Bi2Sr2CaCu2Oy Single Crystals Research Articles

Fabrications of Small and High-quality Intrinsic Josephson Junctions by Combinatorial Method of Ar-ion and Focused Ga-ion Etchings

We present a study on the fabrication of the intrinsic Josephson junctions (IJJs), which are made of Bi2Sr2CaCu2Oy (Bi2212) single crystals, by using the focused Ga-ion etching and the Ar-ion etching. We made sure that the current-voltage (I-V) characteristics of small IJJs was never influenced by the direction of a Ga-ion beam in the focused ion beam (FIB) etchings. In addition, it was found that Ar-ion etching after FIB process is quite useful to improve the performance of the I-V curves of IJJs. By using a substrate with a small hole, we succeeded in employing the double-side Ar-ion etching after the FIB microfabrication. These results strongly support that the combinatorial method of FIB and Ar-ion etchings is very important to fabricate the small and high-quality IJJs.

Spin injection effect in thin Bi2212 single crystal

The influence of spin-injection on the in-plane transport properties of thin Bi2Sr2CaCu2Oy (BSCCO) single crystal has been investigated. The in-plane transport measurements without and with spin injection were carried out at 77 K by four terminal method. The in-plane critical current was strongly reduced by injecting the current from Co/Au electrodes formed on the BSCCO bridge with 50 mm wide and 450 nm thick. Furthermore, it was observed that magnetic field dependence of the magnetoresistance shows a hysteresis loop. These results indicate that the in-plane superconductive transport property is affected by the spin-injection related to the magnetization of Co.

Generation and Detection of THz Radiation Using Intrinsic Josephson Junctions

Abstract We present the generation and detection of terahertz radiation using intrinsic Josephson junctions (IJJs) in Bi2Sr2CaCu2Oy single crystals. This approach allows us to detect THz radiation from large stacks consisting of a few hundred intrinsic Josephson junctions. The lateral dimensions of the fabricated IJJ oscillator mesa range from 290×50 to 290×90 μm2 and the number of IJJs which constitute the mesas is between 100 and 450, while the small mesa with the lateral dimensions of 5 × 5 μm2 is used as the high sensitive THz detector. The largest emission is always observed when the oscillator is biased at the negative resistance region of the current-voltage characteristics. We find that the emission frequency cor–responds to the second harmonics of the in-phase cavity resonance mode. This is consistent with the emission condition of the case of thick IJJ stacks reported previously.

Effects of junction geometry in crossover temperature to macroscopic quantum tunneling regime of intrinsic Josephson junctions

We investigated the phase dynamics of long intrinsic Josephson junctions, which were fabricated on a narrow bridge structure of Bi2Sr2CaCu2Oy (Bi-2212) single crystals by using a focused ion-beam etching. We measured the probability distribution of the switching events from the zero-voltage state for two types of junction geometry. One is a junction where the bridge width (L1) is larger than the Josephson penetration depth, λJ, and the distance between two slits (L2) is comparable to λJ, while the other is a junction where L1 is comparable to λJ and L2 is larger than λJ. We found that a crossover temperature from the thermally activated regime to the macroscopic quantum tunneling regime was quite different between the two types of junction geometry. We discuss the observed behavior in terms of an edge effect in long Josephson junctions dependent on the junction geometry.

Measurements of low-frequency noise spectral densities for a small-sized stack of intrinsic Josephson junctions of Bi2Sr2CaCu2Oy single crystal

We measured the low-frequency-voltage noise properties of a small-sized stack of intrinsic Josephson junctions formed on the surface of a cleaved Bi2Sr2CaCu2Oy (Bi-2212) single crystal, containing a few intrinsic Josephson junctions. A stack with an area of 2×2μm was fabricated by electron-beam lithography and argon-ion-milling. The current–voltage (I–V) characteristics along the c-axis direction of the stack were measured by a three-terminal method. The stack showed (I–V) and dV∕dI-V curves without backbending due to heating effects. Five discrete resistive branches with hysteresis were observed in the low-voltage region of these curves. The measured noise-voltage spectral density SV(f) had a 1∕f dependence on frequency. We estimated the magnitude of the 1∕f noise parameter η from SV(f) in a Bi-2212 stack and found that above the sum-gap voltage in a small-sized stack η was almost the same as that for low-Tc Nb∕Al+AlOx∕Nb high-quality tunnel junctions.

