Sergey Fedoseev - Academia.edu (original) (raw)

Papers by Sergey Fedoseev

Superconductor Science and Technology, 2020

© 2020 IOP Publishing Ltd. In order to enhance and/or control critical current density (J c) in s... more © 2020 IOP Publishing Ltd. In order to enhance and/or control critical current density (J c) in superconducting YBa2Cu3O7-δ (YBCO) thin films, different arrays of 3 µm large ferromagnetic (iron) dots with differing configurations and shapes have been deposited on top of high-quality YBCO thin films post-buffered with a layer of CeO2. Some tremendous J c enhancement of up to nearly 100% have been obtained at high temperatures and low fields. However, the J c performance is strongly dependent on the array configurations, shape and amount of ferromagnetic iron involved. We show that it is possible to enhance J c at high or low magnetic field ranges. The results are clearly different to similar non magnetic array structures used to previously manipulate J c in YBCO films, which proves the magnetic origin of the changes in J c we observed. The enhancement is likely due to the flux localization and magnetic pinning effects, rather than magnetic shielding alone, which is effective at relatively low fields only. The results also suggest that the observed J c changes depend on rather minor variations in initial pinning and corresponding J c levels of the films. At the same time a common trend for all of the various investigated magnetic arrays could not be established due to explained factors

Annalen der Physik, 2017

Uniform, graded and spaced arrays of 3 μm triangular antidots in pulsed laser deposited YBa 2 Cu ... more Uniform, graded and spaced arrays of 3 μm triangular antidots in pulsed laser deposited YBa 2 Cu 3 O 7 (YBCO) superconducting thin films are compared by examining the improvements in the critical current density J c they produced. The comparison is made to establish the role of their lithographically defined (non-)uniformity and the effectiveness to control and/or enhance the critical current density. It is found that almost all types of non-uniform arrays, including graded ones enhance J c over the broad applied magnetic field and temperature range due to the modified critical state. Whereas uniform arrays of antidots either reduce or produce no effect on J c compared to the original (asdeposited) thin films.

APL Materials, 2019

High quality robust two-dimensional electron gas (2DEG) LaAlO 3 /SrTiO 3 (LAO/STO) interfaces are... more High quality robust two-dimensional electron gas (2DEG) LaAlO 3 /SrTiO 3 (LAO/STO) interfaces are produced using pulsed laser deposition and an acid-free substrate Ti-termination process, resulting in single unit cell terraces. Temperature dependent resistance measurements show two hysteresis anomalies around 80 K and 160 K. By using Hall measurements, we find an Arrhenius dependence in charge carrier density describing a partial carrier freeze-out below ∼80 K. We show that these two resistance anomalies are unrelated to the temperature dependence of the charge carrier density despite the tempting coincidence of the low temperature hysteresis feature and the freeze-out process. A twocarrier model is required to accurately estimate the activation energy of the thermally activated type charge carriers, which are found to be ∼5 to 7 meV. These results support the theory that oxygen vacancy defects contribute to the metallic conductivity at the 2DEG LAO/STO interface even for annealed samples.

Pricm 7, Pts 1-3, 2010

Abstract Multilayered approach to YBCO-based Josephson junction manufacture is introduced. Proper... more Abstract Multilayered approach to YBCO-based Josephson junction manufacture is introduced. Properties of monolayer YBa2Cu3O7 (YBCO) and multilayer YBCO/NdBCO/YBCO junctions of similar thickness grown by pulsed-laser deposition on ...

Journal of Physics D: Applied Physics, 2013

Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets... more Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets. Dramatic structure variations are obtained by varying the laser frequency while keeping the thickness of the films constant. A new theoretical model based on the mean field approach is proposed, which quantitatively describes the structural changes obtained experimentally. The experiment and the model exhibit the opposite growth development to the trend reported for the modulated flux in the literature. The new model considers different growth rates in the lateral and transverse directions due to different responses of the normal and tangential film surfaces being deposited to the incident flux and the migration kinetics of adatoms and clusters. The quantitative results obtained confirm that the migration kinetics and self-assembly can easily be controlled by the PLD frequency which is consistent with the experiments. Magnetic properties of the films are shown to be extremely sensitive to the structure variations allowing practical tunability.

