Peter A. Sheldon

Short-pulse excitation and spectroscopy of KNbO3, LiNbO3 and KTiOPO4

Abstract Light-induced optical absorption in KNbO3, LiNbO3, and KTiOPO4 (KTP) is of concern for the performance of these important nonlinear optical materials. Gray track formation in KTP and LiNbO3, and blue-light-induced infrared absorption in KNbO3 have been the focus of recent research on photo-damage. Picosecond growth and decay of optical absorption produced by ∼ 0.8 ps pulses of blue laser light are described in the present work on crystals of KNbO3, LiNbO3, and KTP in comparison to measurements made with electron pulses and X-ray irradiation.

Third sound in3He-4He mixture films

We report third-sound studies of3He-4He mixture films for 0.3≲T≲1.8 K carried out in two different experimental situations. In one experiment detailed measurements of the amplitude of both the temperature and thickness variation of the film were made for pure4He and the results for |ΔT/Δd| are in good agreement with the predictions of Bergman. Mixture studies in this apparatus were made difficult due to the presence of capillary condensation. In spite of this a number of interesting results are reported. Subsequent measurements were made in a new apparatus where capillary condensation effects could be demonstrably avoided. Helium films of 5.7 layers of4He were studied as a function of added amounts of3He. BelowT≃0.7 K the third-sound measurements are consistent with model calculations based on a bilayer film. We conclude that for 0.3≲T≲0.6 K films of the type we have studied are isotopically layered in the van der Waals field provided by the substrate.

3He spin diffusion in thin4He films

Abstract We report measurements of the transverse and longitudinal spin diffusion coefficients for 0.1 – 0.24 monolayers of3He adsorbed on thin3He films as a function of temperature and4He coverage. A strong4He coverage dependence is observed in the spin diffusion which suggests that the3He quasiparticles in the film are strongly scattered by substrate inhomogeneities for4He coverages below 0.5A˚-2.

Spin diffusion and magnetization studies of3He in3He-4He mixture films

The longitudinal spin diffusion coefficient and magnetization of a submonolayer coverage of3He, n3=0.0066 Å−2, in a thin adsorbed4He film has been measured as a function of temperature0.024 K ≤ T≤0.5 K and4He coverage0.21 Å−2≤n4≤0.32 Å−2.

Ultrafast spectroscopy of hole and exciton self-trapping in halide crystals

Abstract Subpicosecond and picosecond optical absorption spectra following band-gap excitation of alkali halide crystals have been measured and analyzed. It is found that a simple rule relates the low-energy edge of the “N band” previously attributed to the one-center (Landau-type) hole polaron, the photon energy of the (two-photon) excitation pulse, and the band gap. The rule explains the quantitative photon-energy dependence of the N-band position and its presence or absence in all of the data compiled from the present work and other published works, within the framework of two-photon cross-correlated absorption of the excitation and probe pulses as well as pulse dispersion considered separately in the optics and in the sample. It is concluded that the one-center self-trapped hole in alkali halides has not yet been identified by absorption spectroscopy in the visible-uv spectral range. The self-trapped hole and/or associated self-trapped exciton is observed in the two-center (V k ) configuration.

Experiments to search for a substrate state for3he adjacent to surfaces in3He-4He mixtures

We describe three experiments designed to search for the bound state for3He adjacent to a substrate which has been predicted by Treiner and co-workers. In each case a bulk3He-4He solution is in contact with the substrate of interest. For a Nuclepore substrate, NMR techniques are used to establish that there is no 2D behavior for the3He in the cell; thus, no substrate state is present. For a cesium or hydrogen substrate the technique involves the measurement of the velocity of ordinary sound in the bulk solution as a function of temperature. The cesium and hydrogen experiments are incomplete.

Localization of 2D3He on a4He film

We report measurements of the3He spin diffusion, magnetization and relaxation times for3He−4He mixture films on Nuclepore. For a3He coverage ofn3=0.006 Å− (∼0.10 layers), we observe a ∼104 increase of the diffusion coefficient over the one-layer4He thickness range, 0.15≤n4≤0.23Å−2 and Curie-like behavior for the magnetization forn≤0.20 Å−2. Disorder due to surface roughness may result in localization of the3He. This type of localization has been predicted to be accompanied by a vanishing diffusion constant and diverging magnetization.

Third sound and NMR studies of helium mixtures in nuclepore

We report measurements of the third sound properties of3He–4He mixture films on Nuclepore. Using third sound resonance techniques, we measure the frequency, damping, amplitude and phase of third sound modes on a mixture film with a fixed coverage of4He, n4 = 0.39Å−2, as a function of drive power, Pdr, 0.044 ≤ Pdr ≤ 440 nW, temperature, 40 ≤T ≤ 500 mK, and3He coverage, 0.01 ≤n3 ≤ 0.06Å−2.Near T = 200 mK there is a sharp shift in the difference in phase, δΦ, between the drive voltage fluctuation and the film thickness fluctuation.

Binding energy of3He in thin4He films

Abstract We report measurements of the magnetization of 0.1 monolayers of 3 He in a thin 4 He film as a function of temperature and from these we determine the binding energies of the 3 He in the mixture film as a function of the 4 He coverage. For 4 He coverages between 0.21 and 0.53A˚ -2 the 3 He binding energies obtained are in reasonable agreement with predictions of Pavloff and Treiner for the 4 He coverage dependence of the binding energy.