Formation of Ca-doped YBa2Cu4O8 phase in thin films

Ca-doped YBa₂Cu₄O₈ (124) thin films are prepared on (100) SrTiO₃ substrates by annealing the amorphous films deposited using a pulsed laser deposition technique. The X-ray diffraction measurements show that the Ca-doped YBa₂Cu₄O₈ phase is formed by annealing below 800°C at a oxygen pressure of 1 atm. The 124 films have c-axis orientation normal to the substrates. As the Ca content increases, the proportion of the 123 impurity phase in the samples increases. The onset temperature of superconductivity of the Y(Ca)Ba₂Cu₄O₈ films increases from 79 K to 88 K with an increase Ca-substitution for 5 to 10% of Y.     

The pseudogap and superconducting gap in the ab-plane electronic Raman continuum of YBa2Cu4O8

We report plane-polarised Raman spectra from YBa₂Cu₄O₈ single crystals between 300 and 10 K. In the normal state we observe a gap-like depletion of intensity from the electronic continuum extending to around 1200 cm⁻¹ with an onset temperature of around 225 K. We remove the phonons and pseudogap depletion from the spectra using a simple model and recover a characteristic high-T_c superconductor continuum. In the superconducting state, intensity returns to the continuum in the form of a very broad pair-breaking peak.     

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Highly dense sintered YBa₂Cu₄O₈ has been produced by hot isostatic pressing (HIP). The electrical resistivity of this material has been measured as a function of temperature T and pressure in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of are found to be well described by a model based on the standard Bloch-Grüneisen theory. It is pointed out that is liner in T only under isobaric conditions, while is strongly nonlinear in all high-T_c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm² at 4 K, while the resistivity is 630 μΩ cm at 294 K.     

Anomalous changes in CuO2 plane in YBa2Cu4O8 under pressure

We report the results of an atomistic simulation study of the pressure-induced structural changes in YBa₂Cu₄O₈ from 0 to 20 GPa. It is found that the crystal has similar compressibilities in the a- and b-directions over the whole pressure range, whereas in the c-direction there are quite different compressibilities at different pressures. Our results suggest that there exists a correlation between the charge transfer and the change in the Cu(2)---O(1) bond length, but we do not support the suggestion of a direct dependence of T_c on the length of this bond. However, we predict that there are anomalous changes in the CuO₂ plane with pressure, which we propose may lead to a charge carrier redistribution on in-plane copper and in-plane oxygens, which in turn may be related to the optimum carrier concentration in the CuO₂ plane.     

Intergrain ordering processes of YBa2Cu4O8 and related complex ceramic systems

Intergrain ordering phenomena of frustrated ceramic YBa₂Cu₄O₈ (Y124) and dual phase mixed ceramic containing CuO are studied by both magnetic and resistivity observations. Magnetically estimated intergrain ordering temperature T_c2 and zero-resistivity attainment temperature T_z are both decreased with CuO content. However, it is also revealed that mutual location of T_c2 and T_z is changed. For the mixed ceramic with torn intergrain network structure, T_z stays above T_c2, indicating local dissolution of frustration by cluster structure. On the other side, remaining resistivity tails blow T_c2, indicating intrinsic fluctuation effect on the resistivity caused by frustration in the ordered state. It is known then that, superconductive ceramic samples which is free from structural phase inhomogeneity is essential in order to clarify the frustration effect.     

Phase diagrams of YBa2Cu4O8 and YBa2Cu3.5O7.5 in the pressure range 1 bar≤PO2≤3000 bar

The P-T-x phase diagram of the pseudobinary system (Y-Ba-Cu-O)-O₂ has been further investigated in the oxygen pressure range between 1 and 3000 bar. The stability ranges of the phases YBa₂Cu₄O₈ (124), YBa₂Cu_3.5O_7.5−x (123.5) and YBa₂Cu₃O_7−x (123) have been determined. Long duration experiments showed that the 123 phase is not stable at least down to 7 bar≤P≤20 bar oxygen and 900°C. It is not clear whether at lower pressures and temperatures the 123 phase is thermodynamically stable or metastable due to low reaction rates. In the presence of excess CuO, the 124 is the stable phase. The melting of 124 pellets at P_O2=2800 bar shows that even at this pressure the 124 compound melts incongruently. Using the phase diagram data we could change the T_c of 123.5 from 16 to 70 K by varying systematically the nonstoichiometry. Due to a narrow homogeneity range the T_c of 124 remained constant but is different for powder pellets (81 K) and for crystals (70 K), probably due to the influence of the flux. Single crystals of both 124 and 123.5 with dimensions up to 4 mm were grown from the flux under high oxygen pressure.     

