Low-temperature Scanning Laser Microscopy Research Articles

Current Limiting Phenomena in $\hbox{SmBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7}$ Coated Conductors Observed by Laser-Induced Thermoelectric Imaging and Low-Temperature Laser Scanning Microscopy

SmBa2Cu3O7 (SmBCO) coated conductors were grown on IBAD-MgO templates the evaporation using a drum-in-dual-chambers (EDDC) deposition system. The material showed a critical temperature of 93 ~ 94 K and a critical current of 200 ~ 250 A/cm-width at 77 K in a self-field. The current-limiting properties and local structure of the films were examined by laser-induced thermoelectric (LITE) imaging and low temperature laser scanning microscopy (LTLSM). Surface morphology and chemical composition of the superconducting layers were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. A 100μm × 1 mm-microbridge was fabricated using a photolithographic technique and etching process. The measured LITE images of the SmBCO coated conductor showed that some grains on the surface, surface inhomogeneity, second phases, defects and tilted grains, cause different thermal gradients. The LTLSM images revealed two dimensional variations of local flux flow on the films as a function of the local critical current. The findings are consistent with the LITE images, indicating that both methods can visualize the local structures and phases.

Local Investigation of Spatial Homogeneity in 600 A/cm-w Class ${\hbox {GdBa}}_{2}{\hbox {Cu}}_{3}{\hbox {O}}_{7-\delta}$ PLD Coated Conductor

The spatial homogeneity of PLD GdBCO coated conductors with high critical current of 620 A/cm-w was studied. By using laser-induced thermoelectric-imaging and low-temperature laser scanning microscopy we investigate film structure homogeneity, Tc and Jc variation. Our results imply that critical temperature variation along the sample of less than 0.2 K and Ic variation is fairly small. Our spatially resolved measurements reveal good in-plane homogeneity and current carrying capability. This shows a potential to use such conductors in various types of applications even with striated multi-filamentary conductors in which long-length narrow strips are required for AC-loss reduction.

Interaction of hot spots and terahertz waves inBi2Sr2CaCu2O8intrinsic Josephson junction stacks of various geometry

At high enough input power in stacks of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent terahertz (THz) emission. We investigate the interplay of hot spots and standing electromagnetic waves by low-temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. Standing electromagnetic wave patterns and THz emission are observed for a disk-shaped sample. The growth of a hot spot with increasing input power is monitored by small detector junctions surrounding a large rectangular mesa. For two rectangular mesas equipped with two current injectors and one arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot-spot position. The results support the picture of the hot spot acting as a reflective termination of the cavity, formed by the cold part of the mesa.

A superconducting 180° hybrid ring coupler for circuit quantum electrodynamics

Superconducting circuit quantum electrodynamics experiments with propagating microwaves require devices acting as beam splitters. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180° microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6 GHz. For the magnitude of the coupling and isolation, we find −3.5±0.5 dB and at least −15 dB, respectively, in a bandwidth of 2 GHz. We also investigate the effect of reflections at the superconductor-normal conductor contact by means of low temperature laser scanning microscopy. Our measurements show that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.

Effect of LaAlO3 twin-domain topology on local dc and microwave properties of cuprate films

Different imaging modes of low temperature laser scanning microscopy (LTLSM) have been applied to probe local optical and superconducting properties, as well as the spatial variations in thermoelectric and electronic (both dc and rf) transport, in a YBa2Cu3O6.95/LaAlO3 (YBCO/LAO) superconducting microstrip resonator with micron-range resolution. Additionally, the local sources of microwave nonlinearity (NL) were mapped in two-dimensions simultaneously by using the LTLSM in two-tone rf intermodulation distortion contrast mode as a function of (x,y) position of the laser beam perturbation on the sample. The influence of the direction of individual twin-domain YBCO blocks on its NL properties was analyzed in detail. The result shows the direct spatial correlation between NL microwave and dc electronic transport properties of the YBCO film that are imposed by the underlying twin-domain topology of the LAO substrate. In these circumstances, the scale of local NL current densities JIM(x,y) in different areas of the YBCO microstrip quantitatively coincide with the scale of local critical current densities Jc(x,y) measured at the same positions.

Spatial distribution of the local resistive transition and the critical current density in YBCO coated conductors using Low-temperature Scanning Laser Microscopy

Low-temperature Scanning Laser Microscopy (LTSLM) was carried out to reveal the spatial distribution of the local resistive transition temperature and the local current density in commercial YBCO coated conductors near the superconducting transition. The result of the sample with an array of holes of various sizes shows that the signal δV is proportional to the current density. The distribution of the critical temperature of the sample with two parallel bridges is quite homogeneous and the transport current flows mainly along the outer edge of the sample. Using LTSLM we directly image the current path in YBCO coated conductors of different patterns.

