Delta Single Crystals Research Articles

NMR studies of spin excitations in superconducting Bi2Sr2CaCu2O8+ delta single crystals.

NMR studies of spin excitations in superconducting Bi2Sr2CaCu2O8+ delta single crystals.

Growth of bulk YBa2Cu3O7- delta single crystals and their properties

The effects of the charge composition, temperature-time conditions, value and direction of temperature gradients in a melt on the crystallization of bulk YBa2Cu3O7- delta single crystals have been studied. The morphology, chemical composition, structural, magnetic and resistive properties of these crystals have been characterized. The width of a metastable zone of YBCO is found to be about 0.8 K. At overcooling less than 0.8 K a bulk crystal of polyhedral type grows on a seed. At overcooling greater than 0.8 K a spontaneous crystallization takes place and crystals in the form of thin plates are produced. Orientation figures are defined for as-grown YBCO faces by a chemical etching method. The distribution of the main impurity (Al) is shown to be homogeneous in the volume of the growing crystal (the homogeneity level is 3%), its concentration varying from 0.05 to 0.11 at/FU in different runs. The absence of the block structure in (001) planes of the YBCO single crystal is established using X-ray topography. The contrast unevenness on the (100) topogram indicates that a certain amount of the block structure occurs on (100) planes, comparable with the resolution of the method of 0.02 degrees . The lattice parameters and the M(T) and rho (T) dependences are measured before and after the thermal treatment of crystals.

Comparative study of the peak effect in YBa2Cu3O7- delta single crystals by transport and magnetic measurements.

The peak effect in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals is observed by transport-current measurements. Comparison with the shielding current from magnetization shows good agreement between the results, which exclude granularity as being necessarily involved in the origin of the peak effect. The behavior of the current-voltage characteristics in a wide voltage range (${10}^{\mathrm{\ensuremath{-}}9}$E${10}^{\mathrm{\ensuremath{-}}4}$ V/cm) is analyzed. Different types of scaling are established in magnetic fields below and above the position of the current peak. Possible mechanisms responsible for the peak effect are discussed.

Photoinduced second-harmonic generation of YBa2Cu3O7- delta single crystals

Complex investigations of the photoinduced changes in YBa2Cu3O7- delta ( delta >0.5) single crystals have been carried out using the time-resolved dependences of the photocurrent, photoresistivity, photovoltage and optical second-harmonic generation (SHG). Simultaneously the spectroscopic parameters were monitored by time-resolved Raman and UV-visible reflection spectra. A time delay shift between the SHG and photocurrent dependences was found. The contributions of two different mechanisms in the formation of the superconducting behaviour were unambiguously shown. It was clearly shown that only by simultaneously taking into account the anharmonicity due to the specific electron-phonon interaction together with the traditional Anderson metal-insulator transition we can clarify the main physical mechanisms of the high-temperature superconductivity. Magnetic fields of up to 1 T suppress the SHG signal. The different delay times for the electronic and phonon spectra indicate the essential role of the electron-phonon anharmonic coupling in the high-temperature superconductivity. Some different approaches to explain the observed phenomena are discussed.

3D X-Y scaling of the specific heat of YBa2Cu3O7- delta single crystals.

The specific heat of a single crystal sample of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ has been measured with magnetic fields up to 8 T applied parallel to the c axis of the crystal. The results provide strong evidence for the existence of a critical regime within which there is scaling behavior characteristic of the three-dimensional X-Y model with critical exponents consistent with those observed in superfluid $^{4}\mathrm{He}$.

Magnetic hysteresis loops and fast relaxation of vortex lattice in YBaCuO single crystal

The magnetic moment values /spl Mscr/ at conventional magnetic moment relaxation at constant magnetic field and in magnetic hysteresis loop experiment are directly related and are the same if /spl musub 0/(d/spl Mscrdt)/sub CR/=/spl chisub 0(dB/sub edt)/sub MHL/. Dependencies of /spl Mscr/(dB/sub edt) measured by vibrating sample magnetometer on YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta single crystals at temperatures ranging from 2.5 K to 54 K are analyzed. Activation energy U/sub e/=-kT[ln(dB/sub edt)-C] vs. /spl Mscr/ is plotted with parameter C being adjusted to form a smooth curve. In this way the critical sweep rate (dB/sub edt)c=exp(C) corresponding to the critical state U/sub e/(/spl Mscrsub c/)=0 is determined. A detailed analysis shows that In(dB/sub edt)c increases approximately linearly with temperature. This dependence can be removed by scaling the activation energy by a temperature dependent factor. At temperatures below 5 K departure from the thermally activated relaxation occurs, the origin of which can be the quantum tunnelling effect.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Strong magnetic relaxation toward diamagnetism and evidence for glassy behavior in YBa2Cu3O7- delta single crystals close to Tc.

