Oxygen Deficient YBa2Cu3Ox Research Articles

Modification of the Kosterlitz–Thouless transition to the temperature dependent resistance for anisotropic superconductors

Abstract It is found that the temperature dependence of the resistance transition can be analyzed by the modified Coulomb–Gas scaling law for anisotropic superconductors [Y.Z. Zhang, R. Deltour, Z.X. Zhao, Phys. Rev. Lett. 85 (2000) 3492]. In this Letter, through introducing the thermal activation energy and the pinning energy in the correlation length, the modified Coulomb–Gas scaling law is deduced from the phenomenological Kosterlitz–Thouless transition theory. This modified Kosterlitz–Thouless model is applied to the re-analysis of the temperature dependence of the resistance transition of an oxygen-deficient YBa2Cu3Ox crystal reported in literature. Through this model, the mean height of pinning U 0 can be directly calculated from the resistance transition. It is found the mean height of pinning can be described as U 0 ( T , H ) ∝ ( 1 − T / T c ) H − β , where T c is the mean field critical temperature, and β = 1.08 . These experimental results are consistent with the proposed model.

Photo-induced increase of the superconducting coherence lengths in oxygen-deficient YBa2Cu3Ox thin films

\(\) (\(\)) thin films were photodoped with white light at various temperatures from 70 K to 290 K. Before and after the excitation, the magnetoconductivity was measured in a magnetic field B = 0.5 T, and the experimental results were fitted to the Aslamazov-Larkin theory of superconducting order-parameter fluctuations to determine the superconducting coherence lengths, \(\) and \(\). We observed that the photodoping process enhanced \(\) and \(\) and that the amount increased with the photodoping temperature increase. On the other hand, the superconducting anisotropy \(\)/ \(\) decreased with increasing temperature. The photodoping effect enhances superconducting properties of partially oxygen-deficient \(\) samples and is considerably increased by high doping temperatures.

Is There a Unified Description of Conductivity of Layered Cuprates

We present a novel approach to the analysis of the normal state in-plane $$\sigma _{ab} $$ and out-of-plane σc conductivities of anisotropic layered crystals such as oxygen deficient YBa 2 Cu 3 O x . It can be shown that the resistive anisotropy is determined by the ratio of the phase coherence lengths in the respective directions; i.e., $$\sigma _{ab} /\sigma _c = \ell _{ab}^2 /\ell _c^2 $$ . From the idea that at all doping levels and temperatures T the out-of-plane transport in these crystals is incoherent, follows that $$\ell _c $$ is T-independent, equal to the spacing $$\ell _0 $$ between the neighboring bilayers. Thus, the T-dependence of $$\ell _{ab} $$ is given by the measured anisotropy, and $$\sigma _{ab} (\ell _{ab} )$$ dependence is obtained by plotting $$\sigma _{ab} {\text{ }}vs{\text{ }}\ell = {\text{ (}}\sigma _{ab} /\sigma _c )^{1/2} \ell _0 $$ .The analysis of several single crystals of YBa 2 Cu 3 O x (6.35 < x < 6.93) shows that for all of them $$\sigma _{ab} (\ell ) $$ is described by a universal dependence $$\sigma _{ab} /\overline \sigma = f(\ell /\overline \ell ) $$ with doping dependent parameters $$\overline \sigma {\text{ }}and{\text{ }}\overline \ell $$ .

Photoinduced enhancement of the c-axis conductivity in oxygen-deficient YBa2Cu3Ox thin films

High quality thin films of oxygen-depleted YBa2Cu3Ox (x≈6.6) were prepared by pulsed-laser deposition on SrTiO3, substrates that were cut with tilt angles of 10° and 20° with respect to the [001] direction, resulting in a steplike growth of the layers. The resistance showed a semiconducting behavior along the projection of the c axis to the film surface, but a metallic behavior in the perpendicular direction, indicating that the former is dominated by the c-axis resistivity and the latter by the ab-plane resistivity of YBa2Cu3Ox. Long-term illumination of the samples with a 100 W halogen lamp resulted in a significant conductivity enhancement in both directions. The photoinduced change of the out-of-plane resistance vs temperature characteristics is comparable to the effect of large hydrostatic pressure, introducing structural changes similar to that of photoexcitation.

Resistivity and Hall effect of metallic oxygen-deficient YBa2Cu3Ox films in the normal state.

We present a systematic study of normal-state transport properties in a series of $c$-axis-oriented $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (YBCO) epitaxial thin films and Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$/Pr${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (YBCO/PrBCO) superlattices. The hole doping level in the YBCO films is varied from the optimum-doped metallic down to the underdoped insulating regime by changes in the oxygen content $x$. We find that the magnitude of the resistivity $\ensuremath{\rho}$ and Hall coefficient ${R}_{H}$ increases monotonically with decreasing $x$ and that their respective temperature dependences undergo marked changes. The ${R}_{H}(T)$ behavior is reminescent of the Hall effect behavior in heavy fermion metals, taking into account a difference in temperature by a factor of 100. The Hall angle ${cot\ensuremath{\theta}}_{H}=\frac{\ensuremath{\rho}}{{R}_{H}B}$ shows a quadraticlike temperature dependence, with systematic deviations at high and low doping levels. Transport measurements in YBCO/PrBCO superlattices, with the YBCO layers in the two-dimensional regime, indicate that the deviations of a ${T}^{2}$ dependence of the Hall angle are intrinsic and not related to the dimensionality of the system. A method of analyzing the transport data is presented, revealing a striking scaling behavior of the respective properties. A comparison with reported transport data in the literature suggests that the observed scaling behavior may be universal for underdoped cuprates. Furthermore, we show that reported NMR Knight shift data for oxygen-deficient YBCO samples can also be mapped on a single scaling curve, by using the same scaling parameter derived from our transport measurements. This finding strongly indicates that the dominant scattering mechanism in these materials is of magnetic origin. Going one step further, we present a qualitative analysis of the conductivity under the assumption that we may use the expression for a two-dimensional quantum liquid and that the inelastic scattering length may be replaced by the magnetic correlation length $\ensuremath{\xi}(T)$. Expressions for the latter are taken from reported calculations for undoped and doped cuprates. A good qualitative agreement is obtained between the calculated and experimentally observed temperature-dependent conductivity.

