Single Crystals Of YBa2Cu3O7 Research Articles

EFFECT OF PRESSURE ON THE METAL-INSULATOR TRANSITION IN YBa2Cu3O7-δ SINGLE CRYSTALS

In present work we have investigated the effect of high hydrostatic pressure on conductivity in the ab-plane of the oxygen underdoped HTSC YbaCuO single crystals. It is shown, that the decreasing of oxygen concentration in sumples of YBa2Cu3O7-δ leads to enhancement of the localization and realization in the system of the metal – insulator transition, which always precedes the superconducting transition. Increasing the preassure leads to a significant shift of the metal – insulator transition to low temperatures.

Pressure effect on the electrical resistance of Y0.77Pr0.23Ba2Cu3O7-δ single crystals

The effect of hydrostatic pressure up to 12 kbar on the electrical resistance in the basal ab-plane of optimally oxygen-doped (δ<0.1) single crystals Y1–xPrxBa2Cu3O7–δ moderately doped with praseodymium (x≈0.23) with a critical temperature Tc≈67 K. Compared to undoped single-crystal YBa2Cu3O7–δ, doping with praseodymium led to a decrease in the critical temperature by ≈24 K with a simultaneous increase in ρab (300 K) by ≈130 μΩcm. In the region of the transition to the superconducting state, several clearly pronounced peaks are observed on the dρ/dT – T curves, which indicates the presence of several phases with different critical temperatures in the sample. The application of high hydrostatic pressure leads to an increase in Tc by about 3 K. This increase slows down with increasing pressure, and the baric derivatives, dTc/dP, decrease from 0.44 K/kbar at atmospheric pressure to 0.14 K/kbar at 11 kbar. The comparatively weak change in the critical temperature under the action of hydrostatic pressure is due to the relatively small value of the orthorhombic distortion, (a–b)/a. The change in the baric derivative dTc/dP upon all-round compression of the sample is due to the fact that, along with an increase in the Debye temperature, the matrix element of the electron-phonon interaction also increases. Possible mechanisms of the effect of high pressure on Tc are discussed taking into account the presence of features in the electronic spectrum of carriers.

Redistribution of oxygen ions in single crystal YBa2Cu3O7-x owing to external hydrostatic pressure

The effect of high hydrostatic pressure on the temperature dependences of the electrical resistance in the basal plane of single crystal YBa2Cu3O7-x with an oxygen deficit is studied. It is found that an external hydrostatic pressure P ≈ 7 kbar substantially intensifies the diffusive coalescence of oxygen clusters, i.e., causes an increase in their average size. This, in turn, produces an increased number of negative U-centers whose presence leads to the appearance of a phase capable of generating paired carriers of electrical charge and is, therefore, characterized by a higher transition temperature Tc. Changes in the form of the temperature and time dependences of the electrical resistivity under external hydrostatic pressure are discussed in terms of this same hypothesis regarding the mechanism of diffusive coalescence of oxygen clusters.

Investigating the Effect of the Magnetic Field on the Thermal Conductivity of a Single Crystal of YBa 2 Cu 3 O 7−δ

The effect of the magnetic field on thermal conductivity of a single crystal of YBa2Cu3 O 7 − δ was investigated using London’s equation. The result of the study shows that the thermal conductivity of YBa2Cu3 O 7 − δ decreases exponentially as the applied magnetic field increases at a uniform temperature and then ‘saturates’. This indicates that magneto-thermal conductivity of YBa2Cu3 O 7 − δ is necessitated by electrons which are scattered by the core of vortices in the mixed state.

Networks of superconducting nano-puddles in 1/8 doped YBa2Cu3O6.5 + y controlled by thermal manipulation

