Intrinsic Josephson Junctions Research Articles

Intrinsic Josephson junction characteristics of Nd2-xCexCuO4 /SrTiO3 epitaxial films

The current-voltage (I-V) properties along the c axis on Nd2-xCexCuO4/SrTiO3 epitaxial films with x = 0.145, 0.15 were investigated. For all the samples it has been established that the I-V characteristics exhibit several resistive branches, which correspond to the resistive states of individual Josephson junctions. The results confirm the idea of a tunneling mechanism between the CuO2 layers (superconductor - insulator - superconductor junction) for the investigated Nd2-xCexCuO4 compound. The I-V dependence of this compound with x = 0.15 points out on the nonmonotonic nature of the d-wave or anisotropic s-wave symmetry order parameter associated with the coexistence of superconductivity and antiferromagnetic fluctuations.

Chaotic behavior in Josephson junction for high-quality random-number generation

The configuration of a high-quality security system or computer using a Monte Carlo simulation high-quality random-number generator has been expected. The high-quality random-number generators are obtained using physical randomness phenomena. In particular, chaos is an appropriate random-number generator. First, we investigate the chaotic behavior in the Josephson junction (JJ) under irradiation with external radio frequency (RF) at 77 K using an equivalent circuit of the JJ. JJ is assumed to be an intrinsic JJ in a Bi2Sr2CaCu2O8+δ single crystal. The output voltages of the JJ behave chaotic under some conditions and are evaluated using the Lyapunov exponent, Poincaré section, and attractors. Next, random numbers were generated from the chaotic output voltage in rgw JJ under irradiation with RF and evaluated by a verification test. Thus, random-number generators can be applied using the output voltage of theJJ under irradiation with external RF.

Study on the effect of Bi/Sr ratio on the device characteristics of high-Tc superconducting terahertz wave emitters made of Bi2Sr2CaCu2O8+δ single crystals

To obtain high-performance THz-wave-emitting devices made of single crystals of Bi2Sr2CaCu2O8+δ (Bi2212), a high-temperature superconductor, an understanding of the device characteristics based on crystal characteristics can be a key issue because, in principle, the electrical properties of the intrinsic Josephson junctions (IJJs) constructed in Bi2212 crystals highly depend on crystal conditions, such as carrier concentration, crystal homogeneities, and crystal defects. To evaluate the tendencies of the device characteristics associated with crystal characteristics, we prepared Bi2212 crystals with different Bi/Sr ratios (x=0.05, 0.15, and 0.25) and δ values (annealed under N2 or O2 gas flow conditions). The unit cell parameter c decreased as the Bi/Sr ratio or δ increased. For the same annealing conditions under N2 gas flow, the superconducting transition temperature as well as the size of the hysteresis loop of the current–voltage characteristics and emission characteristics were significantly suppressed for the sample with x=0.25 compared with the corresponding values for the samples with x=0.05 and 0.15. The experimental results clearly indicate that parameters, such as the Bi/Sr ratio and annealing conditions, are crucial factors in determining the electrical characteristics of a device. This information can be a useful guide for the preparation of crystals for IJJ THz-wave devices that can be fine-tuned according to the desired device characteristics.

Wide-band frequency modulation of a terahertz intrinsic Josephson junction emitter of a cuprate superconductor

Wide-band frequency modulation of a terahertz intrinsic Josephson junction emitter of a cuprate superconductor

Time-reversal symmetry breaking superconductivity between twisted cuprate superconductors.

Twisted interfaces between stacked van der Waals (vdW) cuprate crystals present a platform for engineering superconducting order parameters by adjusting stacking angles. Using a cryogenic assembly technique, we construct twisted vdW Josephson junctions (JJs) at atomically sharp interfaces between Bi2Sr2CaCu2O8+x crystals, with quality approaching the limit set by intrinsic JJs. Near 45° twist angle, we observe fractional Shapiro steps and Fraunhofer patterns, consistent with the existence of two degenerate Josephson ground states related by time-reversal symmetry (TRS). By programming the JJ current bias sequence, we controllably break TRS to place the JJ into either of the two ground states, realizing reversible Josephson diodes without external magnetic fields. Our results open a path to engineering topological devices at higher temperatures.

Twisted cuprate van der Waals heterostructures with controlled Josephson coupling

Twisted van der Waals (vdW) heterostructures offer a unique platform for engineering the efficient Josephson coupling between cuprate thin crystals harboring the nodal superconducting order parameter. Preparing the vdW heterostructures-based Josephson junction comprising stacked cuprates requires maintaining an ordered interface with preserved surface superconductivity. Here, we report the preparation of the Josephson junction out of the stacked Bi2Sr2CaCu2O8+d crystals using the cryogenic dry transfer technique and encapsulating the junction with an insulating layer, that protects the interface during the electrical contacts evaporation at the 1 × 10−6 mbar base pressure. We find that the Josephson critical current Ic has a maximum at low twist angles, comparable to that of the bulk intrinsic Josephson junctions, and is reduced by two orders of magnitude at twist angles close to 45°. The reduction of Ic occurs due to a mismatch between superconducting d-wave order parameters, which suppresses the direct Cooper pair tunneling.

