Granular High-temperature Superconductors Research Articles

Kinetics of topological phase transitions and topological phases in the Josephson medium of granular high-temperature superconductors under the action of temperature, magnetic field and transport current

The paper we presented is essentially the logical conclusion of a large-scale work - from developing a strategy for detecting topological BKT phase transitions in a Josephson medium of granular high-temperature superconductors to establishing the identity of topological phases arising from two fundamentally different types of external influence. The work is devoted to the study of the topological phases arising in the two-dimensional Josephson medium of granular high-temperature superconductors YBa2Cu3O7-δ during the Berezinskii-Kosterlits-Thouless phase transitions (BKT transition) under the influence of temperature, external magnetic field and transport current. The features of the behavior of magnetoresistance isotherms R(Нext)T=const and current – voltage characteristics (CVCs) E(j)T=const in the vicinity of the temperatures of the BKT phase transitions have been discovered. It has been experimentally established that granular HTSC YBa2Cu3O7-δ is a unique object for studying the influence of external actions (magnetic field, transport current) on topological phase transitions and topological phases, since in its crystal structure there are no sources of an internal magnetic field.

Universal Behavior and Temperature Evolution of the Magnetoresistance Hysteresis in Granular High-Temperature Superconductors Y–Ba–Cu–O

Universal Behavior and Temperature Evolution of the Magnetoresistance Hysteresis in Granular High-Temperature Superconductors Y–Ba–Cu–O

New Evidence of Interaction Between Grain and Boundaries Subsystems in Granular High-Temperature Superconductors

Granular high-temperature superconductors (HTSs) exhibit magnetotransport properties, including the clockwise magnetoresistance hysteresis. The hysteresis is explained by the concept of an effective field in the subsystem of grain boundaries, where the observed dissipation occurs. The effective field in the intergrain medium is determined by a superposition of the external field H and the field induced by the magnetic response of HTS grains. The magnetic flux compression in the intergrain medium provides an increase of the effective field. The magnetoresistance hysteresis of polycrystalline YBa2Cu3O7-δ has a bright feature: In a fairly wide external field range, the hysteresis width ΔH(H) is found to be almost linear, ΔH ≈ H. This behavior is considered to be universal over the entire temperature range corresponding to the superconducting state (the investigations have been carried out at temperatures of 77 K and 4.2 K). The analysis of the magnetoresistive and magnetic properties has shown that the upper boundary of the field range of the ΔH ≈ H regime is consistent with the field of complete penetration into HTS grains. This is indicative of the strong interrelation between the magnetotransport and magnetic properties of granular HTSs.

Механизмы, определяющие гистерезис магнитосопротивления гранулярного ВТСП в присутствии парамагнитного вклада, на примере HoBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=-

The hysteretic magnetoresistance of granular high-temperature superconductor (HTSC) HoBa2Cu3O7-δ is investigated. Superconductors of the YBCO family with magnetic rare earth elements (Nd, Ho, Er, Sm, Yb, Dy) in place of yttrium are characterized by a significant paramagnetic contribution to the total magnetization. Impact of this paramagnetic contribution on the magnetotransport properties is analyzed using the concept of an effective field in an intergranular medium. Lines of magnetic induction from paramagnetic moments do not concentrate in intergranular boundaries, and, thus, have an insignificant effect on magnetotransport properties of granular HTSC. At the same time, there are strong concentration of magnetic flux in the intergranular boundaries due to Meissner currents and Abrikosov vortices. This magnetic flux compression determines the magnetotransport properties of granular HTSCs, including YBCO with magnetic rare earth elements.

Универсальность поведения гистерезиса магнитосопротивления и его температурной эволюции для гранулярных высокотемпературных сверхпроводников Y-Ba-Cu-O

