Intrinsic Josephson Junctions Research Articles

Comparative Study of First and Second Switches with Nearly Identical Switching Current in Bi2Sr2Ca0.85Y0.15Cu2Oy Intrinsic Josephson Junctions

We present a study of the first and second switches (SWs) with nearly identical switching current $I_{\rm SW}$ in Bi$_2$Sr$_2$Ca$_{0.85}$Y$_{0.15}$Cu$_2$O$_y$ intrinsic Josephson junctions (IJJs). In contrast to early studies where a large difference between $I_{\rm SW}^{\rm 1st}$ and $I_{\rm SW}^{\rm 2nd}$ was observed, a crossover temperature $T_{\rm cr}$ to the quantum phase escapes for the 2nd SW is strongly reduced $(\sim$2.8~K) and agrees with that for the 1st SW. This confirms that the anomalous enhancement of $T_{\rm cr}^{\rm 2nd}$, commonly observed in the IJJs with $I_{\rm SW}^{\rm 1st} < I_{\rm SW}^{\rm 2nd}$, originates from the phase running state after the 1st SW. Detailed analysis of the microwave-induced escapes strongly suggests that nonlinear bifurcated phenomena possibly occurs in IJJs.

Local heating effects on the radiation intensity of high-Tc superconducting terahertz emitters

The terahertz electromagnetic waves can be generated by the mesa device made of a piece of high-Tc superconductor Bi2Sr2CaCu2O8+δ single crystal, which is known to be comprised of multi-stacks of intrinsic Josephson junctions. In the case of a thick mesa containing the large number of junctions, large amount of Joule heat is generated, resulting in the formation of the inhomogeneous temperature distribution T(r) in the mesa. It is known from previous studies that T(r) strongly affects the emission intensity and line width. When the mesa is irradiated partly by a focused laser beam, the terahertz radiation intensity grows almost two times as compared with the standard biases without laser irradiation as was found previously. It is also found that the enhanced emission remains without losing the intensity even after switching off the laser power. This result suggests that the normal current distribution in the mesa is modified irreversibly when the device is biased under laser irradiation, according to the thermoreflectance measurement.

Power enhancement of the high-Tc superconducting terahertz emitter with a modified device structure

Continuous and monochromatic terahertz electromagnetic waves can be generated with sizable power (∼30 μW) by the mesa-shaped device made of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, by synchronizing the phase of the Josephson currents between multi-stacked intrinsic Josephson junctions. From the previous experimental results on the line width, spatial distribution of the emission, etc., we think that the relatively weaker power may originate from the partial failure of the phase synchronization of the Josephson currents in the mesa device. This may be improved by modifying the device structure to enhance the Josephson current density inside the mesa. In the present study, an array of four rectangle-shaped mesas with the dimensions of 80±5 μm×350 μm×3.2 μm was fabricated on a thin Bi2Sr2CaCu2O8+δ single crystal, where several slots were made around the mesas by photolithography and chemical etching techniques in order to weaken electrical connection between mesa and superconductor base. The best result of the emission power for the array so far obtained is ~80 μW at 0.42 THz, which is about 2.7 times bigger than the previous champion data in our group.

Resonant Cavity Modes in Bi2Sr2CaCu2O8+x Intrinsic Josephson Junction Stacks

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency ${f}_{e}$ is continuously tunable via changing the bias current and the bath temperature, but at low bias ${f}_{e}$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.

Spontaneous and Controlled Chaos Synchronization in Intrinsic Josephson Junctions

Synchronization properties of model systems containing large numbers of phase oscillators have many potential biophysical and other applications. Biophysical examples include networks of pacemaker cells in the heart and suprachiasmatic nucleus of the brain. In physical systems, the phase dynamics of high- $T_{\rm c}$ superconducting materials continue to attract attention, since systems of intrinsic Josephson junctions form natural arrays of coupled phase oscillators. As such, they have the potential to act as systems in which various exotic synchronization effects may be observed. In this paper, we make a more detailed exploration of the parameter space containing regions of spontaneous chaos synchronization in the CCJJ + DC model of intrinsic Josephson junctions. Extensive regions of phase synchronization—corresponding to the Shapiro steps with zero charge density in the S-layers—are found through numerical simulation. By computing the Lyapunov exponent spectra, we see that the spontaneous chaos synchronization occurs within certain subregions that overlap with uncharged steps in the IV-characteristics. We tried to stabilize the spontaneous chaos synchronization over a wider range of parameters, by applying a global, noninvasive, and proportional control scheme. The control is affected by applying a time-dependent phase shift to the external electromagnetic radiation, proportional to the total voltage. The effect of the control is found to be three-fold. First, it tends to broaden the current interval over which lower harmonic Shapiro steps occur. Second, it does not change the width in the current range over which the chaos synchronization occurs. Third, it makes the chaos synchronization more robust to thermal noise. The chaos synchronization we report here may be useful in any applications requiring more powerful, high-frequency, and chaotic signals, such as in secure communication.

