5,837 research outputs found
On the dimensional dependence of duality groups for massive p-forms
We study the soldering formalism in the context of abelian p-form theories.
We develop further the fusion process of massless antisymmetric tensors of
different ranks into a massive p-form and establish its duality properties. To
illustrate the formalism we consider two situations. First the soldering mass
generation mechanism is compared with the Higgs and Julia-Toulouse mechanisms
for mass generation due to condensation of electric and magnetic topological
defects. We show that the soldering mechanism interpolates between them for
even dimensional spacetimes, in this way confirming the Higgs/Julia-Toulouse
duality proposed by Quevedo and Trugenberger \cite{QT} a few years ago. Next,
soldering is applied to the study of duality group classification of the
massive forms. We show a dichotomy controlled by the parity of the operator
defining the symplectic structure of the theory and find their explicit
actions.Comment: Reference [8] has been properly place
On duality of the noncommutative extension of the Maxwell-Chern-Simons model
We study issues of duality in 3D field theory models over a canonical
noncommutative spacetime and obtain the noncommutative extension of the
Self-Dual model induced by the Seiberg-Witten map. We apply the dual projection
technique to uncover some properties of the noncommutative Maxwell-Chern-Simons
theory up to first-order in the noncommutative parameter. A duality between
this theory and a model similar to the ordinary self-dual model is
estabilished. The correspondence of the basic fields is obtained and the
equivalence of algebras and equations of motion are directly verified. We also
comment on previous results in this subject.Comment: Revtex, 9 pages, accepted for publication PL
Steady-state entanglement between distant quantum dots in photonic crystal dimers
We show that two spatially separated semiconductor quantum dots under
resonant and continuous-wave excitation can be strongly entangled in the
steady-state, thanks to their radiative coupling by mutual interaction through
the normal modes of a photonic crystal dimer. We employ a quantum master
equation formalism to quantify the steady-state entanglement by calculating the
system {\it negativity}. Calculations are specified to consider realistic
semiconductor nanostructure parameters for the photonic crystal dimer-quantum
dots coupled system, determined by a guided mode expansion solution of Maxwell
equations. Negativity values of the order of 0.1 ( of the maximum value)
are shown for interdot distances that are larger than the resonant wavelength
of the system. It is shown that the amount of entanglement is almost
independent of the interdot distance, as long as the normal mode splitting of
the photonic dimer is larger than their linewidths, which becomes the only
requirement to achieve a local and individual qubit addressing. Considering
inhomogeneously broadened quantum dots, we find that the steady-state
entanglement is preserved as long as the detuning between the two quantum dot
resonances is small when compared to their decay rates. The steady-state
entanglement is shown to be robust against the effects of pure dephasing of the
quantum dot transitions. We finally study the entanglement dynamics for a
configuration in which one of the two quantum dots is initially excited and
find that the transient negativity can be enhanced by more than a factor of two
with respect to the steady-state value. These results are promising for
practical applications of entangled states at short time scales.Comment: 10 pages, 7 figure
Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride
We present a fast method to fabricate high quality heterostructure devices by
picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with
hexagonal boron nitride to demonstrate this approach, showing good electronic
quality with mobilities ranging from 17 000 cm^2/V/s at room temperature to 49
000 cm^2/V/s at 4.2 K, and entering the quantum Hall regime below 0.5 T. This
method provides a strong and useful tool for the fabrication of future high
quality layered crystal devices.Comment: 5 pages, 3 figure
24 \textmu m length spin relaxation length in boron nitride encapsulated bilayer graphene
We have performed spin and charge transport measurements in dual gated high
mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results
show spin relaxation lengths up to 13~\textmu m at room temperature
with relaxation times of 2.5~ns. At 4~K, the diffusion coefficient
rises up to 0.52~m/s, a value 5 times higher than the best achieved for
graphene spin valves up to date. As a consequence, rises up to
24~\textmu m with as high as 2.9~ns. We characterized 3 different
samples and observed that the spin relaxation times increase with the device
length. We explain our results using a model that accounts for the spin
relaxation induced by the non-encapsulated outer regions.Comment: 5 pages and 4 figure
Controlling spin relaxation in hexagonal BN-encapsulated graphene with a transverse electric field
We experimentally study the electronic spin transport in hBN encapsulated
single layer graphene nonlocal spin valves. The use of top and bottom gates
allows us to control the carrier density and the electric field independently.
