Single Crystals Of YBa2Cu3O7 Research Articles

Q-Dependent neutron scattering: A signature of the gap anisotropy in high-T c superconductors

The influence of the superconducting gap anisotropy on the phonon dispersion is discussed for tetragonal and orthorhombic crystals. Superconducting order parameters with different types ofs-wave, as well asd- and (s+d)-wave anisotropies, are studied. We show that along specific directions of the Brillouin zone the lineshape (energy and linewidth) of phonons strongly depends on the anisotropy of the gap. In this context, we discuss recent experimental results performed on YBa2Cu3O7 single crystals. The data can be well described in terms of an anisotropics-wave gap function obtained from the nontotally screened interaction of charge carriers with long-range optical phonons. The calculations suggest, furthermore, that it is generally insufficient to describe experiments that involve the anisotropy of the superconducting order parameter solely in terms of its symmetry. Not only the symmetry, but also the specific structure of the gap as a function of k is important. This conclusion holds for those experiments that are not restricted to the pure determination of the phase of the gap.

Hall effect in detwinned single crystals of YBa2(Cu1?x Zn x )3O7 ??in the mixed state

The effect of zinc doping on the mixed-state Hall effect in detwinned single crystals of YBa2Cu3O7 −δ has been studied. We observe a sign reveral of the Hall effect in crystals with a superconducting transition temperature (Tc) as low as 71 K. The Hall conductivity in the mixed state follows theC1/H+C2H field dependence. The absolute value of the coefficientC1 increases faster than (1–T/Tc) with decreasing temperature.C2 increases rapidly asT falls belowTc. The temperature dependence of the ratioC1/C2 scales approximately asT/Tc for different levels of zinc doping.

Improvement of the superconducting transition and demagnetization factor in YBa2Cu3O7−δ single crystals by laser cutting

Single crystals of YBa2Cu3O7−δ have been cut into disc shapes using an excimer laser. The bulk superconducting transition was found to be sharpened in all cases measured. This is associated with the removal of material at the edges which may be depleted of oxygen or otherwise damaged. The magneto-optical images for one crystal have been correlated with the optical image. The angular dependence of the demagnetizing factor for field applied in the plane was drastically reduced in disc-shaped crystals relative to rectangular ones. The field of first flux entry was also increased.

Compact-heating stage for use in sputtering in active oxygen gas environments

An easily movable compact heating stage for heating substrates in an oxidizing atmosphere up to a temperature of about 700 °C is described. A resistive heating element of Kanthal (72.5Fe-22Cr-5.5A1 wt %) wire is used. The heating element is mounted inside a stainless steel vessel to isolate it from active oxygen gas environments. The vessel is assembled in air of one atmospheric pressure at room temperature. Substrates are contacted for heating on the top plane of the vessel with a stainless steel substrate holder. In order to show the performance of the heating stage, the resistive transition property of a single crystal YBa2Cu3O7–x thin film formed by rf magnetron sputtering on a MgO (100) substrate heated by this stage is demonstrated.

High resolution characterization of structural inhomogeneities in YBa2Cu3O7−δ crystals with sharp superconducting transitions

Structural inhomogeneities in two single crystals of YBa2Cu3O7−δ are characterized using high resolution x-ray diffraction. Results of these analyses show a distribution in the c-axis lattice parameters. These are interpreted as evidence of a distribution in oxygen content. Values of 7−δ range from 6.71 to 7.00 in one crystal and from 6.80 to 6.99 in the other. Nevertheless, the measured superconducting transition widths for both crystals are less than 0.5 K, as determined by ac susceptibility. When examined with conventional x-ray diffraction techniques, both crystals also appeared to be free of gross defects, but high resolution rocking curves reveal variations in defect density. These results suggest that crystals presumed to be homogeneous based on narrow superconducting transition widths or conventional x-ray diffractometry, may not be. Therefore, crystals found to exhibit exotic physical properties should be carefully examined for the presence of structural anomalies. If such defects are present, the experimental results may be attributed to sample imperfections, rather than intrinsic phenomena.

Irreversibility line of YBaCuO sintered ceramics, YBaCuO single crystal and superconducting composites studied by AC susceptibility and transport measurements

It is shown that the onset of the third harmonic susceptibility is the only appropriate criterium, when using AC susceptibility measurements, to determine the IL. Then, we show that the IL strongly depends on the crystallinity of superconductors and on the superconductor volume fraction in the composites. Besides, it is demonstrated, from transport measurements, that the IL corresponds to the J c = 0 line.

