Search Results (79 total)

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 study the photoinduced ferromagnetism in the dilute magnetic semiconductors by solving a Hamiltonian model that consists of localized magnetic moments interacting with the photoexcited itinerant carriers. The spin states of the itinerant carriers are split due to the interaction with the localized magnetic moments, which are assumed to be in thermal equilibrium in the local magnetic field due to the carriers. The time dependence of the light-matter interaction term is eliminated by a unitary transformation and the resulting Hamiltonian is solved by making a Bogoliubov-Valatin transformation or by a variational approach using a Bardeen-Cooper-Schrieffer-type wave function. Without incident light, there are no carriers present to mediate magnetic interaction between the localized spins, so that the system is nonmagnetic. When light is present, the photoexcited carriers mediate a ferromagnetic interaction between the localized moments resulting in a ferromagnetic state, with a transition to a paramagnetic state as temperature is increased beyond T_c. The magnitude of T_c is determined by the parameters of the system such as the strength of the light-matter coupling, the frequency of light, interaction strength of the carriers with the localized moments, etc. Even for a sub-band-gap light frequency, there are induced carriers, primarily due to the Rabi oscillations, leading to a small but nonzero T_c. We find that for typical parameters, T_c is about a fraction of a Kelvin or so, which is sizable. In systems which are already ferromagnetic such as GaAs(Mn), the incident light would enhance the T_c by this amount, an effect which has been recently observed.

The paper presents the dynamic compound wavelet method (dCWM) for modeling the time evolution of multiscale and/or multiphysics systems via an “active” coupling of different simulation methods applied at their characteristic spatial and temporal scales. Key to this “predictive” approach is the dynamic updating of information from the different methods in order to adaptively and accurately capture the temporal behavior of the modeled system with higher efficiency than the (nondynamic) “corrective” compound wavelet matrix method (CWM), upon which the proposed method is based. The system is simulated by a sequence of temporal increments where the CWM solution on each increment is used as the initial conditions for the next. The numerous advantages of the dCWM method such as increased accuracy and computational efficiency in addition to a less-constrained and a significantly better exploration of phase space are demonstrated through an application to a multiscale and multiphysics reaction-diffusion process in a one-dimensional system modeled using stochastic and deterministic methods addressing microscopic and macroscopic scales, respectively.

We report here an anomalous peak in the composition dependence of the dielectric permittivity of non-ferroelectric (1−x)NaNbO₃-xCaTiO₃ ceramics for the composition range 0.10<x<0.20. This is reminiscent of a similar phenomenon in the ferroelectric morphotropic phase boundary ceramics. Rietveld analysis of the powder x-ray diffraction data for various compositions reveals that this peak is linked with a change of crystal structure from one orthorhombic phase in the Pbma space group for 0≤x≤0.10 to another orthorhombic phase in the Pbnm space group for x≥0.20 through an intermediate long period modulated orthorhombic phase whose lattice parameter is ∼14 times the lattice parameter of the Pbnm phase of CaTiO₃ in the [010] direction (q∼[0,1∕14,0]).

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.).

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.

Neutron diffraction studies using powder samples have been used to understand the complex sequence of low temperature phase transitions of NaNbO₃ in the temperature range from 12  to  350  K. Detailed Rietveld analysis of the diffraction data reveals that the antiferroelectric to ferroelectric phase transition occurs on cooling around 73  K, while the reverse ferroelectric to antiferroelectric transition occurs on heating at 245  K. However, the former transformation is not complete until it reaches 12  K and there is unambiguous evidence for the presence of the ferroelectric R3c phase coexisting with an antiferroelectic phase (Pbcm) over a wide range of temperatures. The coexisting phases and reported anomalous smearing of the dielectric response akin to dipole glasses and relaxors observed in the same temperature range are consistent with competing ferroelectric and antiferroelectric interactions in NaNbO₃. We have carried out theoretical lattice dynamical calculations which reveal that the free energies of the antiferroelectric Pbcm and ferroelectric R3c phases are nearly identical over a wide range of temperature. The small energy difference between the two phases is of interest as it explains the observed coexistence of these phases over a wide range of temperature. The computed double well depths and energy barriers from paraelectric Pm3̅ m to antiferroelectric Pbcm and ferroelectric R3c phases in NaNbO₃ are also quite similar, although the ferroelectric R3c phase has a slightly lower energy.