While it is known that the nature and the arrangement of defects in complex oxides have an impact on the material functionalities, little is known about control of superconductivity by oxygen interstitial organization in cuprates. Here we report direct compelling evidence for the control of Tc by manipulation of the superconducting granular networks of nanoscale puddles, made of ordered oxygen stripes, in a single crystal of YBa2Cu3O6.5 + y with average formal hole doping p close to 1/8. Upon thermal treatments we were able to switch from a first network of oxygen defect striped puddles with OVIII modulation (qOVIII(a*) = (h + 3/8, k, 0) and qOVIII(a*) = (h + 5/8, k, 0)) to a second network characterized by OXVI modulation (qOXVI(a*) = (h + 7/16, k, 0) and qox-VI(a*) = (h + 9/16, k, 0)) and finally to a third network with puddles of OV periodicity (qOV(a*) = (4/10, 1, 0) and qOV(a*) = (6/10, 1, 0)). We map the microscopic spatial evolution of the out of plane OVIII, OXVI and OV puddle nano-size distribution via scanning micro-diffraction measurements. In particular, we calculated the number of oxygen chains (n) and the charge density (hole concentration p) inside each puddle, analyzing areas of 160 × 80 μm2, and recording 12 800 diffraction patterns to reconstruct each spatial map. The high spatial inhomogeneity shown by all the reconstructed spatial maps reflects the intrinsic granular structure that characterizes cuprates and iron chalcogenides, disclosing the presence of several complex networks of coexisting superconducting domains with different lattice modulations, charge densities and gaps as in the proposed multi-gap scenario called superstripes.

Vortex lock-in transition coinciding with the 3D to 2D crossover in YBa2Cu3O7

A vortex lock-in transition was directly detected by torque magnetometry in an overdoped YBa2Cu3O7 single crystal of low anisotropy (\approx 7). The locked-in state was observed below the 3D to 2D crossover temperature Tcr = 76 K, independently of extrinsic pinning effects thanks to a high quality clean crystal and the use of a vortex shaking technique. The shape of the torque signal as a function of the angle between the applied magnetic field and the crystallographic c-axis is in very good agreement with the model developped by Feinberg and Ettouhami [Int. J. Mod. Phys. B 7, 2085 (1993)] for quasi-2D superconductors, despite the low anisotropy of the material.

Magnetic torque study of the temperature-dependent anisotropy parameter in overdoped superconducting single-crystal YBa2Cu3O7

An overdoped YBa2Cu3O7 single crystal was studied by SQUID and torque magnetometry in order to investigate the temperature dependence of the anisotropy parameter close to the transition temperature T_c (0.87 T_c < T < T_c). Angle dependent torque measurements were performed and analyzed with the widely used Kogan model [Phys. Rev. B 38, 7049 (1988)] as well as with an extended model by Hao and Clem [Phys. Rev. Lett. 67, 2371 (1991)], taking into account the influence of the vortex cores on the magnetization. Both approaches yield similar results, with an out-of-plane anisotropy parameter around 6.5 which slightly increases with decreasing temperature, and a temperature independent in-plane anisotropy parameter gamma_ab}= 1.12(5).

Paramagnetic Meissner effect at high fields in YCaBaCuO single crystal and melt-textured YBaCuO

We report on systematic field-cooled magnetization experiments in an Y0.98Ca0.02Ba2Cu3O7−δ single crystal and melt-textured YBa2Cu3O7−δ samples. Magnetic fields up to 50kOe were applied and a paramagnetic response related to the superconducting state was observed. We discuss our results in terms of the flux compression scenario into the sample.

Magnetoresistance and 2D-3D crossover in aluminum-doped YBa2Cu3−zAlzO7−δ single crystals with a system of unidirectional twinning boundaries

The effect of a constant magnetic field to 12.7kOe on the temperature dependences of the electric conductivity of aluminum-doped YBaCuO single crystals with a system of unidirectional twinning boundaries is investigated. The temperature dependences of the excess paraconductivity and the field dependence of the coherence length ξc(0,H) along the c axis are determined in the basal ab plane. It is determined that the twinning boundaries are effective centers of scattering of fluctuation carriers. The temperature dependences of the excess paraconductivity are interpreted on the basis of the Hikami-Larkin theoretical model of fluctuation conductivity for layered superconducting systems. The reasons for the suppression of three-dimensional superconducting fluctuations and the nonmonotonic dependence of ξc(0,H) in weak magnetic fields with the magnetic field vector oriented along the c axis are discussed.

Temperature dependence of the pseudogap in aluminum and praseodymium-doped single crystals

Abstract We have investigated the conductivity in the basis plane of YBaCuO single crystals slightly doped by Al and Pr with a pre-specified topology of twin boundaries. The excess conductivity for the analyzed samples shows dependence Δ σ ∼ ( 1 - T / T * ) exp ( Δ ab * / T ) in a wide temperature range T f T T * , where T * is the mean field temperature of superconducting transition. The temperature dependence of the pseudogap can be satisfactorily described in terms of the BCS–BEC crossover theoretical model.

