194 research outputs found
Optical and Infrared Spectroscopy
Contains report on one research project.Joint Services Electronics Program (Contract DA36-039-AMC-03200(E)
Optical and Infrared Spectroscopy
Contains research objectives and reports on one research project.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 36-039-AMC-03200(E
Optical and Infrared Spectroscopy
Contains research objectives and reports on one research project
Molecular spintronics: Coherent spin transfer in coupled quantum dots
Time-resolved Faraday rotation has recently demonstrated coherent transfer of
electron spin between quantum dots coupled by conjugated molecules. Using a
transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the
Faraday rotation signal as a function of the probe frequency in a pump-probe
setup using neutral quantum dots. Additionally, we study the signal of one
spin-polarized excess electron in the coupled dots. We show that, in both
cases, the Faraday rotation angle is determined by the spin transfer
probabilities and the Heisenberg spin exchange energy. By comparison of our
results with experimental data, we find that the transfer matrix element for
electrons in the conduction band is of order 0.08 eV and the spin transfer
probabilities are of order 10%.Comment: 13 pages, 6 figures; minor change
Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a Structural Transition
We report a first-order phase transition at T_M=357 K in single crystal
Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous
decrease in electrical resistivity signals the near destruction of the Mott
insulating phase and is triggered by a structural transition from the low
temperature orthorhombic to a high temperature tetragonal phase. The magnetic
susceptibility, which is temperature dependent but not Curie-like decreases
abruptly at TM and becomes less temperature dependent. Unlike most insulator to
metal transitions, the system is not magnetically ordered in either phase,
though the Mott insulator phase is antiferromagnetic below T_N=110 K.Comment: Accepted for publication in Phys. Rev. B (Rapid Communications
Fermi velocity engineering in graphene by substrate modification
The Fermi velocity is one of the key concepts in the study of a material, as
it bears information on a variety of fundamental properties. Upon increasing
demand on the device applications, graphene is viewed as a prototypical system
for engineering Fermi velocity. Indeed, several efforts have succeeded in
modifying Fermi velocity by varying charge carrier concentration. Here we
present a powerful but simple new way to engineer Fermi velocity while holding
the charge carrier concentration constant. We find that when the environment
embedding graphene is modified, the Fermi velocity of graphene is (i) inversely
proportional to its dielectric constant, reaching ~2.5 m/s, the
highest value for graphene on any substrate studied so far and (ii) clearly
distinguished from an ordinary Fermi liquid. The method demonstrated here
provides a new route toward Fermi velocity engineering in a variety of
two-dimensional electron systems including topological insulators.Comment: accepted in Scientific Report
Functionalized boron nitride membranes with ultrafast solvent transport performance for molecular separation
Pressure-driven, superfast organic solvent filtration membranes have significant practical applications. An excellent filtration membrane should exhibit high selectivity and permeation in aqueous and organic solvents to meet increasing industrial demand. Here, we report an amino functionalized boron nitride (FBN) based filtration membrane with a nanochannel network for molecular separation and permeation. This membrane is highly stable in water and in several organic solvents and shows high transport performance for solvents depending on the membranes' thickness. In addition, the FBN membrane is applicable for solute screening in water as well as in organic solvents. More importantly, the FBN membranes are very stable in acidic, alkaline and oxidative media for up to one month. The fast-flow rate and good separation performance of the FBN membranes can be attributed to their stable networks of nanochannels and thin laminar structure, which provide the membranes with beneficial properties for practical separation and purification processes
Vibrational properties of single-wall nanotubes and monolayers of hexagonal BN
We report a detailed study of the vibrational properties of BN single-walled
nanotubes and of the BN monolayer. Our results have been obtained from a
well-established Tight-Binding model complemented with an electrostatic model
to account for the long-range interactions arising from the polar nature of the
material, and which are not included in the Tight-Binding model. Our study
provides a wealth of data for the BN monolayer and nanotubes, such as phonon
band structure, vibrational density of states, elastic constants, etc. For the
nanotubes we obtain the behavior of the optically active modes as a function of
the structural parameters, and we compare their frequencies with those derived
from a zone-folding treatment applied to the phonon frequencies of the BN
monolayer, finding general good agreement between the two.Comment: 14 pages with 10 postscript figures, to appear in PRB, January 15th
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