The spin relaxation times in our devices range up to 2 ns with spin relaxation
lengths exceeding 12 m even at room temperature. We obtain that the ratio
of the spin relaxation time for spins pointing out-of-plane to spins in-plane
is 0.75 for zero applied perpendicular
electric field. By tuning the electric field this anisotropy changes to
0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane
Rashba spin-orbit coupling
Consequências da redução de espaçamentos entre as linhas e entre as plantas na linha de plantio sobre os componentes vegetativos de cafeeiros (Coffea arabica L.) cultivar Catuaí.
O experimento foi instalado na Fazenda Experimental da EPAMIG em Machado, sul de Minas Gerais, em 1992, com o objetivo de avaliar as conseqüências da redução de espaçamentos entre as linhas e entre as plantas na linha de plantio sobre a produção e a fenologia do cafeeiro. O delineamento experimental utilizado foi um fatorial 4 x 3 com parcela subdividida, sendo quatro distâncias entre as linhas (2,0? 2,5? 3,0 e 3,5 m) e três distâncias entre as plantas na linha de plantio (0,5? 0,75? 1,0 m), e totalizando 12 tratamentos dispostos em blocos ao acaso em três repetições. Em julho de 2002, logo após a colheita e antes da primeira recepa, foram avaliadas as características relativas à arquitetura dos cafeeiros (altura da planta, diâmetro da copa, altura dos ramos plagiotrópicos baixeiros e diâmetro do caule). O adensamento entre linhas proporciona plantas mais altas, com inserção dos primeiros ramos plagiotrópicos mais altos, menores comprimento e diâmetro da copa. O adensamento entre linhas não afeta o diâmetro do caule. Já o adensamento das plantas na linha de plantio linha proporciona também plantas mais altas e com inserção dos primeiros ramos plagiotrópicos mais altos, não influencia o comprimento da copa e apresenta diâmetros da copa e do caule menores
Carbon nanotube: a low-loss spin-current waveguide
We demonstrate with a quantum-mechanical approach that carbon nanotubes are
excellent spin-current waveguides and are able to carry information stored in a
precessing magnetic moment for long distances with very little dispersion and
with tunable degrees of attenuation. Pulsed magnetic excitations are predicted
to travel with the nanotube Fermi velocity and are able to induce similar
excitations in remote locations. Such an efficient way of transporting magnetic
information suggests that nanotubes are promising candidates for memory devices
with fast magnetization switchings
Massive scalar field near a cosmic string
The function of a massive scalar field near a cosmic string is
computed and then employed to find the vacuum fluctuation of the field. The
vacuum expectation value of the energy-momentum tensor is also computed using a
point-splitting approach. The obtained results could be useful also for the
case of self-interacting scalar fields and for the finite-temperature Rindler
space theory.Comment: 15 pages, standard LaTeX, no figures. Reference [14] correcte
Evolução da produção individual de cafeeiros (Coffea arabica L.) cultivar catuaí submetidos a espaçamentos crescentes ao longo de nove colheitas.
O experimento foi instalado na Fazenda Experimental da EPAMIG em Machado, sul de Minas Gerais, em 1992, com o objetivo de avaliar as conseqüências da redução de espaçamentos entre as linhas e entre as plantas na linha de plantio sobre a produção do cafeeiro. O delineamento experimental utilizado foi um fatorial 4 x 3 com parcela subdividida, sendo quatro distâncias entre as linhas (2,0? 2,5? 3,0 e 3,5 m) e três distâncias entre as plantas na linha de plantio (0,5? 0,75? 1,0 m), e totalizando 12 tratamentos dispostos em blocos ao acaso em três repetições. Entre os anos de 1994 e 2002 foram avaliadas a produção individual de cada um dos tratamentos. A produção de café beneficiado por planta foi afetada negativamente com a redução dos espaçamentos entre as plantas e não foi influenciada pela redução dos espaçamentos entre as linhas de plantio
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