The noncentrosymmetry requirement of sum frequency generation (SFG) spectroscopy allows selective detection of crystalline cellulose in plant cell walls and lignocellulose biomass without spectral interferences from hemicelluloses and lignin. In addition, the phase synchronization requirement of the SFG process allows noninvasive investigation of spatial arrangement of crystalline cellulose microfibrils in the sample. This paper reviews how these principles are applied to reveal structural information of crystalline cellulose in plant cell walls and biomass.
Femtosecond optical reflectivity measurements of La2-xSrxCuO4, La2CuO4+y, Bi2Sr2CuO6+z and Bi2Sr2CaCu2O8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.
Epitaxial strain in La2-xSrxCuO4+δ thin films (0 ⩽ x ⩽ 0.30) is controlled by using SrLaAlO4 buffer layers of different thicknesses on SrTiO3 substrates. We found that compressive epitaxial strain results in higher Tc for all the Sr concentrations. Better oxygenation by cooling the films in ozone/molecular oxygen mixture also leads to higher Tc. In undoped and lightly-doped ultrathin films, the samples are insulating under tensile strain, but superconducting when the strain is sufficiently compressive. We suggest that the epitaxial strain affects the insertion of interstitial oxygen, which is responsible for the observed effects. Hall measurements confirm the change in carrier density in films of different strain. The Hall angle also changes with epitaxial strain. The epitaxial strain dependence of the slope in the T2 dependence of the cotangent of the Hall angle is different for underdoped and optimally-doped samples.
We describe a new pled laser deposition (PLD) system that is linked to an angle-resolved photoemission (ARPES) chamber at the Synchrotron Radiation Center (SRC) in Wisconsin, USA. We also discuss our first results on epitaxially grown YBa2Cu3O7-(delta ) (YBCO) films. The core level photoemission data indicate that a Ba-oxide layer is the dominant surface layer. We were not able to reproducibly detect a sharp fermi edge in the photoemission spectra and thus conclude that the surface layer is non-metallic, probably due to oxygen loss at the surface. The absence of screening of the Y and Ba core levels is a further argument for this conclusion. Further experiments with ozone treated film surfaces are currently under way.
Pulsed laser deposition has been used to deposit multilayer heterostructures consisting of high temperature superconductor (HTS) and ferroelectric layers for tunable microwave device applications. The dielectric nonlinearity and loss of the ferroelectric thin films are the key material parameters determining the performance of the tunable devices and hence the feasibility of this technology. In this paper, I summarize the current understanding of these issues and outline the strategies to study these problems.
We present a brief review of results on the surface impedance of cuprate superconductors, focusing mainly on YBa2Cu3O7-(delta ) (YBCO) and evidence of d-wave superconductivity in that material. We then discuss our recent results on Ba-K-Bi-O thin films, and the effects of DC electric fields on the surface impedance of YBCO films. A summary of our data on high quality thin films and single crystals of the electron-doped Nd1.85Ce0.15CuO4-(delta ) (NCCO) cuprate superconductor follows. Surprisingly, the measurements on NCCO are consistent with the behavior of an s-wave BCS superconductor, in striking contrast to recent results on YBCO. Finally we discuss some of the interesting potential implications of d-wave superconductivity for microwave applications of the cuprates.
Ultrathin ReBa2Cu3O7-(delta ) (ReBCO) (Re equals Y, Nd) films in the form of trilayer sandwiches between (PrxY1-x)Ba2Cu3O7- (delta )((PrxY1-x)BCO) (1 >= x >= 0) and YBCO/(PrxY1-x)BCO superlattices with nominal YBCO layer thickness as thin as 1 unit- cell were grown by pulsed laser deposition. It was found that although the 1-unit-cell thick YBCO films were superconducting, the Tc value depends strongly on the doping level x in the adjacent layers. The Tc value increases when x is reduced. A number of effects, either extrinsic or intrinsic in origin, have been examined by various experiments. The results indicate that the lattice mismatch, interdiffusion between Y and Pr atoms, and oxygen deficiency are unlikely to be the primary reason. Charge redistribution and the possibility of K-T transition were suggested by Raman and transport measurement, but the effects observed may not be sufficient to explain the change of zero resistance temperature and the broadening of the transition in these sample structures.
We have investigated the optical response of single grain-boundary weak-links in superconducting YBCO thin films. At the superconducting transition of a film containing a single grain-boundary weak-link we observed an enhanced optical response from the weak- link. Below the transition temperature, we measure the bias current and the temperature dependence of the optical response of the grain-boundary in the temperature range of 15 K < T < 80 K. Using a Resistively-Shunted-Junction model for the grain-boundary weak- link, we find that the optical response in this temperature range can be described by radiation induced thermal modulation of the critical currents.
High Tc superconductor jitter-free-opening switches, made of YBCO films, with switching risetime of 100 ps and switching efficiency of 80% have been successfully demonstrated. The switches were used in an inductive energy storage pulsed power system (IESPPS). Ultrashort electrical pulses with pulse compression and peak power gain of 40 were obtained.
An anomalous optical response was observed in YBa2Cu3O7-x thin films showing percolative behavior in the superconducting transitions. Despite the fact that the temperature derivative of the resistance had increased considerably, no pronounced increase of optical response was found in the high resistivity regime of the transition. This nonbolometric response is likely related to inhomogeneities in superconducting properties of the films, but its nature is not well understood.
We report studies of a thin high-Ta film operating as a fast bolometric detector of infrared radiation. The film has a response of several mV when exposed to a 1 W, 1 ns duration broadband infrared pulse. The decay after the pulse was about 4 ns. The temperature dependence of the response accurately tracked dR/dT. A thermal model, in which the film's temperature varies relative to the substrate, provides a good description of the response. We find no evidence for other (non-bolometric) response mechanisms for temperatures near or well below T.