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See detailOptical properties of topological flat and dispersive bands
Habibi, Alireza UL; Musthofa, Ahmad Z.; Adibi, Elaheh et al

E-print/Working paper (2021)

We study the optical properties of topological flat and dispersive bands. Due to their topological nature, there exists an anomalous Hall response which gives rise to a transverse current without applied ... [more ▼]

We study the optical properties of topological flat and dispersive bands. Due to their topological nature, there exists an anomalous Hall response which gives rise to a transverse current without applied magnetic field. The dynamical Hall conductivity of systems with flat bands exhibits a sign change when the excitation energy is on resonance with the band gap, similar to the magnetotransport Hall conductivity profile. The sign change of the Hall conductivity is located at the frequency corresponding to the singularity of the joint density of states, i.e., the van Hove singularity (VHS). For perfectly flat bands, this VHS energy matches the band gap. On the other hand, in the case of dispersive bands, the VHS energy is located above the band gap. As a result, the two features of the Hall conductivity, i.e., the resonant feature at the band gap and the sign change at the VHS energy, become separated. This anomalous Hall response rotates the polarization of an electric field and can be detected in the reflected and transmitted waves, as Kerr and Faraday rotations, respectively, thus allowing a simple optical characterization of topological flat bands. [less ▲]

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See detailOptical spectroscopy on the photo-response in multiferroic BiFeO3 at high pressure
Meggle, F.; Ebad-Allah, J.; Kreisel, Jens UL et al

in JOURNAL OF APPLIED PHYSICS (2019), 126(16),

The pressure dependence of light-induced effects in single-crystalline BiFeO3 is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just ... [more ▼]

The pressure dependence of light-induced effects in single-crystalline BiFeO3 is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just below the two crystal-field excitations and the absorption onset, respectively. These absorption features were previously ascribed to excitons, possibly connected with the ultrafast photostriction effect in BiFeO3. The pressure-induced redshift of the absorption features follows the pressure dependence of the corresponding crystal-field excitations and absorption onset, suggesting the link between them. Above the structural phase transition at P-c1 approximate to 3.5 GPa, the three absorption features disappear, suggesting their connection to the polar phase in BiFeO3. The pressure-induced disappearance of the photoinduced features is irreversible upon pressure release. Published under license by AIP Publishing. [less ▲]

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See detailOptical spectroscopy study on the photo-response in multiferroic BiFeO3
Burkert, F.; Kreisel, Jens UL; Kuntscher, C. A.

in Applied Physics Letters (2016), 109(18),

We investigate the underlying mechanism of the photostriction effect in single-crystalline BiFeO3 by transmission measurements in the infrared and visible frequency range under continuous illumination ... [more ▼]

We investigate the underlying mechanism of the photostriction effect in single-crystalline BiFeO3 by transmission measurements in the infrared and visible frequency range under continuous illumination with a green 532 nm). The small photo-induced changes in the transmission spectrum reveal three well-defined absorption features at 1.22 eV, 1.66 eV, and 2.14 eV, which we assign to charge-transfer excitons and in-gap defect states probably related to oxygen vacancies. The intensity of the three absorption features follows a linear dependence on the illumination intensity for an irradiance above 90 W/m(2). Published by AIP Publishing. [less ▲]

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See detailOptical studies of ferroelectric and ferroelastic domain walls
Nataf, G. F.; Guennou, Mael UL

in Journal of Physics: Condensed Matter (2020), 32(18), 183001

Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the ... [more ▼]

Recent studies carried out with atomic force microscopy or high-resolution transmission electron microscopy reveal that ferroic domain walls can exhibit different physical properties than the bulk of the domains, such as enhanced conductivity in insulators, or polar properties in non-polar materials. In this review we show that optical techniques, in spite of the diffraction limit, also provide key insights into the structure and physical properties of ferroelectric and ferroelastic domain walls. We give an overview of the uses, specificities and limits of these techniques, and emphasize the properties of the domain walls that they can probe. We then highlight some open questions of the physics of domain walls that could benefit from their use. [less ▲]

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See detailOptimal and approximate Q-value functions for decentralized POMDPs
Oliehoek, Frans A.; Spaan, Matthijs T. J.; Vlassis, Nikos UL

in Journal of Artificial Intelligence Research (2008), 32

Decision-theoretic planning is a popular approach to sequential decision making problems, because it treats uncertainty in sensing and acting in a principled way. In single-agent frameworks like MDPs and ... [more ▼]

