Single Crystalline Bi2Sr2CaCu2O8 Research Articles

Single-crystalline Bi2Sr2CaCu2O8+x detectors for direct detection of microwave radiation

We test radiation detectors made from single-crystalline Bi2Sr2CaCu2O8+x flakes put on oxidized Si substrates. The 100-nm-thick flakes are lithographically patterned into 4×12 μm2 large rectangles embedded in thin-film log-spiral antennas. The SiO2 layer weakens the thermal link between the flakes and the bath. Two modes of radiation detection have been observed. For a bolometric type of sensors a responsivity of ∼300 V/W and a noise equivalent power of 30 nW/Hz has been deduced at 70 K. Much more sensitive is the non-bolometric device showing characteristics similar to a Golay-type detector while being at least a thousand times faster. Making smaller (sub-μm) structures is expected to significantly improve the performance of these devices and makes them very competitive among other microwave and terahertz detectors.

Reflection type of terahertz imaging system using a high-Tc superconducting oscillator

A reflection type of imaging system is shown at sub-terahertz frequencies generated from high-Tc superconducting intrinsic Josephson junction mesa structures fabricated by single crystalline Bi2Sr2CaCu2O8+δ to demonstrate how the sub-terahertz imaging technique using monochromatic radiation is powerful and unique for the variety of practical applications. Several examples are discussed in detail and are compared to other terahertz imaging systems.

Effects of magnetic fields on the coherent THz emission from mesas of single crystal Bi2Sr2CaCu2O8+δ

We have measured the magnetic field effect of THz radiation emitted from a mesa structure fabricated from high-quality high-transition temperature (Tc) superconductor single crystalline Bi2Sr2CaCu2O8+δ using a newly developed measurement system in magnetic fields up to 6T. The results show that the THz radiation was strongly suppressed in magnetic fields with a considerable anisotropy: about 20Oe magnetic field is sufficient for the total suppression of the THz radiation for the field being parallel to the c-axis, while for the field being parallel to the ab-plane the radiation has first a slight hump at about 50Oe, then has a sudden drop at about 150Oe, and shows a step-wise structure between about 200 and 300Oe before the intensity diminishes completely below the detection sensitivity limit at about 450Oe. These characteristic features are discussed in comparison with recent theoretical works.

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.

Excitation mode characteristics in Bi2212 rectangular mesa structures

The rectangular mesa structure of single crystalline Bi2Sr2CaCu2O8+δ with the dimensions of 64 × 135 × 1.35 μm3 was fabricated by using Ar-ion milling with a metallic mask and focused ion beam. From this mesa, the radiation frequencies of 24.226 cm−1 and 17.296 cm−1 were observed at the different temperatures of 25 K and 40 K, respectively. The change of the radiation frequencies is also obviously reflected in the radiation pattern measurement. The radiation pattern observed at lower temperature seems to have an asymmetric shape and its intensity as a whole is slightly weak. In the higher temperature, however, the radiation intensity increases and the radiation pattern becomes almost symmetric shape. The radiation characteristics observed at higher temperature is well explained by the fundamental cavity resonance mode expected from the width of the mesa. This result may give us not only the suggestion of the existence of the adequate matching condition between the geometrical cavity and the ac Josephson current in the mesa but also the guide for the designing of the high efficiency of the radiation.

Terahertz imaging system using high-Tc superconducting oscillation devices

Microwatt power oscillation devices at sub-terahertz frequency region between 0.3 and 1.0 terahertz (THz) were fabricated from high-Tc superconducting single crystalline Bi2Sr2CaCu2O8+δ and used as a source of the transmission terahertz imaging system. As test examples, terahertz images of coins and a razor blade placed inside the brownish paper envelopes with the spatial resolution of 1 mm are presented. The signal-to-noise ratio exceeds 130 in these images. Using a simple wedge-shaped interferometer and analysing the interference fringe pattern, the wavelength of the terahertz wave is calibrated within 0.1% accuracy. This interferometer also provides a simple method to measure the absorption coefficient of the liquid sample. Two test measurements for distilled water and ethanol are demonstrated and their absorption coefficients are obtained with 99.2% accuracy. This suggests that our terahertz imaging system can be applied to many practical applications, such as biological and biomedical imaging, environmental monitoring, microanalysis of impurities, structure and dynamical analyses of large molecules and ions in solution.

