Low-temperature Scanning Laser Microscopy Research Articles

Imaging Local Dissipation and Magnetic Field in YBCO Films With Artificial Defects

We used Low Temperature Laser Scanning Microscopy (LTLSM) to investigate dc current flow in high-J/sub c/ YBCO thin films with artificially prepared defects and correlated the LTLSM response to Magneto Optical Images (MOI). Artificial defects model current blockages such as cracks, high angle grain boundaries or voids. Because the LTLSM voltage response is associated with the local electric field, while MOI shows the local perpendicular magnetic field, the combination of techniques gives complementary local information about the effect of current-limiting defects in superconductors.

Microwave-induced flow of vortices in long Josephson junctions

We report an experimental and numerical study of microwave-induced flow of vortices in long Josephson junctions at zero dc magnetic field. Our intriguing observation is that applying an ac bias of a small frequency $f⪡{f}_{p}$ and sufficiently large amplitude changes the current-voltage characteristics ($I\text{\ensuremath{-}}V$ curve) of the junction in a way similar to the effect of dc magnetic field, well known as the flux-flow behavior. The characteristic voltage $V$ of this low voltage branch increases with the power $P$ of microwave radiation as ${V}_{s}\ensuremath{\propto}{P}^{\ensuremath{\alpha}}$ with the index $\ensuremath{\alpha}\ensuremath{\simeq}0.5$. Experiments using a low-temperature laser scanning microscope unambiguously indicate the motion of Josephson vortices driven by microwaves. Numerical simulations agree with the experimental data and show strongly irregular vortex motion. We explain our results by exploiting an analogy between the microwave-induced vortex flow in long Josephson junctions and incoherent multiphoton absorption in small Josephson junctions in the presence of large thermal fluctuations. In the case of long Josephson junctions the spatially temporal chaos in the vortex motion mimics the thermal fluctuations. In accordance with this analogy, a control of the intensity of chaos in a long junction by changing its damping constant leads to a pronounced change in the shape of the $I\text{\ensuremath{-}}V$ curve. Our results provide a possible explanation to previously measured but not yet understood microwave-driven properties of intrinsic Josephson junctions in high-temperature superconductors.

Scanning laser imaging of dissipation in YBa2Cu3O7−δ-coated conductors

We investigate dc-current flow in high-jc YBa2Cu3O7−δ-coated conductors by low-temperature laser scanning microscopy (LTLSM) and correlate the LTLSM response to magneto-optical imaging (MOI) and grain boundary (GB) misorientation. Because the voltage response measured by LTLSM is associated with the local electric field, while MOI shows the local magnetic field, the combination of these two techniques unambiguously shows that the dominant sources of dissipation and easy flux flow occur at and near GBs. By correlating LTLSM images to grain misorientation maps determined by electron backscatter diffraction (EBSD), we can directly observe the overloading of current paths through low-angle GBs neighboring higher-angle GBs.

Josephson behavior of phase-slip lines in wide superconducting strips.

Phase-slip lines can be viewed as dynamically created Josephson junctions in a homogeneous superconducting film. In contrast to phase-slip centers, phase-slip lines occur in wide superconducting strips, where the order parameter may vary in two dimensions. We investigated phase-slip lines in two different materials using several methods. We observed Shapiro steps under microwave radiation, which shows that the frequency of the order parameter oscillation is equal to Josephson frequency. A periodic oscillation of a critical current versus the applied magnetic field was found in strips with a hole in the middle. The latter effect provides a clear evidence of macroscopic quantum interference across a phase-slip line. We have used low temperature scanning laser microscopy to visualize the phase-slip lines and to distinguish them from possible local inhomogeneities in the films.

