An Effective Method for Improving Emissions from Intrinsic Josephson Junction Arrays of Misaligned Tl2Ba2CaCu2O8+δ Thin Films

In this paper, an effective method for improving emissions from Josephson junction arrays was proposed. We took the substrate as a dielectric resonance antenna to improve the coupling of intrinsic junctions to electromagnetic waves. Electromagnetic simulations were performed in parallel with experiment investigations. Sharp radiation peaks at 76.9?GHz were detected from intrinsic Josephson junction arrays in misaligned Tl₂Ba₂CaCu₂O_8+?? thin films at 78?K. Experimental results proved the effectiveness of this method, which offered a competitive choice for coherent emission from intrinsic Josephson junction arrays.     

Radiation from Intrinsic Josephson Junctions in Misaligned Tl2Ba2CaCu2O8 Thin Films

We measure the current?Cvoltage (I?CV) curves of intrinsic Josephson junctions (IJJs) in misaligned Tl₂Ba₂CaCu₂O₈ thin films embedded in a Fabry?CPérot (F-P) resonator at liquid nitrogen temperature. Regarding the substrate as a dielectric resonator, electromagnetic coupling between the intrinsic Josephson junctions is improved. By adjusting the location of the substrate in the F-P resonator, the critical current of the IJJs under microwave irradiation (75.6?GHz) is suppressed almost to zero. Two pairs of symmetrical radiation (75.6?GHz) peaks are detected. Possible explanations for this experimental result are discussed.     

Josephson Bicrystal Junctions on Sapphire Substrates for THz Frequency Application

Thin film metal oxide superconducting bicrystal junctions on sapphire substrates with I _c R _N products up to 2.5 mV at 4.2 K for width 4 μm and normal-state junction resistance 10–60 Ω were fabricated and characterized at dc and THz frequency. Three types of samples—one with broadband log-periodic antenna, another with double-slot antenna for 300 GHz and third one with double-slot antenna for 400 GHz—have been investigated at THz frequency. New design of antenna coupling with Josephson junction was elaborated for minimization of THz frequency losses in superconducting film. For a particular case of f=320 GHz double-slot antenna, a ratio for bandwidth Q = f/Δf ≈ 10 was measured.     

A Superconducting Cold-Electron Bolometer with SIS’ and Josephson Tunnel Junctions

A novel concept of a Superconducting Cold-Electron Bolometer (SCEB) with Superconductor-Insulator-Weak Superconductor (SIS’) Tunnel Junction and Josephson Junction has been proposed. The main innovation of this concept is utilizing the Josephson Junction for DC and HF contacts, and for thermal isolation. The SIS’ junction is used also for electron cooling and dc readout of the signal. The SIS’ junction is designed in loop geometry for suppression of the critical current by a weak magnetic field. The key moment of this concept is that the critical current of the Josephson junction is not suppressed by this weak magnetic field and can be used for dc contact. Due to this innovation, a robust two layer technology can be used for fabrication of reliable structures. A direct connection of SCEBs to a 4-probe antenna has been proposed for effective RF coupling.      

Power handling capabilities of superconducting YBCO thin films: Thermally induced nonlinearity effects

We have measured the microwave power dependence of the surface impedance Z_s of YBa₂Cu₃O_x thin films up to very high microwave power levels. Films with different crystal qualities, including one with a bicrystal Josephson junction, were investigated. The experiments included both frequency-domain and pulsed time-domain measurements using a 14 GHz TE₀₁₁ dielectric cavity. Our results demonstrate that the dissipation of heat, generated by rf currents in the superconducting film, contributes to the observed nonlinearities in the surface resistance. The relative extent of this contribution is determined primarily by the film quality. A simple Fabry-Perot resonator model, combined with a cavity heat transfer model, was used to analyze the effects of such nonlinearities on the electromagnetic response of the dielectric cavity to a pulsed input signal.     

Experimental Studies to Realize Josephson Junctions and Qubits in Cuprate and Fe-based Superconductors

Cuprate superconductors are very promising in terms of Josephson junction device because of the large energy scale of high-T _c superconductivity. In particular, fabrication of qubits attracts lots of attention because of its primary importance for future computer technology. We will present some of our recent activities pointing to this purpose. (1) We succeeded in fabricating Josephson junction of cuprate without making any extra oxide barrier layer, rather putting Fe islands on the small strip of cuprate superconductors. This is very promising, since the fabrication of good Josephson junction was unsuccessful so far. Together with the introduction of the data, we discuss the possible mechanism of the weak-link fabrication in this structure. (2) We investigated the switching events in the I–V characteristics of the intrinsic Josephson junctions of Bi-cuprate superconductor, where macroscopic quantum tunneling (MQT) observation is well established recently. In addition to confirm the MQT for the first switching at 1 K in the multiple-branched current–voltage characteristics, we found that the temperature independence of the switching distribution for the second switching up to higher temperatures (10 K) is not due to the trivial Joule heating. We discuss the mechanism of the phenomena, including the possibility of MQT. (3) New Fe-based superconductors are also promising in terms of the application of superconductivity, since the anisotropy looks rather weak, in contrast to cuprates. We will introduce our trial to fabricate epitaxial thin films as the initial step to fabricate Josephson junction of this material.     

