Electron-phonon induced superconductivity in highT c superconductors

It is proposed that phonon induced mechanism is capable of explaining the observed range ofT _c for the newly discovered high temperature superconductor within the Allen and Dynes strong coupling theory.     

Double phase transition in highT c superconductors

Experiments have revealed that some samples of high-T _c ceramic superconductors possess a double peak structure in the specific heat. We shall show that within a BCS formalism it is possible to explain this double peak as a pure superconduction phenomenon. An important feature of the model is the onsite pairing interaction term which turns out to determine the difference between the two peaks.     

Critical temperature of layered highT c superconductors

We present and investigate a model for layered superconductors with single-electron interlayer tunneling and a unit cell containing an arbitrary number of sheets. For an effective pairing of two-dimensional origin,T _c is found to be independent of the layer spacings. However, pairing of three-dimensional nature, allowing for spacing dependent coupling strength, fits remarkably well the observed climb inT _c for the Bi–O and Tl–O layered systems, as the number of CuO₂ planes in the unit cell is increased from two to four and six. This agreement points to a weak coupling pairing mechanism of three-dimensional origin.     

On the pairing interaction in highT c superconductors

It is argued, from the dependence of the critical current density and ofT _c on applied magnetic field, that the pairing interaction in highT _c superconductors must be strongly field-dependent.     

Physical properties of GaAs on glass

Physical properties of GaAs layers grown by molecular beam epitaxy on glass substrates are investigated by Raman light scattering, Photothermal Deflection Spectroscopy (PDS), optical absorption and Scanning Electron Microscopy (SEM) measurements. The results indicate that the layers are polycrystalline and strain-free. Raman spectra exhibit GaAs-TO and LO modes at 260 and 283 cm^–1, respectively. The peaks are shifted by about 10 cm^–1 with respect to bulk GaAs which we attribute to local heating effects induced by laser excitation. The phonon lines are strong and have a bandwidth of about 5 to 8 cm^-1 indicating a good crystalline quality. However, neither photoluminescence nor the Hall effect could be observed that is suggestive of the presence of a large number of electronic defects. SEM micrographs taken from the surface and the interface exhibit a granular structure with the polycrystal sizes of well under 1 m. PDS results show about 10¹⁸ cm^–3 defects and some disorder at the band gap.     

Mössbauer spectroscopy in highT c superconductors

Mossbauer spectroscopy has recently been applied to study the new high T_c superconducting compounds RBa₂Cu₃O_z, using isotopes of rare earths mainly¹⁵⁵Gd and¹⁵¹Eu, and⁵⁷Fe, with different amounts of Fe ions replacing Cu. It was shown that magnetic moments on the rare earth site do not interfere with superconductivity. Fe at low concentrations (<1%) was found to replace Cu mainly in the Cu(1) site, and the Mossbauer spectra reveal different quadrupole doublets-fol lowing the different oxygen coordination around the Fe ion. The change of the relative intensities of the different doublets with z can easily be followed. For higher iron concentrations, it seems that increasing amounts of iron replace Cu in the Cu(2) site. For z<6.5, the iron reflects the magnetic ordering of Cu in this site, and the ordering temperature as function of z can be obtained. The agreement between neutron diffraction and Mossbauer measurements prove that Fe is a good probe for the magnetic behaviour of the Cu(2) ions. At low temperatures, Fe Mossbauer spectra of Fe in the Cu(1) site are also magnetically broadened, for all z. Superconducting-magnetic phase diagrams are also obtained in Y_1−x Pr_x Ba₂ Cu₃O_z as function of x and z. For z=7.1, T_N changes sharply with x. T_N=300, 230 and 35 K for x=0.8, 0.6 and 0.4 respectively, whereas for z=6.1 T_N changes very little with x. Mossbauer measurements performed on 5 at %⁵⁷Fe doped in CalaBaCu₃O_z show that most of the la occupy the Ba site. For z=7 about half the iron in the Cu(2) sites are magnetically ordered, with H_eff=520 kOe and T_N=400 K, even though the sample is superconducting with T_c=35 K. The possibility of coexistence between superconductivity and magnetic order in these systems will be discussed.     

