7,482 research outputs found
Geometric scaling of purely-elastic flow instabilities
We present a combined experimental, numerical and theoretical investigation
of the geometric scaling of the onset of a purely-elastic flow instability in a
serpentine channel. Good qualitative agreement is obtained between experiments,
using dilute solutions of flexible polymers in microfluidic devices, and
two-dimensional numerical simulations using the UCM model. The results are
confirmed by a simple theoretical analysis, based on the dimensionless
criterion proposed by Pakdel-McKinley for onset of a purely-elastic
instability
Radiative cascade from quantum dot metastable spin-blockaded biexciton
We detect a novel radiative cascade from a neutral semiconductor quantum dot.
The cascade initiates from a metastable biexciton state in which the holes form
a spin-triplet configuration, Pauli-blockaded from relaxation to the
spin-singlet ground state. The triplet biexciton has two photon-phonon-photon
decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in
which the two photons are co-linearly polarized, in the triplet biexciton
cascade they are crosslinearly polarized. We measured the two-photon
polarization density matrix and show that the phonon emitted when the
intermediate exciton relaxes from excited to ground state, preserves the
exciton's spin. The phonon, thus, does not carry with it any which-path
information other than its energy. Nevertheless, entanglement distillation by
spectral filtering was found to be rather ineffective for this cascade. This
deficiency results from the opposite sign of the anisotropic electron-hole
exchange interaction in the excited exciton relative to that in the ground
exciton.Comment: 6 pages, 4 figure
Bi-Laplacian Growth Patterns in Disordered Media
Experiments in quasi 2-dimensional geometry (Hele Shaw cells) in which a
fluid is injected into a visco-elastic medium (foam, clay or
associating-polymers) show patterns akin to fracture in brittle materials, very
different from standard Laplacian growth patterns of viscous fingering. An
analytic theory is lacking since a pre-requisite to describing the fracture of
elastic material is the solution of the bi-Laplace rather than the Laplace
equation. In this Letter we close this gap, offering a theory of bi-Laplacian
growth patterns based on the method of iterated conformal maps.Comment: Submitted to PRL. For further information see
http://www.weizmann.ac.il/chemphys/ander
An Upper Bound on the Higgs Boson Mass from a Positivity Condition on the Mass Matrix
We impose the condition that the eigenvalues of the mass matrix in the
shifted Lagrangian density be positive at \phi=\phi_{0}, the vacuum expectation
value of the scalar field. Using the one-loop effective potential of the
standard model, this condition leads to an upper bound on the Higgs boson mass
m_{H}: m_{H}<230GeV, for a top quark mass of 175GeV.Comment: LaTex, 5 page
Drift and Diffusion in Periodically Driven Renewal Processes
We consider the drift and diffusion properties of periodically driven renewal
processes. These processes are defined by a periodically time dependent waiting
time distribution, which governs the interval between subsequent events. We
show that the growth of the cumulants of the number of events is asymptotically
periodic and develop a theory which relates these periodic growth coefficients
to the waiting time distribution defining the periodic renewal process. The
first two coefficients, which are the mean frequency and effective diffusion
coefficient of the number of events are considered in greater detail. They may
be used to quantify stochastic synchronization.Comment: 29 pages, 6 figures, submitted to Journal of Statistical Physic
Attosecond double-slit experiment
A new scheme for a double-slit experiment in the time domain is presented.
Phase-stabilized few-cycle laser pulses open one to two windows (``slits'') of
attosecond duration for photoionization. Fringes in the angle-resolved energy
spectrum of varying visibility depending on the degree of which-way information
are observed. A situation in which one and the same electron encounters a
single and a double slit at the same time is discussed. The investigation of
the fringes makes possible interferometry on the attosecond time scale. The
number of visible fringes, for example, indicates that the slits are extended
over about 500as.Comment: 4 figure
Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film
We report on the observation of cyclotron resonance induced photocurrents,
excited by continuous wave terahertz radiation, in a 3D topological insulator
(TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent
formation is supported by complimentary measurements of magneto-transport and
radiation transmission. We demonstrate that the photocurrent is generated in
the topologically protected surface states. Studying the resonance response in
a gated sample we examined the behavior of the photocurrent, which enables us
to extract the mobility and the cyclotron mass as a function of the Fermi
energy. For high gate voltages we also detected cyclotron resonance (CR) of
bulk carriers, with a mass about two times larger than that obtained for the
surface states. The origin of the CR assisted photocurrent is discussed in
terms of asymmetric scattering of TI surface carriers in the momentum space.
Furthermore, we show that studying the photocurrent in gated samples provides a
sensitive method to probe the effective masses and the mobility of 2D Dirac
surface states, when the Fermi level lies in the bulk energy gap or even in the
conduction band
A Discrete and Bounded Envy-free Cake Cutting Protocol for Four Agents
We consider the well-studied cake cutting problem in which the goal is to
identify a fair allocation based on a minimal number of queries from the
agents. The problem has attracted considerable attention within various
branches of computer science, mathematics, and economics. Although, the elegant
Selfridge-Conway envy-free protocol for three agents has been known since 1960,
it has been a major open problem for the last fifty years to obtain a bounded
envy-free protocol for more than three agents. We propose a discrete and
bounded envy-free protocol for four agents
An ultra-low field SQUID magnetometer for measuring antiferromagnetic and weakly remanent magnetic materials at low temperatures
A novel setup for the measurement of magnetic fields external to certain antiferromagnets and generally weakly remanent magnetic materials is presented. The setup features a highly sensitive Super Conducting Quantum Interference Device (SQUID) magnetometer with a magnetic field resolution of approx. 10 fT, non-electric thermalization of the sample space for a temperature range of 1.5 - 65 K with a non-electric sample movement drive and optical position encoding. To minimize magnetic susceptibility effects, the setup components are degaussed and realized with plastic materials in sample proximity. Running the setup in magnetically shielded rooms allows for a well-defined ultra low magnetic background field well below 150 nT in situ. The setup enables studies of inherently weak magnetic materials which cannot be measured with high field susceptibility setups, optical methods or neutron scattering techniques, giving new opportunities for the research on e.g. spin-spiral multiferroics, skyrmion materials and spin ices
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