Süperiletken Durumda Cıva Bazlı Bakır Oksit Katmanlı Süperiletkenin Özgül Kapasitanslarının Tespiti

Mercury based copper oxide layered high temperature superconductor, which consists of superconductor–insulator–superconductor (SIS) layers, can be considered as a stack of nearly ideal, intrinsic Josephson junctions (IJJ). The SIS junction, where the electrical field is confined, topologically resembles a parallel-plate capacitor. As is known, the coupling between junctions in superconductors is capacitive. Hence, the determination of the specific capacitance (Cs) of the IJJ at the superconducting state has a crucial importance in order to give information about superconductivity mechanism. In this study, the Cs values of the investigated sample have been calculated by means of the critical current density, Jcand plasma frequency, that have been obtained from magnetic measurements taken at the below temperatures than the critical transition temperature, Tc. Moreover, Cs values at superconducting temperature have been compared to that of the normal temperature.

Circularly polarized terahertz radiation monolithically generated by cylindrical mesas of intrinsic Josephson junctions

We report emissions of circular polarized electromagnetic waves from cylindrically shaped mesa structures of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. The frequency range of circularly polarized emission of a cylindrical mesa with notched sides is between 400 and 430 GHz, which is wider than expected by the patch antenna theory. Three maxima recognized in emission intensity are presumably attributed to excitations of fundamental orthogonal modes and circularly polarized modes. Along with the demonstration of circularly polarized emission from truncated edge square mesas, the obtained results provide a wide variety of engineering designs of compact and solid-state electromagnetic sources which are able to generate circularly-polarized terahertz waves.

Fabrications of Small and High-quality Intrinsic Josephson Junctions by Combinatorial Method of Ar-ion and Focused Ga-ion Etchings

We present a study on the fabrication of the intrinsic Josephson junctions (IJJs), which are made of Bi2Sr2CaCu2Oy (Bi2212) single crystals, by using the focused Ga-ion etching and the Ar-ion etching. We made sure that the current-voltage (I-V) characteristics of small IJJs was never influenced by the direction of a Ga-ion beam in the focused ion beam (FIB) etchings. In addition, it was found that Ar-ion etching after FIB process is quite useful to improve the performance of the I-V curves of IJJs. By using a substrate with a small hole, we succeeded in employing the double-side Ar-ion etching after the FIB microfabrication. These results strongly support that the combinatorial method of FIB and Ar-ion etchings is very important to fabricate the small and high-quality IJJs.

Dynamics of Phase Switch in the Intrinsic Josephson Junctions Made of Bi2212 with Perfectly-stoichiometric Cation Compositions

We study the phase switching dynamics in the intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2Oy (Bi2212) cuprate superconductors, focusing on the effect of stoichiometry of cation composition. The current-voltage characteristics for the IJJs with the cation composition approaching to be perfectly stoichiometric (Bi:Sr:Ca:Cu=2:2:1:2) show the highly-underdamped nature with a very small retrapping current. We found that the switching current distribution P(I) for the IJJs with perfectly stoichiometric composition and less damage had double peaks or broadened. This suggests that the phase switched junction is not always specified in the case of IJJs with stoichiometric composition, since more than one junction with almost the same switching current is associated with the observed switching events. We obtained the magnitude of a critical current density Jc and an effective temperature Tesc for the phase escape composed of a single component, by analysing the bias current dependence of the switching rate Γ(I). The results indicate that the perfectly-stoichiometric cation composition can greatly enhance the magnitude of Jc, although a crossover temperature to the macroscopic quantum tunnelling (MQT) for the higher-ordered phase switches is not influenced by the enhancement of Jc, in contrast to the conventional MQT theory.

Effect of RF Isolation of Intrinsic Josephson Junctions Made of Solid Bi-2212 Film for Terahertz Radiation

This is the first report of terahertz (THz) radiation from intrinsic Josephson junction devices made of solid Bi 2 Sr 2 CaCu 2 O 8+δ thin films. It is shown that in situ laser trimming of the current lead structure could activate initially THz inactive devices. The trimming increased inductive reactance and improved RF isolation of intrinsic Josephson junctions. It is considered that the RF isolation is a key component to maintain a cavity resonance within intrinsic Josephson junctions essential to create THz waves from solid Bi-2212 film IJJ devices.