Confining Tiny MoO2 Clusters into Reduced Graphene Oxide for Highly Efficient Low Frequency Microwave Absorption

Herein, a supermolecular‐scale cage‐confinement pyrolysis strategy is proposed to build two dielectric electromagnetic wave absorbents, in which MoO₂ nanoparticles are sandwiched uniformly between porous carbon shells and reduced graphene oxide (RGO). Both sandwich structures are derived from hybrid hydrogels doped by two different crosslinkers (with/without oxygen bridge), which can precisely confine Mo source (e.g., PMo₁₂). Without adding magnetic components, both absorbents exhibit excellent low frequency absorption performance in combination with electrically tunable ability and enhanced reflection loss value, which is superior over other relative 2D dielectric absorbers and satisfies the requirements of portable electronics. Notably, introducing oxygen bridges in the crosslinker generates a more stable confining configuration, which in turn renders its corresponding derivative exhibiting an extra multifrequency electromagnetic wave absorption trait. The intrinsic electromagnetic wave adjustment mechanism of the ternary hybrid absorbent is also explored. The result reveals that the elevated electromagnetic wave absorbing property is attributed to moderate attenuation constant and glorious impendence matching. The cage‐confinement pyrolysis route to fabricate 2D MoO₂‐based dielectric electromagnetic wave absorbents opens a new path for the design of electromagnetic wave absorbents used in multi/low frequency.     

Cross-Whisker Intrinsic Josephson Junction as a Probe of Symmetry of the Superconducting Order Parameter

As a test of the superconducting order parameter, we have developed an intrinsic Josephson junction by the name of cross-whisker junction. This junction was made using two Bi₂Sr₂CaCu₂O_8+d single crystal whiskers. Two whiskers were connected at their c planes with various cross-angles. Angular dependence of the critical current densities shows d-wave-like fourfold-symmetry. However, the angular dependence is much stronger than that of the conventional d_x ^{2-y 2} wave. The Jc shows its smallest value around 45 deg, which suggests that the Josephson penetration depth becomes longer. We have successfully observed a Fraunhofer pattern in the cross-whisker junction with cross-angle 45 deg.     

Intrinsic Josephson effects in YBa2Cu3O7-films

We have developed a standard photolithographic process to fabricate intrinsic Josephson effect (IJE) devices on the basis of YBa₂Cu₃O₇ (YBCO) thin films. The two key issues to observe IJE in YBCO were structure sizes in the micron range and a reduction of the interlayer coupling by removal of chain oxygen.     

Josephson Vortex Flow and Plasma Amplification in High-T c Superconductors

We investigate Josephson vortex flow states in two device arrangements considered to amplify the excited Josephson plasma. First, we study an intrinsic Josephson junction embedded inside a wave guide. The simulation result in the system reveals that the amplitude of the excited electric field almost linearly grows along the junction plane at the resonant voltage with the Josephson plasma. Second, we simulate a system composed of two separated Josephson junctions embedded inside a long wave guide and confirm that the excited electric field is amplified by the synchronization between two junction sites.     

Theory of the AC Josephson effect in triplet superconducting junction

The quasiparticle and AC Josephson current in superconductor / insulator / superconductor (S/I/S) junctions including a triplet p-wave superconductor are calculated for arbitrary transmission of the junction. As a triplet pairing state we select one of the unitary pairing states which is a promising candidate for the superconducting state in Sr ₂ RuO ₄ . In p-wave superconductor / insulator /p-wave superconductor junction, both quasiparticle current and AC Josephson current are enhanced near zero-bias voltage due to the existence of zero energy state of p-wave superconductors. For the s-wave superconductor/ insulator / p-wave superconductor junction, the quasiparticle current at low voltage is suppressed due to the energy gap of s-wave superconductor. The first Fourier component of the AC Josephson current vanishes due to the difference of the parity between the two superconductors.     

