I. M. Fishman

Thermal boundary resistance and diffusivity measurements on thin YBa2Cu3O7−x films with MgO and SrTiO3 substrates using the transient grating method

Interface selective transient grating experiments are performed on oriented thin films (∼100 nm) of YBa2Cu3O7−x, with MgO and SrTiO3 substrates. The anisotropic YBa2Cu3O7−x thermal diffusivity constants and the thermal boundary resistance between the thin film and substrate are measured. Four different excitation and probe geometries are utilized such that each geometry results in a unique temporal decay. The grating has a significant amplitude on both sides of the film–substrate interface with a grating wave vector parallel to the interface. The four experimental geometries comprise an over‐determined system that can be used to confirm the validity of the model assumptions. Numerical fits to the experimental data, using a straightforward diffusive model, are performed to obtain information on thermal diffusivity and to demonstrate the applicability of the technique to monitor anisotropic thermal relaxation processes in thin film–substrate structures.

Surface selectivity in four-wave mixing : transient gratings as a theoretical and experimental example

A theoretical treatment of transient grating diffraction is derived for gratings that are spatially nonuniform in the direction perpendicular to the sample surface. This treatment is readily generalized to any four-wave mixing experiment. Both reflection and transmission geometries of diffraction are examined for the standard transient grating case, in which both grating excitation beams are incident upon the same side of the sample For samples in which the grating amplitude perpendicular to the sample surface varies slowly relative to the optical wavelength, the reflection geometry is shown to probe only the surface or the interface, while the transmission geometry probes the bulk of the sample. An experimental example using four transient grating geometries (two reflection, two transmission) is shown to yield significantly different temporal responses, illustrating the nature of the theoretical predictions. The sample is a thin molecular crystal upon a substrate Both electronic excitations (excitons) and wave-guided acoustic modes are generated and probed Distinct signals are obtained from the bulk, the crystal–substrate interface, and the free-crystal face. Model calculations are presented that illuminate the behavior of the experimental example.

Symmetry of differential optical responses of normal and superconducting phases in single-crystal YBCO

Abstract We measured differential optical reflectivity (DOR) of YBCO single domains. DOR measurements reject the temperature independent bulk reflectivity and reveal dR dT . These measurements detect the superconducting transition, and reveal specific anisotropy associated with high-T c superconductivity.

Interface selective transient grating spectroscopy: theory and applications to thermal flow and acoustic propagation in superconducting thin films

A general theoretical and experimental treatment of transient grating diffraction is presented for interfacial holographic gratings in thin film structures. The theoretical treatment allows for gratings that have nonuniform spatial amplitude throughout the sample. Interface selective transient gratings experiments are performed on oriented thin films (approximately 100 nm) of YBa2Cu3O7-x, with MgO and SrTiO3 substrates. Four different excitation and probe geometries are utilized such that each geometry results in a unique temporal decay. The grating has a significant amplitude on both sides of the film-substrate interface with a grating wave-vector parallel to the interface. The four experimental geometries comprise an over determined system that can be used to confirm the validity of the model assumptions. Numerical fits to the experimental data, using a straight forward diffusive model, are performed to obtain information on thermal diffusivity and to demonstrate the applicability of the technique to monitor anisotropic thermal relaxation processes in thin film- substrate structures. This analysis yields the anisotropic YBa2Cu3O7-x thermal diffusivity constants and the thermal boundary resistance between the thin film and substrate.

Experimental determination of the absolute temperature modulation in photothermal microscopy of Y1Ba2Cu3O7 crystals

We present a technique to experimentally determine the value of the temperature perturbation δT in photothermal microscopy. Knowledge of δT is necessary for proper analysis of the differential optical reflectivity (∂R/∂T) data obtained from untwinned Y1Ba2Cu3O7 crystals. We measure δT to be 0.2 K, consistent with previous theoretical predictions from thermal diffusion data.

High-Frequency Optical Excitations in YBCO Measured from Differential Optical Reflectivity

YBCO differential optical reflectivity (DOR) responses are measured in the temperature range of 50–160 K at several optical frequencies. It is found that the background response relates to temperature variation of the scattering rate. Above Tc the full DOR response may be attributed to a Drude plasma with a low mobility. Below Tc, accumulation of the superconducting condensate is accompanied by appearance of a new optical spectrum in the range of 1.2–1.5 eV. We present a model for a plasma having parameters close to that of a Mott transition that demonstrates qualitatively the features observed experimentally.

Photothermal measurements of differential reflectivity of YBCO related to the symmetry of the superconducting order parameter

Abstract We have investigated symmetry of the differential optical response of single YBCO domains in the a-b plane. As a function of temperature, this response consists of a smooth background and a sharp peak in the vicinity of Tc related to the normal and superconducting phases, respectively. For the probe light polarized along the a axis, we have observed for the first time that the superconducting phase response is a sharp peak near Tc which is only ∼ 1 K wide. It is difficult to explain this peak in terms of a single second-order phase transition, or to arrive at a suitable model to explain the experimental results. We have also measured cross-polarized DOR signals which showed no off-diagonal reflection components.