Intrinsic Josephson Junctions Of Bi2Sr2CaCu2O8 Research Articles

Cavity mode identification for coherent terahertz emission from high-Tc superconductors.

Stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ emit intense and coherent terahertz waves determined by the internal electromagnetic cavity resonance. We identify the excited transverse magnetic mode by observing the broadly tunable emissions from a nearly square stack and simulating the scattering spectrum. We employ a wedge-type interferometer to measure spatially-integrated power independently of the far-field pattern. The simulation results are in good agreement with observed resonance behaviors as a function of frequency.

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%.

Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks with improved cooling: Coherent emission above 1 THz

We report on Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junction stacks with improved cooling, allowing for a remarkable increase in emission frequency compared to the previous designs. We started with a BSCCO stack embedded between two gold layers. When mounted in the standard way to a single substrate, the stack emits in the range of 0.43–0.82 THz. We then glued a second, thermally anchored substrate onto the sample surface. The maximum voltage of this better cooled and dimension-unchanged sample was increased and, accordingly, both the emission frequencies and the tunable frequency range were significantly increased up to 1.05 THz and to 0.71 THz, respectively. This double sided cooling may also be useful for other “hot” devices, e.g., quantum cascade lasers.

Terahertz emission and detection both based on high-Tc superconductors: Towards an integrated receiver

We have combined a stand-alone Bi2Sr2CaCu2O8 intrinsic Josephson junction stack, emitting terahertz radiation, with a YBa2Cu3O7 grain boundary Josephson junction acting as detector. The detector is mounted on a lens, positioned 1.2 cm away from the emitter on a similar lens. With the emitter radiating at 0.5 THz, we observed up to 7 Shapiro steps on the current-voltage characteristic of the detector. The ac current induced in this junction was 0.9 mA, and the dissipated power was 1.8 μW. The setup, although far from being optimized, may be considered as a first step towards an integrated high-Tc receiver.

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.

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.

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.

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.

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.

Investigation of critical current density of submicron intrinsic Josephson junctions ofBi2Sr2CaCu2O8 + δ at 30 K

The Cooper pair tunneling in the intrinsic Josephson junctions (IJJs)can be affected by quantum phase fluctuations when the resistance of asubmicron stack is in the range of the quantum resistance. To observe thisquantum effect we have fabricated stacks of IJJs of various in-plane area from4 µm2 downto 0.16 µm2 inBi2Sr2CaCu2O8 + δ single-crystalwhiskers using a three-dimensional focused-ion-beam etching technique. For stacks of in-plane area lessthan 1 µm2 a strong suppression in critical current density (Jc) isnoticed in current–voltage characteristics at 30 K. The possible mechanisms in this suppression ofJc arediscussed and the data analysis points to the quantum phase fluctuations as the most likely mechanismof this effect. We have achieved quantum fluctuations at 30 K reported for the first time ever forBi2Sr2CaCu2O8 + δ (Bi-2212) single-crystal whiskers.

Anomalously strong doping dependence of the maximum Josephson current in intrinsic Josephson junctions and inhomogeneous superconducting state in Bi2Sr2CaCu2O8+δ

Using intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ of which the doping level p is varied in a wide range, we have measured the magnitude of the superconducting order parameter Δ, the normal tunneling resistance RN, and the maximum Josephson current density Jc. It is found that Jc decreases quite rapidly by more than two orders of magnitude as p decreases, while Δ and RN change only moderately. This implies that the superconducting state of Bi2Sr2CaCu2O8+δ is inhomogeneous and nonsuperconducting regions coexist on a fine scale when p is small. Based on the Ambegaokar–Baratoff theory, the volume fraction of the superconducting regions is estimated to be 10% or less when p∼0.1.

Maximum Josephson current and inhomogeneous superconductivity in Bi2Sr2CaCu2O8+δ

The maximum Josephson current (MJC) is measured for a small thin stack of approximately 10 intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ (BSCCO) single crystals with various carrier doping levels. By interlayer tunneling spectroscopy, tunneling spectra are also measured to obtain values for the superconducting energy gap 2Δ/e. It is found that the MJC decreases with decreasing doping by more than two orders of magnitude from almost 104 A/cm2 to less than 102 A/cm2. The anomalous decrease of two orders of magnitude in the MJC is inexplicable in terms of the Ambegaokar-Baratoff theory. It is explained only when the superconducting state is inhomogeneous. This leads to an astonishing implication that the superconducting state in BSCCO is inhomogeneous and phase-separated on a fine scale.

