23,101 research outputs found

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

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

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

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

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?

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

We report high-resolution measurements of the in-plane thermal expansion
anisotropy in the vicinity of the electronic nematic phase in Sr$_3$Ru$_2$O$_7$
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

The electronic nematic phase of Sr$_3$Ru$_2$O$_7$ is investigated by
high-resolution in-plane thermal expansion measurements in magnetic fields
close to 8 T applied at various angles $\Theta$ off the c-axis. At
$\Theta<10^\circ$ we observe a very small ($10^{-7}$) lattice distortion which
breaks the four-fold in-plane symmetry, resulting in nematic domains with
interchanged $a$- and b-axis. At $\Theta \gtrsim 10^\circ$ the domains are
almost fully aligned and thermal expansion indicates an area-preserving lattice
distortion of order $2\times 10^{-6}$ 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 Sr$_3$Ru$_2$O$_7$ for an in-plane field

We study the relationship between the nematic phases of Sr$_3$Ru$_2$O$_7$ 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 Sr$_3$Ru$_2$O$_7$

### Anisotropy of the low-temperature magnetostriction of Sr3Ru2O7

We use high-resolution capacitive dilatometry to study the low-temperature
linear magnetostriction of the bilayer ruthenate Sr$_3$Ru$_2$O$_7$ as a
function of magnetic field applied perpendicular to the ruthenium-oxide planes
($B\parallel c$). The relative length change $\Delta L(B)/L$ 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 $\lambda(B)=d(\Delta
L/L)/dB$ at three subsequent metamagnetic transitions are observed. For $\Delta
L\perp c$, 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 Sr$_3$Ru$_2$O$_7$

Bilayer strontium ruthenate Sr$_3$Ru$_2$O$_7$ 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 $\Gamma_{\rm H}$, 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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