Search Results (25 total)

We have made pressure and NMR measurements during the evolution of phase separation in solid helium isotopic mixtures. Our observations indicate clearly all three stages of the homogeneous nucleation-growth process: (1) creation of nucleation sites; (2) growth of the new-phase component at these nucleation sites; and (3) coarsening: the dissolution of subcritical droplets with the consequent further late-stage growth of the supercritical droplets. The time exponent for the coarsening, a=1/3, is consistent with the conserved order parameter Lifshitz-Slezov evaporation-condensation mechanism.

We present the design and simulation of a three-dimensional photonic crystal waveguide for linear laser-driven acceleration in vacuum. The structure confines a synchronous speed-of-light accelerating mode in both transverse dimensions. We report the properties of this mode, including sustainable gradient and optical-to-beam efficiency. We present a novel method for confining a particle beam using optical fields as focusing elements. This technique, combined with careful structure design, is shown to have a large dynamic aperture and minimal emittance growth, even over millions of optical wavelengths.

We report the first observations of spinodal decomposition in solid helium isotopic mixtures, using NMR measurements. The experiments were performed at a ³He concentration of 50% where the transition proceeds through the critical point. We used an initial pressure such that the system remained solid. Our observations indicate that the transition occurs by the mechanism of spinodal decomposition and we are able to study its evolution in real time.

We recently achieved the first experimental observation of laser-driven particle acceleration of relativistic electrons from a single Gaussian near-infrared laser beam in a semi-infinite vacuum. This article presents an in-depth account of key aspects of the experiment. An analysis of the transverse and longitudinal forces acting on the electron beam is included. A comparison of the observed data to the acceleration viewed as an inverse transition radiation process is presented. This is followed by a detailed description of the components of the experiment and a discussion of future measurements.

We present the first direct observation of a higher-order inverse-free-electron-laser (IFEL) interaction. Interaction at the fourth, fifth, and sixth harmonics is observed from an IFEL operating at 800 nm. The harmonic spacing, relative harmonic strength, and transverse beam overlap of the interaction are all in good agreement with tracking simulations.

We demonstrate a new particle acceleration mechanism using 800 nm laser radiation to accelerate relativistic electrons in a semi-infinite vacuum. The experimental demonstration is the first of its kind and is a proof of principle for the concept of laser-driven particle acceleration in a structure loaded vacuum. We observed up to 30 keV energy modulation over a distance of 1000λ, corresponding to a 40  MeV/m peak gradient. The energy modulation was observed to scale linearly with the laser electric field and showed the expected laser-polarization dependence. Furthermore, as expected, laser acceleration occurred only in the presence of a boundary that limited the laser-electron interaction to a finite distance.

A high precision torsional oscillator has been used to study ³He films of thickness from 100 to 350 nm, in the temperature range 5<T<200  mK. This thickness is much smaller than the viscous penetration depth, and also much smaller than the low temperature inelastic mean-free path of ³He quasiparticles. Such films are observed to decouple from the motion of the oscillator below 60 mK. We interpret the frequency shift and dissipation data in terms of an interfacial friction model.

Photonic crystals provide a method of confining a synchronous speed-of-light mode in an all-dielectric structure, likely a necessary feature in any optical accelerator. We explore computationally a class of photonic crystal structures with translational symmetry in a direction transverse to the electron beam. We demonstrate synchronous waveguide modes and discuss relevant parameters of such modes. We then explore how accelerator parameters vary as the geometry of the structure is changed and consider trade-offs inherent in the design of an accelerator of this type.

NMR and pressure have been measured in a solid ³He-⁴He mixture as the temperature was lowered in steps through phase separation. The spin-echo method was used to detect the behavior typical for bounded diffusion and to estimate the diffusion coefficient, size and cluster concentration in the ³He-enriched phase. The characteristic phase separation time constant of the mixture was found to decrease at lower temperatures. The results convincingly support homogeneous nucleation. From a comparison with theory, the surface tension at the boundary of the phase-separated clusters is found either from the cluster concentration, determined by NMR, or from the separation time constant, determined by pressure measurements. The results of the two independent determinations agree well and yield a surface tension coefficient of 1.27×10^-2 erg/cm² (1.27×10^-5 J/m²).

