L. Miu - Academia.edu (original) (raw)

Papers by L. Miu

Physical Review B, 2012

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts ... more The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the pinning barriers by thermal fluctuations. In the case of YB 2 Cu 3 O 7 theoretical estimates give a depinning temperature T dp very close to the critical temperature T c (T dp ∼ 0.95T c), while the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T dp (∼0.5T c). We determined the temperature T variation of the normalized magnetization relaxation rate S for YB 2 Cu 3 O 7 films containing BaZrO 3 nanorods preferentially oriented along the c axis, with the external magnetic field applied along the nanorods. By extending the T interval up close to T c , below the matching field a rich nonmonotonous S(T) variation was observed. It is shown here that the often analyzed S(T) maximum occurring at relatively low T (which was connected to a disappointing T dp) has an extrinsic origin, related to thermomagnetic instabilities. The accommodation of vortices to the columnar pins in the presence of the T-dependent macroscopic currents induced in the sample during experiments is actually signaled by a pronounced S(T) deep located at high T , indicating that T dp remains close to T c , in agreement with the theoretical prediction.

Modern Physics Letters B, 2020

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificia... more We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO3 (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy [Fo...

Journal of Superconductivity and Novel Magnetism, 2016

We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys therm... more We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 ∘C in vacuum and cold rolling (∼ 50 % thickness reduction). Starting with a rectangular piece (∼ 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak (where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

Physical Review B, 1995

A continuous transition of the vortex ensemble into a Bose-glass state in epitaxial Bi2Sr2CaCu2O8... more A continuous transition of the vortex ensemble into a Bose-glass state in epitaxial Bi2Sr2CaCu2O8+delta films with columnar defects along the c axis, induced by irradiation with 2.7 GeV 238U ions was observed. The temperature variation of the resistivity, measured with the magnetic field B=mu0H applied parallel to the c axis, fits the expression rho(T)~(T-TBG)nu(z-2) at low levels, with a field-independent exponent up to B~=2 T. At very low fields, the magnetic-field dependence of the Bose-glass transition temperature TBG is in good agreement with the predictions of theory of boson localization in the presence of correlated disorder, 1/TBG-1/Tc0~B1/4. However, for applied field values significantly larger than the dose-equivalent field Bphi=1 T, the vortex delocalization in our samples seems to proceed via melting, the magnetic-field exponent becoming very close to 1/2. The occurrence of the Bose-glass transition in the case of intrinsically very anisotropic superconductors, as Bi2Sr2CaCu2O8+delta, can be understood through an increase of the tilt modulus in the vortex ensemble in the presence of correlated disorder, which promotes vortex localization.

Superconductor Science and Technology, 2012

ABSTRACT

Superconductor Science and Technology, 2011

Page 1. Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxati... more Page 1. Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxation measurements This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Supercond. Sci. Technol. 24 045014 ...

Superconductor Science and Technology, 1997

ABSTRACT

Physica C: Superconductivity, 2008

The relaxation of the irreversible magnetization of MgB 2 bulk samples obtained by electric-field... more The relaxation of the irreversible magnetization of MgB 2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H / T À2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low-T limit.

Philosophical Magazine Letters, 1988

It is shown that in the high-Tc YBa2 Cu3 O∼7 superconductor the critical temperature is a functio... more It is shown that in the high-Tc YBa2 Cu3 O∼7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against Tc, a maximum critical temperature of 66 K for the tetragonal phase of YBa2 Cu3 O∼7 was predicted. From the correlation between the transition

Recent models for the occurrence of high-temperature superconductivity predict that the supercond... more Recent models for the occurrence of high-temperature superconductivity predict that the superconducting transition of underdoped cuprates is determined by the phase fluctuations of the order parameter, while the modulus of the wave function remains finite up to the pseudogap temperature T*. Here we show that the mean-field transition temperature cannot be identified with T*. Exhibiting superconductivity at surprisingly high temperatures (1), the cuprates became the best studied materials outside the semiconductor family. There are hundreds of such high-temperature superconductors (HTS), but they all shear a layered structure made up of one or more Cu-O planes. It is now generally accepted that the parent compound is a Mott insulator, which describes the situation where the material should be metallic according to the band theory, but is insulating due to the strong electron-electron repulsion. It also results that this Mott insulator should be an antiferromagnetic insulator, since w...

