Roger Ward

Abstract:

Cerium usually comprises a mixture of polycrystalline FCC and DHCP allotropes. Single-crystal Ce has been stabilised in Ce/Pr superlattices grown using molecular beam epitaxy. It is found that FCC or DHCP phases can be obtained depending on superlattice composition and growth conditions. Low-temperature neutron scattering was performed on Ce/Pr samples using the triple-axis spectrometer D10 at the ILL. Such measurements revealed one sample, [Ce20Pr20]60, to be a single crystal with a DHCP unit cell; while another, [Ce30Pr10]56, was a mixture of FCC and DHCP phases. Antiferromagnetic ordering of magnetic moments was observed in the DHCP sample (TN = 11.1 K) with a magnetic structure similar to that found in bulk β-Ce. Surprisingly, the magnetic ordering was found to be confined to single Ce blocks. Furthermore, it was found that, at low temperatures, the lattice contraction observed for bulk FCC Ce was suppressed in Ce/Pr superlattices.

Abstract:

The magneto-optical absorption of the quantum Ising ferrornagnet LiHoF4 has been measured in the frequency range 38 to 140 GHz, the temperature range 0.5 to 20 K and magnetic fields of tip to 15 T. The sample was studied in two orientations, with the external magnetic field aligned perpendicular or parallel to the uniaxial direction, i.e., along the quantum and classical directions. In each case, several families of resonances are observed, including linear EPR branches and a series of non-linear fast moving resonances. We present this data and propose all interpretation for the origin of these resonances.

Abstract:

Single-frequency diode lasers have been stabilized to 200Hz at 1.5μm and independently to 20 Hz at 793 nm over 10-ms integration times using narrow spectral holes in the absorption lines of Er³⁺- and Tm³⁺-doped cryogenic crystals as frequency references. Kilohertz stability over 100-s integration times is provided by these techniques, and that performance should be extendable to long integration times with further development. The achieved frequency stabilization provides ideal lasers for high-resolution spectroscopy in the time and frequency domains, real time analog optical signal processing based on spatial-spectral holography, interferometry, and other applications requiring ultra-narrow-band light sources or coherent detection. The stabilized lasers have enabled demonstrations of analog optical signal processing in Er³⁺ materials at 0.5 GHz bandwidths at temperatures of 4.2K, and they will be important for electromagnetically induced transparency and quantum information demonstrations. © 2002 Elsevier Science B.V. All rights reserved.