Fatih Dogan

The structure of the high-energy spin excitations in a high-transition-temperature superconductor

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-Tc) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at Tc. The most obvious feature in the magnetic excitations of high-Tc superconductors such as YBa2Cu3O6+x is the so-called ‘resonance’. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)xCuO4 family and is not universally present in Bi2Sr2CaCu2O8+δ (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at Tc, and have spectral weight far exceeding that of the ‘resonance’. The discovery of similar excitations in La2–xBaxCuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

Spin fluctuations in YBa2Cu3O6.6

An important feature of the high-transition-temperature (high-Tc) copper oxide superconductors is the magnetism that results from the spins associated with the incomplete outer electronic shells (3d9) of the copper ions. Fluctuations of these spins give rise to magnetic excitations of the material, and might mediate the electron pairing that leads to superconductivity. If the mechanism for high-Tc superconductivity is the same for all copper oxide systems, their spin fluctuations should be universal. But so far, theopposite has seemed to be the case: neutron scattering data reveal clear differences between the spin fluctuations for two major classes of high-Tc materials, La2−xSrxCuO4 (refs 1-3) and YBa2Cu3O7−x (refs 4-6), whose respective building blocks are CuO2 layers and bilayers. Here we report two-dimensional neutron-scattering imaging of YBa2Cu3O6.6, which reveals that the low-frequency magnetic excitations are virtually identical to those of similarly doped La2−xSrxCuO4. Thus, the high-temperature (Tc ≲ 92 K) superconductivity of the former materials may be related to spatially coherent low-frequency spin excitations that were previously thought to be unique to the lower-Tc (<40 K) single-layer La2−xSrxCuO4 family.

Resonance as a measure of pairing correlations in the high-T-c superconductor YBa2Cu3O6.6

One of the most striking universal properties of the high-transitiontemperature (high-Tc) superconductors is that they are all derived from the hole-doping of their insulating antiferromagnetic (AF) parent compounds. From the outset, the intimate relationship between magnetism and superconductivity in these copper-oxides has intrigued researchers [1–4]. Evidence for this link comes from neutron scattering experiments that show the unambiguous presence of short-range AF correlations (excitations) in cuprate superconductors. Even so, the role of such excitations in the pairing mechanism and superconductivity is still a subject of controversy [5]. For YBa2Cu3O6+x, where x controls the hole-doping level, the most prominent feature in the magnetic excitations spectra is the “resonance” [6–11]. Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below the optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular rather than parallel to the CuO2 planes. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in the superconductivity of cuprates. The persistence of a field effect above Tc favors mechanisms with preformed pairs in the normal state ofOne of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu 3O6+x, where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6,7,8,9,10,11). Here we show that for underdoped YBa2Cu 3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.

One-dimensional nature of the magnetic fluctuations in YBa2Cu3O6.6

There is increasing evidence that inhomogeneous distributions of charge and spin—so-called ‘striped phases’—play an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa2Cu 3O7-x family of materials show both spin and charge fluctuations that are consistent with the striped-phase picture. But the fluctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic fluctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa2Cu3O6.6 reveal the one-dimensional character of the magnetic fluctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the [specialb] crystal axis, where the superfluid density is found to be substantially larger than for the [speciala] direction.

INCOMMENSURATE MAGNETIC FLUCTUATIONS IN YBA2CU3O6.6

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in YBa2Cu3O6.6 (T-c = 62.7 K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above T-c. For the energies studied, the susceptibility at incommensurate positions increases on cooling below T-c, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.

Dielectric Properties of Polymer-particle Nanocomposites Influenced by Electronic Nature of Filler Surfaces

The interface between the polymer and the particle has a critical role in altering the properties of a composite dielectric. Polymer-ceramic nanocomposites are promising dielectric materials for many electronic and power devices, combining the high dielectric constant of ceramic particles with the high dielectric breakdown strength of a polymer. Self-assembled monolayers of electron rich or electron poor organophosphate coupling groups were applied to affect the filler-polymer interface and investigate the role of this interface on composite behavior. The interface has potential to influence dielectric properties, in particular the leakage and breakdown resistance. The composite films synthesized from the modified filler particles dispersed into an epoxy polymer matrix were analyzed by dielectric spectroscopy, breakdown strength, and leakage current measurements. The data indicate that significant reduction in leakage currents and dielectric losses and improvement in dielectric breakdown strengths resulted when electropositive phenyl, electron-withdrawing functional groups were located at the polymer-particle interface. At a 30 vol % particle concentration, dielectric composite films yielded a maximum energy density of ~8 J·cm(-3) for TiO2-epoxy nanocomposites and ~9.5 J·cm(-3) for BaTiO3-epoxy nanocomposites.

