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arXiv2009

Electrode Polarization in Glassy Electrolytes: Large Interfacial Capacitance Values and Indication for Pseudocapacitive Charge Storage

We study the electrode polarization behaviour of different Na-Ca-phosphosilicate glasses by measuring the differential capacitance between blocking Pt electrodes. At low applied dc bias voltages, we detect a linear capacitance regime with interfacial capacitance values considerably larger than expected from double layer theories and also considerably larger than found for ionic liquids with similar ion concentrations. With increasing bias voltages, the differential capacitance of interfacial layer exhibits a maximum around 1 V and a strong drop at higher voltages. We suggest that these features are caused by pseudocapacitive processes, namely by the adsorption of mobile Na+ ions at the electrodes followed by electronic charge transfer. While pseudocapacitive processes are well known in liquid electrochemistry, more detailed studies on solid electrolytes should offer perspectives for improved energy storage in solid-state supercapacitors

cond-mat.dis-nn↗

arXiv2009

Edge disorder and localization regimes in bilayer graphene nanoribbons

A theoretical study of the magnetoelectronic properties of zigzag and armchair bilayer graphene nanoribbons (BGNs) is presented. Using the recursive Green's function method, we study the band structure of BGNs in uniform perpendicular magnetic fields and discuss the zero-temperature conductance for the corresponding clean systems. The conductance quantized as 2(n+1)G_ for the zigzag edges and nG_0 for the armchair edges with G_{0}=2e^2/h being the conductance unit and $n$ an integer. Special attention is paid to the effects of edge disorder. As in the case of monolayer graphene nanoribbons (GNR), a small degree of edge disorder is already sufficient to induce a transport gap around the neutrality point. We further perform comparative studies of the transport gap E_g and the localization length in bilayer and monolayer nanoribbons. While for the GNRs E_{g}^{GNR}is proportional to 1/W, the corresponding transport gap E_{g}^{BGN} for the bilayer ribbons shows a more rapid decrease as the ribbon width W is increased. We also demonstrate that the evolution of localization lengths with the Fermi energy shows two distinct regimes. Inside the transport gap, xi is essentially independent on energy and the states in the BGNs are significantly less localized than those in the corresponding GNRs. Outside the transport gap ξgrows rapidly as the Fermi energy increases and becomes very similar for BGNs and GNRs.

cond-mat.mes-hall↗

arXiv2009

Extra condition is necessary to have a unique cluster wave vectors set in the periodic cluster approximations

We added an extra condition, original lattice symmetry of chosen cluster around cluster central site, to the cluster approximation methods with periodic boundary condition such as dynamical cluster approximation (DCA), effective medium approximation (EMSCA) and nonlocal coherent potential approximation (NLCPA). For each cluster size, this condition leads to a unique cluster wave vectors set in the first Brillouin zone (FBZ) where they preserve full symmetry of first Brillouin zone around ${\bf K=0}$. In this case whole cluster wave vectors are restricted to the FBZ and when number of sites in the cluster is equal to the whole lattice sites, these approximations recover original lattice symmetry in real and k-spaces.

cond-mat.dis-nn↗

arXiv2009

Correlation between Organic Magnetoresistance (OMAR) and Ferromagnetic ordering

We report observation of ferromagnetic (FM) ordering in organic semiconductors, namely regio-regular poly (3-hexyl thiophene) (RRP3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM), in the temperature range of 5-300 K in addition to magnetoresistance (OMAR) observed in the diodes made from the same materials. Particle induced x-ray emission spectroscopy confirms the presence of dilute magnetic impurities in the materials mainly as residues from the synthesis process. However, upon blending these two materials with FM signal, the FM ordering is suppressed by a huge paramagnetic (PM) signal indicating ground state charge transfer formation in the blend. Together with the magneto-transport studies, these results indicate that OMAR response is observed in a device only when the corresponding active materials are FM. In the diodes with P3HT:PCBM complex, that as a blend shows PM response, OMAR vanishes almost completely. We propose that ferromagnetism in the active material can have important correlation with the OMAR response in the diodes.

cond-mat.mtrl-sci↗

arXiv2013

Comparison of electron and phonon transport in disordered semiconductor carbon nanotubes

Charge and thermal conductivities are the most important parameters of carbon nanomaterials as candidates for future electronics. In this paper we address the effects of Anderson type disorder in long semiconductor carbon nanotubes (CNTs) to electron charge conductivity and lattice thermal conductivity using the atomistic Green function approach. The electron and phonon transmissions are analyzed as a function of the length of the disordered nanostructures. The thermal conductance as a function of temperature is calculated for different lengths. Analysis of the transmission probabilities as a function of length of the disordered device shows that both electrons and phonons with different energies display different transport regimes, i.e. quasi-ballistic, diffusive and localization regimes coexist. In the light of the results we discuss heating of the semiconductor device in electronic applications.

