10,059 research outputs found
Optimal Invariant Similar Tests for Instrumental Variables Regression
This paper considers tests of the parameter on endogenous variables in an instrumental variables regression model. The focus is on determining tests that have some optimal power properties. We start by considering a model with normally distributed errors and known error covariance matrix. We consider tests that are similar and satisfy a natural rotational invariance condition. We determine tests that maximize weighted average power (WAP) for arbitrary weight functions among invariant similar tests. Such tests include point optimal (PO) invariant similar tests. The results yield the power envelope for invariant similar tests. This allows one to assess and compare the power properties of existing tests, such as the Anderson-Rubin, Lagrange multiplier (LM), and conditional likelihood ratio (CLR) tests, and new optimal WAP and PO invariant similar tests. We find that the CLR test is quite close to being uniformly most powerful invariant among a class of two-sided tests. A new unconditional test, P*, also is found to have this property. For one-sided alternatives, no test achieves the invariant power envelope, but a new test -- the one-sided CLR test -- is found to be fairly close. The finite sample results of the paper are extended to the case of unknown error covariance matrix and possibly non-normal errors via weak instrument asymptotics. Strong instrument asymptotic results also are provided because we seek tests that perform well under both weak and strong instruments.Instrumental variables regression, invariant tests, optimal tests, similar tests, weak instruments, weighted average power
Spin resonance in the d-wave superconductor CeCoIn5
Neutron scattering is used to probe antiferromagnetic spin fluctuations in
the d-wave heavy fermion superconductor CeCoIn (T=2.3 K).
Superconductivity develops from a state with slow (=0.3 0.15
meV) commensurate (=(1/2,1/2,1/2)) antiferromagnetic spin
fluctuations and nearly isotropic spin correlations. The characteristic
wavevector in CeCoIn is the same as CeIn but differs from the
incommensurate wavevector measured in antiferromagnetically ordered
CeRhIn. A sharp spin resonance ( meV) at
= 0.60 0.03 meV develops in the superconducting state removing spectral
weight from low-energy transfers. The presence of a resonance peak is
indicative of strong coupling between f-electron magnetism and
superconductivity and consistent with a d-wave gap order parameter satisfying
.Comment: (5 pages, 4 figures, to be published in Phys. Rev. Lett.
Continuation of a deep borehole stress measurement profile near the San Andreas Fault: 2. Hydraulic fracturing stress measurements at Black Butte, Mojave Desert, California
Hydraulic fracturing stress measurements were obtained in the Black Butte drill hole, 18 km northeast of the San Andreas fault in the Mojave Desert, at depths from 251 to 635 m. In all tests the least and greatest horizontal principal stresses (S_h and S_H, respectively) exceeded the vertical stress (S_ν), indicating a thrust faulting stress regime. A single good-quality hydraulic fracture impression from 309 m depth indicates an S_H direction of N41°E ± 10°. This S_H direction should be interpreted with caution because it is based on only one observation. This orientation is fairly compatible with nearby surface stress measurements but is incompatible with most of the hydraulic fracturing stress orientations reported from comparable depths in the Mojave Desert and is not favorable for right-lateral slip on either the San Andreas fault or NW striking faults present farther to the east. The stress regime measured in the Black Butte hole is comparable to that measured at nearby shallow depths but differs from the strike-slip or transitional (strike-slip to thrust faulting) stress regime present at similar depths in two nearby holes: Crystallaire, 4 km northeast of fhe San Andreas fault, and Hi Vista, 32 km northeast of the San Andreas fault. The S_H direction measured in these holes is approximately 60° counterclockwise of that observed in the Black Butte hole. The differences in stress magnitudes and orientation among these holes substantiate previous indications of local variations in stress in the upper kilometer of the crust in this area and cast doubt on the validity of linear elastic models in which the effects of the San Andreas fault dominate the stress field in the western Mojave Desert
Dynamic treatment of vibrational energy relaxation in a heterogeneous and fluctuating environment
A computational approach to describe the energy relaxation of a
high-frequency vibrational mode in a fluctuating heterogeneous environment is
outlined. Extending previous work [H. Fujisaki, Y. Zhang, and J.E. Straub, J.