Electron-fluctuating distortion interaction in the new highT c superconductors

A mechanism involving interaction of conduction electrons with the distortion field and the phonon modes is considered for the newly discovered highT _c superconducting materials. This is capable of explaining the observed range ofT _c.     

On the valence band photoemission of highT c superconductors

The valence band photoemission spectrum of highT _c -superconductors is discussed based on the half-filled single band Hubbard Hamiltonian with the strong Coulomb interaction. We discuss how to analyze these valence band and deep core level excitation spectra, concerning particularly with which orbital, Cud or Op state, a hole will occupy in the CuO₂ plane.     

Fluctuations of order parameters in the highT c superconductors

Recently we have proposed a phenomenological approach in terms of two coexisting macroscopic order parameters corresponding to the superconducting and insulating states and have discussed the electrodynamical responses of the superconducting ceramics. In this paper we discuss the fluctuations of the order parameters both in static and dynamical situations in the mean field approach and obtain results for the electrical conductivity which possesses anomalies as in granular materials.     

Study of the gas-phase parameters affecting the silicon-oxide film deposition induced by an ArF laser

A study of the gas-phase parameters involved in ArF laser induced chemical vapour deposition of silicon-oxide thin films is presented. A complete set of experiments has been performed showing the influence of the concentration of the precursor gases, N₂O and SiH₄, and their influence on total and partial pressures on film growth and properties. In this paper we demonstrate the ability of this LCVD method to deposit silicon oxide films of different compositions and densities by appropriate control of gas composition and total pressure. Moreover, a material specific calibration plot comprising data obtained using different preparation techniques is presented, allowing determination of the stoichiometry of SiO _x films by using FTIR spectroscopy independently of the deposition method. For the range of processing conditions examined, the experimental results suggest that chemical processes governing deposition take place mainly in the gas phase.     

Mössbauer study of Tl containing highT c superconductors

Mössbauer spectroscopy, X-ray diffractometry, ESR and electrical resistivity measurements, were used to study highT _c (above 100 K) superconducting materials in order to get information about the effect of the preparation circumstances as well as about the anomalous behaviour in⁵⁷Fe and¹¹⁹Sn doped TlBaCaCu(⁵⁷Fe)O_4.5+δ and TlBaCuCu(¹¹⁹Sn)O_4.5+δ superconductors. We have found that the Mössbauer parameters strongly depend on the preparation circumstances. In many cases the presence of Mössbauer lines of nonsuperconducting oxide phases indicated that the iron and tin could not entirely built in into the lattice of superconductor. Four valence state tin and four valence state iron sites were identified in the superconducting phases. We have found anomalous temperature dependent changes in the⁵⁷Fe Mössbauer spectra of TlBaCaCu(⁵⁷Fe)O_4.5+δ samples around theT _c between 105–135 K. In this temperature range the total area of the spectrum temporary increases.     

On the role of acoustic plasmons in highT c superconductors

The role of acoustic plasmons in the recently discovered highT _c superconductors is discussed. It is shown that the exchange of acoustic plasmons together with the usual phonon exchange between electrons can give rise to aT _c ∼ 100 K.     

Fluorescence interference-contrast microscopy on oxidized silicon using a monomolecular dye layer

A silicon chip is covered by a monomolecular film of a fluorescence dye with silicon dioxide used as a spacer. The fluorescence depends on the distance of the dye from the silicon. The modulation of the intensity is described quantitatively by an optical theory which accounts for interference of the exciting light and of the emitted light. The effect is used to obtain a microscopic picture of the surface profile with a precision of a few Angströms. The perspectives for an application in wet systems such as neuron-silicon junctions and lipid membranes on silicon are pointed out.     

