Experimental Evidence for Defect Tolerance in Pb-Halide Perovskites

The term defect tolerance (DT) is used often to rationalize the exceptional optoelectronic properties of Halide Perovskites, HaPs, and their devices. Even though DT lacked direct experimental evidence, it became fact in the field. DT in semiconductors implies tolerance to structural defects without the electrical and optical effects (e.g., traps), associated with such defects. We present first direct experimental evidence for DT in Pb HaPs by comparing the structural quality of 2D, 2D_3D, and 3D Pb HaP crystals with their optoelectronic characteristics using high sensitivity methods. Importantly, we get information from the material bulk, because we sample at least a few 100 nm, up to several micrometer, from the sample surface, which allows assessing intrinsic bulk (and not only surface) properties of HaPs. The results point to DT in 3D, to a lesser extent in 2D_3D, but not in 2D Pb HaPs. We ascribe such dimension dependent DT to the higher number of (near)neighboring species, available to compensate for structural defect effects in the 3D than in the 2D HaP crystals. Overall, our data provide an experimental basis to rationalize DT in Pb HaPs. These experiments and findings can guide the search for, and design of other materials with DT

Monitoring Electron Spin Fluctuations with Paramagnetic Relaxation Enhancement

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1 and T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMgMnPO4 and Fe(3+) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300K

Efficient graph topologies in network routing games

A topology is efficient for network games if, for any game over it, every Nash equilibrium is socially optimal. It is well known that many topologies are not efficient for network games. We characterize efficient topologies in network games with a finite set of players, each wishing to transmit an atomic unit of unsplittable flow. We distinguish between two classes of atomic network routing games. In network congestion games a player's cost is the sum of the costs of the edges it traverses, while in bottleneck routing games, it is its maximum edge cost. In both classes, the social cost is the maximum cost among the players' costs. We show that for symmetric network congestion games the efficient topologies are Extension Parallel networks, while for symmetric bottleneck routing games the efficient topologies are Series Parallel networks. In the asymmetric case the efficient topologies include only trees with parallel edges.Network routing games Congestion games Price of anarchy

Strong price of anarchy

A strong equilibrium is a pure Nash equilibrium which is resilient to deviations by coalitions. We define the strong price of anarchy (SPoA) to be the ratio of the worst strong equilibrium to the social optimum. Differently from the Price of Anarchy (defined as the ratio of the worst Nash Equilibrium to the social optimum), it quantifies the loss incurred from the lack of a central designer in settings that allow for coordination. We study the SPoA in two settings, namely job scheduling and network creation. In the job scheduling game we show that for unrelated machines the SPoA can be bounded as a function of the number of machines and the size of the coalition. For the network creation game we show that the SPoA is at most 2. In both cases we show that a strong equilibrium always exists, except for a well defined subset of network creation games.Strong equilibrium Price of anarchy Strong price of anarchy Coalitions Congestion games Network formation Job scheduling

Strong equilibrium in cost sharing connection games

We study network games in which each player wishes to connect his source and sink, and the cost of each edge is shared among its users either equally (in Fair Connection Games--FCG's) or arbitrarily (in General Connection Games--GCG's). We study the existence and quality of strong equilibria (SE)--strategy profiles from which no coalition can improve the cost of each of its members--in these settings. We show that SE always exist in the following games: (1) Single source and sink FCG's and GCG's. (2) Single source multiple sinks FCG's and GCG's on series parallel graphs. (3) Multi source and sink FCG's on extension parallel graphs. As for the quality of the SE, in any FCG with n players, the cost of any SE is bounded by H(n) (i.e., the harmonic sum), contrasted with the [Theta](n) price of anarchy. For any GCG, any SE is optimal.Strong equilibrium Price of anarchy Strong price of anarchy Coalitions Cost sharing Network design

Designing Committees for Mitigating Biases

It is widely observed that individuals prefer to interact with others who are more similar to them (this phenomenon is termed homophily). This similarity manifests itself in various ways such as beliefs, values and education. Thus, it should not come as a surprise that when people make hiring choices, for example, their similarity to the candidate plays a role in their choice. In this paper, we suggest that putting the decision in the hands of a committee instead of a single person can reduce this bias. We study a novel model of voting in which a committee of experts is constructed to reduce the biases of its members. We first present voting rules that optimally reduce the biases of a given committee. Our main results include the design of committees, for several settings, that are able to reach a nearly optimal (unbiased) choice. We also provide a thorough analysis of the trade-offs between the committee size and the obtained error. Our model is inherently different from the well-studied models of voting that focus on aggregation of preferences or on aggregation of information due to the introduction of similarity biases

Endogenous Metal-Ion Dynamic Nuclear Polarization for NMR Signal Enhancement in Metal Organic Frameworks

Acknowledgements: We thank Dr Alexander C. Forse for access to the CO2 adsorption measurements. I. B. M. is grateful to the Sustainability and Energy Research Initiative (SAERI) fellowship for financial support. This research was funded by the European Research Council (MIDNP, Grant 803024), the European Union's Horizon 2020 research and innovation program (Pan-European Solid-State NMR Infrastructure for Chemistry-Enabling Access, PANACEA, Grant Agreement 101008500) and the Clore Institute for High Field Magnetic Resonance Spectroscopy and Imaging. Dr Daniel Jardón-Álvarez is acknowledged for valuable discussions and the help with data processing. We also thank Dr Ayan Maity for acquiring the SEM images. The work was made possible in part by the historic generosity of the Harold Perlman family.Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy...</jats:p

Endogenous metal-ion dynamic nuclear polarization for NMR signal enhancement in metal organic frameworks.

Acknowledgements: We thank Dr Alexander C. Forse for access to the CO2 adsorption measurements. I. B. M. is grateful to the Sustainability and Energy Research Initiative (SAERI) fellowship for financial support. This research was funded by the European Research Council (MIDNP, Grant 803024), the European Union's Horizon 2020 research and innovation program (Pan-European Solid-State NMR Infrastructure for Chemistry-Enabling Access, PANACEA, Grant Agreement 101008500) and the Clore Institute for High Field Magnetic Resonance Spectroscopy and Imaging. Dr Daniel Jardón-Álvarez is acknowledged for valuable discussions and the help with data processing. We also thank Dr Ayan Maity for acquiring the SEM images. The work was made possible in part by the historic generosity of the Harold Perlman family.Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4',4''-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold 13C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research