Temperature dependence of the magnetic hysteresis of a YBa2Cu3O7−δ single crystal

Magnetic hysteresis loops (MHL) have been measured on a single crystal of YBa2Cu3O7−δ using a QD MPMS SQUID magnetometer. The MHLs were recorded in the temperature range 4.4 to 70 K in magnetic fields up to ±5 T. A ‘fishtail’ shape of the MHL was observed at temperatures between 10 and 65 K. Position of the fishtail maximum scales well with (1−T/Tc)1.78 but the corresponding magnetic moment value does not scale to a single temperature function. The MHL size drops much more rapidly with increasing temperature atT 55 K the character of MHL changes strongly and it seems to become controlled by surface barrier effects.

Different flux pinning regimes in YBa2Cu4O8 single crystals

Magnetization studies on single crystals of YBa2Cu4O8 were performed in a wide range of temperature, in magnetic fields up to 50 kOe. The studied crystals exhibit weak pinning-FC susceptibility 4πχ=−0.75. The different flux pinning mechanisms dominate in different field and temperature regimes. In higher magnetic fields the pronounced fishtail like shape of the magnetization hysteresis loops is observed. In low magnetic fields strong influence of surface barriers on flux pinning is noticed. Pronounced differences in relaxation rates are found for different flux pinning regimes.

Temperature dependence and anisotropy due to twin planes of the critical current inab-plane

We report the observation at different currents of voltage-angle characteristics (VAC) of flux creep in YBaCuO single crystals with unidirectional twins in the presence of the magnetic fieldH=1.5 T applied ina−b plane and directed in a small angle range near twin planes (TP). From the simple phenomenological analysis of the VAC measured at fixed temperature nearT c we derive the activation energy of flux creep, the critical angle θ*, the transport critical current angle dependencej c (ϕ) and also the value of critical current anisotropy for H‖TP and HτTP.

Observation of defect-independent ab-plane vortex dynamics in YBa2Cu3O7 single crystals with the third harmonic transmittivity measurements

Anisotropic vortex dynamics of YBa2Cu3O7 single crystals with heavy-ion induced parallel and canted columnar defects are investigated and compared with that of the as-grown single crystals. Novel vortex dynamics are observed when magnetic field is applied close to the ab-plane\((\vec H \bot \hat c)\). The amplitude of the third harmonic transmittivity exhibits a secondary peak at lower temperatures in addition to a sharper peak at higher temperatures. The possible relevance of this observation to the theoretical model of “smectic crystal” to vortex-liquid phase transitions is discussed.

Weakly increasing anisotropy in Zn doped YBa2Cu3O7−δ

The electrical resistivity of single crystals of YBa2Cu3O7−δ with 1 and 3.5% Zn replacing copper has been studied in magnetic field up to 12 T parallel to thec-axis. From the onset ofc-axis resistance, ocurring at a higher temperature than for the in-plane resistivity, the phase diagram can be determined which separates the region of intact vortex tubes from that of decorrelated vortex segments moving independently in differentab-planes of the crystal. This phase separation occurs at a larger magnetic field for comparable temperatures and concentrations when compared to Fe-doped Y-123. This result and a quantitative description in terms of a published model for Josephson tunneling between parallel planes, show that Zn-doping increases anisotropy only weakly in contrast to Fe-doped Y-123.

In-plane torque and gap symmetry of untwinned YBa2Cu3O7 crystals

The reversible magnetic torque of untwinned YBa2Cu3O7 single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thedx2−y2 symmetry and rule out the possibility of thedxy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\).

The absolute efficiency of magnetic Raman scattering in RBa2Cu3O x (R=Y,Pr)

Wide range Raman spectra have been measured on single crystals of YBa2Cu3O6.3 and PrBa2Cu3O6.9 for several scattering geometries and laser excitations. The absolute spectral density was obtained by comparing the phonomic resonances with reported absolute Raman efficiencies. In B1g symmetry the 2-magnon Raman signal was observed in both materials. Integrating the spectral densities the Raman efficiencies of the 2-magnon signal is determined for the different laser excitations. The experimental values are compared to absolute scattering rates calculated in the 3-band Hubbard model. A Raman vertex containg charge transfer excitations into the oxygen band and two copper spin exchange processes is used.

Electromagnetic field induced dielectric resonance in the YBa2Cu3O7 single crystal

First observations of the microwave dielectric resonance in a single crystal of the YBa2Cu3O7 are presented. With the YBa2Cu3O7 single crystal inserted into the microwave (f∼9 GHz) resonator we observe the appearance of the new intense and broad transmission band. The transmittivity value of this band exhibits remarkable temperature hysteresis. The data show that the observed birefringence is the intrinsic property of the HTSC material resulting from the spatial dispersion and nonlinearity of HTSC electric properties due to the metal-insulator-type instability.