Hysteresis of the magnetic properties and the irreversibility line in Bi2Sr2CaCu2Oy layered superconductors

Measurements of the irreversibility line in Bi2Sr2CaCu2Oy single crystals are made by three experimental methods: by dc magnetization measurements, microwave absorption measurements, and registration of the harmonics in the high-frequency response of the superconductor. It is found that the position of the irreversibility line on the magnetic phase diagram depends substantially on the method of measurement and the frequency used. Analysis of the results shows that the discrepancy in the irreversibility lines is due to both a difference in the mechanisms of hysteresis of the magnetic properties and to the different scales of the measurement times. It is shown that the position of the irreversibility line is determined at low temperatures by the limiting value of the vortex creep rate for the given method of registration and at high temperatures by the time required for two-dimensional vortices to overcome the surface barrier. It is concluded that the irreversibility line is a dynamic phase transition. The first experimental evidence of the presence of a surface barrier for 2D vortices is obtained.

Electronic structures of two-phase microstructures α and β in heavily Pb-doped Bi2Sr2CaCu2Oy single crystals investigated by scanning tunneling microscopy/spectroscopy

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) of a high-Jc superconductor, heavily Pb-doped Pb0.6Bi1.4Sr2CaCu2Oy, at low temperatures below Tc. The obtained STM images showed a lamella-type phase separation into Pb-poor and Pb-rich regions. At 4.3 K, values of energy gap Δ, deduced from STS spectra, showed considerable inhomogeneity in the range of 20–80 meV, while Δ variations across the phase boundaries were less significant. At 63 K, in contrast, we found that Δ altered sharply at the boundaries particularly in the annealed samples, resulting from the suppression of electronic inhomogeneity in each phase. This suggests that the phase boundaries act as strong pinning centers at higher temperatures.

Improvement of superconductivity in Bi-2212 single crystals grown by a vertical Bridgman method

Abstract We grew Bi2Sr2CaCu2Oy (Bi-2212) single crystals by the modified vertical Bridgman method using the starting materials of Bi:Sr:Ca:Cu=x:2:1:2 (x=2–3), and investigated the optimum annealing conditions for the improvement of the superconductivity. From the results, we found that the optimum composition of starting materials for the large and plate-like Bi-2212 single crystal with good superconductivity and crystallinity was Bi:Sr:Ca:Cu=2:2:1:2, and that the Tc, the full-width-half-maximum value of (0 0 1 0 ) X-ray diffraction peak and the size of the single crystal were 82 K, 0.05° and 18 × 5 mm2, respectively. In the single crystal annealed at 600 °C for 10 h in an atmosphere of oxygen gas, the Tc of the as-grown single crystals increased with annealing time and changed from 82 to 86.3 K. On the other hand, the ΔT of the single crystal annealed at 600 °C for 5 h tended to decrease with increasing annealing time and changed from 6.2 to 4.2 K.

Inhomogeneous electronic structures in heavily Pb-doped Bi2Sr2CaCu2Oy single crystals probed by low temperature STM/STS

We have performed cryogenic scanning tunneling microscopy/spectroscopy (STM/STS) of heavily Pb-doped Bi2Sr2CaCu2Oy single crystals to investigate local electronic structures in the overdoped regime. The obtained STM/STS results at 4.3 K clearly showed local inhomogeneity of gap structure Δ (Δ=20–60 meV) in a scale of several nm, suggesting the coexistence of sperconducting and pseudogap-like regions, even in the overdoped regime.

