YBCO Single Crystals Research Articles

Evolution of the pseudogap temperature dependence in YBa2Cu3O7–δ films under the influence of a magnetic field

The evolution of the temperature dependence of the pseudogap Δ*(T) in optimally doped (OD) YBa2Cu3O7–δ (YBCO) films with the superconducting critical temperature Tc = 88.7 K under the influence of a magnetic field B has been studied in detail. It has been established that the shape of Δ*(T) for various B over the entire range from the pseudogap opening temperature T* to T01, below which superconducting fluctuations occur, has a wide maximum at the BEC-BCS crossover temperature Tpair, which is typical for OD films and untwinned YBCO single crystals. T* was shown to be independent on B, whereas Tpair shifts to the low-temperature region along with the increase in B, while the maximum value of Δ*(Tpair) remains practically constant regardless of B. It was revealed that as the field increases, the low-temperature maximum near the 3D-2D transition temperature T0 is blurred and disappears at B > 5 T. Moreover, above the Ginzburg temperature TG, which limits superconducting fluctuations from below, for B > 0.5 T, a minimum appears on Δ*(T) at Tmin, which becomes very pronounced with a further increase in B. As a result, the overall value of Δ*(T) decreases noticeably most likely due to the pair-breaking effect. A comparison of Δ*(T) near Tc with the Peters–Bauer theory shows that the density of fluctuating Cooper pairs actually decreases from ⟨n↑n↓⟩ ≈ 0.31 at B = 0 to ⟨n↑n↓⟩ ≈ 0.28 in the field of 8 T. The observed behavior of Δ*(T) around Tmin is assumed to be due to the influence of a two-dimensional vortex lattice created by the magnetic field, which prevents the formation of fluctuating Cooper pairs near Tc.

High-pressure effects on basal-plane conductivity of YPrBCO single crystals

Influence of high hydrostatic pressure up to 12 kbar on the basal-plane electrical resistance of Y1−xPrxBa2Cu3O7−δ single crystals is investigated for moderate Pr doping levels (x ≈ 0.23). The application of high pressure leads to a threefold increase of the derivative dTc/dP in comparison with optimally-doped YBCO single crystals. Possible mechanisms of Tc suppression are discussed with account for features in the electronic spectrum of charge carriers, typical for lattices with strong coupling. The excess conductivity Δσ(T) obeys an exponential temperature dependence in the broad temperature range Tf < T < T*. The dependence Δσ(T)∼(1−T/T*)exp(Δab*/T) is interpreted in terms of the mean-field theory, where T* is the mean-field temperature of transition to the pseudogap state and the temperature dependence of the pseudogap is described satisfactory within the BCS-BEC crossover theory. All in all, an increase of the pressure results in a narrowing of the pseudogap temperature interval, expanding the temperature range of resistance linear dependence.

The in-plane and cross-plane thermal conductivity of RBa 2 Cu 3 O 7−x (R = Eu, Gd, Dy, Er, Y) superconductors

We present an analysis of thermal conductivity of a series of rare earth substituted high temperature superconductors (HTS) of the form RBa 2 Cu 3 O 7−x (R = Eu, Gd, Dy, Er, Y). Interaction of lattice vibrations with structural defects, surface boundaries, dislocations or point defects, phonons and electrons can lead to substantial thermal resistance. This work deals with the combined impact of all these scattering processes on the thermal conductivity of these HTSs and thus, presents a complete picture. The role of various scattering processes in thermal transport has been explored based on the many-body quantum dynamics for the bulk material. We have tried to interpret which scattering processes are important in different temperature ranges. We note that the boundary scattering is the sole contributor to thermal resistance at very low temperatures, in excellent agreement with experimental observations. Further, we also studied the anisotropic nature of thermal conductivity in single-crystal YBCO. Our results bear excellent agreement with the available experimental data for both the in-plane and cross-plane thermal conductivity. It is observed that the in-plane thermal conductivity is much higher than the cross-plane thermal conductivity imparting a 2 − D nature to the thermal transport in such materials. We conclude that of the various scattering processes, anisotropy in the thermal transport is largely owing to the defect scattering, phonon-phonon scattering and interference scattering processes. Such an analysis addresses the possibility of limiting the heat flow in a particular direction while allowing it in the other; a feature that can be exploited for the development of exotic technological high-temperature superconductor crystals for industrial use.

Magnetic Field- and Frequency-Dependent Study of the AC Susceptibility of High-Tc YBCO Single Crystal

Magnetic Field- and Frequency-Dependent Study of the AC Susceptibility of High-Tc YBCO Single Crystal

Evolution of excess conductivity in YBCO single crystals after irradiation with fast electrons

Evolution of the excess conductivity in YBCO single crystals is investigated after their step-by-step irradiation with fast electrons. The dependence of the coherence length ξc(0) along the c-axis on the irradiation dose dependence is obtained. It is revealed that an increase of the irradiation dose from 1.4 to 8.8 × 1018 cm−2 leads to a considerable extension of the temperature range of the excess conductivity. This is accompanied by a factor of about 1.5 increase of ξc(0) and a shift ot the 2D-3D crossover point in temperature.