Magnetic-relaxation measurements have been performed on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals. In a temperature range of \ensuremath{\approxeq}2 K below the transition the diamagnetic magnetization increases with time. This increase is barely noticeable for more than ten minutes and then develops in a nonlogarithmic manner for hours. Controlled small temperature changes applied during a relaxation run cause sharp magnetization responses that are fully analogous to findings for nonmetallic spin glasses. The effect correlates with a quasihysteretic anomaly in field-cooled magnetization found recently.

Comment on "Anomalous peak in the thermopower of YBa2Cu3O7- delta single crystals: A possible fluctuation effect"

In an experiment of Howson et al. the temperature dependence of the thermopower of a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystal was measured with an oscillating heat source. A sharp peak in the temperature dependence of the thermopower in the vicinity of the critical temperature was observed. In contrast, Ri et al., who used a dc technique, could not observe such a peak. We show that the observation of the peak may be an artifact of the ac measuring technique used by Howson et al.

Reply to "Comment on 'Anomalous peak in the thermopower of YBa2Cu3O7- delta single crystals: A possible fluctuation effect' "

Howson et al. reported measurements of the thermopower using an ac optical heating technique. In this reply we show how the signal measured in this experiment is proportional to only the thermopower of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ sample with respect to the Pb reference leads. The argument of Logvenov et al. that the signal has a contribution from the derivative of the thermopower, is shown to be incorrect.

Growth of CaLnBaCu3O7- delta (Ln=La, Pr and Nd) single crystals by the flux technique

Single crystals of CaLnBaCu3O7- delta (Ln=La, Pr and Nd) were grown by the flux technique from a BaO-CuO eutectic melt. The morphological features of the grown crystals were studied using optical microscopy and SEM. Phase formation and single crystallinity were confirmed by X-ray diffraction studies. Superconductivity was characterized by four-probe electrical resistivity measurements.

Effect of gold impurities on the superconducting fluctuations and the upper critical field of YBa2Cu3O7- delta single crystals.

High-resolution measurements of the reversible magnetization of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystals which were grown in gold crucibles and zirconia crucibles reveal that, within the experimental accuracy, gold impurities do not influence the normal-state susceptibility, the upper critical field, nor the superconducting fluctuation phenomena.

Hall effect in the mixed state of Y1-xPrxBa2Cu3O7- delta single crystals.

The magnetic field and the temperature dependence of the mixed-state Hall effect have been measured on single crystals in the system Y[sub 1[minus][ital x]]Pr[sub [ital x]]Ba[sub 2]Cu[sub 3]O[sub 7[minus][delta]] (0.0[le][ital x][le]0.3) with the magnetic field perpendicular to the [ital ab] plane and the current flowing in the [ital ab] plane. A sign change of the Hall resistivity is observed at low Pr concentrations, with the negative Hall resistivity disappearing between the concentrations of [ital x]=0.1 and [ital x]=0.2. The field dependence of the Hall resistivity for various Pr concentrations can be qualitatively explained by incorporating a finite pinning force [bold F][sub [ital p]] in the Nozieres and Vinen (NV) model.

YBa2Cu3O7- delta single-crystal microstructure related to transport critical current density

The structure of YBa2Cu3O7- delta crystals has been investigated by means of high-resolution electron microscopy. The cooperative growth of a number of spirals causes the 'interference' of stresses and the formation of microblocks. It is suggested that the screw dislocations inside the growing spirals as well as the edge dislocations on subgrain boundaries are responsible for the high Jc values. Actually, the highest transport critical current density Jc(77 K) and Jc(H) have been measured on these YBa2Cu3O7- delta single crystals.

The thermopower of YBa2Cu3O7- delta single crystals

The authors present results for the measured thermopower of a number of YBa2Cu3O7- delta single crystals. One of the crystals was untwinned and allowed to measure the thermopower in the a and b crystallographic directions and discuss the contributions to the thermopower from the chains and planes of Cu-O. The thermopower near Tc is fitted to recent models of superconducting fluctuation effects in the thermopower and it is found that the a axis coherence length is almost five times the b axis coherence length. A sharp peak near Tc is nearly always observed and they argue that while this may be a superconducting fluctuation effect it may also be the result of a combination of the a axis and b axis thermopowers which are of opposite sign. The thermopower in the b direction is nearly temperature independent well above Tc, which is consistent with conduction in a narrow band. The a axis thermopower, due essentially to the Cu-O2 planes, is metallic-like and similar to results on other high-Tc systems.

