306 research outputs found
Thermodynamics of the two-dimensional Falicov-Kimball model: a classical Monte Carlo study
The two-dimensional Falicov-Kimball (FK) model is analyzed using Monte Carlo
method. In the case of concentrations of both itinerant and localized particles
equal to 0.5 we determine temperature dependence of specific heat, charge
density wave susceptibility and density-density correlation function. In the
weak interaction regime we find a first order transition to the ordered state
and anomalous temperature dependence of the correlation function. We construct
the phase diagram of half-filled FK model. Also, the role of
next-nearest-neighbor hopping on the phase diagram is analyzed. Lastly, we
discuss the density of states and the spectral functions for the mobile
particles in weak and strong interaction regime.Comment: 15 pages, RevTe
Ising t-J model close to half filling: A Monte Carlo study
Within the recently proposed doped-carrier representation of the projected
lattice electron operators we derive a full Ising version of the t-J model.
This model possesses the global discrete Z_2 symmetry as a maximal spin
symmetry of the Hamiltonian at any values of the coupling constants, t and J.
In contrast, in the spin anisotropic limit of the t-J model, usually referred
to as the t-J_z model, the global SU(2) invariance is fully restored at J_z=0,
so that only the spin-spin interaction has in that model the true Ising form.
We discuss a relationship between those two models and the standard isotropic
t-J model. We show that the low-energy quasiparticles in all three models share
the qualitatively similar properties at low doping and small values of J/t. The
main advantage of the proposed Ising t-J model over the t-J_z one is that the
former allows for the unbiased Monte Carlo calculations on large clusters of up
to 10^3 sites. Within this model we discuss in detail the destruction of the
antiferromagnetic order by doping as well as the interplay between the AF order
and hole mobility. We also discuss the effect of the exchange interaction and
that of the next nearest neighbour hoppings on the destruction of the AF order
at finite doping. We show that the short-range AF order is observed in a wide
range of temperatures and dopings, much beyond the boundaries of the AF phase.
We explicitly demonstrate that the local no double occupancy constraint plays
the dominant role in destroying the magnetic order at finite doping. Finally, a
role of inhomogeneities is discussed.Comment: 24 pages, 10 figure
Completing the triad: Synthesis and full characterization of homoleptic and heteroleptic carbonyl and nitrosyl complexes of the group VI metals
Oxidation of M(CO) (M = Cr, Mo, W) with the synergistic oxidative system Ag[WCA]/0.5 I yields the fully characterized metalloradical salts [M(CO)]+˙[WCA]− (weakly coordinating anion WCA = [F-{Al(OR)}], R = C(CF)). The new metalloradical cations with M = Mo and W showcase a similar structural fluxionality as the previously reported [Cr(CO)]˙. Their reactivity increases from M = Cr < Mo < W and their syntheses allow for in-depth insights into the properties of the group 6 carbonyl triad. Furthermore, the reaction of NO[WCA] with neutral carbonyl complexes M(CO) gives access to the heteroleptic carbonyl/nitrosyl cations [M(CO)(NO)] as salts of the WCA [Al(ORF)], the first complete transition metal triad of their kind
The planar thermal Hall conductivity in the Kitaev magnet {\alpha}-RuCl3
We report detailed measurements of the Onsager-like planar thermal Hall
conductivity in -RuCl, a spin-liquid candidate of
topical interest. With the thermal current and magnetic field
(zigzag axis), the observed varies strongly
with temperature (1-10 K). The results are well-described by bosonic edge
excitations which evolve to topological magnons at large . Fits to
yield a Chern number and a band energy 1
meV, in agreement with sharp modes seen in electron spin-resonance experiments.
