Nanodimensional effects in the structure of epitaxial YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">y</Emphasis></Subscript> films

We have studied the structure of YBa₂Cu₃O_7−y (YBCO) high-T _c superconductor films obtained by deposition for various times under otherwise identical conditions. Thin YBCO films prepared by the nonaxial laser shadow deposition technique are epitaxial and have microstrains below 1×10⁻³. The main factor determining variations of the unit cell parameter c in the films is the difference of the crystal block size 〈D〉 in the direction normal to the film surface, which reflects the nanodimensional effects known in oxides.     

Simulated Hysteretic Loops for YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>

This paper presents modification for a model to retrace magnetic hysteresis loop and calculate hysteresis energy loss (HEL) in YBa₂Cu₃O₇ (YBCO). In comparison with reported experimental results, it is revealed that the modified model could retrace successfully magnetic hysteresis loop for YBCO. As an application of this model, the calculation formula of HEL has been included in this article. The results show also that HEL in YBCO increases by 14% with the addition of the magnetic NiW substrate. This leads to conclude that the type of substrate has an effect on HEL of YBCO. This modified model can help us to explain experimental results and expect the functioning of superconductor tapes.     

Quantum dots in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">x</Emphasis></Subscript> films with a tetragonal structure

Experimental evidence is presented for the existence of clusters possessing metallic conductivity in epitaxial YBa₂Cu₃O_6+x (x<0.4) films with a tetragonal structure. Despite the overall dielectric state, the optical transmission spectra reveal the regions of absorption due to free charge carriers. The presence of metallic clusters is also confirmed by the X-ray diffraction data. At low temperatures, these metallic clusters are capable of converting into quantum dots responsible for the θ(T) = const regions in the temperature dependence of the electric resistance. The process of cluster formation can be enhanced by high-power laser (KrF) radiation.     

Microwave Response of Perfect YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films Deposited on CeO<Subscript>2</Subscript>-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z _s(T), of epitaxial YBCO thin films deposited on CeO₂-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R _s(T) at low temperatures obeys the exponential law: R _s(T) = R _res+R ₀⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T _c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R _s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R _s(T) behavior.     

Formation of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + <Emphasis Type="Italic">x</Emphasis></Subscript> clusters in laser-generated plasma

We have studied the influence of YBa₂Cu₃O_{6 + x} clusters formed in the plasma generated by laser ablation of a YBa₂Cu₃O_{7 ? δ} target on the optical transmission spectra of amorphous YBaCuO films deposited on glass substrates arranged along the direction of predominant plasma expansion in the laser plume. It is established that intense cluster formation begins in the region of rapid decrease in the film thickness, where the temperature of plasma decreases to a level at which stable atomic complexes characteristic of the target composition can form (under the experimental conditions studied, this was observed at as distance of L > 6 cm from the target). As the amount of clusters in the deposit increases, the magnitude of the interference fringes, which are characteristic of optically homogeneous media, gradually decreases and eventually almost vanishes. At the same time, features typical of the electron structure of YBa₂Cu₃O_{7 ? δ} appear and grow in the optical transmission spectra of the YBaCuO films, including the absorption due to free charge carriers at ?ω < 1.2 eV (characteristic of “metallic” clusters) and the minima at ?ω = 1.4 and 1.75 eV (characteristic of a dielectric state).     

High Field ESR Measurements of <Emphasis Type=Italic>S</Emphasis>=1/2 Quasi One-Dimensional Antiferromagnet Cu<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript>

The tunneling of a Bose-Einstein condensate in a double-well potential causes the coherent atomic oscillations. The damping mechanism is employed to enhance the tunneling. Our numerical simulations show that the damping mechanism can firstly cause the atoms return to the initial well and form the metastable state. After that, the continuative dissipation of the energy of the system causes the unidirectional quantum coherent atomic tunneling, which just induces the system to transfer from the metastable state to the ground state.     

