Single Crystals Of YBa2Cu3O7 Research Articles

Optical conductivity in orthorhombic d-wave superconductors

The large anisotropies observed in the in-plane transport properties of untwinned single crystals of YBa2Cu3O6.95reside in the orthorhombic nature of its band structure which includes CuO chains as well as CuO2planes. An infinite band model with distinct effective mass in a and b-direction is used to study the a.c. conductivity of a d-wave BCS superconductor. A subdominant s-wave admixture is also included in the gap because both symmetries belong to the same irreducible representation in an orthorhombic system. Parameters are fit to experimental data.

Intrinsic pinning in the layered superconductor YBa2Cu3O7 studied by the conversion of ultrasonic to electromagnetic waves

Intrinsic pinning in a single-crystal YBa2Cu3O7 superconductor has been investigated through the conversion of ultrasonic to electromagnetic waves. The interaction with a longitudinal ultrasonic wave was dramatically enhanced as a result of the intrinsic pinning of the vortex so as to form a sharp peak when a dc magnetic field was applied nearly parallel to the superconducting plane. Not only the height but also the angle width of the peak decreased with the temperature increase because of the weakening two-dimensionality. The crossover between the intrinsic pinning and the point-defect pinning is discussed by taking into account the pinning strength.

Experimental observation of a new galvanomagnetic effect in YBaCuO single crystals with unidirected twins

The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in magnetic fields up to 12 T. Unidirected twin planes are the cause of anisotropic pinning of vortices. In contrast with the measurements of the usual longitudinal magnetoresistance of the isotropic sample which is even with respect to reversal of the direction of the magnetic field, we have observed an additional, odd with respect to reversal of the magnetic field, contribution to the longitudinal resistivity which is due to the presence of unidirected twins in the YBaCuO crystals. A theoretical discussion of the effect is presented.

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.

Observation of an odd longitudinal magnetoresistance in YBa 2Cu 3O 7− δ single crystals with unidirected twins

Unidirected twin planes are the cause of anisotropic pinning of vortices in YBa 2Cu 3O 7− δ (below YBaCuO) single crystals. In contrast to the case of only isotropic pinning, two new effects may appear due to the pinning anisotropy: guiding of vortices along twin planes and anisotropy of the Hall resistivity. The former leads to the appearance of the even (with respect to the reversal of a magnetic field direction) transverse resistivity and influences the Hall angle, while the last causes new additional odd contribution to the longitudinal resistivity. The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in the magnetic fields up to 12 T. Field inversion was used for separation of the even and odd components of the measured transverse and longitudinal voltages in order to select new contributions to the resistivity in the case of anisotropic pinning. The results are consistent with the model of anisotropic pinning where the anisotropy of the Hall drag coefficient is caused by twins.

Surface barrier and magnetic hysteresis of ac permeability in YBaCuO single crystal

The nature of hysteretic behavior of the flux line lattice (FLL) contribution to ac magnetic permeability ( μ v) is analyzed for the case of YBa 2Cu 3O x single crystal (at applied magnetic field H⊥ c axis). It is shown that hysteresis loops μ v( H) corresponding to different temperatures ( T=70–84 K) are scaled to a universal curve in normalized coordinates. Such a behavior is interpreted in terms of the FLL interaction with the crystal surface. The explicit relationship between μ v and magnetic induction B is found for the near-surface region of the superconductor. It is shown that the μ v( H) loops are closely related to the hysteresis of B at cycling of applied magnetic field. The latter hysteresis stems from the Bean–Livingston surface barrier. The estimates demonstrate strong suppression of the surface barrier in YBa 2Cu 3O x crystal in comparison to that expected for the ideal surface. As a result, the lower branch of the hysteresis loop corresponding to the increasing field is very close to the equilibrium μ v( H) curve and the surface barrier appreciably affects only the upper branch when magnetic flux leaves the sample. The comparison of theoretical predictions and experimental data provides an opportunity to refine the actual range of stability H max( B)– H min( B) for the FLL at fixed B for YBa 2Cu 3O x crystal in the case of H⊥c.

Raman scattering in YBa2Cu3O7 single crystals: Anisotropy in normal and superconductivity states

Precise temperature and polarization dependences of Raman spectra have been investigated for fully oxygenated twin-free YBa2Cu3O7 single crystals. We have found a striking superconductivity-induced x−y anisotropy in the temperature behavior of the 340 cm−1 line: the magnitudes of the softening and broadening are quite different in the xx-and yy-polarizations. This anisotropy suggests a contribution of the CuO-chain superconductivity with a pairing symmetry different from that for the CuO2 plane, or indicates that the superconducting gap amplitudes are different in the kx and ky directions. The d+s gap symmetry is the only realistic symmetry in the case of Δx≠Δy.