Percolative transition and scaling of transport E - J characteristics in YBCO coated ibad tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Spread of critical currents in thin-film YBa/sub 2/Cu/sub 3/O/sub 7-x/ bicrystal junctions and faceting of grain boundary

The statistical distributions of critical currents in series arrays of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary junctions have been studied by low-temperature laser scanning microscopy. A set of arrays has been fabricated on [110] NdGaO/sub 3/ bicrystal substrates with misorientation angles from 2 /spl times/ 10/spl deg/ up to 2 /spl times/ 26/spl deg/ and patterned to the widths from 1.7 up to 5 micrometers. The critical current values of the individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the bias current at which the maximum laser-induced voltage response has appeared on the array. The measured critical current distributions have been demonstrated to be close to a log-normal Gauss function. A spread of this distribution has been found to increase with a bicrystal angle. YBa/sub 2/Cu/sub 3/O/sub 7-x/ grain-boundary topography has been studied by atomic force microscopy. From results of these measurements we suppose that the maximum values of critical current density might be assigned to the symmetrical facets of grain boundary.

Imaging of discrete breathers.

In this paper I review our experiments on visualization of discrete breathers (intrinsic localized modes) in nonlinear lattices made of Josephson junctions. Properties of Josephson junctions and arrays made of such junctions are discussed in the Introduction. The visualization technique based on low temperature laser scanning microscopy (LSM) is described in detail. Images of discrete breathers in Josephson junction arrays of various geometries are presented. Possible further experiments that can be done using LSM technique are envisioned.

Imaging sub-millimeter waves in planar cryoelectronic circuits by scanning laser microscopy

Low temperature scanning laser microscopy (LTSLM) is demonstrated to be capable of imaging sub-THz electromagnetic fields in cryoelectronic integrated structures. This method allows one to evaluate the spatial distribution of time-averaged field amplitudes with a resolution of about one micrometer for samples with characteristic dimensions of order millimeters. Using LTSLM, cryoelectronic devices with both passive and active superconducting elements can be characterized. Local heating of superconducting structures by a laser beam introduces extra loss for the propagating and standing sub-millimeter waves. We present LTSLM images of two-dimensional 400 to 500 standing GHz wave patterns in integrated superconducting receiver chips.

Spatially resolved measurements of critical parameters in superconducting filaments by laser scanning technique

We present an experimental study of local transport properties of superconducting filaments by means of low temperature scanning laser microscopy (LTSLM). A novel theoretical model that treats the laser-beam-induced voltage response is proposed and applied to analyze the measured data. The method of LTSLM allows to visualize the local inhomogeneities in individual filaments extracted from (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x//Ag tapes. Using the proposed theoretical analysis we extract the spatial distributions of the critical currents and critical temperatures over the sample. Tile data obtained by LTLSM show good agreement with standard non-local transport measurements. Using autocorrelation analysis of the voltage response, the characteristic length of the critical current variation is retrieved for filaments that belong to different parts of the tape.

Inner Structure of Intact Chloroplasts Observed by a Low Temperature Laser Scanning Microscope

Inner structure of isolated intact chloroplasts was observed for the first time by a method of laser scanning microscopy at the temperature of liquid nitrogen at 77 K. The microscope, based on gradient index optics, has a maximum resolution of 440 nm at the wavelength of 650 nm. Chloroplasts were excited into the Q-band of chlorophyll b by a krypton laser line at 647.6 nm and fluorescence was detected using two different interference filters. The 680 nm interference filter detects the regions where photosystem (PS) 2 mainly occurs, the 730 nm interference filter detects domains with predominant location of PS1. Since PS1 occurs mainly in stroma lamellae, whereas PS2 occurs mainly in grana regions we were able to view the structure of thylakoid membrane in isolated intact chloroplast that is the closest to in vivo state.

Spatial distribution of critical current and supercurrent density in individual filaments extracted from Ag-sheathed Bi-2223 tapes

The critical current density J c distributions along individual filaments of Bi-2223/Ag tapes are studied by low-temperature scanning laser microscopy. The filaments are extracted from the tapes by cutting them with a high-power UV laser. A relation between the supercurrent density distribution along cross section and the response amplitude is determined. The spatial resolution of this imaging method for bulk filaments is about few micrometers.

Low-temperature scanning laser microscopy of individual filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x tapes

The method of low-temperature scanning laser microscopy is applied to visualize the resistive state in individual superconducting filaments extracted from (Bi, Pb)2Sr2Ca2Cu3O10+x/Ag tapes. This technique is capable of imaging the distributions of the critical currents over a sample. Using the nonbolometric response, a spatial resolution of about 1 μm is demonstrated for 10-μm-thick filaments. Some of the resistively visualized grain boundaries between crystallites show Josephson behavior.

