F. Araujo-moreira - Academia.edu (original) (raw)
Papers by F. Araujo-moreira
Physical Review B
The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1... more The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1222͒ have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ϳ40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization ͑TRM͒ measurements at Tϭ60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.
Physical Review B, 2000
The distribution of magnetic induction in Meissner state with finite London penetration depth is ... more The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.
Physical Review B, 2000
We study the magnetic response of superconductors in the presence of low values of a uniform appl... more We study the magnetic response of superconductors in the presence of low values of a uniform applied magnetic field. We report measurements of DC magnetization and AC magnetic susceptibility performed on niobium cylinders of different length-to-radius ratios, which show a dramatic enhance of the initial magnetization for thin samples, due to the demagnetizing effects. The experimental results are analyzed by applying a model that calculates the magnetic response of the superconductor, taking into account the effects of the demagnetizing fields. We use the results of magnetization and current and field distributions of perfectly diamagnetic cylinders to discuss the physics of the demagnetizing effects in the Meissner state of type-II superconductors.
2012 IEEE International Conference on Systems, Man, and Cybernetics (SMC), 2012
ABSTRACT Several works have been done worldwide and are dedicated to the advancement of software ... more ABSTRACT Several works have been done worldwide and are dedicated to the advancement of software that enables the comparison and reading of fingerprints. Literature shows several works related to the development of scientific techniques that allow the improvement in the collection and disclosure of fingerprints left at crime scenes. Some of those techniques are based on the use of high-tech materials and devices, such as gold nanoparticles and thin films made of copper and gold. However, the cost of these techniques is very high for everyday use, especially when considering the financial budgets for police apparatus in developing countries. Attempting to improve economically such viable and efficient techniques in order to permit the disclosure of latent fingerprints left at crime scenes, this paper aims the studying and development of the ninhydrin colorimetric method, by performing numerous tests on fingerprints deposited on porous surfaces, such as paper and walls. Once confirmed the viability of ninhydrin employment, both in laboratory and on-site, and also having adjusted the optimum temperature for rapid revelation; this study provides the use of image-processing software, enabling the expert to compare, in real time, even at the site of criminal traces, the fingerprint image found with the official registry database, thus, obtaining the identity of the offender within a short period of time.
Josephson junction arrays (JJA) have been actively studied for decades. However, they continue to... more Josephson junction arrays (JJA) have been actively studied for decades. However, they continue to contribute to a wide variety of intriguing and peculiar phenomena. To name just a few recent examples, it suffice to mention the so-called paramagnetic Meissner effect and related reentrant temperature behavior of AC susceptibility, observed both in artificially prepared JJA and granular superconductors. Employing mutual-inductance measurements and using a high-sensitive home-made bridge, we have thoroughly investigated the temperature and magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays (2D-JJA) of both shunted and unshunted Nb-AlO x-Nb tunnel junctions In this Chapter, we report on three phenomena related to the magnetic properties of 2D-JJA: (a) the influence of non-uniform critical current density profile on magnetic field behavior of AC susceptibility; (b) the origin of dynamic reentrance and the role of the Stewart-McCumber parameter, β C , in the observability of this phenomenon, and (c) the manifestation of novel geometric effects in temperature behavior of AC magnetic response. Firstly, we present evidences for the existence of local type nonuniformity in the periodic (globally uniform) unshunted 2D-JJA. Specifically, we found that in the mixed state region AC susceptibility χ(T, h AC) can be rather well fitted by a single-plaquette approximation of the overdamped 2D-JJA model assuming a non-uniform distribution of the critical current density within a single junction. According to the current paradigm, paramagnetic Meissner effect (PME) can be related to the presence of π-junctions, either resulting from the presence of magnetic impurities in the junction or from unconventional pairing symmetry. Other possible explanations of this phenomenon are based on flux trapping and flux compression effects including also an important role of the surface of the sample. Besides, in the experiments with unshunted 2D-JJA, we have previously reported that PME manifests itself through a dynamic reentrance (DR) of the AC magnetic susceptibility as a function of temperature. Using an analytical expression we successfully fit our experimental data and demonstrate that the dynamic reentrance of AC susceptibility is directly linked to the value of β C. By simultaneously varying the parameter β L , a phase diagram β C-β L is plotted which demarcates the border between the reentrant and nonreentrant behavior. We show that only arrays with sufficiently large value of β C will exhibit the dynamic reentrance behavior and hence PME. The last topic reviewed in this Chapter is related to the step-like structure observed when the resolution of home-made mutual-inductance bridge is improved. That structure (with the number of steps n = 4 for all AC fields) has been observed in the temperature dependence of AC susceptibility in unshunted 2D-JJA with β L (4.2K) = 30. We were able to successfully fit our data assuming that steps are related to the geometric properties of the plaquette. The number of steps n corresponds to the number of flux quanta that can be screened by the maximum critical current of the junctions. The steps are predicted to manifest themselves in arrays with the inductance related parameter β L (T) matching a "quantization" condition β L (0)=2π(n+1).
