Search Results (35 total)

Using a variety of thermodynamic measurements made in magnetic fields, we show evidence that the diffusionless transition (DT) in many shape-memory alloys is related to significant changes in the electronic structure. We investigate three alloys that show the shape-memory effect (In-24 at. % Tl, AuZn, and U-26 at. % Nb). We observe that the DT is significantly altered in these alloys by the application of a magnetic field. Specifically, the DT in InTl-24 at. % shows a decrease in the DT temperature with increasing magnetic field. Further investigations of AuZn were performed using an ultrasonic pulse-echo technique in magnetic fields up to 45 T. Quantum oscillations in the speed of the longitudinal sound waves propagating in the [110] direction indicated a strong acoustic de Haas–van Alphen-type effect and give information about part of the Fermi surface.

We report high magnetic field linear magnetostriction experiments on CeCoIn₅ single crystals. Two features are remarkable: (i) a sharp discontinuity in all the crystallographic axes associated with the upper superconducting critical field B_c2 that becomes less pronounced as the temperature increases and (ii) a distinctive second orderlike feature observed only along the c axis in the high field (10  T≲B≤B_c2) low temperature (T≲0.35  K) region. This second order transition is observed only when the magnetic field lies within 20° of the ab planes and there is no signature of it above B_c2, which raises questions regarding its interpretation as a field induced magnetically ordered phase. Good agreement with previous results suggests that this anomaly is related to the transition to a possible Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.

The γ→α isostructural transition in the Ce_0.9-xLa_xTh_0.1 system is measured as a function of La alloying using specific heat, magnetic susceptibility, resistivity, thermal expansivity or striction measurements. A line of discontinuous transitions, as indicated by the change in volume, decreases exponentially from 118 K to close to 0 K with increasing La doping, and the transition changes from being first-order to continuous at a critical concentration, x_c≈0.14. At the tricritical point, the coefficient of the linear T term in the specific heat γ and the magnetic susceptibility increase rapidly near x_c and approach large values at x=0.35 signifying that a heavy Fermi-liquid state evolves at large doping. The Wilson ratio reaches a value above 2 for a narrow range of concentrations near x_c, where the specific heat and susceptibility vary most rapidly with the doping concentration.

We report on the results of a three-flavor oscillation analysis using Super-Kamiokande I atmospheric neutrino data, with the assumption of one mass scale dominance (Δm₁₂²=0). No significant flux change due to matter effect, which occurs when neutrinos propagate inside the Earth for θ₁₃≠0, has been seen either in a multi-GeV ν_e-rich sample or in a ν_μ-rich sample. Both normal and inverted mass hierarchy hypotheses are tested and both are consistent with observation. Using Super-Kamiokande data only, 2-dimensional 90% confidence allowed regions are obtained: mixing angles are constrained to sin⁡²θ₁₃<0.14 and 0.37<sin⁡²θ₂₃<0.65 for the normal mass hierarchy. Weaker constraints, sin⁡²θ₁₃<0.27 and 0.37<sin⁡²θ₂₃<0.69, are obtained for the inverted mass hierarchy case.

The details of Super-Kamiokande-I’s solar neutrino analysis are given. Solar neutrino measurement in Super-Kamiokande is a high statistics collection of ⁸B solar neutrinos via neutrino-electron scattering. The analysis method and results of the 1496 day data sample are presented. The final oscillation results for the data are presented also.

Plasma waveguides generated by focusing a moderate intensity laser into neutral gas with an axicon lens can be unstable to the generation of axial modulations in the channel parameters. A model is proposed in which the modulations are due to the nonlinear coupling between the axicon field and a scattered mode in the evolving channel. Good agreement is found with experimental measurements of these modulations.

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline α-uranium. In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature Θ_D=256  K. In the polycrystalline sample the heat capacity curve is thermally broadened (Θ_D=184  K), but no excess heat capacity was observed. The excess heat capacity C_ϕ (taken as the difference between the single-crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency ν₀ for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal.

The Cryogenic Dark Matter Search (CDMS) is an experiment to detect weakly interacting massive particles (WIMPs), which may constitute the universe’s dark matter, based on their interactions with Ge and Si nuclei. We report the results of an analysis of data from the first two runs of CDMS at the Soudan Underground Laboratory in terms of spin-dependent WIMP-nucleon interactions on ⁷³Ge and ²⁹Si. These data exclude new regions of WIMP parameter space, including regions relevant to spin-dependent interpretations of the annual modulation signal reported by the DAMA/NaI experiment.

We report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Underground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days, giving spectrum-weighted exposures of 34 (12) kg d for the Ge (Si) targets after cuts, averaged over recoil energies 10–100 keV for a weakly interacting massive particle (WIMP) mass of 60  GeV/c². A blind analysis was conducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceeding expected backgrounds was observed. When combined with our previous results from Soudan, the 90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6×10^-43  cm² from Ge and 3×10^-42  cm² from Si, for a WIMP mass of 60  GeV/c². The combined limit from Ge (Si) is a factor of 2.5 (10) lower than our previous results and constrains predictions of supersymmetric models.

Phonon densities of states were measured on pure uranium and solutions U-0.4% C, U-1.5% Si, and U-0.91% Fe (atomic) using inelastic neutron scattering. The solute atoms stiffened the phonons, resulting in large decreases in vibrational entropy. The vibrational entropy decrease for carbon was four times the configurational entropy increase, showing that the mixing entropy is not only negative but is dominated by vibrations. Comparison with single-crystal dispersion curves indicates that the phonon stiffening for all solutes involved the transverse optic branch propagating along (001) and displacing atoms along (010). The magnitudes of the changes were too large to be explained in terms of short-range force constant models but may originate with impurity pinning of collective modes associated with α-U’s charge density wave transitions.

The results of a search for point sources of high energy neutrinos in the northern hemisphere using data collected by AMANDA-II in the years 2000, 2001, and 2002 are presented. In particular, a comparison with the single-year result previously published shows that the sensitivity was improved by a factor of 2.2. The muon neutrino flux upper limits on selected candidate sources, corresponding to an E_ν^-2 neutrino energy spectrum, are included. Sky grids were used to search for possible excesses above the background of cosmic ray induced atmospheric neutrinos. This search reveals no statistically significant excess for the three years considered.

A high-quality electron beam can be extracted from a channel guided laser wakefield accelerator without confining the injected particles to a small region of phase. By careful choice of the injection energy, a regime can be found where uniformly phased particles are quickly bunched by the accelerator itself and subsequently accelerated to high energy. The process is particularly effective in a plasma channel because of a favorable phase shift that occurs in the focusing fields. Furthermore, particle-in-cell simulations show that the self-fields of the injected bunches actually tend to reduce the energy spread on the final beam. The final beam characteristics can be calculated using a computationally inexpensive Hamiltonian formulation when beam-loading effects are minimal.

We present the results of a search for point sources of high-energy neutrinos in the northern hemisphere using AMANDA-II data collected in the year 2000. Included are flux limits on several active-galactic-nuclei blazars, microquasars, magnetars, and other candidate neutrino sources. A search for excesses above a random background of cosmic-ray-induced atmospheric neutrinos and misreconstructed downgoing cosmic-ray muons reveals no statistically significant neutrino point sources. We show that AMANDA-II has achieved the sensitivity required to probe known TeV γ-ray sources such as the blazar Markarian 501 in its 1997 flaring state at a level where neutrino and γ-ray fluxes are equal.

Muon spin spectroscopy has been used to study in detail the onset of large-moment antiferromagnetism (LMAF) in UPt₃ as induced by Th substitution. Zero-field experiments have been carried out on a series of polycrystalline U_1-xTh_xPt₃ (0<~x<~0.05) samples in the temperature range 0.04–10 K. At low Th content (x<~0.002) magnetic ordering on the time scale of the μSR experiment (10^-8 s) is not detected. For x=0.005 a weak magnetic signal appears below T=2 K, while for 0.006<~x<~0.05, spontaneous oscillations in the μSR spectra signal the presence of the LMAF phase. The data are well described by a two-component depolarization function, combining the contribution of a polycrystalline antiferromagnet and a Kubo-Lorentzian response. However, the transition into the antiferromagnetic phase is quite broad. For x=0.01 and 0.02, a weak magnetic signal appears below about 7 K, which is well above the mean-field transition temperatures. The broadening may be a result of the effects of disorder on the time fluctuations associated with anomalous small-moment antiferromagnetism.

