On the universality of compact polymers

Fully packed loop models on the square and the honeycomb lattice constitute new classes of critical behaviour, distinct from those of the low-temperature O(n) model. A simple symmetry argument suggests that such compact phases are only possible when the underlying lattice is bipartite. Motivated by the hope of identifying further compact universality classes we therefore study the fully packed loop model on the square-octagon lattice. Surprisingly, this model is only critical for loop weights n < 1.88, and its scaling limit coincides with the dense phase of the O(n) model. For n=2 it is exactly equivalent to the selfdual 9-state Potts model. These analytical predictions are confirmed by numerical transfer matrix results. Our conclusions extend to a large class of bipartite decorated lattices.Comment: 13 pages including 4 figure

Simulations of energetic beam deposition: from picoseconds to seconds

We present a new method for simulating crystal growth by energetic beam deposition. The method combines a Kinetic Monte-Carlo simulation for the thermal surface diffusion with a small scale molecular dynamics simulation of every single deposition event. We have implemented the method using the effective medium theory as a model potential for the atomic interactions, and present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to 35 eV. The method is capable of following the growth of several monolayers at realistic growth rates of 1 monolayer per second, correctly accounting for both energy-induced atomic mobility and thermal surface diffusion. We find that the energy influences island and step densities and can induce layer-by-layer growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag), which correlates with where the net impact-induced downward interlayer transport is at a maximum. A high step density is needed for energy induced layer-by-layer growth, hence the effect dies away at increased temperatures, where thermal surface diffusion reduces the step density. As part of the development of the method, we present molecular dynamics simulations of single atom-surface collisions on flat parts of the surface and near straight steps, we identify microscopic mechanisms by which the energy influences the growth, and we discuss the nature of the energy-induced atomic mobility

Dense loops, supersymmetry, and Goldstone phases in two dimensions

Loop models in two dimensions can be related to O(N) models. The low-temperature dense-loops phase of such a model, or of its reformulation using a supergroup as symmetry, can have a Goldstone broken-symmetry phase for N<2. We argue that this phase is generic for -2< N <2 when crossings of loops are allowed, and distinct from the model of non-crossing dense loops first studied by Nienhuis [Phys. Rev. Lett. 49, 1062 (1982)]. Our arguments are supported by our numerical results, and by a lattice model solved exactly by Martins et al. [Phys. Rev. Lett. 81, 504 (1998)].Comment: RevTeX, 5 pages, 3 postscript figure

Inelastic Scattering in Metal-H2-Metal Junctions

We present first-principles calculations of the dI/dV characteristics of an H2 molecule sandwiched between Au and Pt electrodes in the presence of electron-phonon interactions. The conductance is found to decrease by a few percentage at threshold voltages corresponding to the excitation energy of longitudinal vibrations of the H2 molecule. In the case of Pt electrodes, the transverse vibrations can mediate transport through otherwise non-transmitting Pt $d$-channels leading to an increase in the differential conductance even though the hydrogen junction is characterized predominately by a single almost fully open transport channel. In the case of Au, the transverse modes do not affect the dI/dV because the Au d-states are too far below the Fermi level. A simple explanation of the first-principles results is given using scattering theory. Finally, we compare and discuss our results in relation to experimental data.Comment: Accepted in Phys. Rev.

Loop Model with Generalized Fugacity in Three Dimensions

A statistical model of loops on the three-dimensional lattice is proposed and is investigated. It is O(n)-type but has loop fugacity that depends on global three-dimensional shapes of loops in a particular fashion. It is shown that, despite this non-locality and the dimensionality, a layer-to-layer transfer matrix can be constructed as a product of local vertex weights for infinitely many points in the parameter space. Using this transfer matrix, the site entropy is estimated numerically in the fully packed limit.Comment: 16pages, 4 eps figures, (v2) typos and Table 3 corrected. Refs added, (v3) an error in an explanation of fig.2 corrected. Refs added. (v4) Changes in the presentatio