Magnetic properties of Bi2Sr2CaCu2O8+ delta single crystals with variable oxygen content.

The reversible magnetization of Bi 2 Sr 2 CaCu 2 O 8+δ crystals was measured for the Hc direction. Crystals with different stoichiometry were obtained by quenching from different annealing temperatures between 450 o C and 725 o C, resulting in critical temperatures varying from 84 to 93 K. No large variations in the magnetization with T c were found

Temperature dependence of electronic Raman-scattering spectra in superconducting Bi2Sr2CaCu2O8+ delta single crystals.

The electronic Raman scattering of both tetragonal ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ symmetries has been measured in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals as a function of the temperature. We observe that a redistribution of the electronic continuum spectrum below ${\mathit{T}}_{\mathit{c}}$ exhibits two aspects of the temperature dependence: One of them is associated with its energy scale, which has a significant temperature variation only just below ${\mathit{T}}_{\mathit{c}}$. This indicates an abrupt opening of the superconducting gap. The other is associated with the variation of the spectral weight, which shows a more gradual temperature dependence compared with the former. This indicates that a superconducting condensation of thermally excited quasiparticles occurs gradually with decreasing temperature. We also find that the ${\mathit{A}}_{1\mathit{g}}$ and ${\mathit{B}}_{1\mathit{g}}$ spectra exhibit a similar temperature dependence below ${\mathit{T}}_{\mathit{c}}$.

Field-dependence anomaly in critical currents and granularity for flux-grown single-crystal YBa2Cu3O7- delta

Magnetic hysteresis has been measured for a flux-grown YBa2Cu3O7- delta single crystal over the temperature range 4.2-80 K and in applied fields up to 12 T. Critical current densities jc have been derived from these data with the use of the Bean model. At temperatures between 20 and 70 K, it is found that the field dependence of the critical current has a peak in the medium field region. Successive division of the crystal gives a linear dependence of the magnetization M on the length scale R, which indicates that the current circulates around the whole sample. The shape of the magnetic hysteresis curve remains unchanged. Therefore the authors conclude that granularity is not the reason for the anomalous field dependence of the critical current density in their sample.

Time and orientation dependent magnetic measurements and increase of the critical current by fast neutron irradiation of Y1Ba2Cu3O7- delta single crystals

The authors report on magnetic measurements with a SQUID-magnetometer where the tilt angle between the direction of the c-axis and the magnetic field was varied from 0 degrees (B//c) to 90 degrees (B perpendicular to c). The hysteresis loop was recorded in the range of 0 T to 5.5 T at 77 K. Intense flux creep measurements were performed between 0.03 T and 2 T at 77 K. The increase of the magnetic critical current and the change of flux creep were examined after fast neutron irradiation with different doses.

Theoretical analysis of the thermal conductivity of YBa2Cu3O7- delta single crystals.

We analyze the in-plane thermal conductivity of single-crystal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Our theoretical fit provides an excellent description of the experimental data throughout the temperature range 10 KT200 K. From this fit, we can extract information about the lattice microstructure and the electronic contribution to the total thermal conductivity. Our calculations indicate that long-wavelength phonons, with mean free paths of the order of the crystal thickness, contribute significantly to the thermal conductivity up to \ensuremath{\sim}75 K. The phonon-electron coupling strength \ensuremath{\lambda} for the phonons which dominate in the heat-conduction process is calculated to lie in the range 0.030.08. In addition, we extract a value of (1.6\ifmmode\pm\else\textpm\fi{}0.3)${\mathrm{\ensuremath{\Delta}}}_{\mathrm{BCS}}$ for the superconducting energy gap.

Effect of magnetic field on thermal conductivity of YBa2Cu3O7- delta single crystals.

We report measurements of the thermal conductivity in a magnetic field on single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$. Unlike in sintered samples, the zero-field thermal conductivity of these crystals above the superconducting transition temperature shows a marked decrease with increasing temperature, demonstrating the importance of phonon-phonon processes and the high quality of the crystals. We find that the thermal conductivity \ensuremath{\kappa} is very sensitive to the presence of fluxoids which are strong scatterers of phonons. The degradation of \ensuremath{\kappa} is particularly large when the magnetic field H is oriented perpendicular to the Cu-O planes. The anisotropic behavior of \ensuremath{\kappa}(H) is in qualitative agreement with a geometric argument based on the anisotropy of the superconducting coherence length. We detect hysteresis at the level of several percent in the dependence of \ensuremath{\kappa} on the field.