The bosonic character is incompatible with half-quantization of
.Comment: 7 pages, 3 figure
Phase transitions in the spinless Falicov-Kimball model with correlated hopping
The canonical Monte-Carlo is used to study the phase transitions from the
low-temperature ordered phase to the high-temperature disordered phase in the
two-dimensional Falicov-Kimball model with correlated hopping. As the
low-temperature ordered phase we consider the chessboard phase, the axial
striped phase and the segregated phase. It is shown that all three phases
persist also at finite temperatures (up to the critical temperature )
and that the phase transition at the critical point is of the first order for
the chessboard and axial striped phase and of the second order for the
segregated phase. In addition, it is found that the critical temperature is
reduced with the increasing amplitude of correlated hopping in the
chessboard phase and it is strongly enhanced by in the axial striped and
segregated phase.Comment: 17 pages, 6 figure
Physicochemical and Antibacterial Characterisation of a Novel Fluorapatite Coating
Peri-implantitis remains the major impediment to the long-term use of dental
implants. With increasing concern over growing antibiotic resistance there is
considerable interest in the preparation of antimicrobial dental implant coatings that
also induce osseointegration. One such potential coating material is fluorapatite
(FA). The aim of this study was to relate the antibacterial effectiveness of FA
coatings against pathogens implicated in peri-implantitis to the physicochemical
properties of the coating. Ordered and disordered FA coatings were produced on the
under and upper surface of stainless steel (SS) discs respectively, using a
hydrothermal method. Surface charge, surface roughness, wettability and fluoride
release were measured for each coating. Surface chemistry was assessed by X-ray
photoelectron spectroscopy and FA crystallinity by X-ray diffraction. Antibacterial
activity against periodontopathogens was assessed in vitro using viable counts,
confocal and scanning electron (SEM) microscopies. SEM showed that the
hydrothermal method produced FA coatings predominately aligned perpendicular to
the SS substrate or disordered FA coatings consisting of randomly aligned rod-like
crystals. Both FA coatings significantly reduced the growth of all the examined
bacterial strains in comparison to the control. The FA coatings, and especially the
disordered ones, presented significantly lower charge, higher roughness and area
when compared to the control, enhancing bacteria–material interactions and
therefore bacterial deactivation by fluoride ions. The ordered FA layer reduced not
only bacterial viability but adhesion too. Ordered FA crystals produced as a potential
novel implant coating showed significant antibacterial activity against bacteria
implicated in peri-implantitis which could be explained by a detailed understanding of
their physicochemical properties
In Situ Small-Angle X-ray Scattering Studies During Reversible Addition–Fragmentation Chain Transfer Aqueous Emulsion Polymerization
Polymerization-induced self-assembly (PISA) is a powerful platform technology for the rational and efficient synthesis of a wide range of block copolymer nano-objects (e.g., spheres, worms or vesicles) in various media. In situ small-angle X-ray scattering (SAXS) studies of reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization have previously provided detailed structural information during self-assembly (see M. J. Derry et al., Chem. Sci. 2016, 7, 5078–5090). However, conducting the analogous in situ SAXS studies during RAFT aqueous emulsion polymerizations poses a formidable technical challenge because the inherently heterogeneous nature of such PISA formulations requires efficient stirring to generate sufficiently small monomer droplets. In the present study, the RAFT aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA) has been explored for the first time. Chain extension of a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block leads to the formation of sterically-stabilized PGMA-PMOEMA spheres, worms or vesicles, depending on the precise reaction conditions. Construction of a suitable phase diagram enables each of these three morphologies to be reproducibly targeted at copolymer concentrations ranging from 10 to 30% w/w solids. High MOEMA conversions are achieved within 2 h at 70 °C, which makes this new PISA formulation well-suited for in situ SAXS studies using a new reaction cell. This bespoke cell enables efficient stirring and hence allows in situ monitoring during RAFT emulsion polymerization for the first time. For example, the onset of micellization and subsequent evolution in particle size can be studied when preparing PGMA29-PMOEMA30 spheres at 10% w/w solids. When targeting PGMA29-PMOEMA70 vesicles under the same conditions, both the micellar nucleation event and the subsequent evolution in the diblock copolymer morphology from spheres to worms to vesicles are observed. These new insights significantly enhance our understanding of the PISA mechanism during RAFT aqueous emulsion polymerization
Nuclear Quantum Effects in Water and Aqueous Systems: Experiment, Theory, and Current Challenges
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 properties. These have been combined with theoretical developments such as the introduction of the principle of competing quantum effects 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 this area of research
Thermodynamic studies of the two dimensional Falicov-Kimball model on a triangular lattice
Thermodynamic properties of the spinless Falicov-Kimball model are studied on
a triangular lattice using numerical diagonalization technique with Monte-Carlo
simulation algorithm. Discontinuous metal-insulator transition is observed at
finite temperature. Unlike the case of square lattice, here we observe that the
finite temperature effect is not able to smear out the discontinuous
metal-insulator transition seen in the ground state. Calculation of specific
heat (C_v) shows single and double peak structures for different values of
parameters like on-site correlation strength (U), f-electron energy (E_f) and
temperature.Comment: 6 pages, 7 figure
Extending SMT Solvers to Higher-Order Logic
International audienceSMT solvers have throughout the years been able to cope with increasingly expressive formulas, from ground logics to full first-order logic (FOL). In contrast, the extension of SMT solvers to higher-order logic (HOL) is mostly un-explored. We propose a pragmatic extension for SMT solvers to support HOL reasoning natively without compromising performance on FOL reasoning, thus leveraging the extensive research and implementation efforts dedicated to efficient SMT solving. We show how to generalize data structures and the ground decision procedure to support partial applications and extensionality, as well as how to reconcile quantifier instantiation techniques with higher-order variables. We also discuss a separate approach for redesigning an HOL SMT solver from the ground up via new data structures and algorithms. We apply our pragmatic extension to the CVC4 SMT solver and discuss a redesign of the veriT SMT solver. Our evaluation shows they are competitive with state-of-the-art HOL provers and often outperform the traditional encoding into FOL
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