Journal of Superconductivity and Novel Magnetism, 2017

The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves ... more The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves of superconducting single crystals with random pinning is still under debate. Many interesting SMP models and mechanisms were proposed and considered so far, and it is believed at present that this effect is system dependent. We analyzed the dc magnetization curves and the magnetic relaxation in the SMP domain for various single-crystal specimens (superconducting cuprates and ironbased superconductors), of different pinning strengths, with the external magnetic field H oriented along the crystallographic c axis or perpendicular to it. The sample independent aspects revealed by the relaxation results are the absence of singlevortex collective pinning around the SMP onset field and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior supports strongly the pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

Superconductor Science and Technology, 2018

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2... more We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field H dc lower than the matching field H Φ. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential U c from the characteristics of electric field E versus J. We show that the dynamic critical current J d induced at the depinning frequency f d can be experimentally obtained by measuring m '(T) and m''(T) at different frequencies and amplitudes. It was found that the value of f d obtained by extrapolation of J to J d in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements.

Superconductor Science and Technology, 2017

We report an enhancement of critical current density (Jc) and unusual behaviour of its dependence... more We report an enhancement of critical current density (Jc) and unusual behaviour of its dependence on field orientation in YBa2Cu3O7-x (YBCO) nanostructured films by a combination of substrate decoration with Ag nano-dots, of incorporation of BaZrO3 (BZO) nano-particles and nano-rods, and of multilayer architecture (a thin SrTiO3 layer separating two 1.5 m-thick YBCO layers). SrTiO3 insulating layers were 15, 30 or 45 nm thick. The highest improvement of Jc in applied magnetic fields smaller than 1 T occurs in the bi-layer with 30 nm thick STO, while a 45 nm layer of SrTiO3 gives highest Jc in fields higher than 1 T. Our thick nanostructured films show significant improvement of Jc in magnetic field along both the c axis and the ab-plane direction. The presence of BZO nano-rods, ab-plane defects and nano particles of BZO and Y2O3 was observed in Transmission Electron Microscopy (TEM) images of the film. The peculiarities of artificial pinning centres revealed in the TEM images of the nanostructured films are used to explain an unusual split of the peak in the Jc dependence on the magnetic field along the ab-plane of YBCO. Effective pinning potentials in high magnetic fields have rather high values for such thick films.

AIP Advances, 2016

Dependence of the flux-creep activation energy on current density and magnetic field for a Ca 10 ... more Dependence of the flux-creep activation energy on current density and magnetic field for a Ca 10 (Pt 3 As 8)[(Fe 1−x Pt x) 2 As 2 ] 5 single crystal

Physical Review B, 1998

The supercurrent transport properties of epitaxial Bi 2 Sr 2 CaCu 2 O 8ϩ␦ films in zero applied m... more The supercurrent transport properties of epitaxial Bi 2 Sr 2 CaCu 2 O 8ϩ␦ films in zero applied magnetic field were investigated in a temperature interval of Ϸ20 K below the mean-field critical temperature T c0. The modification of the shape of the I-V curves observed by varying the temperature was explained in terms of vortexfluctuation-induced layer decoupling and vortex-antivortex unbinding, revealing a strong probing-length dependence. The change of the effective dimensionality of thermally excited vortices involved in the dissipation process leads to the appearance of a few characteristic regions in the current-temperature diagram. Above a temperature value T*ϽT c0 , the superconducting layers are decoupled, as predicted by Monte Carlo simulations and renormalization-group analyses. In this region, the resistivity exhibits two-dimensional ͑2D͒ behavior corresponding to the superconducting ͑CuO 2) 2 layers ͑2D-layer behavior͒. However, the resistive transition seems to be mainly related to the 2D behavior at the film level. In the sensitivity window of our measurements, finite resistance in the limit of small transport currents was detected to occur above a temperature value T c ϽT*, through the dissociation of vortex-string-antivortex-string pairs. By decreasing the temperature and/or by increasing the transport current, the I-V curves in the double logarithmic plot show a clear downward curvature. This can be described in terms of current-induced quasi-2D vortex pair unbinding, with a nonzero critical-current density resulting from the interlayer Josephson coupling. At even lower temperatures and/or higher transport currents, the I-V curves exhibit a crossover from quasi-2D to 2D-layer behavior, due to the decrease of the probing length below the Josephson length, where the interlayer Josephson coupling becomes irrelevant. The temperature dependence of the 2D I-V exponent is in good agreement with recent Langevin simulations of the Coulomb gas model, revealing an anomalous diffusion of vortex fluctuations.