Improvement of spatial homogeneity in GdBCO/IBAD-MgO coated conductor

Recent process optimization allows improving homogeneity and a significant increase of Jc in GdBa2Cu3O7-δ deposited on ion-beam assisted deposited MgO template (GdBCO/IBAD-MgO). We applied low-temperature laser scanning microscopy and laser scanning thermo-electric microscopy (LSTEM) to investigate local dissipation and defects simultaneously in recent GdBCO/IBAD-MgO coated conductor. By using high-resolution LSTEM, we could detect current blocking obstacles which are responsible for the large scale local dissipation. Data on the present GdBCO/IBAD-MgO coated conductors point out a significant reduction of current blocking obstacles compared to the previous process. We have shown the improvement of spatial homogeneity in the recent GdBCO/IBAD-MgO coated conductors. Current blocking obstacles are much less densely distributed as compared to previous YBCO/IBAD-GZO coated conductor. Obstacles larger than several micrometer significantly increase local dissipation whereas smaller defects do not influence it noticeably. 2D map of the phase delay component of the thermoelectric voltage lock-in signal is effective to analyze current blocking obstacles having action upon local dissipation.

鉄系超伝導多結晶体の微細組織と電流輸送特性

Foreseeing practical applications of recently discovered new series of superconductor —Fe-pnictides, one of the most important parameters is current transport across grain boundaries. We combined SQUID measurements, magneto-optical (MO) imaging, scanning and transmission electron microscope (SEM and TEM) and low temperature laser scanning microscope (LTLSM) in order to understand the relationship between the microstructure and intergrain current transport, so called global current, in a random polycrystalline SmFeAsO0.85 (Sm1111) bulk. Our Sm1111 bulk showed significant global critical current density (Jc) which is more than one magnitude higher compared to random polycrystalline pure YBCO bulks at self field and the same temperature. However there was different temperature dependence of intergrain and intragrain Jc, exhibiting granularity at low temperature which was caused by large difference of Jc on two distinct scales. Strikingly most of intergrain current transport at self field relied on SNS Josephson weak links where supercurrent passed across the conductive impurity phase of FeAs, strongly suggesting the need of eliminating such a wetting phase in order to explore internal blocking effects at grain boundaries which are not fully understood yet.

Fast Josephson vortex ratchet made of intrinsic Josephson junctions inBi2Sr2CaCu2O8

We demonstrate the operation of a deterministic fluxon ratchet made of a stack of 30 intrinsic ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8}$ Josephson junctions. The ratchet has the shape of a gear with 20 asymmetric teeth (periods). It produces a rectified voltage of about $100\text{ }\ensuremath{\mu}\text{V}$ at a 12 GHz drive frequency. The effect of coupling between intrinsic junctions, i.e., the mode of fluxon motion for ratchet operation, has been studied within the framework of the two-dimensional coupled sine-Gordon equations. Further, we used low-temperature scanning laser microscopy to demonstrate that voltage rectification indeed is due to directed fluxon motion, in agreement with numerical simulations.

Intergrain current flow in a randomly oriented polycrystalline SmFeAsO0.85 oxypnictide

We report a direct current transport study of the local intergrain connections in a polycrystalline SmFeAsO0.85 (Sm1111) bulk, for which we earlier estimated significant intergranular critical current density Jc. Our combined low temperature laser scanning microscopy and scanning electron microscopy observations revealed only few grain-to-grain transport current paths, most of which switched off when a magnetic field was applied. These regions typically occur where current crosses Fe–As, which is a normal-metal wetting-phase that surrounds Sm1111 grains, producing a dense array of superconducting-normal-superconducting contacts. Our study points out the need to reduce the amount of grain boundary-wetting Fe–As phase, as well as the crack density within pnictide grains, as these defects produce a multiply connected current-blocking network.

Hot Spots and Waves inBi2Sr2CaCu2O8Intrinsic Josephson Junction Stacks: A Study by Low Temperature Scanning Laser Microscopy

Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.

Visualizing Transport Properties in IBAD Based YBCO Coated Conductors by Multiple Analysis Techniques

Current transport properties in Y1Ba2Cu3O7-delta (YBCO) coated conductors obtained by pulsed laser deposition process on a CeO2 capped Gd2Zr2O7-IBAD template have been studied by spatially resolved measurements. We utilized low temperature scanning laser microscopy along with laser induced Seebeck effect imaging and scanning SQUID microscopy that allow us to visualize 1) distributed flux flow dissipation, 2) current blocking obstacles and 3) local current flow, respectively. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the dissipation, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens mum to hundreds mum. Detailed correlation between local current flow and dissipation has been observed. Present methods have great potential as tools for basic understanding of current limiting mechanisms in the coated conductors.

Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)

We have used the Low-Temperature Laser Scanning Microscope (LTLSM) technique for a spatially resolved investigation of the microwave transport properties, nonlinearities, and material inhomogeneities in an operating coplanar waveguide YBa2Cu3O_{7‐δ}(YBCO) microwave resonator on a LaAlO3 (LAO) substrate. The influence of twin-domain blocks, in-plane rotated grains, and micro-cracks in the YBCO film on the nonuniform radiofre-quency (rf) current distribution was measured with a micrometer-scale spatial resolution. The impact of the peaked edge currents and rf field penetration into weak links on the linear device performance were also studied. The LTLSM capabilities and its future potential for nondestructive characterization of the microwave properties of superconducting circuits are discussed.