Chem. Phys. {\bf 124}, 144910 (2006)], second-order time-dependent perturbation
theory is employed which includes the fluctuations of the parameters in the
Hamiltonian within the vibrational adiabatic approximation. This means that the
time-dependent vibrational frequencies along an MD trajectory are obtained via
a partial geometry optimization of the solute with fixed solvent and a
subsequent normal mode calculation. Adopting the amide I mode of
N-methylacetamide in heavy water as a test problem, it is shown that the
inclusion of dynamic fluctuations may significantly change the vibrational
energy relaxation. In particular, it is found that relaxation occurs in two
phases, because for short times ( 200 fs) the spectral density
appears continuous due to the frequency-time uncertainty relation, while at
longer times the discrete nature of the bath becomes apparent. Considering the
excellent agreement between theory and experiment, it is speculated if this
behavior can explain the experimentally obtained biphasic relaxation the amide
I mode of N-methylacetamide.Comment: 24 pages, 7 figures, submitted to J. Chem. Phy
Rapid transition from continental breakup to igneous oceanic crust in the South China Sea
Continental breakup represents the successful process of rifting and thinning of the continental lithosphere, leading to plate rupture and initiation of oceanic crust formation. Magmatism during breakup seems to follow a path of either excessive, transient magmatism (magma-rich margins) or of igneous starvation (magma-poor margins). The latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation. Discovery of magma-poor margins has raised fundamental questions about the onset of ocean-floor type magmatism, and has guided interpretation of seismic data across many rifted margins, including the highly extended northern South China Sea margin. Here we report International Ocean Discovery Program drilling data from the northern South China Sea margin, testing the magma-poor margin model outside the North Atlantic. Contrary to expectations, results show initiation of Mid-Ocean Ridge basalt type magmatism during breakup, with a narrow and rapid transition into igneous oceanic crust. Coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers. Asthenospheric upwelling yielding Mid-Ocean Ridge basalt-type magmatism from normal-temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre-rift lithosphere
Report on televiewer log and stress measurements in core hole USW G-2, Nevada Test Site, October-November, 1982
Hydraulic fracturing stress measurements and a borehole televiewer
log were obtained in hole USW G-2 at Yucca Mountain, Nevada, to depths of
1200 m. Results indicate that at the depths tested, the minimum and
maximum horizontal stresses are less than the vertical stress,
corresponding to a normal faulting stress regime. Drilling-induced
hydrofractures seen in the televiewer log imply a least horizontal
principal stress direction of N 60° W to N 65° W. For reasonable
values of the coefficient of friction, the magnitude of the least
horizontal stress is close to the value at which slip would occur on
preexisting faults of optimal orientation (strike N 25° E to N 30° E
and dipping 60° to 67°).
The prominent drilling-induced fractures seen in the televiewer log
are believed to have been caused by excess downhole pressures applied
during drilling the hole. Many throughgoing fractures are also seen in
the televiewer log; most of these are high angle, stringking N 10° E
to N 40° E. These fractures show a general decrease in angle of dip
with depth. Stress-induced wellbore breakouts are seen at depths below
1050 m. The average N 60° W azimuth of these breakouts agrees very
closely with the N 60° W to N 65° W direction of least horizontal
principal stress inferred from the drilling-induced hydrofracs
Different Ways of Reading, or Just Making the Right Noises?
What does reading look like? Can learning to read be reduced to the acquisition of a set of isolable skills, or proficiency in reading be equated with the independence of the solitary, silent reader of prose fiction? These conceptions of reading and reading development, which figure strongly in educational policy, may appear to be simple common sense. But both ethnographic data and evidence from literary texts suggest that such paradigms offer, at most, a partial and ahistorical picture of reading. An important dimension, neglected in the dominant paradigms, is the irreducibly social quality of reading practices
From incommensurate correlations to mesoscopic spin resonance in YbRh2Si2
Spin fluctuations are reported near the magnetic field driven quantum
critical point in YbRh2Si2. On cooling, ferromagnetic fluctuations evolve into
incommensurate correlations located at q0=+/- (delta,delta) with delta=0.14 +/-
0.04 r.l.u. At low temperatures, an in plane magnetic field induces a sharp
intra doublet resonant excitation at an energy E0=g muB mu0 H with g=3.8 +/-
0.2. The intensity is localized at the zone center indicating precession of
spin density extending xi=6 +/- 2 A beyond the 4f site.Comment: (main text - 4 pages, 4 figures; supplementary information - 3 pages,
3 figures; to be published in Physical Review Letters
Magnetic field splitting of the spin-resonance in CeCoIn5
Neutron scattering in strong magnetic fields is used to show the
spin-resonance in superconducting CeCoIn5 (Tc=2.3 K) is a doublet. The
underdamped resonance (\hbar \Gamma=0.069 \pm 0.019 meV) Zeeman splits into two
modes at E_{\pm}=\hbar \Omega_{0}\pm g\mu_{B} \mu_{0}H with g=0.96 \pm 0.05. A
linear extrapolation of the lower peak reaches zero energy at 11.2 \pm 0.5 T,
near the critical field for the incommensurate "Q-phase" indicating that the
Q-phase is a bose condensate of spin excitons.Comment: 5 pages, 4 figure
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