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

Terahertz Radiations and Switching Phenomena of Intrinsic Josephson Junctions in High-Temperature Superconductors: Josephson Phase Dynamics in Long- and Short-Ranged Interactions

Publisher's Note: “Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals” [J. Appl. Phys. 133, 163904 (2023)

Publisher's Note: “Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals” [J. Appl. Phys. 133, 163904 (2023)

Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals

We fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystals annealed under oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device, we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, the O2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hysteresis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresis loops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygen content along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin films during the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitive than that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.

Tailoring Bi2Sr2CaCu2O8+δ surface Josephson junctions

Bi2Sr2CaCu2O8+δ (BSCCO) single crystals are promising for high-temperature superconducting electronic devices with ultimate performance. Recently, tailoring superconductivity in ultra-thin BSCCO locally has received wide attention and interest. In this work, we focus on controlling the superconductivity of the uppermost CuO2 double layer of the BSCCO crystal. Gold electrodes are deposited onto the freshly cleaved BSCCO surface by a high-vacuum in situ evaporation technique at room temperature, and then surface Josephson junctions (SJJs) between the two outermost superconducting CuO2 double layers are fabricated as a probe to evaluate the superconductivity of the uppermost double layer. We find that the junction transition temperature Tc′ as well as its critical current density jc depend systematically and reproducibly on the deposition rate of the gold electrode. The electric properties of the SJJs, spanning the range from completely degraded to almost as good as the intrinsic Josephson junctions inside the bulk, systematically reveal the evolutionary process of superconductivity of the uppermost BSCCO layer, extending the range for practical applications.

Compact High-Tc Superconducting Terahertz Emitter with Tunable Frequency from 0.15 to 1 THz

A compact cryogenic terahertz emitter is highly desired for applications in terahertz astronomy with a broad frequency range of emissions and relatively high emission power. In this paper, we report on a terahertz emitter based on Bi2Sr2CaCu2O8 + δ (BSCCO) intrinsic Josephson junctions, with a frequency range from 0.15 to 1.01 THz. The emitter is a square gold-BSCCO-gold mesa on a sapphire substrate fabricated by a simple and efficient method. The highest emission power of 5.62 μW at 0.35 THz was observed at 50 K. A record low emission frequency of 0.15 THz was achieved at 85 K, extending the applicability of BSCCO terahertz emitters in the low-frequency range.

Polarization analysis of terahertz emission from Bi-2212 cross-whisker intrinsic Josephson junction devices and their refractive index

Polarization analyses of the terahertz (THz) emission from Bi2Sr2CaCu2O8+δ whisker crystals used for superconducting THz emitters were conducted. The THz emission mode was estimated by a simple polarization measurement, and a simulation study was conducted to examine the validity of the polarization analysis. The refractive index of whisker crystals revealed through the polarization analyses was greater than that of bulk single crystals and agreed well with our previous THz emission report. The simulation study suggested the complex plasma excitation mode of the THz emission, and an interpretation of the refractive index obtained in this study is provided.

Circuit models of simultaneously biased intrinsic Josephson junction stacks for terahertz radiations in high-bias regime

Experiment-based circuit models for Josephson plasma emitters (JPEs) made of cuprate superconductor Bi2Sr2CaCu2O8+δ single crystals are proposed. Introducing an LCR resonator shunting an RCSJ parallel circuit allows us to reproduce experimental radiation intensity as a function of device voltage. Mismatch in the resonance frequencies of the total device and the LCR part is related to the wide-range tuning of the radiation frequency of JPE. It is found that coupling capacitances for simultaneously biased operations provide significant increases in radiation intensities, which have been reported as synchronized radiation from multiple mesas in the literature.

Simulation of switching to voltage state in an intrinsic Josephson junction

When a current applied to an intrinsic Josephson junction (IJJ) in a copper oxide high-temperature superconductor is increased, the IJJ enters a voltage state and then the voltage increases stepwise. After these voltage states, when the current is lowered, THz oscillation occurs. The voltage state is described as a running state of a particle in a washboard potential, using the RCSJ model of the Josephson junction (JJ). In this study, we investigate these running states in the IJJ, using a coupled layered JJ model with the finite element method. Especially, we focus on time evolution of phase differences in all JJs in the IJJ under weak external current.