Decision-theoretic planning is a popular approach to sequential decision making problems, because it treats uncertainty in sensing and acting in a principled way. In single-agent frameworks like MDPs and POMDPs, planning can be carried out by resorting to Q-value functions: an optimal Q-value function Q* is computed in a recursive manner by dynamic programming, and then an optimal policy is extracted from Q*. In this paper we study whether similar Q-value functions can be defined for decentralized POMDP models (Dec-POMDPs), and how policies can be extracted from such value functions. We define two forms of the optimal Q-value function for Dec-POMDPs: one that gives a normative description as the Q-value function of an optimal pure joint policy and another one that is sequentially rational and thus gives a recipe for computation. This computation, however, is infeasible for all but the smallest problems. Therefore, we analyze various approximate Q-value functions that allow for efficient computation. We describe how they relate, and we prove that they all provide an upper bound to the optimal Q-value function Q*. Finally, unifying some previous approaches for solving Dec-POMDPs, we describe a family of algorithms for extracting policies from such Q-value functions, and perform an experimental evaluation on existing test problems, including a new firefighting benchmark problem. [less ▲]

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See detailOptimal and Suboptimal Transmit Beamforming
Bengtsson, Mats; Ottersten, Björn UL

in Godara, Lal C. (Ed.) Handbook of Antennas in Wireless Communications (2001)

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See detailOptimal Berry-Esseen bounds on the Poisson space.
Peccati, Giovanni UL; Azmoodeh, Ehsan UL

E-print/Working paper (2015)

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See detailOptimal Berry-Esseen rates on the Wiener space: the barrier of third and fourth cumulants
Biermé, Hermine; Bonami, Aline; Nourdin, Ivan UL et al

in Latin American Journal of Probability and Mathematical Statistics (2012), 9(2), 473--500

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See detailOptimal Channel Selection for Simultaneous RF Energy Harvesting and Data Transmission in Cognitive Radio Networks
Hooshiary, Akram; Azmi, Paeiz; Mokari, Nader et al

in Transactions on Emerging Telecommunications Technologies (2018)

In this paper, an RF-powered cognitive radio network is considered, in which the secondary users are powered by an RF energy harvester (Rectenna). Unlike most existing works, we consider a realistic ... [more ▼]

In this paper, an RF-powered cognitive radio network is considered, in which the secondary users are powered by an RF energy harvester (Rectenna). Unlike most existing works, we consider a realistic Rectenna characteristic function, and derive the actual amount of harvested energy and thus, the resulting actual energy level of the secondary users. We consider a system architecture at which simultaneous energy harvesting and data transmission for each secondary user is possible. We introduce a strategy to manage the challenge of network throughput decreasing due to lack of the secondary users’ energy, via selecting the best possible channels for energy harvesting and simultaneously by allocating the best channels for data transmission. Therefore, we implement cognition in spectrum utilization and in energy harvesting. We show that the amount of harvested energy affects the available energy of the secondary user and consequently the throughput, therefore, the channels selection to maximize energy harvesting affects the network throughput. To maximize the network throughput, the Hungarian algorithm is employed, and then, an algorithm with lower complexity based on the matching theory is proposed. Finally, we compare our proposed approach with some existing benchmarks and show its high performance in energy harvesting and system throughput. [less ▲]

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See detailOptimal coalition splitting with heterogenous strategies
Boucekkine, Raouf; Camcho, Carmen; Ruan, Weihua et al

E-print/Working paper (2022)

We consider a group of players initially members of a coalition managing cooperatively a public bad, in this case, the stock of pollution. Countries are technologically heterogeneous but the pollution ... [more ▼]

We consider a group of players initially members of a coalition managing cooperatively a public bad, in this case, the stock of pollution. Countries are technologically heterogeneous but the pollution damage is uniform. We essentially attempt to characterize the conditions under which a country may eventually split and when it splits within an infinite horizon multi-stage differential game. In contrast to the existing literature, we do not assume that after splitting, the splitting player and the remaining coalition will adopt Markovian strategies. Instead, we assume that the latter will remain committed to the collective control of pollution and play open-loop, while the splitting player plays Markovian. Within a full linear-quadratic model, we characterize the optimal strategies. We later compare with the outcomes of the case where the splitting player and the remaining coalition play both Markovian. We highlight several interesting results in terms of the implications for long- term pollution levels and the duration of coalitions with heterogeneous strategies. [less ▲]

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See detailAn Optimal Control Approach to Portfolio Optimisation with Conditioning Information
Boissaux, Marc; Schiltz, Jang UL

