Light scattering from quantum spin fluctuations in R2CuO4 (R=La, Nd, Sm).

Raman spectra reveal spin fluctuation scattering over the energy range 08000 cm-1 (01 eV) in single crystal La2CuO4 (T phase), Nd2CuO4, and Sm2CuO4 (T' phase). From the B1g symmetry spectra, values of the exchange parameter J for these crystals are determined to be 1030 50, 871 50, and 888 50 cm-1, respectively. All three materials also exhibit broad spectral features of A1g and B2g symmetry, which are forbidden within the classical spin-pair excitation model, but allowed by quantum fluctuations. The line shapes for spectra of all three symmetries agree with recent model calculations for the planar quantum antiferromagnet. While the spectra for crystals with the T' structure are nearly identical, subtle differences from the spectra of La2CuO4 (T structure) indicate a sensitivity to the structure outside of the CuO2 planes. © 1990 The American Physical Society

Light scattering from quantum spin fluctuations in R2CuO4 (R=La, Nd, Sm).

Raman spectra reveal spin fluctuation scattering over the energy range 08000 cm-1 (01 eV) in single crystal La2CuO4 (T phase), Nd2CuO4, and Sm2CuO4 (T' phase). From the B1g symmetry spectra, values of the exchange parameter J for these crystals are determined to be 1030 50, 871 50, and 888 50 cm-1, respectively. All three materials also exhibit broad spectral features of A1g and B2g symmetry, which are forbidden within the classical spin-pair excitation model, but allowed by quantum fluctuations. The line shapes for spectra of all three symmetries agree with recent model calculations for the planar quantum antiferromagnet. While the spectra for crystals with the T' structure are nearly identical, subtle differences from the spectra of La2CuO4 (T structure) indicate a sensitivity to the structure outside of the CuO2 planes. © 1990 The American Physical Society

Far-infrared absorptivity of UPt3.

The absorptivity of the heavy-fermion compound UPt3 is measured from 2 to 1000 cm-1 (0.25"124 meV) at temperatures between 1.2 K and room temperature. Above 50 cm-1 (6.2 meV) the absorptivity is relatively temperature independent, while below that frequency the absorptivity is very temperature dependent, in accord with the dc resistivity. By performing a Kramers-Kronig transformation of the data, augmented with recently published results at higher frequencies, the complex conductivity is obtained. The low-temperature conductivity may be characterized by free carriers which undergo frequency-dependent scattering and have a concomitant frequency-dependent mass enhancement, ≫(), with ≫(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×104 cm-1 (2.6 eV). Combining these results with the measured specific heat for UPt3 gives for the low-frequency effective mass m*=240m, and for the optical band mass mb=3.7m. The carrier density is close to one electron per formula unit. The far-infrared absorptivity measurement indicates that the scattering rate begins to rise with an 2 dependence, while the measured dc resistivity has a T2 dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate "(,T) can be written as "(,T)1/42+(pT)2, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2. © 1988 The American Physical Society

Far-infrared absorptivity of UPt3.

The absorptivity of the heavy-fermion compound UPt3 is measured from 2 to 1000 cm-1 (0.25"124 meV) at temperatures between 1.2 K and room temperature. Above 50 cm-1 (6.2 meV) the absorptivity is relatively temperature independent, while below that frequency the absorptivity is very temperature dependent, in accord with the dc resistivity. By performing a Kramers-Kronig transformation of the data, augmented with recently published results at higher frequencies, the complex conductivity is obtained. The low-temperature conductivity may be characterized by free carriers which undergo frequency-dependent scattering and have a concomitant frequency-dependent mass enhancement, ≫(), with ≫(0)=65. The data indicate a bare free-carrier plasma frequency of 2.1×104 cm-1 (2.6 eV). Combining these results with the measured specific heat for UPt3 gives for the low-frequency effective mass m*=240m, and for the optical band mass mb=3.7m. The carrier density is close to one electron per formula unit. The far-infrared absorptivity measurement indicates that the scattering rate begins to rise with an 2 dependence, while the measured dc resistivity has a T2 dependence at temperatures below 2 K. To account for the far-infrared data, the carrier scattering rate "(,T) can be written as "(,T)1/42+(pT)2, with an experimental upper limit of p=1. This is not consistent with electron-electron scattering, for which p=2. © 1988 The American Physical Society

Is there a role for Hedgehog genes in Hirschsprung's diseases?

There has been considerable interest lately in imaging techniques that employ thermal waves [1–4]. In thermal-wave imaging, a beam of energy, usually a laser or electron beam, is focused and scanned across the surface of a sample. This beam is generally intensity-modulated at a frequency in the range of 10kHz to 10MHz. As the beam scans across the sample it is absorbed at or near the surface, and periodic surface heating results at the beam modulation frequency. This periodic heating is the source of thermal waves, which propagate from the heated region. The thermal waves are diffusive waves similar to eddy current waves, evanescent waves, and other critically damped phenomena that travel only one to two wavelengths before their intensity becomes negligibly small. Nevertheless, within their range, the thermal waves interact with thermal features in a manner that is mathematically similar to the scattering and reflection processes of conventional propagating waves [5], Thus any features on or beneath the surface of the sample that are within the range of these thermal waves and that have thermal characteristics different from their surroundings will reflect and scatter the waves and thus become visible

High-energy spin and charge excitations in La2CuO4.

Inelastic light scattering studies of La2CuO4, both pure and doped with lead, over the energy range 01 have revealed the presence of high-frequency A1g scattering components, in the spectral region 35006000 cm-1, while also providing the complete line shape for the previously reported spin-pair mode of B1g symmetry. Two main bands, broad and overlapping, are observed in this region with maxima near 3500 and 5200 cm-1. The various spectral features show different resonance behavior. We suggest that a model based upon coupled spin and charge excitations will be required to describe these spectra. © 1989 The American Physical Society