Effects of Zn and Mg in Cu sites of YBa_ {2} Cu_ {3} O_ {7-δ} single crystals on the resistive transition, fluctuation conductivity, and magnetic irreversibilities (original) (raw)
Related papers
2002
We report on the resistive transition and fluctuation conductivity of an YBa 2 Cu 2.97 Zn 0.03 O 7Ϫ␦ single crystal and on the magnetic irreversibilities of this compound in two different oxygen states and another YBa 2 Cu 3 O 7Ϫ␦ single crystal in which ͑1 at. %͒ of Cu was substituted by Mg. Our measurements show a very drastic decrease of the superconducting transition temperature and the growth of granularity effects already in this low dopand concentration. In the temperature region immediately above T c our results of fluctuation conductivity reveal the occurrence of critical and Gaussian regimes. The resistive transition of the YBa 2 Cu 2.97 Zn 0.03 O 7Ϫ␦ single crystal occurs in two steps. First superconductivity sets in within the grains and at somewhat lower temperature long-range superconducting order takes place over the whole sample. Between the zero resistance temperature T c0 (H) and the pairing transition temperature T c (H) lays the magnetic irreversibility line T irr (H), which follows the power law predicted by the flux creep theories in most of the high field range. However, in a low field region, dominated by Josephson flux dynamics, the irreversibility line exhibits two different regimes dominated by disorder and frustration and characterized by AT ͑de Almeida-Thouless͒ and GT ͑Gabay-Toulouse͒ power law behaviors. This Josephson flux dynamics is squeezed into a considerably smaller field region than in other granular YBa 2 Cu 3 O 7Ϫ␦ systems. We attribute the origin of the observed stepwise resistive transition and low field magnetic irreversibility regimes mainly to the granularity of the superconducting state, which is induced by the impurities at the Cu sites and increased by shortage of oxygen.
Physical Review B, 2008
We report on magnetization and magnetoresistance measurements of single-crystal, melt-processed, and polycrystalline YBa 2 Cu 3 O 7−␦ samples with the purpose of disclosing the connection between the magnetic irreversibility limit and the zero resistance temperature point as a function of applied field in the above samples with very different superconducting granularities. Another goal is to find out how much the applied fields degrade the grain couplings and the electric conductivity for the different field-current configurations. In homogeneous superconductors, the magnetic irreversibility line is well known as a limit below which an electric current different from zero can flow without resistance. Our data for the single crystal with a weak superconducting granularity closely follow this rule. However, the results on the other samples are quite different. Normally, good-quality melt-processed YBa 2 Cu 3 O 7−␦ samples do not exhibit the signature of superconducting granularity. Nevertheless, x-ray analysis of our melt-processed sample, containing 30 wt % of the Y 2 BaCuO 5 phase, shows considerable misalignment of the crystallite c axis, which weakens the grain couplings and hence is expected to result in superconducting granularity. Effectively, the magnetic irreversibility lines for H ʈ c axis as well as for H ʈ ab plane exhibit, in the low-field region, the usual signature of superconducting granularity. On the other hand, the zero resistance data for increasing fields along the c axis and J ʈ ab plane split away from the irreversibility line toward the lower-temperature side. Nevertheless, for H ʈ J ʈ ab plane, the zero resistance data closely follow the irreversibility line up to the highest applied fields. In the polycrystalline YBa 2 Cu 3 O 7−␦ sample, in which the grain junctions are much weaker, the effect of an increasing applied field on the grain couplings is much stronger. The zero resistance line is already split from the irreversibility line and is lower by more than 17 K at 8 kOe. Our present measurements provide good quantitative data for the discrepancy between the zero resistance and magnetic irreversibility lines. We explain these features in terms of the superconducting glass model.
Physica C: Superconductivity, 2001
The granular superconducting character and the magnetic irreversibility of YBa 1:75 Sr 0:25 Cu 3 O 7Àd single crystal and oxygen depleted polycrystalline samples are studied in great detail from low ®eld magnetoresistance and zero-®eldcooled and ®eld-cooled DC magnetization as a function of temperature, for applied ®elds from 3 Oe up to 50 kOe. The results are discussed together with those of a very pure and high quality YBCO-123 single crystal. Coarsely the pro®les of the irreversibility curves are all similar, but the irreversibility data of the granular superconductors develop in the low ®eld region a structure, that systematically falls o the general trend and is recognized as the signature of the intergrain Josephson-¯ux dynamics, that is strongly marked by the frustrated grain coupling physics.
Physical Review B, 2007
We investigate the effect of the divalent Ca atoms at the trivalent Y site of YBa 2 Cu 3 O 7−␦ on the magnetoconductivity and magnetic irreversibility of single crystals and a polycrystalline sample. The magnetic irreversibility displays the de Almeida-Thouless and Gabay-Toulouse power laws, the signature of a frustrated superconducting grain aggregate for the H ʈ c axis as well as for the H ʈ ab plane. The resistive transition for measuring current along the ab plane and H ʈ c axis is a clear two step process. While the upper temperature step is only weakly affected by applied fields, the lower temperature transition is visibly broadened and shifted down to lower temperatures. We impute the upper temperature step to the pairing transition within grains while the lower temperature step is due to grain coupling and coherence transition. For the J ʈ ab and H ʈ ab planes, the resistive transition is a one step process and the effect of the applied fields is very weak for H ʈ J as well as for H Ќ J. We explain these observations in terms of the directional anisotropy of the phase displacements between the superconducting grains and their weakening effect on the grain coupling, caused by the applied field. We also compare our present results with those of YBa 2 Cu 3 O 7−␦ single crystals doped with the trivalent Pr at the Y site and with divalent elements such as Zn or Mg at the Cu site and Sr at the Ba site and suggest that the valence as well as the local breaking of the orbital symmetry by impurities plays an important role in the induction of superconducting granularity.