Effect of Photodoping on the Fiske Resonances of YBa2Cu3O x Grain Boundary Josephson Junctions

We have expanded our studies on illuminated YBa₂Cu₃O _x grain boundary Josephson junctions (GBJJ) which show both dc Josephson properties (Fraunhofer pattern) and ac Josephson properties (Fiske resonance). Illuminating GBJJs with visible light changes the Josephson coupling. This change is characterized by an increase of the critical current and a large shift in the voltage position of the Fiske resonances. This effect is due to persistent photoinduced superconductivity (PPS) of the oxygen-depleted YBa₂Cu₃O _x barrier, similar to the PPS found in illuminated oxygen-deficient YBa₂Cu₃O _x thin films. From Fiske resonance experiments in GBJJs of different lengths, it is possible to study the velocity of the electromagnetic wave in the barrier and its change after illumination. Information on the parameters of the barrier, before and after illumination, is obtained from this study.     

The Josephson Effect in Nanoscale Tunnel Junctions

In nanoscale Josephson junctions, the Josephson coupling energy is usually comparable with the charging energy of the junction and with the typical energy of thermal fluctuations. Under these circumstances, phase fluctuations imposed by the electromagnetic environment of the junction crucially affect the junction electrical behavior. In particular, they determine the maximum supercurrent the junction can sustain. We discuss this quantity in the case where the junction is not resistively shunted, so that the I V characteristics of the junction remains hysteretic. For a simple, yet realistic, unshunted junction model, we obtain detailed predictions of the shape of the supercurrent branch of the I V characteristic. Finally, we present experimental results supporting the theoretical analysis and which demonstrate that the supercurrent in an unshunted nanoscale Josephson junction can indeed be of the order of its critical current.     

Effect of Photodoping on the Fiske Resonances of YBa2Cu3Ox Grain Boundary Josephson Junctions

We have expanded our studies on illuminated YBa₂Cu₃O _x grain boundary Josephson junctions (GBJJ) which show both dc Josephson properties (Fraunhofer pattern) and ac Josephson properties (Fiske resonance). Illuminating GBJJs with visible light changes the Josephson coupling. This change is characterized by an increase of the critical current and a large shift in the voltage position of the Fiske resonances. This effect is due to persistent photoinduced superconductivity (PPS) of the oxygen-depleted YBa₂Cu₃O _x barrier, similar to the PPS found in illuminated oxygen-deficient YBa₂Cu₃O _x thin films. From Fiske resonance experiments in GBJJs of different lengths, it is possible to study the velocity of the electromagnetic wave in the barrier and its change after illumination. Information on the parameters of the barrier, before and after illumination, is obtained from this study.     

Effect of strain and grain boundaries on dielectric properties in La0.7Sr0.3MnO3 thin films

High dielectric constant and its dependence on structural strain and grain boundaries (GB) in La_0.7Sr_0.3MnO₃ (LSMO) thin films are reported. X-ray diffraction, magnetization, and magneto-transport measurements of the LSMO films, made by pulsed laser deposition on two different substrates—MgO and SrTiO₃ (STO), were compared to co-relate magnetic properties with dielectric properties. At room temperature, in the ferromagnetic phase of LSMO, a high dielectric constant (6 × 10⁴) was observed up to 100 kHz frequency for the films on MgO, with polycrystalline properties and more high-angle GB related defects, while for the films on STO, with single-crystalline properties but strained unit cells, high dielectric constant (≈10⁴) was observed until 1 MHz frequency. Also, a large dielectric relaxation time with significant broadening from the Debye single-dielectric relaxation model has been observed in samples with higher GB defects. Impedance spectroscopy further shows that large dielectric constant of the single-crystalline, strained LSMO film is intrinsic in nature while that in the polycrystalline films are mainly extrinsic due to higher amount of GBs. The presence of high dielectric constant value until high frequency range rules out the possibility of “apparent giant dielectric constant” arising from the sample-electrode interface. Coexistence of ferromagnetism and high dielectric constant can be very useful for different microelectronic applications.     