Resistive studies of creep controlled by plastic deformation of a vortex lattice

Current–voltage characteristics (IVC) are measured for YBaCuO single crystals in a magnetic field applied at 45° to twinning planes. The current- and field dependences of the pinning potential are derived from the IVC curves. The obtained dependences indicate that the flux creep in the range of relatively strong magnetic fields is controlled by the plastic deformation of the vortex lattice.

Pinning and dynamics of magnetic flux in YBaCuO single crystals for vortex motion along twin boundaries

Current-voltage characteristics of YBaCuO single crystals are studied for a Lorentz force aligned along the twin planes. The temperature dependences of the depinning critical currents are determined in magnetic fields that are parallel or tilted with respect to the twin planes for magnetic field vectors oriented almost along axis c. A nearly 50% decrease in the critical current in parallel magnetic fields is attributed to the plastic flow of vortices along channels formed by twin boundaries. It is shown that the resistance to the viscous flux flow is described quite correctly by the Bardeen–Stephen model. The peculiarities in the current and temperature dependences of the differential resistivity at temperatures higher than the melting point but lower than the temperature of onset of vortex pinning by twin boundaries are attributed to the coexistence of solid and liquid vortex phases in this temperature region. A large number of peaks observed in the current dependences of the differential resistivity in a parallel field at temperatures below the melting point of the vortex lattice is associated with nonuniform indirect vortex pinning caused by the fluctuations of the separation between twins.

Local magnetometry at high fields and low temperatures using InAs Hall sensors

We characterize the temperature (0.3⩽T⩽300 K), magnetic field (0⩽H⩽80 kOe), and thickness (0.1, 0.5, and 2.5 μm) dependence of the Hall response of high purity InAs epilayers grown using metalorganic chemical vapor deposition. The high sensitivity, linearity, and temperature independence of the response make them attractive for local Hall probe magnetometry, and uniquely qualified for high field applications below liquid helium temperatures. As a stringent test of performance, we use a six element micron-sized array to monitor the internal field gradient during vortex avalanches at milliKelvin temperatures in a single crystal of YBa2Cu3O7−δ.

Anomalous behavior of the structural parameters of YBa2Cu4O8 single crystals at the transition to the superconducting state

High-precision x-ray crystallographic studies of YBa2Cu4O8 single crystals (T c =70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state.

Investigation of the current-induced nonlinear vortex dynamics in YBa2Cu3O7 single crystals using transport and transmittivity measurements

The current-induced nonlinear vortex dynamics of high-temperature superconductors are investigated in two YBa2Cu3O7 single crystals: one untwinned and the other with canted columnar defects. Both the electric transport properties and harmonic transmittivities demonstrate unique current-driven effects associated with the intrinsic pinning forH¯⊥ĉin layered superconductors, in contrast to the defect-dependent current-driven effects forH¯|ĉ.

Influence of twins in the vortex pinning in YBaCuO single crystals

We have studied the magnetization hysteresis loop of twined and detwined YBaCuO single crystals. We investigated the temperature dependence, angular and time dependence of the magnetization. Comparisons between the hysteresis loops of the two samples are made and conclusions concerning the twin pinning are drawn.

Bose-glass and entanglement - disentanglement transitions in YBa2Cu3O7

The Bose-glass transition field BBG in YBa2Cu3O7 single crystals with columnar defects is tracked by the onset of the third harmonic generation. The progressive upward shift of the BBG(T)-line with the irradiation dose is followed by a saturation for the matching fields BΦ = 20 kG – 100 kG. The uppermost position of the Bose-glass transition is interpreted as a boundary between entangled and disentangled vortex liquids in virgin YBa2Cu3O7.

Mechanical measurement for phase transition of vortex at low field in high-Tc cuprates

The resonance frequency ω and internal friction (IF) Q−1 of YBaCuO single crystal, Bi system, Hg system and Tl system polycrystalline were measured between 77k and 120k in different applied field up to 0.5T. A dissipation peak can be found below Tc in all samples. In order to clarify the mechanism of the dissipation peak, the measurement were performed as functions of different vibrating frequency and vibrating amplitude. No obvious dissipation peak shift occurs in the frequency range from 102 to 103 Hz, the dissipation peak height is obviously frequency dependent and satisfies the power law Q−1 ∝ω−1. This is speculated to originate from phase transition associated with the melting or a glass-transition of vortex. These results provide a fundamental measurement of the phase diagram of vortex in high-Tc cuprates.

