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 Excitonic lasing of strainfree InP(As) quantum dots in AlInAs microdisk (2017.03.20)
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 Optimal working conditions for thermoelectric generators with realistic thermal coupling (3886)
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MEMBERS' PUBLICATIONS
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Electrostatic interaction of neutral semipermeable membranes
 Uploaded:
 17.10.12
 Modified:
 17.10.12
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 566 KB
Olga I. Vinogradova, Lyderic Bocquet, Artem N. Bogdanov, Roumen Tsekov and Vladimir Lobaskin
We consider an osmotic equilibrium between bulk solutions of polyelectrolyte bounded by semipermeable membranes and separated by a thin film of saltfree liquid. Although the membranes are neutral, the counterions of the polyelectrolyte molecules permeate into the gap and lead to a steric charge separation. This gives rise to a distancedependent membrane potential, which translates into a repulsive electrostatic disjoining pressure. From the solution of the nonlinear Poisson–Boltzmann equation, we obtain the distribution of the potential and of ions. We then derive an explicit formula for the pressure exerted on the membranes and show that it deviates from the classical van’t Hoff expression for the osmotic pressure. This difference is interpreted in terms of a repulsive electrostatic disjoining pressure originating from the overlap of counterion clouds inside the gap. We also develop a simplified theory based on a linearized Poisson–Boltzmann approach. A comparison with simulation of a primitive model for the electrolyte is provided and does confirm the validity of the theoretical predictions. Beyond the fundamental result that the neutral surfaces can repel, this mechanism not only helps to control the adhesion and longrange interactions of living cells, bacteria, and vesicles, but also allows us to argue that electrostatic interactions should play enormous role in determining behavior and functions of systems bounded by semipermeable membranes. 
Anisotropic flow in striped superhydrophobic channels
 Uploaded:
 17.10.12
 Modified:
 17.10.12
 File Size:
 504 KB
Jiajia Zhou, Aleksey V. Belyaev, Friederike Schmid and Olga I. Vinogradova
We report results of dissipative particle dynamics simulations and develop a semianalytical theory of an anisotropic flow in a parallelplate channel with two superhydrophobic striped walls. Our approach is valid for any local slip at the gas sectors and an arbitrary distance between the plates, ranging from a thick to a thin channel. It allows us to optimize area fractions, slip lengths, channel thickness, and texture orientation to maximize a transverse flow. Our results may be useful for extracting effective slip tensors from global measurements, such as the permeability of a channel, in experiments or simulations, and may also find applications in passive microfluidic mixing. 
Effective slip boundary conditions for arbitrary onedimensional surfaces
 Uploaded:
 17.10.12
 Modified:
 17.10.12
 File Size:
 659 KB
Evgeny S. Asmolov and Olga I. Vinogradova
In many applications it is advantageous to construct effective slip boundary conditions, which could fully characterize flow over patterned surfaces. Here we focus on laminar shear flows over smooth anisotropic surfaces with arbitrary scalar slip b(y), varying in only one direction. We derive general expressions for eigenvalues of the effective sliplength tensor, and show that the transverse component is equal to half of the longitudinal one, with a two times larger local slip, 2b(y). A remarkable corollary of this relation is that the flow along any direction of the onedimensional surface can be easily determined, once the longitudinal component of the effective slip tensor is found from the known spatially nonuniform scalar slip. 
Tensorial slip of superhydrophobic channels
 Uploaded:
 17.10.12
 Modified:
 17.10.12
 File Size:
 1 MB
Sebastian Schmieschek, Aleksey V. Belyaev, Jens Harting and Olga I. Vinogradova
We describe a generalization of the tensorial slip boundary condition, originally justified for a thick (compared to texture period) channel, to any channel thickness. The eigenvalues of the effective sliplength tensor, however, in general case become dependent on the gap and cannot be viewed as a local property of the surface, being a global characteristic of the channel. To illustrate the use of the tensor formalism we develop a semianalytical theory of an effective slip in a parallelplate channel with one superhydrophobic striped and one hydrophilic surface. Our approach is valid for any local slip at the gas sectors and an arbitrary distance between the plates, ranging from a thick to a thin channel. We then present results of lattice Boltzmann simulations to validate the analysis. Our results may be useful for extracting effective slip tensors from global measurements, such as the permeability of a channel, in experiments or simulations. 