7th Joint MMM-Intermag Conference. Abstracts (Cat. No.98CH36275), 1998
Revista Brasileira de Ensino de Física, 2005
TRIZé a sigla para as palavras russas que, em português, significam Teoria para a Resolução de Pr... more TRIZé a sigla para as palavras russas que, em português, significam Teoria para a Resolução de Problemas Criativos. O estudo de mais de dois milhões de patentes no mundo tem permitido identificar princípios universais de invenção. Ensinando estes princípios e com a ajuda de técnicas psicológicas de geração de idéias, a metodologia TRIZ pretende aprofundar e dinamizar o processo criativo. Neste artigo apresentamos algumas sugestões de como utilizar o método TRIZ na formação de estudantes de Física. Palavras-chave: TRIZ, criatividade, inovação, ensino. TRIZ is the acronym for the Russian phrase Theory of Inventive Problem Solving. Over two million patents have been examined, classified by level of inventiveness, and analyzed to look for principles of innovation. If these principles could be identified and codified, they could be taught to people to make the process of invention more predictable. In this paper we discuss some ideas to use this method in the preparation of Physics students.
Physics Letters A, 2006
Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magne... more Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magnetic penetration depth λ(T) in the optimally-doped Sm1.85Ce0.15CuO4 thin films have been extracted. The low-temperature behavior of λ(T) is found to be best-fitted by linear ∆λ(T)/λ(0) = ln(2)kBT /∆0 and quadratic ∆λ(T)/λ(0) = Γ −1/2 ∆ −3/2 0 T 2 laws above and below T = 0.22TC , respectively, which clearly indicates the presence of d-wave pairing mechanism dominated by strong paramagnetic scattering at the lowest temperatures. The best fits produce ∆0/kBTC = 2.07 and Γ/TC = 0.25(TC /∆0) 3 for the estimates of the nodal gap parameter and impurity scattering rate.
Solid State Communications, 2004
Employing mutual-inductance measurements, we study the magnetic field dependence of complex AC su... more Employing mutual-inductance measurements, we study the magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays of unshunted Nb-AlO x-Nb junctions. The observed behavior can be explained assuming single-plaquette approximation of the overdamped model with an inhomogeneous critical current distribution within a single junction.
Physical Review Letters, 2009
At the charge neutral point, graphene exhibits a very unusual high resistance metallic state and ... more At the charge neutral point, graphene exhibits a very unusual high resistance metallic state and a transition to a complete insulating phase in a strong magnetic field. We propose that the current carriers in this state are the charged vortices of the XY valley-pseudospin order-parameter, a situation which is dual to a conventional thin superconducting film. We study energetics and the stability of this phase in the presence of disorder.
