Single Crystal Superconductor Research Articles

Complex conductivity in high‐Tc single crystal superconductors

AbstractThe electromagnetic surface impedance of single crystal high‐Tc superconductors has been examined within the framework of the two‐fluid model and the hypothesis that the e m field modulates the partial concentrations of both normal and condensate fluids. A comparison with experimental data is reported.

A low-temperature ultrahigh vacuum scanning tunneling microscope system and tunneling spectra of the Bi2212 superconductor

We designed and constructed an ultrahigh vacuum scanning tunneling microscope (UHV-STM) which is available for the measurement of tunneling spectra at low temperature. The sample holder is cooled by thermal conduction to the stage that thermally connects with the refrigerator head. We cleaved Bi2Sr2CaCu2Oy (Bi2212 high Tc superconductor) single crystals under ultrahigh vacuum conditions, and measured tunneling spectra of the Bi–O surface at 35 K. The spectra were smeared due to the fact that the STM tip was at high temperature (≳100 K).

Studies of Single Crystal Superconductors by Magnetically Modulated Resistance

Studies of Single Crystal Superconductors by Magnetically Modulated Resistance

Electronic Structure and Symmetry of the Itinerant States in High Tc Superconductors

The linearly polarized Cu L3 XANES (X-ray Absorption Near Edge Structure) spectra of several high Tc superconductor single crystals and of their parent insulating compounds have been measured. The spectra have been interpreted in the frame of the full multiple scattering (MS) theory in the real space. Over a range of 40 eV, the polarized Cu L3 XANES spectra have been predicted in term of a one-electron dipole (Δl=+1) transitions, probing the partial density of states of the conduction bands projected on the Cu site with the selected orbital angular momentum l=2 and their projections ml. The oscillator strenght for the dipole allowed transitions Δl=-1 has been found to be a factor 100 weaker than the Cu 2p to 3d transitions. The physical origin of all features in the continuum XANES spectra and the effect of the electron-hole Coulomb interaction in the final states have been identified. The difference spectra between the metal and the insulating phase allows to identify the symmetry of the itinerant 3d9L states induced by doping, that is deduced to be a mixture of Cu 3d holes with ml=0, and 2 orbital angular momentum and oxygen 2p holes on planar oxygen in the molecular orbital combination of local a1 and b1 symmetry. Evidence for the more delocalized character of (3d9x2-y2) L(b1) states versus the more localized character of (3d3z2-r2) L(a1) states has been found.

Measurement of magnetic forces on small high Tc superconductor single crystals in magnetic field gradients

In this paper a brief account is given of a new instrument which allows quantitative measurements of magnetic forces in very small single crystals. The crucial element of the instrument is a capacitor with plane and parallel plates, one of which can be elastically displaced to change the distance between them. Forces in the range of 10 −9 N can be measured with this set-up. Good quality single crystals of Nd 1.85Ce 0.85CuO 4, placed in low magnetic field gradients, have been studied at different fields and temperatures. To prove the capability of the method for studying magnetic relaxation phenomena, results are presented of d M/dln t for this material at several low magnetic fields, H c1 < H < H ∗ (where H ∗ is the field for complete field penetration) at 4.2 K.

Luminescence of high-temperature single-crystal superconductors cleaved in ultrahigh vacuum.

Experimental confirmation of the existence of intrinsic luminescence in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ high-temperature superconductors has been obtained by investigation of luminescence and reflectivity spectra of single crystals cleaved at liquid-helium temperatures in ultrahigh vacuum. A correlation between the radiation instability (photon-stimulated desorption of oxygen molecules) and the quenching of the intrinsic luminescence has also been obtained from the dependence on the synchrotron-radiation photon flux. We conclude that the luminescence originates from an oxygen quasimolecule formed as a result of electron-excitation localization in a Cu-O plane.

Modulated microwave absorption at low magnetic fields in a Tl2Ba2CaCu2O8+x single crystal.

We have studied the low-field microwave absorption in a ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{x}}$ single crystal using an electron-spin-resonance spectrometer. Two different series of well-resolved periodic lines that are ascribed to superconducting current loops interrupted by Josephson junctions are observed. The angular dependences of the main features of the spectra as well as the orientations of both superconducting current loops are discussed. The results are in accordance with a theoretical model describing low-field microwave absorption in oxide superconductor single crystals. The spectra exhibit magnetic hysteresis effects depending on the magnetic flux through the superconducting current loops.

