Stacks Of Intrinsic Josephson Junctions Research Articles

Output from a Josephson stimulated terahertz amplified radiation emitter

The angular dependence of the radiation-zone output power and electric polarizationof stimulated terahertz amplified radiation (STAR) emitted from a dc voltageapplied across cylindrical and rectangular stacks of intrinsic Josephson junctions iscalculated. The boundary conditions are obtained from Love’s equivalence principles.During coherent emission, a spatially uniform ac Josephson current density inthe stack acts as a surface electric current density antenna source, leading toa harmonic radiation frequency spectrum, as in experiment, but absent in allcavity models of cylindrical mesas. Spatial fluctuations of the ac Josephson currentallow its fundamental mode to lock onto the lowest finite energy cylindrical cavitymode, causing it to resonate, leading to a non-uniform magnetic surface currentdensity radiation source, and a non-trivial combined fundamental frequency outputpower with linear polarization for general radiation directions, which may be fullyor partially coherent. The higher ac Josephson harmonics do not excite othercylindrical cavity modes. For rectangular mesas, the lowest energy modes areempirically not excited, but the non-uniform ac Josephson current can excitethe harmonic sequence of modes with spatial variation across the rectangularwidths, leading to combined radiation outputs both for the fundamental andthe higher harmonics, which combinations also may be either fully or partiallycoherent. The superconducting substrate is modeled as a perfect magnetic conductor,greatly reducing the STAR emitter power and modifying its angular dependence,especially parallel to the substrate. Based upon this substrate model, existingBi2Sr2CaCu2O8 + δ crystals atop perfect electric conductors could have STAR emitter power in excess of 5 mW,acceptable for many device applications.

Publisher’s Note: Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime [Phys. Rev. Lett.105, 057002 (2010)

Publisher’s Note: Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime [Phys. Rev. Lett.<b>105</b>, 057002 (2010)

Shunt-capacitor-assisted synchronization of oscillations in intrinsic Josephson junctions stack

We show that a shunt capacitor, by coupling each Josephson junction to all the other junctions, stabilizes synchronized oscillations in an intrinsic Josephson junction stack biased by a dc current. This synchronization mechanism is similar to the previously discussed radiative coupling between junctions, however, it is not defined by the geometry of the stack. It is particularly important in crystals with smaller numbers of junctions (where the radiation coupling is weak), and is comparable with the effect of strong super-radiation in crystals with many junctions. The shunt also helps to enter the phase-locked regime in the beginning of the oscillations, after switching on the bias current. Furthermore, it may be used to tune radiation power, which drops as the shunt capacitance increases.

Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime

We report on THz emission measurements and low temperature scanning laser imaging of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the ``cold'' part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

Response and amplification of terahertz electromagnetic waves in intrinsic Josephson junctions of layered high-Tcsuperconductor

We investigate the response of a stack of intrinsic Josephson junctions (IJJs) to terahertz (THz) electromagnetic (EM) irradiation. A significant amplification of the EM wave can be achieved by the IJJs stack when the incident frequency equals to one of the cavity frequencies. The irradiation excites pi phase kinks in the junctions, which stimulate the cavity resonance when the bias voltage is tuned. A large amount of dc energy is then pumped into the Josephson plasma oscillation, and the incident wave gets amplified. From the profound current step in IV characteristics induced at the cavity resonance, the system can also be used for detection of the THz wave.

First-principles calculations for anisotropy of iron-based superconductors

We perform first-principles calculations for anisotropy of various iron-based superconductors. The anisotropy ratio γλ of the c-axis penetration depth to the ab-plane one is relatively small in BaFe2As2, LiFeAs and FeSe, indicating that the transport applications are promising in these superconductors. On the other hand, in those having perovskite-type blocking layers such as Sr2ScFePO3 we find a very large value, γλ≳200, comparable to that in strongly anisotropic high-Tc cuprate Bi2Sr2CaCu2O8−δ. Thus, the intrinsic Josephson junction stacks are expected to be formed along the c-axis, and novel Josephson effects due to the multi-gap nature are also suggested in these superconductors. We also examine the doping effect on the anisotropy of LaFeAsO.