Physical Review Letters, 1997
The paramagnetic Meissner effect ͑PME͒ measured in high-T C granular superconductors has been att... more The paramagnetic Meissner effect ͑PME͒ measured in high-T C granular superconductors has been attributed to the presence of junctions between the grains. Here we present measurements of complex ac magnetic susceptibility from two-dimensional arrays of conventional ͑non-͒ Nb/Al/AlOx/Nb Josephson junctions. We measured the susceptibility as a function of the temperature T, the ac amplitude of the excitation field h ac and the external magnetic field H dc. The experiments show a strong paramagnetic contribution from the multijunction loops, which manifests itself as a reentrant screening at low temperature, for values of h ac higher than 50 mOe. A highly simplified model, based on a single loop containing four junctions, accounts for this paramagnetic contribution and the range of parameters in which it appears. This model offers an alternative explanation of PME that does not involve junctions. ͓S0163-1829͑99͒02234-1͔
Physical Review B, 2007
By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response... more By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response to an effective stress field, we study (both analytically and numerically) the temperature and magnetic field dependence of TE coefficient α in a single small junction and in a square array. In particular, we found that in addition to field oscillations due to Fraunhofer-like dependence of the critical current, α of a small single junction also exhibits strong flux driven temperature oscillations near T C. We also numerically simulated stress induced response of a closed loop with finite self-inductance (a prototype of an array) and found that α of a 5 × 5 array may still exhibit temperature oscillations provided the applied magnetic field is strong enough to compensate for the screening induced effects.
Physica C: Superconductivity, 2001
This contribution presents a selected set of results, obtained as part of a systematic investigat... more This contribution presents a selected set of results, obtained as part of a systematic investigation, evidencing that many eects exhibited by superconductors are distinct manifestations of granularity which, in turn, is envisaged as a break of symmetry. The Wohlleben eect, the``®shtail anomaly'', the magnetic remanence exhibited by Josephson junction arrays, and the jumps on the magnetic moment of superconducting samples of mesoscopic dimensions, are examples which we brie¯y review and discuss taking granularity as the basic ingredient. The emphasis of the present approach is to recognize the importance of granularity in every scenario intended to explain the magnetic properties of superconducting systems.
Physica C: Superconductivity, 2007
Polycrystalline samples of Ru 1Àx Nb x Sr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Àd , 0 6 x 6 0.5, have been sy... more Polycrystalline samples of Ru 1Àx Nb x Sr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Àd , 0 6 x 6 0.5, have been synthesized and studied by resistivity, magnetization and ac susceptibility measurements. It was identified a strong suppression of the spin glass (SG) transition, which was totally suppressed in samples for x P 0.2. The hysteresis loops at low temperatures are the result of the contribution of two distinct magnetic phases: a canted AFM phase and the SG phase. More importantly, the significant changes in the magnetic response of the material affect the superconducting properties of the samples. It was found that both T c and the superconducting fraction are reduced in samples that show the spin glass phase, possibly due to the magnetic pair breaking effect.
Physica C: Superconductivity, 2005
In this work we present the study of the elastic properties of polycrystalline samples of superco... more In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa 2 Cu 3 O 7-δ prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensiticlike phase above the superconducting critical temperature T C. We argue that this peculiar behavior can be attributed to the granularity present in the samples.
Physica C: Superconductivity, 2010
By improving resolution of home-made mutual-inductance measurements technique, a pronounced reson... more By improving resolution of home-made mutual-inductance measurements technique, a pronounced resonance-like structure has been observed in the current dependence of AC susceptibility in artificially prepared two-dimensional array of unshunted Nb-AlO x-Nb Josephson junctions (JJA). Using a single-plaquette approximation for JJA model, we were able to successfully fit our data assuming that resonance structure is related to the geometric (inductive) properties of the array.
Physical Review B
The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1... more The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1222͒ have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ϳ40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization ͑TRM͒ measurements at Tϭ60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.
Physical Review B, 2000
The distribution of magnetic induction in Meissner state with finite London penetration depth is ... more The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.