Incommensurate Magnetic Fluctuations in YBa{sub 2}Cu{sub 3}O{sub 6.6}

We use inelastic neutron scattering to demonstrate that the low-frequency magnetic fluctuations in YBa2Cu3O6.6 (T-c = 62.7 K) change from commensurate to incommensurate on cooling with the incommensurability first appearing at temperatures above T-c. For the energies studied, the susceptibility at incommensurate positions increases on cooling below T-c, accompanied by a suppression of the spin fluctuations at the commensurate points. These results suggest that incommensurate spin fluctuations may be a common feature for all cuprate superconductors.

Performance of a Porous Electrolyte in Single-Chamber SOFCs

A cell which consists of a porous 18 μm thick Y-doped ZrO 2 (YSZ) electrolyte (23 ′ 3 vol % open porosity) on a NiO-YSZ anode substrate and a cathode using (La, Sr)(Co, Fe)O 3 has been investigated in the single-chamber configuration. The cell performance and catalytic activity of the anode was measured in a flowing air-methane gas mixture with various flow rates. The results showed that the open-circuit voltage and the power density increased as the gas flow rate increased. The cell generated an open-circuit voltage of about 0.78 V, which was only about 0.1 V lower than that observed with dense electrolyte specimens. A maximum power density of 660 mW cm - 2 (0.44 V) was obtained at set temperature = 606°C (cell temperature = 744°C) in the flow rate of 900 cm 3 min - 1 , where the current efficiency was about 5% determined from fuel consumption.

Freeze Casting of Porous Hydroxyapatite Scaffolds -- II. Sintering, Microstructure, and Mechanical Behavior

In Part I, the influence of processing parameters on the general microstructure of freeze-cast hydroxyapatite (HA) constructs was explored. This work is an extension of Part I to investigate the effect of sintering conditions on the microstructure and mechanical behavior of freeze-cast HA. For constructs prepared from aqueous suspensions (5-20 vol % HA), sintering for 3 h at temperatures from 1250 degrees C to 1375 degrees C produced a decrease in porosity of <5% but an increase in strength of nearly 50%. Constructs with a porosity of 52% had compressive strengths of 12 +/- 1 MPa and 5 +/- 1 MPa in the directions parallel and perpendicular to the freezing direction, respectively. The mechanical response showed high strain tolerance (5-10% at the maximum stress), high strain to failure (>20%), and high strain rate sensitivity. Manipulation of the freeze-cast microstructure, achieved by additions of glycerol and 1,4-dioxane to the aqueous suspensions, produced changes in the magnitude of the mechanical response, but little change in the general nature of the response. The favorable mechanical behavior of the porous constructs, coupled with the ability to modify their microstructure, indicates the potential of the present freeze-casting route for the production of porous scaffolds for bone tissue engineering.

Freeze-cast hydroxyapatite scaffolds for bone tissue engineering applications

Freeze casting of aqueous suspensions was investigated as a method for preparing porous hydroxyapatite (HA) scaffolds for eventual application to bone tissue engineering. Suspensions of HA particles (10-20 volume percent) were frozen unidirectionally in a cylindrical mold placed on a cold steel substrate (-20 degrees C). After sublimation of the ice, sintering for 3 h at 1350 degrees C produced constructs with dense HA lamellae, with porosity of approximately 50%, and inter-lamellar pore widths of 5-30 microm. These constructs had compressive strengths of 12 +/- 1 MPa and 5 +/- 1 MPa in the directions parallel and perpendicular to the freezing direction, respectively. Manipulation of the microstructure was achieved by modifying the solvent composition of the suspension used for freeze casting. The use of water-glycerol mixtures (20 wt% glycerol) resulted in the production of constructs with finer pores (1-10 microm) and a larger number of dendritic growth connecting the HA lamellae, and higher strength. On the other hand, the use of water-dioxane mixtures (60 wt% dioxane) resulted in a cellular-type microstructure with larger pores (90-110 microm). The mechanical response showed high strain tolerance (5-10% at the maximum stress), high strain for failure (>20%) and sensitivity to the loading rate. The favorable mechanical behavior of the porous constructs, coupled with the ability to modify their microstructure, indicates the potential of the present freeze casting route for the production of porous scaffolds for bone tissue engineering.