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We carried out surface cleaning of Bi2Sr2CaCu2Oy (Bi-2212) single crystals grown by a modified vertical Bridgman method (modified VB method), and investigated the chemical state with X-ray photoelectron spectroscopy. From the results, we found that carbon impurities on the surface of the single crystals can be removed almost completely by cleaving them under high vacuum (7.3 × 10-6 Pa). The O-1s peak from the cleaved surface of the single crystal was very sharp and its FWHM value was about 2.0 eV. Moreover, we found that the crystals do not include any carbon impurities, and that they have few mutual substitutions between Ca and Sr. Therefore, the single crystals grown by a modified VB method is found to have high quality.

1/f noise of Bi2Sr2CaCu2Oy intrinsic Josephson junctions

We have measured the low-frequency voltage noise properties across mesa stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2Oy (BSCCO) single crystals as a function of bias voltage. The BSCCO mesas with area of 160×40 μm2 were fabricated on the surface of cleaved BSCCO single crystals using standard photolithography and Ar ion-milling techniques. The measured noise voltage spectrum density SV(f ) had a 1/f dependence on frequency. The magnitude of the 1/f noise spectra was found to increase monotonically with the branch number in the I–V characteristic at fixed bias current, and no anomalously enhanced 1/f noise was observed.

Polarized X-Ray Absorption Spectroscopic Study on Mercuric Bromide Intercalated Bi2Sr2CaCu2Oy Single Crystal

Polarized X-ray absorption spectroscopic experiments have been carried out for the mercuric bromide intercalated Bi2Sr2CaCu2Oy single crystal, in order to understand the variation of anisotropic resistivities upon HgBr2 intercalation as well as to probe the electronic structure of HgBr2 intercalate. The Br K- and Bi LIII-edge spectra indicate a significant electron transfer from bismuth to bromine, implying an overlap between Bi 6s and Br 4p orbitals. From the Hg LIII- and Br K-edge data, it is also found that HgBr2 intercalation gives rise to an electron localization in the (Hg–Br) bond, which leads to poor electronic conduction along the intercalant layer. In this context, the semiconducting out-of-plane resistivity of HgBr2 intercalate can be explained by the poor conductivity of the intercalated HgBr2 layer, even though the orbital overlap between Bi and Br facilitates the electron conduction between host and guest along the c-axis.

Growth of large Bi2Sr2CaCu2Oy single crystals by a vertical Bridgman method

We prepared Bi2Sr2CaCu2Oy (Bi-2212) single crystals by the vertical Bridgman method. The amount of the starting material was varied and alumina crucibles of different bottom angles were used. We found that the area of plate-like single crystals was proportional to the amount of starting materials and that the optimum bottom angle of the crucible for obtaining large single crystals was about 60°. Optimizing the growth conditions leads to plate-like single crystals with sizes up to 25×5mm2 which have a zero-resistance temperature of about 80K and a Bi-2212 single phase. Furthermore, the transition temperature width of an as-grown single crystal decreases to about 3K by annealing at 500°C for 106h in flowing oxygen.

HTS Josephson plasma oscillator

Recent observations on novel electromagnetic emissions from the microwave to THz range due to tunnel injection of quasiparticles into the c-axis direction of high-Tc superconductors are briefly reviewed. The results for YBa2Cu3O7-y, ErBa2Cu3O7-y and Bi2Sr2CaCu2Oy (BSCCO) thin-film tunnel samples and BSCCO single crystals are reported. The observed emission phenomena are qualitatively different between the high-voltage injection regime (V>>/e, where is the gap or gap-sum parameter) and the low-voltage injection regime (V /e). For V>>/e, to which regime most thin-film samples might belong, the nonequilibrium broadband emissions extending to the THz range due to the incoherent plasmon scattering are observable. For V /e, as realized in the case of a BSCCO crystal, in addition to the broadband emissions, a sharp microwave emission peak corresponding to the coherent Josephson plasma peak is observable.