Physical Review B, 2000
We study the magnetic response of superconductors in the presence of low values of a uniform appl... more We study the magnetic response of superconductors in the presence of low values of a uniform applied magnetic field. We report measurements of DC magnetization and AC magnetic susceptibility performed on niobium cylinders of different length-to-radius ratios, which show a dramatic enhance of the initial magnetization for thin samples, due to the demagnetizing effects. The experimental results are analyzed by applying a model that calculates the magnetic response of the superconductor, taking into account the effects of the demagnetizing fields. We use the results of magnetization and current and field distributions of perfectly diamagnetic cylinders to discuss the physics of the demagnetizing effects in the Meissner state of type-II superconductors.
2012 IEEE International Conference on Systems, Man, and Cybernetics (SMC), 2012
ABSTRACT Several works have been done worldwide and are dedicated to the advancement of software ... more ABSTRACT Several works have been done worldwide and are dedicated to the advancement of software that enables the comparison and reading of fingerprints. Literature shows several works related to the development of scientific techniques that allow the improvement in the collection and disclosure of fingerprints left at crime scenes. Some of those techniques are based on the use of high-tech materials and devices, such as gold nanoparticles and thin films made of copper and gold. However, the cost of these techniques is very high for everyday use, especially when considering the financial budgets for police apparatus in developing countries. Attempting to improve economically such viable and efficient techniques in order to permit the disclosure of latent fingerprints left at crime scenes, this paper aims the studying and development of the ninhydrin colorimetric method, by performing numerous tests on fingerprints deposited on porous surfaces, such as paper and walls. Once confirmed the viability of ninhydrin employment, both in laboratory and on-site, and also having adjusted the optimum temperature for rapid revelation; this study provides the use of image-processing software, enabling the expert to compare, in real time, even at the site of criminal traces, the fingerprint image found with the official registry database, thus, obtaining the identity of the offender within a short period of time.
Josephson junction arrays (JJA) have been actively studied for decades. However, they continue to... more Josephson junction arrays (JJA) have been actively studied for decades. However, they continue to contribute to a wide variety of intriguing and peculiar phenomena. To name just a few recent examples, it suffice to mention the so-called paramagnetic Meissner effect and related reentrant temperature behavior of AC susceptibility, observed both in artificially prepared JJA and granular superconductors. Employing mutual-inductance measurements and using a high-sensitive home-made bridge, we have thoroughly investigated the temperature and magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays (2D-JJA) of both shunted and unshunted Nb-AlO x-Nb tunnel junctions In this Chapter, we report on three phenomena related to the magnetic properties of 2D-JJA: (a) the influence of non-uniform critical current density profile on magnetic field behavior of AC susceptibility; (b) the origin of dynamic reentrance and the role of the Stewart-McCumber parameter, β C , in the observability of this phenomenon, and (c) the manifestation of novel geometric effects in temperature behavior of AC magnetic response. Firstly, we present evidences for the existence of local type nonuniformity in the periodic (globally uniform) unshunted 2D-JJA. Specifically, we found that in the mixed state region AC susceptibility χ(T, h AC) can be rather well fitted by a single-plaquette approximation of the overdamped 2D-JJA model assuming a non-uniform distribution of the critical current density within a single junction. According to the current paradigm, paramagnetic Meissner effect (PME) can be related to the presence of π-junctions, either resulting from the presence of magnetic impurities in the junction or from unconventional pairing symmetry. Other possible explanations of this phenomenon are based on flux trapping and flux compression effects including also an important role of the surface of the sample. Besides, in the experiments with unshunted 2D-JJA, we have previously reported that PME manifests itself through a dynamic reentrance (DR) of the AC magnetic susceptibility as a function of temperature. Using an analytical expression we successfully fit our experimental data and demonstrate that the dynamic reentrance of AC susceptibility is directly linked to the value of β C. By simultaneously varying the parameter β L , a phase diagram β C-β L is plotted which demarcates the border between the reentrant and nonreentrant behavior. We show that only arrays with sufficiently large value of β C will exhibit the dynamic reentrance behavior and hence PME. The last topic reviewed in this Chapter is related to the step-like structure observed when the resolution of home-made mutual-inductance bridge is improved. That structure (with the number of steps n = 4 for all AC fields) has been observed in the temperature dependence of AC susceptibility in unshunted 2D-JJA with β L (4.2K) = 30. We were able to successfully fit our data assuming that steps are related to the geometric properties of the plaquette. The number of steps n corresponds to the number of flux quanta that can be screened by the maximum critical current of the junctions. The steps are predicted to manifest themselves in arrays with the inductance related parameter β L (T) matching a "quantization" condition β L (0)=2π(n+1).