Observation of breathers in Josephson ladders.

We report on the observation of spatially localized excitations in a ladder of small Josephson junctions. The excitations are whirling states which persist under a spatially homogeneous force due to the bias current. These states of the ladder are visualized using a low temperature scanning laser microscopy. We also compute breather solutions with high accuracy in corresponding model equations. The stability analysis of these solutions is used to interpret the measured patterns in the I-V characteristics.

Equivalence of LTSLM and LTSEM analyses: study of a YBa 2Cu 3O 7 multi-layer device

Low-temperature scanning electron microscopy (LTSEM) and low-temperature scanning laser microscopy (LTSLM) are both methods for analyzing local electrical properties of superconducting devices. With both techniques we have investigated a YBa 2Cu 3O 7 (YBCO) multi-layer device on a SrTiO 3 (100) substrate namely a micro-bridge containing a set of crossovers. We find that critical current density and critical temperature of this device are reduced at the edges of the crossovers. Complete equivalence of the results obtained for LTSEM and LTSLM has been found, in particular concerning the spatial distribution, the temperature dependence, and the strength of the signal.

Spatially resolved characterization of superconducting films and cryoelectronic devices by means of low temperature scanning laser microscope

The work describes the low temperature scanning laser microscopy technique used for spatially resolved characterization of superconducting films and film-based cryoelectronic circuits in the temperature range from 2 to 300 K. The determination of superconducting parameters for separate elements of a high T c Josephson junctions array and imaging of the resistive transition in a high T c superconducting polycrystalline film are demonstrated. The spatial evolution of the resistive state of a Sn thin film strip associated with the phase slip lines formation is visualized.

Spatially resolved analyses of epitaxial and electrical properties of YBa2Cu3O7 devices

We use scanning micro-Raman spectroscopy (SMRS) to characterize the epitaxial film quality as well as low-temperature scanning laser microscopy (LTSLM) to image electrical properties of YBa2Cu3O7 devices locally in both lateral dimensions. Investigations have been performed on crossovers as used in flux transformers of magnetometers, patterned in a multilayer process on SrTiO3 (100) substrates. We find a correlation between an inhomogeneous superconducting performance and a disturbance of epitaxy on locations where the crossovers pass edges of the underlying bridge. The distribution of the oxygen is not affected at these points.

Low temperature scanning laser microscope for studies in high magnetic fields

The low temperature scanning laser microscope (LTSLM) for investigation of thin film superconducting objects in the temperature range of 2.5–300 K and magnetic field up to 5 T is described.

Laser scanning visualization of evolution of vortex instability in current-carrying superconducting strips

The low temperature scanning laser microscopy is used to study spatial characteristics of current-induced resistive states in wide superconducting Sn films. The irregularities of edge potential barrier and the current distribution in a film cross section are visualized. The results are in good qualitative agreement with Aslamazov-Lempitsky theory.

Spatially resolved study of transition to the phase-slip lines resistive state in wide superconducting strips

The voltage images of nonequilibrium resistive state associated with a phase-slip lines (PSLs) formation in a current-carrying superconducting Sn strip were obtained using low-temperature scanning laser microscopy (LTSLM). The transition to the PSLs started from dynamic-mixed state by single PSL nucleation. The number of PSLs increases rapidly while increasing the current density, the spatial rearrangement of the whole resistive structure being observed.

Laser scanning imaging and local characterization of superconducting properties in high- Tc thin film multiturn coil

Low-temperature scanning laser microscopy has been used to investigate the spatial variation of the critical temperature T c and critical current I c in thin-film high- T c multilayer structures that include dielectric layers. The method is described and measurements are presented on an YBa 2Cu 3O 7− x -based multiturn coil with SrTiO 3 insulating layer. We found that the critical temperature T c of the YBa 2Cu 3O 7− x top layer, from which the return strip of the coil is formed, is higher than that of the YBa 2Cu 3O 7− x base layer. The critical current of the coil is limited by the quality of the YBa 2Cu 3O 7- x base layer and not by the edges of the crossovers.