Measurement of magnetic field penetration depth in YBa 2Cu 3O 7− gd single crystals by surface paramagnetic probes

The applicability of electron spin resonance line broadening of a diphenylpicrylhydrazyl paramagnetic probe for measuring the magnetic field penetration depth into the surface of a YBa 2Cu 3O 7−δ single crystal superconductor is studied. The temperature dependence follows a [1−( T/ T c) 4] relation where T c is the superconducting transition temperature, and the average magnetic field penetration depth in YBa 2Cu 3O 7−δ single crystals is determined to be 6250 Å.

Neutron Depolarisation by YBa2Cu5O6.85, Superconductors

ABSTRACTWe report a study of magnetic flux penetration into a single crystal superconductor, using the technique of 1-dimensional neutron depolarisation. In these experiments we have applied magnetic fields of up to 1.7 kOe to the crystal, perpendicular to the polarisation direction of the neutron beam. By analysing the polarisation of the transmitted neutron beam, after removal of the applied field, we are able to determine the remanent magnetisation in the crystal as well as its decay over periods from 10 to 105 seconds. This technique allows us to study both field and time dependent behaviour.We are developing a model to explain these observations, using the Bean critical state model to calculate the bulk trapped flux with the assumption of simple spin precession in a uniaxial magnetic environment. We have been able to relate the field dependence of the remanent magnetisation to the lower critical field Hcl and the field at which magnetisation saturation is reached 2H*-Hcl. We also relate the time dependence to the flux pinning energy U which determines the rate of flux relaxation. Measurements of this type could be used to plot the magnetic phase diagram of high temperature superconductors, to study anisotropy and flux creep in such crystals.

Flux trapping in high Tc superconductor single crystals in a tilted magnetic field

Zero-field-cooled, field-cooled and remanent magnetization of Pb 2Sr 2Y 0.3Ca 0.7Cu 3O 8 single crystals has been studied for various orientations between the applied magnetic field and the crystallographic axes. One of the crystals demonstrated a demagnetizing factor that was nearly angle-independent. We therefore separated the material anisotropy from the sample shape anisotropy in our measurements. It is shown that the field-cooled magnetization is independent of the shape of the sample and reflects the anisotropy of the magnetic flux trapping. We observe that the vortices are mostly trapped in the direction parallel to the Cu-O planes. We have found a similar qualitative character of the anisotropy in a YBa 2Cu 3O 6 + x crystals. Our results are consisdent with the anisotropy of the irreversibility line observed in high- T c superconductors.

GROWTH AND CHARACTERISATION OF Bi2Sr2Ca1Cu2−xNixOy SINGLE CRYSTALS

Effect of partial substitution of Ni in the high Tc superconductor single crystals Bi-Sr-Ca-Cu-O has been studied. Single crystals of Bi-Sr-Ca-Cu/Ni-O are grown from its stoichiometric melt. The addition of Nickel in this system ( Bi 2 Sr 2 Ca 1 Cu 2−x Ni x O y, x=0.1, 0.3) is found to change the growth temperature, superconducting transition temperature, surface morphology, and mechanical property.

Thermal expansion measurements in a La 1.85Sr 0.15CuO 4 single crystal superconductor by X-ray diffraction

Lattice parameter measurements have been made in a La 1.85Sr 0.15CuO 4 single crystal superconductor between 4.2 K and room temperature by the X-ray Bond method. A structural distortion near the superconducting transition temperature of 33 K has been observed. The lattice spacing in the c-direction increases below the superconducting transition temperature with decreasing temperature. The change in the lattice parameter, Δc/c 0, between 4.2 K and 33 K, is 2×10 −5. Along a direction perpendicular to the c-axis, on other hand, the lattice spacing decreases more rapidly than expected from normal thermal contraction. The possible importance of these results for existing theories of high temperature superconductivity is noted.

Quantitative Auger electron spectroscopy of superconducting YBaCuO single crystals and thin films

Quantitative Auger electron spectroscopy (AES) measurements of stoichiometric YBa 2Cu 3O 7 single-crystal superconductors have been performed to determine the relative elemental Auger sensitivity factors used for internal standards. This quantitative method was applied to investigate the compositional structures of YBa 2Cu 3O 7− x thin films sputtered onto (100)-oriented SrTiO 3, MgO, and Al 2O 3 substrate materials. For YBa 2Cu 3O 7− x films with these typical substrates the resulting stoichiometric structur and epitaxial growth behaviour are fairly different. Interdiffusion effects at the superconductor-substrate interfaces observed by application of an AES depth profiling technique are remarkable for YBa 2Cu 3O 7− x films sputtered onto Al 2O 3 substrates. The experimental results presented here demonstrate that AES can be a very useful method in the quantitative analysis study of high- T c superconductors.