Ferromagnet/superconducting YBa2Cu3O7−δ bilayer devices

A magnetic fringe-field effect has been investigated for a simple bilayer device structure consisting of a Co_0.9Fe_0.1 film and an epitaxial YBa₂Cu₃O_7−δ (YBCO) film patterned as a microbridge. The resistance of the bridge is measured with a four-probe technique and is found to depend on the orientation of a magnetic field, which is externally applied in the device plane. A maximum (minimum) of the resistance occurs when the magnetic field is applied in parallel (perpendicular) to the bridge axis. The difference between the maximum and the minimum is very large for a small range of temperature below the critical temperature of the YBCO film. The observed features in the resistance are qualitatively explained by vortex motion in the YBCO bridge under the influence of the magnetic fringe-field of the Co_0.9Fe_0.1 film.     

Coherence transition in granular YBa2Cu3O7-δ, YBa2Cu2.95Zn0.05O7-δ, and YBa1.75Sr0.25Cu3O7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa₂Cu₃O_7-δ, YBa₂ (Cu_2.95Zn_0.05)O_7-δ and Y(Ba_1.75Sr_0.25)Cu₃O_7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature T_c, where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature T_c0. This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of T_c, but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa₂Cu₃O_7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition.     

YBa2Cu3O7-x的金属-绝缘体转变

借助于局域化的标度理论,我们对层状结构系统引进一个由电导各向异性或层间耦合决定的有效维度,认为电导的减小和有效维度的降低是YBa₂Cu₃O_7-x和其它氧化物金属-绝缘体转变的原因,同时也讨论了有限温度的标度理论以解释正常态电阻率的温度行为。 关键词：     

Infrared optical response of YBa2Cu3O7 and PrBa2Cu3O7 An ellipsometric study

Ellipsometry was used to study (110) and (001) oriented films of YBa₂Cu₃O₇ and PrBa₂Cu₃O₇ in the mid-and near-infrared spectral regions. Below a photon energy of 0.1 eV, the in-plane component of the dielectric tensor of YBa₂Cu₃O₇ is dominated by a Drude term with a squared plasma energy of (3.0 ± 0.3) eV². This “oscillator strength”, and the Lorentzian broadening energy of (0.104 ± 0.005) eV at room temperature, are confirmed by the changes induced in Pr-substituted material, and by low-temperature measurements in the near-infrared. The deviation from the Drude behavior observed above 0.1 eV is accounted for by a broad absorption band with an oscillator strength of (3.6 ± 0.1) eV² in YBa₂Cu₃O₇ which shifts to higher energies and takes over almost all of the oscillator strength of the Drude term when Y is substituted by Pr. The response to electric fields perpendicular to the planes is much weaker, with an upper bound of 0.63 eV² for the squared plasma energy. The in-plane loss function computed from the measured dielectric function follows the Drude-like lineshape, modified by the bound-state absorption band, down to the lowest energy reached in our measurements (0.058 eV).     

Site selection NMR study on electronic state in and near vortex core of YBa2Cu4O8

We report a site-selective ¹⁷O spin-lattice relaxation rate T₁⁻¹ in the vortex state of YBa₂Cu₄O₈. We found that T₁⁻¹ at the planar sites exhibits an unusual nonmonotonic NMR frequency dependence. Based on T₁⁻¹ in the vortex core region, we establish strong evidence that the local density of states within the vortex core is strongly reduced.     

Theoretical and experimental investigation of the linear optical response of YBa2Cu4O8 The relevance of LDA calculations

We report results of ellipsometric measurements of the dielectric tensor of YBa₂Cu₄O₈ as well as calculations employing density functional theory in the local density approximation (LDA). For the computation of the one-electron eigenvalues and states the linear-muffin-tin-orbital method (LMTO) was used. Measured and calculated results show good agreement. In particular, the calculation predicts different band structures for YBa₂Cu₃O₇ and YBa₂Cu₄O₈ in the vicinity of the Fermi energy which lead to significant differences in the optical spectra. One such difference is a region of small ε₂ in the near infrared which is a fingerprint of the high (relative to the top of the highest fully occupied band) Fermi energy as compared to the situation in YBa₂Cu₃O₇. Our experimental results confirm the theoretical findings.     

Oxygen defects relaxation in high-temperature YBa2Cu3O7 superconductors

The temperature dependences of normal state static magnet susceptibility χ(T) for high-temperature YBa₂Cu₃O₇ superconductors (T _c ≤ T ≤ 400 K) measured for the same sample before and after 22 years of storage are compared. It is shown that during longime storage of a maximally doped sample, its electronic system is more in equilibrium than it was in the initial state, while a similar underdoped sample decomposed completely. Comparing the χ(T) curves for YBa₂Cu₃O₇, we draw conclusions as to the nature of the Curie contribution and the fluctuation-induced contribution to the magnetic susceptibility of the investigated sample, and the experimental procedure for determining T _c.     