Journal of Optoelectronics and Advanced Materials

Well-compacted MgB 2 specimens with the density higher than 90 % of the theoretical value were ob... more Well-compacted MgB 2 specimens with the density higher than 90 % of the theoretical value were obtained by electric-field assisted sintering. This method assures a good grain connectivity, which leads to the appearance of efficient pinning centres at the grain boundaries. We measured the DC magnetization curves and the relaxation of the irreversible magnetization using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. The critical current density is of the order of 10 10 A/m 2 at H = 20 kOe and T = 10 K. A crossover plastic creep at high temperatures T- elastic creep at low T described by H α T -2 in the low T- high H domain was observed. This is caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can be explained through the occurrence of micro flux jumps, which seem to be responsible for the finite magnetization relaxation rate in the low-T limit. The relaxation of the irreversible magnetization allowed us the precise determination of the characteristic pinning energy barrier.

Journal of Superconductivity, 2001

We investigated the superconducting critical temperature, transport critical-current density, and... more We investigated the superconducting critical temperature, transport critical-current density, and the thermo-power properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)y samples. It was found that the midpoint critical temperature increases with increasing y, attaining ˜ 119.5 K for y = 0.15. The transition width shows a minimum and the critical temperature (zero electrical resistance) exhibits a maximum as a function of y. The observed behavior

Thin Solid Films, 1988

Abstract X-ray and transmission electron microscopy structural investigations were performed as w... more Abstract X-ray and transmission electron microscopy structural investigations were performed as well as elastoresistance and magnetoresistance measurements on polycrystalline nickel thin films, sputtered by r.f. and d.c. methods, at different stages of annealing. Shape magnetic anisotropy, related to the plate-like morphology of the crystallites, seems to prevail in the as-deposited r.f. sputtered nickel films, while it is absent in d.c. films.

Physical Review B, 2012

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts ... more The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the pinning barriers by thermal fluctuations. In the case of YB 2 Cu 3 O 7 theoretical estimates give a depinning temperature T dp very close to the critical temperature T c (T dp ∼ 0.95T c), while the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T dp (∼0.5T c). We determined the temperature T variation of the normalized magnetization relaxation rate S for YB 2 Cu 3 O 7 films containing BaZrO 3 nanorods preferentially oriented along the c axis, with the external magnetic field applied along the nanorods. By extending the T interval up close to T c , below the matching field a rich nonmonotonous S(T) variation was observed. It is shown here that the often analyzed S(T) maximum occurring at relatively low T (which was connected to a disappointing T dp) has an extrinsic origin, related to thermomagnetic instabilities. The accommodation of vortices to the columnar pins in the presence of the T-dependent macroscopic currents induced in the sample during experiments is actually signaled by a pronounced S(T) deep located at high T , indicating that T dp remains close to T c , in agreement with the theoretical prediction.

Modern Physics Letters B, 2020

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificia... more We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa2Cu3O7 films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO3 (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy [Fo...

Journal of Superconductivity and Novel Magnetism, 2016

We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys therm... more We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 ∘C in vacuum and cold rolling (∼ 50 % thickness reduction). Starting with a rectangular piece (∼ 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak (where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.

Physical Review B, 1995

A continuous transition of the vortex ensemble into a Bose-glass state in epitaxial Bi2Sr2CaCu2O8... more A continuous transition of the vortex ensemble into a Bose-glass state in epitaxial Bi2Sr2CaCu2O8+delta films with columnar defects along the c axis, induced by irradiation with 2.7 GeV 238U ions was observed. The temperature variation of the resistivity, measured with the magnetic field B=mu0H applied parallel to the c axis, fits the expression rho(T)~(T-TBG)nu(z-2) at low levels, with a field-independent exponent up to B~=2 T. At very low fields, the magnetic-field dependence of the Bose-glass transition temperature TBG is in good agreement with the predictions of theory of boson localization in the presence of correlated disorder, 1/TBG-1/Tc0~B1/4. However, for applied field values significantly larger than the dose-equivalent field Bphi=1 T, the vortex delocalization in our samples seems to proceed via melting, the magnetic-field exponent becoming very close to 1/2. The occurrence of the Bose-glass transition in the case of intrinsically very anisotropic superconductors, as Bi2Sr2CaCu2O8+delta, can be understood through an increase of the tilt modulus in the vortex ensemble in the presence of correlated disorder, which promotes vortex localization.

