44,591 research outputs found
Statistical physics of cerebral embolization leading to stroke
We discuss the physics of embolic stroke using a minimal model of emboli
moving through the cerebral arteries. Our model of the blood flow network
consists of a bifurcating tree, into which we introduce particles (emboli) that
halt flow on reaching a node of similar size. Flow is weighted away from
blocked arteries, inducing an effective interaction between emboli. We justify
the form of the flow weighting using a steady flow (Poiseuille) analysis and a
more complicated nonlinear analysis. We discuss free flowing and heavily
congested limits and examine the transition from free flow to congestion using
numerics. The correlation time is found to increase significantly at a critical
value, and a finite size scaling is carried out. An order parameter for
non-equilibrium critical behavior is identified as the overlap of blockages'
flow shadows. Our work shows embolic stroke to be a feature of the cerebral
blood flow network on the verge of a phase transition.Comment: 11 pages, 11 figures. Major rewrite including improved justification
of the model and a finite size scalin
Angular Normal Modes of a Circular Coulomb Cluster
We investigate the angular normal modes for small oscillations about an
equilibrium of a single-component coulomb cluster confined by a radially
symmetric external potential to a circle. The dynamical matrix for this system
is a Laplacian symmetrically circulant matrix and this result leads to an
analytic solution for the eigenfrequencies of the angular normal modes. We also
show the limiting dependence of the largest eigenfrequency for large numbers of
particles
Non-equilibrium spatial distribution of Rashba spin torque in ferromagnetic metal layer
We study the spatial distribution of spin torque induced by a strong Rashba
spin-orbit coupling (RSOC) in a ferromagnetic (FM) metal layer, using the
Keldysh non-equilibrium Green's function method. In the presence of the s-d
interaction between the non-equilibrium conduction electrons and the local
magnetic moments, the RSOC effect induces a torque on the moments, which we
term as the Rashba spin torque.
A correlation between the Rashba spin torque and the spatial spin current is
presented in this work, clearly mapping the spatial distribution of Rashba Spin
torque in a nano-sized ferromagnetic device. When local magnetism is turned on,
the out-of-plane (Sz) Spin Hall effect (SHE) is disrupted, but rather
unexpectedly an in-plane (Sy) SHE is detected. We also study the effect of
Rashba strength (\alpha_R) and splitting exchange (\Delta) on the
non-equilibrium Rashba spin torque averaged over the device. Rashba spin torque
allows an efficient transfer of spin momentum such that a typical switching
field of 20 mT can be attained with a low current density of less than 10^6
A/cm^2
MODELING THE EFFECTS OF THE FOOD STAMP PROGRAM ON PARTICIPATING HOUSEHOLDS' PURCHASES: AN EMPIRICAL APPLICATION
Food Security and Poverty,
Induced Lorentz- and CPT-violating Chern-Simons term in QED: Fock-Schwinger proper time method
Using the Fock-Schwinger proper time method, we calculate the induced
Chern-Simons term arising from the Lorentz- and CPT-violating sector of quantum
electrodynamics with a term. Our
result to all orders in coincides with a recent linear-in- calculation
by Chaichian et al. [hep-th/0010129 v2]. The coincidence was pointed out by
Chung [Phys. Lett. {\bf B461} (1999) 138] and P\'{e}rez-Victoria [Phys. Rev.
Lett. {\bf 83} (1999) 2518] in the standard Feynman diagram calculation with
the nonperturbative-in- propagator.Comment: 11 pages, no figur
Quantum criticality in Kondo quantum dot coupled to helical edge states of interacting 2D topological insulators
We investigate theoretically the quantum phase transition (QPT) between the
one-channel Kondo (1CK) and two-channel Kondo (2CK) fixed points in a quantum
dot coupled to helical edge states of interacting 2D topological insulators
(2DTI) with Luttinger parameter . The model has been studied in Ref. 21,
and was mapped onto an anisotropic two-channel Kondo model via bosonization.
For K<1, the strong coupling 2CK fixed point was argued to be stable for
infinitesimally weak tunnelings between dot and the 2DTI based on a simple
scaling dimensional analysis[21]. We re-examine this model beyond the bare
scaling dimension analysis via a 1-loop renormalization group (RG) approach
combined with bosonization and re-fermionization techniques near weak-coupling
and strong-coupling (2CK) fixed points. We find for K -->1 that the 2CK fixed
point can be unstable towards the 1CK fixed point and the system may undergo a
quantum phase transition between 1CK and 2CK fixed points. The QPT in our model
comes as a result of the combined Kondo and the helical Luttinger physics in
2DTI, and it serves as the first example of the 1CK-2CK QPT that is accessible
by the controlled RG approach. We extract quantum critical and crossover
behaviors from various thermodynamical quantities near the transition. Our
results are robust against particle-hole asymmetry for 1/2<K<1.Comment: 17 pages, 9 figures, more details added, typos corrected, revised
Sec. IV, V, Appendix A and
Tunnelling current and emission spectrum of a single electron transistor under optical pumping
Theoretical studies of the tunnelling current and emission spectrum of a
single electron transistor (SET) under optical pumping are presented. The
calculation is performed via Keldysh Green's function method within the
Anderson model with two energy levels. It is found that holes in the quantum
dot (QD) created by optical pumping lead to new channels for the electron
tunnelling from emitter to collector. As a consequence, an electron can tunnel
through the QD via additional channels, characterized by the exciton, trion and
biexciton states. It is found that the tunnelling current as a function of the
gate voltage displays a series of sharp peaks and the spacing between these
peaks can be used to determine the exciton binding energy as well as the
electron-electron Coulomb repulsion energy. In addition, we show that the
single-photon emission associated with the electron-hole recombination in the
exciton complexes formed in the QD can be controlled both electrically and
optically.Comment: 24 pages, 10 figure
Nanoscale, Phonon-Coupled Calorimetry with Sub-Attojoule/Kelvin Resolution
We have developed an ultrasensitive nanoscale calorimeter that enables heat capacity measurements upon minute, externally affixed (phonon-coupled) samples at low temperatures. For a 5 s measurement at 2 K, we demonstrate an unprecedented resolution of ΔC ~ 0.5 aJ/K (~36 000 k_B). This sensitivity is sufficient to enable heat capacity measurements upon zeptomole-scale samples or upon adsorbates with sub-monolayer coverage across the minute cross sections of these devices. We describe the fabrication and operation of these devices and demonstrate their sensitivity by measuring an adsorbed ^4He film with optimum resolution of ~3 × 10^(-5) monolayers upon an active surface area of only ~1.2 × 10^(-9) m^2
Peeling from a patterned thin elastic film
Inspired by the observation that many naturally occurring adhesives arise as
textured thin films, we consider the displacement controlled peeling of a
flexible plate from an incision-patterned thin adhesive elastic layer. We find
that crack initiation from an incision on the film occurs at a load much higher
than that required to propagate it on a smooth adhesive surface; multiple
incisions thus cause the crack to propagate intermittently. Microscopically,
this mode of crack initiation and propagation in geometrically confined thin
adhesive films is related to the nucleation of cavitation bubbles behind the
incision which must grow and coalesce before a viable crack propagates. Our
theoretical analysis allows us to rationalize these experimental observations
qualitatively and quantitatively and suggests a simple design criterion for
increasing the interfacial fracture toughness of adhesive films.Comment: 8 pages, To appear in Proceedings of Royal Society London, Ser.
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