E-print/Working paper (2010)

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See detailAn Optimal Control Approach to Portfolio Optimisation with Conditioning Information
Boissaux, Marc; Schiltz, Jang UL

Scientific Conference (2010, May 04)

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See detailOptimal control for rough differential equations
Mazliak, Laurent; Nourdin, Ivan UL

in Stochastics and Dynamics (2008), 8(1), 23-33

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See detailAn Optimal Control Solution to the Constrained Weight Portfolio Optimisation Problem with Conditioning Information
Boissaux, Marc; Schiltz, Jang UL

Scientific Conference (2010, June 04)

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See detailOptimal Convergence Rates and One-Term Edgeworth Expansions for Multidimensional Functionals of Gaussian Fields
Campese, Simon UL

in ALEA: Latin American Journal of Probability and Mathematical Statistics (2013)

We develop techniques for determining the exact asymptotic speed of convergence in the multidimensional normal approximation of smooth functions of Gaussian fields. As a by-product, our findings yield ... [more ▼]

We develop techniques for determining the exact asymptotic speed of convergence in the multidimensional normal approximation of smooth functions of Gaussian fields. As a by-product, our findings yield exact limits and often give rise to one-term generalized Edgeworth expansions increasing the speed of convergence. Our main mathematical tools are Malliavin calculus, Stein's method and the Fourth Moment Theorem. This work can be seen as an extension of the results of arXiv:0803.0458 to the multi-dimensional case, with the notable difference that in our framework covariances are allowed to fluctuate. We apply our findings to exploding functionals of Brownian sheets, vectors of Toeplitz quadratic functionals and the Breuer-Major Theorem. [less ▲]

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See detailOptimal convergence rates for the invariant density estimation of jump-diffusion processes
Amorino, Chiara UL; Nualart, Eulalia

E-print/Working paper (2021)

We aim at estimating the invariant density associated to a stochastic differential equation with jumps in low dimension, which is for d = 1 and d = 2. We consider a class of jump diffusion processes whose ... [more ▼]

We aim at estimating the invariant density associated to a stochastic differential equation with jumps in low dimension, which is for d = 1 and d = 2. We consider a class of jump diffusion processes whose invariant density belongs to some Hölder space. Firstly, in dimension one, we show that the kernel density estimator achieves the convergence rate 1/T, which is the optimal rate in the absence of jumps. This improves the convergence rate obtained in [Amorino, Gloter (2021)], which depends on the Blumenthal-Getoor index for d = 1 and is equal to log T/T for d = 2. Secondly, we show that is not possible to find an estimator with faster rates of estimation. Indeed, we get some lower bounds with the same rates { 1/T , log T/T } in the mono and bi-dimensional cases, respectively. Finally, we obtain the asymptotic normality of the estimator in the one-dimensional case. [less ▲]

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See detailOptimal day-ahead trading and storage of renewable energies—an approximate dynamic programming approach
Löhndorf, Nils UL; Minner, Stefan

in Energy Systems (2010), 1(1), 61--77

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See detailOptimal Deployment of Base Stations in Cognitive Satellite Terrestrial Networks
Vuppala, Satyanarayana UL; Sellathurai, Mathini; Chatzinotas, Symeon UL

in 2018 9th Advanced Satellite Multimedia Systems Conference and the 15th Signal Processing for Space Communications Workshop (ASMS/SPSC) (2018)

The application of linear precoding at the gateway side enables broadband multibeam satellite systems to use more aggressive frequency reuse patterns increasing the overall capacity of future High ... [more ▼]

The application of linear precoding at the gateway side enables broadband multibeam satellite systems to use more aggressive frequency reuse patterns increasing the overall capacity of future High Throughput Satellites (HTS). However, although some previous works about precoding consider imperfect CSIT (Chanel State Information at the Transmitter) adding some CSI estimation errors, that is not the main cause of CSI degradation. In practice, receivers can only detect and estimate a few coefficients of the CSI vector being the other nullified, replaced by zeros. This introduces errors in the SINR calculation by the gateway that lead to the assignment of Modulation and Coding Schemes (MCS) over the decoding possibilities of the users, increasing the rate of erroneous frames. In this work, the errors in the SINR calculation caused by the nullification of the CSI are analyzing statistically and geographically using a radiation diagram of 245 beams over Europe. Furthermore, a solution based on a link adaptation algorithm with a per user adaptive margin is proposed, helping to achieve the QEF (Quasi-error Free) target of DVB-S2X systems. [less ▲]

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