Nuclear quantum effects influence the structure and dynamics of hydrogen bonded systems, such as water, which impacts their observed properties with widely varying magnitudes. This review highlights the recent significant developments in the experiment, theory and simulation of nuclear quantum effects in water. Novel experimental techniques, such as deep inelastic neutron scattering, now provide a detailed view of the role of nuclear quantum effects in water's 2 properties. These have been combined with theoretical developments such as the introduction of the competing quantum effects principle that allows the subtle interplay of water's quantum effects and their manifestation in experimental observables to be explained. We discuss how this principle has recently been used to explain the apparent dichotomy in water's isotope effects, which can range from very large to almost nonexistent depending on the property and conditions. We then review the latest major developments in simulation algorithms and theory that have enabled the efficient inclusion of nuclear quantum effects in molecular simulations, permitting their combination with on-the-fly evaluation of the potential energy surface using electronic structure theory. Finally, we identify current challenges and future opportunities in the area.3

We review the interplay of frustration and strong electronic correlations in quasi-two-dimensional organic charge transfer salts, such as (BEDT-TTF) 2 X and EtnMe 4−n P n[Pd(dmit) 2 ] 2 . These two forces drive a range of exotic phases including spin liquids, valence bond crystals, pseudogapped metals, and unconventional superconductivity. Of particular interest is that in several materials with increasing pressure there is a first-order transition from a spin liquid Mott insulating state to a superconducting state. Experiments on these materials raise a number of profound questions about the quantum behaviour of frustrated systems, particularly the intimate connection between spin liquids and superconductivity. Insights into these questions have come from a wide range of theoretical techniques including first principles electronic structure, quantum many-body theory and quantum field theory. In this review we introduce some of the basic ideas of the field by discussing a simple frustrated Heisenberg model with four spins. We then describe the key experimental results, emphasizing that for two materials, κ-(BEDT-TTF) 2 Cu 2 -(CN) 3 and EtMe 3 Sb[Pd(dmit) 2 ] 2 , there is strong evidence for a spin liquid ground state, and for another, EtMe 3 P[Pd(dmit) 2 ] 2 , there is evidence of a valence bond crystal ground state. We review theoretical attempts to explain these phenomena, arguing that they can be captured by a Hubbard model on the anisotropic triangular lattice at half filling, and that Resonating Valence Bond (RVB) wavefunctions capture most of the essential physics. We review evidence that this Hubbard model can have a spin liquid ground state for a range of parameters that are realistic for the relevant materials. In particular, spatial anisotropy and ring exchange are key to destabilising magnetic order. We conclude by summarising the progress made thus far and identifying some of the key questions still to be answered. (S,S13,S24)= J J' J J J 1 2 3 4

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We review the role of strong electronic correlations in quasi-twodimensional organic charge transfer salts such as (BEDT-TTF)2X, (BETS)2Y and β ′ -[Pd(dmit)2]2Z. We begin by defining minimal models for these materials. It is necessary to identify two classes of material: the first class is strongly dimerised and is described by a half-filled Hubbard model; the second class is not strongly dimerised and is described by a quarter filled extended Hubbard model. We argue that these models capture the essential physics of these materials. We explore the phase diagram of the half-filled quasi-two-dimensional organic charge transfer salts, focusing on the metallic and superconducting phases. We review work showing that the metallic phase, which has both Fermi liquid and 'bad metal' regimes, is described both quantitatively and qualitatively by dynamical mean field theory (DMFT). The phenomenology of the superconducting state is still a matter of contention. We critically review the experimental situation, focusing on the key experimental results that may distinguish between rival theories of superconductivity, particularly probes of the pairing symmetry and measurements of the superfluid stiffness. We then discuss some strongly correlated theories of superconductivity, in particular, the resonating valence bond (RVB) theory of superconductivity. We conclude by discussing some of the major challenges currently facing the field. These include: parameterising minimal models; the evidence for a pseudogap from nuclear magnetic resonance (NMR) experiments; superconductors with low critical temperatures and extremely small superfluid stiffnesses; the possible spin-liquid states in κ-(ET)2Cu2(CN)3 and β ′ -[Pd(dmit)2]2Z; and the need for high quality large single crystals.

The temperature dependence of the transport properties of the metallic phase of a frustrated Hubbard model on the hypercubic lattice at half-filling is calculated. Dynamical mean-field theory, which maps the Hubbard model onto a single impurity Anderson model that is solved self-consistently, and becomes exact in the limit of large dimensionality, is used. As the temperature increases there is a smooth crossover from coherent Fermi liquid excitations at low temperatures to incoherent excitations at high temperatures. This crossover leads to a nonmonotonic temperature dependence for the resistance, thermopower, and Hall coefficient, unlike in conventional metals. The resistance smoothly increases from a quadratic temperature dependence at low temperatures to large values which can exceed the Mott-Ioffe-Regel value បa/e 2 ͑where a is a lattice constant͒ associated with mean free paths less than a lattice constant. Further signatures of the thermal destruction of quasiparticle excitations are a peak in the thermopower and the absence of a Drude peak in the optical conductivity. The results presented here are relevant to a wide range of strongly correlated metals, including transition metal oxides, strontium ruthenates, and organic metals.

