YBa2Cu3O7-δ Single Crystals Research Articles

Crystallization features of YBa2Cu3O7-δ single crystals in 2YBa4Cu3O9-δ + BaCu2O2 + CuO2 system

In this article, a consistent study of phase transformations during the crystallization of YBa2Cu3O7-δ compound was carried out using XRD, thermogravimetric and differential thermal analyzes, as well as optical microscopy. When studying the microstructure and elemental composition in the reaction zone in the process of obtaining single crystals by the crucible-less method, the products of chemical reactions were identified depending on the composition of the reacting components and synthesis conditions. It has been established that the use of precursors Y2BaCuO5, YBa4Cu3O9-δ and ВаCu2O2 as initial reagents has made it possible to carry out the direct synthesis of YBa2Cu3O7-δ single crystals without the formation of intermediate phases. The superconductor has been synthesized at 1270 K on single-crystal MgO substrates with the (001) orientation, since their surface is poorly wetted by the melt solution and stimulates the YBa2Cu3O7-δ nucleation process. This ensures the minimum loss of the liquid fraction formed in the sample. The growth conditions for YBa2Cu3O7-δ single crystals have been studied and optimized. It has been experimentally revealed that the use of combined cooling conditions leads to an increase in the size of single crystals and a reduction in the time of their growth without changing the quality and crystal structure. The investigation showed that the largest volume (50 mm3) was achieved for single-phase YBa2Cu3O7-δ single crystals grown at a cooling rate of 0.5 deg/h in the temperature range 1260–1240 K and at a rate of 1.2 deg/h in the range 1240–1210 K. An analysis of the Laue rotation lines obtained in this work indicates the presence of blocks in single crystals cooled in the temperature range 1243–1193 K at a cooling rate of 1.5 deg/h and their absence in crystals cooled at 1.2 deg/h. An assessment of the degree of perfection of the structure by the width of the rocking curves at half-height of the X-ray reflection (006) showed that the width of the rocking curves of 0.36 deg indicates the absence of structural defects, such as twins, blockiness, and shear defects.

Enhanced stability of floating-zone by modifying its liquid wetting ability and fluidity for YBa2Cu3O7-δ crystal growth

As an ideal material platform for deep investigations on superconductivity, high purity YBa2Cu3O7-δ (Y123) single crystals have been highly concerned and requested recently. However, it is very difficult to grow such crystals by the conventional travelling-solvent floating-zone (TSFZ) process due to the intrinsic instability of floating zone caused by the liquid loss. To overcome this obstacle, the feed rod with modified precursor powder (MPP) of 0.5Y2O3+ Ba2Cu3O5 was adopted, instead of the conventional one (Y123). Our experimental results proved that less Ba3Cu7Oy solvent penetrated to the MPP-rod due to its low wettability. Furthermore, it was also observed that less melt dropped down from the floating zone owning to its low fluidity, caused by low dimension of bubbles and Y211 particles generated from the MPP-rod. Consequently, the molten zone was remarkably stabilized and the Y123 single crystal was successful grown. The effective size of the grown boule was about 5 mm (Ø) × 30 mm. This new conception of altering precursor components for continuous TSFZ growth is universally worth of producing other functional materials.

Electrical conductivity of YBa2Cu3O7-δ single crystals under conditions of anionic ordering in Cu(1)O1-δ layers

The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in YBa2Cu3O7-δ single crystals has been studied. It has been shown that an increase in the critical onset temperature of the transition to the superconducting state during thermocycling annealing procedures is consistent with the decrease of the σс/σаb parameter. This fact indicates a redistribution of the electronic density between the structurally inhomogeneous Cu(2)O2 and Cu(1)O1-δ planes, due to the formation of oxygen long-range order in the O(4)–Cu(1)–O(4) linear groups along the (b) crystal structure axis of the unit cell, and elimination of oxygen defects in the square nets of the Cu(2)O2 planes. The existence of the critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Тс, has been confirmed. In this case an increase in Тс and orthorhombic distortion of the crystal structure during isothermal annealing are caused by the amplification of the “interlayer” interaction between the Cu(2)О2 and Cu(1)О1-δ planes. As a result, the contribution of the Cu(1)О1-δ chain layers to the electron state density at the Fermi level increases. These layers can acquire superconducting properties due to tunneling of Cooper pairs from the Cu(2)О2 planes resulting in the formation of the induced superconductivity in these planes.

