23,101 research outputs found

    Estimation of within-stratum variance for sample allocation: Foreign commodity production forecasting

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    The problem of determining the stratum variances required for an optimum sample allocation for remotely sensed crop surveys is investigated with emphasis on an approach based on the concept of stratum variance as a function of the sampling unit size. A methodology using the existing and easily available information of historical statistics is developed for obtaining initial estimates of stratum variances. The procedure is applied to variance for wheat in the U.S. Great Plains and is evaluated based on the numerical results obtained. It is shown that the proposed technique is viable and performs satisfactorily with the use of a conservative value (smaller than the expected value) for the field size and with the use of crop statistics from the small political division level

    Stratum variance estimation for sample allocation in crop surveys

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    The problem of determining stratum variances needed in achieving an optimum sample allocation for crop surveys by remote sensing is investigated by considering an approach based on the concept of stratum variance as a function of the sampling unit size. A methodology using the existing and easily available information of historical crop statistics is developed for obtaining initial estimates of tratum variances. The procedure is applied to estimate stratum variances for wheat in the U.S. Great Plains and is evaluated based on the numerical results thus obtained. It is shown that the proposed technique is viable and performs satisfactorily, with the use of a conservative value for the field size and the crop statistics from the small political subdivision level, when the estimated stratum variances were compared to those obtained using the LANDSAT data

    Operator Evolution via the Similarity Renormalization Group I: The Deuteron

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    Similarity Renormalization Group (SRG) flow equations can be used to unitarily soften nuclear Hamiltonians by decoupling high-energy intermediate state contributions to low-energy observables while maintaining the natural hierarchy of many-body forces. Analogous flow equations can be used to consistently evolve operators so that observables are unchanged if no approximations are made. The question in practice is whether the advantages of a softer Hamiltonian and less correlated wave functions might be offset by complications in approximating and applying other operators. Here we examine the properties of SRG-evolved operators, focusing in this paper on applications to the deuteron but leading toward methods for few-body systems. We find the advantageous features generally carry over to other operators with additional simplifications in some cases from factorization of the unitary transformation operator.Comment: 33 pages, 19 figures. Improved figures 17 and 18. Expanded comments on OPE in tex

    Operator Evolution via the Similarity Renormalization Group I: The Deuteron

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    Similarity Renormalization Group (SRG) flow equations can be used to unitarily soften nuclear Hamiltonians by decoupling high-energy intermediate state contributions to low-energy observables while maintaining the natural hierarchy of many-body forces. Analogous flow equations can be used to consistently evolve operators so that observables are unchanged if no approximations are made. The question in practice is whether the advantages of a softer Hamiltonian and less correlated wave functions might be offset by complications in approximating and applying other operators. Here we examine the properties of SRG-evolved operators, focusing in this paper on applications to the deuteron but leading toward methods for few-body systems. We find the advantageous features generally carry over to other operators with additional simplifications in some cases from factorization of the unitary transformation operator.Comment: 33 pages, 19 figures. Improved figures 17 and 18. Expanded comments on OPE in tex

    Are low-energy nuclear observables sensitive to high-energy phase shifts?

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    Conventional nucleon-nucleon potentials with strong short-range repulsion require contributions from high-momentum wave function components even for low-energy observables such as the deuteron binding energy. This can lead to the misconception that reproducing high-energy phase shifts is important for such observables. Interactions derived via the similarity renormalization group decouple high-energy and low-energy physics while preserving the phase shifts from the starting potential. They are used to show that high-momentum components (and high-energy phase shifts) can be set to zero when using low-momentum interactions, without losing information relevant for low-energy observables.Comment: 13 pages, 5 figures; reference and acknowledgment adde

    Electronic nematicity and its relation to quantum criticality in Sr_3Ru_2O_7 studied by thermal expansion