Anisotropy of the vortex structure and resistivity in the basal plane of YBa2Cu4O8 single crystals

The vortex lattice of the YBa2Cu4O8 high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρa/ρb measured at temperatures from Tc to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.

Investigation of YBa2Cu4O8 single crystal growth by KOH flux

The growth of YBa2Cu4O8 single crystals was performed using KOH flux under ambient pressure. By testing five sorts of cruciblematerial, ZrO2 and Al2O3 are found to be suitable for the growth. A stepwise-cooling method was applied to increasethe size of the crystals. The as-grown crystals could be separated from the residual flux bymethanol solvent. The crystals showed a plate-like morphology with dimensions1.2 × 0.5 × 0.3 mm3. Study of the structure in the (001) plane indicated that the formation of the crystal occurredby a two-dimensional nucleation or spiral growth mechanism, depending on the supersaturationof the solution. Defects in the crystals were revealed by etching, which exhibited aninteresting feature of etch pits with an orthorhombic structure consisting of {001}, {100} and{010} faces. The superconducting property of the crystals shows very sharp transition at aTc of 82.9 K.

Effect of the redistribution of labile oxygen on the pseudogap state in untwinned single crystal YBa2Cu3O7−x

The influence of room-temperature annealing on the excess conductivity of untwinned single crystal YBa2Cu3O7−x with a small deviation of from oxygen stoichiometry is investigated. It is found that the range of existence of the pseudogap regime undergoes a relative expansion of the range of existence of the pseudogap regime during a four-day period of annealing. The absolute magnitude of the pseudogap decreases by approximately 10%.

On the through-thickness critical current density of an YBa2Cu3O7−x film containing a high density of insulating, vortex-pinning nanoprecipitates

Using sequential ion milling the authors have studied the thickness dependence of the critical current density Jc(H) of a single crystal 1μm thick YBa2Cu3O7−x thin film containing ∼5vol% of insulating Y2BaCuO5 (Y211) nanoparticles in order to better understand how to obtain high critical currents in thick films. Except very near the interface where the defect density was enhanced, Jc(H) in the body of the film was uniform and independent of thickness with a high maximum pinning force of 8.8GN∕m3 at 77K. The authors conclude that the nanoscale Y211 precipitates result in strong, three-dimensional pinning characterized by a pin spacing of ∼30nm, much smaller than the film thickness.

Vortex phase diagram near the lower critical point in untwinned YBa2Cu3Oy single crystals

In order to study the vortex phase diagram near the lower critical point Hlcp of the first-order melting transition, we have measured resistivity ρ(H,T) and magnetization M(H) in untwinned YBa2Cu3O7 single crystals. Below Hlcp, the resistive transition is described by the vortex glass theory and the relatively small value of the critical exponent can be explained by the recent theory of the vortex glass scaling which takes into account the pinning disorder strength. The partial loop measurements of M(H) with repeated field cycling provide evidence of the first-order transition at the onset of the peak effect H∗(T) between the Bragg glass and the vortex glass phases.

Excess conductivity and pseudo-gap state in YBCO single crystals slightly doped with Al and Pr

In this work we investigate the conductivity in the base plane of YBa2Cu3O7 (YBCO) single crystals with slight (up to 5%) aluminium (Al) and praseodymium (Pr) doping and with a given topology of plane defects. The results indicate that the doping with Al and Pr results to the increase of the number of the effective scattering centers of the normal carriers. The excess conductivity for the analyzed single crystals, within a wide temperature interval, is described by an exponential temperature dependence. Near the critical temperature (T c) it is well approximated by the Aslamazov–Larkin theoretical model. A small doping with Pr enhances a significant narrowing regarding the temperature interval in which the pseudo-gap (PG) is realised. On the contrary, the Al doping with the same concentration results to the expansion of the temperature interval of excess conductivity existence.

