We study electronic inhomogeneities in manganites using simulations on a microscopic model with Coulomb interactions amongst two electronic fluids-one localized (polaronic), the other extended-and dopant ions. The long range Coulomb interactions frustrate phase separation induced by the large on site repulsion between the fluids. A single phase ensues which is inhomogeneous at a nanoscale, but homogeneous on mesoscales, with many features that agree with experiments. This, we argue, is the origin of nanoscale inhomogeneities in manganites, rather than phase competition or disorder effects.

We studied the behavior of H 2 and CH 4 flowing through various pore geometries of nanoporous graphene using molecular dynamics method. Ten different geometries of pore-18, with different eccentricities, were prepared. It was found that the gas permeance and adsorption layer were heavily influenced by the eccentricity of the pores. On further investigation, it was also found that the jaggedness of the pore geometry played a role as well. It was also noted that at specific eccentricities, pore-18 exhibited hydrogen selective behavior, which was found to extend to pore-12, -14, -16, -20, -24, and -30 as well. Furthermore, it was shown that the H 2 permeance of these pores can reach 9 times the value of that of pore-10 (which was previously found to be the only selective pore). Hence, these pores show H 2 selectivity with high H 2 yields. Thus, this study demonstrates the exciting possibility of creating highly efficient H 2 separators by pore geometry variation. Recent experimental studies, which involve an atom-by-atom removal technique to create nanopores, point to the possibility of obtaining these geometries in the lab.

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PACS 71.10.-w -Electronic structure: theories and models of condensed matter PACS 71.30.+h -Insulator-metal transitions PACS 75.10.Kt -Magnetic ordering: quantum spin liquids Abstract -Dynamics of magnetic moments near the Mott metal-insulator transition is investigated by a combined slave-rotor and Dynamical Mean-Field Theory solution of the Hubbard model with additional fully-frustrated random Heisenberg couplings. In the paramagnetic Mott state, the spinon decomposition allows to generate a Sachdev-Ye spin liquid in place of the collection of independent local moments that typically occurs in the absence of magnetic correlations. Cooling down into the spin-liquid phase, the onset of deviations from pure Curie behavior in the spin susceptibility is found to be correlated to the temperature scale at which the Mott transition lines experience a marked bending. We also demonstrate a weakening of the effective exchange energy upon approaching the Mott boundary from the Heisenberg limit, due to quantum fluctuations associated to zero and doubly occupied sites. serge.florens@grenoble.cnrs.frIntroduction. -The Mott metal-insulator transition, wherein electronic waves are localized by short-range electron-electron interactions (see [1] for a review), is one of the most complex phenomenon observed in strongly correlated electronic systems. Even though the appearance of a Mott gap is purely driven by the charge degrees of freedom, it is expected that magnetic fluctuations play a very crucial role in determining the true nature of this phase transition. In the paramagnetic Mott insulator, local moments are indeed well defined objects after their creation at high temperature (at a scale set by the local Coulomb interaction) and before their ultimate antiferromagnetic ordering at the Néel temperature, offering a window in which complex behavior of the spin excitations is yet to be clearly understood. Experimentally, the simplest situation in this respect occurs when the low-temperature magnetic ordering is first order, as in the case of Cr-doped V 2 O 3 . Since the magnetic correlations are expected to be weak in this case, many predictions can be made from a single-site approach like the Dynamical Mean Field Theory (DMFT) [2], where local moments are described as freely fluctu-

Electronic, magnetic, or structural inhomogeneities ranging in size from nanoscopic to mesoscopic scales seem endemic and are possibly generic to colossal magnetoresistance manganites and other transition metal oxides. They are hence of great current interest and understanding them is of fundamental importance. We show here that an extension, to include long-range Coulomb interactions, of a quantum two-fluid ᐉ-b model proposed recently for manganites ͓Phys. Rev. Lett. 92, 157203 ͑2004͔͒ leads to an excellent description of such inhomogeneities. In the ᐉ-b model two very different kinds of electronic states, one localized and polaronic ͑ᐉ͒ and the other extended or broad band ͑b͒ coexist. For model parameters appropriate to manganites and even within a simple dynamical mean-field theory ͑DMFT͒ framework, it describes many of the unusual phenomena seen in manganites, including colossal magnetoresistance ͑CMR͒, qualitatively and quantitatively. However, in the absence of long-ranged Coulomb interaction, a system described by such a model would actually phase separate, into macroscopic regions of l and b electrons, respectively. As we show in this paper, in the presence of Coulomb interactions, the macroscopic phase separation gets suppressed and instead nanometer scale regions of polarons interspersed with band electron puddles appear, constituting a kind of quantum Coulomb glass. We characterize the size scales and distribution of the inhomogeneity using computer simulations. For realistic values of the long-range Coulomb interaction parameter V 0 , our results for the thresholds for occupancy of the b states are in agreement with, and hence support, the earlier approach mentioned above based on a configuration averaged DMFT treatment which neglects V 0 ; but the present work has features that cannot be addressed in the DMFT framework. Our work points to an interplay of strong correlations, long-range Coulomb interaction, and dopant ion disorder, all inevitably present in transition metal oxides as the origin of nanoscale inhomogeneities rather than disorder frustrated phase competition as is generally believed. As regards manganites, it argues against explanations for CMR based on disorder frustrated phase separation and for an intrinsic origin of CMR. Based on this, we argue that the observed micrometer ͑meso͒ scale inhomogeneities owe their existence to extrinsic causes, e.g., strain due to cracks and defects. We suggest possible experiments to validate our speculation.