Electrical conductivity of YBa2Cu3O7-δ single crystals under conditions of anionic ordering in Cu(1)O1-δ layers

The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in theYBa2Cu3O7-δ single crystals have been studied. It was determined that an increase in the critical temperature of the onset of the transition to the superconducting state during step annealing procedures is consistent with decrease of the σс/σаb parameter. This fact indicates the redistribution of the electronic density between the Cu(2)O2 and Cu(1)O1-d structurally-inhomogeneous planes, due to the formation of the oxygen long-range ordering in the O(4)—Cu(1)—O(4) linear groups along the (b) crystal structure axis of the unit cell, and removal of the oxygen defects in the square nets of the Cu(2)O2 planes. The existence of a critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Тс, has been proved. In this case, the increase of Тс and the orthorhombic distortion of the crystal structure at the isothermal annealing processes occur due to the amplification of the «interlayer» interaction between the Cu(2)О2 and Cu(1)О1-δ planes. As a result, the contribution of the Cu(1)О1-δ chain layers in the electronstate density on the Fermi level increases. These layers could be the superconducting ones by means of the Cooper pairs tunneling from the Cu(2)О2 planes, forming the induced superconductivity there.

The effect of low divalent chemical doping on the high field diamagnetic thermodynamic fluctuations of the YBa2Cu3O7-δ single crystals

Abstract We report on reversible dc magnetization data of the YBa2Cu3O7-δ, YBa2Cu2.97Zn0.03O7-δ, YBa2Cu2.97Mg0.03O7-δ and YBa2Cu2.97Ni0.03O7-δ single crystals with the aim of studying the contribution of the divalent chemical doping on the diamagnetic thermodynamic fluctuations (DTF) of the YBa2Cu3O7-δ. The dc zero field cooled [MZFC(T)] and field cooled cooling [MFCC(T)] diamagnetic magnetizations were measured with a SQUID magnetometer. The DTF behavior of our samples was interpreted in light of the 3D lowest Landau level formalism (3D-LLL). The results highlight that the amount of the Cu partially substituted individually for Zn, Mg or Ni in the YBa2Cu3O7-δ structure do not change the 3D character of its vortices dynamics. In addition, they show an increase in the temperature range dominated by 3D-DTF of ours doped single crystals. This increase could be connected to the intensification of their inhomogeneous superconducting state due to the chemical doping.

Effect of hydrostatic pressure on the conductivity of YBa2Cu3O7-δsingle crystals in a broad range of temperature and oxygen content

The effect of external hydrostatic pressure on the electrical resistance of optimally doped and underdoped YBa2Cu3O7-δ single crystals is investigated in a broad temperature range, which includes the normal state and the region of superconducting fluctuations. The temperature dependences of the resistivity in the normal state are determined by scattering of charge carriers by phonons and defects. The application of pressure leads to significant changes in the electronic structure of the sample and reduces the degree of their defectiveness. This is accompanied by changeû in the lattice characteristics as the sample volume decreases. The fluctuation conductivity only exists within the range of ∼0.1Tc. With increasing δ the superconducting characteristics are close to the values typical for conventional low-temperature superconductors.