EPL (Europhysics Letters), 2013

Different experimental techniques are employed to evaluate the critical current density (Jc), nam... more Different experimental techniques are employed to evaluate the critical current density (Jc), namely transport current measurements and two different magnetisation measurements forming quasi-equilibrium and dynamic critical states. Our technique-dependent results for superconducting YBa 2Cu3O7 (YBCO) film and MgB2 bulk samples show an extremely high sensitivity of Jc and associated interpretations, such as irreversibility fields and Kramer plots, which lose meaning without a universal approach. We propose such approach for YBCO films based on their unique pinning features. This approach allows us to accurately recalculate the magnetic-field-dependent Jc obtained by any technique into the Jc behaviour, which would have been measured by any other method without performing the corresponding experiments. We also discovered low-frequency-dependent phenomena, governing flux dynamics, but contradicting the considered ones in the literature. The understanding of these phenomena, relevant to applications with moving superconductors, can clarify their dramatic impact on the electric-field criterion through flux diffusivity and corresponding measurements.

Journal of Applied Physics, 2013

IEEE Transactions on Applied Superconductivity, 2013

ABSTRACT High-temperature superconducting thin films (YBa2Cu3O7-x) are emerging in superconductin... more ABSTRACT High-temperature superconducting thin films (YBa2Cu3O7-x) are emerging in superconducting single photon detector (SSPD) research as a novel replacement for conventional and semiconductor detectors. The major hindrance for this is the degradation of the superconducting properties of YBa2Cu3O7-x (YBCO) thin film with reduction of its lateral and longitudinal dimensions (i.e., film thickness and width of the stripe). Furthermore, the surface of the film should be smooth to enable fabrication of the SSPD device. In order to improve the quality of YBCO thin films, we exploited various buffer layers (i.e., SrTiO3, CeO2, and PrBa2Cu3O7) with thickness of 30 +/- 5 nm. We have also investigated the properties of (65 +/- 5-nm-thick) YBCO films grown simultaneously on different substrates (i.e., SrTiO3, LaAlO3, MgO, and yttrium stabilized zirconia). For some substrate/buffer material combinations, the surface morphology of the YBCO film has been effectively improved. Also, there was only a small or no degradation of their critical temperature values. These structures give a precursor for further development of fabrication technology for YBCO-based SSPD devices.

IEEE Transactions on Applied Superconductivity, 2011

Abstract Application of Josephson effect in sensors, electronic and metrological devices working ... more Abstract Application of Josephson effect in sensors, electronic and metrological devices working at temperature of 77 K requires fabrication of efficient and reproducible high temperature superconducting (HTS) Josephson junctions. In order to enhance the ...

2001 IEEE EMC International Symposium. Symposium Record. International Symposium on Electromagnetic Compatibility (Cat. No.01CH37161), 2001

The grand challenge in twenty-first-century physical and material sciences is related to enabling... more The grand challenge in twenty-first-century physical and material sciences is related to enabling continued advances in information processing and data storage beyond conventional silicon based electronics. Recent researches in oxide electronics suggest that it may be plausible to implement complex systems at the device level, thereby drastically increasing computational density and power efficiency. High temperature superconducting (HTS) ceramics YBa 2 Cu 3 O 7-δ (YBCO) is currently one of the most promising candidates for the creation of sophisticated cryo-electronic devices such as: Josephson junctions, microwave filters, single photon detectors, and etc. One of the important problems in YBCO thin film technology is an enhancement of superconducting properties of epitaxial films which allows the creation of reliable cryo-electronic devices with high performances. Heteroepitaxial structures consisting, for example, of YBCO layers and layers of different superconducting v ACKNOWLEDGEMENTS I would like to express my deepest sincere appreciation my supervisor Professor Alexey Pan for his valuable advices, suggestions, guidance, patience and constructive critics through the completion of my thesis work. I also would like to thank all Thin Films Team members especially Dr. Olga Shcherbakova and Igor Golovchanskiy, as well as the other ISEM staff. It had been a pleasure working with all of you. My sincerest thanks are extended to all my friends in our group. Thanks for your constant support and contributions. vi TABLE OF CONTENTS

The influence of laser frequency on the structure and physical properties of thin films grown by ... more The influence of laser frequency on the structure and physical properties of thin films grown by pulsed laser deposition has been studied. Different types of thin films, hard ferromagnetic FePt L10 and quasi-single crystal superconducting YBa2Cu3O7 (YBCO), have been used for demonstration of the effect. Significant structural modifications have been obtained for the films with similar thicknesses. These modifications are shown to dramatically control their corresponding properties, providing an instrumental ability for tuning the practical characteristics of the films by changing the laser frequency of their deposition. In particular, 20-fold increase of coercive field and modification of demagnetization mechanism are obtained for FePt films by varying the frequency from 1 Hz to 6 Hz. Over a similar frequency range, a strong dependence on the laser frequency is discovered for the YBCO films for the critical current density behavior as a function of the applied magnetic field [ Jc(Ba)] with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications and corresponding property variations are proposed.