7th Joint MMM-Intermag Conference. Abstracts (Cat. No.98CH36275), 1998
Revista Brasileira de Ensino de Física, 2005
TRIZé a sigla para as palavras russas que, em português, significam Teoria para a Resolução de Pr... more TRIZé a sigla para as palavras russas que, em português, significam Teoria para a Resolução de Problemas Criativos. O estudo de mais de dois milhões de patentes no mundo tem permitido identificar princípios universais de invenção. Ensinando estes princípios e com a ajuda de técnicas psicológicas de geração de idéias, a metodologia TRIZ pretende aprofundar e dinamizar o processo criativo. Neste artigo apresentamos algumas sugestões de como utilizar o método TRIZ na formação de estudantes de Física. Palavras-chave: TRIZ, criatividade, inovação, ensino. TRIZ is the acronym for the Russian phrase Theory of Inventive Problem Solving. Over two million patents have been examined, classified by level of inventiveness, and analyzed to look for principles of innovation. If these principles could be identified and codified, they could be taught to people to make the process of invention more predictable. In this paper we discuss some ideas to use this method in the preparation of Physics students.
Physics Letters A, 2006
Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magne... more Using a highly-sensitive home-made mutual-inductance technique, temperature profiles of the magnetic penetration depth λ(T) in the optimally-doped Sm1.85Ce0.15CuO4 thin films have been extracted. The low-temperature behavior of λ(T) is found to be best-fitted by linear ∆λ(T)/λ(0) = ln(2)kBT /∆0 and quadratic ∆λ(T)/λ(0) = Γ −1/2 ∆ −3/2 0 T 2 laws above and below T = 0.22TC , respectively, which clearly indicates the presence of d-wave pairing mechanism dominated by strong paramagnetic scattering at the lowest temperatures. The best fits produce ∆0/kBTC = 2.07 and Γ/TC = 0.25(TC /∆0) 3 for the estimates of the nodal gap parameter and impurity scattering rate.
Solid State Communications, 2004
Employing mutual-inductance measurements, we study the magnetic field dependence of complex AC su... more Employing mutual-inductance measurements, we study the magnetic field dependence of complex AC susceptibility of artificially prepared highly ordered (periodic) two-dimensional Josephson junction arrays of unshunted Nb-AlO x-Nb junctions. The observed behavior can be explained assuming single-plaquette approximation of the overdamped model with an inhomogeneous critical current distribution within a single junction.
Physical Review Letters, 2009
At the charge neutral point, graphene exhibits a very unusual high resistance metallic state and ... more At the charge neutral point, graphene exhibits a very unusual high resistance metallic state and a transition to a complete insulating phase in a strong magnetic field. We propose that the current carriers in this state are the charged vortices of the XY valley-pseudospin order-parameter, a situation which is dual to a conventional thin superconducting film. We study energetics and the stability of this phase in the presence of disorder.