I–V characteristics in multi-gap intrinsic Josephson junction stacks

We develop a theory for the AC Josephson effect in 2-gap intrinsic Josephson junction stacks. The coupled dynamical equations which the phase differences obey are derived from the phenomenological effective Lagrangian. The equations can describe the longitudinal Josephson plasma and the Leggett mode propagating in the c-direction. Numerical results for the I–V characteristics are presented. The switching current to the voltage state is greatly reduced. The multiple-branch structure appears in the I–V characteristics (74 words)

Kink State in a Stack of Intrinsic Josephson Junctions in Layered High-T c Superconductors and Terahertz Radiation

A new family of dynamic states are found in a stack of inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, $(2m_l+1)\pi$ phase kinks with integers $m_l$'s stack along the c axis and lock neighboring junction together. Large dc power is pumped into plasma oscillation via kinks at the cavity resonance. The plasma oscillation is uniform along the c axis with the frequency satisfying the ac Josephson relation. Thus this state supports strong terahertz radiation and seems to be compatible with the recent experimental observations.

First-Principles Study for the Anisotropy of Iron-Based Superconductors toward Power and Device Applications

Performing the first-principles calculations, we investigate the anisotropy in the superconducting state of iron-based superconductors to gain an insight into their potential applications. The anisotropy ratio γ λ of the c -axis penetration depth to the a b -plane one is relatively small in BaFe 2 As 2 and LiFeAs, i.e., γ λ ∼3, indicating that the transport applications are promising in these superconductors. On the other hand, in those having perovskite type blocking layers such as Sr 2 ScFePO 3 we find a very large value, \(\gamma_{\lambda}\gtrsim 200\), comparable to that in strongly anisotropic high- T c cuprate Bi 2 Sr 2 CaCu 2 O 8-δ . Thus, the intrinsic Josephson junction stacks are expected to be formed along the c -axis, and novel Josephson effects due to the multi-gap nature are also suggested in these superconductors.

Stability of the kink state in a stack of intrinsic Josephson junctions

A new dynamic state characterized by (2ml+1)π static phase kink with integers {ml} is proposed recently in a stack of inductively coupled Josephson junctions. In the present paper, the stability of the phase kink state is investigated against various perturbations and it is shown that the kink state is stable. It is also discussed that the suppression of the amplitude of superconducting order parameter caused by the kink is weak.

Numerical simulation for THz wave emission from high-[formula omitted] intrinsic Josephson junctions

Emission of THz electromagnetic waves from BSCCO mesas covered with normal electrodes is numerically studied. We discuss the electromagnetic excitations corresponding to π- and 2π-cavity modes in a stack of intrinsic Josephson junctions with a thickness of ∼1μm and a width of ∼100μm, using the “coarse-grained”xz-model. It is shown that the AC electromagnetic field excited inside the junctions is nearly perfect standing-wave mode and then the emission takes place mainly from the electrodes.

Cavity phenomena in mesas of cuprate high-Tcsuperconductors under voltage bias

Modeling a single crystal of cuprate high-${T}_{c}$ superconductor, such as ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$, as a stack of intrinsic Josephson junctions, we formulate explicitly the cavity phenomenon of plasma oscillations and electromagnetic (EM) waves in mesas of cylindrical and annular shapes. The phase differences of the junctions are governed by the inductively coupled sine-Gordon equations, with the Neumann-type boundary condition for sample thickness much smaller than the EM wavelength, which renders the superconductor single crystal a cavity. Biasing a dc voltage in the $c$ direction, a state with $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ kinks in the superconductivity phase difference piled up alternatively along the $c$ axis is stabilized. The $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ phase kinks provide interlock between superconductivity phases in adjacent junctions, taking the advantage of huge inductive couplings inherent in the cuprate superconductors, which establishes the coherence across the whole system of more than $\ensuremath{\sim}600$ junctions. They also permit a strong coupling between the lateral cavity mode of the transverse Josephson plasma and the $c$-axis bias, and enhance the plasma oscillation significantly at the cavity modes which radiates EM waves in the terahertz band when the lateral size of mesa is set to tens of micrometers. It is discussed that the cavity mode realized in a very recent experiment using a cylindrical mesa can be explained by the present theory. In order to overcome the heating effect, we propose to use annular geometry. The dependence of frequency on the radius ratio is analyzed, which reveals that the shape tailor is quite promising for improving the present technique of terahertz excitation. The annular geometry may be developed as a waveguide resonator, mimicking the fiber lasers for visible lights.

Effect of interjunction coupling on superconducting current and charge correlations in intrinsic Josephson junctions

Charge formations on superconducting layers and creation of the longitudinal plasma wave in the stack of intrinsic Josephson junctions change crucially the superconducting current through the stack. Investigation of the correlations of superconducting currents in neighboring Josephson junctions and the charge correlations in neighboring superconducting layers allows us to predict the additional features in the current-voltage characteristics. The charge autocorrelation functions clearly demonstrate the difference between harmonic and chaotic behavior in the breakpoint region. Use of the correlation functions gives us a powerful method for the analysis of the current-voltage characteristics of coupled Josephson junctions.