Radiation from Flux Flow in Josephson Junction Structures

We derive the radiation power from a single Josephson junction (JJ) and from layered superconductors in the flux-flow regime. For JJ case, we formulate the boundary conditions for the electric and magnetic fields at the edges of the superconducting leads using the Maxwell equations in the dielectric media and find dynamic boundary conditions for the phase difference in JJ which account for the radiation. We derive the fraction of the power fed into JJ transformed into the radiation. In a finite-length JJ this fraction is determined by the dissipation inside JJ and it tends to unity as dissipation vanishes independently of mismatch of the junction and dielectric media impedances. We formulate also the dynamic boundary conditions for the phase difference in intrinsic JJs in highly anisotropic layered superconductors of the Bi₂Sr₂CaCu₂O₈ type at the boundary with free space. Using these boundary conditions, we solve equations for the phase difference in the linear regime of Josephson oscillations for rectangular and triangular lattices of Josephson vortices. In the case of rectangular lattice for crystals with the thickness along the c-axis much larger than the radiation wavelength, we estimate the radiation power per unit length in the direction of magnetic field at the frequency 1 THz as ∼N μW/cm for Tl₂Ba₂CaCu₂O₈ and ∼0.04 N μW/cm for Bi₂Sr₂CaCu₂O₈. For crystals with thickness smaller than the radiation wavelength, we found that the radiation power in the resonance is independent on number of layers and can be estimated at 1 THz as 0.5 W/cm (Tl₂Ba₂CaCu₂O₈) and 24 mW/cm (Bi₂Sr₂CaCu₂O₈). For rectangular lattice, due to superradiation regime, up to half of power fed into the crystal may be converted into the radiation. In the case of triangular or random lattice in the direction perpendicular to the layers, the fraction of power converted into the radiation depends on the dissipation rate and is much lower than for rectangular lattice in the case of high-temperature superconductors with nodes in the gap.     

Ferroelectric and optical properties of Ba5Li2Ti2Nb8O30 ceramics potential for memory applications

Giant grained (42 μm) translucent Ba₅Li₂Ti₂Nb₈O₃₀ ceramic was fabricated by conventional sintering technique using the powders obtained via solid state reaction route. These samples were confirmed to possess tetragonal tungsten bronze structure (P4bm) at room temperature. The scanning electron microscopy established the average grain size to be close to 20 μm. The photoluminescence studies carried out on these ceramics indicated sharp emission bands around 433 and 578 nm at an excitation wavelength of 350 nm which were attributed to band-edge emission as the band gap was 2.76 eV determined by Kubelka–Munk function. The dielectric properties of these ceramics were studied over wide frequency range (100–1 MHz) at room temperature. The decrease in dielectric constant with frequency could be explained on the basis of Koops theory. The dielectric constant and the loss were found to decrease with increasing frequency. The Curie temperature was confirmed to be ~370 °C based on the dielectric anomaly observed when these measurements were carried out over a temperature range of 30–500 °C. This shows a deviation from Curie–Weiss behaviour and hence an indicator of the occurrence of disordering in the system, the γ = 1.23 which confirms the diffuse ferroelectric transition. These ceramics at room temperature exhibited P–E hysteresis loops, though not well saturated akin to that of their single crystalline counterparts. These are the suitable properties for ferroelectric random access memory applications.     

Plasmonic coupling of bow tie antennas with Ag nanowire

Ag nanowire with the receiving and transmitting Ag bow tie antenna pairs at its incident and emission ends was patterned on the SiO(2) substrate to realize an enhanced surface plasmon emission with a factor of 45 compared to the single Ag nanowire without antenna pairs. The receiving and transmitting bow tie antenna pairs enhanced the plasmon coupling and emission efficiencies of the Ag nanowire. And the maximum plasmon emission sensitively depended on the length of Ag nanowire, the arm length of bow tie antennas, and the incident angle of optical excitation. This enhanced plasmon emission was confirmed by finite-difference time-domain simulations and explored with analytical calculations using the impedance matching theory at optical frequency.     

Sintering characteristic,crystal structure and microwave dielectric properties of a novel thermally stable ultra-low-firing Na2BiMg2V3O12 ceramic

A novel ultra low-firing microwave dielectric ceramic with the composition of Na₂BiMg₂V₃O₁₂ was fabricated by a solid state reaction method. The phase structure, sintering behavior and microwave dielectric properties of ceramics were investigated. The ceramic can be well densified at 660 °C for 4 h. X-ray diffraction data show that Na₂BiMg₂V₃O₁₂ has a cubic garnet structure with lattice parameters of a = 12.4929 Å, V = 1,949.80 Å³, Z = 8 and ρ = 4.42 g/cm³. The microwave dielectric properties are strongly related to the density and morphology of the samples. The ceramic sintered 660 °C exhibits good microwave dielectric properties with ε_r = 23.2, Q × f = 3,700 GHz and τ _f  = 8.2 ppm/°C. These results indicate that Na₂BiMg₂V₃O₁₂ ceramic is a candidate for low temperature co-fired ceramics devices, such as chip multi-layer LC filter, microstrip bandpass filter, multilayer antenna.