Interactions of neutral semipermeable shells in asymmetric electrolyte solutions
 Uploaded:
 17.10.12
 Modified:
 17.10.12
 File Size:
 423 KB
Vladimir Lobaskin, Artem N. Bogdanov and Olga I. Vinogradova
We study the ionic equilibria and interactions of neutral semipermeable spherical shells immersed in electrolyte solutions, including polyions. Although the shells are uncharged, only one type of ion of the electrolyte can permeate them, thus leading to a steric charge separation in the system. This gives rise to a charge accumulation inside the shell and a build up of concentrationdependent shell potential, which converts into a disjoining pressure between the neighboring shells. These are quantified using the Poisson–Boltzmann and integral equations theories. In particular, we show that in a case of low valency electrolytes, interactions between shells are repulsive and can be sufficiently strong to stabilize the shell dispersion. In contrast, the charge correlation effects in solutions of polyvalent ions result in attractions between the shells, with can lead to their aggregation. 
Optimum inhomogeneity of local lattice distortions in La2CuO4+y
 Uploaded:
 03.09.12
 Modified:
 03.09.12
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 1 MB
Nicola Poccia, Alessandro Ricci, Gaetano Campi, Michela Fratinia, Alessandro Puri, Daniele Di Gioacchino, Augusto Marcelli, Michael Reynolds, Manfred Burghammer, Naurang Lal Saini, Gabriel Aeppli, and Antonio Bianconi
Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the "glue" binding the ordered regions together. Here we use scanning Xray microdiffraction (with a beam 300 nm in diameter) to show that for La_{2}CuO_{4+y} , the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La_{2}CuO_{4+y} layers intercalated between the CuO_{2} layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature T_{c} is actually underpinned by a fundamental relation between T_{c} and the distribution of ordered defect networks supported by the materials.

Spontaneous Symmetry Breaking in a Polariton and Photon Laser
 Uploaded:
 04.07.12
 Modified:
 04.07.12
 File Size:
 2 MB
H. Ohadi, E. Kammann, T. C. H. Liew, K. G. Lagoudakis, A.V. Kavokin and P. G. Lagoudakis
We report on the simultaneous observation of spontaneous symmetry breaking and longrange spatial coherence both in the strong and the weakcoupling regime in a semiconductor microcavity. Under pulsed excitation, the formation of a stochastic order parameter is observed in polariton and photon lasing regimes. Singleshot measurements of the Stokes vector of the emission exhibit the buildup of stochastic polarization. Below threshold, the polarization noise does not exceed 10%, while above threshold we observe a total polarization of up to 50% after each excitation pulse, while the polarization averaged over the ensemble of pulses remains nearly zero. In both polariton and photon lasing regimes, the stochastic polarization buildup is accompanied by the buildup of spatial coherence. We find that the Landau criterion of spontaneous symmetry breaking and PenroseOnsager criterion of longrange order for BoseEinstein condensation are met in both polariton and photon lasing regimes. 
Quantum Theory of Spin Dynamics of ExcitonPolaritons in Microcavities
 Uploaded:
 31.08.10
 Modified:
 12.10.10
 File Size:
 134 KB
K.V. Kavokin, I. A. Shelykh, A.V. Kavokin, G. Malpuech and P. Bigenwald
We present the quantum theory of momentum and spin relaxation of excitonpolaritons in microcavities. We show that giant longitudinaltransverse splitting of the polaritons mixes their spin states, which results in beats between right and leftcircularly polarized photoluminescence of microcavities, as was recently experimentally observed [M. D. Martin et al., Phys. Rev. Lett. 89, 077402 (2002)]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.

Optical Spin Hall Effect
 Uploaded:
 31.08.10
 Modified:
 12.10.10
 File Size:
 207 KB
A. Kavokin, G. Malpuech and M. Glazov
A remarkable analogy is established between the wellknown spin Hall effect and the polarization dependence of Rayleigh scattering of light in microcavities. This dependence results from the strong spin effect in elastic scattering of exciton polaritons: if the initial polariton state has a zero spin and is characterized by some linear polarization, the scattered polaritons become strongly spin polarized. The polarization in the scattered state can be positive or negative dependent on the orientation of the linear polarization of the initial state and on the direction of scattering. Very surprisingly, spin polarizations of the polaritons scattered clockwise and anticlockwise have different signs. The optical spin Hall effect is possible due to strong longitudinaltransverse splitting and finite lifetime of exciton polaritons in microcavities. 
ExcitonPolariton Mediated Superconductivity
 Uploaded:
 01.09.10
 Modified:
 12.10.10
 File Size:
 499 KB
F. P. Laussy, A. V. Kavokin and I. A. Shelykh
We revisit the exciton mechanism of superconductivity in the framework of microcavity physics, replacing virtual excitons as a binding agent of Cooper pairs by excitations of an excitonpolariton Bose Einstein condensate. We consider a model microcavity where a quantum well with a twodimensional electron gas is sandwiched between two undoped quantum wells, where a polariton condensate is formed. We show that the critical temperature for superconductivity dramatically increases with the condensate population, opening a new route towards hightemperature superconductivity.