15,920 research outputs found
Measuring mechanical motion with a single spin
We study theoretically the measurement of a mechanical oscillator using a
single two level system as a detector. In a recent experiment, we used a single
electronic spin associated with a nitrogen vacancy center in diamond to probe
the thermal motion of a magnetized cantilever at room temperature {Kolkowitz et
al., Science 335, 1603 (2012)}. Here, we present a detailed analysis of the
sensitivity limits of this technique, as well as the possibility to measure the
zero point motion of the oscillator. Further, we discuss the issue of
measurement backaction in sequential measurements and find that although
backaction heating can occur, it does not prohibit the detection of zero point
motion. Throughout the paper we focus on the experimental implementation of a
nitrogen vacancy center coupled to a magnetic cantilever; however, our results
are applicable to a wide class of spin-oscillator systems. Implications for
preparation of nonclassical states of a mechanical oscillator are also
discussed.Comment: 17 pages, 6 figure
Distance metric choice can both reduce and induce collinearity in geographically weighted regression
This paper explores the impact of different distance metrics on collinearity in local regression models such as geographically weighted regression. Using a case study of house price data collected in Hà Nội, Vietnam, and by fully varying both power and rotation parameters to create different Minkowski distances, the analysis shows that local collinearity can be both negatively and positively affected by distance metric choice. The Minkowski distance that maximised collinearity in a geographically weighted regression was approximate to a Manhattan distance with (power = 0.70) with a rotation of 30°, and that which minimised collinearity was parameterised with power = 0.05 and a rotation of 70°. The results indicate that distance metric choice can provide a useful extra tuning component to address local collinearity issues in spatially varying coefficient modelling and that understanding the interaction of distance metric and collinearity can provide insight into the nature and structure of the data relationships. The discussion considers first, the exploration and selection of different distance metrics to minimise collinearity as an alternative to localised ridge regression, lasso and elastic net approaches. Second, it discusses the how distance metric choice could extend the methods that additionally optimise local model fit (lasso and elastic net) by selecting a distance metric that further helped minimise local collinearity. Third, it identifies the need to investigate the relationship between kernel bandwidth, distance metrics and collinearity as an area of further work
Direct transition from a disordered to a multiferroic phase on a triangular lattice
Competing interactions and geometric frustration provide favourable
conditions for exotic states of matter. Such competition often causes multiple
phase transitions as a function of temperature and can lead to magnetic
structures that break inversion symmetry, thereby inducing ferroelectricity
[1-4]. Although this phenomenon is understood phenomenologically [3-4], it is
of great interest to have a conceptually simpler system in which
ferroelectricity appears coincident with a single magnetic phase transition.
Here we report the first such direct transition from a paramagnetic and
paraelectric phase to an incommensurate multiferroic in the triangular lattice
antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric
polarization when the symmetry of the magnetic order changes and
ferroelectricity is only observed when the magnetic structure has chirality and
breaks inversion symmetry. Multiferroic behaviour in RFMO provides a
theoretically tractable example of ferroelectricity from competing spin
interactions. A Landau expansion of symmetry-allowed terms in the free energy
demonstrates that the chiral magnetic order of the triangular lattice
antiferromagnet gives rise to a pseudoelectric field, whose temperature
dependence agrees with that observed experimentally.Comment: 16 pages pdf including 3 figure
Infrared signatures of the spin-Peierls transition in CuGeO3
We investigated the infrared reflectivity of several Mg- and Si-substituted
CuGeO3 single crystals. The temperature dependent b-axis and c-axis optical
response is reported. For T<Tsp we detected the activation of zone-boundary
phonons along the b axis of the crystal on the pure sample and for 1% Mg and
0.7% Si concentrations. From a detailed analysis of the phonon parameters the
redshift of the B2u mode at 48 cm^-1 is observed and discussed in relation to
the soft mode expected to drive the spin-Peierls phase transition in CuGeO3.
Moreover, the polarization dependence of a magnetic excitation measured in
transmission at 44 cm^-1 has been investigated.Comment: Revtex, 3 pages, 5 postscript pictures, submitted to PRB Rapid
Communication
The CDF dijet excess from intrinsic quarks
The CDF collaboration reported an excess in the production of two jets in
association with a . We discuss constraints on possible new particle state
interpretations of this excess. The fact of no statistically significant
deviation from the SM expectation for {+dijet} events in CDF data disfavors
the new particle explanation. We show that the nucleon intrinsic strange quarks
provide an important contribution to the boson production in association
with a single top quark production. Such {+t} single top quark production
can contribute to the CDF {+dijet} excess, thus the nucleon intrinsic quarks
can provide a possible explanation to the CDF excess in {+dijet} but not in
{+dijet} events.Comment: 4 latex pages, 1 figure. Version for journal publicatio
Experimental investigation of the initial regime in fingering electrodeposition: dispersion relation and velocity measurements
Recently a fingering morphology, resembling the hydrodynamic Saffman-Taylor
instability, was identified in the quasi-two-dimensional electrodeposition of
copper. We present here measurements of the dispersion relation of the growing
front. The instability is accompanied by gravity-driven convection rolls at the
electrodes, which are examined using particle image velocimetry. While at the
anode the theory presented by Chazalviel et al. describes the convection roll,
the flow field at the cathode is more complicated because of the growing
deposit. In particular, the analysis of the orientation of the velocity vectors
reveals some lag of the development of the convection roll compared to the
finger envelope.Comment: 11 pages, 15 figures, REVTEX 4; reference adde
Competing Magnetic Phases on a "Kagome Staircase"
We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a
kagome staircase. The extreme degeneracy of the kagome antiferromagnet is
lifted to produce two incommensurate phases at finite T - one amplitude
modulated, the other helical - plus a commensurate canted antiferromagnet for T
->0. The H-T phase diagram is described by a model of competing first and
second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus
provides an elegant example of order from sub leading interactions in a highly
frustrated systemComment: 4 pages, 3 figure
Quantum Interference on the Kagom\'e Lattice
We study quantum interference effects due to electron motion on the Kagom\'e
lattice in a perpendicular magnetic field. These effects arise from the
interference between phase factors associated with different electron
closed-paths. From these we compute, analytically and numerically, the
superconducting-normal phase boundary for Kagom\'e superconducting wire
networks and Josephson junction arrays. We use an analytical approach to
analyze the relationship between the interference and the complex structure
present in the phase boundary, including the origin of the overall and fine
structure. Our results are obtained by exactly summing over one thousand
billion billions () closed paths, each one weighted by its
corresponding phase factor representing the net flux enclosed by each path. We
expect our computed mean-field phase diagrams to compare well with several
proposed experiments.Comment: 9 pages, Revtex, 3 figures upon reques
Polariton Analysis of a Four-Level Atom Strongly Coupled to a Cavity Mode
We present a complete analytical solution for a single four-level atom
strongly coupled to a cavity field mode and driven by external coherent laser
fields. The four-level atomic system consists of a three-level subsystem in an
EIT configuration, plus an additional atomic level; this system has been
predicted to exhibit a photon blockade effect. The solution is presented in
terms of polaritons. An effective Hamiltonian obtained by this procedure is
analyzed from the viewpoint of an effective two-level system, and the dynamic
Stark splitting of dressed states is discussed. The fluorescence spectrum of
light exiting the cavity mode is analyzed and relevant transitions identified.Comment: 12 pages, 9 figure
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