Growth of High-T&lt;sub&gt;c&lt;/sub&gt; Superconductor Single Crystals and Investigation of the Effect of Thermobaric Treatment in an Oxygen Atmosphere on their Critical Temperature

Growth of High-T&lt;sub&gt;c&lt;/sub&gt; Superconductor Single Crystals and Investigation of the Effect of Thermobaric Treatment in an Oxygen Atmosphere on their Critical Temperature

Effect of annealing on superconductivity of Pb-doped Bi-Sr-Ca-Cu-O single-crystal superconductors

The microstructure of high quality Pb-doped Bi2Sr2CaCu2Ox single crystals grown by the flux technique has been studied by transmission electron microscopy (TEM). The single crystals have a superconducting transition temperature (Tc) of about 90 K as grown; Tc drops to about 83 K after annealing in air at 600 °C. TEM results show that the as-grown crystals exhibit twinning. The twin boundaries were found to be parallel to the (100) and (010) lattice planes. However, upon annealing the twin structure disappeared. This microstructure change is discussed in terms of a possible cause for the Tc decrease in the annealed crystals.

Proximity effects and critical fields in high- T C superconductors

We show, within proximity effect theory, that the upper critical field H C 2 ⊥ ( T ) can have a positive curvature when imperfections exist in high-TC superconductor single crystals. Positive or negative curvature in the lower critical field HC1(T) can result from penetration of vortices through normal defects, depending on average defect size.

Growth and Characterization of High T. Superconductor Single Crystals

Abstract Single crystals of oxide superconductor (ErBa2Cu307-δ) were grown from a liquid phase using an excess CuO + BaO as a flux. The crystals were square thin plates (3 × 3 × 0.1mm3) with large (001) faces on which various kinds of growth layers were developed including growth spirals. There was a certain correlation between Tc and the twinning. The measurements of Tc showed a large scatter in the value of Tc ranging from 65 to 90 K, in spite of a prolonged annealing and slow cooling in air or oxygen atmosphere. However, we found out for the first time a method to improve Tc by treating the solidified ingot in molten CrO3. It turned out that the value of Tc before this treatment was improved to the extent of 85 to 92 K.

Quasiparticle trapping from a single-crystal superconductor into a normal-metal film via the proximity effect.

We have studied the dynamics of the nonequilibrium state produced in single-crystal superconducting indium by pulsed laser irradiation using tunnel junctions. By a suitable choice of a thin metal film in intimate contact with the indium crystal, we have been able to produce a pronounced change in the time evolution of the excess quasiparticle density which indicates that the quasiparticles are trapped in the film. This effect can take place both when the film is another superconductor of lower energy gap or is a normal metal made superconducting by the proximity effect.

Growth of HIGH- Tc superconductor single crystals and the effect of thermobaric treatment in oxygen on critical temperatures

By means of slow cooling from the melt, using a rotatable crucible, bulk single crystals of YBa 2Cu 3O 7−δ, La 2CuO 4 and La 1.85Sr 0.15CuO 4−δ were synthesized. To determine the crystallization conditions, phase diagrams of La 2O 3CuOCuO 0.5 and YBa 2Cu 3O 7−δ- BaO CuO with a solvent composition of BaO 28%–CuO 72% were studied in air. The bulk single crystals of YBa 2Cu 3O 7− δ , measuring 3× 3× 1 mm 3, exhibited a non-uniform oxygen distribution. Superconductivity in La 1.85Sr 0.15CuO 4− δ was obtained by growth under non-equilibrium conditions. The influence of annealing at high oxygen pressure on the critical temperature T c in the synthesized crystals was studied.

Crystal growth of YBa 2Cu 3O x

YBCO superconductor single crystals were grown by means of high temperature solution method and by a technique which eliminates superfluous crystallites. Crucibles of various materials and sizes, different kinds of solutions, and growth parameters such as soaking temperature, cooling rate and ambient atmosphere were tried. Size of crystals obtained is about 3–4 mm long and wide, 0.1 mm thick, and zero-resistance transition temperature T c (0) is about 86K.