Physical Review Letters, 1997
The paramagnetic Meissner effect ͑PME͒ measured in high-T C granular superconductors has been att... more The paramagnetic Meissner effect ͑PME͒ measured in high-T C granular superconductors has been attributed to the presence of junctions between the grains. Here we present measurements of complex ac magnetic susceptibility from two-dimensional arrays of conventional ͑non-͒ Nb/Al/AlOx/Nb Josephson junctions. We measured the susceptibility as a function of the temperature T, the ac amplitude of the excitation field h ac and the external magnetic field H dc. The experiments show a strong paramagnetic contribution from the multijunction loops, which manifests itself as a reentrant screening at low temperature, for values of h ac higher than 50 mOe. A highly simplified model, based on a single loop containing four junctions, accounts for this paramagnetic contribution and the range of parameters in which it appears. This model offers an alternative explanation of PME that does not involve junctions. ͓S0163-1829͑99͒02234-1͔
Physical Review B, 2007
By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response... more By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response to an effective stress field, we study (both analytically and numerically) the temperature and magnetic field dependence of TE coefficient α in a single small junction and in a square array. In particular, we found that in addition to field oscillations due to Fraunhofer-like dependence of the critical current, α of a small single junction also exhibits strong flux driven temperature oscillations near T C. We also numerically simulated stress induced response of a closed loop with finite self-inductance (a prototype of an array) and found that α of a 5 × 5 array may still exhibit temperature oscillations provided the applied magnetic field is strong enough to compensate for the screening induced effects.
Physica C: Superconductivity, 2001
This contribution presents a selected set of results, obtained as part of a systematic investigat... more This contribution presents a selected set of results, obtained as part of a systematic investigation, evidencing that many eects exhibited by superconductors are distinct manifestations of granularity which, in turn, is envisaged as a break of symmetry. The Wohlleben eect, the``®shtail anomaly'', the magnetic remanence exhibited by Josephson junction arrays, and the jumps on the magnetic moment of superconducting samples of mesoscopic dimensions, are examples which we brie¯y review and discuss taking granularity as the basic ingredient. The emphasis of the present approach is to recognize the importance of granularity in every scenario intended to explain the magnetic properties of superconducting systems.
Physica C: Superconductivity, 2007
Polycrystalline samples of Ru 1Àx Nb x Sr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Àd , 0 6 x 6 0.5, have been sy... more Polycrystalline samples of Ru 1Àx Nb x Sr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Àd , 0 6 x 6 0.5, have been synthesized and studied by resistivity, magnetization and ac susceptibility measurements. It was identified a strong suppression of the spin glass (SG) transition, which was totally suppressed in samples for x P 0.2. The hysteresis loops at low temperatures are the result of the contribution of two distinct magnetic phases: a canted AFM phase and the SG phase. More importantly, the significant changes in the magnetic response of the material affect the superconducting properties of the samples. It was found that both T c and the superconducting fraction are reduced in samples that show the spin glass phase, possibly due to the magnetic pair breaking effect.
Physica C: Superconductivity, 2005
In this work we present the study of the elastic properties of polycrystalline samples of superco... more In this work we present the study of the elastic properties of polycrystalline samples of superconducting YBa 2 Cu 3 O 7-δ prepared by the sol-gel method. The quality of all samples was checked by x-ray diffraction and scanning electronic microscopy while their physical properties were verified by transport and magnetic measurements. The elastic study was performed using the standard pulse-echo technique through measuring the phase velocity and the attenuation of ultrasonic waves (in the range of a few MHz) as a function of temperature. We have focused this study on the low temperatures interval (T < 200K). The obtained results show a strong hysteretic behavior in the ultrasonic attenuation (in addition to usually observed hysteretic behavior for the velocity) which strongly supports the existence of a martensiticlike phase above the superconducting critical temperature T C. We argue that this peculiar behavior can be attributed to the granularity present in the samples.
Physica C: Superconductivity, 2010
By improving resolution of home-made mutual-inductance measurements technique, a pronounced reson... more By improving resolution of home-made mutual-inductance measurements technique, a pronounced resonance-like structure has been observed in the current dependence of AC susceptibility in artificially prepared two-dimensional array of unshunted Nb-AlO x-Nb Josephson junctions (JJA). Using a single-plaquette approximation for JJA model, we were able to successfully fit our data assuming that resonance structure is related to the geometric (inductive) properties of the array.