Microwave response of single crystal YBa2Cu3O7−δ films as a probe for pairing symmetry

Temperature dependences of the microwave surface impedance, Zs(T), are measured in the c-axis oriented single-crystal high-Tc superconducting cuprate YBa2Cu3O7−δ (YBCO) thin films deposited by the off-axis dc magnetron sputtering onto CeO2-buffered single-crystal sapphire substrates (film thickness is d≈150, 300, 480nm). Measurements are performed by a use of the coplanar resonator as well as the end-plate cylindrical cavity resonator techniques at several discrete frequencies within the range 5–134GHz. The measurements have revealed unexpected peculiarities on the Zs(T)-dependences for the most perfect films under study. The peculiarities appear to be most strongly pronounced on the temperature dependences of the film surface resistance Rs(T)=Re{Zs(T)}. The most important features of the unusual surface resistance behavior are: (i) the temperature dependence Rs(T) of the YBCO films under study at low temperatures obeys the exponential law: Rs(T)=Rres+R0exp[−Δs∕T] with a small energy gap Δs(Δs≈0.5Tc at f=5GHz); (ii) the most perfect films reveal a distinct two-peak structure of the Rs(T) dependence with peaks positioned at 27–30K and 48–51K, while such peaks are not observed in less perfect films. The peaks are mostly pronounced at moderate (e.g., 34GHz) frequencies and gradually disappear both at higher and lower frequencies, while their temperature positions remain unchanged. These features of perfect single-crystalline YBCO films are believed to reveal their intrinsic electron properties. Taking into account the possibility of a mixed (s+id)-type pairing symmetry as well as a significant role of extended out-of-plane crystal defects (such as dislocation lines or twin planes) in Bogolyubov’s quasiparticle scattering within the most perfect YBCO films, one can suggest a consistent explanation for the anomalies observed in the Zs(T) behavior.

Measurements of the fluctuation magnetization around TC in a YBa2Cu3O7 crystal: Determination of the critical-region width

We present measurements of the fluctuation-induced magnetization around the superconducting transition in a YBa2Cu3O7 single crystal. The data were analyzed in terms of the Ginzburg–Landau theory for multilayered superconductors. The simultaneous analysis in the low magnetic field region above TC0 and in the high magnetic field region around TC(H) allowed us to obtain accurate information on the critical region width (which was found to be in excellent agreement with the prediction of the Ginzburg criterion) and also on the ratio of Josephson couplings between the different superconducting layers.

Surface modifications in single crystal surfaces of YBa2Cu3O7 − δ upon high energy ion irradiation

Atomic force microscopy investigations on swift heavy ion (200MeV Au) irradiated surfaces of a high Tc single crystal YBa2Cu3O7−δ are presented. Results obtained revealed an ion-induced erosion/sputtering clearly confirming our earlier observation on grain boundary dominated thin films. Apart from sputtering, notable effects were seen with many defect structures like dikes/hillocks surrounded by craters, dikes, holes, pearl like structures and ripple formation of sub-micron undulations, all in one crystal. Results are discussed in the light of co-operative phenomena of material re-distribution mechanism related to mass transfer and crater formations.

High-resolution measurements of the magnetic penetrationdepth on Yba2Cu4O8 single crystals

Abstract In order to unveil the intrinsic electrodynamic response of the CuO2 layers of cuprate superconductors and to differentiate this response from that of the charge reservoir layers, we measured the in-plane (ab) magnetic penetration depth on Yba2Cu4O8 (Y124) single crystals by using a high-resolution mutual inductance technique. Among the various high-Tc families of cuprate superconductors, Y124 is a stoichiometric compound characterized by a double CuO chain per unit cell acting as charge reservoir and by the absence of any disorder or structural inhomogeneities arising from chemical substitutions and/or oxygen vacancies. This feature makes Y124 an ideal system for our study. The experimental data show, in the low temperature region down to 4.2 K, a marked linear dependence of the variation Δλab≡λab(T)−λab (4.2 K) with a small downturn at 9 K. This behavior confirms the existence of zero-energy thermally activated excitations, that are also observed in the Yba2Cu3O7−δ (Y123) counterpart possessing only one CuO chain per unit cell. Whilst such linear behavior is consistent with the d-wave symmetry picture previously established for cuprates, the measured thermal activation rate, ∝∂λab/∂T, is however, more than a factor 4 larger in comparison with Y123. This result is consistent with a proximity model of superconducting planes and metallic chains and suggests that the reservoir layers strongly affect the low temperature behavior of the magnetic penetration depth in cuprates.