Optical constants of thin CoSi2 films on silicon

The optical transmission of CoSi₂ films of thickness 2.6–15 nm is measured in the wavelength range 1–20 m. The optical constants are evaluated by taking into account multiple reflections in the film and by fitting a Drude model. The plasma frequency _p=5.4–7.6 eV is equivalent to a carrier density n _eff=3×10²² cm^–3 and one carrier per unit cell. The relaxation frequency of the plasma resonance assumes high values =2 eV near the interface to silicon and decreases into the bulk film over several nanometers. Films grown off-axis from the (111) Si orientation exhibit an enhanced relaxation frequency.     

Anomalous magnetic hysteresis loops and smallH c1 values in highT c superconductors

We have studied the hysteresis loops of RBa₂Cu₃O₇ (R=Gd, Ho and Y) and detected anomalies in some of them. The observed anomalies support a recent prediction by Ravi Kumar and Chaddah based on an extension of Bean’s model. The anomalies indicate lowH _c1 values and we have confirmed this by studying the onset of low-field hysteresis in less than 10 Oe at 77 K for these highT _c superconductors.     

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

Influence of carrier gas pressure and flow rate on atomic layer deposition of HfO2 and ZrO2 thin films

Influence of the carrier gas on HfCl₄-H₂O and ZrCl₄-H₂O atomic layer processes was investigated. The growth rates of HfO₂ and ZrO₂ decreased with increasing flow rate and pressure of the N₂ carrier gas. Data of real-time quartz crystal microbalance measurements demonstrated that the effect observed was mainly due to influence of carrier gas on surface reactions and the role of overlapping the precursor pulses was negligible. At the same increase of the carrier gas mass flow, the increase of the linear flow rate led to more significant changes of thin-film properties than the increase of the carrier gas pressure did. Thin films with higher density, higher refractive index and, particularly, lower concentration of residual chlorine were obtained at higher carrier gas flow rates. Increase of the carrier gas flow rate also resulted in a higher concentration of a metastable phase in HfO₂ thin films deposited at 300 °C.     

Shifting of the thermal properties of amorphous germanium films upon relaxation and crystallization

The modification of the thermal conductivity and melting temperature of unrelaxed amorphous Ge films on Si substrates upon laser-induced relaxation and crystallization is presented. Real-Time Reflectivity (RTR) measurements are used to determine experimentally both the melting threshold and the melt durations, and the finite element method is used to simulate the laser-induced heat-flow process. A thermal conductivity ofk=0.010 W dem K is determined for the unrelaxed material by fitting the experimental melting thresholds of unrelaxed films of different thicknesses. A similar procedure applied to the amorphous relaxed and crystallized materials lead to a shift to higher values of both the thermal conductivity and the melting temperature. In order to achieve a good fit of the experimental melt durations, it was necessary to assume a large degree of undercooling prior to solidification. The role of undercooling in the solidification process is finally discussed in terms of its dependence on the faser energy density and the high thermal conductivity of the substrate.     

On the lattice properties and electron-lattice interaction in highT c superconductors

The experimental results on the unusual crystalline lattice properties in high temperature superconductors are reviewed. Special attention is paid to their dependence on temperature and doping and their anomalous behaviour associated with the superconducting and metal-insulator transition. The most physically relevant features are: certain atoms fluctuating between different positions in the unit cell, unusually large oscillator strength of certain vibrational modes strongly coupled to a broad spectrum of electronic excitations and the polaronic nature of charge carriers. A synthesis of these experimental results is attempted in view of constructing a coherent picture for the metal-insulator transition and the superconducting state. We conclude that in the insulating materials the holes are localized on the CuO₂ units in the immediate vicinity of the dopant ions. In the metallic materials we expect charge-fluctuations of holes between the units containing the O(4) apex ions (Cu(1)-20(4)-20(1) for YBa₂Cu₃O_7–) and the Cu(2)-20(2)-20(3) complex in the CuO₂ layers. This charge transfer being linked to large Cu(1)-O(4) bond fluctuations ultimately leads — via a polaronic mechanism — to pairing of holes in the Cu(2)-2O(2)-2O(3) units.     

A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE

We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994