The heat capacity and magnetization of a fluid ³He monolayer adsorbed on graphite plated with a bilayer of HD have been measured in the temperature range 1–60 mK. Approaching the density at which the monolayer solidifies into a sqrt[7]×sqrt[7] commensurate solid, we observe an apparent divergence of the effective mass and magnetization corresponding to a T=0 Mott-Hubbard transition between a 2D Fermi liquid and a magnetically disordered solid. The observations are consistent with the Brinkman-Rice-Anderson-Vollhardt scenario for a metal-insulator transition. We observe a leading order T² correction to the linear term in heat capacity.

We present pulse NMR measurements on ³He adsorbed on hexagonal boron nitride powder in the registered phase. We observe strong coupled magnetic relaxation with substrate ¹¹B spins, mediated by ¹⁴N surface-layer spins via the level-crossing or quadrupole-dip (QD) effect at 4.5 MHz. Ab initio electronic structure calculations of the BN surface-layer electric field gradients, dramatically modified by the ³He were essential in justifying our QD explanation. BN crystallite geometry plays a key role in producing the QD. Stretched exponential fits to the QD relaxation data may provide a new tool for studying specifically the commensurate phases on BN and related materials.

Measurements are reported of the heat capacity of ³He bound to an atomically layered ⁴He film on the surface of graphite. At ³He coverages n₃<4 nm^-2, the ³He forms a uniform 2D Fermi system. Thereafter with increasing n₃ we resolve two steps in the heat capacity, each arising from the formation of a further 2D continuum built on excited surface-normal states. We infer information on the hydrodynamic mass, quasiparticle interactions, and energetics of these states. Both short range repulsive interactions and ripplon mediated quasiparticle interactions are important.

The superfluidity of ⁴He films adsorbed on the atomically flat surface of graphite, preplated with HD to tune the surface binding potential, has been studied using a torsional oscillator. The superfluidity of a single uniform fluid layer of ⁴He shows an intrinsic coverage dependent suppression, while the fluid bilayer is fully superfluid at T  =  0. The contribution of nonvortex excitations in the film to the normal density shows a strong dependence on coverage, arising from the atomic layering of the film.

Measurements of the magnetization and heat capacity of the second layer of ³He films adsorbed on graphite indicate that the evolution of the exchange from antiferromagnetic to ferromagnetic arises from a tuning of the competing exchange processes. At certain coverages the coexistence of an antiferromagnetic heat capacity with a ferromagnetic magnetization is a clear manifestation, predicted by theory, of frustration. At the ferromagnetic anomaly the system is well described by series expansions for a 2D Heisenberg ferromagnet on a triangular lattice.

We have measured the heat capacity of the second layer solid in a ³He film adsorbed on the surface of graphite to temperatures below 1 mK, in the regime where the nuclear exchange interaction is antiferromagnetic. The temperature dependence of the data is described by high temperature series expansions for a 2D Heisenberg antiferromagnet on a triangular lattice. There is no evidence for a phase transition down to J/3. Comparison with J inferred from the magnetization provides a measure of the frustration due to competing antiferromagnetic and ferromagnetic cyclic exchanges.

Nuclear magnetic resonance experiments are reported on multilayer ³He films adsorbed on the surface of graphite. Initially the second layer exhibits antiferromagnetic exchange, which has been measured as a function of coverage. Measurements of the nuclear magnetic susceptibility and resonant frequency for coverages around the third layer promotion precisely identify the onset of ferromagnetic exchange in the film. The evolution of the structure of the film is discussed in the light of these results and previous heat-capacity and NMR data.

