No many-scallop theorem: Collective locomotion of reciprocal swimmers

To achieve propulsion at low Reynolds number, a swimmer must deform in a way that is not invariant under time-reversal symmetry; this result is known as the scallop theorem. We show here that there is no many-scallop theorem. We demonstrate that two active particles undergoing reciprocal deformations can swim collectively; moreover, polar particles also experience effective long-range interactions. These results are derived for a minimal dimers model, and generalized to more complex geometries on the basis of symmetry and scaling arguments. We explain how such cooperative locomotion can be realized experimentally by shaking a collection of soft particles with a homogeneous external field

The Extended Loop Representation of Quantum Gravity

A new representation of Quantum Gravity is developed. This formulation is based on an extension of the group of loops. The enlarged group, that we call the Extended Loop Group, behaves locally as an infinite dimensional Lie group. Quantum Gravity can be realized on the state space of extended loop dependent wavefunctions. The extended representation generalizes the loop representation and contains this representation as a particular case. The resulting diffeomorphism and hamiltonian constraints take a very simple form and allow to apply functional methods and simplify the loop calculus. In particular we show that the constraints are linear in the momenta. The nondegenerate solutions known in the loop representation are also solutions of the constraints in the new representation. The practical calculation advantages allows to find a new solution to the Wheeler-DeWitt equation. Moreover, the extended representation puts in a precise framework some of the regularization problems of the loop representation. We show that the solutions are generalized knot invariants, smooth in the extended variables, and any framing is unnecessary.Comment: 27 pages, report IFFC/94-1

Effect of carrier recombination on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3

Transient reflectivity (TR) from thin films (6 - 40 nm thick) of the topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest the existence of both radiative and non-radiative recombination between electrons residing in the upper cone of initially unoccupied high energy Dirac surface states (SS) and holes residing in the lower cone of occupied low energy Dirac SS. The modeling of measured TR traces allowed us to conclude that recombination is induced by the depletion of bulk electrons in films below ~20 nm thick due to the charge captured on the surface defects. We predict that such recombination processes can be observed using time-resolved photoluminescence techniques

Primordial black holes as a tool for constraining non-Gaussianity

Primordial Black Holes (PBH's) can form in the early Universe from the collapse of large density fluctuations. Tight observational limits on their abundance constrain the amplitude of the primordial fluctuations on very small scales which can not otherwise be constrained, with PBH's only forming from the extremely rare large fluctuations. The number of PBH's formed is therefore sensitive to small changes in the shape of the tail of the fluctuation distribution, which itself depends on the amount of non-Gaussianity present. We study, for the first time, how quadratic and cubic local non-Gaussianity of arbitrary size (parameterised by f_nl and g_nl respectively) affects the PBH abundance and the resulting constraints on the amplitude of the fluctuations on very small scales. Intriguingly we find that even non-linearity parameters of order unity have a significant impact on the PBH abundance. The sign of the non-Gaussianity is particularly important, with the constraint on the allowed fluctuation amplitude tightening by an order of magnitude as f_nl changes from just -0.5 to 0.5. We find that if PBH's are observed in the future, then regardless of the amplitude of the fluctuations, non-negligible negative f_nl would be ruled out. Finally we show that g_nl can have an even larger effect on the number of PBH's formed than f_nl.Comment: 9 pages, 5 figures, v2: version to appear in Phys. Rev. D with minor changes, v3: typos corrected (including factor of 1/2 in erfc prefactor), no changes to result

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

Traffic jams and intermittent flows in microfluidic networks

We investigate both experimentally and theoretically the traffic of particles flowing in microfluidic obstacle networks. We show that the traffic dynamics is a non-linear process: the particle current does not scale with the particle density even in the dilute limit where no particle collision occurs. We demonstrate that this non-linear behavior stems from long range hydrodynamic interactions. Importantly, we also establish that there exists a maximal current above which no stationary particle flow can be sustained. For higher current values, intermittent traffic jams form thereby inducing the ejection of the particles from the initial path and the subsequent invasion of the network. Eventually, we put our findings in the broader context of the transport proccesses of driven particles in low dimension

Forster energy transfer signatures in optically driven quantum dot molecules

The Forster resonant energy transfer mechanism (FRET) is investigated in optically driven and electrically gated tunnel coupled quantum dot molecules. Two novel FRET induced optical signatures are found in the dressed excitonic spectrum. This is constructed from exciton level occupation as function of pump laser energy and applied bias, resembling a level anticrossing spectroscopy measurement. We observe a redistribution of spectral weight and splitting of the exciton spectral lines. FRET among single excitons induces a splitting in the spatially-direct exciton lines, away from the anticrossing due to charge tunneling in the molecule. However, near the anticrossing, a novel signature appears as a weak satellite line following an indirect exciton line. FRET signatures may also occur among indirect excitons, appearing as split indirect lines. In that case, the signatures appear also in the direct biexciton states, as the indirect satellite mixes in near the tunneling anticrossing region

Coherent control of indirect excitonic qubits in optically driven quantum dot molecules

We propose an optoelectronic scheme to define and manipulate an indirect neutral exciton qubit within a quantum dot molecule. We demonstrate coherent dynamics of indirect excitons resilient against decoherence effects, including direct exciton spontaneous recombination. For molecules with large interdot separation, the exciton dressed spectrum yields an often overlooked avoided crossing between spatially indirect exciton states. Effective two level system Hamiltonians are extracted by Feshbach projection over the multilevel exciton configurations. An adiabatic manipulation of the qubit states is devised using time dependent electric field sweeps. The exciton dynamics yields the necessary conditions for qubit initialization and near unitary rotations in the picosecond time scale, driven by the system internal dynamics. Despite the strong influence of laser excitation, charge tunneling, and interdot dipole-dipole interactions, the effective relaxation time of indirect excitons is much longer than the direct exciton spontaneous recombination time, rendering indirect excitons as potential elemental qubits in more complex schemes.Comment: Submitted to PRB, 11 pages and 6 figure

Infinitesimal and local convexity of a hypersurface in a semi-Riemannian manifold

Given a Riemannian manifold M and a hypersurface H in M, it is well known that infinitesimal convexity on a neighborhood of a point in H implies local convexity. We show in this note that the same result holds in a semi-Riemannian manifold. We make some remarks for the case when only timelike, null or spacelike geodesics are involved. The notion of geometric convexity is also reviewed and some applications to geodesic connectedness of an open subset of a Lorentzian manifold are given.Comment: 14 pages, AMSLaTex, 2 figures. v2: typos fixed, added one reference and several comments, statement of last proposition correcte

Classical Loop Actions of Gauge Theories

Since the first attempts to quantize Gauge Theories and Gravity in the loop representation, the problem of the determination of the corresponding classical actions has been raised. Here we propose a general procedure to determine these actions and we explicitly apply it in the case of electromagnetism. Going to the lattice we show that the electromagnetic action in terms of loops is equivalent to the Wilson action, allowing to do Montecarlo calculations in a gauge invariant way. In the continuum these actions need to be regularized and they are the natural candidates to describe the theory in a ``confining phase''.Comment: LaTeX 14 page