The work is devoted to the establishment of regularities in the behavior of the magnetoresistance hysteresis R(H) of granular high-temperature superconductors (HTS) of the yttrium system. We carried out a comparative study of the magnetotransport properties of granular HTSC samples, which have (i) approximately the same magnetic properties and temperatures of the onset of the superconducting transition (90.5–93.5 K, which characterizes HTS grains) and (ii) different values of the transport critical current JC (which characterizes the intergrain boundaries). Despite the significant scatter in the JC values (more than an order of magnitude) of the three samples studied, a universal behavior of the magnetoresistance hysteresis was found, apparently inherent in all granular Y-Ba-Cu-O. The R(H) hysteresis is very wide, and in a sufficiently large interval of the external field, the dependence of the hysteresis width of the magnetoresistance H on the field Hdec (external field H = Hdec for a decreasing hysteresis branch) is close to a linear function: H ≈ Hdec. This behavior is observed for the entire temperature range of realization of the superconducting state (the studies were carried out at temperatures of 77–88 K and 4.2 K). The explanation of the obtained result is based on the concept of consideration the effective field in the intergrain boundaries. This effective field is the superposition of the external field and the field induced by the magnetic moments of the grains. The field induced by the grains, in turn, is significantly enhanced in the region of intergrain boundaries due to the effect of the magnetic flux compression (the length of the intergrain boundaries is several orders of magnitude smaller than the size of the HTS grains). The above is confirmed from the analysis of the R(H) hysteresis for a composite HTS sample based on Y-Ba-Cu-O and CuO, in which the length of the intergranular boundaries is purposefully increased, as a result, the effect of the flux compression is less pronounced, and the hysteresis R(H) narrows.

Model of the Behavior of a Granular HTS in an External Magnetic Field: Temperature Evolution of the Magnetoresistance Hysteresis

A model for describing the magnetoresistance behavior in a granular high-temperature superconductor (HTS) that has been developed in the last decade explains a fairly extraordinary form of the hysteretic R(H) dependences at T = const and their hysteretic features, including the local maximum, the negative magnetoresistance region, and the local minimum. In the framework of this model, the effective field Beff in the intergrain medium has been considered, which represents a superposition of the external field and the field induced by the magnetic moments of HTS grains. This field can be written in the form Beff(H) = H + 4παM(H), where M(H) is the experimental field dependence of the magnetization and α is the parameter of crowding of the magnetic induction lines in the intergrain medium. Therefore, the magnetoresistance is a function of not simply an external field, but also the “internal” effective field R(H) = f(Beff(H)). The magnetoresistance of the granular YBa2Cu3O7 – δ HTS has been investigated in a wide temperature range. The experimental hysteretic R(H) dependences obtained in the high -temperature range (77–90 K) are well explained using the developed model and the parameter α is 20–25. However, at a temperature of 4.2 K, no local extrema are observed, although the expression for Beff(H) predicts them and the parameter α somewhat increases (~30–35) at this temperature. An additional factor that must be taken into account in this model can be the redistribution of the microscopic current trajectories, which also affects the dissipation in the intergrain medium. At low temperatures under the strong magnetic flux compression (α ~ 30–35), the microscopic trajectories of the current Im can change and tunneling through the neighboring grain is preferred, but the angle between Im and Beff will be noticeably smaller than 90°, although the external (and effective) field direction is perpendicular to the macroscopic current direction.

The Berezinskii–Kosterlitz–Thouless topological phase transition in the Josephson medium of granular high-temperature superconductors under the effect of magnetic and electric fields

This study establishes the identity of the topological phases that emerge in the Josephson medium of granular high-temperature superconductors YBa2Cu3O7–δ in the process of the Berezinskii–Kosterlitz–Thouless (BKT) phase transitions, under the influence of an external magnetic field and transport current. The nature of the topological phases arising as a result of the BKT phase transition was found to be independent of the type of external effect.

Модель поведения гранулярного ВТСП во внешнем магнитном поле: температурная эволюция гистерезиса магнитосопротивления

Abstract A model for describing the magnetoresistance behavior in a granular high-temperature superconductor (HTS) that has been developed in the last decade explains a fairly extraordinary form of the hysteretic R ( H ) dependences at T = const and their hysteretic features, including the local maximum, the negative magnetoresistance region, and the local minimum. In the framework of this model, the effective field B _eff in the intergrain medium has been considered, which represents a superposition of the external field and the field induced by the magnetic moments of HTS grains. This field can be written in the form B _eff( H ) = H + 4πα M ( H ), where M ( H ) is the experimental field dependence of the magnetization and α is the parameter of crowding of the magnetic induction lines in the intergrain medium. Therefore, the magnetoresistance is a function of not simply an external field, but also the “internal” effective field R ( H ) = f ( B _eff( H )). The magnetoresistance of the granular YBa_2Cu_3O_7 – δ HTS has been investigated in a wide temperature range. The experimental hysteretic R ( H ) dependences obtained in the high -temperature range (77–90 K) are well explained using the developed model and the parameter α is 20–25. However, at a temperature of 4.2 K, no local extrema are observed, although the expression for B _eff( H ) predicts them and the parameter α somewhat increases (~30–35) at this temperature. An additional factor that must be taken into account in this model can be the redistribution of the microscopic current trajectories, which also affects the dissipation in the intergrain medium. At low temperatures under the strong magnetic flux compression (α ~ 30–35), the microscopic trajectories of the current I _ m can change and tunneling through the neighboring grain is preferred, but the angle between I _ m and B _eff will be noticeably smaller than 90°, although the external (and effective) field direction is perpendicular to the macroscopic current direction.