Thermal conductivity of oxygen-deficient YBCO: Evidence for the spin-phonon coupling

We have studied the in-plane thermal conductivity in oxygen-deficient single crystals of YBa2Cu3O6+x (0<x<1). Strong temperature dependence of K(T) observed in antiferromagnetic YBa2Cu3O6 near the Neel temperature indicates the strong coupling of the acoustic phonons with localized Cu spins. In the underdoped superconducting crystals the systematic evolution of the metallic normal state K(T) correlates with pseudo-gap behavior of the spin excitation spectrum. This suggests that the upturn in K(T) below Tc may arise from the spin-gap opening.

Effects of impurities on the in-plane thermal conductivity of insulating YBa2Cu3O6

We have measured the in-plane thermal conductivity K(T) for insulating single crystals of YBa2Cu3O6 doped with Er atoms in the Y site and Ni and Zn in the Cu(2) site. Thermal conductivity of pure YBa2Cu3O6 indicates a significant additional heat resistance at temperatures near the antiferromagnetic transition and below 200 K. Erbium impurities increase the phonon scattering, the effect being most pronounced at T 15 K even for concentrations less than 2.5 at %. The experimental data suggest that acoustic phonons are strongly coupled with spin excitations in CuO2 planes.

Splayed configurations of columnar defects in YBa2Cu3O7, (Tl, Bi)Sr2CaCu2O7 and Bi2Sr2CaCu2O8 single crystals study of the enhanced pinning efficiencies for different characteristic angles

Splayed configurations of columnar defects with different characteristic angles θi relative to the c-axis have been achieved in single crystals of YBCO, (Tl, Bi)-1212 and Bi-2212. Their pinning efficiencies for Hc have been studied by magnetic measurements. An increased pinning efficiency consistent with the splayed model of Hwa et al. [Phys. Rev. Lett. 71 (1993) 3545] has been observed with θi=10° for YBCO and (Tl, Bi)-1212. On the other hand, the effect is deterimental for the more anisotropic compound Bi-2212. Moreover, higher angles up to θi=75° have yielded further pinning improvements in all the compounds for high temperatures and low fields, but these latter results are more delicate to interpret.

High-Q oscillator measurements of vortex properties in YBa2Cu3O7??

High-Q oscillators, vibrating with extremely small tilt angles, ∼10−7 radians, were used to study vortex behavior in single crystal YBa2Cu3O7−δ. Systematic changes in the dissipation and frequency of the oscillator were explored for various fields both parallel and perpendicular to twin boundaries. At the small tilt oscillation amplitudes employed here, a number of features were observed which are not evident at the more typically utilized large oscillation amplitudes.

Thermal conductivity of Zn, Pr and Tb doped YBCO single crystals: Theory and experiment

The thermal conductivity K(T) vs temperature has been measured for (Y,Pr)Ba2Cu3O7, (Y,Tb)Ba2Cu3O7 and YBa2(Cu,Zn)3O7 single crystals from 10≤T≤280K. For YBa2Cu3O7(YBCO), a strong enhancement in K(T) is observed for T≤Tcwith a peak in K(T) at approximately T≈Tc/2 with a sharp change in slope of K(T) at Tc. These results are similar to those reported previously. For the selectively doped YBCO, the peak in K(T) is shifted to lower temperatures and occurs at T≈ 20K independent of Tc and the impurity concentration. In addition, the sharp change in slope of K(T) at Tc is not apparent for the doped crystals. These results are discussed along with a theoretical model for electronic conduction including both impurity and inelastic scattering from spin fluctuations.

Long-lived induced Raman bands, appearance and ? 4 cm?1 shift of ?500? band as a result of pulsed IR(520?640 cm?1) laser irradiation on YBaCuO single crystals

Single crystal YBa2Cu3O6.4¯E∥c irradiation by CO2-Ar laser caused (1) a shift of the “500” Raman band (480–490 cm−1), which means the appearance of the superconducting phase in the focus spot (proved by preliminary results of the inductive method); (2) the increase in intensity of many weakly lived Raman bands that usually look like “pedestals” of the “500” band (they were observed before as strictly reversible Ar+ laser photoinduced bands). The additional (∼10 components) structure was developed in the region 400–600 cm−1 on the Raman spectra by comparison of spectra for samples before and after irradiation. By reconstruction of such spectra we analyzed the composition of the sample, which was assumed to be a mixture of microregions with different oxygen content.