Critical Current of Intrinsic Josephson Junctions in Co/Au/BSCCO/Au/Co Hybrid Structure

Critical Current of Intrinsic Josephson Junctions in Co/Au/BSCCO/Au/Co Hybrid Structure

Enhancement of macroscopic quantum tunneling in the higher-order phase switches of Bi2212 intrinsic Josephson junctions

The macroscopic quantum tunneling (MQT) in the current-biased intrinsic Josephson junctions (IJJs) of high-Tc cuprates has attracted much attention for decades. Although the MQT for the phase switches from the zero to the first voltage state (1st SW) in the multiple-branched I-V curves is well explained by the conventional theory, the occurrence of MQT for the higher order switches such as the switch from the 1st to 2nd voltage state (2nd SW) has been still debated. Here, we present an experimental study on the phase switches of small IJJs fabricated from underdoped Bi2Sr2(Ca,Y)Cu2Oy. We observed the single photon transition between quantized energy levels in the 3rd phase switches at 59.15 GHz and 2 K. The comparison with the previous studies on the nearly optimal-doped Bi2Sr2CaCu2Oy clearly suggests a possibility that the MQT rate for the higher-order phase switches is commonly enhanced by the effective suppression of the energy barrier for the higher-order phase escape due to the phase-running state after the 1st SW, in spite of the large difference in a critical current density and Tc.

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.

Detailed investigation of the bifurcation diagram of capacitively coupled Josephson junctions in high-Tc superconductors and its self similarity

We study an array of coupled Josephson junction of superconductor/insulator/superconductor type (SIS junction) as a model for high temperature superconductors with layered structure. In the current-voltage characteristics of this system there is a breakpoint region in which a net electric charge appear on superconducting layers, S-layers, of junctions which motivate us to study the charge dynamics in this region. In this paper first of all we show a current voltage characteristics (CVC) of Intrinsic Josephson Junctions (IJJs) with N=3 Junctions, then we show the breakpoint region in that CVC, then we try to investigate the chaos in this region. We will see that at the end of the breakpoint region, behavior of the system is chaotic and Lyapunov exponent become positive. We also study the route by which the system become chaotic and will see this route is bifurcation. Next goal of this paper is to show the self similarity in the bifurcation diagram of the system and detailed analysis of bifurcation diagram.

Shift of Shapiro Step in High Critical Temperature Superconductors

The influence of the charge imbalance effect on a system of intrinsic Josephson junctions of high temperature superconductors under external electromagnetic radiation is investigated. We find that the charge imbalance is responsible for the slope of the Shapiro step in the IV-characteristic. The nonperiodic boundary conditions shift the Shapiro step from the canonical position which is determined by the frequency of the external radiation. We also find how the system parameters influence the shift of the Shapiro step.

Spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks

Spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks

Terahertz emission from the intrinsic Josephson junctions of high-symmetry thermally-managed Bi 2 Sr 2 CaCu 2 O 8+δ microstrip antennas

Klemm, RA (reprint author), Show less Univ Cent Florida, Dept Phys, 4111 Libra Dr, Orlando, FL 32816 USA, richard.klemm@ucf.edu

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.

Thermoreflectance microscopy measurements of the Joule heating characteristics of high-Tc superconducting terahertz emitters

Joule heating is the central issue in order to develop high-power and high-performance terahertz (THz) emission from mesa devices employing the intrinsic Josephson junctions in a layered high transition-temperature Tc superconductor. Here, we describe a convenient local thermal measurement technique using charge-coupled-device-based thermoreflectance microscopy, with the highest spatial resolution to date. This technique clearly proves that the relative temperature changes of the mesa devices between different bias points on the current-voltage characteristics can be measured very sensitively. In addition, the heating characteristics on the surface of the mesa devices can be detected more directly without any special treatment of the mesa surface such as previous coatings with SiC micro-powders. The results shown here clearly indicate that the contact resistance strongly affects the formation of an inhomogeneous temperature distribution on the mesa structures. Since the temperature and sample dependencies of the Joule heating characteristics can be measured quickly, this simple thermal evaluation technique is a useful tool to check the quality of the electrical contacts, electrical wiring, and sample defects. Thus, this technique could help to reduce the heating problems and to improve the performance of superconducting THz emitter devices.

Terahertz emission from the intrinsic Josephson junctions of high-symmetry thermally-managed Bi2Sr2CaCu2O8+δ microstrip antennas

We show for high-symmetry disk, square, or equilateral triangular thin microstrip antennas of any composition respectively obeying C∞v, C4v, and C3v point group symmetries, that the transverse magnetic electromagnetic cavity mode wave functions are restricted in form to those that are one-dimensional representations of those point groups. Plots of the common nodal points of the ten lowest-energy non-radiating two-dimensional representations of each of these three symmetries are presented. For comparison with symmetry-broken disk intrinsic Josephson junction microstrip antennas constructed from the highly anisotropic layered superconductor Bi2Sr2CaCu2O8+δ (BSCCO), we present plots of the ten lowest frequency orthonormal wave functions and of their emission power angular distributions. These results are compared with previous results for square and equilateral triangular thin microstrip antennas.