superconducting properties were investigated systematically as a function of the doping level. The doping level of one and the same sample was changed by current injection in very small steps from an underdoped state up to a slightly overdoped state. We have observed that Tc versus log (jc) exhibits a dome-shaped characteristic, which can be fitted by a parabola. As Tc versus carrier concentration has a parabolic form, too, it can be concluded that the critical current density jc increases exponentially with the doping level. The electron-trapping mechanism is interpreted in the framework of Phillips’ microscopic theory. In addition, the Joule heating effect in the intrinsic Josephson junction (IJJ) was controlled by carrier injection, and the effect of the non-equilibrium quasiparticle on the I–V curves of the IJJs was also discussed.

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.

Solution of Sine-Gordon Equations with Weak and Moderate Inductive Couplings under Bias Current and Dissipations

It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when a single crystal of cuprate high-Tc superconductor Bi2Sr2CaCu2O8+δ (BSCCO) is biased by a dc voltage. In this work we study the inductively coupled sine-Gordon equations which compose a good model for intrinsic Josephson junctions (IJJs) realized in BSCCO due to the strong layer structure. We derive a general solution for the coupled sine-Gordon equations valid for weak and moderate inductive couplings, which can enhance injection of dc energy into IJJs and convert it to THz EM radiation. This solution evolves into the π phase kink state known before for strong inductive coupling.

Terahertz Waves Emission From Two <formula formulatype="inline"><tex Notation="TeX">$\hbox{Bi}_{2}\hbox{Sr}_{2} \hbox{CaCu }_{2} \hbox{O} _{8}$</tex></formula> Mesas in Series

The dependence of terahertz (THz) emission power on the thickness of series Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> (BSCCO) mesas is studied in this letter. An intrinsic Josephson junction (IJJ) array with two BSCCO mesas in series is proposed. The current-voltage characteristics of the IJJs, the powers, and Fourier spectra of the THz emission are measured for different thicknesses of the mesas, which is gradually mounted by the argon ion beam milling. Experiment results indicate that the THz emission power grows as the thickness of the mesas is increasing. The power reaches a maximum value of about 0.502 μW at a frequency of 0.708 THz when the thickness is 1200 nm. However, the THz emission power decreases while the thickness of the mesas keeps increasing. This nonlinearity between the THz emission power and the thickness of the mesas is due to the cavity resonance of the mesas and the self-heating generated by the IJJs in the mesas. The structure of connecting two mesas in series can be extended to form an IJJ array of multiple series mesas, which could further improve the THz emission power.

Effect of Bias Electrode Position on Terahertz Radiation From Pentagonal Mesas of Superconducting &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\hbox{Bi}}_{2}{\hbox{Sr}}_{2}{\hbox{CaCu}}_{2}{\hbox{O}}_{8+\delta}$&lt;/tex&gt; &lt;/formula&gt;

We examined the effect of the bias current position on the radiation of intense and coherent terahertz electromagnetic waves from the intrinsic Josephson junctions (IJJs) in regular pentagonal mesas of single crystalline high- $T_{\rm c}$ superconducting ${\hbox{Bi}}_{2}{\hbox{Sr}}_{2}{\hbox{CaCu}}_{2}{\hbox{O}}_{8+\delta}$ . The emission from Sample 1, a mesa with a current bias feed on its middle, is strong and sharply peaked at 0.47 THz, whereas that from Sample 2, a mesa with its feed along an edge, shows a comparably strong, sharp peak at 0.43 THz. The common backbending in the current-voltage characteristics due to Joule heating was seen for Sample 1 but almost not for Sample 2. The experimental results are in good agreement with numerical simulations and calculations, both for the cavity resonances and the angular emission patterns. The resonance peaks obtained from these pentagonal mesas are both stronger and narrower than those found previously from differently shaped mesas fabricated by focused ion beam milling. The overall devices are small enough to be cooled to their operational temperatures by Stirling refrigerators, so that properly designed devices could be held in one hand as for laser pointers. Therefore, our IJJ-based emitter devices are promising candidates to fill the THz gap with compact, continuous-wave quantum solid-state sources.

Current Filamentation in LargeBi2Sr2CaCu2O8+δMesa Devices Observed via Luminescent and Scanning Laser Thermal Microscopy

Stacked intrinsic Josephson junctions in the well studied superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8}$ are promising as a compact source of coherent terahertz radiation, which would have applications ranging from bioimaging to security screening. Understanding localized heating in such stacks is essential to optimizing their performance, and to this end the authors directly image the complex self-heating behaviors of these devices under bias conditions typical for THz emission. For good heat removal, narrow (compared to device size) filaments of current nucleate hot spots in asymmetric locations, which is consistent with theoretical predictions and suggests a means to enhance emission power.

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.