Imaging of Microscopic Sources of Resistive and Reactive Nonlinearities in Superconducting Microwave Devices

The technique of low-temperature laser scanning microscopy (LSM) has been applied to the investigation of local microwave properties in operating YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> /LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin-film resonators patterned into a meandering strip transmission line. By using a modified newly developed procedure of spatially-resolved complex impedance partition, the influence of inhomogeneous current flow on the formation of nonlinear (NL) microwave response in such planar devices is analysed in terms of the independent impact from resistive and inductive components. The modified procedure developed here is dramatically faster than our previous method. The LSM capability to probe the spatial variations of two-tone, third-order intermodulation photoresponse on micron length scales is used to find the 2D distribution of the local sources of microwave NL. The results show that the dominant sources of microwave NL are strongly localized in the resistive domains.

Laser scanning microscopy of guided vortex flow in microstructured high-Tc films

We report the visualization of guidance of vortices by artificial microholes (antidots) in superconducting thin films using a low-temperature laser scanning microscope. Previously, guided motion of vortices via tilted rows of antidots in YBa2Cu3O7 films was detected indirectly by using resistive Hall-type measurements. Here we prove that vortices are steered between antidots into a priori chosen direction by imaging of resistive photoresponse with a spatial resolution down to about 1μm. We observe predominant paths for vortex motion. Vortices are nucleated and annihilated at antidots, i.e., antidots define starting and ending points of predominant vortex paths. Depending on the misorientation angle between rows of antidots and the current-driven direction of vortex motion, different channels dominate in antidot-guided vortex motion. Our experimental results can be explained by the n-channel model. Finally, we present direct measurements of the local critical currents. This technique can be used as a quantitative method for the analysis of vortex motion in micropatterned thin films.

Local Observation of Reverse-Domain Superconductivity in a Superconductor-Ferromagnet Hybrid

Nanoscale magnetic and superconducting properties of the superconductor-ferromagnet Nb/PbFe12O19 hybrid were studied as a function of applied magnetic fields. Low-temperature scanning laser microscopy (LTSLM) together with transport measurements were carried out in order to reveal local variations of superconductivity induced by the magnetic field template produced by the ferromagnetic substrate. Room temperature magnetic force microscopy (MFM) was performed and magnetization curves were taken at room and low temperature to investigate the magnetic properties of the hybrid. Comparative analysis of the LTSLM and the MFM images has convincingly demonstrated the presence of the reverse-domain superconductivity.

Laser Microscopy of Tunneling Magnetoresistance in Manganite Grain-Boundary Junctions

Using low-temperature scanning laser microscopy we directly image electric transport in a magnetoresistive element, a manganite thin film intersected by a grain boundary (GB). Imaging at variable temperature allows reconstruction and comparison of the local resistance versus temperature for both the manganite film and the GB. Imaging at low temperature also shows that the GB switches between different resistive states due to the formation and growth of magnetic domains along the GB. We observe different types of domain wall growth; in most cases a domain wall nucleates at one edge of the bridge and then proceeds towards the other edge.

Study on local inhomogeneity in TFA-MOD YBCO coated conductor by use of low temperature scanning laser microscopy

We investigated the local inhomogeneity of flux flow dissipation by use of low temperature scanning laser microscopy (LTSLM) in a Y1Ba2Cu3O7−δ (YBCO) coated conductor obtained by the Metal-Organic Deposition process using metal trifuluoroacetate (TFA-MOD process). The LTSLM images showed that the highest Jc value in local region reached almost two times higher than the minimum value, while the global Jc was limited by the weakest spot. Typical length scale of the spatial inhomogeneity of Jc was about several tens to hundreds μm.

Imaging of trapped vortices in YBCO coated conductor by scanning SQUID microscope

We investigated trapped vortices in Y1Ba2Cu3O7−δ (YBCO) coated conductor by using a scanning SQUID microscope (SSM). Several peculiar properties of the vortices were observed such as a large bundle of vortices at a certain position and a coexistence of positive- and negative-sign vortices. Comparing the SSM images with that of low temperature scanning laser microscopy (LTSLM) and SEM, we identified the relationship between local dissipation and position of current blocking defect. Furthermore, regular vortex images were also observed by the SSM in the region where higher Jc has been identified by the LTSLM.

Low Temperature Scanning Laser Microscopy of YBCO Coated IBAD Tapes

Spatially resolved measurements of dissipative state in YBCO coated conductors have been carried out by use of the low temperature scanning laser microscopy (LTSLM). It has been shown that the current dependent dissipation peak is well scaled by the Gaussian distribution, and its normalized distribution becomes narrower in inversely proportion to the square root of power index in the local current-voltage characteristic. From the LTSLM analysis, we can collect positional information of current limiting coordinate as well as spatial variation of local critical current. Optimizing the measurement condition, we also succeeded to visualize the dissipation even in the tape which is covered by a 10 /spl mu/m thick Ag stabilization layer.