7,717 research outputs found
Supersymmetric contributions to B --> K pi in the view of recent experimental result
Supersymmetric contributions to the branching ratios and CP asymmetries of
B--> K pi decays are analyzed in the view of recent experimental measurements.
We show that supersymmetry can still provide a natural solution to the apparent
discrepancy between theses results and the standard model expectations. We
emphasize that chargino contributions may enhance the electroweak penguin
effects that can resolve to the B--> K pi puzzle. We also point out that a
non-universal -terms is an essential requirement for this solution.Comment: 9 pages, 2 figure
Universal transport near a quantum critical Mott transition in two dimensions
We discuss the universal transport signatures near a zero-temperature
continuous Mott transition between a Fermi liquid (FL) and a quantum spin
liquid in two spatial dimensions. The correlation-driven transition occurs at
fixed filling and involves fractionalization of the electron: upon entering the
spin liquid, a Fermi surface of neutral spinons coupled to an internal gauge
field emerges. We present a controlled calculation of the value of the zero
temperature universal resistivity jump predicted to occur at the transition.
More generally, the behavior of the universal scaling function that collapses
the temperature and pressure dependent resistivity is derived, and is shown to
bear a strong imprint of the emergent gauge fluctuations. We further predict a
universal jump of the thermal conductivity across the Mott transition, which
derives from the breaking of conformal invariance by the damped gauge field,
and leads to a violation of the Wiedemann-Franz law in the quantum critical
region. A connection to organic salts is made, where such a transition might
occur. Finally, we present some transport results for the pure rotor O(N) CFT.Comment: 27 pages, 16 figure
Nonequilibrium quantum criticality in bilayer itinerant ferromagnets
We present a theory of nonequilibrium quantum criticality in a coupled
bilayer system of itinerant electron magnets. The model studied consists of the
first layer subjected to an inplane current and open to an external substrate.
The second layer is closed and subject to no direct external drive, but couples
to the first layer via short-ranged spin exchange interaction. No particle
exchange is assumed between the layers. Starting from a microscopic fermionic
model, we derive an effective action in terms of two coupled bosonic fields
which are related to the magnetization fluctuations of the two layers. When
there is no interlayer coupling, the two bosonic modes possess different
dynamical critical exponents z with z=2 (z=3) for the first (second) layer.
This results in multi-scale quantum criticality in the coupled system. It is
shown that the linear coupling between the two fields leads to a low energy
fixed point characterized by the larger dynamical critical exponent z=3. The
perturbative renormalization group is used to compute the correlation length in
the quantum disordered and quantum critical regimes. We also derive the
stochastic dynamics obeyed by the critical fluctuations in the quantum critical
regime. Comparing the nonequilibrium situation to the thermal equilibrium
scenario, where the whole system is at a temperature T, we find that the
nonequilibrium drive does not always play the role of temperature.Comment: 20+ pages, 3 figures; Revised version as accepted by PRB, added
figure of mean field phase diagra
Hybridization-driven gap in U3Bi4Ni3: a 209Bi NMR/NQR study
We report 209Bi NMR and NQR measurements on a single crystal of the Kondo
insulator U3Bi4Ni3. The 209Bi nuclear spin-lattice relaxation rate ()
shows activated behavior and is well-fit by a spin gap of 220 K. The 209Bi
Knight shift (K) exhibits a strong temperature dependence arising from 5f
electrons, in which K is negative at high temperatures and increases as the
temperature is lowered. Below 50 K, K shows a broad maximum and decreases
slightly upon further cooling. Our data provide insight into the evolution of
the hyperfine fields in a fully gapped Kondo insulator based on 5f electron
hybridization.Comment: 4 pages, 4 figures, submitted to Phys. Rev.