Negative thermal expansion (NTE) is known in Cu₂O below 300 K and in Ag₂O up to its decomposition temperature of about 500 K. Inelastic neutron scattering measurements in Cu₂O and lattice dynamics calculations of NTE in both Cu₂O and Ag₂O are reported. The phonon density of states from a polycrystalline sample of Cu₂O is measured using the triple-axis spectrometer at Trombay. A lattice dynamical model is used for the calculations of phonon frequencies and their pressure dependence in the entire Brillouin zone. The experimental phonon spectrum is in fair agreement with calculations. The calculated Grüneisen parameters for Cu₂O have lower negative values in comparison with Ag₂O. This results in a smaller value of negative thermal expansion coefficient in Cu₂O, in fair agreement with the experimental data from the literature. An important librational mode is identified that is related to NTE. Variance of bond lengths (⟨u_bond²⟩) and their temperature dependence have been calculated and compared with extended x-ray absorption fine structure data that illustrates the nature of the M-O and M-M (M=Cu,Ag) bonds in these compounds.

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.

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.).

Two-dimensional nonequilibrium nematic steady states, as found in agitated granular-rod monolayers or films of orientable amoeboid cells, were predicted [Europhys. Lett. 62, 196 (2003)] to have giant number fluctuations, with the standard deviation proportional to the mean. We show numerically that the steady state of such systems is macroscopically phase separated, yet dominated by fluctuations, as in the Das-Barma model [Phys. Rev. Lett. 85, 1602 (2000)]. We suggest experimental tests of our findings in granular and living-cell systems.

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.

We study the spin dependent tunneling of electrons through a zinc-blende semiconductor with the indirect X (or Δ) minimum serving as the tunneling barrier. The basic difference between tunneling through the Γ vs the X barrier is the linear-k spin-orbit splitting of the two spin bands at the X point, as opposed to the k³ Dresselhaus splitting at the Γ point. The linear coefficient of the spin splitting at the X point is computed for several semiconductors using density-functional theory and the transport characteristics are calculated using the barrier tunneling model. We show that both the transmission coefficient as well as the spin polarization can be large, suggesting the potential application of these materials as spin filters.

We study quantum transport through a C₆₀-X molecule with the extra atom X placed at the center of C₆₀ by computing the transmission coefficient via the Green’s function technique and relating it to the conductance via the Landauer-Büttiker formula. Depending on the energy of the extra atom at the center and its coupling to the rest of the system, the transmission is predicted to show qualitatively different features. The current could pass mainly through the extra atom or through the surface of the molecule, depending on the electronic structure of the extra atom at the center of the C₆₀.

Measurements of the local susceptibility and 3d spin relaxation rate of single Fe impurities embedded in Cu and Ag nanoparticles indicate a strong influence of lattice size on the magnetism and Kondo temperature, T_K. With a reduction in particle size, T_K increases in nanocrystalline Ag, but decreases in nanocrystalline Cu. Supported by macroscopic host susceptibility data that indicate enhanced Pauli paramagnetism for Cu nanoparticles, we suggest that size-induced host spin polarization stronglyinfluences the Kondo behavior.

Dark currents are calculated in Al_0.27Ga_0.73As/GaAs heterojunction based quantum well taking the thermionic field emission current into account. In our model the currents are calculated by taking emission of electrons from the bound state inside the well to the continuum scattered by the coupled plasmon-phonon modes. The calculated dark currents are compared with those obtained taking the sum of thermionic field emission currents arising individually from plasmon- and phonon-scattering processes and experiments. Compared to previous theoretical results the present dark currents agree reasonably well with experiments due to the inclusion of dynamical screening processes.

We formulate the photonic band-structure problem for a one-dimensional photonic crystal in terms of the reflection and transmission coefficients, obtaining a transcendental photonic band equation. The reflection and the transmission coefficients may be evaluated by using the standard transfer-matrix method. The structure of the equation reveals the existence of gaps, analogous to the Kronig-Penney model in the electronic band-structure problem. As an example, the photonic band equation is solved for the simple case of the “Kronig-Penney” dielectric structure, consisting of alternating slabs of refractive indices n₁ and n₂.

Ranjan et al. [Phys. Rev. B 65, 060102(R) (2002)] have recently presented results of a powder neutron-diffraction study of the high-temperature monoclinic (F_M^HT) to low-temperature monoclinic (F_M^LT) phase transition in Pb(Ti_1-xZr_x)O₃ discovered by Ragini et al. [Phys. Rev. B 64, 054101 (2001)]. They attribute the presence of superlattice reflections in the diffraction data to tilting of oxygen octahedra and propose a monoclinic space group Pc for the F_M^LT phase. It is shown that for the model proposed by Ranjan et al., the correct space group of the F_M^LT phase should be Cc. This has also been corroborated by a group-theoretical approach to the problem. A different set of refined structural parameters for the Cc space group obtained from the Rietveld analysis of the powder neutron-diffraction data of Ranjan et al. is also presented.