Enhancement of the field of first flux penetration due to Andreev bound states in [formula omitted

First flux penetration into single crystalline Bi2Sr2CaCu2O8+δ is investigated using magneto-optical imaging and Hall probe array magnetometry. Below 50K, a slight enhancement of the field of first flux penetration, Hp, is found when the edge normals are oriented at 45° with respect to the principal (ab) crystal axes, rather than parallel to them. We discuss this effect in terms of a role of Andreev bound states (Iniotakis et al., 2008 [1]).

Microwave responses on locally modified [formula omitted] by near-field microwave microscope

We have locally modified a cleaved surface of single crystalline Bi2Sr2CaCu2O8+δ by local anodic oxidation using scanning tunneling microscope-assisted near-field scanning microwave microscope (STM-assisted NSMM). After the modification, we have measured the resonant frequency and the quality factor of the microwave responses near the modified region at 7.31GHz. The resonant frequency and the quality factor increase at the modified region. It has been confirmed that the STM-assisted NSMM enables us both to modify the sample surface and to measure the microwave impedance on the modified region in nanoscale.

Disorder and c-axis quasiparticle dynamics in underdoped Bi2Sr2CaCu2O8

We present measurements of the Josephson plasma frequency and the in-plane penetration depth of underdoped single crystalline Bi2Sr2CaCu2O8 with varying degrees of disorder introduced by irradiation with 2.3 MeV electrons. Increasing disorder drives Tc down, in agreement with in all model descriptions of high Tc superconductivity. However, the manner in which the JPR frequency, the square of which represents the zero-frequency spectral weight of the c-axis conductivity in the superconducting state, is driven down by disorder depends more strongly on the model description. We show that only the model of impurity assisted quasiparticle hopping in a d-wave superconductor, together with strongly scattering point defects in the superconducting layers, can explain the disorder dependence of the c-axis plasma frequency, the in-plane penetration depth, and Tc consistently. From the data, we extract the energy scale governing nodal quasiparticle excitations, Δ0 ∼ 2.5kB Tc.

Direct observation of tetrahertz electromagnetic waves emitted from intrinsic Josephson junctions in single crystalline Bi2Sr2CaCu2O8+δ

We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν=c/nλ=c/2nw and ν=2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437–438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5μW from single mesa with w=60μm at 648GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960GHz in the fundamental mode in the w=40μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437–438 (2006) 111 , M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.

Dynamical properties of Josephson vortices in mesoscopic intrinsic Josephson junctions in single crystalline Bi2Sr2CaCu2O8+δ

Dynamical nature of Josephson vortices confined within a micron sized mesoscopic intrinsic Josephson junction made of single crystalline Bi2Sr2CaCu2O8+δ has been investigated by dc-resistivity and I–V characteristics measurements of the junction. Two types of periodic oscillations of the dc-resistivity of the junction as a function of applied magnetic field H has been confirmed as observed previously, but interpreted differently, i.e., by the two purely quantum mechanical effects: one is the interference effect which gives a period of H0=ϕ0/sL and the other is the diffraction effect which gives the same frequency of ϕ0/sL but phase shifted by ϕ0/2sL, where s is the interlayer distance, L the width of the junction perpendicular to the magnetic field and ϕ0 the quantum magnetic flux. The characteristic features of the oscillation are summarized based on the experimental findings. At higher currents, where such an oscillating behavior diminishes, we succeeded in directly observing electromagnetic radiation emitted from the intrinsic Josephson junction system towards outgoing direction of the vortices from the sample. The integrated power detected by the bolometric detector is estimated to be of the order of 100W/cm2. The condition for radiation is discussed.

Observation of Vortices and Columnar Defects by 1-MV Lorentz Microscopy I

ABSTRACTA single crystalline Bi2Sr2CaCu2O8+8 which has columnar defects in its inside are observed by Lorentz microscopy using the newly developed 1-MV field emission electron microscope at the first time. The superconducting vortices are observed with higher contrast than ever. Simultaneous observation of vortices and columnar defect is succeeded in real time.

