Intrinsic Josephson Junctions Research Articles

Numerical study for electromagnetic wave emission from intrinsic Josephson junction stacks with a dielectric cover

Electromagnetic (EM) wave emission from the intrinsic Josephson junction stacks (IJJ’s) covered with a thin dielectric medium is numerically investigated, using the multi-scale simulation method developed in our previous paper. It is shown that the power of emitted EM waves is considerably increased in the IJJ’s with a dielectric cover. The emission from the n=2 resonance mode is greatly enhanced. The enhancement is caused by the excitation of a solitonic mode.

New lock-in phenomena in intrinsic Josephson junctions of Bi2Sr2CaCu2O8+y with hole-array

Dynamical behaviour of Josephson and pancake vortices (JVs and PVs) in intrinsic Josephson junctions of Bi2Sr2CaCu2O8+y (Bi-2212) single crystal with a nano-size hole-array has been studied to measure the flow-resistance of the vortices. In the magnetic field perpendicular to the superconducting layers, flow resistance of PVs measured with the in-plane current shows a matching behaviour as usually observed at the matching fields. After the measurements, the sample was fabricated into the in-line shaped structure for the c-axis current measurements to obtain the JV flow-resistance. Instead of the usually-observed lock-in phenomenon of JVs in Bi-2212, several peaks are observed with changing the angle from the in-plane magnetic field to show the enhancement of the flow-resistance at some typical angles. When PVs are introduced into the sample with changing the angle, are trapped into holes, and are interacted with JVs, it is clearly seen that the well-aligned PVs cause the enhancement of the JV flow-resistance.

Tunable terahertz emission from the intrinsic Josephson junctions in acute isosceles triangular Bi_2Sr_2CaCu_2O_8+δ mesas

In order to determine if the mesa geometry might affect the properties of the coherent terahertz (THz) radiation emitted from the intrinsic Josephson junctions in mesas constructed from single crystals of the high-temperature superconductor, Bi₂Sr₂CaCu₂O₈+δ, we studied triangular mesas. For equilateral triangular mesas, the observed emission was found to be limited to the single mesa TM(1,0) mode. However, tunable radiation over the range from 0.495 to 0.934 THz was found to arise from an acute isosceles triangular mesa. This 47% tunability is the widest yet observed from the outer current-voltage characteristic branch of such mesas of any geometry. Although the radiation at a few of the frequencies in the tunable range appear to have been enhanced by cavity resonances, most frequencies are far from such resonance frequencies, and can only be attributed to the ac-Josephson effect.

Temperature and current dependencies of terahertz emission from stacks of intrinsic Josephson junctions with thin electrodes revealed by a high-resolution FT-IR spectrometer

We report on emission of electromagnetic wave in a frequency range of 1012 hertz (THz) from stacks of intrinsic Josephson junctions (IJJ) made of superconducting Bi2Sr2CaCu2O8+δ single crystals. A home-built high-resolution Fourier-transfer-infrared spectrometer reveals that the emission spectrum is monochromatic and the width is as sharp as its resolution limit (∼1GHz). The THz emission is obtained in a broad temperature and current range depending on the mesa. The emission frequency is tuned from 0.55 to 0.45THz by changing temperature from 20 to 55K.

Electric Field Effect in Intrinsic Josephson Junctions

Abstract The electric field effect in intrinsic Josephson junction stacks (IJJ's) is investigated on the basis of the capacitively-coupled IJJ model. We clarify the current-voltage characteristics of the IJJ's in the presence of an external electric field. It is predicted that the IJJ's show a dynamical transition to the voltage state as the external electric field is increased.

An Approach to Use a Raw Single Crystal Whisker as Intrinsic Josephson Junctions Stack

We have developed a technique to observe the intrinsic Josephson effect in Bi2Sr2CaCu2O8+δ (Bi-2212) single crystal whiskers. In this technique, a raw Bi-2212 single crystal whisker used as intrinsic Josephson junctions (IJJs) along the c-axis. The technique is simple, quick, and less-cost processing. First, a whisker made to stand on ac-plane and then two electrodes were made in ab-planes on either side. The standing whisker with this configuration worked as IJJs. The standing whisker (1.6 mm×40 μm×3 μm) showed the transition temperature of about 84 K. The critical current was about 15 mA at 8 K (critical current density ∼23 A/cm2). We observed voltage gap of about 500 mV in current–voltage (I–V) characteristics. This corresponds to a few hundred of IJJs out of ∼2,000 IJJs in the whisker thickness. Observations reflect that the technique can be further improved with the single crystal quality, shape of single crystal whisker, and annealing conditions.