High-Field Paramagnetic Meissner Effect in Ga-Implanted Single Crystal

We report on dc magnetization data collected from a modified superficial pinning potential YBa 2 Cu 3 O 7-δ single crystal with the goal of checking the majority contribution of the magnetic flux compression scenario to the high-field paramagnetic Meissner effect (HFPME). The additional superficial pinning centers were introduced at the YBa 2 Cu 3 O 7-δ [YBCO(Ga)] single crystal surface by Ga ion beam bombardment. The field-cooled warming (M FCW (T)), field-cooled cooling (M FCC (T)), and zero-field-cooled (M ZFC (T)) dc magnetizations were recorded, while magnetic fields H up to 50 kOe were applied parallel to the single crystal c-axis. M FCW (T) and M FCC (T) data show the usual superconductor diamagnetic response to H <;5 kOe. In contrast, when 5 kOe ≤ H ≤ 50 kOe was applied, M FCW (T) and M FCC (T), particularly for T≪T irr (H) (magnetic irreversibility temperature), become paramagnetic. The magnetic-field-dependent paramagnetic behavior of M FCW (T) and M FCC (T) is the physical signature of HFPME in superconductor materials. We contrast the HFPME observed to the YBCO(Ga) sample with that observed to a not irradiated YBa 2 Cu 3 O 7-δ (YBCO) single crystal. We suggest that the enhancement of superficial pinning potential collaborates definitely to the strengthening of the magnetic flux compression scenario and consequently to the reinforcement of the HPME in YBa 2 Cu 3 O 7-δ single crystals.

Effect of Long Aging on the Resistivity Properties of Optimally Doped &lt;inline-formula&gt; &lt;tex-math notation="TeX"&gt;$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Single Crystals

We investigate conducting properties in the basal abplane before and after a long time exposure in air atmosphere of optimally oxygen doped YBa 2 Cu 3 O 7-δ single crystals. Prolonged exposure leads to an increase of the effective scattering centers of the normal carriers. The excess conductivity in a wide temperature range has exponential temperature dependence and near the critical temperature is well described within the AslamazovLarkin theoretical model. The prolonged exposure increases to a great extent the temperature range of the implementation of the pseudogap state, narrowing the linear section of the temperature dependence of the resistivity in the ab-plane, ρ ab (T).

Low Sr doping effects on critical current density and pinning mechanism of YBa2Cu3O7-δ single crystals

We report on isotherm dc magnetization hysteresis loops, M(H) to 70K ≤ T ≤ 82.5K of YBa2-xSrxCu3O7-δ (x = 0, 0.02 and 0.1) single crystals with the purpose to study the influence of the low Sr doping effects on the YBa2Cu3O7-δ critical current density, JC(H) and normalized flux pinning force density, f(h). The M(H) measurements were performed in a commercial SQUID magnetometer with H ≤ 50kOe applied parallel to the c axis of the samples. The extended Bean critical state model was applied to JC(H) determination. The magnetic irreversibility fields, Hirr(T) were obtained from M(H) loops. The f(h) behavior was determined from FP/FP,max versus H/Hirr plots where FP is the pinning force density (FP = J × μ0H). The results show that the low Sr doped samples transport higher JC(H) than the pure one. The application of the Dew-Hughes scaling functions to the f(h) plots designates the core normal punctual type as majority pinning mechanism of samples. We suggest that the core normal punctual pinning mechanism, responsible to the enhancement of the JC(H) transported by YBa2-xSrxCu3O7-δ samples, is possibly connected to the segregation of Sr atoms precipitates at the crystalline structure of these samples.

Effects of hole doping and chemical pressure on the average superconducting kinetic energy of YBa2Cu3O7-δ single crystals

