Hydrogen Absorption Properties of Metal-Ethylene Complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties

Quantum Fluctuations Driven Orientational Disordering: A Finite-Size Scaling Study

The orientational ordering transition is investigated in the quantum generalization of the anisotropic-planar-rotor model in the low temperature regime. The phase diagram of the model is first analyzed within the mean-field approximation. This predicts at $T=0$ a phase transition from the ordered to the disordered state when the strength of quantum fluctuations, characterized by the rotational constant $\Theta$, exceeds a critical value $\Theta_{\rm c}^{MF}$. As a function of temperature, mean-field theory predicts a range of values of $\Theta$ where the system develops long-range order upon cooling, but enters again into a disordered state at sufficiently low temperatures (reentrance). The model is further studied by means of path integral Monte Carlo simulations in combination with finite-size scaling techniques, concentrating on the region of parameter space where reentrance is predicted to occur. The phase diagram determined from the simulations does not seem to exhibit reentrant behavior; at intermediate temperatures a pronounced increase of short-range order is observed rather than a genuine long-range order.Comment: 27 pages, 8 figures, RevTe

Melting of Colloidal Molecular Crystals on Triangular Lattices

The phase behavior of a two-dimensional colloidal system subject to a commensurate triangular potential is investigated. We consider the integer number of colloids in each potential minimum as rigid composite objects with effective discrete degrees of freedom. It is shown that there is a rich variety of phases including ``herring bone'' and ``Japanese 6 in 1'' phases. The ensuing phase diagram and phase transitions are analyzed analytically within variational mean-field theory and supplemented by Monte Carlo simulations. Consequences for experiments are discussed.Comment: 10 pages, 4 figure

Dual-path state reconstruction scheme for propagating quantum microwaves and detector noise tomography

Quantum state reconstruction involves measurement devices that are usually described by idealized models, but not known in full detail in experiments. For weak propagating microwaves, the detection process requires linear amplifiers which obscure the signal with random noise. Here, we introduce a theory which nevertheless allows one to use these devices for measuring all quadrature moments of propagating quantum microwaves based on cross-correlations from a dual-path amplification setup. Simultaneously, the detector noise properties are determined, allowing for tomography. We demonstrate the feasibility of our novel concept by proof-of-principle experiments with classical mixtures of weak coherent microwaves.Comment: 11 pages, 3 figure

Subexponential Parameterized Directed Steiner Network Problems on Planar Graphs: A Complete Classification

In the Directed Steiner Network problem, the input is a directed graph G, asubset T of k vertices of G called the terminals, and a demand graph D on T.The task is to find a subgraph H of G with the minimum number of edges suchthat for every edge (s,t) in D, the solution H contains a directed s to t path.In this paper we investigate how the complexity of the problem depends on thedemand pattern when G is planar. Formally, if \mathcal{D} is a class ofdirected graphs closed under identification of vertices, then the\mathcal{D}-Steiner Network (\mathcal{D}-SN) problem is the special case wherethe demand graph D is restricted to be from \mathcal{D}. For general graphs,Feldmann and Marx [ICALP 2016] characterized those families of demand graphswhere the problem is fixed-parameter tractable (FPT) parameterized by thenumber k of terminals. They showed that if \mathcal{D} is a superset of one ofthe five hard families, then \mathcal{D}-SN is W[1]-hard parameterized by k,otherwise it can be solved in time f(k)n^{O(1)}. For planar graphs an interesting question is whether the W[1]-hard cases canbe solved by subexponential parameterized algorithms. Chitnis et al. [SICOMP2020] showed that, assuming the ETH, there is no f(k)n^{o(k)} time algorithmfor the general \mathcal{D}-SN problem on planar graphs, but the special casecalled Strongly Connected Steiner Subgraph can be solved in time f(k)n^{O(\sqrt{k})} on planar graphs. We present a far-reaching generalization andunification of these two results: we give a complete characterization of thebehavior of every $\mathcal{D}$-SN problem on planar graphs. We show thatassuming ETH, either the problem is (1) solvable in time 2^{O(k)}n^{O(1)}, andnot in time 2^{o(k)}n^{O(1)}, or (2) solvable in time f(k)n^{O(\sqrt{k})}, butnot in time f(k)n^{o(\sqrt{k})}, or (3) solvable in time f(k)n^{O(k)}, but notin time f(k)n^{o({k})}.<br