Electron irradiation and annealing effects on the pseudogap in optimally doped YBCO single crystals

The effect of room-temperature annealing on the basal-plane conductivity of YBCO single crystals is investigated after their irradiation with fast electrons. It is revealed that the excess conductivity [Formula: see text] of the samples obeys an exponential temperature dependence in a broad temperature range [Formula: see text]. Here, [Formula: see text] is the crossover temperature from the pseudogap to the fluctuation conductivity regime. The description of the excess conductivity with the expression [Formula: see text] can be interpreted in terms of the mean-field theory where [Formula: see text] corresponds to the mean-field transition temperature to the pseudogap state and the dependence [Formula: see text] is described within the framework of the BCS-BEC crossover theory.

Annealing of defects after irradiation of YBCO single crystals with fast electrons

The effect of annealing on the conductivity of optimally doped YBa2Cu3O7−δ single crystals irradiated with fast electrons is investigated. The variation of width of the superconducting transition with increase of the irradiation dose and in the course of annealing is associated with the evolution of the sample inhomogeneity. A recovery of the critical temperature Tc after annealing to some intermediate value between those in the initial and irradiated states is observed. This recovery of Tc is accompanied by a substantial narrowing of the superconducting transition whose width nicely matches that in the non-irradiated state. The observed transition narrowing is attributed to an improvement of the sample homogeneity in consequence of the diffusion-induced growth and eventual coalescence of superconducting domains. In addition, the complete recovery of the normal conductivity after annealing points to a reversible change of parameters of the charge carriers scattering on phonons and defects.

Transverse conductivity and the pseudogap in YBCO single crystals irradiated with fast electrons

Temperature dependences of the electrical resistance along the c-axis of [Formula: see text] single crystals are investigated before and after irradiation with electrons at 0.5–2.5 MeV energies. The irradiation was done at [Formula: see text] K and the irradiation dose was varied from [Formula: see text] to [Formula: see text]. An enhancement of the charge carriers localization with increase of the irradiation dose has been revealed. A crossover from the pseudogap (PG) regime to the variable-range hopping conductivity has been observed.

Effect of annealing on a pseudogap state in untwinned YBa2Cu3O7\u2212\u03b4 single crystals

The effect of annealing both in the oxygen atmosphere and at room temperatures on physical properties such as the pseudogap (Δ*(T)) and excess conductivity (σ′(T)) of untwined YBa2Cu3O7−δ (YBCO) single crystal with a small deviation from oxygen stoichiometry is studied. It was revealed that as the charge carrier density, nf, increases, Тс also slightly increases, whereas the temperature of the pseudogap opening, T*, decreases noticeably, which is consistent with the phase diagram (PD) of cuprates. The excess conductivity in the vicinity of Tc is represented by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, illustrating the three-dimensional to two-dimensional (i.e. 3D-2D) crossover with an increase in temperature. The crossover temperature T0 determines the coherence length along the c axis is ξc(0) = 0.86 Å, that is 2.6 times larger than for optimally doped YBCO single crystals with defects. Taking into account the short coherence length in high-temperature superconductors, in the model of free charge carriers the phase relaxation time of fluctuating Cooper pairs is determined, τφ (100 K) = (4.55 ± 0.4) · 10−13 s, which is slightly (1.2 times) larger than in well-structured YBCO films, and as in films, does not depend on nf. It is shown that Δ*(T) at different annealing stages practically does not change its shape. As in the well-structured YBCO films, Δ*(T) demonstrates maximum at Tpair~124 K which depends weakly on nf. However, the maximum value of Δ*(Tpair) increases with increasing nf, as it follows from the PD of cuprates. Comparing the experimental data with the Peters-Bauer theory we estimated the density of local pairs <n↑n↓> ≈ 0.3 near Tc that is a common value for high-temperature superconductors.

Resistivity anisotropy in YBCO single crystals irradiated with fast electrons

Temperature dependences of the longitudinal and transverse resistivity of YBa2Cu3O7−δ single crystals irradiated with fast electrons are investigated. It is revealed that the ratio of the normal-state resistivity along the c-axis and in the ab-plane, ρc∕ρab, markedly decreases with increase of the number of structural defects induced by the electron irradiation. Importantly, the temperature dependence of the ratio ρc∕ρab can be described well by both, the universal “law of 1/2” for the variable-range hopping conductivity as well as the conventional exponential expression for the thermally activated conductivity.