Induced Voltage Measurements on a YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconducting Thin Film Due to a Pair of Rotating Magnets

In this study, the induced voltages due to the rotation of a pair of magnets on an YBCO superconducting thin film sample (SS) with and without bias current were measured. It was found that the induced root mean square voltage (V _rms) was a constant (V ₀) at temperatures higher than the critical temperature (T _c ) of the SS, as expected from Faraday’s law. At a temperature in the superconducting transition region, the induced V _rms is a sensitive function of both the motion of the magnet and the bias current applied to the SS. These results can be understood by considering the superposition of the two kinds of induced voltages. One is induced according to Faraday’s law, and the other one is induced by the vortex movements inside the SS which is caused by the bias current. At temperatures below the transition region, the induced V _rms had a value equal to V ₀ and remained unchanged as the temperature further decreased. An explanation based on the distribution of the magnetic flux inside the SS was given, and it was concluded that the superconductor-normal conductor loop acted like a normal-normal conductor loop to the moving magnetic fields in this low temperature region.     

PTCR Characteristics in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript>—Linear Low Density Polyethylene (LLDPE) Composite Materials

A series of superconductor/polymer composites is made using high T _c YBa₂Cu₃O_7−x (Y-123) phase and Linear Low Density Polyethylene (LLDPE). The resistivity of the composites is measured as a function of temperature between 293 K and 400 K. A large positive temperature coefficient (PTC) of resistance is observed near 395 K. An attempt has been made to relate the percolation behavior of the YBCO–LLDPE composites and the thermal expansion of the LLDPE matrix. The intensity of the PTC effect for these composites was found to be as large as eight orders of magnitude.     

Near IR diffuse reflectance spectroscopy for <Emphasis Type="Italic">in situ</Emphasis> monitoring of the degree of oxidation of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> powders

We have studied the evolution of the diffuse reflectance spectra of YBa₂Cu₃O_7−x powders with 0.1≤x≤∼1.0 in the spectral interval of 0.5 eV≤hν≤6.0 eV. Combination of the optical spectroscopy with thermal desorption measurements revealed a band at 1.25 eV that reversibly varied in the redox cycle. This band, assigned to interband absorption in the CuO₂ planes with low coordination of Cu(2), can be used for in situ monitoring of the degree of oxidation of YBa₂Cu₃O_7−x powders with x≥0.5.     

The Effects of Sb Substitution on Structural Properties in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> Superconductors

In this study, the effects of partial Sb³⁺ ion substitutions for the Y sites and the Cu sites on the superconducting properties of YBa₂Cu₃O _y (Y123) ceramic superconductors were investigated. The samples were prepared by the conventional solid-state reaction method and the properties of the samples were investigated by means of X-ray diffraction, AC magnetic susceptibility measurements, scanning electron microscope, and energy dispersive X-ray analysis. The critical temperatures were determined to be in the range of 80–92 K for both Systems I and II. It was found that Sb-addition leads to the formation of the non-superconducting YBa₂SbO₆ phase, which has a negative effect on the critical temperature, since the highest critical temperature was measured for pure Y123. However, the increasing substitution level has a negligible effect on the X-ray diffraction analysis peak intensities of the superconducting phases. In addition, SEM images showed that Sb substitution decreases the grain size and modifies the microstructure development, which makes the samples denser.     

On the Origin of a Small Hole Pocket in the Fermi Surface of Underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The comprehensive experimental studies of the quantum oscillations (QOs) in YBa₂Cu₃O _y (YBCO) lead N. Doiron-Leyraud et al. (Nature Communications 6:6034, 2015) to detection of small hole pockets below the transition into the charge-ordered (CO) phase. Their interpretation of the observed energy spectrum imposes serious constraints for the possible origin of electron and hole pockets—both of them should be due to the Fermi surface (FS) reconstruction at charge ordering with biaxial order parameter. However, the results of recent X-ray experiments which reveal the uniaxial CO in the underdoped YBCO make such interpretation questionable. We show that uniaxial CO parameter in combination with preexisting electron pocket at the Γ-point of Brillouin zone suggests a plausible interpretation of QOs associated with small hole pockets in YBCO.     