Physical Review B

In this paper the origin of flux-jumps in Nb thin films is established during magnetization measu... more In this paper the origin of flux-jumps in Nb thin films is established during magnetization measurements using a vibrating sample magnetometer (VSM). Magnetization measurements of the flux avalanche activity show its strong dependence on frequency and amplitude of VSM vibration. In particular, under certain conditions the vibrations induce a transition from a stable superconducting critical state to an undercritical state, accompanied by the 20-fold drop in the magnetic moment. These features allow the elucidation of the origin of the flux-jumps. In contrast to the commonly assumed thermomagnetic instabilities to be responsible for the flux-jumps in Nb films, our results provide solid support for an alternative explanation being due to criticalitybuilt instability well represented by a sandpile. Considering properties of the flux-flow during a flux avalanche regime allows us to estimate nonuniformity of a magnetic field in a VSM sample space developed as a result of vibrations.

IEEE Transactions on Applied Superconductivity

Nanoscale

A giant ferromagnetic/superconducting proximity effect is demonstrated to be controllable by an e... more A giant ferromagnetic/superconducting proximity effect is demonstrated to be controllable by an external magnetic field.

Advanced Materials Interfaces

Scientific Reports, 2017

The dynamics of transient current distributions in superconducting YBa 2 Cu 3 O 7−δ thin films we... more The dynamics of transient current distributions in superconducting YBa 2 Cu 3 O 7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile. The field and current profiles of YBa 2 Cu 3 O 7−δ (YBCO) and other superconducting samples are well understood for different magnetic states and histories under static conditions 1-10. However, many of the most interesting magnetic effects in superconductors are transient and occur at extremely high speed: the spectacular dendritic flux avalanches appear to occur nearly instantaneously-their actual speed could be between 5 km/s 11 and 180 km/s 12 ; while smaller localised flux jumps can occur over timescales less than 0.1 s 13. The final penetration depth of flux into YBCO films has been shown to be dependent on the ramping rate of applied field 14,15 , suggesting that transient flux dynamics during penetration could be a strong determining factor in the measured final field state of any superconducting sample. In order to investigate such transient and dynamic events in superconductors, new high-speed magnetic measurement techniques are required. Most local magnetic measurements on superconductors are carried out using various scanning techniques 16 , which are fundamentally limited in speed due to their scanning mechanism 17. On the other hand, laser pulsed magneto-optical imaging is able to obtain pairs of images with nanosecond-scale variable delay 12 , but it cannot record continuously in order to fully investigate superconducting magneto-dynamics.

Superconductor Science and Technology, 2016

In this work the effect of vibrations on critical current density (Jc) of superconductors has bee... more In this work the effect of vibrations on critical current density (Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency (f) and/or amplitude (A) leads to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on Jc (Ba) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field (Birr) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of Birr for Jc modeling is demonstrated.

Superconductor Science and Technology, 2016

We present a novel method for analysis of superconducting thin films using dynamic magneto-optica... more We present a novel method for analysis of superconducting thin films using dynamic magneto-optical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O${}_{7-\delta }$ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage.

Superconductor Science and Technology, 2020

© 2020 IOP Publishing Ltd. In order to enhance and/or control critical current density (J c) in s... more © 2020 IOP Publishing Ltd. In order to enhance and/or control critical current density (J c) in superconducting YBa2Cu3O7-δ (YBCO) thin films, different arrays of 3 µm large ferromagnetic (iron) dots with differing configurations and shapes have been deposited on top of high-quality YBCO thin films post-buffered with a layer of CeO2. Some tremendous J c enhancement of up to nearly 100% have been obtained at high temperatures and low fields. However, the J c performance is strongly dependent on the array configurations, shape and amount of ferromagnetic iron involved. We show that it is possible to enhance J c at high or low magnetic field ranges. The results are clearly different to similar non magnetic array structures used to previously manipulate J c in YBCO films, which proves the magnetic origin of the changes in J c we observed. The enhancement is likely due to the flux localization and magnetic pinning effects, rather than magnetic shielding alone, which is effective at relatively low fields only. The results also suggest that the observed J c changes depend on rather minor variations in initial pinning and corresponding J c levels of the films. At the same time a common trend for all of the various investigated magnetic arrays could not be established due to explained factors

