YBa2Cu3Ox Single Crystals Research Articles

High field paramagnetic effect in YBCO single crystals with different oxygen contents

Magnetization vs. temperature measurements were carried out on superconducting YBa2Cu3Ox (YBCO) single crystals. Samples with oxygen concentrations corresponding to x = 6.90, 6.85, 6.78 and 6.72 were studied. Zero field cooling (ZFC), field cooled cooling (FCC) and field cooled warming (FCW) prescriptions were employed. Measurements were performed in the temperature interval between 20 K and 120 K in constant magnetic fields ranged from 0.05 T to 5 T. A negative slope was observed in the FCC and FCW curves below a field-dependent minimum located in temperatures less than the irreversibility point (Tirr). This negative slope is identified as the high-field paramagnetic effect (HFPE). With a fixed magnetic field, the magnitude of the observed HFPE increases as the temperature decreases. The field dependence of the HFPE at fixed temperatures is not trivial. The overall magnitude of the measured HFPE depends on the oxygen content and becomes smaller when x decreases. The results are discussed on the basis of the effects of flux compression and spin moment polarization inside the vortex cores.

Averaged effective pinning potential in YBCO single crystals nearTc

In this paper we present our results on the low density magnetic flux pinning characteristics, obtained though HTSC samples magnetization relaxation measurements M(t) near the phase transition temperatures (0,8 < T/Tc > 0,99).As the main object of investigation we have chosen pure YBa2Cu3O7-x (YBCO) single crystal samples with optimal oxygen saturation. We note the presence of strong pinning in the system of unidirectional planar defects and a significant nonmonotonicity of the Up(T) dependence behavior for low density trapped magnetic flux.

Scattering of electrons in oxygen underdoped YBa2Cu3O7-x single crystals

Scattering of electrons in oxygen underdoped YBa2Cu3O7-x single crystals

Microwave surface impedance anisotropy of YBa2Cu3Ox single crystals with different oxygen content

The linear microwave response of ultra high-quality YBa2Cu3Ox single crystals grown in BaZrO3 crucibles is measured at 9.4 GHz in rf magnetic fields parallel and perpendicular to the ab-plane in the temperature range 5⩽T⩽200 K. Having found the analytic solution for the magnetic field distribution on the sample surface we determine both the surface impedance Zab=Rab+iXab in the ab-plane and Zc=Rc+iXc along the c-axis of the crystals. For the first time the evolution of the Zab(T) and Zc(T) dependences on the same sample and in a wide range of oxygen content is obtained. For x=6.95 (optimum oxygen content) the temperature dependence of the imaginary part σc″(T)/σc″(0) of the c-axis conductivity is found to be strikingly similar to that of σab″(T)/σab″(0) and becomes more convex with x lowering.

Phase fluctuations and the pseudogap in YBa(2)Cu(3)O (x).

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa(2)Cu(3)O (x) single crystals and Monte Carlo simulations of the anisotropic 3D- XY model. We directly show that T(c) of underdoped YBa(2)Cu(3)O (x) is strongly suppressed from its mean-field value (T(MF)(c)) by phase fluctuations of the superconducting order parameter. For overdoped YBa(2)Cu(3)O (x) fluctuation effects are greatly reduced and T(c) approximately T(MF)(c). We find that T(MF)(c) exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.

Magnetoresistivity in strongly underdoped YBa2Cu3Ox single crystals

We report magnetoresistivity measurements on YBa 2 Cu 3 O 6.36 single crystals in applied magnetic fields H∥ c up to 14 T. We observe several unusual features: the out-of-plane magnetoresistivity Δρ c /ρ c is negative at high temperatures T and becomes positive for T < 150 K. The in-plane magnetoresistivity Δρ ab /ρ ab is positive and displays a maximum at T ≈ 180 K. There is a direct correlation between the sign and T-dependence of magnetoresistivities and the corresponding temperature coefficients of resistivities; i.e. Δρ c, ab ∼ dρ c,ab / dT over the measured T range for the in-plane component and for T > 150 K for the out-of-plane component.

Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: A glasslike transition near room temperature

We present high-resolution thermal expansion data from 5-500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x approximately 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O ordering are found for x approximately 6.95, which disappear below a kinetic glasslike transition near 280 K. The kinetics at this glass transition is governed by an energy barrier of 0.98+/-0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O ordering in the Y123 system is particularly sensitive to uniaxial stress along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.