cond-mat.mes-hall↗

arXiv2009

Degree-based graph construction

Degree-based graph construction is an ubiquitous problem in network modeling, ranging from social sciences to chemical compounds and biochemical reaction networks in the cell. This problem includes existence, enumeration, exhaustive construction and sampling questions with aspects that are still open today. Here we give necessary and sufficient conditions for a sequence of nonnegative integers to be realized as a simple graph's degree sequence, such that a given (but otherwise arbitrary) set of connections from a arbitrarily given node are avoided. We then use this result to present a swap-free algorithm that builds {\em all} simple graphs realizing a given degree sequence. In a wider context, we show that our result provides a greedy construction method to build all the $f$-factor subgraphs embedded within $K_n\setminus S_k$, where $K_n$ is the complete graph and $S_k$ is a star graph centered on one of the nodes.

math.CO↗

arXiv2009

Effect of platy- and leptokurtic distributions in the random-field Ising model: Mean field approach

The influence of the tail features of the local magnetic field probability density function (PDF) on the ferromagnetic Ising model is studied in the limit of infinite range interactions. Specifically, we assign a quenched random field whose value is in accordance with a generic distribution that bears platykurtic and leptokurtic distributions depending on a single parameter $τ< 3$ to each site. For $τ< 5/3$, such distributions, which are basically Student-$t$ and $r$-distribution extended for all plausible real degrees of freedom, present a finite standard deviation, if not the distribution has got the same asymptotic power-law behavior as a $α$-stable Lévy distribution with $α= (3 - τ)/(τ- 1)$. For every value of $τ$, at specific temperature and width of the distribution, the system undergoes a continuous phase transition. Strikingly, we impart the emergence of an inflexion point in the temperature-PDF width phase diagrams for distributions broader than the Cauchy-Lorentz ($τ= 2$) which is accompanied with a divergent free energy per spin (at zero temperature).

cond-mat.stat-mech↗

arXiv2009

Coulomb Disorder Effects on ARPES and NQR Spectra in Cuprates

The role of Coulomb disorder, either of extrinsic origin or introduced by dopant ions in undoped and lightly-doped cuprates, is studied. We demonstrate that charged surface defects in an insulator lead to a Gaussian broadening of the Angle-Resolved Photoemisson Spectroscopy (ARPES) lines. The effect is due to the long-range nature of the Coulomb interaction. A tiny surface concentration of defects about a fraction of one per cent is sufficient to explain the line broadening observed in Sr$_2$CuO$_2$Cl$_2$, La$_2$CuO$_{4}$, and Ca$_{2}$CuO$_{2}$Cl$_{2}$. Due to the Coulomb screening, the ARPES spectra evolve dramatically with doping, changing their shape from a broad Gaussian form to narrow Lorentzian ones. To understand the screening mechanism and the lineshape evolution in detail, we perform Hartree-Fock simulations with random positions of surface defects and dopant ions. To check validity of the model we calculate the Nuclear Quadrupole Resonance (NQR) lineshapes as a function of doping and reproduce the experimentally observed NQR spectra. Our study also indicates opening of a substantial Coulomb gap at the chemical potential. For a surface CuO$_2$ layer the value of the gap is of the order of 10 meV while in the bulk it is reduced to the value about a few meV.

cond-mat.str-el↗

arXiv2009

Maintaining the local temperature below the critical value in thermally out of equilibrium superconducting wires

A generalized theory of open quantum systems combined with mean-field theory is used to study a superconducting wire in contact with thermal baths at different temperatures. It is shown that, depending on the temperature of the colder bath, the temperature of the hotter bath can greatly exceed the equilibrium critical temperature, and still the local temperature in the wire is maintained below the critical temperature and hence the wire remains in the superconducting state. The effects of contact areas and disorder are studied. Finally, an experimental setup is suggested to test our predictions.

cond-mat.mes-hall↗

arXiv2009

Fractal dimension and threshold properties in a spatially correlated percolation model

We consider the effects of spatial correlations in a two-dimensional site percolation model. By generalizing the Newman-Ziff Monte Carlo algorithm to include spatial correlations, percolation thresholds and fractal dimensions of percolation clusters are obtained. For a wide range of spatial correlations, the percolation threshold differs little from the uncorrelated result. In contrast, the fractal dimension differs sharply from the uncorrelated result for almost all types of correlation studied. We interpret these results in the framework of long-range correlated percolation.

cond-mat.dis-nn↗

arXiv2009

Role of thermal vibrations in phase transitions

All theoretical models (Heisenberg, Ising etc.) assume a negligible role for thermal vibrations in alloy and magnetic phase transitions. Analysis of diffraction data conclusively proves that this assumption is incorrect. A simple criterion emerges that theoretical models can ignore the role of thermal vibrations only if the Debye-Waller Factor is ignored in the analysis of diffraction data. Diffraction data constrain all theoretical models to incorporate the role of thermal vibrations. This conclusion is also supported by other experimental results, the effect of thermal vibrations on ordering energy that is of the same order of magnitude as ordering energy and an isotope effect on magnetic phase transitions. An electron-phonon interaction (EPI) formalism that incorporates the Debye-Waller Factor in electronic structure calculations already exists and must be adopted for a correct understanding of phase transitions as it can account for all the different experimental results mentioned above. The discrepancy between experimental and theoretical ordering energy in Ni3V is direct evidence for the role of thermal vibrations in altering ordering energy. The inter-nuclear potential energy term converges if zero point vibrations are incorporated and this method can replace the Ewald sum method. The three dimensional Ising model cannot represent order-disorder transition in beta brass, CuZn. An isotope effect is predicted for magnetic phase transitions if the transition temperature is below Debye temperature. The long range order parameter obtained from diffraction data can only be compared with predictions of models that incorporate the role of thermal vibrations and not otherwise.