Reduction of the Superfluid Density in the Vortex-Liquid Phase ofBi2Sr2CaCu2Oy

In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was measured in the mixed state at 40.8 GHz.The surface reactance, which is proportional to the real part of the effective penetration depth, increased rapidly just above the first-order vortex-lattice melting transition field and the second magnetization peak field.This increase is ascribed to the decrease in the superfluid density rather than the loss of pinning.This result indicates that the vortex melting transition changes the electronic structure as well as the vortex structure.

Observation of Josephson plasma emission by quasiparticle injection into a Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub y} single crystal

The possible first observation of Josephson plasma emission from a quasiparticle-injected Bi2Sr2CaCu2Oy(BSCCO) single crystal is reported. The quasiparticles are injected into the c-axis direction of the crystal. The emitted microwaves in the range 10 - 47GHz are detected by a superheterodyne mixer technique. Three different modes of emissions are observed by biasing the current in the hysteretic curve of the intrinsic junctions. In addition to Josephson self-emission at low bias voltage, we find the two novel emissions due to nonequilibrium effects: incoherent broadband emissions at quasiparticle branches and a sharp coherent emission in the negative resistance region at the gap edge. The latter two emissions are almost identified as Josephson plasma emissions.

Studies of Both Intralayer and Interlayer Electrodynamics of Bi 2 Sr 2 CaCu 2 O y in the Superconducting State

We studied the intralayer and interlayer electrodynamics of the superconducting Bi2Sr2CaCu2Oysingle crystals in the microwave and millimeter wave regions. The c-axis superfluid density fraction ρscshows a more rapid growth than ρsabjust below Tcand a weaker temperature dependence at low temperatures. These features suggest the anisotropic growth mechanism of ρs. The temperature dependence of σab1shows a large increase below Tcwith a broad peak around 20 K. However, to discuss the detailed behavior of σ1cor the anisotropy of conductivity σab1 / σc1in the superconducting state, much more careful measurement is required in the c-axis direction.

Effects of first-order vortex phase transition on the electronic states of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub y}

We measured the complex surface impedance Zs= Rs+iXsof optimally- and over-doped Bi2Sr2CaCu2Oysingle crystals around the vortex-phase boundaries. In both doping states, the surface reactance Xsincreased rapidly just above the first-order melting transition field and the so-called second magnetization peak field, while no obvious anomalies were observed in the surface resistance Rs. From the analysis of the relationship between Xsand Rs, we showed that the origin of this anomaly is not the loss of pinning but the reduction of the superfluid density above the transition.

Fiske and flux-flow modes of the intrinsic Josephson junctions in Bi2Sr2CaCu2Oy mesas

We report the observation of the highest velocity mode of Josephson vortex motion in mesas consisting of 5–15 intrinsic Josephson junctions with an area of 37×30 μm2 on Bi2Sr2CaCu2Oy single crystals. By applying the dc magnetic field parallel to CuO2 planes in the mesas, resonant steps and flux flow branches have been observed on the current–voltage characteristics along the c-axis of the mesas. The characteristic velocities of the intrinsic Josephson junctions are estimated to be (1.5–1.8)×106 m/s, which corresponds to the highest velocity mode, from the voltages of Fiske steps and the maximum voltages of the flux-flow branches.

Surface of Bi2Sr2CaCu2O y Single Crystals Heated in Air

We treated Bi2Sr2CaCu2Oy (BSCCO) superconducting single crystals at various temperatures in air and investigated their surfaces with X-ray photoelectron spectroscopy. From the results, we determined that the carbon and oxygen impurities on the surfaces of BSCCO single crystals were removed by heating them in air at 400°C for 1 h. This was also confirmed by an Auger electron spectroscopy. In addition, the results of the half-width of the Cu-2p main peak and the intensity ratio of satellite to main peaks indicated that the optimum heat treatment temperature for obtaining a clean surface of BSCCO single crystal was approximately 400°C. The method is thought to be effective for cleaning the surfaces of superconducting oxides with various formations.