Incoherent charge transport induced by irradiation of YBCO single crystals with MeV electrons

Irradiation with electrons is an efficient approach to induce a large number of defects with minimal impact on the material composition. In high-temperature superconducting cuprates, electrons with MeV energies produce point defects in the form of interstitial ions and vacancies that act as efficient scattering centers. Here, we analyze the changes in the electronic transport along the c-axis of $$\hbox {YBa}_2\hbox {Cu}_3\hbox {O}_{7-\delta }$$ single crystals after their irradiation with $$0.5-2.5$$ MeV electrons at $$T \lesssim 10$$ K and a dose of $$10^{18}$$ cm $$^{-2}$$ . While a variable-range hopping conductivity associated with some mesoscopic length scale is observed in the non-irradiated samples, we reveal a two-step superconducting transition in the irradiated samples, that attests to the presence of an additional, macroscopic inhomogeneity induced by the electron irradiation.

Tuning electric charge scattering in YBCO single crystals via irradiation with MeV electrons

Irradiation with electrons is an efficient approach to inducing a large number of defects with a minimal impact on the material itself. Analysis of the energy transfer from an accelerated particle smashing into the crystal lattice shows that only electrons with MeV energies produce point defects in the form of interstitial ions and vacancies that form perfect scattering centers. Here, we investigate the changes in the resistive characteristics of YBCO single crystals from the 1-2-3 system after several steps of low-temperature irradiation with $0.5-2.5$\,MeV electrons and irradiation doses of up to $8.8\times10^{18}$\,cm$^{-2}$. The penetration depth of such electrons is much larger than the crystal thickness. We reveal that defects appearing in consequence of such electron irradiation not only increase the residual resistance, but they affect the phonon spectrum of the system and lower the superconducting transition temperature linearly with increase of the irradiation dose. Furthermore, the irradiation-induced defects are distributed non-uniformly, that manifests itself via a broadening of the superconducting transition. Interestingly, the excess conductivity remains almost unaffected after such electron irradiation.

Effect of electron irradiation and Pr doping on the charge transport in YBCO single crystals

The influence of irradiation by electrons with energies of 0.5 … 2.5 MeV at temperatures of about 10 K on the basal-plane resistivity of the YBa2Cu3O7−δ single crystals is investigated in the range from Tc to 300 K. The resistivity temperature dependence is determined by defects arising due to the irradiation. These defects directly affect the superconducting transition, decreasing Tc and increasing the transition width without significant distortions of its shape. The resulting defects also lead to an increase in the Debye temperature due to a reduction of the anisotropy, and a noticeable increase in the scattering by phonons in the sample. The excess conductivity does not change with the irradiation used.

Influence of annealing on the electrical resistance of YBCO single crystals

The effect of annealing on the basal-plane electrical resistivity of the YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals is studied in a broad range of oxygen contents. Within the framework of s-d scattering of electrons by phonons, an increase in the oxygen deficit index, $\delta$, leads to a significant increase in the Debye temperature, $\theta$, which is associated with the isotropization of the phonon spectrum as the concentration of oxygen vacancies increases. Near the optimal doping, the role of the paraconductivity becomes crucial, whereas its contribution decreases with increasing $\delta$. At large values of $\delta$ some deviations from the s-d model of electron scattering by phonons are observed at room temperature, while no paraproductivity is observed. In the superconducting transition region, a 2D-3D crossover is observed, which shifts in the direction of $T_c$ with increasing $\delta$. The estimate for the transverse coherence length is about $1$\,\AA.

Peculiarities in the pseudogap behavior in optimally doped YBa2Cu3O7−δ single crystals under pressure up to 1 GPa

The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity σ′(T) and the pseudogap Δ∗(T) in optimally doped YBa2Cu3O7−δ single crystals (Tc ≃ 91.1 K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of dlnΔ∗/dP ≈ 0.32, which is only a factor of 1.12 smaller than in slightly doped single crystals, is revealed for the first time. This implies a somewhat more moderate increase of the coupling strength in optimally doped cuprates with increasing pressure. Simultaneously, the ratio 2Δ∗(Tc)/kBTc ≈ 5 at P = 0 GPa, which is typical for high-temperature superconductors with strong coupling, increases by 16% with increasing pressure. At the same time, the pressure effect on Tc is minor: dTc/dP ≈ +0.73 K GPa−1, whereas dlnρ/dP ≈ (−17 ± 0.2)% GPa−1 is comparable with that in lightly doped YBCO single crystals. This suggests that the mechanisms of the pressure effect on ρ(T) and Tc are noticeably different. Independently of pressure, near Tc, σ′(T) is well described by the Aslamazov-Larkin (3D-AL) and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D–2D crossover with increasing temperature. However, the temperature interval Tc < T < T01, in which σ′(T) obeys the classical fluctuation theories, is exceptionally narrow (≈1.16 K). Nevertheless, a peculiarity at the temperature T01, up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Δ∗(T). Below T01 a fast growth of Δ∗(T) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, ns, that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches Tc.