Hall effect of as-grown oxygen-deficient YBa2Cu3Ox thin films.

The resistivity and Hall effect have been measured in a number of samples of as-grown, oxygen deficient, c-axis-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ thin films, both as a function of temperature and oxidation state. The temperature dependence of the longitudinal resistivity and Hall coefficient were generally similar to films prepared by ex-situ oxidation processes with the R-T and 1/${\mathit{R}}_{\mathit{H}}$-T characteristics nearly linear and cot${\mathrm{\ensuremath{\theta}}}_{\mathit{H}}$\ensuremath{\propto}${\mathit{T}}^{2}$. Small deviations from these laws were investigated and a quadratic fit obtained for the resistivity data and a cubic fit for cot${\mathrm{\ensuremath{\theta}}}_{\mathit{H}}$ but with dominant linear and quadratic terms, respectively. From the empirical data, expressions for the effective carrier relaxation times and mobilities were obtained and compared with other possible scattering mechanisms. In contrast to films reoxidized after growth, we find that for the as-grown films the Hall mobility ${\mathrm{\ensuremath{\mu}}}_{\mathit{H}}$ increases sharply and cot${\mathrm{\ensuremath{\theta}}}_{\mathit{H}}$ decreases with increasing oxygen content. This is discussed in terms of different oxidation conditions that lead to different lattice defects for the two cases.

Photoinduced superconductivity and structural changes in high temperature superconducting films

The illumination of PryGd1−yBa2Cu3Ox semiconducting and superconducting thin films increases their critical temperatures and decreases their normal state resistivities if and only if the films are oxygen deficient. Moreover, these changes are enhanced near the Pr-induced metal-insulator transition. Light also causes a contraction of the c-axis in YBa2Cu3Ox which is correlated with the observed photoinduced resistivity changes. These changes are similar to those observed when oxygen-deficient YBa2Cu3Ox is enriched with oxygen or annealed at room temperature after quenching from high temperatures.

Photoinduced changes in high temperature superconducting films

Photoexcitation experiments performed in oxygen deficient but metallic YBa2Cu3Ox c-axis oriented films revealed that the persistent photoinduced enhancement of conductivity and superconductivity are not restricted to the insulating side of the metal-insulator transition. A clear increase of the critical temperature Tc, which is accompanied by a decrease of the resistivity in the normal state ρab, is observed for all studied oxygen contents (6.4 ⩽ x ⩽ 6.6). In this oxygen content range, the persistent photoinduced changes in Tc and ρab have a strong dependence on the oxygen stoichiometry of the film. Several mechanisms which may explain the persistent photoinduced phenomena are discussed.

Controlled preparation of oxygen deficient YBa2Cu3Ox films

We describe a simple method based on the oxygen pressure-temperature phase diagram to prepare oxygen deficient YBa2Cu3Ox thin films. Systematic critical temperature and x-ray diffraction experiments clearly show that films with different oxygen contents (6.6≤x≤7) are obtained in a controlled, reproducible, and reversible way.

Pressure effect onT c and resistivity: Localization in YBa2Cu3O x oxides

We have studied the effect of hydrostatic pressures up to 20 Kbar on the temperature dependence of the resistivity σ(T) and the effect of quasihydrostatic pressure up to 200 Kbar on the lattice parameters of YBa2Cu3O x for different oxygen concentrations (x=6.95–6.2). Pressure produces a decrease of resistivity in normal state, an increase ofT c , and a suppression of semiconducting-like resistivity σ(T) at lowx. The dependence of dT c (x)/dP onx is nonmonotonic; the record values of dT c /dP=(1.0±0.1) K/Kbar are observed forx=6.7 and 6.8. The derivative dlnσ/dP atT=293 K differs by the factor 1.8 between superconducting and nonsuperconducting compounds. The compressibility and its pressure derivative alonga, b andc-directions in YBa2Cu3O x have been determined. The most remarkable variation is alongc-direction. A nonmonotonic dependence of dk c (x)/dP onx has been observed. The results are discussed in the context of localized effects in disordered oxygen-deficient YBa2Cu3O x .

Antiferromagnetism and superconductivity in oxygen-deficient YBa2Cu

Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2Cu3Ox have revealed local antiferromagnetic order for 6.0≲x≲6.4 with a Néel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with 6.35≲x≲6.5 the superconducting transition temperature Tc increases smoothly with x from 25 K at x=6.348 to 60 K at x=6.507. Two such samples with x=6.348 and x=6.400 appear to ‘‘switch’’ from superconductivity to antiferromagnetic order at lower temperatures.Received 31 December 1987DOI:https://doi.org/10.1103/PhysRevLett.60.1073©1988 American Physical Society