Mott transition between a spin-liquid insulator and a metal in three dimensions
We study a bandwidth controlled Mott metal-insulator transition (MIT) between
a Fermi liquid metal and a quantum spin-liquid insulator at half-filling in
three dimensions (3D). Using a slave rotor approach, and incorporating gauge
field fluctuations, we find a continuous MIT and discuss the finite temperature
crossovers around this critical point. We show that the specific heat C=T ln ln
(1/T) at the MIT and argue that the electrical transport on the metallic side
near the transition should exhibit a `conductivity minimum' as a function of
temperature. A possible candidate to test these predictions is the 3D spin
liquid insulator Na4Ir3O8 which exhibits a pressure-tuned transition into a
metallic phase. We also present the electron spectral function of Na4Ir3O8 at
the transition.Comment: 5 pages, 3 figure
Polarity control of carrier injection at ferroelectric/metal interfaces for electrically switchable diode and photovoltaic effects
We investigated a switchable ferroelectric diode effect and its physical
mechanism in Pt/BiFeO3/SrRuO3 thin-film capacitors. Our results of electrical
measurements support that, near the Pt/BiFeO3 interface of as-grown samples, a
defective layer (possibly, an oxygen-vacancy-rich layer) becomes formed and
disturbs carrier injection. We therefore used an electrical training process to
obtain ferroelectric control of the diode polarity where, by changing the
polarization direction using an external bias, we could switch the transport
characteristics between forward and reverse diodes. Our system is characterized
with a rectangular polarization hysteresis loop, with which we confirmed that
the diode polarity switching occurred at the ferroelectric coercive voltage.
Moreover, we observed a simultaneous switching of the diode polarity and the
associated photovoltaic response dependent on the ferroelectric domain
configurations. Our detailed study suggests that the polarization charge can
affect the Schottky barrier at the ferroelectric/metal interfaces, resulting in
a modulation of the interfacial carrier injection. The amount of
polarization-modulated carrier injection can affect the transition voltage
value at which a space-charge-limited bulk current-voltage (J-V) behavior is
changed from Ohmic (i.e., J ~ V) to nonlinear (i.e., J ~ V^n with n \geq 2).
This combination of bulk conduction and polarization-modulated carrier
injection explains the detailed physical mechanism underlying the switchable
diode effect in ferroelectric capacitors.Comment: Accepted for publication in Phys. Rev.
Variability study of Si nanowire FETs with different junction gradients
Random dopant fluctuation effects of gate-all-around Si nanowire field-effect transistors (FETs) are investigated in terms of different diameters and junction gradients. The nanowire FETs with smaller diameters or shorter junction gradients increase relative variations of the drain currents and the mismatch of the drain currents between source-drain and drain-source bias change in the saturation regime. Smaller diameters decreased current drivability critically compared to standard deviations of the drain currents, thus inducing greater relative variations of the drain currents. Shorter junction gradients form high potential barriers in the source-side lightly-doped extension regions at on-state, which determines the magnitude of the drain currents and fluctuates the drain currents greatly under thermionic-emission mechanism. On the other hand, longer junction gradients affect lateral field to fluctuate the drain currents greatly. These physical phenomena coincide with correlations of the variations between drain currents and electrical parameters such as threshold voltages and parasitic resistances. The nanowire FETs with relatively-larger diameters and longer junction gradients without degrading short channel characteristics are suggested to minimize the relative variations and the mismatch of the drain currents. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).1163Ysciescopu
Police research, officer surveys, and response rates
In recent years, policing scholars have increasingly used survey methods to gain insight into officers’ attitudes and behaviours. Yet, surprisingly, methodological research analysing surveys of police officers is rare. We analysed the extent and correlates of response rates in police surveys, providing insights about the survey design features and study characteristics associated with higher rates of officer participation. We examined the response rates to 497 police surveys reported in 390 articles published in 15 journals from 2008 to 2017. Findings included the following: (1) the average response rate was 64%, but there was a great deal of variation, (2) in-person surveys achieved substantially higher response rates, (3) inviting a greater number of officers to participate in surveys was associated with lower response rates, and (4) response rates have declined over time (though primarily among surveys not administered in-person). Given the weight of the evidence suggesting response rates are typically a poor predictor of nonresponse bias, we argue that a low response rate on its own is an insufficient reason to dismiss a study’s merit. Furthermore, we recommend minimally acceptable reporting standards and discuss avenues for future research
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