3,426 research outputs found
Dynamics of Holes and Universality Class of the Antiferromagnetic Transition in the Two Dimensional Hubbard Model
The dynamics of a single hole (or electron) in the two dimensional Hubbard
model is investigated. The antiferromagnetic background is described by a
N\`eel state, and the hopping of the carrier is analyzed within a configuration
interaction approach. Results are in agreement with other methods and with
experimental data when available. All data are compatible with the opening of a
mean field gap in a Fermi liquid of spin polarons, the so called Slater type of
transition. In particular, this hypothesis explains the unusual dispersion
relation of the quasiparticle bands near the transition. Recent photoemission
data for CaCuOCl are analyzed within this context.Comment: New results and comparison with recent data adde
The politics of the apolitical: Irony, sincerity, engagement
What do political engagement and apathy mean against the backdrop of a rapidly changing society? What kinds of problems are being expressed when people complain that young individuals are too ironic or too sincere? This dissertation explores the problems of understanding contemporary (progressive) citizenship by analyzing the cultural preoccupation with irony and sincerity in post-millennial Anglosphere Western culture. It presents a framework of philosophical and cultural-historic analysis in order to understand the valuation of the Modern ideal of âsincerityâ for citizenship.While central to modern culture, there is a lack of clarity in how âsincerityâ can be an ideal that revitalizes citizenship. For this reason, the work of Stanley Cavell, Charles Taylor and Lionel Trilling provide specific criteria. Additionally, the problems of individualized and fragmented societies are difficult to counter via the appeal to sincerity and authenticity. The risk of a âWordsworthianâ personalization of politics risks obscuring the necessary procedural (temporally unfolding) dimensions of political engagement and democracy. An exploration of different âstructures of feelingâ analyses the dangers of the ideal within specific Western conditions.Its conclusion points to the need to create balance between the personalized and procedural aspects of political imagination and self-understanding, and offers insights as to how to navigate this imbalance
Force measurements with optical tweezers inside living cells
The force exerted by optical tweezers can be measured by tracking the momentum changes of the trapping beam, a method which is more general and powerful than traditional calibration techniques as it is based on first principles, but which has not been brought to its full potential yet, probably due to practical difficulties when combined with high-NA optical traps, such as the necessity to capture a large fraction of the scattered light. We show that it is possible to measure forces on arbitrary biological objects inside cells without an in situ calibration, using this approach. The instrument can be calibrated by measuring three scaling parameters that are exclusively determined by the design of the system, thus obtaining a conversion factor from volts to piconewtons that is theoretically independent of the physical properties of the sample and its environment. We prove that this factor keeps valid inside cells as it shows good agreement with other calibration methods developed in recent years for viscoelastic media. Finally, we apply the method to measuring the stall forces of kinesin and dynein in living A549 cells.Publisher PD
Compositeness effects, Pauli's principle and entanglement
We analyse some compositeness effects and their relation with entanglement.
We show that the purity of a composite system increases, in the sense of the
expectation values of the deviation operators, with large values of the
entanglement between the components of the system. We also study the validity
of Pauli's principle in composite systems. It is valid within the limits of
application of the approach presented here. We also present an example of two
identical fermions, one of them entangled with a distinguishable particle,
where the exclusion principle cannot be applied. This result can be important
in the description of open systems
The role of pressure on the magnetism of bilayer graphene
We study the effect of pressure on the localized magnetic moments induced by
vacancies in bilayer graphene in the presence of topological defects breaking
the bipartite nature of the lattice. By using a mean-field Hubbard model we
address the two inequivalent types of vacancies that appear in the Bernal
stacking bilayer graphene. We find that by applying pressure in the direction
perpendicular to the layers the critical value of the Hubbard interaction
needed to polarize the system decreases. The effect is particularly enhanced
for one type of vacancies, and admits straightforward generalization to
multilayer graphene in Bernal stacking and graphite. The present results
clearly demonstrate that the magnetic behavior of multilayer graphene can be
affected by mechanical transverse deformation
Harvesting Thermal Fluctuations: Activation Process Induced by a Nonlinear Chain in Thermal Equilibrium
We present a model in which the immediate environment of a bistable system is
a molecular chain which in turn is connected to a thermal environment of the
Langevin form. The molecular chain consists of masses connected by harmonic or
by anharmonic springs. The distribution, intensity, and mobility of thermal
fluctuations in these chains is strongly dependent on the nature of the springs
and leads to different transition dynamics for the activated process. Thus, all
else (temperature, damping, coupling parameters between the chain and the
bistable system) being the same, the hard chain may provide an environment
described as diffusion-limited and more effective in the activation process,
while the soft chain may provide an environment described as energy-limited and
less effective. The importance of a detailed understanding of the thermal
environment toward the understanding of the activation process itself is thus
highlighted
Hole Pairs in the Two-Dimensional Hubbard Model
The interactions between holes in the Hubbard model, in the low density,
intermediate to strong coupling limit, are investigated. Dressed spin polarons
in neighboring sites have an increased kinetic energy and an enhanced hopping
rate. Both effects are of the order of the hopping integral and lead to an
effective attraction at intermediate couplings. Our results are derived by
systematically improving mean field calculations. The method can also be used
to derive known properties of isolated spin polarons.Comment: 4 page
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