Study of coherent and continuous terahertz wave emission in equilateral triangular mesas of superconducting Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions

We report on intense and coherent terahertz (THz) electromagnetic (EM) waves emitted from equilateral triangular mesa structures of the intrinsic Josephson junctions (IJJs) in single crystalline high-Tc superconducting Bi2Sr2CaCu2O8+δ. The focused ion beam milling technique is used for mesa fabrication. THz radiation is observed when the emission frequency is in the vicinity of the primary cavity resonance frequency determined by the mesa geometry. We also investigated numerically the THz radiation from such mesas using the finite difference time domain method. We found an apparent EM mode similar to the known TM(1, 0) = TM(0, 1) cavity mode during the THz emission.

An analysis of three dimensional radiation patterns from intrinsic Josephson junctions with hot spot

In this study, we investigate numerically the terahertz radiation from the mesa-structured intrinsic Josephson junctions (IJJs). The Sine–Gordon equation for inside of the mesa, and the Maxwell equation for outside of the mesa, are solved simultaneously. We consider the influence of a hot spot in the mesa where jc locally decreases, and calculate the radiation power as a function of the voltage. We observe strong radiation when the ac Josephson frequency satisfies the cavity resonance condition. For this strong radiation from the IJJs mesa, we calculate the radiation patterns. The radiation patterns reflect the existence of two types of internal modes: a uniform background mode and a cavity resonance mode. Furthermore, the radiation patterns are strongly affected by the diffraction at substrate edges.

Terahertz Wave Emission from Intrinsic Josephson Junctions in Bi2Sr2CaCu2O8+δ

We have studied the conditions for the THz radiation in many systems with different shapes and dimensions such as rectangles, squares, cylinders and (equilateral and isosceles including right-angled isosceles) triangles and rectangles with non-uniform width (asymmetric rectangles) fabricated from single crystals of Bi2Sr2CaCu2O8+δ, which are known as a typical intrinsic Josephson junction system. As a result two conditions have been deduced to be fulfilled: one is the ac-Josephson effect always working at each intrinsic Josephson junction. Another one is the standing wave formation of the THz waves excited inside the mesa due to the cavity resonance for the radiation. The former condition is always necessary for the radiation but the latter may have only the secondary role for the radiation because the Q-value of the resonance cavity in some cases seems to be quite low and of order of unity. Therefore, the radiation frequency can be controlled by the voltage effectively and widely varied by the ac-Josephson effect only. In addition, we show a new phenomenon observed at high current region and a comprehensive physical picture for the extraordinary THz radiation phenomenon is given.

THz-wave emission from inner I-V branches of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ

We have observed emissions of intense, continuous and monochromatic EM waves at THz frequencies from intrinsic Josephson junctions in the single crystalline Bi2Sr2CaCu2O8+δ mesa structure, which take place deep inside the multiple branch structures of the current-voltage characteristics. We measured a rectangular groove-type mesa sample with a nearly square shape fabricated by focused ion beam milling. The detection of emitted THz-waves and the spectroscopic measurement by the FT-IR spectrometer for obtaining the radiation frequency were performed simultaneously during the current-voltage measurement. By sweeping the voltage on a particular branch, we have observed that the radiation frequency varied in a relatively wider range than expected from the conventional cavity resonance condition, and that the frequency obeys the ac Josephson relation, in which the Josephson frequency is universally proportional to the voltage. These experimental facts may provide us significant information to understand the mechanism of the THz-wave emission from intrinsic Josephson junctions in order to make use of them as useful THz sources for many practical applications.

Terahertz Wave Emission from Intrinsic Josephson Junctions in Bi2Sr2CaCu2O8+δ

We have studied the conditions for the THz radiation in many systems with different shapes and dimensions such as rectangles, squares, cylinders and (equilateral and isosceles including right-angled isosceles) triangles and rectangles with non-uniform width (asymmetric rectangles) fabricated from single crystals of Bi2Sr2CaCu2O8+δ, which are known as a typical intrinsic Josephson junction system. As a result two conditions have been deduced to be fulfilled: one is the ac-Josephson effect always working at each intrinsic Josephson junction. Another one is the standing wave formation of the THz waves excited inside the mesa due to the cavity resonance for the radiation. The former condition is always necessary for the radiation but the latter may have only the secondary role for the radiation because the Q-value of the resonance cavity in some cases seems to be quite low and of order of unity. Therefore, the radiation frequency can be controlled by the voltage effectively and widely varied by the ac-Josephson effect only. In addition, we show a new phenomenon observed at high current region and a comprehensive physical picture for the extraordinary THz radiation phenomenon is given.

Magnetic field effects and dynamic control of terahertz electromagnetic wave emission from high-Tc superconducting Bi2Sr2CaCu2O8+δ mesa structures

Terahertz electromagnetic waves are very useful for a number of security and medical applications. Recently, intense and coherent terahertz emission from high-Tc superconducting Bi2Sr2CaCu2O8+δ (Bi2212) intrinsic Josephson junctions (IJJs) has been intensively investigated. In this paper, we report magnetic field effects and dynamic control of terahertz electromagnetic wave emission generated by the rectangular mesa structure of Bi2212. The magnetic field affects anisotropically the emission intensity at T = 25 K Terahertz emission is strongly suppressed by an external magnetic field (less than 200 Oe) parallel to the c axis. In contrast, a 20% enhancement of radiated power is observed when an appropriate magnetic field (H = φ0) is applied parallel to the ab plane. This anisotropy seems to result from the anisotropic vortex formation inside Bi2212. We also demonstrate that the intensity of terahertz emission can be controlled dynamically by applying a weak magnetic field, suggesting that we can modulate the continuous power of terahertz emission by applying weak magnetic field pulses.