Magnetoresistance Hysteresis Evolution in the Granular Y–Ba–Cu–O High-Temperature Superconductor in a Wide Temperature Range

The temperature evolution of the magnetoresistance hysteresis in the granular YBa2Cu3O7-δ high-temperature (TC ≈ 92 K) superconductor has been investigated. The measurements have been performed in the high-temperature region (78–90 K) and at the liquid helium temperature (4.2 K). The results obtained have been analyzed using the developed model of the behavior of transport properties of a granular high-temperature superconductor in an external magnetic field. Within the discussed model, the dissipation of the grain boundary subsystem is determined by the intergrain spacing-averaged effective field Beff, which is a superposition of external field H and the field induced by the magnetic moments of superconducting grains. Such a consideration yields the expression Beff(H) = H − 4πM(H) α for the effective field in the intergrain medium, where M(H) is the experimental hysteretic dependence of magnetization and α is the parameter of magnetic flux crowding in the intergrain medium. Here, the magnetoresistance is assumed to be proportional to the absolute value of the effective field: R(H) ~ |Beff(H)|. Analysis of the experimental R(H) and M(H) dependences obtained under the same conditions for the investigated high-temperature superconductor sample showed that in the high-temperature region this parameter is α ≈ 25. At the low temperature (4.2 K), we may state that the degree of flux crowding increases and the estimated α value is ~ 50. The estimates made are indicative of the strong effect of flux compression in the intergrain medium on the magnetotransport properties of the investigated granular high-temperature superconductor system. Possible reasons for a discrepancy between the developed model concepts and experimentally observed low-temperature R(H) hysteresis are analyzed.

Dissipation in granular high-temperature superconductors: New approach to describing the magnetoresistance hysteresis and the resistive transition in external magnetic fields

An approach to describing the R(H) magnetoresistance hysteresis in granular high-temperature superconductors and behavior of the R(T) resistive transition in these objects in an external magnetic field is proposed. The dissipation is attributed to the subsystem of intergrain boundaries, which form a Josephson junction network. The approach is based on accounting for the effect of magnetic moments of superconducting grains on the resulting (effective) field in the intergrain medium. The described procedure includes (i) establishing of the degree of magnetic flux crowding in the intergrain medium by comparing the experimental data on the R(H) magnetoresistance hysteresis and magnetization M(H), (ii) determining the effective field Beff in the intergrain medium as a function of external field H and temperature T with regard to the thermomagnetic prehistory, and (iii) fitting the experimental R(H) and R(T) dependences using the Arrhenius expression R ∼ exp(–EJ/ kB T), where EJ is the parameter corresponding to the Josephson coupling energy. The fundamental novelty of the proposed approach is the extraction of the functional dependences of EJ on the effective field Beff in the intergrain medium rather than on the external field H, as was made in many previous works. It is shown that the proposed approach makes it possible to adequately describe both the R(H) hysteretic dependences and R(T) dependences of the Y-Ba-Cu-O high-temperature superconductor samples with different morphologies and critical current densities.

Visualization of a Berezinskii–Kosterlitz–Thouless Topological Phase Transition in a Josephson Medium: Detection of an Anomalous Temperature Dependence of the Magnetoresistance of YBa2Cu3O7–δ Granular High-Temperature Superconductors

The possibility of implementing a Berezinskii–Kosterlitz–Thouless topological phase transition in a Josephson medium of two-level high-temperature superconductors induced by a magnetic field has been studied. The effect of temperature and external magnetic field on the transport properties of the YBa2Cu3O7–δ granular high-temperature superconductor considered as a model object is studied experimentally. For the first time, an anomalous behavior of the magnetoresistance isotherms is detected in a narrow range of temperature and external magnetic field indicating a topological phase transition in the Josephson medium of granular high-temperature superconductors.