Low-temperature resistivity of YBa2Cu3O6+x single crystals in the normal state

A scan of the superconductor-nonsuperconductor transformation in single crystals of YBa2Cu3O6+x (x≈0.37) is done in two alternative ways, namely, by applying a magnetic field and by reducing the hole concentration through oxygen rearrangement. The in-plane normal-state resistivity ρab obtained in the two cases is quite similar; its temperature dependence can be fitted by a logarithmic law in a temperature range of almost two decades. However, an alternative representation of the temperature dependence of σab=1/ ρ ab by a power law, typical for a 3D material near a metal-insulator transition, is also plausible. The vertical conductivity σc=1/ρc followed a power law, and neither σc(T), nor ρc(T) could be fitted by log T. It follows from the ρc measurements that the transformation at T=0 is split into two transitions: superconductor-normal-metal and normal-metal-insulator. In our samples, they are separated in oxygen content by Δx≈0.025.

Thermodynamic properties of YBa2Cu4O8 crystal near upper critical fields Hc2(T)

High-field magnetization with external magnetic fields parallel to the c-axis has been measured in high-quality YBa2Cu4O8 single crystal. The fluctuation-induced magnetization above Tc and a crossover of magnetization curves M(T) at T*=77.4 K were clearly observed for various magnetic fields, and analyzed by the high-field scaling law and the vortex fluctuation theory for three-dimensional systems. In the reversible temperature region, the magnetization is well described by the model of Hao et al. [Phys. Rev. B 43, 2884 (1991)]. From this analysis, the thermodynamic parameters such as upper critical field slope −dHc2/dT=1.2 T/K near Tc, penetration depth λab(0)=1943 Å, and coherence length ξab(0)=21.7 Å were extracted.

Seeded Crystal Growth of Yba2Cu3O6.5 in Semisolid Melts

Melt textured single crystals of YBa2Cu3O6.5 (123) superconductors were produced by isothermal solidification from a semisolid melt using single crystal NdBa2Cu3O6.5 or SmBa2Cu3O6.5 seeds. The microstructure within the single crystals shows an inhomogeneous segregation of Y2BaCO5 (211) particles trapped in the 123 crystals during solidification. The concentration of 211 particles varies with the crystal axes in 123 crystals produced from precursors with compositions of 80 wt.% 123 powder and 20 wt.% excess 211. The 211 particle concentration along the c-axis in the crystal is much lower than that along the a- or b-axes during initial crystallization. This concentration increases in both directions as the crystal grows larger. The 211 concentration along the c-axis increases more quickly than the concentration along the other axes during solidification, which allows the 211 concentration to approach that on the other axes as the solidification continues. 211 particle segregation in stoichiometric 123 samples formed “X”-shaped tracks instead of the variations in 211 concentration described above. A single crystal growth model of 123 is proposed and employed to interpret these experimental observations. Quenched samples were prepared to investigate the volume fraction of 211 particles in the liquid phase. A constant distribution of 211 particles was observed in the liquid, except very near the crystal interface, where the 211 concentration decreased rapidly. Copper oxide content in the liquid was also measured. It is found that the copper content is lower at the (001) interface compared with (100) or (010) interfaces.

Temperature and magnetic field dependences of the London penetration depth and coherence length in the vortex state of YBa 2 Cu 3 O 7-\delta

We report TF‐μ+SR measurements of the London penetration depth in the high‐Tc superconductor YBa2Cu3O7-\delta as a function of temperature T and magnetic field H. The magnetic field distribution in high quality single crystals of YBa2Cu3O6.95 was fit assuming a triangular vortex lattice with an average in‐plane penetration depth \lambdaab=1155(3) A, extrapolated to zero temperature and magnetic field. We find that \lambdaab(T,H) increases linearly both as a function of magnetic field and temperature in the field range studied. This unusual behaviour is attributed to an unconventional pairing scheme in which there are line nodes in the superconducting energy gap function. Measurements have also been made on a deoxygenated sample, YBa_2Cu_3O6.60. The lineshapes for YBa2Cu3O6.60 clearly show the high‐field cutoff due to the finite coherence length \xiab. A preliminary analysis indicates that \xiab(T,H) increases linearly as a function of temperature and decreases with increasing magnetic field.

Evidence for vortex lattice melting and softening in untwinned YBa2Cu3O7 single crystal

Real part of the ac susceptibility X′1 of an untwinned YBa2 Cu3O7 single crystal jumps at the onset temperature in the narrow width (0.1 K in 0.7 T), suggesting the first-order melting transition. A sharp X′1 dip and a sharp X″ peak can be interpreted as synchronous trapping of vortices due to lattice softening prior to melting. At lower temperatures, the X′1-X″1 chart shows a single-parameter behavior while the melting occurs as a strong deviation from this.