Josephson Junctions In 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.

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.

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.

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

Resistive state of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ

We have measured current-voltage (I – V) characteristics of Bi2Sr2CaCu2O8+δ in c-axis direction. Exclusion of self-heating by applying pulse currents of less than 1-μs-duration time enabled I – V measurements up to normal region.