We reported on MZFC(T) and MFCC(T) reversible dc magnetizations of YBa2Cu3O7-δ, Y0.99Ca0.01Ba2Cu3O7-δ and YBa1.75Sra0.25Cu3O7-δ single crystals with a strong focus on the effects of Ca and Sr doping on the average superconducting kinetic energy density, k(T) of the YBa2Cu3O7-δ The k(T) is used as a relevant tool to provide physical information about the HTSC paring mechanism. The determination of the k(T) from MZFC(T) and MFCC(T) data is supported by virial theorem of superconductivity [k(T) = – MB]. The MZFC(T) and MFCC(T) measurements were performed with a SQUID magnetometer to H ≤ 50kOe applied parallel to the c axis of the samples. The results show that the samples present a common k(T) behavior that is characterized by a maximum value for T ≤ Tc that gradually decreases as the temperature rises towards to the Tc, becoming null to T ≤ Tc. The magnetic field affects smoothly the k(T) data behavior. The k(T) results contrasting of our samples shows that the Ca and Sr doping promotes a reduction of its amplitude. A possible explanation to this feature could be associated to the fact that the hole doping character promoted by Ca doping and the chemical pressure effect motivated by Sr doping affects considerably the superconducting paring mechanism of the YBa2Cu3O7-δ.

Peculiarities of microstructure of YBa2Cu3O7-δ single crystals heat treated under different conditions

The microstructure of single-crystal YBa2Cu3O7-δ samples has been studied after a reaction with water vapor at 200–300°C and after restoring annealings at 400–930°C. The absorption of water from air during low-temperature annealing was found to lead to the disappearance of twin structure and appearance of stress fields around precipitated particles. During restoring annealing at 930°C, the primary recrystallization of single crystals takes place. The initial twin structure was not formed in the course of subsequent oxidation.

Critical State Stability and Flux Jumps' Dynamics in a Single Crystal of YBa2Cu3O7-δ

Critical State Stability and Flux Jumps' Dynamics in a Single Crystal of YBa₂Cu₃O_7-δ

Irreversibility line and anisotropy of SmBa2Cu3[formula omitted] with varying oxygen content

We report on magnetization and torque measurements of SmBa2Cu3O7-δ (Sm123) single crystals with an oxygen content 7-δ=6.60,6.70and6.78. The anisotropy increases from 40.1±0.4 to 56.4±0.7 with decreasing oxygen content from 6.78 to 6.70. The crossover field, Hcr, where the irreversibility line changes slope, is lowered for crystal with a lower oxygen content. The Hcr values are higher for Sm123 than for YBa2Cu3O7-δ (Y123) single crystals. The anisotropy dependence on the oxygen content is compared to that of Y123.

The change of anisotropy by Zn or Ca substitution in YBCO single crystals

Zn and Ca substitution effects in YBa2Cu3O7-δ single crystals are presented, focusing on the anisotropy in overdoped regime. Overdoping by Ca substitution lowers the pseudogap temperature T*, T* vanishes for heavily overdoped samples. The effect of oxygen deficiency by Ca-substitution is negligible. Zn substitution effect on residual part of c-axis resistivity is hardly observed, which is in contrast to the steady increase for in-plane resistivities.

Manifestation of oxygen ordering in the magnetic relaxation of YBa2Cu3O7-δ single crystals

The temperature dependence of the relaxation rate S = -dln M/dlnt for YBa2Cu3O7_δ samples with different oxygen content was measured. The most important result consists in the qualitative change of the topology of the curve S(T) when the oxygen deficiency δ passes through some threshold value δth ∼ 0.37. For the oxygen contents corresponding to δ < 0.37 (Tc > 60 K), the function S(T) has a usually observed maximum at T/Tc ∼ 0.4. For a sample with Tc = 51 K, this maximum transforms to a plateau and a new sharp maximum at T/Tc ∼ 0.1 arises. It is known that the transition from disordered to ordered oxygen state occurs in YBa2Cu3O7_δ samples with δ ≊ δth (Tc ≊ 60 K). We consider that the change in the form of our experimental curve S(T) is a macroscopic manifestation of this transition.

Influence of low-level Pr substitution on the superconducting properties of YBa2Cu3O7-δ single crystals

We report on measurements on Y1-xPrxBa2Cu3O7-δ single crystals, with x varying from 0 to 2.4%. The upper and the lower critical fields, Hc2 and Hc1, the Ginzburg-Landau parameter, κ, and the critical current density, Jc (B), were determined from magnetization measurements and the effective media approach scaling method. We present the influence of Pr substitution on the pinning force density as well as on the trapped field profiles analyzed by Hall probe scanning.

