Interdependence of chemical structure, thin-film morphology, and transport properties is a key, yet often elusive aspect characterizing the design and development of high-mobility, solution-processed polymers for large-area and flexible electronics applications. There is a specific need to achieve >1 cm 2 V −1 s −1 field-effect mobilities (μ) at low processing temperatures in combination with environmental stability, especially in the case of electron-transporting polymers, which are still lagging behind hole transporting materials. Here, the synthesis of a naphthalene-diimide based donor-acceptor copolymer characterized by a selenophene vinylene selenophene donor moiety is reported. Optimized field-effect transistors show maximum μ of 2.4 cm 2 V −1 s −1 and promising ambient stability. A very marked film structural evolution is revealed with increasing annealing temperature, with evidence of a remarkable 3D crystallinity above 180 °C. Conversely, transport properties are found to be substantially optimized at 150 °C, with limited gain at higher temperature. This discrepancy is rationalized by the presence of a surface-segregated prevalently edge-on packed polymer phase, dominating the device accumulated channel. This study therefore serves the purpose of presenting a promising, high-electron-mobility copolymer that is processable at relatively low temperatures, and of clearly highlighting the necessity of specifically investigating channel morphology in assessing the structure-property nexus in semiconducting polymer thin films.

The ground state of hard-core bosons on the square lattice with nearest and next-nearest neighbor repulsion is studied by Quantum Monte Carlo simulations. A supersolid phase with vacancy condensation and 'star' diagonal ordering is found for filling ρ < 0.25. At fillings ρ > 0.25 a supersolid phase exists between the star and the stripe crystal at ρ=0.5. No supersolid phase occurs for ρ > 0.25 if the ground state at half-filling is either a checkerboard crystal or a superfluid. No commensurate supersolid phase is observed.PACS numbers: 75.10. Jm, 05.30.Jp, 67.40.Kh, 74.25.Dw Experimental advances in the field of ultracold atoms in optical lattices [1] have given renewed impetus to the investigation of novel phases of matters, especially those displaying simultaneously different types of order. One such phase is the supersolid, featuring both diagonal and off-diagonal long range order, which has been the subject of much theoretical speculation [2]. Recent claims of observation of this phase in solid helium [3] have generated some controversy [4].Theoretical studies have yielded strong evidence of supersolid phases of lattice bosons, for various types of model interactions among atoms, as well as of lattice geometries [5,6,7,8]. In the presence of nearest-neighbor particle hopping and on-site hard core repulsion, the supersolid phase is only observed on the interstitial side of a commensurate solid phase (e.g., for 1/3 < ρ < 2/3 on the triangular lattice) [8]. In contrast, doping with vacancies results in the coexistence of an insulating crystal and a superfluid by the formation of a domain wall [9]. Phase separation of vacancies is also observed in ab initio simulations of helium crystals [10]. This seems remarkable, as Bose condensation of vacancies has long been regarded as the paradigm for supersolidity [2]. The purpose of this work is to gain understanding in the asymmetry between the behavior of vacancies and interstitials, and explore physical conditions that underlie a vacancy supersolid phase in lattice bosons.Our starting point is the well-known HamiltonianA square lattice with periodic boundary conditions of N = L × L sites is assumed. The boson density (filling) is ρ = N B /N , where the number of particles N B is determined by the chemical potential µ. The operatorb † i (b i ) creates (annihilates) a hard-core boson on site i, with a maximum occupation numbern i =b † ib i of one particle per site. The first term of (1) describes particle hopping to a nearest-neighboring site with amplitude t, which is our energy scale, t = 1. The second and third terms represent repulsive interactions between bosons on nearest and next-nearest neighboring sites, respectively.Previous studies of the ground state of (1) have yielded evidence of three possible phases at half-filling ( ρ=0.5): a superfluid, a checkerboard solid and a stripe solid. The latter two are commensurate and insulating phases. Doping the stripe crystal away from half filling yields a supersolid phase, whereas a first-order quantum phase transit...

We study the ground state phase diagram of a two-dimensional kagome lattice spin-1/2 XY model (J) with a four-site ring exchange interaction (K) using quantum Monte Carlo simulations. We find that the superfluid phase, existing in the regime of small ring exchange, undergoes a direct transition to a Z2 quantum spin liquid phase at (K/J)c ≈ 22, which is related to the phase proposed by Balents, Girvin and Fisher [Phys. Rev. B, 65 224412 (2002)]. The quantum phase transition between the superfluid and the spin liquid phase has exponents z and ν falling in the 3D XY universality class, making it a candidate for an exotic XY* quantum critical point, mediated by the condensation of bosonic spinons.The coveted quantum spin liquid state, proposed more than three decades ago by Fazekas and Anderson [1], remains surprisingly elusive. Although recent experiments [2-4] provide tantalizing evidence of their existence, the scarcity of spin liquid states in microscopic models belies their resistance to theoretical characterization. Even a basic unifying notion for which microscopic ingredients are required to promote spin liquid states in models is muddied. Geometric frustration is the main suspect; recent Density-Matrix Renormalization Group measurements have championed the case for a spin liquid as the long-debated groundstate of the kagome lattice Heisenberg antiferromagnet [5]. However, the apparent discovery of a gapped spin liquid state in quantum Monte Carlo (QMC) simulations of the honeycomb lattice Hubbard model at half filling seems to contradict some longheld notions for where spin liquids might lie [6]. The extensive theoretical framework [7][8][9][10], developed over decades, continues to undergo refinement [11][12][13] motivated by these and other discoveries from large-scale computer simulations of a relatively small number of models.Part of the difficulty in finding models that harbor spin liquids in two-dimensions (2D) and higher is the fact the important ingredient of frustration typically leads to the infamous sign problem and preclusion of the model from being studied by scalable QMC techniques. A significant development in this front came in 2002, when Balents et. al.[14] proposed a sign-problem free Hamiltonian of spins on the kagome lattice which gives rise to a Z 2 spin liquid phase. Away from their exactly soluble point, the spin liquid appears to be robust, as several models containing XY terms and constrained potentials have now shown evidence of this spin liquid ground state [15][16][17]. In this paper, we demonstrate that a Z 2 spin liquid phase can also be stabilized in a model with competition between purely kinetic terms, namely two-site XY and four-site "ring" exchange interactions. This Hamiltonian is amenable to large-scale quantum Monte Carlo (QMC) studies, which allows us to characterize the spin liquid state -and the quantum phase transition into this state -in great detail. We find that there exists a single di- rect quantum phase transition into the spin liquid from the superfluid phas...

We use quantum Monte Carlo simulations to study the phase diagram of hard-core bosons with short-ranged {\it attractive} interactions, in the presence of uniform diagonal disorder. It is shown that moderate disorder stabilizes a glassy superfluid phase in a range of values of the attractive interaction for which the system is a Mott insulator, in the absence of disorder. A transition to an insulating Bose glass phase occurs as the strength of the disorder or interactions increases.Comment: 5 pages, 6 figure

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