Results of a powder neutron diffraction study of the high-temperature monoclinic (F_M^HT) to a low-temperature monoclinic (F_M^LT) phase transition in Pb(Ti_1-xZr_x)O₃ discovered recently by Ragini et al. are presented for x=0.520. The powder neutron diffraction pattern of the F_M^LT phase contains superlattice reflections which have all odd hkl (h≠k≠l) Miller indices with respect to a doubled elementary perovskite cell, characteristic of antiphase tilting of oxygen octahedŕa. In analogy to other perovskite systems, the appearance of these superlattice reflections is attributed to an instability at the R point of the cubic Brillouin zone. We show that the most plausible space group of the F_M^LT phase is Pc. Results of Rietveld refinement for this space group are presented to show that the oxygen octahedra indeed undergo antiphase rotations about [001] direction.

We report on the extraction of R  =  σ_L / σ_T from CCFR ν_μ-Fe and ν̅ _μ-Fe differential cross sections. The CCFR differential cross sections do not show the deviations from the QCD expectations that are seen in the CDHSW data at very low and very high x. R as measured in ν_μ scattering is in agreement with R as measured in muon and electron scattering. All data on R for Q²>1 GeV² are in agreement with a NNLO QCD calculation which uses NNLO parton distribution functions and includes target mass effects. We report on the first measurements of R in the low x and Q²<1 GeV² region (where an anomalous large rise in R for nuclear targets has been observed by the HERMES Collaboration).

The Raman scattering measurements on ceramic pellets of Sr_0.70Ca_0.30TiO₃ (SCT 30) have been carried out as a function of temperature from 290 K to 8 K. It is shown that below (236±5 K), two additional lines appear around 79 and 128 cm^-1. Further, there is enhancement in the intensity of the modes near 170 and 541 cm^-1 below (236±5 K). The temperature at which these changes occur in the Raman spectra of SCT 30 correlates very well with the temperature at which the antiferroelectric superlattice reflections appear in the x-ray diffraction patterns. We propose that the additional modes are due to q≠0 phonons that have become Raman active due to the folding of the corresponding special points into the zone center below the cell-doubling antiferroelectric phase-transition temperature.

Results of dielectric and resonance frequency (f_r) measurements below room temperature are presented for Pb(Zr_xTi_1-x)O₃, x=0.515 and 0.520. It is shown that the temperature coefficient of f_r changes sign from negative to positive around 210 and 265 K for x=0.520 and 200 and 260 K for x=0.515. Anomalies in the real part of the dielectric constant (ɛ^′) are observed around the same temperatures at which the temperature coefficient of f_r changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the ɛ^′ vs T and f_r vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of Pb(Zr_xTi_1-x)O₃ below the second transition temperature.

A new structure function analysis of CCFR deep inelastic ν-N and ν̅ -N scattering data is presented for previously unexplored kinematic regions down to Bjorken x  =  0.0045 and Q²  =  0.3 GeV². Comparisons to charged lepton scattering data from NMC and E665 experiments are made and the behavior of the structure function F₂^ν is studied in the limit Q²→0.

By applying perturbed angular-correlation spectroscopy we have investigated spin correlations in the “heavy-fermion” compounds U(In_1-xSn_x)₃, with x=1.0, 0.7, and 0.5, by measuring the induced magnetic hyperfine field B_ind at diamagnetic ¹¹¹Cd probe nuclei as a function of temperature and applied magnetic field. In zero applied field, the absence of any detectable magnetic hyperfine field at Cd reveals the absence of static magnetic correlations down to 4.2 K. However, from the field dependence of B_ind we find evidence for the presence of field-induced, short-ranged, and dynamic spin correlation between U f electrons at all compositions. The strength and dynamics of these correlated spins strongly depend on x, temperature, and applied magnetic field. As an important feature, for compositions near x=0.5, classified as a heavy-fermion material with electronic specific heat coefficient γ=500 mJ/mol K², these U spin correlations seem to set in from a relatively high temperature (>~37 K), and become very large on lowering temperature and/or increasing magnetic field, reflected in the measured Knight shift value K of about -32% at 4.2 K and B_app=7 T. We believe that these short-range spin correlations and their relaxation dynamics are responsible for the low temperature increase in magnetic susceptibility and electronic specific heat, previously considered to be an indication of heavy-fermion behavior in this system.

We report on the extraction of the structure functions F₂ and ΔxF₃  =  xF₃^ν-xF₃^ν̅ from CCFR ν_μ-Fe and ν̅ _μ-Fe differential cross sections. The extraction is performed in a physics model-independent (PMI) way. This first measurement of ΔxF₃, which is useful in testing models of heavy charm production, is higher than current theoretical predictions. The ratio of the F₂ (PMI) values measured in ν_μ and μ scattering is in agreement (within 5%) with the predictions of next-to-leading-order parton distribution functions using massive charm production schemes, thus resolving the long-standing discrepancy between the two sets of data.