Temperature behavior of the magnetoresistance hysteresis in a granular high-temperature superconductor: Magnetic flux compression in the intergrain medium

Granular high-temperature superconductors (HTSs) are characterized by the hysteretic behavior of magnetoresistance. This phenomenon is attributed to the effective field in the intergrain medium of a granular HTS. At the grain boundaries, which are, in fact, weak Josephson couplings, the dissipation is observed. The effective field in the intergrain medium is a superposition of the external field and the field induced by magnetic moments of HTS grains. Meanwhile, analysis of the field width of the R(H) magnetoresistance hysteresis ΔH = Hdec − Hinc at Hdec = const, where Hinc and Hdec are increasing and decreasing branches of the R(H) hysteretic dependence, shows that the effective field in the intergrain medium exceeds by far both the external field and the field induced by magnetic moments of HTS grains. This situation suggests the magnetic flux compression in the intergrain medium because of the small length of grain boundaries, which amounts to ∼1 nm, i.e., is comparable with the coherence length and corresponds to Josephson tunneling in HTS materials. In this work, using the previously developed approach, we examine experimental data on the magnetoresistance and magnetization hysteresis in the granular YBa2Cu3O7 HTS compound in the range from 77 K to the critical temperature. According to the results obtained, the degree of magnetic flux compression determined by the parameter α in the expression for the effective field Beff(H) = H − 4π M(H) α in the intergrain medium remains constant over the investigated temperature range. All the features of the observed evolution of the R(H) hysteretic dependences are explained well within the proposed approach when the expression for Beff(H) contains the experimental M(H) magnetization data and the parameter α of about 20–25. The latter is indicative of the dominant effect of magnetic flux compression in the intergrain medium on the transport properties of granular HTS materials.

Effect of the Temperature, External Magnetic Field, and Transport Current on Electrical Properties, Vortex Structure Evolution Processes, and Phase Transitions in Subsystems of Superconducting Grains and “Weak Links” of Granular Two-Level High-Temperature Superconductor YBa2Cu3O7–δ

The temperature dependences of the resistivity of granular high-temperature superconductor YBa2Cu3O7–δ ρ(T) are measured at various transverse external magnetic fields 0 ≤ Hext ≤ 100 Oe in the temperature range from the resistivity onset temperature Tρ = 0 to the superconducting transition critical temperature T c at the transport current density from 50 to 2000 mA/cm2. The effect of the external magnetic field and transport current density on the kinetics of phase transitions in both subsystems of granular two-level HTSC (T = Tc2J, Tc1g, T c ) is determined. The feasibility of the topological phase transition, i.e., the Berezinsky–Kosterlitz–Thouless transition, in the Josephson medium at Tc2J < TBKT < Tc1g “in transport current” is established, and its feasibility conditions are studied.

The Berezinskii-Kosterlitz-Thouless phase transition in the Josephson medium of a two-level granular high-temperature superconductor YBa2Cu3O7-δ

The temperature dependences ρ(T) of the electrical resistivity of the granular high-temperature superconductor YBa2Cu3O7-δ are measured in different transverse external magnetic fields 0 ≤ Hext ≤ 100 Oe for temperatures ranging from the temperature Tρ=0 at which resistivity appears to the critical superconducting transition temperature Tc and transport current densities ranging from 50 to 2000 mA/cm2. The effects of temperature, external magnetic field, and transport current density on the kinetics of phase transitions in both subsystems of the granular HTSC are studied. The possibility of a topological phase transition—a Berezinskii-Kosterlitz-Thouless phase transition in a Josephson medium—is established and the conditions for its occurrence are studied.

Влияние температуры, внешнего магнитного поля и транспортного тока на электрофизические свойства, процессы эволюции вихревой структуры и фазовые переходы в подсистемах сверхпроводящих гранул и "слабых связей" гранулярного двухуровневого высокотемпературного сверхпроводника YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-7-delta-=/SUB=-

AbstractThe temperature dependences of the resistivity of granular high-temperature superconductor YBa_2Cu_3O_7–δ ρ( T ) are measured at various transverse external magnetic fields 0 ≤ H _ext ≤ 100 Oe in the temperature range from the resistivity onset temperature T _ρ = 0 to the superconducting transition critical temperature T _ c at the transport current density from 50 to 2000 mA/cm^2. The effect of the external magnetic field and transport current density on the kinetics of phase transitions in both subsystems of granular two-level HTSC ( T = T _ c 2J, T _ c 1g, T _ c ) is determined. The feasibility of the topological phase transition, i.e., the Berezinsky–Kosterlitz–Thouless transition, in the Josephson medium at T _ c 2J &lt; T _BKT &lt; T _ c 1g “in transport current” is established, and its feasibility conditions are studied.