Restoring the surface layer of Bi2Sr2CaCu2O8+δ single crystals by current injection

In 2010, we reported a new method to electronically tailor the hole concentration of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212). We discovered that current injection along the c-axis of Bi-2212 provides a convenient tool to reversibly increase the carrier concentration. In this study, we have investigated the current injection effect on the surface junction of Bi-2212 by a series of current injection experiments carried out in samples of slightly overdoped and of extremely underdoped Bi-2212 crystals. The c-axis transport characteristics of bulk and surface intrinsic Josephson junctions are examined in a wide range of doping by current injection. Before injection treatment, typically, our samples demonstrate non-superconducting characteristics across their surface junctions, because standard preparation procedures cause the topmost CuO2 layers to be heavily degraded by oxygen loss. The electronic doping enables to substantially restore the hole concentration of the topmost CuO2 layers. The contact resistance is significantly reduced, and the intrinsic junction at the surface gets back its superconducting tunneling characteristic. We examine systematically the doping results of surface and bulk junction stacks and discuss the enhancement of their superconducting properties.

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5 with T c ≃ 12 K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature T* ≃ 7 K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers θ = 0°. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below T*, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above T* expected for Abrikosov vortices. Furthermore, when θ is larger than 0.5°, the flux-flow resistance and critical current have no anomalous behaviors across T*. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers ξ ⊥ becomes shorter than half of the interlayer distance d/2. These experimental findings indicate the existence of intrinsic Josephson junctions below T* and thus quasi-two-dimensional superconductivity in Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5, similar to those in the cuprate superconductors.

Spontaneous Frequency Shift and Phase Delay of Coupled Terahertz Radiation Mediated by the Josephson Plasmon in a Cuprate Superconductor

We examine coupling interactions used to synchronize macroscopic Josephson oscillations induced in intrinsic Josephson-junction (IJJ) mesa stacks made of a $\mathrm{Bi}2212$ single crystal. Synchronized radiations of terahertz electromagnetic (EM) waves are detected under common voltage and current bias operations of two connected mesas with close individual radiation frequencies, while uncoupled and bimodal radiations are frequently observed in two mesas with different individual radiation frequencies. Detailed observations of the polarizations of the EM waves emitted when two mesas are biased in parallel or series allow us to reveal the coupling matrix components, which include ratios of synchronized IJJs in the mesas and phase delay between the macroscopic Josephson oscillations. A frequency evolution of the phase delay implies that the coupling between the Josephson oscillations is mediated by the small amplitude Josephson plasmon inside the superconducting substrate. This finding stimulates systematic survey on polarization of EM wave emitted from synchronized multiple mesa devices in order to realize powerful terahertz emissions from superconductors.

Chaos along the rc-branch of [formula omitted]-shunted intrinsic Josephson junctions

The appearance of chaos along the “charged” resonance circuit branch of the current-voltage characteristics is examined in a model of intrinsic Josephson junctions shunted by resistive, inductive and capacitive circuit elements. Detailed numerical simulations of the electric charge density, current-voltage characteristics and various chaos indicators, such as Lyapunov exponents and Poincaré sections are performed over a wide range of relevant system parameters. In mapping the parameter space, several distinctly different regions are found: some completely regular, while others are dominated by chaos, where the rc-frequency determines the appearance and properties of those regions. At higher values of the rc-frequency chaos appears at first over relatively narrow regions at the lower-current end of the rc-branch. As the rc-frequency is lowered, the chaotic region at the lowered-current end of the rc-branch may becomes wider, depending sensitively on the combination of system parameters. Examination of the metric entropy and maximal Kaplan–Yorke dimension shows that the dimensions of the chaotic attractors associated with the chaos do not plateau, as in the overdamped case, but surprisingly increase indefinitely with the number of junctions, indicating that the system is capable of truly high-dimensional chaos. The onset of the chaos along the rc-branch is found to occur via a two-frequency quasi-periodic route. Our results provided a guide for specific parameter combinations that could minimize the chaos, thereby making certain applications potentially more viable.

2D Bifurcation diagram of capacitively coupled Josephson junctions in high-Tc superconductors

We study the nonlinear dynamics in an array capacitively coupled Josephson junctions with N=3 junctions. We consider the superconductor/ insulator/ superconductor, SIS, type junctions as a model for high temperature superconductors with layered structure. In this study, we introduce the 2D bifurcation diagrams for the Josephson junctions. First we show a current voltage characteristics (CVC) of Intrinsic Josephson Junctions (IJJs) with N=3 Junctions, then we show the breakpoint region,BPR (a region in which a net electric charge appear on superconducting layers, (S-layers), of junctions), in the CVC of the system, then we try to investigate the chaos in BPR region by the 2D bifurcation diagram of the system. We show the 2D bifurcation diagrams in two cases in which either the coupling factor, α or the β parameter which is related to dissipation, be variable. In the later case we showed there is a range of β parameter in which the chaotic region of BPR is disappeared, also we show that there is another range of β in which the BPR structure disappeared.

Possibilities and Challenges of Superconducting Qubits in the Intrinsic Josephson Junctions

Possibilities and Challenges of Superconducting Qubits in the Intrinsic Josephson Junctions