Annalen der Physik, 2017

Uniform, graded and spaced arrays of 3 μm triangular antidots in pulsed laser deposited YBa 2 Cu ... more Uniform, graded and spaced arrays of 3 μm triangular antidots in pulsed laser deposited YBa 2 Cu 3 O 7 (YBCO) superconducting thin films are compared by examining the improvements in the critical current density J c they produced. The comparison is made to establish the role of their lithographically defined (non-)uniformity and the effectiveness to control and/or enhance the critical current density. It is found that almost all types of non-uniform arrays, including graded ones enhance J c over the broad applied magnetic field and temperature range due to the modified critical state. Whereas uniform arrays of antidots either reduce or produce no effect on J c compared to the original (asdeposited) thin films.

APL Materials, 2019

High quality robust two-dimensional electron gas (2DEG) LaAlO 3 /SrTiO 3 (LAO/STO) interfaces are... more High quality robust two-dimensional electron gas (2DEG) LaAlO 3 /SrTiO 3 (LAO/STO) interfaces are produced using pulsed laser deposition and an acid-free substrate Ti-termination process, resulting in single unit cell terraces. Temperature dependent resistance measurements show two hysteresis anomalies around 80 K and 160 K. By using Hall measurements, we find an Arrhenius dependence in charge carrier density describing a partial carrier freeze-out below ∼80 K. We show that these two resistance anomalies are unrelated to the temperature dependence of the charge carrier density despite the tempting coincidence of the low temperature hysteresis feature and the freeze-out process. A twocarrier model is required to accurately estimate the activation energy of the thermally activated type charge carriers, which are found to be ∼5 to 7 meV. These results support the theory that oxygen vacancy defects contribute to the metallic conductivity at the 2DEG LAO/STO interface even for annealed samples.

Pricm 7, Pts 1-3, 2010

Abstract Multilayered approach to YBCO-based Josephson junction manufacture is introduced. Proper... more Abstract Multilayered approach to YBCO-based Josephson junction manufacture is introduced. Properties of monolayer YBa2Cu3O7 (YBCO) and multilayer YBCO/NdBCO/YBCO junctions of similar thickness grown by pulsed-laser deposition on ...

Journal of Physics D: Applied Physics, 2013

Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets... more Pulsed laser deposition (PLD) is employed to fabricate FePt L10 thin films from elemental targets. Dramatic structure variations are obtained by varying the laser frequency while keeping the thickness of the films constant. A new theoretical model based on the mean field approach is proposed, which quantitatively describes the structural changes obtained experimentally. The experiment and the model exhibit the opposite growth development to the trend reported for the modulated flux in the literature. The new model considers different growth rates in the lateral and transverse directions due to different responses of the normal and tangential film surfaces being deposited to the incident flux and the migration kinetics of adatoms and clusters. The quantitative results obtained confirm that the migration kinetics and self-assembly can easily be controlled by the PLD frequency which is consistent with the experiments. Magnetic properties of the films are shown to be extremely sensitive to the structure variations allowing practical tunability.

EPL (Europhysics Letters), 2013

Different experimental techniques are employed to evaluate the critical current density (Jc), nam... more Different experimental techniques are employed to evaluate the critical current density (Jc), namely transport current measurements and two different magnetisation measurements forming quasi-equilibrium and dynamic critical states. Our technique-dependent results for superconducting YBa 2Cu3O7 (YBCO) film and MgB2 bulk samples show an extremely high sensitivity of Jc and associated interpretations, such as irreversibility fields and Kramer plots, which lose meaning without a universal approach. We propose such approach for YBCO films based on their unique pinning features. This approach allows us to accurately recalculate the magnetic-field-dependent Jc obtained by any technique into the Jc behaviour, which would have been measured by any other method without performing the corresponding experiments. We also discovered low-frequency-dependent phenomena, governing flux dynamics, but contradicting the considered ones in the literature. The understanding of these phenomena, relevant to applications with moving superconductors, can clarify their dramatic impact on the electric-field criterion through flux diffusivity and corresponding measurements.

