6,359 research outputs found
Precision Spectroscopy of Polarized Molecules in an Ion Trap
Polar molecules are desirable systems for quantum simulations and cold
chemistry. Molecular ions are easily trapped, but a bias electric field applied
to polarize them tends to accelerate them out of the trap. We present a general
solution to this issue by rotating the bias field slowly enough for the
molecular polarization axis to follow but rapidly enough for the ions to stay
trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman sublevels in
180Hf19F+ with a coherence time of 100 ms. Frequency shifts arising from
well-controlled topological (Berry) phases are used to determine magnetic
g-factors. The rotating-bias-field technique may enable using trapped polar
molecules for precision measurement and quantum information science, including
the search for an electron electric dipole moment.Comment: Accepted to Scienc
Comparison of the Electronic Structures of Two Non-cuprate Layered Transition Metal Oxide Superconductors
Comparison is made of the electronic structure of the little-studied layered
transition metal oxide LiNbO with that of NaCoO, which has
attracted tremendous interest since superconductivity was discovered in its
hydrate. Although the active transition metal states are quite different
due to different crystal fields and band filling, both systems show a strong
change of electronic structure with changes in the distance between the
transition metal ion layer and the oxygen layers. The niobate is unusual in
having a large second-neighbor hopping amplitude, and a nearest neighbor
hopping amplitude that is sensitive to the Nb-O separation. LiNbO also
presents the attractive simplicity of a single band triangular lattice system
with variable carrier concentration that is superconducting.Comment: 5 pages, 3 embedded figures (Proceedings in third Hiroshima
international workshop
Potential energy surface of the 2A' Li2+Li doublet ground state
The lowest doublet electronic state for the lithium trimer (2A') is
calculated for use in three-body scattering calculations using the valence
electron FCI method with atomic cores represented using an effective core
potential. It is shown that an accurate description of core-valence correlation
is necessary for accurate calculations of molecular bond lengths, frequencies
and dissociation energies. Interpolation between 2A' ab initio surface data
points in a sparse grid is done using the global interpolant moving least
squares method with a smooth radial data cutoff function included in the
fitting weights and bivariate polynomials as a basis set. The Jahn-Teller
splitting of the 2E' surface into the 2A1 and 2B2 states is investigated using
a combination of both CASSCF and FCI levels of theory. Additionally,
preliminary calculations of the 2A'' surface are also presented using second
order spin restricted open-shell Moller-Plesset perturbation theory.Comment: 7 pages, 5 figure
Characterization of a novel angular dioxygenase from fluorene-degrading Sphingomonas sp. strain LB126
In this study, the genes involved in the initial attack on fluorene by
Sphingomonas sp. LB126 were investigated. The ? and ? subunits of a dioxygenase
complex (FlnA1A2), showing 63% and 51% sequence identity respectively, with the
subunits of an angular dioxygenase from Gram-positive Terrabacter sp. DBF63,
were identified. When overexpressed in E. coli, FlnA1A2 was responsible for the
angular oxidation of fluorene, fluorenol, fluorenone, dibenzofuran and
dibenzo-p-dioxin. Moreover, FlnA1A2 was able to oxidize polycyclic aromatic
hydrocarbons and heteroaromatics, some of which were not oxidized by the
dioxygenase from Terrabacter sp. DBF63. Quantification of resulting oxidation
products showed that fluorene and phenanthrene were preferred substrates
Magnetotransport properties of AlxGa1-xN/AlN/GaN heterostructures grown on epitaxial lateral overgrown GaN templates
We studied the low-temperature magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures with a two-dimensional electron gas(2DEG). Structures with different Al compositions were grown by metal-organic vapor-phase epitaxy on three types of templates: conventional undoped GaN, in situ epitaxial lateral overgrown GaN using a SiNx nanomask layer, and ex situe pitaxial lateral overgrown GaN (ELO-GaN) using a stripe-patterned SiO2 mask. All of the samples display Shubnikov–de Haas (SdH) oscillations that confirm the existence of 2DEGs. Field-dependent magnetoresistance and Hall measurements further indicate that the overgrown heterostructures have a parallel conducting layer in addition to the 2DEG. To characterize the parallel channel, we repeated the measurements after the 2DEG was etched away. 2DEGcarrier density values were then extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total. The quantitative mobility spectrum analysis could not be applied in some cases, due to a large contact resistance between the parallel channels. The resulting 2DEG mobility is about a factor of 2 higher in the ELO-GaN and SiN–GaN samples as compared to the standard control samples. The mobility enhancement is attributed to a reduction of threading dislocations by the two ELO techniques employed
Controlling the quantum stereodynamics of ultracold bimolecular reactions
Chemical reaction rates often depend strongly on stereodynamics, namely the
orientation and movement of molecules in three-dimensional space. An ultracold
molecular gas, with a temperature below 1 uK, provides a highly unusual regime
for chemistry, where polar molecules can easily be oriented using an external
electric field and where, moreover, the motion of two colliding molecules is
strictly quantized. Recently, atom-exchange reactions were observed in a
trapped ultracold gas of KRb molecules. In an external electric field, these
exothermic and barrierless bimolecular reactions, KRb+KRb -> K2+Rb2, occur at a
rate that rises steeply with increasing dipole moment. Here we show that the
quantum stereodynamics of the ultracold collisions can be exploited to suppress
the bimolecular chemical reaction rate by nearly two orders of magnitude. We
use an optical lattice trap to confine the fermionic polar molecules in a
quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along
the tight confinement direction. With the combination of sufficiently tight
confinement and Fermi statistics of the molecules, two polar molecules can
approach each other only in a "side-by-side" collision, where the chemical
reaction rate is suppressed by the repulsive dipole-dipole interaction. We show
that the suppression of the bimolecular reaction rate requires quantum-state
control of both the internal and external degrees of freedom of the molecules.
The suppression of chemical reactions for polar molecules in a
quasi-two-dimensional trap opens the way for investigation of a dipolar
molecular quantum gas. Because of the strong, long-range character of the
dipole-dipole interactions, such a gas brings fundamentally new abilities to
quantum-gas-based studies of strongly correlated many-body physics, where
quantum phase transitions and new states of matter can emerge.Comment: 19 pages, 4 figure
Aharonov-Bohm interferences from local deformations in graphene
One of the most interesting aspects of graphene is the tied relation between
structural and electronic properties. The observation of ripples in the
graphene samples both free standing and on a substrate has given rise to a very
active investigation around the membrane-like properties of graphene and the
origin of the ripples remains as one of the most interesting open problems in
the system. The interplay of structural and electronic properties is
successfully described by the modelling of curvature and elastic deformations
by fictitious gauge fields that have become an ex- perimental reality after the
suggestion that Landau levels can form associated to strain in graphene and the
subsequent experimental confirmation. Here we propose a device to detect
microstresses in graphene based on a scanning-tunneling-microscopy setup able
to measure Aharonov-Bohm inter- ferences at the nanometer scale. The
interferences to be observed in the local density of states are created by the
fictitious magnetic field associated to elastic deformations of the sample.Comment: Some bugs fixe
Bias dependent two-channel conduction in InAlN/AlN/GaN structures
Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that although the additional unintended layer may have beneficial aspects, we discuss a strategy to prevent its occurrence
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