8,413 research outputs found
Cavity-enhanced optical Hall effect in two-dimensional free charge carrier gases detected at terahertz frequencies
The effect of a tunable, externally coupled Fabry-P\'{e}rot cavity to
resonantly enhance the optical Hall effect signatures at terahertz frequencies
produced by a traditional Drude-like two-dimensional electron gas is shown and
discussed in this communication. As a result, the detection of optical Hall
effect signatures at conveniently obtainable magnetic fields, for example by
neodymium permanent magnets, is demonstrated. An AlInN/GaN-based high electron
mobility transistor structure grown on a sapphire substrate is used for the
experiment. The optical Hall effect signatures and their dispersions, which are
governed by the frequency and the reflectance minima and maxima of the
externally coupled Fabry-P\'{e}rot cavity, are presented and discussed. Tuning
the externally coupled Fabry-P\'{e}rot cavity strongly modifies the optical
Hall effect signatures, which provides a new degree of freedom for optical Hall
effect experiments in addition to frequency, angle of incidence and magnetic
field direction and strength
On class visualisation for high dimensional data: Exploring scientific datasets
Parametric Embedding (PE) has recently been proposed as a general-purpose
algorithm for class visualisation. It takes class posteriors produced by a
mixture-based clustering algorithm and projects them in 2D for visualisation.
However, although this fully modularised combination of objectives (clustering
and projection) is attractive for its conceptual simplicity, in the case of
high dimensional data, we show that a more optimal combination of these
objectives can be achieved by integrating them both into a consistent
probabilistic model. In this way, the projection step will fulfil a role of
regularisation, guarding against the curse of dimensionality. As a result, the
tradeoff between clustering and visualisation turns out to enhance the
predictive abilities of the overall model. We present results on both synthetic
data and two real-world high-dimensional data sets: observed spectra of
early-type galaxies and gene expression arrays.Comment: to appear in Lecture notes in Artificial Intelligence vol. 4265, the
(refereed) proceedings of the Ninth International conference on Discovery
Science (DS-2006), October 2006, Barcelona, Spain. 12 pages, 8 figure
First Use of a Longitudinally Polarized Target with CLAS12
Run Group C comprises eight experiments utilizing the CLAS12 detector system in Hall B at Jefferson Lab to study the multidimensional partonic structure of nucleons. The experiments scatter electrons from polarized protons and neutrons in samples of solid NH3 and ND3, dynamically polarized at a temperature of 1 K in a 5 T magnetic field. After a brief description of the target system, the current status of the target and preliminary results of its performance in the experiment are presented
Thermal and magnetic properties of spin-1 magnetic chain compounds with large single-ion and in-plane anisotropies
The thermal and magnetic properties of spin-1 magnetic chain compounds with
large single-ion and in-plane anisotropies are investigated via the integrable
su(3) model in terms of the quantum transfer matrix method and the recently
developed high temperature expansion method for exactly solved models. It is
shown that large single-ion anisotropy may result in a singlet gapped phase in
the spin-1 chain which is significantly different from the standard Haldane
phase. A large in-plane anisotropy may destroy the gapped phase. On the other
hand, in the vicinity of the critical point a weak in-plane anisotropy leads to
a different phase transition than the Pokrovsky-Talapov transition. The
magnetic susceptibility, specific heat and magnetization evaluated from the
free energy are in excellent agreement with the experimental data for the
compounds NiC_2H_8N_2)_2Ni(CN)_4 and Ni(C_{10}H_8N_2)_2Ni(CN)_4.H_2O.Comment: 18 pages, 6 figures, to appear in PR
Evolution of the silicon bottom cell photovoltaic behavior during III-V on Si multi-junction solar cells production