Journal of Applied Physics, 2013

IEEE Transactions on Applied Superconductivity, 2013

ABSTRACT High-temperature superconducting thin films (YBa2Cu3O7-x) are emerging in superconductin... more ABSTRACT High-temperature superconducting thin films (YBa2Cu3O7-x) are emerging in superconducting single photon detector (SSPD) research as a novel replacement for conventional and semiconductor detectors. The major hindrance for this is the degradation of the superconducting properties of YBa2Cu3O7-x (YBCO) thin film with reduction of its lateral and longitudinal dimensions (i.e., film thickness and width of the stripe). Furthermore, the surface of the film should be smooth to enable fabrication of the SSPD device. In order to improve the quality of YBCO thin films, we exploited various buffer layers (i.e., SrTiO3, CeO2, and PrBa2Cu3O7) with thickness of 30 +/- 5 nm. We have also investigated the properties of (65 +/- 5-nm-thick) YBCO films grown simultaneously on different substrates (i.e., SrTiO3, LaAlO3, MgO, and yttrium stabilized zirconia). For some substrate/buffer material combinations, the surface morphology of the YBCO film has been effectively improved. Also, there was only a small or no degradation of their critical temperature values. These structures give a precursor for further development of fabrication technology for YBCO-based SSPD devices.

IEEE Transactions on Applied Superconductivity, 2011

Abstract Application of Josephson effect in sensors, electronic and metrological devices working ... more Abstract Application of Josephson effect in sensors, electronic and metrological devices working at temperature of 77 K requires fabrication of efficient and reproducible high temperature superconducting (HTS) Josephson junctions. In order to enhance the ...

2001 IEEE EMC International Symposium. Symposium Record. International Symposium on Electromagnetic Compatibility (Cat. No.01CH37161), 2001

The grand challenge in twenty-first-century physical and material sciences is related to enabling... more The grand challenge in twenty-first-century physical and material sciences is related to enabling continued advances in information processing and data storage beyond conventional silicon based electronics. Recent researches in oxide electronics suggest that it may be plausible to implement complex systems at the device level, thereby drastically increasing computational density and power efficiency. High temperature superconducting (HTS) ceramics YBa 2 Cu 3 O 7-δ (YBCO) is currently one of the most promising candidates for the creation of sophisticated cryo-electronic devices such as: Josephson junctions, microwave filters, single photon detectors, and etc. One of the important problems in YBCO thin film technology is an enhancement of superconducting properties of epitaxial films which allows the creation of reliable cryo-electronic devices with high performances. Heteroepitaxial structures consisting, for example, of YBCO layers and layers of different superconducting v ACKNOWLEDGEMENTS I would like to express my deepest sincere appreciation my supervisor Professor Alexey Pan for his valuable advices, suggestions, guidance, patience and constructive critics through the completion of my thesis work. I also would like to thank all Thin Films Team members especially Dr. Olga Shcherbakova and Igor Golovchanskiy, as well as the other ISEM staff. It had been a pleasure working with all of you. My sincerest thanks are extended to all my friends in our group. Thanks for your constant support and contributions. vi TABLE OF CONTENTS

The influence of laser frequency on the structure and physical properties of thin films grown by ... more The influence of laser frequency on the structure and physical properties of thin films grown by pulsed laser deposition has been studied. Different types of thin films, hard ferromagnetic FePt L10 and quasi-single crystal superconducting YBa2Cu3O7 (YBCO), have been used for demonstration of the effect. Significant structural modifications have been obtained for the films with similar thicknesses. These modifications are shown to dramatically control their corresponding properties, providing an instrumental ability for tuning the practical characteristics of the films by changing the laser frequency of their deposition. In particular, 20-fold increase of coercive field and modification of demagnetization mechanism are obtained for FePt films by varying the frequency from 1 Hz to 6 Hz. Over a similar frequency range, a strong dependence on the laser frequency is discovered for the YBCO films for the critical current density behavior as a function of the applied magnetic field [ Jc(Ba)] with the unexpected reversal of Jc(Ba) curves with temperature. The mechanisms of structure modifications and corresponding property variations are proposed.

Physical Review B

In this paper the origin of flux-jumps in Nb thin films is established during magnetization measu... more In this paper the origin of flux-jumps in Nb thin films is established during magnetization measurements using a vibrating sample magnetometer (VSM). Magnetization measurements of the flux avalanche activity show its strong dependence on frequency and amplitude of VSM vibration. In particular, under certain conditions the vibrations induce a transition from a stable superconducting critical state to an undercritical state, accompanied by the 20-fold drop in the magnetic moment. These features allow the elucidation of the origin of the flux-jumps. In contrast to the commonly assumed thermomagnetic instabilities to be responsible for the flux-jumps in Nb films, our results provide solid support for an alternative explanation being due to criticalitybuilt instability well represented by a sandpile. Considering properties of the flux-flow during a flux avalanche regime allows us to estimate nonuniformity of a magnetic field in a VSM sample space developed as a result of vibrations.