Vortex pinning in YBa2Cu3O7?x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa₂Cu₃O_7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T _c and magnetic field 0<B<6 T. Below the vortex lattice melting transition B_g(T) but above a threshold field B_p≈8(1-T/T _c ) T, the inductance of vortices increases as B², much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region B_g(T<B<B^*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ^*≡ξ(B=B^*)≈450?. The value of ξ^* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at B_g.     

Pinning potential of stacking fault-related dislocations in YBa2Cu3O7

The pinning potential of the 031 type dislocation has been calculated. This dislocation bounds the excess Cu---O layer defect commonly observed in YBa₂Cu₃O₇. It was found that there is a strong pinning anisotropy between edge dislocations lying along [100] and [010]. For the former, a repulsive potential of about 30 meV is found, while the latter exhibits an attractive interaction of less than this magnitude. This potential is short-range (about 2ξ), indicating that these dislocations would act as distributed point-like pinning sites.     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.     

Suppression of antiferromagnetic spin fluctuation in Zn-substituted YBa2Cu3O7

We have prepared Zn-substituted YBa₂Cu_3−xZn_xO₇ (YBCO, x=0.0–0.09) and performed ^63,65Cu nuclear quadrupole resonance (NQR) measurements for the plane site at 300 and 100 K as a function of Zn concentration. The substitutional effects are observed in resonant frequencies and linewidths of spectra, and relaxation times as well as in the superconducting transition temperature. The spin–lattice relaxation rate 1/T₁ is reduced for the higher Zn concentration and the reduction is more significant at 100 K. The ratio of ^63,65Cu spin–lattice relaxation rates suggests that a magnetic contribution due to the antiferromagnetic spin fluctuation becomes weak as the Zn concentration increases. These effects confirm that the antiferromagnetic spin fluctuation of Cu 3d spins is suppressed by the Zn substitution due to the absence of local moment at the zinc site.     

The nuclear contribution to the specific heat of single crystals of YBa2Cu3O7 and Bi2Sr2CaCu2O8

Specific heat data below 1 K of Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O₇ are analyzed. For YBa₂Cu₃O₇ the nuclear specific heat, C_N, amounts to 38T⁻² μJ/mol K. C_N for Bi₂Sr₂CaCu₂O₈ exceeds that of YBa₂Cu₃O₇ by a factor of 15. The nuclear quadrupolar specific heat contribution alone is insufficient to explain the data for YBa₂Cu₃O₇, while lack of NQR data does not allow such a comparison in Bi₂Sr₂CaCu₂O₈ to be made. The contribution to C_N from nuclear spins coupled via the contact hyperfine interaction with correlated magnetic spins (in the CuO₂ plane) is derived as a function of the correlation length. This contribution can be treated independently from the quadrupolar term. We show that the excess specific heat in YBa₂Cu₃O₇ likely originates in a few percentage of an impurity (oxygen deficient) phase with a strong hyperfine field on the Cu nuclei.     

Synthesis of YBa2Cu3O7−x superconducting thin films on LaAlO3 by means of PAMOCVD

High-T_c superconducting thin films have been deposited in situ by means of a plasma assisted metal-organic chemical vapour deposition (PAMOCVD) process on LaAlO₃. An EMCORE high-speed rotating disc reactor was used to deposit the films at a substrate temperature of 600°C to 800°C. The system is equipped with a (remote) 120 W microwave plasma generator. The oxidising plasma gas is N₂O and/or O₂ while Ar was used as the inert carrier gas for the different metal-organics. The influence of different process parameters (such as the temperatures of the metal-organics, substrate temperature, and plasma gas composition) on the superconductive properties and on the morphology of the films was investigated. Surface morphology and composition were studied by SEM/EDX or EPMA, and AC susceptibility measurements were used to investigate the superconductive properties (T_c and J_c). X-ray diffraction measurements indicated that single-phase YBa₂Cu₃O_7−x films were epitaxially grown with the 00l orientation perpendicular to the substrate surface. The critical temperature (T_c) of the films is about 90 K and the critical current density (J_c) is higher than 10⁶ A/cm² at 77 K and zero field.     

TEM investigation of YBa2Cu3O7 thin films on SrTiO3 bicrystals

YBa₂Cu₃O₇ films in c-axis orientation on bicrystalline SrTiO₃ substrates are investigated by TEM. The films and the substrates are examined in cross-section and in plane view. The grain boundary of the bicrystal substrate contains (110) faceted voids, but is otherwise straight on a nanometer scale. Contrary to this, the film grain boundary is not straight grain boundary can be up to 100 nm for a 100 nm thick film. The deviation from the intended position of the YBCO grain boundary can already occur at the film/substrate interface where it can be as much as ±50 nm.