Anisotropy of the magnetoresistance hysteresis in the granular superconductor Y-Ba-Cu-O at different magnetic-field and transport-current orientations

Dissipation in granular high-temperature superconductors (HTSs) during the passage of macroscopic transport current j is mainly determined by carrier tunneling through intergrain boundaries (Josephson junctions). In the presence of external magnetic field H, it is necessary to take into account the significant magnetic flux compression, which can lead to the situation when the effective field Beff in the intergrain boundaries exceeds the external field by an order of magnitude. This is observed as a wide hysteresis of the field dependence of magnetoresistance R(H). In this study, we investigate the R(H) hysteresis evolution in granular 1–2-3 HTSs in different j–H orientations. The magnetic flux compression significantly affects the magnetoresistance and its hysteresis for both perpendicular (H ⊥ j) and parallel (H ǁ j) orientations. The obtained experimental data on the R(H) hysteresis at the arbitrary angles θ = ∠H, j are explained using the approach developed for describing the magnetoresistance hysteresis in granular HTSs with regard to the magnetic flux compression and the model representations proposed by Daghero et al. [Phys. Rev. B 66(13), 11478 (2002)]. A concept of the effective field in the intergrain medium explains the well-known anisotropy of the magnetotransport properties of granular HTSs.

Phase transitions and vortex structure evolution in two-level high-temperature granular superconductor YBa2Cu3O7 – δ under temperature and magnetic field

Temperature dependences of the resistivity ρ(T) of samples of granular high-temperature superconductor YBa2Cu3O7 – δ are measured at various transverse external magnetic fields at 0 < H ext < 1900 Оe in the temperature range from the upper Josephson critical temperature of “weak bonds” T c2J to temperatures slightly exceeding the superconducting transition temperature T c . Based on the data obtained, the behavior of the field dependences of the critical temperatures of superconducting grains and “weak bonds,” and temperature and field dependences of the magnetic contribution to the resistivity \(\left[ {\Delta \rho \left( {T,H} \right) = \rho {{\left( T \right)}_{{H_{ext}} = const}} - \rho {{\left( T \right)}_{{H_{ext}} = 0}}} \right]\). It is shown that the behavior of the magnetic contribution to the resistivity Δρ along the line of the phase transition related to the onset of the magnetic field penetration in the form of Abrikosov vortices into the subsystem of superconducting grains T c1g (H ext) is anomalous. The concepts on the magnetic flux redistribution between both subsystems of two-level HTSC near in the vicinity of T c1g : the Josephson vortex decreases, and the Abrikosov vortex density increases.

Anisotropy of the magnetoresistive properties of granular high-temperature superconductors resulting from magnetic flux compression in the intergrain medium

To elucidate the origin of the well-known anisotropy of the magnetoresistive properties of granular high-temperature superconductors (HTSs), which is related to the mutual orientation of magnetic field H and transport current j, we investigate the hysteretic dependences of magnetoresistance R(H) of the yttrium HTS sample at the perpendicular (H ⊥ j) and parallel (H || j) configurations. The hysteretic R(H) dependences are analyzed using the concept of the effective field in the intergrain boundaries through which superconducting current carriers tunnel. The effective degree of magnetic flux compression in the intergrain medium at the perpendicular configuration was found to be twice as much as at the parallel one. This approach explains well the anisotropy of the magnetoresistive properties of granular HTSs, which was previously reported by many authors, and the temperature dependences of the resistance in the resistive transition region.

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi1.8Pb0.3Sr1.9Ca2Cu3Ox have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) H = const have been measured in external magnetic fields H ext (0 ≤ H ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa2Cu3O7 − δ high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) H = 0 for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H ext) T = const. A comparative analysis of the specific features in the behavior of the curves ρ(H) T = const for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H c1g , H c2g ) and Josephson weak links (H c2J ) have been determined, and the H-T phase diagrams of granular YBa2Cu3O7 − δ HTSCs have been recovered.