Effect of Co Substitution on the Structure and Superconducting Properties of GaSr<Subscript>2</Subscript>(Tm,Ca)Cu<Subscript>2</Subscript>O<Subscript><Emphasis Type=Italic>z</Emphasis></Subscript>

The crystal structures of two samples with nominal compositions of GaSr₂(Tm_0.7Ca_0.3)Cu₂O _z and (Ga_0.65Co_0.35)Sr₂(Tm_0.7Ca_0.3)Cu₂O _z have been analyzed by using neutron powder diffraction techniques to clarify the structural mechanisms associated with changes in superconducting behavior. Unlike for the Co-free sample, which does not show superconductivity, the Co-doped sample exhibits superconductivity around 45 K. The Rietveld refinement data reveal that both samples exhibit the 1212 structure with space group Ima2 and the Co atoms exclusively substitute for the Ga atoms in the GaO₄ tetrahedron located on the chain sites of the 123 parent structure. The improvement in T _c of the Co-doped sample, compared to that of the undoped sample, is discussed in connection with the change in structural disorder and the change in hole concentration on the superconducting planes.     

Two-Dimensional Nanogranularity of the Oxygen Chains in the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.33</Subscript> Superconductor

The organization of dopants in high-temperature superconductors provides complex topological geometries that control superconducting properties. This makes the study of dopants’ spatial distribution of fundamental importance. The mobile oxygen ions, y, in the CuO₂ plane of YBa₂Cu₃O_{6 + y} (0.33 < y < 0.67) form ordered chains which greatly affect the transport properties of the material. Here, we visualize and characterize the two-dimensional spatial organization of these oxygen chains using scanning micro X-ray diffraction measurements in transmission mode on a thin single-crystal slab with y = 0.33 (T _c = 7 K) near the critical doping for the insulator-to-metal transition. We show the typical landscape of percolation made of a granular spatial pattern due the oxygen chains segregating in quasi-one-dimensional needles of ortho-II (O-II) phase embedded in an insulating matrix with low density of disordered oxygen interstitials.     

Pulsed Laser Deposition of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> with Scanning Beam: Target to Substrate Composition Transfer and Film Structure

Pulsed laser deposition is often considered a process providing congruent transfer of target composition to the growing film. In fact, many different processes affect compositional preservation, starting from incongruent target ablation, to scattering on the way to the substrate, and to processes of the film formation on the substrate surface. We developed a pulsed laser deposition process trying to minimize the compositional deviations due to the scattering by the ambient gas by applying laser beam scanning across the target surface and substitution of oxygen with argon in the chamber during deposition. Transfer of elemental composition of YBa₂Cu₃O₇ targets with compositions varying from stoichiometric 1/2/3 ratio was tested by deposition of thin films in conditions optimal for high-temperature superconductor formation. Despite all measures, the films still show Ba,Y enrichment due to different efficiencies of scattering on the ambient gas. The Y part in the film followed well the composition of the target, but the Ba enrichment was almost constant for most of the studied target compositions, implying a crucial role of the film growth processes. The YBa₂Cu₃O _x (YBCO) films show a layered structure, with increased density of defects in the topmost layer. We suppose this is due to expelling of the excess Ba into the top layer with formation of a quasi-liquid layer promoting formation of a high-density YBCO film.     

Polaronic Effect and Its Impact on <Emphasis Type=Italic>T</Emphasis><Subscript><Emphasis Type=Italic>c</Emphasis></Subscript> for Novel Layered Superconducting Systems

The crystal lattice of a complex compound may contain a subsystem of ions with each one possessing two close equilibrium positions (double-well structure). For example, the oxygen ions in the cuprates form such a subsystem. In such a situation it is impossible to separate electronic and local vibrational motions. This leads to a large increase in the effective strength of the electron–lattice interaction, which is beneficial for pairing.     

Absence of Magnetic Field Dependence of the Anomalous Bond-Stretching Phonon in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.6</Subscript>

Superlattice diffraction peaks have been recently observed in underdoped YBa₂Cu₃O_6.6. They have been interpreted to originate from a charge density wave (CDW) formation. It is believed that strong phonon anomalies previously observed near the same wavevectors are related to the CDW. Competition with superconductivity is a salient feature of the CDW peaks. We investigated if the same is true of anomalous bond-stretching phonons around 60 meV by measuring the spectrum of this phonon at 10 K with and without an applied magnetic field of 10 T. Applying the field had no effect on the phonon within the experimental uncertainty.