Vortex-flow electromagnetic emission in stacked intrinsic Josephson junctions

We confirmed the existence of the collective transverse plasma modes excited by the motion of the Josephson vortex lattice in stacked intrinsic Josephson junctions of Bi2Sr2CaCu2O8+x by observing the multiple subbranches in the Josephson-vortex-flow current-voltage characteristics. We also observed the symptom of the microwave emission from the resonance between the Josephson vortex lattice and the collective transverse plasma modes, which provides the possibility of developing Josephson-vortex-flow electromagnetic oscillators.

Detailed characterization for YBCO intrinsic Josephson junctions by using small-sized junctions

In order to understand the detailed characteristics of YBa2Cu3O7−X (YBCO) intrinsic Josephson junctions (IJJs) without shunted grain boundary or non-uniformity, junction area should be reduced to sub-micron region. Hence, we fabricated small-sized IJJs on an YBCO whisker with high-quality crystalline, and investigated their transport properties. The small-sized IJJs, with junction areas of 0.12–0.36μm2, were fabricated by 3D focused ion-beam etching method. We measured current–voltage curves, and showed clear multiple branches with hysteresis structure, such as IJJs in Bi2Sr2CaCu2O8+X. Also, from the temperature dependence of critical current, it is found that the junction behavior drastically changes from that of SNS-type to SIS-type when the anisotropy is increased. The critical current densities of junctions were 10–100kA/cm2 orders, and they strongly depend on the critical temperature.

Sub-micron sized intrinsic Josephson junctions inYBa2Cu3O7−X whiskers

To investigate the detailed junction properties of intrinsic Josephson junctions (IJJs) inYBa2Cu3O7−X (YBCO) without shunted grain boundaries and with good uniformity, wefabricated sub-micron sized IJJs in a YBCO whisker by using a focused-ion-beametching. The sub-micron sized junctions showed clear multiple brancheswith a hysteresis structure in the current–voltage curve, like IJJs inBi2Sr2CaCu2O8+X. The temperature dependence of the critical current demonstrates that the junctionbehaviour drastically changes from that of an SNS type to an SIS type when the criticaltemperature is decreased. The critical current density of the junctions, in the order of10–100 kA cm−2, corresponds to Josephson plasma frequencies from a few hundred GHz to 1–2 THz.

Direct observation of self-heating in intrinsic Josephson junction array with a nanoelectrode in the middle

Modifying the double-sided fabrication method we invented a few years ago, a new type of Bi2Sr2CaCu2O8 intrinsic Josephson junction (IJJs) array is explored in which, in addition to the indispensable top and bottom electrodes, there is an electrode only 100 nm thick fabricated in the middle of the array. This electrode provides easy access to the inside of the IJJs, enabling us to understand the device physics better than was possible before. As the first application of such a new device, we have clearly shown how “hot” a junction stack can be under various bias conditions.

Suppression of the superconducting energy gap in intrinsic Josephson junctions ofBi2Sr2CaCu2O8+δ single crystals

We have observed back-bending structures at high bias current in the current–voltagecurves of intrinsic Josephson junctions. These structures may be caused by nonequilibriumquasiparticle injection and/or Joule heating. The energy gap suppression variesconsiderably with temperature. Different levels of the suppression are observed when thesame level of current passes through top electrodes of different sizes. Another effect whichis seen and discussed is a super-current ‘re-entrance’ of a single intrinsic Josephson junctionwith high bias current.

Fabrication and characteristics of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals

Mesa-structured intrinsic Josephson junctions are fabricated in Bi2Sr2CaCu2O8+x single crystals. Typical current-voltage characteristics of intrinsic Josephson junctions are observed, which include multiple quasi-particle branches, surface junction with critical current lower than those of inner junctions, and subgap structures on quasiparticle branches. The corresponding physical explanations are also given. The energy gap voltage of the intrinsic Josephson junctions at 30 K is about 20 mV. Besides, the measuredI c -T relationship agrees quite well with the theoretical computations based on\(d_{x^2 - y^2 } - wave\) superconductor. Our measureddI/dV−V relationship shows theV-shaped gap structure, obviously differing from the U-shaped gap structure of the s-wave superconductor.

Fabrication and characteristics of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals

Fabrication and characteristics of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals