Search Results (172 total)

We report large scale determinant quantum Monte Carlo calculations of the effective bandwidth, momentum distribution, and magnetic correlations of the square lattice fermion Hubbard Hamiltonian at half-filling. The sharp Fermi surface of the noninteracting limit is significantly broadened by the electronic correlations but retains signatures of the approach to the edges of the first Brillouin zone as the density increases. Finite-size scaling of simulations on large lattices allows us to extract the interaction dependence of the antiferromagnetic order parameter, exhibiting its evolution from weak-coupling to the strong-coupling Heisenberg limit. Our lattices provide improved resolution of the Green’s function in momentum space, allowing a more quantitative comparison with time-of-flight optical lattice experiments.

We experimentally study the excitonic Fano resonance in GaAs/Al_0.3Ga_0.7As multiple quantum wells under an external electric field. The resonance results from exciton transitions via localized states and continuum minibands of Wannier-Stark states. In photoreflectance spectra the line shape displays anomalous behavior as a function of increasing field. Our theoretical analysis reveals that this effect, which is absent for the conventional Fano resonance, arises from the field dependence of the nonuniform density of continuum states.

We study the pattern evolution of pre-rippled Au(001) during sputtering by an ion beam that is incident perpendicular to the initial ripple in azimuth at a grazing angle. Prepatterned ripples decay exponentially with time and new ripples develop only after extended flat areas form along the crossing-ion beams. Hence, the superposition of the initial and new ripple patterns does not occur. The kinetic behaviors of new ripples growing on pre-rippled Au(001) by the crossing-ion beams are distinct from those on initially flat Au(001). When comparing the pre-rippled surface to the initially flat surface, the morphological evolution is substantially influenced by enhanced nonlinear effects such as redeposition.

We present the first x-ray scattering measurements of the state of compression and heating in laser irradiated solid beryllium. The scattered spectra at two different angles show Compton and plasmon features indicating a dense Fermi-degenerate plasma state with a Fermi energy above 30 eV and with temperatures in the range of 10–15 eV. These measurements indicate compression by a factor of 3 in agreement with Hugoniot data and detailed radiation-hydrodynamic modeling.

We report quantum Monte Carlo calculations of biexciton binding energies in ideal two-dimensional bilayer systems with isotropic electron and hole masses. We have also calculated exciton-exciton interaction potentials and pair-distribution functions for electrons and holes in bound biexcitons. Comparing our data with results obtained in a recent study using a model exciton-exciton potential [Schindler and Zimmermann, Phys. Rev. B 78, 045313 (2008)], we find a somewhat larger range of layer separations at which biexcitons are stable. We find that individual excitons retain their identity in bound biexcitons for large layer separations.

We study the spatial decay of electron coherence due to electron-electron interaction in a finite-length disorder-free quantum wire. Based on the Luttinger liquid theory, we demonstrate that the coherence length characterizing the exponential decay of the coherence can vary from region to region, and that the coherence can even revive after the decay. This counterintuitive behavior, which is in clear contrast to the conventional exponential decay with single coherence length, is due to the fractionalization of an electron and the finite-size-induced recombination of the fractions.

We obtain the shielding of the nuclear magnetic moment by an electron in the ns_1∕2 state in a hydrogenlike ion with arbitrary nuclear charge Z. The calculation is based on the decomposition of the shielding correction into two terms corresponding to the intermediate states with j=1∕2 and 3∕2. The former is evaluated using the j=1∕2 projection of the correction to the electron wave function due to the external magnetic field, while the latter is calculated by applying the j=3∕2 projection of the correction to the wave function due to the hyperfine interaction between the electron and the nucleus.

A method for direct determination of resonance phase shifts in a (001) CdTe/InSb thin-film system is developed using soft x-ray three-wave resonance diffraction. At the (002) Bragg peaks of CdTe and InSb, two inversion-symmetry related three-wave diffractions are systematically identified according to crystal symmetry and the resonance phase shifts versus photon energies are measured without turning the thin film upside down. The momentum-transfer selectivity at (002) reflections facilitates the quantitative determination of the phase shifts near the Cd L₃, Te L₃, and Sb L₂ edges.

We report the results of a search for ν_μ disappearance by the Main Injector Neutrino Oscillation Search [D. G. Michael (MINOS), Phys. Rev. Lett. 97, 191801 (2006).]. The experiment uses two detectors separated by 734 km to observe a beam of neutrinos created by the Neutrinos at the Main Injector facility at Fermi National Accelerator Laboratory. The data were collected in the first 282 days of beam operations and correspond to an exposure of 1.27×10²⁰ protons on target. Based on measurements in the Near Detector, in the absence of neutrino oscillations we expected 336±14 ν_μ charged-current interactions at the Far Detector but observed 215. This deficit of events corresponds to a significance of 5.2 standard deviations. The deficit is energy dependent and is consistent with two-flavor neutrino oscillations according to |Δm²|=2.74_-0.26^+0.44×10^-3  eV²/c⁴ and sin⁡²2θ>0.87 at 68% confidence level.

We theoretically study Fano resonance in a two-level quantum dot side-coupled to two leads, which are connected by a direct channel. The resonance line shape is found to be deformed, from the conventional Fano form, by interlevel Coulomb interaction and interlevel interference. We derive the connection between the line-shape deformation and the interaction-induced nonmonotonicity of level occupation, which may be useful for experimental study. The dependence of the line shape on the transmission of the direct channel and on the dot-lead coupling matrix elements is discussed.

Elongated Bose-Einstein condensates (BECs) exhibit strong spatial phase fluctuations even well below the BEC transition temperature. We demonstrate that atom interferometers using such condensates are robust against phase fluctuations; i.e., the relative phase of the split condensate is reproducible despite axial phase fluctuations. However, larger phase fluctuations limit the coherence time, especially in the presence of some asymmetries in the two wells of the interferometer.

The velocity of a ∼3  GeV neutrino beam is measured by comparing detection times at the near and far detectors of the MINOS experiment, separated by 734 km. A total of 473 far detector neutrino events was used to measure (v-c)/c=5.1±2.9×10^-5 (at 68% C.L.). By correlating the measured energies of 258 charged-current neutrino events to their arrival times at the far detector, a limit is imposed on the neutrino mass of m_ν<50  MeV/c² (99% C.L.).

X-ray diffraction from a growing film at an anti-Bragg point should exhibit bilayer oscillations caused by interference. In an experiment of TiN film growth by laser ablation onto sapphire, an unexpected beating envelope function is found to modulate the oscillations. The successive nodes and antinodes are identified with the development of new growth domains separated by one atomic layer in thickness. This effect allows atomic layer counting of the film thickness distribution. The results imply that the growth is not characterized by a continuum stochastic process, as usually assumed.

The 5.4 kton MINOS far detector has been taking charge-separated cosmic ray muon data since the beginning of August, 2003 at a depth of 2070 m.w.e. in the Soudan Underground Laboratory, Minnesota, USA. The data with both forward and reversed magnetic field running configurations were combined to minimize systematic errors in the determination of the underground muon charge ratio. When averaged, two independent analyses find the charge ratio underground to be N_μ⁺/N_μ^-=1.374±0.004(stat)_-0.010^+0.012(sys). Using the map of the Soudan rock overburden, the muon momenta as measured underground were projected to the corresponding values at the surface in the energy range 1–7 TeV. Within this range of energies at the surface, the MINOS data are consistent with the charge ratio being energy independent at the 2 standard deviation level. When the MINOS results are compared with measurements at lower energies, a clear rise in the charge ratio in the energy range 0.3–1.0 TeV is apparent. A qualitative model shows that the rise is consistent with an increasing contribution of kaon decays to the muon charge ratio.

The dynamic magnetic properties of two-dimensional periodic Co antidot arrays were studied by X-band ferromagnetic resonance. The experimental results on geometrically scaled antidot arrays reveal a strong attenuation of the uniform ferromagnetic resonance mode in comparison to a continuous film, but an excitation of nonuniform in-plane spin-wave modes. Micromagnetic finite-element simulations show that the static magnetic structure in an antidot array depends on the direction of the external field with respect to the symmetry axes of the antidot lattice, even if the external field is strong enough to enforce a technically saturated magnetization state. The analysis gives evidence that characteristic inhomogeneities in the magnetization distribution around the antidots give rise to the changes of the resonance modes with the in-plane direction of the magnetization.

We found 140 neutrino-induced muons in 854.24 live days in the MINOS far detector, which has an acceptance for neutrino-induced muons of 6.91×10⁶  cm² sr. We looked for evidence of neutrino disappearance in this data set by computing the ratio of the number of low momentum muons to the sum of the number of high momentum and unknown momentum muons for both data and Monte Carlo expectation in the absence of neutrino oscillations. The ratio of data and Monte Carlo ratios, R, is R=0.65_-0.12^+0.15(stat)±0.09(syst), a result that is consistent with an oscillation signal. A fit to the data for the oscillation parameters sin⁡²2θ₂₃ and Δm₂₃² excludes the null oscillation hypothesis at the 94% confidence level. We separated the muons into μ^- and μ⁺ in both the data and Monte Carlo events and found the ratio of the total number of μ^- to μ⁺ in both samples. The ratio of those ratios, R-^ _CPT, is a test of CPT conservation. The result R-^ _CPT=0.72_-0.18^+0.24(stat)_-0.04^+0.08(syst) is consistent with CPT conservation.

The recombination of two split Bose-Einstein condensates on an atom chip is shown to result in heating which depends on the relative phase of the two condensates. This heating reduces the number of condensate atoms between 10% and 40% and provides a robust way to read out the phase of an atom interferometer without the need for ballistic expansion. The heating may be caused by the dissipation of dark solitons created during the merging of the condensates.