Interplay between quantum criticality and geometrical frustration in Fe3Mo3N with stella quadrangula lattice

In the eta-carbide-type correlated-electron metal Fe3Mo3N, ferromagnetism is abruptly induced from a nonmagnetic non-Fermi-liquid ground state either when a magnetic field (~14 T) applied to it or when it is doped with a slight amount of impurity (~5% Co). We observed a peak in the paramagnetic neutron scattering intensity at finite wave vectors, revealing the presence of the antiferromagnetic (AF) correlation hidden in the magnetic measurements. It causes a new type of geometrical frustration in the stellla quadrangula lattice of the Fe sublattice. We propose that the frustrated AF correlation suppresses the F correlation to its marginal point and is therfore responsible for the origin of the ferromagnetic (F) quantum critical behavior in pure Fe3Mo3N

Theory of the optical conductivity of (TMTSF)2_2PF6_6 in the mid-infrared range

We propose an explanation of the mid-infrared peak observed in the optical conductivity of the Bechgaard salt (TMTSF)$_2$PF$_6$ in terms of electronic excitations. It is based on a numerical calculation of the conductivity of the quarter-filled, dimerized Hubbard model. The main result is that, even for intermediate values of $U/t$ for which the charge gap is known to be very small, the first peak, and at the same time the main structure, of the optical conductivity is at an energy of the order of the dimerization gap, like in the infinite $U$ case. This surprising effect is a consequence of the optical selection rules.Comment: 10 pages, 9 uuencoded figure

A study of 15N14N isotopic exchange over cobalt molybdenum nitrides

The 14N/15N isotopic exchange pathways over Co3Mo3N, a material of interest as an ammonia synthesis catalyst and for the development of nitrogen transfer reactions, have been investigated. Both the homomolecular and heterolytic exchange processes have been studied, and it has been shown that lattice nitrogen species are exchangeable. The exchange behavior was found to be a strong function of pretreatment with ca. 25% of lattice N atoms being exchanged after 40 min at 600 °C after N2 pretreatment at 700 °C compared to only 6% following similar Ar pretreatment. This observation, for which the potential contribution of adsorbed N species can be discounted, is significant in terms of the application of this material. In the case of the Co6Mo6N phase, regeneration to Co3Mo3N under 15N2 at 600 °C occurs concurrently with 14N15N formation. These observations demonstrate the reactivity of nitrogen in the Co–Mo–N system to be a strong function of pretreatment and worthy of further consideration

Duality and Multicritical Point of Two-Dimensional Spin Glasses

Determination of the precise location of the multicritical point and phase boundary is a target of active current research in the theory of spin glasses. In this short note we develop a duality argument to predict the location of the multicritical point and the shape of the phase boundary in models of spin glasses on the square lattice.Comment: 4 pages, 1 figure; Reference updated, definition of \tilde{V} added; to be published in J. Phys. Soc. Jp

Distinct nature of static and dynamic magnetic stripes in cuprate superconductors

We present detailed neutron scattering studies of the static and dynamic stripes in an optimally doped high-temperature superconductor, La$_2$CuO$_{4+y}$. We find that the dynamic stripes do not disperse towards the static stripes in the limit of vanishing energy transfer. We conclude that the dynamic stripes observed in neutron scattering experiments are not the Goldstone modes associated with the broken symmetry of the simultaneously observed static stripes, but rather that the signals originate from different domains in the sample. These domains may be related by structural twinning, or may be entirely different phases, where the static stripes in one phase are pinned versions of the dynamic stripes in the other. Our results explain earlier observations of unusual dispersions in underdoped La$_{2-x}$Sr$_x$CuO$_{4}$ ($x=0.07$) and La$_{2-x}$Ba$_x$CuO$_{4}$ ($x=0.095$). Our findings are relevant for all compounds exhibiting magnetic stripes, and may thus be a vital part in unveiling the nature of high temperature superconductivity