Raman studies of the high pressure effects in high Tc superconductors

The effect of a hydrostatic pressure (up to 6GPa) on the Raman spectra of YBa2Cu3Ox(x = 6.5 and overdoped) single crystals is investigated in the temperature range 77-300 K in connection with all available data in the literature for the other oxygen concentrations. All five strong modes (4Ag+1B1g) are examined for both oxygen doping and compared with previously published results. From the systematic investigation of the phonon spectral characteristics with the application of pressure, the formation or disappearance of the phases is examined. The results provide a spectroscopic support for the changes induced by the hydrostatic pressure on the chain ordering and the corresponding phases.

Superconducting transitions of YBa2Cu3Ox superconductors inhigh fields

Superconducting transitions were measured in field-cooled coolingand field-cooled warming modes in external magnetic fields ranging between1 mT and 7 T on YBa2Cu3Ox (Y123) single crystals withand without the fishtail effect. The Y123 crystal with the fishtail effectexhibited a two-step transition when being cooled in high fields, while theY123 crystal without the fishtail showed a single step transition.Furthermore, an upturn in magnetic moment was observed at lower temperaturesfor Y123 with the fishtail effect. Such an upturn is ascribed to the local Cumagnetic moment due to oxygen vacancies. The fishtail effect is thus caused byoxygen vacancies.

Surface an Interface of Double Oxides. AFM Observation of Single-Crystal Surface of High-Tc Superconducting Cuprates.

We examined the surface of a YBa2Cu3Ox (YBCO) single crystal by an atomic force microscope (AFM) in order to use the surface as a substrate for homoepitaxial growth. A step-and-terrace feature was revealed on the YBCO (001) surface after mechanical polishing, however YBCO at the topmost layer showed an unusual c-axis length (appeared in step height) much greater than normal one in precise AFM observations. Instability of the surface at high temperatures and poor reproducibility of homoepitaxial films obtained were caused by this degraded YBCO layer. The damaged layer was removed by the chemical etching using HCl/methanol solution. This treatment brought a topmost surface of YBCO single crystal, which made it possible to grow homoepitaxial YBCO films reproducibly.

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 $$\overline \sigma {\text{ }}and{\text{ }}\overline \ell $$ .

Mixed state stability range in a YBaCuO single crystal

The range of applied magnetic field H corresponding to the stability of mixed state with a fixed magnetic induction B is determined for a high-Tc superconducting YBa2Cu3Ox single crystal for H⊥c-axis. For this purpose, the field dependences of the contribution μv to the dynamic magnetic permeability μac′ are analyzed for the hysteresis loops μv(H) in the temperature range from 70 to 84 K. It is shown that the μv(H) hysteresis can be interpreted in terms of the interaction between the vortex lattice and the surface and is associated with the B(H) hysteresis as a manifestation of the Bean–Livingston surface barrier. As a result, the μv(H) hysteresis loops corresponding to different temperatures can be described by a universal curve in dimensionless coordinates. The obtained estimates and the available experimental data reveal a significant suppression of the surface barrier in actual YBaCuO single crystals as compared to the barrier predicted for a perfect surface. The lower branch of the μv(H) hysteresis loop, which corresponds to increasing H, is found to be close to the equilibrium μv(H) curve, and the surface barrier appreciably affects the behavior of μv(H) only for decreasing field, i.e., the emergence of magnetic vortices from the sample surface.

Preparation and Electronic Properties of YBa2Cu3Ox Films with Controlled Oxygen Stoichiometries

We describe a novel technique capable of controlling the oxygen content of YBa2Cu3Ox (YBCO) films in a precise and reversible manner. The temperature dependence of resistivity and the distinct two-plateau behavior in critical temperature Tco versus oxygen content plot of these films are consistent with those observed in the bulk and single crystals of YBCO. The O 1s and Cu 2p absorption spectra of these films were measured by polarization-dependent X-ray absorption spectroscopy (XAS). The intensity variations of the pre-edge peaks as a function of oxygen content are discussed. We also used these films to systematically study the electron-phonon coupling strength and the position of Fermi level by using a femtosecond pump-probe technique. A clear sign-reversal of the transient reflectivity, which was consistently explained by the thermomodulation model, was observed. Both of these optical measurements support the idea that the electronic structure of YBCO cuprates is based on the charge transfer model with hybridization between the Cu and O sites.