We consider the competition between superconducting, charge ordered, and metallic phases in layered molecular crystals with the u and b 00 structures. Applying slave-boson theory to the relevant extended Hubbard model, we show that the superconductivity is mediated by charge fluctuations and the Cooper pairs have d xy symmetry. This is in contrast to the k-͑BEDT-TTF͒ 2 X family, for which theoretical calculations give superconductivity mediated by spin fluctuations and with d x 2 2y 2 symmetry. We predict several materials that should become superconducting under pressure. DOI: 10.1103/PhysRevLett.87.237002 PACS numbers: 74.70.Kn, 71.10.Fd, 71.27. +a The issue of the interplay of superconductivity, magnetism, and charge ordering is relevant to a wide range of strongly correlated electron materials. Examples include the copper-oxide (high-temperature) superconductors [1], colossal magnetoresistance materials [2], heavy fermion compounds [3], vanadium oxides [4], and organic molecular crystals [5][6][7]. In particular, for the cuprate superconductors there is controversy about the relative importance of charge fluctuations (associated with dynamical "stripes") and antiferromagnetic spin fluctuations (associated with the Mott insulator which occurs when there is an average of one electron or hole for every lattice site). For some heavy fermion compounds recent experiments support the idea that the superconductivity is mediated by spin fluctuations [3].The family k-͑BEDT-TTF͒ 2 X [8] of molecular crystals has similarities to the cuprates [6] including the proximity of superconductivity to a Mott insulator in the phase diagram. Although there is an average of half a hole per molecule the necessary condition of one hole per lattice site is met because the molecules are paired up (dimerized) within the k-type crystal structure. Theoretical calculations [9] suggest that the superconductivity has d x 2 2y 2 symmetry (as in the cuprates) and is mediated by antiferromagnetic spin fluctuations. However, there is controversy about whether experiments support this [10]. In this Letter, we show theoretically that the organic superconductors listed in Table I crystal structures the donor molecules are not dimerized and so noninteracting electron models (band structure calculations) predict a metallic state due to a band which is one-quarter filled with holes. However, some of these materials are insulators at low temperatures. Mott insulators (resulting from the Coulomb repulsion between electrons on a single site) occur only for a half-filled band. However, the localization of charge (and associated insulating behavior) could result from charge ordering due to the Coulomb repulsion between electrons on neighboring sites. Indeed, such charge ordering is observed in some of these materials and is reflected in a disproportion of charge between neighboring donor molecules (see Ref.[7] for a brief review of how this is determined experimentally). Depending on the anion, temperature, and pressure, the materials can be either a ...

We investigate the entanglement characteristics of two general bimodal Bose-Einstein condensates -a pair of tunnel-coupled Bose-Einstein condensates and the atom-molecule Bose-Einstein condensate. We argue that the entanglement is only physically meaningful if the system is viewed as a bipartite system, where the subsystems are the two modes. The indistinguishibility of the particles in the condensate means that the atomic constituents are physically inaccessible and thus the degree of entanglement between individual particles, unlike the entanglement between the modes, is not experimentally relevant so long as the particles remain in the condensed state. We calculate the entanglement between the two modes for the exact ground state of the two bimodal condensates and consider the dynamics of the entanglement in the tunnel-coupled case.

We use series expansion methods to calculate the dispersion relation of the one-magnon excitations for the spin-1/2 triangular-lattice nearest-neighbor Heisenberg antiferromagnet above a threesublattice ordered ground state. Several striking features are observed compared to the classical (large-S) spin-wave spectra. Whereas, at low energies the dispersion is only weakly renormalized by quantum fluctuations, significant anomalies are observed at high energies. In particular, we find roton-like minima at special wave-vectors and strong downward renormalization in large parts of the Brillouin zone, leading to very flat or dispersionless modes. We present detailed comparison of our calculated excitation energies in the Brillouin zone with the spin-wave dispersion to order 1/S calculated recently by Starykh, Chubukov, and Abanov [cond-mat/0608002]. We find many common features but also some quantitative and qualitative differences. We show that at temperatures as low as 0.1J the thermally excited rotons make a significant contribution to the entropy. Consequently, unlike for the square lattice model, a non-linear sigma model description of the finite-temperature properties is only applicable at extremely low temperatures.