We present the first collective x-ray scattering measurements of plasmons in solid-density plasmas. The forward scattering spectra of a laser-produced narrow-band x-ray line from isochorically heated beryllium show that the plasmon frequency is a sensitive measure of the electron density. Dynamic structure calculations that include collisions and detailed balance match the measured plasmon spectrum indicating that this technique will enable new applications to determine the equation of state and compressibility of dense matter.

This Letter reports results from the MINOS experiment based on its initial exposure to neutrinos from the Fermilab NuMI beam. The rates and energy spectra of charged current ν_μ interactions are compared in two detectors located along the beam axis at distances of 1 and 735 km. With 1.27×10²⁰ 120 GeV protons incident on the NuMI target, 215 events with energies below 30 GeV are observed at the Far Detector, compared to an expectation of 336±14 events. The data are consistent with ν_μ disappearance via oscillations with |Δm₃₂²|=2.74_-0.26^+0.44×10^-3  eV² and sin⁡²(2θ₂₃)>0.87 (68% C.L.).

We report the details of the recent x-ray back diffraction experiments, in which interference fringes due to x-ray cavity resonance are unambiguously observed. The Fabry-Pérot type cavities, the tested crystal devices of reflectivity R≃0.5 and finesse F≃2.3, consist of monolithic two-plate and eight-plate silicon crystals. They were prepared by using x-ray lithographic techniques. The thicknesses of the crystal plates and the gaps between the two adjacent plates are a few tens to hundreds μm. The (12 4 0) back reflection and synchrotron x-radiation of energy resolution ΔE=0.36 meV at 14.4388 keV are employed. Interference fringes in angle- and photon-energy scans for two-plate and eight-plate cavities are shown. Considerations on the temporal and spatial coherence for observable resonance interference fringes using synchrotron x-rays are presented. The details about the accompanied simultaneous 24-beam diffraction in relation to x-ray photon energy are also described.

In this paper, we study the effect of the position of a positively charged impurity, or so-called donor, in a single-electron two-dimensional (2D) magnetic quantum ring, where the magnetic field is zero within the ring and constant elsewhere, on the low-lying spectra of such a neutral donor D⁰ system (involving a positively charged impurity surrounded by a single electron) under perpendicular magnetic fields. The optical absorption spectra with their corresponding intensities between the ground state and the first excited state are calculated. The donor is located at a distance d as measured vertically from the plane of the ring along the z direction. The ground-state orbital angular momentum (L) transitions induced by magnetic fields are overall presented in terms of a magnetic field versus donor position (B-d) phase diagram. Dependences of the absorption spectra and their intensities on the layer thickness for the quasi-two-dimensional (q2D) magnetic quantum ring are also discussed.

We employ angle-resolved photoemission to study the electronic structure of atomically uniform films of Ag grown on Ge(111). A new kind of quantum well state is observed near a specific emission direction away from the surface normal. In contrast with the usual quantum well state arising from electron confinement by specular reflections at the surface and interface of the film, the new kind involves retroreflections, or umklapp reflections, at the interface. It requires four reflections, instead of the usual two reflections, to complete a coherent interference path.

The complete 5.4 kton MINOS far detector has been taking data since the beginning of August 2003 at a depth of 2070 meters water-equivalent in the Soudan mine, Minnesota. This paper presents the first MINOS observations of ν_μ and ν̅ _μ charged-current atmospheric neutrino interactions based on an exposure of 418 days. The ratio of upward- to downward-going events in the data is compared to the Monte Carlo expectation in the absence of neutrino oscillations, giving R_up/down^data/R_up/down^MC=0.62_-0.14^+0.19(stat.)±0.02(sys.). An extended maximum likelihood analysis of the observed L/E distributions excludes the null hypothesis of no neutrino oscillations at the 98% confidence level. Using the curvature of the observed muons in the 1.3 T MINOS magnetic field ν_μ and ν̅ _μ interactions are separated. The ratio of ν̅ _μ to ν_μ events in the data is compared to the Monte Carlo expectation assuming neutrinos and antineutrinos oscillate in the same manner, giving R_{ν̅ μ/νμ}^data/R_{ν̅ μ/νμ}^MC=0.96_-0.27^+0.38(stat.)±0.15(sys.), where the errors are the statistical and systematic uncertainties. Although the statistics are limited, this is the first direct observation of atmospheric neutrino interactions separately for ν_μ and ν̅ _μ.

Formulas for the corrections to the energy levels and wave functions of a spin-zero particle bound in a strong field are derived. The general case of the sum of a Lorentz-scalar potential and zero component of a Lorentz-vector potential is considered. The forms of the corrections differ essentially from those for spin-1 / 2 particles. As an example of application of our results, we evaluated the electric polarizability of a ground state of a spin-zero particle bound in a strong Coulomb field.