X-ray irradiation effects on YBa2Cu3O7-x single crystals

In this work as-grown YBCO single crystals were irradiated using the radiation of a Mo anticathode as a X-ray source. The effects of X-ray irradiation were investigated in order to improve the oxygen enrichment during the subsequent annealing in O2. X-ray irradiation was found to have a favourable effect on the homogeneity in the oxygen enrichment of the samples, as stated by the high reproducibility of resistance measurements carried out on specimens grown, irradiated and annealed under the same conditions.

Angular dependences of the peak effect in Yba2Cu3Ox single crystals

Angular dependences of the magnetization hysteresis loops have been studied at T=77 K on YBCO single crystals exhibiting the peak effect. The peak effect is shown to be related to the pinning of longitudinal vortices along the c axis at twin boundary-type ordered defects. The behavior of the peak effect at intermediate angles is explained by anisotropic magnetic field penetration into quasi-two-dimensional superconductors. In thin crystals with a dilute ordered-defect structure this can result in an enhanced peak effect due to formation of a vortex kink structure and to “internal” pinning of transverse vortex segments at Cu-O sheets.

Investigation of the Crystal Shape Controllability of YBa2Cu3O7-x Single Crystals in the Pulling Method

The influence of the geometrical difference on the crystal growth rate of YBa2Cu3O7-x (Y123) single crystals in the modified pulling method was investigated. The growth rate of {001} faces appeared on the bottom of the crystals grown along <001> direction (Rcvc) was about 1.6–2.0 times higher than that of {100} faces appeared on the bottom of the crystals grown along <100> direction (Rava). Rava is more sensitive to the change of supersaturation than Raha (the crystal growth rate of {100} faces appeared on the side of the crystals grown along <100> direction) for the low crystal pulling rate and for the low supersaturation regime, although these faces are crystallographically the same. These difference is attributed to the increase of the temperature at the crystal bottom face during the crystal growth. It is supposed from the experimental results that Y123 single crystals having long and nearly straight body parts with expanding about 25° can be obtained in the <001> direction growth of the modified pulling method.

Anisotropic transport properties in a layered d+s-wave superconductor

Recent measurements of the penetration depth and optical conductivity in YBa2Cu3Ox (YBCO) single crystals suggest the possibility of an order parameter with d+s-wave symmetry. We present results from calculations of both the anisotropic low temperature penetration depth and the frequency-dependent conductivity for a layered superconductor with such a pairing symmetry. The system is modelled as a stack of weakly coupled, two dimensional superconducting planes, and the effect of non-magnetic impurity scattering in both Born and unitarity limits is examined.

Electronic transport and specific heat of Ca-doped YBa2Cu3Ox-single crystals

Measurements of the anisotropic electrical resistivity, the Hall Effect, and the specific heat anomaly at Tc of Ca-doped YBa2Cu3Ox single crystals of various Ca and O contents are presented. The transport properties are discussed in terms of charge and spin separated quasiparticles and confirm that by doping Ca onto Y sites the hole concentration, nh, can be extended beyond the value achieved with O-doping alone at x=7.0. The jump in the specific heat at Tc which increases monotonously with x up to x=7.0 in Ca-free crystals displays a maximum in highly oxygenated Ca-doped crystals. This maximum, however, is displaced from the Tc(nh) maximum to higher values of nh, consistent with the model of induced superconductivity in the CuO chains.

Flux motion in twin boundaries of YBa2Cu3Ox single crystals

The effect of the twin boundaries on flux motion has been studied by using a densely twinned and a perfectly detwinned domain in the same 123 single crystal. The most successful detwinning of the 123 crystals was a thermomechanical treatment under inert gas, which kept the perfectly detwinned domain after oxygen annealing. The influence of the twin boundaries on flux motion strongly depended on temperature. The presence of the twin boundaries brought about a reduction of the flux pinning force in low temperature, but with an increase in temperature, the twin boundaries worked as strong pins.

Influence of domains on the second harmonic generation in YBa2Cu3Oxsingle crystals

Abstract In the present paper the second harmonic generation (SHG) signal in YBa2Cu3Ox single crystals as a function of domain dimensions, experimental geometry, and mechanical and magnetic fields is investigated. It is found that the domain dimensions affect strongly the nonlinear optical susceptibilities by meeting the phase synchronism conditions. The SHG signal is found to be independent of the incident light polarization direction. It is shown how magnetic and mechanical fields applied cause an influence of domain boundary regions on a value of the SHG signal.