Systematic Magnetization Measurements on Single Crystalline Bi2Sr2CaCu2O8+δ with Columnar Defects

We have performed systematic magnetization measurements on single crystalline Bi2Sr2CaCu2O8+δwith columnar defects of BΦ= 0.005 to 1 T by using a SQUID magnetometer. Magnetization hysteresis curves of the pristine sample show a weak irreversible behavior in the vortex liquid state, suggesting the existence of the new vortex state in the vortex liquid state. This weak irreversible region persists systematically in the samples with columnar defects even up to BΦ= 1 T. It is shown that the weak hysteresis of magnetization is sensitive to the disorder level of the sample and shifts systematically to higher temperature and field region with increasing the number of columnar defects. This behavior clearly indicates that effective pinning mechanism exists even in the vortex liquid state and generates a finite critical current.

Longitudinal Josephson plasma excitation in vortex state of Bi2Sr2CaCu2O8+δ

The Josephson plasma absorption at a microwave frequency of 45 GHz in a cavity (E obc//c-axis) has been studied in high quality single crystalline Bi2Sr2CaCu2O8+δ as functions of magnetic field, temperature, doping level, and angle between magnetic field and crystallographic axes. The observed resonance line shape, which is rather symmetric, was analyzed in detail by solving the coupled equations of the Maxwell equation and the Josephson current as well as the charge equations. As a result the observed absorption is unambiguously assigned as the longitudinal plasma mode. The temperature dependence of the absorption agrees with the prediction based on thed-wave symmetry rather than the isotropics-wave one. The resonance line can be scaled by the perpendicular component of the field, indicating that the resonance position is determined by the number of vortices parallel to the c-axis.

Magnetic-field-induced non-linear effects of Josephson coupled superconductor Bi2Sr2CaCu2O8+?

The dc current-voltage (I-V) relation along the c-axis of single crystalline Bi2Sr2CaCu2O8+δ has been measured in magnetic fields parallel and perpendicular to the c-axis. In zero field a clear and sharp jump with large hysteresis in theI-V curve was observed, indicative of the dc-Josephson effect. In magnetic field below a characteristic fieldB*≈0.4 T (at T=0 K) parallel to the c-axis the magnetic field suppresses the hysteresis and reduces the critical currentIc drastically, whereas aboveB* theI-V curve becomes broad and featureless behavior. The characteristic field scaleB* can be interpreted as an energy scale of the Josephson coupling between superconducting layers in Bi2Sr2CaCu2O8+δ and is argued with emphasis on the correlation length of pancake vortices in this system.

Flux-line lattice dynamics and irreversibility line in single crystalline Bi2Sr2CaCu2O8+ delta

High-field magnetisation measurements up to 38 T have been performed on single-crystalline Bi2Sr2CaCu2O8+ delta in order to investigate the irreversible behaviour associated with the flux line dynamics. It was found that the irreversible magnetisation vanishes asymptotically at high fields well below Bc2. As temperature is increased, above approximately 40 K, the magnetisation essentially become reversible. Critical currents Jc and volume pinning forces Fp were calculated and compared with the results of YBa2Cu3O7- delta . The irreversibility line deduced from the present measurements as well as from other techniques is discussed in terms of a possible boundary, above which superconducting phase fluctuations play a dominant role for the dissipation.

Thermally activated flux flow effects in single crystalline Bi2Sr2CaCu2O8, observed via magnetic measurements

In this paper we present time-dependent magnetization and high-temperature, low-field critical-current measurements on single crystalline Bi2Sr2CaCu2O8. A short introduction to thermally activated flux-flow theory (TAFF) is given based on solving a continuity equation for fluxlines with several boundary conditions. A comparison between this theory and our experiments indicates the self consistency of the TAFF model, by relating the temperature dependences of Jc and of the typical relaxation time τ0 of M(t). Furthermore, values for the pinning potential U are obtained at several temperatures.