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    We report high-resolution measurements of the in-plane thermal expansion anisotropy in the vicinity of the electronic nematic phase in Sr3_3Ru2_2O7_7 down to very low temperatures and in varying magnetic field orientation. For fields applied along the c-direction, a clear second-order phase transition is found at the nematic phase, with critical behavior compatible with the two-dimensional Ising universality class (although this is not fully conclusive). Measurements in a slightly tilted magnetic field reveal a broken four-fold in-plane rotational symmetry, not only within the nematic phase, but extending towards slightly larger fields. We also analyze the universal scaling behavior expected for a metamagnetic quantum critical point, which is realized outside the nematic region. The contours of the magnetostriction suggest a relation between quantum criticality and the nematic phase.Comment: 8 pages, 12 Figures, invited paper at QCNP 2012 conferenc

    Symmetry-breaking lattice distortion in Sr_3Ru_2O_7

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    The electronic nematic phase of Sr3_3Ru2_2O7_7 is investigated by high-resolution in-plane thermal expansion measurements in magnetic fields close to 8 T applied at various angles Θ\Theta off the c-axis. At Θ<10\Theta<10^\circ we observe a very small (10710^{-7}) lattice distortion which breaks the four-fold in-plane symmetry, resulting in nematic domains with interchanged aa- and b-axis. At Θ10\Theta \gtrsim 10^\circ the domains are almost fully aligned and thermal expansion indicates an area-preserving lattice distortion of order 2×1062\times 10^{-6} which is likely related to orbital ordering. Since the system is located in the immediate vicinity to a metamagnetic quantum critical end point, the results represent the first observation of a structural relaxation driven by quantum criticality.Comment: 4 pages, 5 figures, PRL accepte

    Pressure study of nematicity and quantum criticality in Sr3_3Ru2_2O7_7 for an in-plane field

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    We study the relationship between the nematic phases of Sr3_3Ru2_2O7_7 and quantum criticality. At ambient pressure, one nematic phase is associated with a metamagnetic quantum critical end point (QCEP) when the applied magnetic field is near the \textit{c}-axis. We show, however, that this metamagnetic transition does not produce the same nematic signatures when the QCEP is reached by hydrostatic pressure with the field applied in the \textit{ab}-plane. Moreover, a second nematic phase, that is seen for field applied in the \textit{ab}-plane close to, but not right at, a second metamagnetic anomaly, persists with minimal change to the highest applied pressure, 16.55 kbar. Taken together our results suggest that metamagnetic quantum criticality may not be necessary for the formation of a nematic phase in Sr3_3Ru2_2O7_7

    Anisotropy of the low-temperature magnetostriction of Sr3Ru2O7

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    We use high-resolution capacitive dilatometry to study the low-temperature linear magnetostriction of the bilayer ruthenate Sr3_3Ru2_2O7_7 as a function of magnetic field applied perpendicular to the ruthenium-oxide planes (BcB\parallel c). The relative length change ΔL(B)/L\Delta L(B)/L is detected either parallel or perpendicular to the c-axis close to the metamagnetic region near B=8 T. In both cases, clear peaks in the coefficient λ(B)=d(ΔL/L)/dB\lambda(B)=d(\Delta L/L)/dB at three subsequent metamagnetic transitions are observed. For ΔLc\Delta L\perp c, the third transition at 8.1 T bifurcates at temperatures below 0.5 K. This is ascribed to the effect of an in-plane uniaxial pressure of about 15 bar, unavoidable in the dilatometer, which breaks the original fourfold in-plane symmetry.Comment: 3 pages, 3 Figures, Manuscript for Proceedings of the International Conference on Quantum Criticality and Novel Phases (QCNP09, Dresden

    Multiple metamagnetic quantum criticality in Sr3_3Ru2_2O7_7

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    Bilayer strontium ruthenate Sr3_3Ru2_2O7_7 displays pronounced non-Fermi liquid behavior at magnetic fields around 8 T, applied perpendicular to the ruthenate planes, which previously has been associated with an itinerant metamagnetic quantum critical end point (QCEP). We focus on the magnetic Gr\"uneisen parameter ΓH\Gamma_{\rm H}, which is the most direct probe to characterize field-induced quantum criticality. We confirm quantum critical scaling due to a putative two-dimensional QCEP near 7.845(5) T, which is masked by two ordered phases A and B, identified previously by neutron scattering. In addition we find evidence for a QCEP at 7.53(2) T and determine the quantum critical regimes of both instabilities and the effect of their superposition
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