Superconductor Science and Technology, 2012

ABSTRACT

Superconductor Science and Technology, 2011

Page 1. Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxati... more Page 1. Vortex creep crossover in YBCO/PrBCO superlattices during standard magnetization relaxation measurements This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Supercond. Sci. Technol. 24 045014 ...

Superconductor Science and Technology, 1997

ABSTRACT

Physica C: Superconductivity, 2008

The relaxation of the irreversible magnetization of MgB 2 bulk samples obtained by electric-field... more The relaxation of the irreversible magnetization of MgB 2 bulk samples obtained by electric-field assisted sintering was investigated using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. We observed a crossover plastic creep at high temperatures T-elastic creep at low T, described by H / T À2 in the low T range, which appears to be caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can easily be explained through the occurrence of micro flux jumps, leading to a finite magnetization relaxation rate in the low-T limit.

Philosophical Magazine Letters, 1988

It is shown that in the high-Tc YBa2 Cu3 O∼7 superconductor the critical temperature is a functio... more It is shown that in the high-Tc YBa2 Cu3 O∼7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against Tc, a maximum critical temperature of 66 K for the tetragonal phase of YBa2 Cu3 O∼7 was predicted. From the correlation between the transition

Recent models for the occurrence of high-temperature superconductivity predict that the supercond... more Recent models for the occurrence of high-temperature superconductivity predict that the superconducting transition of underdoped cuprates is determined by the phase fluctuations of the order parameter, while the modulus of the wave function remains finite up to the pseudogap temperature T*. Here we show that the mean-field transition temperature cannot be identified with T*. Exhibiting superconductivity at surprisingly high temperatures (1), the cuprates became the best studied materials outside the semiconductor family. There are hundreds of such high-temperature superconductors (HTS), but they all shear a layered structure made up of one or more Cu-O planes. It is now generally accepted that the parent compound is a Mott insulator, which describes the situation where the material should be metallic according to the band theory, but is insulating due to the strong electron-electron repulsion. It also results that this Mott insulator should be an antiferromagnetic insulator, since w...

Journal of Superconductivity and Novel Magnetism, 2017

The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves ... more The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves of superconducting single crystals with random pinning is still under debate. Many interesting SMP models and mechanisms were proposed and considered so far, and it is believed at present that this effect is system dependent. We analyzed the dc magnetization curves and the magnetic relaxation in the SMP domain for various single-crystal specimens (superconducting cuprates and ironbased superconductors), of different pinning strengths, with the external magnetic field H oriented along the crystallographic c axis or perpendicular to it. The sample independent aspects revealed by the relaxation results are the absence of singlevortex collective pinning around the SMP onset field and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior supports strongly the pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

Superconductor Science and Technology, 2018

We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2... more We investigated the AC magnetic response of a YBa2Cu3O7 film with embedded BaZrO3 nanorods and Y2O3 nanoparticles at a static magnetic field H dc lower than the matching field H Φ. We proposed a practical formula for the determination of the induced current J during AC susceptibility measurements, which allows us to directly obtain the pinning potential U c from the characteristics of electric field E versus J. We show that the dynamic critical current J d induced at the depinning frequency f d can be experimentally obtained by measuring m '(T) and m''(T) at different frequencies and amplitudes. It was found that the value of f d obtained by extrapolation of J to J d in the PHz domain is much higher than the frequency reported for ordinary YBa2Cu3O7 thin films as determined by microwave impedance measurements.

Superconductor Science and Technology, 2017

We report an enhancement of critical current density (Jc) and unusual behaviour of its dependence... more We report an enhancement of critical current density (Jc) and unusual behaviour of its dependence on field orientation in YBa2Cu3O7-x (YBCO) nanostructured films by a combination of substrate decoration with Ag nano-dots, of incorporation of BaZrO3 (BZO) nano-particles and nano-rods, and of multilayer architecture (a thin SrTiO3 layer separating two 1.5 m-thick YBCO layers). SrTiO3 insulating layers were 15, 30 or 45 nm thick. The highest improvement of Jc in applied magnetic fields smaller than 1 T occurs in the bi-layer with 30 nm thick STO, while a 45 nm layer of SrTiO3 gives highest Jc in fields higher than 1 T. Our thick nanostructured films show significant improvement of Jc in magnetic field along both the c axis and the ab-plane direction. The presence of BZO nano-rods, ab-plane defects and nano particles of BZO and Y2O3 was observed in Transmission Electron Microscopy (TEM) images of the film. The peculiarities of artificial pinning centres revealed in the TEM images of the nanostructured films are used to explain an unusual split of the peak in the Jc dependence on the magnetic field along the ab-plane of YBCO. Effective pinning potentials in high magnetic fields have rather high values for such thick films.