IEEE Transactions on Applied Superconductivity

Nanoscale

A giant ferromagnetic/superconducting proximity effect is demonstrated to be controllable by an e... more A giant ferromagnetic/superconducting proximity effect is demonstrated to be controllable by an external magnetic field.

Advanced Materials Interfaces

Scientific Reports, 2017

The dynamics of transient current distributions in superconducting YBa 2 Cu 3 O 7−δ thin films we... more The dynamics of transient current distributions in superconducting YBa 2 Cu 3 O 7−δ thin films were investigated during and immediately following an external field ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of dynamic current profiles. A number of qualitatively unique and previously unobserved features are seen in this novel analysis of the evolution of supercurrent during penetration. As magnetic field ramps up from zero, the dynamic current profile is characterized by strong peaks, the magnitude of which exceed the conventional critical current density (as determined from static current profiles). These peaks develop close to the sample edges, initially resembling screening currents but quickly growing in intensity as the external field increases. A discontinuity in field and current behaviour is newly observed, indicating a novel transition from increasing peak current toward relaxation behaviour. After this transition, the current peaks move toward the centre of the sample while reducing in intensity as magnetic vortices penetrate inward. This motion slows exponentially with time, with the current distribution in the long-time limit reducing to the expected Kim-model profile. The field and current profiles of YBa 2 Cu 3 O 7−δ (YBCO) and other superconducting samples are well understood for different magnetic states and histories under static conditions 1-10. However, many of the most interesting magnetic effects in superconductors are transient and occur at extremely high speed: the spectacular dendritic flux avalanches appear to occur nearly instantaneously-their actual speed could be between 5 km/s 11 and 180 km/s 12 ; while smaller localised flux jumps can occur over timescales less than 0.1 s 13. The final penetration depth of flux into YBCO films has been shown to be dependent on the ramping rate of applied field 14,15 , suggesting that transient flux dynamics during penetration could be a strong determining factor in the measured final field state of any superconducting sample. In order to investigate such transient and dynamic events in superconductors, new high-speed magnetic measurement techniques are required. Most local magnetic measurements on superconductors are carried out using various scanning techniques 16 , which are fundamentally limited in speed due to their scanning mechanism 17. On the other hand, laser pulsed magneto-optical imaging is able to obtain pairs of images with nanosecond-scale variable delay 12 , but it cannot record continuously in order to fully investigate superconducting magneto-dynamics.

Superconductor Science and Technology, 2016

In this work the effect of vibrations on critical current density (Jc) of superconductors has bee... more In this work the effect of vibrations on critical current density (Jc) of superconductors has been studied. The vibrations are shown to affect Jc of all types of superconductors during their measurements, employing a vibrating sample magnetometer (VSM). Increasing vibration frequency (f) and/or amplitude (A) leads to progressive reduction of Jc as a function of magnetic field (Ba). The effect of vibrations is substantially stronger in thin films. It leads to development of unexpected kinks on Jc (Ba) curves. Analysis of magnetization loops and relaxation of magnetization in YBCO films revealed that the vibration effect can be treated as the effective reduction of pinning potential. The asymmetry of the vibration effect in ascending and descending Ba is observed, indicating differences in free energy of the corresponding vortex structures. Thermal effects induced by vibrations with large f and A are shown to have rather insignificant influence, while the vibrational vortex dynamics exhibit a strong impact. The irreversibility field (Birr) is shown to be instrumentally defined, and its value depends on VSM settings. In addition, the practical importance of Birr for Jc modeling is demonstrated.

Superconductor Science and Technology, 2016

We present a novel method for analysis of superconducting thin films using dynamic magneto-optica... more We present a novel method for analysis of superconducting thin films using dynamic magneto-optical imaging, revealing hallmarks of flux penetration with temporal resolution around 1 ms (in the present work) or better. This method involves investigation of transient field and dynamic current distributions, which are calculated by an inversion procedure on the Biot-Savart Law, which we show to be valid under dynamic conditions. We compare and discuss the flux front penetration speed and evolution of current distribution in high quality YBa2Cu3O${}_{7-\delta }$ thin films with that of samples deliberately damaged in such a way as to reduce critical current density without causing macroscopic damage.