We report measurements of the nuclear magnetic susceptibility of the monolayer two-dimensional solid formed by ³He adsorbed on graphite plated with two atomic layers of HD. A low-density solid is observed, consistent with a √7 × √7 registered structure, which exhibits large antiferromagnetic exchange. A distinct cusp in the susceptibility is observed near 1 mK, suggestive of a magnetic phase transition. The evolution of this and other features with surface density is discussed.

The effects of quantum degeneracy due to the tunneling motion of ³He atoms in a two-dimensional solid isotopic solution have been observed through measurements of the nuclear magnetic susceptibility. This system constitutes a solid phase with the features of a heavy fermion fluid. A phase transition, identified as isotopic phase separation, was observed directly through the time variation of the magnetic susceptibility. The critical temperature was 50 mK with a time constant for phase separation of order 5 h at 10 mK.

The nuclear magnetic susceptibility of ³He adsorbed on the surface of graphite, plated with a monolayer of solid ⁴He, has been measured at surface densities less than 0.055 Å ^-2 and at temperatures down to 6 mK. A temperature-independent susceptibility is observed at low temperatures, which is strongly enhanced over the ideal-gas value, due to quasiparticle interactions. The inferred Landau-Fermi-liquid parameters allow a comparison with microscopic theories of interacting Fermi systems.

We show that the spin-diffusion coefficient in solid ³He may be obtained unambiguously from measurements of spin relaxation at low magnetic fields. For long times, the dipolar autocorrelation function decays as t^-3/2. This follows purely as a consequence of the hydrodynamics of the system. From this we show that the low-frequency form of the dipolar spectral density function is J(ω)=J(0)-Aω^1/2, where the constant A depends on the diffusion coefficient and is independent of the microscopic details of the system. Values of the spin-diffusion coefficient thus obtained are in good agreement with those obtained by conventional means. Moreover this method permits extension to higher densities and lower diffusion coefficients.

The nuclear magnetic susceptibility of submonolayer ³He films adsorbed on a graphite substrate has been measured for temperatures between 6 and 800 mK. Three distinct surface-density regimes are identified for fractional monolayer coverages below perfect registry. These appear to be a fluid phase with some atoms localized by substrate heterogeneity, a solid-fluid coexistence region, and a registered solid phase with defects. T₂ values inferred from NMR linewidth measurements are also reported.

T₁ measurements have been performed on the solid phase of helium-3 adsorbed on Grafoil. A theory has been developed to interpret these data on the basis of the hypothesis of a single correlation time for the exchange-induced spin motion. We infer that the relaxation is mediated by the internuclear dipolar interaction. We show how the T₁-minima data together with exact moment and hydrodynamic calculations may be used to approximate the spectral functions over their whole range. Thus we obtain numerical values for the exchange frequency for all regions of data. Our method is quite general and applicable to other systems.

Photon spectra arising from positrons annihilating at surfaces in exfoliated graphite have been measured over a range of temperatures. The role in the annihilation process of physisorbed equilibrium layers of argon, nitrogen, and oxygen has been observed. With argon and nitrogen the formation of orthopositronium is indicated by a three-to-two γ-ray ratio; for nitrogen maximum o-Ps was found to occur on completion of one monolayer. In contrast, a Doppler analysis shows that oxygen yields parapositronium. In the case of nitrogen, the results allow the estimation of the coverage, the binding energy of the molecule to the carbon substrate, and the activation energy for the process of Ps emission from the surface traps.

Low-temperature NMR studies of solid hydrogen at reduced ortho concentrations X<55% are interpreted in terms of possible quadrupolar glass phases in which the ortho molecules are frozen into random configurations. The quadrupolar glass is an extension of the concept of the dipolar glass of spins introduced by Edwards and Anderson.

Measurements are reported of NMR linewidth, 1 / T₂, for ³He atoms adsorbed on Grafoil at 1 K for various quantities of gas adsorbed. The data show three well-defined regions corresponding to a two-dimensional fluid, a two-dimensional solid, and a region where a second layer of adsorbed atoms is forming. A sharp minimum in T₂ gives a precise indication of monolayer completion. A theory for tunneling in the two-dimensional solid is presented.