AIP Advances, 2016

Dependence of the flux-creep activation energy on current density and magnetic field for a Ca 10 ... more Dependence of the flux-creep activation energy on current density and magnetic field for a Ca 10 (Pt 3 As 8)[(Fe 1−x Pt x) 2 As 2 ] 5 single crystal

Physical Review B, 1998

The supercurrent transport properties of epitaxial Bi 2 Sr 2 CaCu 2 O 8ϩ␦ films in zero applied m... more The supercurrent transport properties of epitaxial Bi 2 Sr 2 CaCu 2 O 8ϩ␦ films in zero applied magnetic field were investigated in a temperature interval of Ϸ20 K below the mean-field critical temperature T c0. The modification of the shape of the I-V curves observed by varying the temperature was explained in terms of vortexfluctuation-induced layer decoupling and vortex-antivortex unbinding, revealing a strong probing-length dependence. The change of the effective dimensionality of thermally excited vortices involved in the dissipation process leads to the appearance of a few characteristic regions in the current-temperature diagram. Above a temperature value T*ϽT c0 , the superconducting layers are decoupled, as predicted by Monte Carlo simulations and renormalization-group analyses. In this region, the resistivity exhibits two-dimensional ͑2D͒ behavior corresponding to the superconducting ͑CuO 2) 2 layers ͑2D-layer behavior͒. However, the resistive transition seems to be mainly related to the 2D behavior at the film level. In the sensitivity window of our measurements, finite resistance in the limit of small transport currents was detected to occur above a temperature value T c ϽT*, through the dissociation of vortex-string-antivortex-string pairs. By decreasing the temperature and/or by increasing the transport current, the I-V curves in the double logarithmic plot show a clear downward curvature. This can be described in terms of current-induced quasi-2D vortex pair unbinding, with a nonzero critical-current density resulting from the interlayer Josephson coupling. At even lower temperatures and/or higher transport currents, the I-V curves exhibit a crossover from quasi-2D to 2D-layer behavior, due to the decrease of the probing length below the Josephson length, where the interlayer Josephson coupling becomes irrelevant. The temperature dependence of the 2D I-V exponent is in good agreement with recent Langevin simulations of the Coulomb gas model, revealing an anomalous diffusion of vortex fluctuations.

Journal of Optoelectronics and Advanced Materials

Well-compacted MgB 2 specimens with the density higher than 90 % of the theoretical value were ob... more Well-compacted MgB 2 specimens with the density higher than 90 % of the theoretical value were obtained by electric-field assisted sintering. This method assures a good grain connectivity, which leads to the appearance of efficient pinning centres at the grain boundaries. We measured the DC magnetization curves and the relaxation of the irreversible magnetization using the SQUID magnetometry for a magnetic field H up to 50 kOe applied in zero-field-cooling conditions. The critical current density is of the order of 10 10 A/m 2 at H = 20 kOe and T = 10 K. A crossover plastic creep at high temperatures T- elastic creep at low T described by H α T -2 in the low T- high H domain was observed. This is caused by the macroscopic currents induced in the sample during magnetization measurements. By decreasing T below this line the determined creep exponent rapidly overcomes the widely accepted theoretical values for elastic (collective) pinning. This behaviour can be explained through the occurrence of micro flux jumps, which seem to be responsible for the finite magnetization relaxation rate in the low-T limit. The relaxation of the irreversible magnetization allowed us the precise determination of the characteristic pinning energy barrier.

Journal of Superconductivity, 2001

We investigated the superconducting critical temperature, transport critical-current density, and... more We investigated the superconducting critical temperature, transport critical-current density, and the thermo-power properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)y samples. It was found that the midpoint critical temperature increases with increasing y, attaining ˜ 119.5 K for y = 0.15. The transition width shows a minimum and the critical temperature (zero electrical resistance) exhibits a maximum as a function of y. The observed behavior

Thin Solid Films, 1988

Abstract X-ray and transmission electron microscopy structural investigations were performed as w... more Abstract X-ray and transmission electron microscopy structural investigations were performed as well as elastoresistance and magnetoresistance measurements on polycrystalline nickel thin films, sputtered by r.f. and d.c. methods, at different stages of annealing. Shape magnetic anisotropy, related to the plate-like morphology of the crystallites, seems to prevail in the as-deposited r.f. sputtered nickel films, while it is absent in d.c. films.