Journal of Superconductivity and Novel Magnetism, 2012
The objective of the work is to establish the contribution of superconductive granules and grain boundaries (weak links) in magnetoresistance ρ value of YBa 2 Cu 3 O 7−δ granular high-temperature superconductor (HTS) at T < T c. The current-voltage characteristics (CVCs) E(j) H ext =const of YBa 2 Cu 3 O 6.95 ceramic samples were measured in H ext (0 ≤ H ext ≤≈ 500 Oe) magnetic fields. The CVCs E(j) H trap =const of the samples magnetized in H treat magnetic fields were measured at H ext = 0. Based on the CVCs, ρ(j) H ext =const , ρ(H ext) j =const and ρ(j) H treat =const dependences were reestablished. The comparative analysis of ρ(j) H ext =const and ρ(H ext) j =const dependences indicates the magnetic field redistribution between grain boundaries and superconductive granules influence on transport and galvanomagnetic properties of granular HTS. The superconductive grain magnetoresistance ρ g was established to be significantly lower than ρ J value of Josephson medium.
Physica C-superconductivity and Its Applications, 1991
We report AC susceptibility and resistivity measurements of the vortex motion in single crystal YBa2Cu3O7−δ, with δ ∼ 0.62. The superconducting transition temperature was varied from 7 K to 17 K by altering the oxygen vacancy ordering in the chains. We find that both the slope of the irreversibility line and its extrapolated zero temperature value scale linearly with Tc, providing strong evidence for the intrinsic nature of the irreversibility line. Moreover, the reduced magnetic field scale of oxygen deficient crystals allows us to obtain new results at previously inaccessible reduced temperatures, well below Tc.
Coherent transition induced by Sr doping on the YBa 2 Cu 3 O 7-δ single crystal magnetoconductivity
Journal of Physics: Conference Series, 2014
We have studied by magnetoresistivity measurements the influence of the low Sr chemical doping on the conductivity thermodynamic fluctuation regimes (TFR) of a YBa 1.98 Sr 0.02 Cu 3 O 7-single crystal. The low-current and low-frequency resistivity, ρ(T,H) measurements were performed in a PPMS while DC magnetic fields up to 50kOe were simultaneously applied perpendicular to the ab plane of the single crystal and to the measurement current density. The dρ(T,H)/dT data shows that the superconducting transition of the sample occur is two stages that corresponds to the signature of a granular superconductor. At this scenario is possible to define two critical temperature transitions T C (H) and T C0 (H) [T C (H) > T C0 (H)]. At T C (H) the superconductivity sets in within the grains and at T C0 (H) a coherent transition takes place and a long-range superconducting order takes place over the whole sample. Special attention is reserved to the paracoherent region [T C0 (H) T < T C (H)] of the ρ(T,H) data. The field and the temperature dependence of the paracoherent TFR behavior is characterized by a power law represented by a critical exponent S 0 ≈ 3. This exponent is an indicative that the coherence transition of the sample belongs to the 3D-XY universality class with relevant disorder that probably is associated to the low Sr doping.
A diffuse phase transition in superconducting YBa2(Cu1 − xMnx)3O7 − δ (MZn, Ni, Cr) compounds
Materials Letters, 1995
Dielectric and magnetic behaviour of the YBa,( Cui _,M,.) sO7 _ a (M = Zn, Ni, Cr and x Q 0.10) system was investigated. The results suggest a diffuse phase transition to a relaxor-type ferroelectric state. Taking into account the double-well model of hightemperature superconductivity and using double-well parameters from the Abrahams-Kurtz-Jamieson (AKJ) relation for ferroelectric phase transition in oxide perovskite, the unrenormalizable barrier heights are calculated. The values could be related to an excitonic mechanism of superconductivity. Small anomalies observed in the static dielectric constants coincide with the N&4 points of the corresponding nonsuperconductive phases in YBa*( Cul _,M,) & _ B compounds.
Magnetic irreversibility and zero resistance in granular Y358 superconductor
2014
We report on magnetization and magnetoresistance measurements of polycrystalline Y3Ba5Cu8O18 superconductor (Y358), prepared by solid state reaction in order to study the correlation between the magnetic irreversibility line and the zero electric resistance as a function of applied field. The magnetization measurements were performed using a MPMS-XL SQUID magnetometer and the magnetoresistance measurements were made using a PPMS, both from Quantum Design, up to 1 T. The granular microstructure was confirmed by scanning electron microscopy. In this case the grain junctions are weaker than in the Y123 system and the effect of the applied field on the grain couplings is much stronger. Consequently, in our sample the zero resistance line shifts away from the irreversibility line and is lower by more than 15 K at 1 T. We explain our results in terms of the superconducting glass model and in comparison with results obtained in other materials from the YBaCuO family.
Physica C: Superconductivity, 1992
Results from AC susceptibility x(H, T) and resistivity p(H, T) measurements on YBa2Cu307 single crystals, in the mixed state by applying magnetic fields H parallel to the c-axis, strongly suggest that the occurrence of the dissipation peak in the imaginary part of the complex susceptibility X" (to, T) is due to the skin effect known for electrical transport in metals in the normal state. At fixed field H, the peak temperature Tp(tn, H) increases with increasing frequency of the superimposed AC probing field, and does not coincide with the irreversibility temperature T* (H) < Tp for magnetization obtained in the static limit to-. 0. A phase diagram is presented which includes the DC irreversibility line H*(T), the peak positions Hp (to, T), the experimentally resolved zero-resistivity fields Hp=0(T), and the crossover fields from activated to diffuse resistive behavior, Hk(T), all measured on the same YBa2Cu307 single crystal.