The evolution of the Si bulk minority carrier lifetime during the heteroepitaxial growth of III-V on Si multi-junction solar cell structures via metal-organic vapor phase epitaxy has been analyzed. Initially, the emitter formation produces important lifetime degradation. Nevertheless, a progressive recovery was observed during the growth of the metamorphic GaAsP/Si structure. A step-wise mechanism has been proposed to explain the lifetime evolution observed during this process. The initial lifetime degradation is believed to be related to the formation of thermally-induced defects within the Si bulk. These defects are subsequently passivated by fast-diffusing atomic hydrogen -coming from precursor (i.e. PH3 and AsH3) pyrolysis- during the subsequent III-V growth. These results indicate that the MOVPE environment used to create the III-V/Si solar cell structures has a dynamic impact on the minority carrier lifetime. Consequently, designing processes that promote the recovery of the lifetime is a must to support the production of high-quality III-V/Si solar cells
Thermal and magnetic properties of integrable spin-1 and spin-3/2 chains with applications to real compounds
The ground state and thermodynamic properties of spin-1 and spin-3/2 chains
are investigated via exactly solved su(3) and su(4) models with physically
motivated chemical potential terms. The analysis involves the Thermodynamic
Bethe Ansatz and the High Temperature Expansion (HTE) methods. For the spin-1
chain with large single-ion anisotropy, a gapped phase occurs which is
significantly different from the valence-bond-solid Haldane phase. The
theoretical curves for the magnetization, susceptibility and specific heat are
favourably compared with experimental data for a number of spin-1 chain
compounds. For the spin-3/2 chain a degenerate gapped phase exists starting at
zero external magnetic field. A middle magnetization plateau can be triggered
by the single-ion anisotropy term. Overall, our results lend further weight to
the applicability of integrable models to the physics of low-dimensional
quantum spin systems. They also highlight the utility of the exact HTE method.Comment: 38 pages, 15 figure
Antiferromagnetic Dimers of Ni(II) in the S=1 Spin-Ladder Na_2Ni_2(C_2O_4)_3(H_2O)_2
We report the synthesis, crystal structure and magnetic properties of the S=1
2-leg spin-ladder compound Na_2Ni_2(C_2O_4)_3(H_2O)_2. The magnetic properties
were examined by magnetic susceptibility and pulsed high field magnetization
measurements. The magnetic excitations have been measured in high field high
frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an
isolated dimer model consistently describes the observations very well. The
analysis of the temperature dependent magnetization data leads to a magnetic
exchange constant of J=43 K along the rungs of the ladder and an average value
of the g-factor of 2.25. From the ESR measurements, we determined the single
ion anisotropy to D=11.5 K. The validity of the isolated dimer model is
supported by Quantum Monte Carlo calculations, performed for several ratios of
interdimer and intradimer magnetic exchange and taking into account the
experimentally determined single ion anisotropy. The results can be understood
in terms of the different coordination and superexchange angles of the oxalate
ligands along the rungs and legs of the 2-leg spin ladder.Comment: 8 pages, 10 figure
Optimal, reliable estimation of quantum states
Accurately inferring the state of a quantum device from the results of
measurements is a crucial task in building quantum information processing
hardware. The predominant state estimation procedure, maximum likelihood
estimation (MLE), generally reports an estimate with zero eigenvalues. These
cannot be justified. Furthermore, the MLE estimate is incompatible with error
bars, so conclusions drawn from it are suspect. I propose an alternative
procedure, Bayesian mean estimation (BME). BME never yields zero eigenvalues,
its eigenvalues provide a bound on their own uncertainties, and it is the most
accurate procedure possible. I show how to implement BME numerically, and how
to obtain natural error bars that are compatible with the estimate. Finally, I
briefly discuss the differences between Bayesian and frequentist estimation
techniques.Comment: RevTeX; 14 pages, 2 embedded figures. Comments enthusiastically
welcomed
Blue lasing at room temperature in high quality factor GaN/AlInN microdisks with InGaN quantum wells
The authors report on the achievement of optically pumped III-V nitride blue microdisk lasers operating at room temperature. Controlled wet chemical etching of an AlInN interlayer lattice matched to GaN allows forming inverted cone pedestals. Whispering gallery modes are observed in the photoluminescence spectra of InGaN∕GaN quantum wells embedded in the GaN microdisks. Typical quality factors of several thousands are found (Q>4000). Laser action at ∼420nm is achieved under pulsed excitation at room temperature for a peak power density of 400kW/cm2. The lasing emission linewidth is down to 0.033nm
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