Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, ΔT _c =0.3 K). A sharp (FWHM 7.2 cm^?1 at 70 K and 10.0 cm^?1 at 110 K) 340 cm^?1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^?1 in theB _1g polarization extends down to 250 cm^?1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^?1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^?1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^?1 mode anomalies have been explained by the electronic spectral function renormalization.     

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa₂Cu₃O_6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm^–1, O(2)-O(3) at 338 cm^–1, and O(4) at 488 cm^–1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequencies, which is in agreement with the enhancement of the static conductivity(0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.     

Electronic Raman scattering in YBa2Cu4O8

We report on the electronic Raman scattering in YBa₂Cu₄O₈ and find that, In contrast to the other superconducting cuprates, the continuum in the normal state is strongly temperature pendent in all polarizations. This temperature dependence is found to follow a Bose-Einstein-like form. We conclude that the absence of the Bose-Einstein factor in the electronic continuum is not essential to high-T_c superconductivity, as is sometimes implied. Additionally, we report on the rearrangement of the continuum which occurs belowT _c.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^–1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

Electronic Raman scattering of superconducting YBa2Cu3Oy single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^–1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^–1 fory7.0 to 550 cm^–1 fory6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

Frequency and temperature dependence of the millimeter wave conductivity of epitaxial YBa2Cu3O7 films

A millimeter wave spectrometer for frequencies between 100 and 350 GHz consisting of continuously tunable backward wave oscillators as sources and a quasioptical interferometer in the Mach-Zehnder configuration was used to measure the transmittivity in phase and amplitude of YBa₂Cu₃O₇ thin films on NdGaO₃ substrates. From the measured spectra we derived the real and imaginary part of the dynamic conductivity= ₁+i ₂ in the superconducting state as a function of temperature. The ₁(T) and ₂(T) values at 300 GHz were compared to corresponding values at 19 GHz determined by surface impedance measurements of the same films using a shielded dielectric resonator. Our observed frequency dependence of both ₁(T) and ₂(T) is consistent with a strong reduction of the quasiparticle scattering rate ^–1(T) with decreasing temperature belowT _c .     

Dynamic local lattice distortion in YBa2Cu4O8

Significant changes in the local atomic structure and lattice dynamics were observed by pulsed neutron inelastic scattering measurements on YBa₂Cu₄O₈ both around the superconducting transition temperature and around the spin-gap temperature. These observations and earlier results of numerical calculations on the enhancement of electron-lattice interaction by electron correlation lead to a novel picture of unconventional lattice-induced superconductivity related to antiferroelectric instability.     

Phonon Dispersion Measurements of YBa2Cu3O6.15 and YBa2Cu3O6.95 by Time-of-Flight Neutron Spectroscopy

We measured the phonon dispersions of YBa₂Cu₃O_6.15 and YBa₂Cu₃O_6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10nY1; 18>nPr 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the linear term is presented and discussed.     

Charge Transport in Spin-Textured YBa2Cu3O6.25

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa₂Cu₃O_6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature T _d 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.     

Temperature dependent Raman scattering of the epitaxial Sr1.9Ca0.1NaNb5O15 film

Epitaxial ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ (SCNN) thin films have been fabricated successfully by pulsed laser deposition on (100)MgO substrates. Temperature dependency of the vibrational modes of these films was investigated for the first time using Raman scattering technique. For temperatures ranging between 50 °C and 450 °C, two strong and broad A₁(TO) phonons around 238 and 608 cm^− 1, and one weak B₁(TO) phonon around 142 cm^− 1 were observed. Changes in the temperature dependency of the peak position, the full-width at half maximum and integrated intensity of the Raman modes were observed in different temperature ranges. These changes are attributed to the structural phase transitions between 200 °C and 325 °C for the SCNN films as well as their relaxor properties.     

Analysis of electronic structure and charge density of the high-temperature superconductor YBa2Cu3O7

Self-consistent linearized augmented plane wave (LAPW) method calculations of the band structure, density of states, Fermi surface, Coulomb potential, charge density, core-level shifts, and electron-phonon interaction are presented for Y₁Ba₂Cu₃O₇. The calculated Sommerfield parameter is 4.35 mJ(mole Cu)^–1 K^–2, roughly about a factor of 2 smaller than experimentally deduced values of the enhanced value=(1 + )₀, suggesting that the Fermi surface mass enhancement is of the order of unity. The crystal charge density is best represented by overlapping spherical ionic densities when the Cu and O ions are assigned charges of +1.62 and –1.69, respectively, corresponding to about 0.3 holes per oxygen atom. Core-level energies for the inequivalent atoms differ by as much as 0.45 eV for Cu and 0.7 eV for O, amounts which may be detectable by core-level spectroscopies. These results provide important information on the character and magnitude of ionic contributions to bonding in these materials. Within the rigid muffin-tin approximation, calculated McMillan-Hopfield parameters yield estimates for the electron-phonon strength that appear to be too small to account for the observedT _c. We point out an unusual band of oxygen-derived chain states below, but within 0.1 eV of, the Fermi level.     

X-Ray and neutron scattering studies of YBa2Cu3O6+x compounds: Oxygen insertion and spin dynamics

In the first part of this paper the results of X-ray and neutron diffraction studies of the structural aspects related to oxygen insertion in the CuO _x planes of YBa₂Cu₃O_6+x compounds are presented. The second part is devoted to the inelastic neutron scattering study of the spin dynamics in the three regimes (weakly doped, heavily doped, and overdoped) of the metallic phases of YBa₂Cu₃O_6+x .     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Insulators for YBa2Cu3O7 Josephson Junction Technology

Because ideal PrBa₂Cu₃O₇ superconducts at 90 K, it should be replaced by RBa_2–u Sr _u Cu₂MO₈ (where R is a light-mass rare earth and M is Nb or, preferably, Ta) as the insulator of choice for YBa₂Cu₃O₇ Josephson junction or microstructure technology. Furthermore, (110)- or (100)-oriented substrates of RSr₂Cu₂MO₈ may provide an opportunity for optimal device fabrication.     

Evaporated YBa2Cu3O7 thin films and device technology

Epitaxial thin films of YBa₂Cu₃O₇ were grown by evaporation of the metals from electron-guns in the presence of atomic oxygen. The films were characterized structurally by X-ray diffraction and electrically by measurements of d.c. magnetization and flux creep. The progress in device technology is discussed. Results are presented on Josephson junctions, and a superconducting microwave filter with the ground plane grown on the back.     

Raman scattering study on ferroelectric (Ba0.32Sr0.68)2Nb2O7 ceramics

Raman scattering technique was applied to examine the Ba-doping effect to the two low temperature phase transitions of Sr₂Nb₂O₇ (SN) in the temperature range from −190 to 600 °C. The line shape of Raman spectra can be well fitted by multidamped harmonic oscillator model. We did not observe any soft mode related to the two low temperature phase transitions corresponding to those of the pure SN. It is correlated to the disappearance of the incommensurate phase in (Ba_0.32Sr_0.68)₂Nb₂O₇ ceramics. However, the temperature dependence behavior of the three low frequency modes indicates another new structural phase transition around 270 °C. It is considered that the reduction of the interlayer interaction caused by partial replacement of Sr-site by Ba-site, whose ionic radius is larger than that of Sr, may be the reason for the disappearance of the incommensurate phase transition in (Ba_0.32Sr_0.68)₂Nb₂O₇ ceramics.