Numerical Study of Radiation Pattern from Intrinsic Josephson Junctions Attached to Finite Size Substrates

In this study, we have investigated numerically three dimensional radiation patterns from mesa-structured intrinsic Josephson junctions (IJJs) attached to finite size substrates. We have calculated electromagnetic fields inside and outside of IJJs simultaneously using three dimensional calculation model. The radiation patterns emitted by the mesa have been calculated for three different substrates. We have found that the radiation patterns reflect the existence of dual radiation source, that is, uniform part of ac Josephson current and non-uniform part of ac Josephson current corresponding to the cavity modes. Moreover, we have found that the radiation pattern changes dramatically with the width of the substrates. The radiation patterns are strongly affected by the diffraction at substrate edges.

THz emission from a triangular mesa structure of Bi-2212 intrinsic Josephson junctions

Coherent, continuous and monochromatic terahertz electromagnetic waves were observed from the isosceles triangular mesa structures made of single crystalline high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) intrinsic Josephson junctions (IJJs), fabricated by the focus ion beam (FIB) technique. We studied experimentally the current-voltage (I–V) characteristics and the emission spectra by the FT-IR spectrometer in order to elucidate the radiation mechanism. The wide range of radiation frequency observed in the case of isosceles triangular mesa implies that the cavity resonance is not working properly but the ac-Josephson effect is working as a primary mechanism of THz radiation.

THz wave emission from intrinsic Josephson junctions controlled by surface impedance and in-plane magnetic field: Numerical study

Motivated by recent experiments on THz wave emission from intrinsic Josephson junctions without or in in-plane magnetic field, effect of surface impedance Z is investigated numerically. Without external field, dynamical phase transitions between the McCumber-like state and various π-phase-kink states occur as the bias current and Z are varied. Emission is optimized for Z ≍ 50 in the n = 1 mode. In this mode dependence of emission intensity on in-plane field shows a crossover behavior around Z = 50: monotonic decrease for Z > 50 and two characteristic peaks for Z < 50, each of which is consistent with a recent experiment.

Simultaneous Observation of Three Types of Terahertz Radiation from Bi2Sr2CaCu2O8 + δ Intrinsic Josephson Junctions

Terahertz radiation from mesa structures of Bi2Sr2CaCu2O8+δ is detected at various bias points with a Si-bolometer. These radiations are classified into these types: W observed at a current close to the critical current, R and IR observed reversibly and irreversibly, respectively. The type W is the most intense among the three and the radiating condition is complicated. Temperature dependence of radiation voltage of the type R and IR is much more significant than ever seen.

Coupling to External Structures: Boundary Conditions for the Bi2212-based Superconducting THz Emitter

It is known that a mesa fabricated from intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ emits intense terahertz radiation at a certain frequency in accordance with the ac-Josephson relation and the general resonance conditions determined by the geometrical shape and dimensions. When more than two mesas are placed on the crystal surface, synchronization phenomenon of two mesas and the cavity resonance due to the coupling to the external structures outside the biased mesa are observed. These phenomena are interpreted by the current propagation effect through the superconducting substrate over the range of a few hundreds of μm even with the phase coherency. This opens up the possibilities of improving the individual device by external antenna and cavity, and of constructing the large-scale planer array of mesas for the purpose of power enhancement.

Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λJ(Jc) and occur at about 0.4 and 1.3 fluxons per λJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

Cavity phenomenon and terahertz radiation of a tall stack of intrinsic Josephson junctions wrapped by a dielectric material

In order to enhance the radiation power in the terahertz band based on intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ single crystal, we investigate a long cylindrical sample embedded in a dielectric material. Tuning the dielectric constant, the radiation power has a maximum which is achieved when it equals the dissipation caused by the Josephson plasma. This yields the optimal dielectric constant of the wrapping material in terms of the properties of a BSCCO single crystal. The maximal radiation power is found proportional to the product of the critical superconducting current squared and the typical normal resistance, which offers a guideline for choosing superconductors as a source of strong radiation.

Effect of microwave irradiation on parametric resonance in intrinsic Josephson junctions

The effect of microwave irradiation on the phase dynamics of intrinsic Josephson junctions in high temperature superconductors is investigated. We demonstrate the influence of microwave’s amplitude variation on the current–voltage characteristics and on the time dependence (temporal oscillations) of the electric charge in the superconducting layers. A remarkable changing of the longitudinal plasma wavelength at parametric resonance is shown. We demonstrate an effect of the microwave radiation on the width of the parametric resonance region.