Irradiation-induced suppression of the critical temperature in high-T c superconductors: Pair breaking versus phase fluctuations

Experiments on the irradiation-induced suppression of the critical temperature in high-T c superconductors are analyzed within the mean-field Abrikosov-Gor’kov-like approach. It is shown that the experimental data for YBa2Cu3O7-δ single crystals can be quantitatively explained by the pair-breaking effects under the assumption of the combined effect of potential and spin-flip scattering on the critical temperature and with an accounting for a nonpure d-wave superconducting order parameter.

Quasi-Homoepitaxial Growth of a-Axis Oriented PrBa2Cu3O7-δ Thick Film on (100) YBa2Cu3O7-δ Single Crystal

The structural characterizations of the quasi-homoepitaxial growth of a-axis oriented PrBa2Cu3O7-δ thick film grown on (100) YBa2Cu3O7-δ single crystal were investigated in comparison with those of the film grown on (001) YBa2Cu3O7-δ single crystal. The a-axis oriented PrBa2Cu3O7-δ films, expected to be a barrier layer, were prepared using a dc-95 MHz hybrid plasma sputtering on (100) and (001) YBa2Cu3O7-δ single crystals that are superconducting ground planes. The atomic force microscopy image revealed that the surfaces of 700-nm-thick a-axis PrBa2Cu3O7-δ films on (100) YBa2Cu3O7-δ single crystals were smooth with a mean roughness of 2.8 nm. X-ray diffraction scans showed that a-axis PrBa2Cu3O7-δ films deposited on (100) YBa2Cu3O7-δ single crystal had good crystallinity. Moreover, the φ-scans of X-ray diffraction for the (102) reflection revealed that a-axis PrBa2Cu3O7-δ film on (100) YBa2Cu3O7-δ single crystal had an in-plane orientation such that the c-axis of PrBa2Cu3O7-δ film aligned along the c-axis of the (100) YBa2Cu3O7-δ single crystal, which suggested the film was well grown epitaxially on (100) YBa2Cu3O7-δ single crystal until it reached 700 nm thickness.

Detwinning YBa2Cu3O7−δ thin films

Single crystals of YBa2Cu3O7-δ (YBCO) have been detwinned under the simultaneous application of heat and uniaxial stress. This approach, however, has proved futile in thin films as they become heavily twinned due to strain from the underlying substrate as well as the orthorhombic unit cell. This lack of control over twinning has made it difficult to determine the role that twins play in determining the transport properties of films. Here, we present a technique for detwinning YBCO thin films. The detwinning in YBCO thin film is achieved by suspending a portion of the YBCO film above the underlying substrate. The suspended material is no longer influenced by the confining effects of the substrate and this allows it to detwin under the application of a uniaxial stress brought about by an anneal. With this technique, we are able to produce high-quality detwinned regions that will help aid the understanding of transport mechanisms in YBCO.

Preparation and Characterization of a-Axis Oriented PrBa2Cu3O7-δ Film Deposited on (001) YBa2Cu3O7-δ Single Crystal

We report on the structural and electrical properties of a-axis oriented PrBa2Cu3O7-δ film deposited on (001) YBa2Cu3O7-δ single crystal. a-Axis oriented PrBa2Cu3O7-δ film, expected to be a barrier layer, was prepared using a dc-95 MHz hybrid plasma sputtered on YBa2Cu3O7-δ single crystals that are superconducting ground planes. The atomic force microscopy image showed that the surface of PrBa2Cu3O7-δ film on (001) YBa2Cu3O7-δ single crystal was smooth with a mean roughness of 2.9 nm. The resistivity of a-axis PrBa2Cu3O7-δ film on (001) YBa2Cu3O7-δ single crystal measured after patterning into the mesa structure was small in comparison with that of a-axis PrBa2Cu3O7-δ film on (100)SrTiO3 substrate and decreased with decreasing temperature.