Data from the AMANDA-B10 detector taken during the austral winter of 1997 have been searched for a diffuse flux of high energy extraterrestrial muon neutrinos. This search yielded no excess events above those expected from background atmospheric neutrinos, leading to upper limits on the extraterrestrial neutrino flux measured at the earth. For an assumed E^-2 spectrum, a 90% classical confidence level upper limit has been placed at a level E²Φ(E)=8.4×10^-7   cm^-2 s^-1 sr^-1 GeV (for a predominant neutrino energy range 6–1000 TeV), which is the most restrictive bound placed by any neutrino detector. Some specific predicted model spectra are excluded. Interpreting these limits in terms of the flux from a cosmological distributions of sources requires the incorporation of neutrino oscillations, typically weakening the limits by a factor of 2.

Phonon densities of states (DOS) were obtained from inelastic neutron scattering measurements on Ce_0.9Th_0.1 at temperatures from 10 to 300 K. The α phase showed a significant softening of its phonon DOS when heated from 10 to 140 K. Despite the 17% volume collapse, the phonon DOS showed little change between the γ phase at 150 K and the α phase at 140 K. This is supported by analysis of the magnetic spectra showing that most of the transition entropy can be accounted for with the crystal field and changes in the ground-state spin fluctuations. We argue that the anomalous behavior of the phonon DOS originates with the volume dependence of the ground-state spin fluctuations.

We report on a search for electromagnetic and/or hadronic showers (cascades) induced by a diffuse flux of neutrinos with energies between 5 TeV and 300 TeV from extraterrestrial sources. Cascades may be produced by matter interactions of all flavors of neutrinos, and contained cascades have better energy resolution and afford better background rejection than throughgoing ν_μ-induced muons. Data taken in 1997 with the AMANDA detector were searched for events with a high-energy cascadelike signature. The observed events are consistent with expected backgrounds from atmospheric neutrinos and catastrophic energy losses from atmospheric muons. Effective volumes for all flavors of neutrinos, which allow the calculation of limits for any neutrino flux model, are presented. The limit on cascades from a diffuse flux of ν_e+ν_μ+ν_τ+ν̅ _e+ν̅ _μ+ν̅ _τ is E²(dΦ/dE)<9.8×10^-6 GeV cm^-2 s^-1 sr^-1, assuming a neutrino flavor flux ratio of 1:1:1 at the detector. The limit on cascades from a diffuse flux of ν_e+ν̅ _e is E²(dΦ/dE)<6.5×10^-6 GeV cm^-2 s^-1 sr^-1, independent of the assumed neutrino flavor flux ratio.

Single crystals of the heavy-fermion superconductor UBe₁₃ doped with 3.5% Th have been studied by neutron scattering techniques. This material undergoes two superconducting transitions at T_c1≅0.55 and T_c2≅0.40 K. Using elastic neutron scattering we found no evidence for either short- or long-range magnetic order down to temperatures of 0.15 K. Similarly, exhaustive scans at reasonably high values of the momentum transfer found no evidence of a recently predicted internal lattice rearrangement of the Be nuclei at low temperatures. However, using neutron inelastic scattering we did find weak signals with a propagation direction q=〈0.5 0.5 0〉, indicating antiferromagnetic fluctuations that are short range in real space, and are also short lived. These correlations disappear only above about 30 K, similar to effects found in pure UBe₁₃. They also show no variation on passing through T_c.

A search for nearly vertical up-going muon-neutrinos from neutralino annihilations in the center of the Earth has been performed with the AMANDA-B10 neutrino detector. The data collected in 130.1 days of live time in 1997, ∼10⁹ events, have been analyzed for this search. No excess over the expected atmospheric neutrino background has been observed. An upper limit at 90% confidence level has been obtained on the annihilation rate of neutralinos in the center of the Earth, as well as the corresponding muon flux limit, both as a function of the neutralino mass in the range 100 GeV–5000 GeV.

We report complex ac magnetic susceptibility measurements of a superconducting transition in very high-quality single-crystal α-uranium using microfabricated coplanar magnetometers. We identify an onset of superconductivity at T≈0.7 K in both the real and imaginary components of the susceptibility which is confirmed by resistivity data. A superconducting volume fraction argument, based on a comparison with a calibration YBa₂Cu₃O_7-δ sample, indicates that superconductivity in these samples may be filamentary. Our data also demonstrate the sensitivity of the coplanar micro-magnetometers, which are ideally suited to measurements in pulsed magnetic fields exceeding 100 T.