Estimation of Nucleation Thermodynamical Parameters of La&lt;sub&gt;2&lt;/sub&gt;Sr&lt;sub&gt;2&lt;/sub&gt;xCuO&lt;sub&gt;4&lt;/sub&gt; (LSCO) and YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7–&amp;delta;&lt;/sub&gt; (YBCO) Crystallizing from High Temperature Solution

The growth kinetics of LSCO and YBCO single crystals from high temperature solution of LSCO-CuO solute-solvent and YBCO-CuO solute-solvent systems has been investigated. Based on regular solution model and classical nucleation theory, the thermodynamical data investigated for the systems are used to determine the nucleation parameters: interfacial free energy, metastable zone-width, volume free energy, critical energy barrier for nucleation and radius of critical nucleus for LSCO and YBCO which leads to the understanding of the nucleation phenomena of LSCO and YBCO.

C-axis hopping conductivity in heavily Pr-doped YBCO single crystals

We investigated the temperature dependence of the longitudinal and transverse conductivity of Y1−zPrzBa2Cu3O7−δ single crystals with different praseodymium contents. The correlation between the experimental results and the predictions of various theoretical models was analyzed. It is observed that the increase of the praseodymium concentration in Y1−zPrzBa2Cu3O7−δ single crystals leads to increased localization effects and suppression of the superconducting state. Interestingly, for single crystals with large praseodymium content the anisotropy of the normal electrical resistivity ρc/ρab(T) is described by the universal ‘law of 1/2’ for thermally activated hopping conductivity. This is in contrast to the case for comparable crystals of YBa2Cu3O7−δ.

Pinning features of the magnetic flux trapped by YBCO single crystals in weak constant magnetic fields

The dynamics of Abrikosov vortices and their bundles was experimentally investigated in weak constant magnetic fields, in the range of Earth's magnetic field. Characteristics of the isothermal magnetization relaxation in YBCO single-crystal samples with strong pinning centers were studied for different sample-field orientation. The obtained values of normalized relaxation rate S allowed us to estimate the effective pinning potential U in the bulk of the YBCO sample and its temperature dependence, as well as the critical current density Jc. A comparison between the data obtained and the results of similar measurements in significantly higher magnetic fields was performed. To compare different techniques for evaluation of Jc, the magnetization loop measurements M(H), which relate the loop width to the critical current, were carried out. These measurements provided important parameters of the samples under study (penetration field Hp and first critical field Hc1), which involve the geometrical configuration of the samples.

Growth of YBCO single crystals by the self-flux technique

Preparation of high purity, highly perfect and homogeneous YBa2Cu3O6+ δ (YBCO) single crystals is a lengthy procedure that consists of five major steps. They are (a) fabrication of BaZrO3 ceramic crucibles, (b) self-flux growth of YBCO crystals using BaZrO3 crucibles, (c) setting of the oxygen content in the crystals, (d) removal of twins and homogenisation of oxygen content, and (e) formation of oxygen vacancy ordered superstructures by low temperature annealing. To obtain BaZrO3 ceramic impervious to the BaO–CuO melt, the volume of the grain boundary glass phase must be reduced to a very low level through the use of high purity starting materials and precise BaO:ZrO mole ratio control. The best quality YBCO crystals are obtained by slow cooling of YO1.5–BaO–CuO melt in the primary crystallisation region of YBCO. Oxygen content in heavily twinned orthorhombic YBCO is inhomogeneous due to the stress caused by twins. Therefore, homogenisation annealing must be carried out after removal of twin boundaries or, alternatively, under conditions where YBCO is tetragonal. In high purity YBCO, randomly distributed oxygen vacancies are the main source of charge carrier scattering. However, ordered superstructures of oxygen vacancies with significantly lower scattering rates can be generated by careful annealing at low temperatures.

Violation of a logarithmic trend of magnetization relaxation in HTSCs with strong pinning centers

The dynamics of magnetic fluxes trapped in small constant magnetic fields, of the order of the Earth’s field, were experimentally investigated in single crystals of YBCO with strong pinning centers formed in the structure of unidirectional twinning boundaries. A significant deviation of the isothermal magnetization relaxation from the logarithmic behavior observed previously only in strong magnetic fields was found for the first time in the region of strong thermal fluctuations near the superconducting phase transition. Within the Anderson-Kim linear model of the thermally activated creep and using experimental data, an effective pinning potential was estimated. It is shown that at the temperatures close to critical one the inverse logarithmic model of Zeldov describes better the magnetization drop, in particular, at long measurements times.