physics.gen-ph↗

arXiv2010

A framework for simulating and estimating the state and functional topology of complex dynamic geometric networks

We present a framework for simulating signal propagation in geometric networks (i.e. networks that can be mapped to geometric graphs in some space) and for developing algorithms that estimate (i.e. map) the state and functional topology of complex dynamic geometric net- works. Within the framework we define the key features typically present in such networks and of particular relevance to biological cellular neural networks: Dynamics, signaling, observation, and control. The framework is particularly well-suited for estimating functional connectivity in cellular neural networks from experimentally observable data, and has been implemented using graphics processing unit (GPU) high performance computing. Computationally, the framework can simulate cellular network signaling close to or faster than real time. We further propose a standard test set of networks to measure performance and compare different mapping algorithms.

cond-mat.dis-nn↗

arXiv2009

Replica Approach in Random Matrix Theory

This Chapter outlines the replica approach in Random Matrix Theory. Both fermionic and bosonic versions of the replica limit are introduced and its trickery is discussed. A brief overview of early heuristic treatments of zero-dimensional replica field theories is given to advocate an exact approach to replicas. The latter is presented in two elaborations: by viewing the $β=2$ replica partition function as the Toda Lattice and by embedding the replica partition function into a more general theory of $τ$ functions.

cond-mat.dis-nn↗

arXiv2009

Filler Dependencies of Electrical Conductivity in Nanotube and Nanofiber Composites

We report on a model of polymer nanocomposites with fibrous fillers which explicitly considers the microscopic filler features and replicates the composites as random distributions of particles interconnected via electron tunneling. By exploiting the critical path method, we are able to obtain simple formulas, applicable to most nanotube and nanofiber composites, which allow to infer the overall composite conductivity starting from few parameters like filler volume fraction, size, and aspect-ratio. The validity of our formulation is assessed by reinterpreting existing experimental results and by extracting the characteristic tunneling length, which is mostly found within its expected value range. These results can be used practically to tailor the electrical properties of nanocomposites.

cond-mat.dis-nn↗

arXiv2009

Thermal conductance in thin wires with surface disorder

Elastic wave characteristics of the heat conduction in low-temperature thin wires can be studied via a wave scattering formalism. A reaction matrix formulation of heat conductance modeled by elastic wave scattering is advocated. This formulation allows us to treat thin wires with arbitrary surface disorder. It is found that the correlation in the surface disorder may significantly affect the temperature dependence of the heat conductance.

cond-mat.mes-hall↗

arXiv2012

Random matrix theory: Wigner-Dyson statistics and beyond. (Lecture notes of a course given at SISSA (Trieste, Italy))

This is a course on Random Matrix Theory which includes traditional as well as advanced topics presented with an extensive use of classical logarithmic plasma analogy and that of the quantum systems of one-dimensional interacting fermions with inverse square interaction (Calogero-Sutherland model). Certain non-invariant random matrix ensembles are also considered with the emphasis on the eigenfunction statistics in them. The course can also be viewed as introduction to theory of localization where the (non-invariant) random matrix ensembles play a role of the toy models to illustrate functional methods based on super-vector/super-matrix representations.

cond-mat.dis-nn↗

arXiv2012

Structural signature of jamming transition in thermal amorphous systems

In thermal amorphous systems, the first peak of the pair correlation function $g(r)$ shows a maximum height $g_1^{\rm max}$ at a volume fraction $ϕ=ϕ_v$ that increases with the temperature. $g_1^{\rm max}$ diverges at the T=0 jamming transition at $ϕ=ϕ_c$. Molecular dynamics simulations show that some typical quantities, such as the pressure, bulk modulus, shear modulus, and boson peak frequency that behave power law scalings with $ϕ-ϕ_c$ in marginally jammed solids at T=0, all show scalings with $ϕ-ϕ_c$ when $ϕ> ϕ_v$, while the scalings break down when $ϕ< ϕ_v$. The presence of $g_1^{\rm max}$ is thus not only a thermal vestige of the T=0 jamming transition, but more importantly the structural signature of the jamming transition.

cond-mat.soft↗

arXiv2009

Interactions Among Agent Variables and Evolution of Social Clusters

In this paper, we first review some basic concepts associated with a model for social interaction previously proposed by us. Each agent is seen as an array of variables that can be found in different states. The agents are then allowed to interact and form groups based on their variables. We discuss how spin-glass type physics may be appropriate for our model. Several types of variables and costs associated with flipping the variables are discussed. Then some simple graphs are presented to understand the formation of various levels of identities within social clusters. In the end, we analyze events from the French revolution and the Russian revolution to to understand how different variables and identities interact within a hierarchical social structure.

physics.soc-ph↗