The Antarctic muon and neutrino detector array (AMANDA) began collecting data with ten strings in 1997. Results from the first year of operation are presented. Neutrinos coming through the Earth from the Northern Hemisphere are identified by secondary muons moving upward through the array. Cosmic rays in the atmosphere generate a background of downward moving muons, which are about 10⁶ times more abundant than the upward moving muons. Over 130 days of exposure, we observed a total of about 300 neutrino events. In the same period, a background of 1.05×10⁹ cosmic ray muon events was recorded. The observed neutrino flux is consistent with atmospheric neutrino predictions. Monte Carlo simulations indicate that 90% of these events lie in the energy range 66 GeV to 3.4 TeV. The observation of atmospheric neutrinos consistent with expectations establishes AMANDA-B10 as a working neutrino telescope.

The microstructural contribution to the heat capacity of α-uranium was determined by measuring the heat-capacity difference between polycrystalline and single-crystal samples from 77 to 320 K. When cooled to 77 K and then heated to about 280 K, the uranium microstructure released (3±1) J/mol of strain energy. On further heating to 300 K, the microstructure absorbed energy as it began to redevelop microstrains. Anisotropic strain-broadening parameters were extracted from neutron-diffraction measurements on polycrystals. Combining the strain-broadening parameters with anisotropic elastic constants from the literature, the microstructural strain energy is predicted in the two limiting cases of statistically isotropic stress and statistically isotropic strain. The result calculated in the limit of statistically isotropic stress was (3.7±0.5) J/mol K at 77 K and (1±0.5) J/mol at room temperature. In the limit of statistically isotropic strain, the values were (7.8±0.5) J/mol K at 77 K and (4.5±0.5) J/mol at room temperature. In both cases the changes in the microstructural strain energy showed good agreement with the calorimetry.

We present a comprehensive report based on recent work [Phys. Rev. Lett. 84, 3085 (2000)] on resonant self-trapping and enhanced absorption of high power Bessel beams in underdense plasmas. The trapping resonance is strongly dependent on initial gas pressure, Bessel-beam geometry, and laser wavelength. Analytic estimates, and simulations using a one-dimensional Bessel-beam-plasma interaction code consistently explain the experimental observations. These results are for longer, moderate intensity pulses where the self-trapping channel is induced by laser-heated plasma thermal pressure. To explore the extension of this effect to ultrashort, intense pulsed Bessel beams, we perform propagation simulations using the code WAKE [Phys. Rev. E 53, R2068 (1996)]. We find that self-trapping can occur as a result of a plasma refractive index channel induced by the combined effects of relativistic motion of electrons and their ponderomotive expulsion.

The current work reports on the specific heat and the electrical resistivity of α-uranium at cryogenic temperatures. Measurements were made on α-uranium single crystals that have some unexpected mechanical properties. Despite the fact that α-uranium normally work hardens and often fails in a brittle manner, these crystals bend easily. Presumably, the combination of flexibility and strength comes from twinning in response to stress, and these twins can run freely during deformation. Because grain boundaries are not present, we anticipated that the characteristics of the charge density wave (CDW) might be more prominent in these crystals. For these reasons, the specific heat was measured from T≈0.5 to 110 K, using semiadiabatic calorimetry in zero field, and the electrical resistivity was measured from T≈0.1 to 0.50 K, in magnetic fields up to 80 mT using a standard four-probe ac technique. An abrupt resistance drop typical of a superconducting transition was observed as the temperature fell below 0.78 K, a temperature at which the resistance fell to 90% from its original value. A residual resistivity ratio RRR≈115 was measured from the low-temperature resistivity data. In addition, three phase transitions were clearly seen in the specific-heat data, located at T=23, 36, and 42 K. These transitions are consistent with the α₃, α₂, and α₁ CDW structures that have been previously observed in uranium metal. Analysis of the specific-heat data give an electronic specific heat (γ)=9.13 mJ K^-2 mol^-1 and a low-temperature limiting Debye temperature (Θ_D)=256 K (±0.25 K). The highest calorimetric value measured previously was 218 K. Our value of 256 K is in favorable agreement with that previously obtained from elastic constants 250 K (±2 K). The agreement between calorimetric and elastic Θ_D values, ductility at room temperature, and a RRR that is three times larger than previously reported values highlight the properties of these α-uranium single crystals.

de Haas–van Alphen measurements made on U_xTh_1-xBe₁₃ for x<~0.1 reveal that U enters the ThBe₁₃ lattice as a mixed valent impurity of average valence ∼4.7, evinced by topological changes in the Fermi surface with x. This implies that U exists in the tetravalent 5f² configuration when diluted and that the strong correlations in UBe₁₃ involve 5f²→5f¹ transitions. Possible scenarios for the nature of the U_xTh_1-xBe₁₃ ground state are discussed.