Emission of Terahertz Waves From Stacks of Intrinsic Josephson Junctions

By patterning mesoscopic crystals of 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) into electromagnetic resonators the oscillations of a large number of intrinsic Josephson junctions can be synchronized into a macroscopic coherent state accompanied by the emission of strong continuous wave THz-radiation. The temperature dependence of the emission is governed by the interplay of self-heating in the resonator and by re-trapping of intrinsic Josephson junctions which can yield a strongly non-monotonic temperature dependence of the emission power. Furthermore, proper shaping of the resonators yields THz-sources with voltage-tunable emission frequencies.

Simulation of the Effect of Temperature on Flux-Flow Behavior in Stacked Intrinsic Josephson Junctions

We have numerically studied the effect of temperature on the flux-flow behavior in a stack of intrinsic Josephson junctions by using the inductive coupling Josephson junctions model taking into account thermal fluctuations. In the absence of noise, the current-voltage characteristic shows a series of clear Fiske and flux-flow steps depending on an external magnetic field. With increasing temperature the Fiske steps corresponding to the out-of-phase mode remain on the current-voltage characteristic but other steps disappear. This suggests that the out-of-phase cavity mode becomes more stable than other modes due to the fluctuations. Furthermore, it is found that the in-phase flux flow mode is also strongly affected by the thermal noise in contrast to the out-of-phase one and fluctuations disturb the formation of the rectangular lattice of vortices even in the flux-flow mode driven by a strong Lorentz force Our results can qualitatively explain recent experimental observations in intrinsic Josephson junctions in the flux-flow state.

Resonant electromagnetic emission from intrinsic Josephson-junction stacks in a magnetic field

We study the effect of an applied magnetic field on the resonant radiation from stacks of intrinsic Josephson junctions. We show that, when applying a moderate dc magnetic field in the plane of the junctions, a huge amplification of terahertz radiation can be obtained. This radiation enhancement occurs due to a modulation of the critical current. Furthermore, a dc magnetic field reducing the ``radiation coupling'' between junctions, produces a peculiar frequency-driven transition from stable to unstable terahertz radiation resonance.

Mechanism of Terahertz Electromagnetic Wave Emission from Intrinsic Josephson Junctions

Using a 3D parallelepiped model of the stack of intrinsic Josephson junctions, we calculate the cavity resonance modes of Josephson plasma waves excited by external electric currents. The cavity modes accompanied by static phase kinks of the order parameter have been intensively investigated. Our calculation shows that the kink phase state is unfavorable, since the static phase kinks reduce the order parameter amplitude and thus the superconducting condensation energy. We point out that the oscillating magnetic field of the cavity mode penetrates the vacuum from the sample surfaces and the energy of the magnetic field plays an important role to determine the orientation of the cavity resonance mode. On the basis of the above discussions, we calculate the I-V characteristic curve, the THz wave emission intensity and the other physical quantities.

Comparison between the first- and the second-junction switchings in a small stack of Bi2Sr2CaCu2Oy intrinsic Josephson junctions

We measured the switching current distributions of the first- and the second-junction switchings in a small stack of Bi2Sr2CaCu2Oy intrinsic Josephson junctions. Switching distribution for the first switching from the zero-voltage state became independent of temperature below 0.8 K suggesting the crossover from thermal activation to macroscopic quantum tunneling. On the other hand, switching distribution for the second switching from the first resistive state showed the similar temperature-independent behavior up to much higher temperature than the crossover temperature for the first switching. We argue that it cannot be explained in terms of self-heating owing to the dissipative currents after the first switching.

Macroscopic quantum effects in capacitively- and inductively-coupled intrinsic Josephson junctions

A theory for macroscopic quantum tunneling (MQT) in intrinsic Josephson junction stacks is formulated. Both capacitive and inductive couplings between junctions are taken into account. We calculate the escape rate in the switching to the first resistive branch in the quantum regime. It is shown that the enhancement of the escape rate is caused mainly by the capacitive coupling between junctions in IJJ's with small in-plane area of ∼ 1μm2.

Observation of macroscopic quantum tunneling in La2-xSrxCuO4 intrinsic Josephson Junctions

We report on our success in (i) fabricating stacks of small intrinsic Josephson junctions using the high temperature superconductor La2-xSrxCuO4 (x ≈ 0.08), and carrying out a reliable measurement of their switching current distributions. The standard deviation of the switching current converged below ∼ 5 K, suggesting the occurrence of macroscopic quantum tunneling (MQT) below this temperature. It was also found that the distribution of switching current at MQT region is quite broader than that expected from the theoy of conventional single Josephson junction model.