25 research outputs found
Dissipative dynamics of spins in quantum dots
We present a theory for the dissipation of electronic spins trapped in
quantum dots due to their coupling to the host lattice acoustic phonon modes.
Based on the theory of dissipative two level systems for the spin dynamics, we
derive a relation between the spin dissipative bath, the electron confinement,
and the electron-phonon interaction. We find that there is an energy scale,
typically smaller than the electronic lateral confinement energy, which sets
the boundary between different dissipative regimes .Comment: 4 pages, 2 eps figure
Bosonização em nÃveis de Landau
Orientador: Amir Ordacgi CaldeiraTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Neste trabalho propomos um novo método teórico de estudo dos sistemas eletrônicos correlacionados de baixa dimensionalidade, na presença de um campo magnético uniforme. Esta técnica trata da bosonização das excitações de baixa energia dos sistemas fermiônicos e foi inspirada em métodos anteriores de bosonização em uma ou duas dimensões sem campo magnético. No esquema proposto, a bosonização das excitações de baixa energia é elaborada a partir da descrição quântica natural da partÃculas carregadas na presença de um campo magnético uniforme, qual seja, a dos NÃveis de Landau. No decorrer, mostraremos como este método pode ser aplicado a problemas recentes, em que as propriedades dos sistemas eletrônicos confinados em duas dimensões, obtidas experimentalmente, indicam a necessidade de novos métodos teóricos de investigaçãoAbstract: Not informed.DoutoradoFÃsicaDoutor em Ciência
Fluctuation induced first-order phase transitions in a dipolar Ising ferromagnet slab
We investigate the competition between the dipolar and the exchange
interaction in a ferromagnetic slab with finite thickness and finite width.
From an analytical approximate expression for the Ginzburg-Landau effective
Hamiltonian, it is shown that, within a self-consistent Hartree approach, a
stable modulated configuration arises. We study the transition between the
disordered phase and two kinds of modulated configurations, namely, striped and
bubble phases. Such transitions are of the first-order kind and the striped
phase is shown to have lower energy and a higher spinodal limit than the bubble
one. It is also observed that striped configurations corresponding to different
modulation directions have different energies. The most stable are the ones in
which the modulation vanishes along the unlimited direction, which is a prime
effect of the slab's geometry together with the competition between the two
distinct types of interaction. An application of this model to the domain
structure of MnAs thin films grown over GaAs substrates is discussed and
general qualitative properties are outlined and predicted, like the number of
domains and the mean value of the modulation as functions of temperature.Comment: 13 pages, 14 figure
Correlated disorder in random block-copolymers
We study the effect of a random Flory-Huggins parameter in a symmetric
diblock copolymer melt which is expected to occur in a copolymer where one
block is near its structural glass transition. In the clean limit the
microphase segregation between the two blocks causes a weak, fluctuation
induced first order transition to a lamellar state. Using a renormalization
group approach combined with the replica trick to treat the quenched disorder,
we show that beyond a critical disorder strength, that depends on the length of
the polymer chain, the character of the transition is changed. The system
becomes dominated by strong randomness and a glassy rather than an ordered
lamellar state occurs. A renormalization of the effective disorder distribution
leads to nonlocal disorder correlations that reflect strong compositional
fluctuation on the scale of the radius of gyration of the polymer chains. The
reason for this behavior is shown to be the chain length dependent role of
critical fluctuations, which are less important for shorter chains and become
increasingly more relevant as the polymer length increases and the clean first
order transition becomes weaker.Comment: 11 pages, 5 figures, submitted to PR
Low-aberration beamline optics for synchrotron infrared nanospectroscopy
MCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕESSynchrotron infrared nanospectroscopy is a recently developed technique that enables new possibilities in the broadband chemical analysis of materials in the nanoscale, far beyond the diffraction limit in this frequency domain. Synchrotron infrared ports have exploited mainly the high brightness advantage provided by electron storage rings across the whole infrared range. However, optical aberrations in the beam produced by the source depth of bending magnet emission at large angles prevent infrared nanospectroscopy to reach its maximum capability. In this work we present a low-aberration optical layout specially designed and constructed for a dedicated synchrotron infrared nanospectroscopy beamline. We report excellent agreement between simulated beam profiles (from standard wave propagation and raytracing optics simulations) with experimental measurements. We report an important improvement in the infrared nanospectroscopy experiment related to the improved beamline optics. Finally, we demonstrate the performance of the nanospectroscopy endstation by measuring a hyperspectral image of a polar material and we evaluate the setup sensitivity by measuring ultra-thin polymer films down to 6 nm thick.2691123811249MCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕESMCTIC - MINISTÉRIO DA CIÊNCIA, TECNOLOGIA, INOVAÇÕES E COMUNICAÇÕE
High-resolution synchrotron-based X-ray microtomography as a tool to unveil the three-dimensional neuronal architecture of the brain
The assessment of neuronal number, spatial organization and connectivity is fundamental for a complete understanding of brain function. However, the evaluation of the three-dimensional (3D) brain cytoarchitecture at cellular resolution persists as a great challenge in the field of neuroscience. In this context, X-ray microtomography has shown to be a valuable non-destructive tool for imaging a broad range of samples, from dense materials to soft biological specimens, arisen as a new method for deciphering the cytoarchitecture and connectivity of the brain. In this work we present a method for imaging whole neurons in the brain, combining synchrotron-based X-ray microtomography with the Golgi-Cox mercury-based impregnation protocol. In contrast to optical 3D techniques, the approach shown here does neither require tissue slicing or clearing, and allows the investigation of several cells within a 3D region of the brain
Dynamical Mean Field Theory for Self-Generated Quantum Glasses
We present a many body approach for non-equilibrium behavior and
self-generated glassiness in strongly correlated quantum systems. It combines
the dynamical mean field theory of equilibrium systems with the replica theory
for classical glasses without quenched disorder. We apply this approach to
study a quantized version of the Brazovskii model and find a self-generated
quantum glass that remains in a quantum mechanically mixed state as T -> 0.
This quantum glass is formed by a large number of competing states spread over
an energy region which is determined within our theory.Comment: 10 pages, 4 figure