Peter Hannaford

Universal Behavior of Pair Correlations in a Strongly Interacting Fermi Gas

We show that short-range pair correlations in a strongly interacting Fermi gas follow a simple universal law described by Tans relations. This is achieved through measurements of the static structure factor which displays a universal scaling proportional to the ratio of Tans contact to the momentum C/q. Bragg spectroscopy of ultracold 6Li atoms from a periodic optical potential is used to measure the structure factor for a wide range of momenta and interaction strengths, providing broad confirmation of this universal law. We calibrate our Bragg spectra using the f-sum rule, which is found to improve the accuracy of the structure factor measurement.

On the widths of atomic resonance lines from hollow-cathode lamps

Abstract The profile of the calcium 4226·73 A resonance line was measured for a standard neon-filled hollow-cathode lamp using a variable gap Fabry-Perot interferometer. Experimental half-intensity widths were corrected for instrumental broadening using theoretical expressions for the experimental and actual line profiles, and a measured value for the width of the instrumental function. The contribution of self-absorption broadening was estimated from measurements of the amount of absorption in the hollow-cathode lamp and from the observed variation with source lamp current of the absorbance of a calcium sputtering cell. For the range of lamp currents 5–15 mA d.c., the actual half-intensity width of the calcium resonance line was found to vary from 0.0092 A to 0.0154 A. These values can be accounted for satisfactorily in terms of Doppler broadening corresponding to the measured operating temperatures of the hollow cathode (347–429°K), self-absorption broadening and (to a lesser extent) Lorentz pressure broadening and natural broadening. The above results were used to calculate the effect of the finite width of the calcium hollow-cathode line on the absorbance-concentration curve for a typical air-acetylene flame absorber. At small lamp currents, errors incurred by the assumption of an infinitely sharp emission line and an unshifted, single Voigt absorption line amounted to about 10 per cent. This result is expected to be typical for resonance lines containing little or no hyperfine structure.

The application of cathodic sputtering to the production of atomic vapours in atomic fluorescence spectroscopy

Abstract Cathodic sputtering is used as a source of atomic vapour for the chemical analysis of metals and alloys by atomic fluorescence spectroscopy. The sputtered vapour is produced in a Pyrex glow-discharge chamber which is suitable for the rapid interchange of flat, metallic samples. The discharge operates with a water-cooled cathode specimen and a flow-through gas control system. Linear calibration curves are obtained for the determination of nickel, chromium, copper, manganese and silicon in some iron-base alloy standards. For nickel, chromium and copper, detection limits are of the order of 20 ppm in the iron, and for manganese and silicon about 70 and 400 ppm respectively. The reproducibility of the fluorescence measurements is about ±1%. The system can be readily adapted to provide simultaneous multi-element analysis.

Temperature dependence of photoluminescence in silicon quantum dots

The optical properties of silicon quantum dots (QDs) embedded in a SiO2 matrix are investigated at various temperatures using photoluminescence (PL) and time-resolved photoluminescence. Two broad luminescence bands, the S-band located at 600–850 nm and the F-band located at 450–600 nm, are observed. In the S-band a stretched exponential time evolution is observed and the short wavelengths have significantly shorter lifetimes than the long wavelengths. In the low temperature regime, the process of carrier delocalization from the defect states and capture into the QDs is dominant, which results in a decrease in PL intensity from the F-band and an increase from the S-band. In the high temperature regime, the carriers captured into the QDs decrease due to competition between the defect states, which results in a PL intensity decrease for both bands. The PL intensity on the high energy side of the S-band decreases more strongly than that on the low energy side due to the state filling effect, which results in a 30 nm red shift. The S-band is attributed mainly to zero-phonon electron–hole recombination due to enhancement of the quantum confinement effect. The F-band has a single exponential evolution with a much shorter lifetime of nanoseconds and is attributed to defect states of silicon oxide.

Crossover From 2D to 3D in a Weakly Interacting Fermi Gas

We have studied the transition from two to three dimensions in a low temperature weakly interacting 6Li Fermi gas. Below a critical atom number N(2D) only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two dimensional. Above N(2D) the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.

One-dimensional lattice of permanent magnetic microtraps for ultracold atoms on an atom chip

We report on the loading and trapping of ultracold atoms in a one-dimensional permanent magnetic lattice of period 10 µm produced on an atom chip. The grooved structure which generates the magnetic lattice potential is fabricated on a silicon substrate and coated with a perpendicularly magnetized multilayered TbGdFeCo/Cr film of effective thickness 960 nm. Ultracold atoms are evaporatively cooled in a Z-wire magnetic trap and then adiabatically transferred to the magnetic lattice potential by applying an appropriate bias field. Under our experimental conditions trap frequencies of up to 90 kHz in the magnetic lattice are measured and the atoms are trapped at a distance of less than 5 µm from the surface with a measured lifetime of about 450 ms. These results are important in the context of studies of quantum coherence of neutral atoms in periodic magnetic potentials on an atom chip.

Generation of high flux, highly coherent extreme ultraviolet radiation in a gas cell

We report the generation of extreme ultraviolet radiation with high photon flux (1010–1012 photon/cm2 s), high spatial coherence (up to 0.95), and good spatial beam profile by high-order harmonic generation in various noble gases (argon, neon, and helium) in a gas cell. The photon flux was determined using an extreme ultraviolet spectrometer equipped with a charge-coupled device camera and the spatial coherence was determined from Young double-slit interference fringes. The high-order harmonic emission is confined to just a few orders because of the small phase mismatch in the cut-off region that allows macroscopic phase matching to be satisfied for just a few harmonics in this region. The efficiency and spatial beam profile are studied as a function of gas pressure and geometrical configuration.

Condensate Splitting in an Asymmetric Double Well for Atom Chip Based Sensors

We report on the adiabatic splitting of a BEC of

Spectrally resolved femtosecond two-color three-pulse photon echoes: Study of ground and excited state dynamics in molecules

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Permanent magnetic lattices for ultracold atoms and quantum degenerate gases

Rb atoms by an asymmetric double-well potential located above the edge of a perpendicularly magnetized TbGdFeCo film atom chip. By controlling the barrier height and double-well asymmetry the sensitivity of the axial splitting process is investigated through observation of the fractional atom distribution between the left and right wells. This process constitutes a novel sensor for which we infer a single shot sensitivity to gravity fields of